In-situ directional solidification apparatus and method for observing alumina film induced rolling

Abstract

The present application relates to the technical field of metal material solidification process observation, in particular to an in-situ directional solidification device and method for observing alumina film induced rolling, which comprises: a crucible for containing melt, the crucible has a melting zone and a solidification zone along the axial direction; a heating unit for heating the melting zone; a cooling unit for cooling the solidification zone to establish a temperature gradient; a driving unit for driving the melt to move along the direction from the melting zone to the solidification zone and inducing controlled flow near the flow guide channel; and a flow guide channel arranged on the melt flow path of the solidification zone. The present application sets the flow guide channel with specific hydraulic diameter d and ratio L / d in the solidification zone of the crucible, uses the inlet throttling effect to generate a local high velocity gradient on the melt flow path, applies precise shear force to the alumina film, makes it occur predictable folding and rolling at a specific position, and thus realizes the inducibility and repeatability of the alumina film rolling process.

Description

Technical Field

[0001] This invention relates to the field of observation technology for the solidification process of metallic materials, and more specifically, to an in-situ directional solidification device and method for observing induced entrapment of alumina thin films. Background Technology

[0002] In high-end equipment fields such as aerospace and weaponry, the metallurgical quality and service reliability of aluminum alloy castings are crucial. Alumina films, as common inclusions in molten aluminum alloys, can become trapped within the melt during the casting filling stage, forming entrainment defects that severely disrupt the matrix and reduce the mechanical properties of the casting. This is one of the core causes of sudden casting failure. Therefore, in-depth research on the entrainment behavior, trajectory, and deformation characteristics of alumina films within the melt is of great significance for revealing defect formation mechanisms and optimizing casting processes. Current technologies for studying alumina film behavior mainly rely on numerical simulation and offline characterization. Numerical simulation methods, such as the VOF model and particle tracking method, often simplify the oxide film to an extremely thin nascent film or rigid particles, neglecting the physical properties and morphological characteristics of the oxide film, resulting in significant deviations between simulation results and actual conditions. Offline characterization methods, such as metallographic analysis, can only obtain the final state results of oxide film entrainment and cannot capture in real time its dynamic behaviors such as folding, agglomeration, and fragmentation after detachment from the melt surface and entrainment beneath the liquid. In recent years, although there have been attempts to use X-ray imaging technology for in-situ observation, existing observation devices usually lack a mechanism to actively induce oxide film entrainment. They can only passively wait for random entrainment events to occur, resulting in uncontrollable and unrepeatable experimental observation processes, and difficulty in capturing dynamic processes at specific locations. Summary of the Invention

[0003] To address the problem in existing technologies that make it difficult to stably induce and observe the dynamic process of alumina films entrained under liquid, this invention provides an in-situ directional solidification device and method for observing the induced entrainment of alumina films.

[0004] To at least partially solve the above problems, the present invention adopts the following technical solution: an in-situ directional solidification device for observing the induced entrapment of alumina thin films, comprising: a crucible for containing melt, the crucible having a molten zone and a solidification zone along the axial direction; a heating unit for heating the molten zone; a cooling unit for cooling the solidification zone to establish a temperature gradient; a driving unit for driving the melt to move in the direction from the molten zone to the solidification zone and inducing controlled flow near the inlet of the guide channel; a guide channel disposed on the melt flow path in the solidification zone, the guide channel having a hydraulic diameter of d and a length of L, the ratio of the length L of the guide channel to the hydraulic diameter d being L / d being 0.8-3.0, the guide channel being used to create a local velocity gradient at the inlet edge of the melt flowing through the guide channel driven by the driving unit, thereby inducing the alumina thin film located on the surface of the melt to fold and entrap; and an observation unit for performing X-ray imaging on the solidification zone. This invention utilizes a flow channel with a specific hydraulic diameter d and a ratio L / d in the solidification zone to generate a local high-velocity gradient in the melt flow path by setting a flow channel inlet throttling effect. This applies precise shear force to the alumina film, causing it to fold and entangle in a predictable manner at a specific location, rather than drifting or breaking randomly. This achieves the induced and repeatable process of the alumina film entanglement, which helps to solve the problem of uncontrollable observation process in the prior art.

[0005] Optionally, the hydraulic diameter d of the guide channel is 5-8 mm, and the ratio L / d of the length L of the guide channel to the hydraulic diameter d is 1.0-2.0. The inlet end of the guide channel is provided with a tapering guide section, and the contraction ratio A1 / A2 of the inlet cross-sectional area A1 of the tapering guide section to the minimum cross-sectional area A2 of the guide channel is 1.5-4.0. The guide channel includes at least one of a circular hole, a tapering hole, a channel with a throat, or a slit-shaped guide structure. By optimizing the hydraulic diameter, aspect ratio, and contraction ratio of the guide channel, the magnitude and distribution range of the velocity gradient at the inlet edge can be further precisely controlled, which can adapt to aluminum alloy melts of different viscosities and alumina films of different thicknesses, thereby improving the success rate and stability of induced entrapment.

[0006] Optionally, the driving unit includes: a pull-down mechanism connected to the crucible or a solidified body within the crucible, used to drive the melt to move axially; and an electromagnetic stirring unit arranged around the crucible, configured to induce a controlled circulating flow in the melt to form a local velocity gradient at the inlet of the flow channel for inducing the entrainment of the alumina film. This invention, through the coordinated driving of the pull-down mechanism and the electromagnetic stirring unit, superimposes the mechanical axial momentum and electromagnetic rotational momentum at the inlet of the flow channel, thereby providing a controlled power source for the local velocity gradient that induces the entrainment of the alumina film, thus reconstructing the dynamic behavior of the oxide film under complex shear flow fields.

[0007] Optionally, the in-situ directional solidification device for observing the induced entrainment of alumina films further includes a control unit, which is connected to an electromagnetic stirring unit and a pull-down mechanism. The control unit is configured to: determine a frequency locking window based on the hydraulic diameter d and the ratio L / d of the guide channel, where the frequency locking window is the inherent frequency range of vortex shedding at the inlet of the guide channel; adjust the stirring frequency of the electromagnetic stirring unit and the pull-down rate of the pull-down mechanism according to the deformation state of the alumina film near the inlet of the guide channel, so that the parameter combination of the stirring frequency and the pull-down rate is maintained on a stable folding entrainment branch within the frequency locking window; when the parameter combination approaches the bifurcation boundary, increase the adjustment range of the stirring frequency and the pull-down rate so that the parameter combination crosses the bifurcation boundary and returns to the stable folding entrainment branch, where the bifurcation boundary is the critical parameter region where the deformation state of the alumina film jumps from stable folding entrainment to breakage or drift; and use the hydraulic diameter d and the ratio L / d of the guide channel as an intermediary to achieve a three-way synergistic coupling of the stirring frequency and the pull-down rate, thereby forming continuous sheet-like folding entrainment near the inlet of the guide channel.

[0008] Optionally, the observation unit includes a radiation source and a detector, and the device also includes an observation window, which is located on the side wall of the crucible or the side wall of the furnace. A radiation transmission membrane is provided on the outside of the observation window, and the periphery of the radiation transmission membrane is sealed to the mounting surface of the observation window through a sealing layer. A clamping frame is provided on the outside of the radiation transmission membrane to clamp and fix the radiation transmission membrane to the observation window.

[0009] Optionally, an insulation layer is provided on the outside of the observation window to keep the X-ray transmission membrane within a preset temperature range; a sealing layer is continuously coated along the circumference of the X-ray transmission membrane to form a closed ring-shaped sealing area; the clamping frame is connected to the observation window or furnace body by fasteners, and the fasteners are evenly arranged along the circumference of the clamping frame.

[0010] The present invention also provides a method for observation using the above-described in-situ directional solidification device for observing induced entrapment of alumina thin films, comprising: The aluminum alloy in the molten zone is heated to form a melt, and a temperature gradient is established in the solidification zone; The observation unit is activated to image the solidification zone; The melt is driven to move in the direction from the molten zone to the solidification zone, and controlled flow is induced near the inlet of the guide channel, so that a local velocity gradient is generated at the inlet edge when the melt flows through the guide channel. The alumina film on the melt surface is folded and rolled up by a local velocity gradient instead of breaking, and the roll-up process is recorded by an observation unit.

[0011] Optionally, the method further includes: Before heating, a crucible with a hydraulic diameter d of 5-8 mm and a length L to hydraulic diameter d ratio of 0.8-3.0 is selected in advance, based on the thickness of the alumina film to be observed. The steps of driving the melt to move in a direction from the molten zone to the solidification zone and inducing controlled flow near the inlet of the guide channel include: The frequency locking window is determined based on the hydraulic diameter d of the guide channel and the ratio L / d. The frequency locking window is the natural frequency range of vortex shedding at the inlet of the guide channel. Adjust the stirring frequency of the electromagnetic stirring unit and the pull-down rate of the pull-down mechanism to maintain the parameter combination of stirring frequency and pull-down rate on the stable folding and winding branch within the frequency locking window, thereby forming a continuous sheet-like folding and winding of the alumina film near the inlet of the flow channel.

[0012] Optionally, the steps of adjusting the stirring frequency of the electromagnetic stirring unit and the pulling speed of the pulling mechanism include: In response to the tendency of alumina films to break, the stirring frequency was reduced; In response to the drift of the alumina film on the melt surface, the stirring frequency is increased; When the combination of stirring frequency and pull-down rate approaches the bifurcation boundary, the bifurcation boundary is the critical region where the deformation state of the alumina film jumps from stable folding and incorporation to breakage or drift. Increasing the adjustment range allows the parameter combination to cross the bifurcation boundary and return to the stable folding and incorporation branch.

[0013] Compared with the prior art, the present invention has at least the following beneficial effects: This invention utilizes a flow channel with a specific hydraulic diameter *d* and a ratio *L / d* in the crucible solidification zone to generate a local high-velocity gradient along the melt flow path through the throttling effect at the channel inlet. This applies precise shear force to the alumina film, causing it to fold and entangle in a predictable manner at a specific location, rather than drifting or breaking randomly. This achieves the induced and repeatable nature of the oxide film entanglement process. Simultaneously, through frequency locking and bifurcation response control logic, the stirring frequency, pull-down rate, and the hydraulic diameter *d* and ratio *L / d* of the flow channel are synergistically coupled, actively avoiding unstable regions where the oxide film breaks or drifts, thus improving experimental repeatability.

[0014] Furthermore, the observation window employs a composite sealing structure consisting of a X-ray transmission film, a sealing layer, a clamping frame, and a heat insulation layer. This ensures reliable sealing under high-temperature conditions while maintaining X-ray transmittance, providing a stable window environment for in-situ observation. This invention not only allows visualization of the oxide film but also enables the active induction and recording of its entrapment process under repeatable conditions, providing an effective experimental method for revealing the formation mechanism of oxide film defects in aluminum alloy castings.

[0015] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the overall structure of the in-situ directional solidification device of the present invention.

[0017] Icons: 1-Furnace lid; 2-Clamping mechanism; 3-Insulation layer; 4-X-ray source; 5-Padded block; 6-Heating unit; 7-Electromagnetic stirring unit; 8-Observation window; 9-X-ray transmission film; 10-Detector; 11-Cooling unit; 12-Solidified body; 13-Pull-down mechanism; 14-Imager; 15-Insulation layer; 16-Crucible; 17-Melting zone; 18-Solidification zone; 19-Alumina film; 20-Flow guide channel; 21-Pressure frame; 22-Sealing layer; 23-Fastener. Detailed Implementation

[0018] To enable those skilled in the art to better understand the technical solutions in this application, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0019] The following is in conjunction with the appendix Figure 1 The present invention will be described in further detail below.

[0020] Example 1: like Figure 1 As shown, this embodiment provides an in-situ directional solidification device for observing the induced entrainment of alumina thin films. The device includes six core components: a crucible 16, a heating unit 6, a cooling unit 11, a driving unit, a flow guiding channel 20, and an observation unit, as well as auxiliary components such as an insulation layer 3 and a pad 5. All components work together to achieve active induction and in-situ observation of the alumina thin film 19 entrainment process. The crucible 16 is configured to contain the melt, and the crucible 16 has a melting zone 17 and a solidification zone 18 along its axial direction. Figure 1In the example, the crucible 16 is divided axially into an upper melting zone 17 and a lower solidification zone 18. The melting zone 17 is used to contain and heat the aluminum alloy raw material, while the solidification zone 18 is used to achieve directional solidification. It should be understood that although the crucible 16 is shown as a cylindrical structure in the figure, in other embodiments, the crucible 16 may also have a square or other irregular cross-section, as long as it can form the melting zone 17 and the solidification zone 18. A spacer block 5 is disposed between the bottom of the crucible 16 and the furnace base, configured to support the crucible 16 and provide thermal insulation. A furnace cover 1 is connected to the top of the furnace body. Figure 1 In the example, the pad 5 is positioned between the bottom of the crucible 16 and the furnace base, providing stable support and positioning for the crucible 16, and forming a thermal insulation layer between the crucible 16 and the furnace base to prevent the high temperature of the crucible 16 from being directly transferred to the pull-down mechanism 13 and the furnace base, thus ensuring the normal operation of the pull-down mechanism 13. The heating unit 6 is used to heat the melting zone 17. Figure 1As shown, the heating unit 6 is arranged around the melting zone 17 of the crucible 16, and heats the melting zone 17 by resistance heating, induction heating or other heating methods to melt the aluminum alloy raw material into a melt. The temperature of the heating unit 6 is controllable to maintain the melt within a preset temperature range. The device also includes a heat insulation layer 3, which is arranged around the upper outer side of the furnace body to reduce heat loss and maintain the temperature stability of the melting zone 17. The heat insulation layer 3 is arranged around the outer side of the melting zone 17 of the furnace body and is located between the heating unit 6 and the furnace shell. By blocking the outward transfer of heat, the temperature of the melting zone 17 is maintained within the preset range, reducing the energy consumption of the heating unit 6 and ensuring the stability of the temperature gradient. The cooling unit 11 is used to cool the solidification zone 18 to establish a temperature gradient. The cooling unit 11 is arranged around the solidification zone 18 of the crucible 16 and cools the solidification zone 18 by water cooling, air cooling or other cooling methods, thereby establishing a temperature gradient along the axial direction from the melting zone 17 to the solidification zone 18 inside the crucible 16. This temperature gradient is a prerequisite for directional solidification, ensuring that the solidification front advances towards the molten zone 17 at a stable speed. The driving unit is configured to drive the melt to move in the direction from the molten zone 17 to the solidification zone 18 and induce controlled flow near the inlet of the guide channel 20. The driving unit can be a pull-down mechanism 13 connected to the crucible 16 or the solidified body 12 inside the crucible 16, driving the melt to move axially. The solidified body 12 can be a cast ingot strip. Alternatively, an electromagnetic stirring unit 7 can be arranged around the crucible 16 to induce flow in the melt. The driving unit provides the power for directional movement of the melt and creates a specific flow field environment near the inlet of the guide channel 20, creating conditions for the entrainment of the alumina film 19. The flow channel 20 is disposed on the melt flow path of the solidification zone 18. The hydraulic diameter of the flow channel 20 is d, and the length of the flow channel is L. The ratio of the length L to the hydraulic diameter d, L / d, is 0.8-3.0. The flow channel 20 is configured to create a local velocity gradient at the inlet edge of the melt flowing through the flow channel 20 driven by the drive unit, so as to induce the alumina film 19 on the surface of the melt to fold and roll in rather than cause the alumina film 19 to break. Specifically, the flow channel 20 is opened on the side wall or bottom of the solidification zone 18 of the crucible 16, and its hydraulic diameter d and length L are specially designed so that the ratio of the length L to the hydraulic diameter d, L / d, is in the range of 0.8-3.0.This embodiment, through extensive experiments, reveals that the L / d parameter is the key bifurcation boundary defining the deformation behavior of the alumina film 19: When L / d < 0.8, the length of the guide channel 20 is too short to form a sufficient velocity gradient at the inlet edge, and the shear force is insufficient to overcome the surface tension of the alumina film 19, causing the alumina film 19 to drift on the melt surface and fail to be entangled; when L / d > 3.0, the length of the guide channel 20 is too long, and the melt is excessively accelerated within the guide channel 20, resulting in excessive shear force, causing the alumina film 19 to break near the inlet of the guide channel 20, forming fragments rather than continuous sheet-like folding and entanglement. Only when L / d is within the range of 0.8-3.0 can the guide channel 20 form a moderate local velocity gradient at the inlet edge, applying precise shear force to the alumina film 19 and inducing stable folding and entanglement behavior. The formation mechanism of this local velocity gradient is that when the melt flows through the guide channel 20, due to the contraction of the hydraulic diameter, the flow velocity increases sharply at the inlet edge, forming a local high velocity gradient region. The velocity gradient applies a non-uniform shear force to the alumina film 19 floating on the melt surface, causing its edges to bend first. Subsequently, under the combined action of melt flow traction and downward directional solidification, it folds and is drawn into the liquid. By controlling the L / d parameter within the range of 0.8-3.0, this embodiment successfully achieved the induced and repeatable process of alumina film 19 entrainment.

[0021] The observation unit is used for X-ray imaging of the solidification zone 18. The observation unit includes a radiation source 4, a detector 10, and an imager 14. The radiation source 4 is located on the outside of the furnace body, opposite to the observation window 8. The emitted X-rays or other rays penetrate the observation window 8 on the side wall of the crucible 16 and enter the solidification zone 18. The detector 10 is located on the other side of the furnace body, opposite to the radiation source 4, and receives the radiation signal after penetrating the solidification zone 18. The imager 14 is signal-connected to the detector 10, receives the signal collected by the detector 10, and processes it to form a real-time visual image of the solidification zone 18, thereby capturing the entrainment process of the alumina film 19 near the inlet of the guide channel 20 in real time. The observation unit works in conjunction with the drive unit and the guide channel 20 to achieve in-situ observation of the dynamic process of the alumina film 19 entrained under the liquid. This embodiment utilizes the local velocity gradient generated at the inlet of the guide channel 20 to actively induce the folding and entrainment of the alumina film 19, and combines the drive unit and the observation unit to achieve repeatable observation of the entire process, providing an effective experimental means to reveal the formation mechanism of oxide film defects in aluminum alloy castings.

[0022] Example 2: Based on Example 1, this example further refines the structural parameters of the flow channel 20 and the hardware layout of the drive unit. Regarding the structural parameters of the flow channel 20, the hydraulic diameter d of the flow channel 20 is 5-8 mm, and the ratio L / d is 1.0-2.0. Specifically, the selection of the hydraulic diameter d needs to match the viscosity of the aluminum alloy melt and the thickness of the alumina film 19. When the hydraulic diameter d is less than 5 mm, the melt flow resistance is too high, easily leading to blockage; when the hydraulic diameter d is greater than 8 mm, the velocity gradient at the inlet edge weakens, making it difficult to form sufficient shear force to induce the folding of the alumina film 19. With a ratio L / d of 1.0-2.0, the flow channel 20 can form the most stable local velocity gradient at the inlet edge, making the folding and roll-up behavior of the alumina film 19 most controllable. The inlet end of the flow channel 20 is provided with a tapered flow section, and the contraction ratio A1 / A2 of the inlet cross-sectional area A1 of the tapered flow section to the minimum cross-sectional area A2 of the flow channel 20 is 1.5-4.0. The tapered guide section is designed to smoothly guide the melt into the guide channel 20, avoiding severe turbulence at the inlet. The contraction ratio A1 / A2 determines the degree of acceleration of the melt within the orifice: when the contraction ratio A1 / A2 is less than 1.5, the acceleration effect is not significant, making it difficult to form an effective velocity gradient; when the contraction ratio A1 / A2 is greater than 4.0, the melt is excessively accelerated within the orifice, and the excessive shear force may cause the alumina film 19 to break. In this embodiment, by controlling the contraction ratio A1 / A2 within the range of 1.5-4.0, precise control of the shear force of the alumina film 19 is achieved. The guide channel 20 includes at least one of a circular orifice, a tapered orifice, a channel with a throat, or a slit-like guide structure.

[0023] Regarding the hardware layout of the driving unit in this invention, as follows: Figure 1 As shown, the drive unit includes a pull-down mechanism 13 and an electromagnetic stirring unit 7. The pull-down mechanism 13 is connected to the crucible 16 or the solidified material 12 inside the crucible 16, and is used to drive the melt to move axially. Figure 1In the example, the pull-down mechanism 13 is connected to the solidified solid 12 inside the crucible 16. By pulling down the solidified solid 12 at a uniform speed, the melt is moved from the melting zone 17 to the solidification zone 18, thereby achieving directional solidification. The pull-down rate can be adjusted within the range of 0.5-1.5 mm / min according to experimental requirements. The electromagnetic stirring unit 7 is arranged around the melting zone 17 of the crucible 16. It induces eddy currents in the melt by generating an alternating magnetic field, thereby driving the melt to generate rotational or convective motion. The stirring frequency and intensity of the electromagnetic stirring unit 7 are adjustable, used to induce controlled flow near the inlet of the guide channel 20. The pull-down mechanism 13 and the electromagnetic stirring unit 7 have a clear division of labor: the pull-down mechanism 13 is responsible for providing axial movement of the melt to achieve directional solidification; the electromagnetic stirring unit 7 is responsible for inducing controlled flow near the inlet of the guide channel 20, creating flow field conditions for the entrainment of the alumina film 19. The two work together to form a specific velocity field distribution in the melt near the inlet of the guide channel 20, thereby achieving active induction of the entrainment behavior of the alumina film 19. This embodiment further improves the controllability and repeatability of the alumina film 19 winding process by refining the structural parameters of the flow channel 20 and the hardware layout of the drive unit. The hydraulic diameter, aspect ratio, and shrinkage ratio of the flow channel 20 are optimized to adapt to aluminum alloy melts of different viscosities and alumina films 19 of different thicknesses. The coordinated operation of the pull-down mechanism 13 and the electromagnetic stirring unit 7 provides a stable power source for the local velocity gradient at the inlet of the flow channel 20, ensuring the reliability of the experimental results.

[0024] Example 3: Building upon Example 2, this example further introduces a control unit to achieve precise control over the alumina film 19 winding process. The device also includes a control unit connected to the electromagnetic stirring unit 7 and the pull-down mechanism 13, configured to execute the following control logic: First, the control unit determines a frequency locking window based on the hydraulic diameter d and ratio L / d of the guide channel 20. This frequency locking window represents the inherent frequency range of vortex shedding at the inlet of the guide channel 20. Specifically, when the melt flows through the guide channel 20, due to hydraulic diameter contraction and boundary layer separation effects, periodic vortex shedding occurs at the inlet edge. The frequency of vortex shedding is closely related to the hydraulic diameter d, ratio L / d, and melt flow velocity of the guide channel 20. The control unit can calculate the inherent frequency range of vortex shedding, i.e., the frequency locking window, based on the current hydraulic diameter d and ratio L / d of the guide channel 20. When the stirring frequency of the electromagnetic stirring unit 7 approaches the inherent frequency range, frequency locking occurs. At this time, the flow field tends to stabilize, and the vortex shedding frequency is locked near the stirring frequency, which is beneficial for the stable folding and winding of the alumina film 19. It should be understood that the determination of the frequency locking window is the basis of the subsequent control strategy, providing a target range for the adjustment of the stirring frequency. Secondly, the control unit adjusts the stirring frequency of the electromagnetic stirring unit 7 and the pulling rate of the pulling mechanism 13 according to the deformation state of the alumina film 19 near the inlet of the guide channel 20, so that the parameter combination of stirring frequency and pulling rate is maintained on the stable folding and winding branch within the frequency locking window. Specifically, the observation unit collects image data near the inlet of the guide channel 20 in real time, and the control unit identifies the deformation state of the alumina film 19 (such as drifting, folding, breaking, etc.) through image processing algorithms. When the alumina film 19 is identified as being in a stable folding and winding state, the control unit maintains the current stirring frequency and pulling rate unchanged; when the alumina film 19 is identified as having a drifting or breaking trend, the control unit fine-tunes the stirring frequency and pulling rate, so that the parameter combination returns to the stable folding and winding branch. This embodiment ensures that the alumina film 19 is always in an ideal folded-in state through real-time monitoring and feedback adjustment, greatly improving the repeatability of the experiment. Furthermore, when the parameter combination approaches the bifurcation boundary, the control unit increases the adjustment range of the stirring frequency and pull-down rate to allow the parameter combination to cross the bifurcation boundary and return to the stable folded-in branch. This bifurcation boundary is the critical parameter region where the deformation state of the alumina film 19 jumps from stable folded-in to breakage or drift. Specifically, the bifurcation boundary is a unique phenomenon in nonlinear dynamic systems. Near the bifurcation boundary, small changes in system parameters can cause drastic changes in the deformation state of the alumina film 19 (such as a sudden jump from stable folded-in to breakage).Therefore, when the control unit detects that the parameter combination is approaching the bifurcation boundary, it no longer adopts a fine-tuning strategy, but instead increases the adjustment range and actively crosses the bifurcation boundary, allowing the system to quickly return to the stable folded entrainment branch. This bifurcation response control strategy can effectively avoid the unstable region and prevent irreversible breakage or drift of the alumina film 19. Finally, the control unit uses the hydraulic diameter d and the length-to-diameter ratio L / d of the guide channel 20 as constraint benchmarks to achieve three-way coordinated control of the stirring frequency, the pull-down rate, and the geometry of the guide channel 20, thereby inducing the formation of continuous sheet-like folded entrainment near the inlet of the guide channel 20. Specifically, the stirring frequency, the pull-down rate, and the hydraulic diameter d and ratio L / d of the guide channel 20 are not independent variables, but are coupled together through flow field dynamics. The control unit uses the hydraulic diameter d and ratio L / d of the guide channel 20 as an intermediary to coordinate the matching relationship between the stirring frequency and the pull-down rate. For example, when the hydraulic diameter of the guide channel 20 is small, the natural frequency of vortex shedding is high, and the control unit correspondingly increases the stirring frequency to achieve frequency locking. Simultaneously, to prevent excessive acceleration of the melt within the hole, which could lead to breakage of the alumina film 19, the control unit needs to appropriately reduce the pull-down rate. Through this three-way synergistic coupling, the stability and controllability of the flow field near the inlet of the guide channel 20 are ensured, thereby achieving continuous sheet-like folding and winding of the alumina film 19. This embodiment, by introducing a control unit and the aforementioned four-stage nonlinear control logic, achieves active intervention and precise control of the alumina film 19 winding process; the determination of the frequency locking window provides the adjustment target, the bifurcation response control strategy avoids the risk of instability, and the three-way synergistic coupling ensures the rationality of parameter matching.

[0025] Example 4: Based on the above embodiments, this embodiment provides a detailed description of the sealing structure of the observation unit. The observation unit includes an X-ray source 4 and a detector 10, as shown below. Figure 1 As shown, the device also includes an observation window 8, which is disposed on the side wall of the crucible 16 or the side wall of the furnace body. Specifically, in Figure 1In this example, the observation window 8 is located on the side wall of the furnace body, directly opposite the solidification zone 18 of the crucible 16, so that the X-rays emitted by the X-ray source 4 can penetrate the observation window 8 to image the solidification zone 18. A X-ray transmission membrane 9 is provided on the outer side of the observation window 8. The periphery of the X-ray transmission membrane 9 is sealed to the mounting surface of the observation window 8 by a sealing layer 22. A clamping frame 21 is provided on the outer side of the X-ray transmission membrane 9, configured to press and fix the X-ray transmission membrane 9 to the observation window 8. The function of the X-ray transmission membrane 9 is to isolate the high-temperature environment inside the furnace from the external environment while allowing X-rays to penetrate. It should be understood that the X-ray transmission membrane 9 can be made of polyimide film, beryllium sheet, or other materials with high X-ray transmittance. In this embodiment, the X-ray transmission membrane 9 specifically uses Kapton film, which has excellent high-temperature resistance and extremely low X-ray absorption, and can remain stable in high-temperature environments above 700°C. The sealing layer 22 fills the gap between the X-ray transmission membrane 9 and the mounting surface of the observation window 8 to prevent gas leakage inside the furnace. The sealing layer 22 can be made of high-temperature resistant sealant, ceramic-based sealant, or high-temperature resistant gasket. In this embodiment, the sealing layer 22 specifically uses a high-temperature resistant sealant, such as a ceramic-based high-temperature resistant sealant, whose temperature resistance can reach over 800℃. The function of the clamping frame 21 is to provide uniform clamping force to ensure that the sealing layer 22 does not shift or fall off at high temperatures. The clamping frame 21 is connected to the observation window 8 or the furnace body by fasteners 23, which are evenly arranged around the circumference of the clamping frame 21. The fasteners 23 can be bolts, screws, or clips. In this embodiment, the fasteners 23 specifically use bolts, which are evenly arranged around the circumference of the clamping frame 21. Tightening the bolts makes the clamping frame 21 evenly press against the periphery of the X-ray transmission membrane 9.

[0026] A heat insulation layer 15 is also provided on the outside of the observation window 8. The heat insulation layer 15 is configured to keep the X-ray transmission membrane 9 within a preset temperature range. Specifically, the heat insulation layer 15 is disposed between the observation window 8 and the X-ray transmission membrane 9, or between the outer wall of the furnace body and the clamping frame 21. The function of the heat insulation layer 15 is to prevent the high temperature inside the furnace from being transmitted to the X-ray transmission membrane 9, keeping the X-ray transmission membrane 9 and the sealing layer 22 in a relatively low temperature region, thereby avoiding seal failure or damage to the X-ray transmission membrane 9 due to excessive temperature. The sealing layer 22 is continuously coated along the circumference of the X-ray transmission membrane 9 to form a closed ring-shaped sealing area. Specifically, before installing the X-ray transmission membrane 9, the mounting surface on the outside of the observation window 8 is cleaned and degreased to remove metal shavings, oil, and oxides. Then, a high-temperature resistant sealant is continuously applied to the circumferential sealing area of ​​the mounting surface to form a closed ring-shaped adhesive layer. The thickness of the adhesive layer is preferably 0.2-1.0 mm to ensure the reliability of the seal. After coating, the X-ray transmission membrane 9 is placed over the outside of the observation window 8, ensuring that the periphery of the X-ray transmission membrane 9 adheres to the high-temperature resistant sealant layer. Then, the clamping frame 21 is installed and tightened gradually in a diagonal sequence using fasteners 23. After tightening, a pre-curing treatment is performed to form a continuous sealing area with the sealing layer 22. Before the experiment, the observation window 8 is subjected to vacuum pressure testing or protective gas pressure testing to confirm that the pressure change is within the allowable range before proceeding with the heating experiment. This circumferentially continuous coating sealing process eliminates sealing gaps and prevents gas from penetrating along the sealing layer 22 at high temperatures, thereby improving the stability of the seal. In this embodiment, the composite sealing structure of the X-ray transmission membrane 9, sealing layer 22, clamping frame 21, and heat insulation layer 15 ensures reliable sealing under high temperature, vacuum, or protective atmosphere conditions while maintaining X-ray transmittance. The heat insulation layer 15 reduces the actual heat temperature of the sealed area, and the circumferentially continuous coating of the sealing layer 22 and the uniformly arranged fasteners 23 further improve the stability and durability of the seal, thus providing a stable window environment for in-situ observation and ensuring the clarity of imaging and the reliability of data during the experiment.

[0027] Example 5: This embodiment also provides a method for observation using the above-described in-situ directional solidification device for observing induced entrapment of alumina thin films. The method includes: Step S100: Heat the aluminum alloy in the melting zone 17 to form a melt and establish a temperature gradient in the solidification zone 18. Specifically, place the aluminum alloy raw material in the melting zone 17 of the crucible 16, and start the heating unit 6 to heat the melting zone 17, causing the aluminum alloy raw material to melt and form a melt. During the heating process, the heating power of the heating unit 6 is controlled to maintain the melt temperature within a preset range, such as 710℃-730℃. At the same time, the cooling unit 11 is started to cool the solidification zone 18, thereby establishing a temperature gradient along the axial direction from the melting zone 17 to the solidification zone 18 inside the crucible 16. This temperature gradient is a prerequisite for achieving directional solidification, ensuring that the solidification front advances towards the melting zone 17 at a stable speed. It should be understood that the grade of aluminum alloy can be selected according to experimental requirements, such as ZL101A, ZL702A, ZL205A, etc. Different grades of aluminum alloy may have different melt viscosities and surface oxide film characteristics, requiring corresponding adjustments to the process parameters in subsequent steps.

[0028] Step S200: Activate the observation unit to image the solidification zone 18. Specifically, before or during heating, activate the X-ray source 4 and detector 10 to perform imaging calibration on the solidification zone 18 of the crucible 16. The X-ray source 4 emits X-rays that penetrate the observation window 8 on the side wall of the crucible 16 to image the solidification zone 18, thereby capturing in real time the entrapment process of the alumina film 19 near the inlet of the flow channel 20. Imaging calibration includes adjusting the emission angle, X-ray intensity, and focusing position of the X-ray source 4 so that the X-ray beam passes through the X-ray transmission membrane 9 through the observation window 8 and is focused on the observation area near the solid-liquid interface of the flow channel 20. After the image is clear, the formal experiment begins.

[0029] Step S300: Drive the melt to move along the direction from the melting zone 17 to the solidification zone 18, and induce controlled flow near the inlet of the guide channel 20, so that a local velocity gradient is generated at the inlet edge when the melt flows through the guide channel 20. Specifically, activate the drive unit, including the pull-down mechanism 13 and the electromagnetic stirring unit 7. The pull-down mechanism 13 is connected to the crucible 16 or the solidified solid 12 inside the crucible 16, driving the melt to move axially from the melting zone 17 to the solidification zone 18 to achieve directional solidification. The electromagnetic stirring unit 7 is arranged around the crucible 16, inducing flow in the melt and providing a power source for the local velocity gradient near the inlet of the guide channel 20. When the melt flows through the inlet of the guide channel 20, the flow velocity increases sharply at the inlet edge due to the hydraulic diameter contraction, forming a local high velocity gradient region. The formation mechanism of this local velocity gradient is that when the melt flows through the inlet of the guide channel 20, the flow velocity increases sharply at the inlet edge due to the hydraulic diameter contraction, forming a local high velocity gradient region. The velocity gradient exerts a non-uniform shear force on the alumina film 19 floating on the surface of the melt, causing its edges to bend first, and then fold and be drawn under the liquid under the combined action of melt flow traction and downward directional solidification.

[0030] Step S400: A local velocity gradient is used to induce the alumina film 19 on the melt surface to fold and entangle, rather than causing it to break. The entanglement process of the alumina film 19 is recorded by an observation unit. Specifically, the local velocity gradient applies a precise shear force to the alumina film 19, causing it to fold and entangle predictably at a specific location, rather than drifting randomly or breaking. In this embodiment, by controlling the ratio L / d of the length L of the flow channel 20 to the hydraulic diameter d within the range of 0.8-3.0, a suitable local velocity gradient is ensured at the inlet edge of the flow channel 20. When L / d < 0.8, the length of the guide channel 20 is too short to form a sufficient velocity gradient at the inlet edge, and the shear force is insufficient to overcome the surface tension of the alumina film 19, causing the alumina film 19 to drift on the melt surface and fail to be entrained. When L / d > 3.0, the length of the guide channel 20 is too long, and the melt accelerates excessively within the hole, resulting in excessive shear force. This causes the alumina film 19 to break near the inlet of the guide channel 20, forming fragments rather than continuous sheet-like folding and entrainment. Only when L / d is within the range of 0.8-3.0 can the guide channel 20 form a suitable local velocity gradient at the inlet edge, applying precise shear force to the alumina film 19 and inducing stable folding and entrainment behavior. The observation unit records in real time the motion trajectory and deformation process of the alumina film 19 as it detaches from the melt surface, is entrained under the liquid near the inlet of the guide channel 20, and interacts with the solid-liquid interface, thus achieving in-situ observation of the dynamic process of the alumina film 19 being entrained under the liquid. Using the above method, this embodiment utilizes the local velocity gradient generated at the inlet of the flow channel 20 to actively induce the folding and winding of the alumina film 19, and combines the driving unit and the observation unit to achieve repeatable observation of the entire process, providing an effective experimental means to reveal the formation mechanism of oxide film defects in aluminum alloy castings.

[0031] Example 6: Building upon Example 5, this example further refines the pre-experiment preparation steps and the specific control strategies for driving melt movement and inducing flow, to achieve more precise control over the alumina film 19 winding process. Before heating, a crucible 16 with a guide channel 20 having a hydraulic diameter d of 5-8 mm and a ratio L / d of 0.8-3.0 is pre-selected based on the thickness of the alumina film 19 to be observed. Specifically, the thickness of the alumina film 19 is closely related to its mechanical properties; a thicker alumina film 19 has higher stiffness and surface tension, requiring greater shear force to induce its folding and winding. Therefore, for thicker alumina films 19, it is advisable to select a flow channel 20 with a larger hydraulic diameter d (e.g., 7-8 mm) and a smaller ratio L / d (e.g., 0.8-1.2) to form a stronger velocity gradient at the inlet edge; for thinner alumina films 19, it is advisable to select a flow channel 20 with a smaller hydraulic diameter d (e.g., 5-6 mm) and a larger ratio L / d (e.g., 2.0-3.0) to avoid excessive shear force leading to film breakage. By pre-selecting the crucible 16, the hydraulic diameter d and ratio L / d of the flow channel 20 are matched with the thickness of the alumina film 19 to be observed, laying the foundation for subsequent frequency locking control. It should be noted that the hydraulic diameter d of the flow channel 20 is an inherent structural parameter of the crucible 16, determined by the physical processing dimensions of the flow channel 20 on the wall of the crucible 16, and remains fixed once the processing is completed. Pre-selection refers to the process where, before heating, the experimenter selects a suitable crucible 16 from a set of candidate crucibles 16 with different hydraulic diameters d, based on the estimated thickness of the alumina film 19 to be observed, and installs it into the furnace before starting heating. Under high-temperature operation at 800°C, the crucible 16 is filled with molten aluminum alloy, and the hydraulic diameter d of the flow channel 20, as an inherent structural parameter of the crucible 16, cannot be adjusted; the only adjustable parameters during operation are the stirring frequency of the electromagnetic stirring unit 7 and the pulling rate of the pull-down mechanism 13. Therefore, the decision on selecting the crucible 16 before the experiment is crucial, as it determines the structural parameters of the flow channel 20 throughout the experiment, and thus the range of the frequency locking window and the target range for adjusting the stirring frequency. The steps of driving the melt to move from the melting zone 17 to the solidification zone 18 and inducing controlled flow near the inlet of the flow channel 20 include: determining the frequency locking window based on the hydraulic diameter d of the flow channel 20 and the ratio L / d, which is the inherent frequency range of vortex shedding at the inlet of the flow channel 20. Specifically, when the melt flows through the inlet of the guide channel 20, periodic vortex shedding occurs at the inlet edge due to hydraulic diameter contraction and boundary layer separation effects. The frequency of vortex shedding is closely related to the hydraulic diameter d of the guide channel 20, the ratio L / d, and the melt flow velocity.Based on the Strouhal number in fluid mechanics (St = f·d / v, where f is the vortex shedding frequency, d is the characteristic length (hydraulic diameter), and v is the flow velocity), the natural frequency range of vortex shedding at the inlet of the guide channel 20, i.e., the frequency locking window, can be calculated. For example, for the guide channel 20 with a hydraulic diameter d = 6 mm, when the melt flow velocity v = 0.1 m / s, the natural frequency of vortex shedding is approximately tens of hertz. Determining the frequency locking window is the basis for subsequent control strategies, providing a target range for adjusting the stirring frequency. By adjusting the stirring frequency of the electromagnetic stirring unit 7 and the pull-down rate of the pull-down mechanism 13, the parameter combination of stirring frequency and pull-down rate is maintained on a stable folding and winding branch within the frequency locking window, thereby causing the alumina film 19 to form continuous sheet-like folding and winding near the inlet of the guide channel 20. Specifically, the stirring frequency and pull-down rate are not independent variables, but are coupled together through the flow field dynamics at the inlet of the guide channel 20. When the stirring frequency of the electromagnetic stirring unit 7 approaches the natural frequency range of vortex shedding, frequency locking occurs. At this time, the flow field tends to stabilize, and the vortex shedding frequency is locked near the stirring frequency, which is beneficial for the stable folding and winding of the alumina film 19. Simultaneously, the pull-down rate determines the residence time of the melt near the inlet of the guide channel 20, thus affecting the stress duration of the alumina film 19. The control unit adjusts the parameter combination of stirring frequency and pull-down rate to maintain it on the stable folding and winding branch within the frequency locking window. For example, when the stirring frequency is 30Hz and the pull-down rate is 1.0mm / min, the parameter combination is exactly located in the central region of the stable folding and winding branch, at which time the alumina film 19 can stably fold and wind up under the liquid. It should be understood that the stable folding and winding branch refers to the region in the parameter space where the deformation state of the alumina film 19 maintains stable folding and winding up; this region is jointly defined by the frequency locking window and the bifurcation boundary. This embodiment further improves the controllability and repeatability of the alumina film 19 winding process by pre-selecting the crucible 16 and using a frequency locking control strategy. The crucible 16 is pre-selected so that the hydraulic diameter d and the ratio L / d of the flow channel 20 are matched with the thickness of the alumina film 19. The frequency locking control strategy ensures that the parameter combination of stirring frequency and pull-down rate is always on the stable folding and winding branch, thereby ensuring that the experimental process is always within the ideal parameter range and avoiding the drift or breakage of the alumina film 19.

[0032] Example 7: Based on Example 6, this example details the specific strategies for adjusting the stirring frequency of the electromagnetic stirring unit 7 and the pull-down rate of the pull-down mechanism 13, as well as the bifurcation boundary avoidance method, to achieve precise control over the alumina film 19 winding process. Adjusting the stirring frequency of the electromagnetic stirring unit 7 and the pull-down rate of the pull-down mechanism 13 includes reducing the stirring frequency in response to a tendency for the alumina film 19 to break. Specifically, when image data acquired by the observation unit shows serrated tears or local fractures at the edge of the alumina film 19, it indicates a tendency for the alumina film 19 to break. At this time, the control unit recognizes this deformation state and determines that the current parameter combination has deviated from the stable folding winding branch, transitioning towards the breaking branch. The mechanism for the breaking tendency is that an excessively high stirring frequency leads to an excessively high vortex shedding frequency near the inlet of the guide channel 20, resulting in excessively high turbulence intensity and applying shear force to the alumina film 19 exceeding its yield strength. Therefore, the control unit reduces the stirring frequency, thereby reducing the vortex shedding frequency and turbulence intensity, thus reducing the shear force and allowing the alumina film 19 to return to a stable folding winding state. It should be understood that the reduction in stirring frequency can be adaptively adjusted according to the severity of the breakage tendency. For example, when the breakage tendency is mild, the reduction is 1-2 Hz; when the breakage tendency is severe, the reduction is 3-5 Hz. The stirring frequency is increased in response to the alumina film 19 drifting on the melt surface. When the image data acquired by the observation unit shows that the alumina film 19 is moving randomly on the melt surface and cannot enter the inlet region of the guide channel 20, it indicates that the alumina film 19 is drifting on the melt surface. At this time, the control unit recognizes this deformation state and determines that the current parameter combination has deviated from the stable folding and entrainment branch, transitioning to the drift branch. The mechanism of the drift tendency is that an excessively low stirring frequency leads to an excessively low vortex shedding frequency near the inlet of the guide channel 20, insufficient flow field velocity gradient, and insufficient shear force applied to the alumina film 19 to overcome its surface tension, thus failing to induce folding and entrainment. Therefore, the control unit increases the stirring frequency, thereby increasing the vortex shearing frequency and the flow field velocity gradient, thus enhancing the shearing force and causing the alumina film 19 to be captured and folded into the inlet region of the guide channel 20. It should be understood that the increase in stirring frequency can be adaptively adjusted according to the degree of drift; for example, when the drift is mild, the increase is 1-2 Hz; when the drift is severe, the increase is 3-5 Hz. When the parameter combination of stirring frequency and pull-down rate approaches the bifurcation boundary, the bifurcation boundary is the critical parameter region where the deformation state of the alumina film 19 jumps from stable folding and entrapment to breakage or drift. Increasing the adjustment range allows the parameter combination to cross the bifurcation boundary and return to the stable folding and entrapment branch. Specifically, the bifurcation boundary is a unique phenomenon in nonlinear dynamic systems, defining the stable and unstable regions of the alumina film 19's deformation state. Near the bifurcation boundary, small changes in system parameters can cause drastic jumps in the deformation state of the alumina film 19.For example, when the parameter combination approaches the bifurcation boundary, even a mere 0.5 Hz increase in stirring frequency can cause the alumina film 19 to suddenly jump from a stable folded-in state to a broken state; similarly, when the parameter combination approaches the drift bifurcation boundary, even a mere 0.5 Hz decrease in stirring frequency can cause the alumina film 19 to suddenly jump from a stable folded-in state to a drifting state. Therefore, when the control unit detects that the parameter combination is approaching the bifurcation boundary (e.g., by monitoring the fluctuation amplitude or frequency characteristics of the deformation state of the alumina film 19), it no longer employs the aforementioned fine-tuning strategy (1-2 Hz), but instead increases the adjustment range (e.g., 5-10 Hz) to actively cross the bifurcation boundary, allowing the system to quickly return to the central region of the stable folded-in branch. This bifurcation boundary avoidance strategy effectively prevents irreversible instability of the alumina film 19 near the bifurcation boundary, ensuring that the experimental process remains under control. This embodiment achieves real-time intervention and precise control of the alumina film 19 winding process through specific adjustment strategies and bifurcation boundary avoidance methods. The strategy of reducing the stirring frequency in response to the breakage trend, increasing the stirring frequency in response to the drift trend, and increasing the adjustment amplitude near the bifurcation boundary together constitute a closed-loop feedback control system, ensuring that the alumina film 19 is always in a stable folded winding state, which greatly improves the repeatability of the experiment and the reliability of the data.

[0033] Example 8: This embodiment provides an in-depth analysis of the physical mechanism of the frequency locking window from a fluid mechanics perspective. In fluid mechanics, when fluid flows through an obstacle or a constricting channel, periodic vortex shedding occurs behind the obstacle or at the channel outlet. This phenomenon can be described by the Strouhal number. For the flow channel 20 structure in this invention, when the melt flows through the inlet of the flow channel 20, periodic vortex shedding occurs at the inlet edge due to hydraulic diameter contraction and boundary layer separation effects. The natural frequency range of vortex shedding can be derived by inversely using the Strouhal number formula, i.e., f = St·v / d. Specifically, for the flow channel 20 with a hydraulic diameter d = 5-8 mm, when the melt velocity v is in the range of 0.05-0.2 m / s, according to experimental measurements and numerical simulations, the Strouhal number St of the aluminum alloy melt at the inlet of the flow channel 20 is typically in the range of 0.1-0.3. Therefore, the natural frequency range of vortex shedding can be calculated to be approximately a few hertz to tens of hertz. This natural frequency range is the frequency locking window. When the stirring frequency of the electromagnetic stirring unit 7 approaches the inherent frequency range, frequency locking occurs: the vortex shedding frequency is no longer determined by the fluid's own dynamic characteristics, but is locked near the stirring frequency. At this time, the flow field tends to be stable, and the vortex structure is regular and persistent. Frequency locking is crucial for the stable folding and entrainment of the alumina film 19. When the flow field is in a frequency-locked state, the flow field structure near the inlet of the guide channel 20 exhibits highly ordered periodic characteristics, and the magnitude and direction of the velocity gradient remain stable in both time and space. This stable flow field environment provides the alumina film 19 with continuous and uniform shear force, enabling it to fold and entrain along a predictable path, rather than drifting randomly or breaking under turbulence. Conversely, if the stirring frequency is far from the frequency locking window, the vortex shedding frequency does not match the stirring frequency, the flow field becomes chaotic, the velocity gradient fluctuates violently, and the alumina film 19 is easily broken or cannot be captured due to uneven force. This embodiment establishes a quantitative relationship between the frequency locking window, the hydraulic diameter d of the guide channel 20, and the ratio L / d. The frequency locking window in this invention is not an empirical parameter range, but rather based on solid fluid dynamics theory. The control unit determines the frequency locking window based on the hydraulic diameter d of the guide channel 20 and the ratio L / d. Essentially, it uses the Strouhal number formula to calculate the inherent frequency range of vortex shedding, thus providing a basis for adjusting the stirring frequency. This design allows the invention to adaptively determine the optimal stirring frequency range based on different guide channel 20 structures and melt flow velocities, ensuring the flow field is always in a frequency-locked state. This achieves stable folding and entrainment of the alumina film 19, greatly improving experimental repeatability and data reliability.

[0034] Example 9: This embodiment provides an in-depth analysis of the physical mechanism of bifurcation boundaries from a nonlinear dynamics perspective. In nonlinear dynamics, Hopf bifurcation refers to the critical phenomenon where changes in system parameters lead to a qualitative change in system stability. When system parameters cross the bifurcation point, the system state jumps from one stable branch to another, or transforms from a stable state to a periodic oscillation state. For the alumina film 19 entrainment system in this invention, the stirring frequency and pull-down rate constitute the bifurcation parameters of the system, and the deformation state of the alumina film 19 (stable folding entrainment, breakage, drift) constitutes the state variables of the system. Specifically, the deformation behavior of the alumina film 19 near the inlet of the guide channel 20 can be described by a nonlinear dynamic system. The state equation of this system includes inertial terms, damping terms, nonlinear restoring force terms, and external driving force terms (determined by the stirring frequency and pull-down rate). When the parameter combination of stirring frequency and pull-down rate is located on the stable folding entrainment branch, the system has a stable attractor, corresponding to the stable folding entrainment state of the alumina film 19. At this point, even with minor disturbances (such as melt temperature fluctuations or uneven thickness of the alumina film 19), the system will automatically return to a stable state, and the alumina film 19 can continuously and stably fold and roll into the liquid. However, when the parameter combination approaches the bifurcation boundary, the system's stability begins to decrease. According to Hopf bifurcation theory, near the bifurcation boundary, the real part of the system's eigenvalues ​​approaches zero, and the system's sensitivity to disturbances increases sharply. At this point, even a small change in parameters can cause a drastic jump in the system state. For example, when the parameter combination approaches the breaking bifurcation boundary, even a tiny increase in the stirring frequency can cause the system state to suddenly jump from the stable folding and rolling branch to the breaking branch, and the alumina film 19 will break instantly; when the parameter combination approaches the drift bifurcation boundary, even a tiny decrease in the stirring frequency can cause the system state to suddenly jump from the stable folding and rolling branch to the drift branch, and the alumina film 19 will be unable to be captured and will drift away. If the control unit still employs a fine-tuning strategy (e.g., an adjustment range of 1-2 Hz) when it detects that the parameter combination is approaching the bifurcation boundary, the system, being in a highly sensitive state, may not be able to cross the bifurcation boundary and return to the stable folded-in branch due to the minute adjustment. Instead, the fluctuations during the adjustment process may cause further instability. Therefore, this invention adopts a bifurcation response control strategy. When the parameter combination is detected to be approaching the bifurcation boundary, the adjustment range is increased (e.g., 5-10 Hz) to actively cross the bifurcation boundary, allowing the system to quickly enter the central region of the stable folded-in branch, thereby avoiding the risk of instability. This embodiment, by introducing Hopf bifurcation theory, reveals the physical mechanism of the jump transition of the deformation state of the alumina film 19 near the bifurcation boundary, demonstrating the necessity of the bifurcation response control strategy. The bifurcation boundary in this invention is not an empirical parameter threshold, but has a solid nonlinear dynamic basis.The control unit actively adjusts parameters to cross the bifurcation boundary, enabling the system to return to a stable folding and winding branch. This allows the invention to effectively address the nonlinear instability phenomenon during the winding process of the alumina film 19, improving the repeatability of the experiment and the reliability of the data.

[0035] Example 10: This embodiment applies the above technical solution to an actual experimental environment, using the alumina film 19 on the surface of the aluminum alloy melt as the observation object, to demonstrate the complete experimental operation process and the specific application of the frequency locking and bifurcation response control strategy, as follows: Step 1: Before heating, perform a sealing check on the directional melting furnace. Specifically, focus on checking the installation status of the X-ray transmission membrane 9, the clamping frame 21, the fasteners 23, and the sealing layer 22. Confirm that the periphery of the X-ray transmission membrane 9 is uniformly compressed, and that the sealing layer 22 forms a continuous closed sealing area without air bubbles or gaps. Subsequently, perform vacuum pressure holding or protective gas pressure holding tests on the observation window 8 to confirm that the pressure change is within the allowable range, in order to ensure the sealing stability of the outer sealing area of ​​the observation window 8 during subsequent high-temperature experiments.

[0036] Step 2: Start the observation unit for imaging calibration. Specifically, start the X-ray source 4 and detector 10, and perform linkage calibration between the X-ray source 4 and imager 14. Adjust the emission angle, X-ray intensity, and focusing position of the X-ray source 4 so that the X-ray beam passes through the X-ray transmission membrane 9 through the observation window 8 and is focused on the observation area near the flow channel 20 and the solid-liquid interface. After the image is clear, the formal experiment begins.

[0037] Step 3: Establish a temperature gradient. Specifically, turn on the cooling unit 11 to cooperate with the heating unit 6 and establish a stable temperature gradient in the lower part of the crucible 16. This temperature gradient is a prerequisite for directional solidification, ensuring that the solidification front advances towards the melting zone 17 at a stable rate.

[0038] Step 4: Heat the aluminum alloy in the melting zone 17 to form a melt. Specifically, open the furnace lid 1, put in the aluminum alloy raw material, and start the heating unit 6 to increase the furnace temperature at a rate of 5℃ / min until the aluminum alloy melt temperature reaches 710℃-730℃. In this embodiment, ZL101A aluminum alloy is selected, which has a moderate melt viscosity, suitable for observing the entrainment behavior of the alumina film 19.

[0039] Step 5: Turn on the electromagnetic stirring unit 7 and place the alumina film 19. Specifically, after the aluminum alloy has completely melted, turn on the electromagnetic stirring unit 7 and adjust the stirring frequency to the initial preset value (e.g., 30Hz) to create a controlled flow of the melt near the inlet of the guide channel 20. Then, use the clamping mechanism 2 to place the pretreated alumina film 19 on the surface of the molten aluminum alloy. At this time, the observation unit acquires image data near the inlet of the guide channel 20 in real time, and the control unit identifies the initial position and deformation state of the alumina film 19 through an image processing algorithm.

[0040] Step 6: Activate the pull-down mechanism 13 and execute the frequency locking control strategy. Specifically, activate the pull-down mechanism 13 to uniformly pull down the solidified solid 12 at a preset rate (e.g., 1.0 mm / min), driving the melt from the melting zone 17 to the solidification zone 18. The control unit determines the frequency locking window based on the geometric parameters of the guide channel 20 (hydraulic diameter d = 6 mm, ratio L / d = 1.5). According to the Strouhal number formula in fluid mechanics, the natural frequency range of vortex shedding at the inlet of the guide channel 20 is calculated to be approximately 25-35 Hz. The control unit adjusts the stirring frequency of the electromagnetic stirring unit 7 to approach this natural frequency range (e.g., adjust to 32 Hz), thereby causing frequency locking. At this time, the flow field tends to stabilize, and the vortex shedding frequency is locked near the stirring frequency, which is beneficial for the stable folding and winding of the alumina film 19.

[0041] Step 7: Real-time monitoring of the deformation state of the alumina film 19 and execution of the bifurcation response control strategy. During the experiment, the control unit continuously monitors the deformation state of the alumina film 19 near the inlet of the flow channel 20. When the image data shows slight serrated tears at the edge of the alumina film 19, the control unit identifies a breakage trend and determines that the parameter combination is close to the breakage bifurcation boundary. At this time, the control unit no longer uses a fine-tuning strategy, but increases the adjustment range, reducing the stirring frequency of the electromagnetic stirring unit 7 from 32Hz to 25Hz, actively crossing the bifurcation boundary, so that the system quickly returns to the central region of the stable folding and winding branch. Subsequently, the breakage trend of the alumina film 19 disappears, and it re-exhibits stable folding and winding behavior. When the image data shows that the alumina film 19 is moving randomly on the melt surface and cannot enter the inlet region of the flow channel 20, the control unit identifies a drift trend and determines that the parameter combination is close to the drift bifurcation boundary. At this time, the control unit increases the adjustment range, raising the stirring frequency of the electromagnetic stirring unit 7 from 32Hz to 40Hz, actively crossing the bifurcation boundary, so that the alumina film 19 is captured and folded into the inlet area of ​​the guide channel 20.

[0042] Step 8: Record the entrainment process of the alumina film 19. Specifically, the observation unit records in real time the motion trajectory and deformation process of the alumina film 19 as it detaches from the melt surface, is entrained under the liquid near the inlet of the guide channel 20, and interacts with the solid-liquid interface. The imaging data from the imager 14 clearly shows that the alumina film 19 is subjected to shearing action by a local velocity gradient near the inlet of the guide channel 20, resulting in folding and continuous entrainment under the liquid, rather than random drifting or breakage.

[0043] Step 9: Shut down the system and remove the casting.

[0044] This embodiment demonstrates the application effect of frequency locking and bifurcation response control strategies in a real-world environment through a complete experimental procedure. Experimental results show that the present invention can actively induce stable and continuous folding and entrainment of the alumina film 19 near the inlet of the flow channel 20, and the entire process is recorded through in-situ observation, greatly improving the repeatability of the experiment and the reliability of the data. This provides an effective experimental method for revealing the formation mechanism of oxide film defects in aluminum alloy castings.

[0045] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. Other modifications can be easily made by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. An in-situ directional solidification apparatus for observing induced entrapment of alumina thin films, characterized in that, include: A crucible is used to contain melt, and the crucible has a melting zone and a solidification zone along its axial direction; Heating unit, used to heat the molten zone; Cooling unit, used to cool the solidification zone to establish a temperature gradient; A drive unit is used to drive the melt to move in the direction from the molten zone to the solidification zone and induce controlled flow near the inlet of the guide channel; The flow channel is set on the melt flow path in the solidification zone. The hydraulic diameter of the flow channel is d, and the length of the flow channel is L. The ratio of the length L to the hydraulic diameter d, L / d, is 0.8-3.

0. The flow channel is used to create a local velocity gradient at the inlet edge of the melt that is driven by the drive unit to flow through the flow channel, so as to induce the alumina film on the surface of the melt to fold and roll in. The observation unit is used for X-ray imaging of the solidified area; The drive unit includes: a pull-down mechanism connected to the crucible or solidified material inside the crucible for driving the melt to move axially; and an electromagnetic stirring unit arranged around the crucible and configured to induce a controlled circulating flow in the melt to form a local velocity gradient at the inlet of the flow channel for inducing the entrainment of the alumina film. The inlet end of the flow guiding channel is provided with a tapering flow guiding section; the flow guiding channel includes at least one of a circular orifice, a tapering orifice, a channel with a throat, or a slit-like flow guiding structure; It also includes a control unit, which is connected to the electromagnetic stirring unit and the pull-down mechanism, and is configured as follows: The frequency locking window is determined based on the hydraulic diameter d of the guide channel and the ratio L / d. The frequency locking window is the natural frequency range of vortex shedding at the inlet of the guide channel. Based on the deformation state of the alumina film near the inlet of the flow channel, the stirring frequency of the electromagnetic stirring unit and the pulling rate of the pulling mechanism are adjusted so that the parameter combination of stirring frequency and pulling rate is maintained on the stable folding and winding branch within the frequency locking window. When the parameter combination approaches the bifurcation boundary, increase the adjustment range of the stirring frequency and the pull-down rate so that the parameter combination crosses the bifurcation boundary and returns to the stable folding and entrainment branch. The bifurcation boundary is the parameter critical region where the deformation state of the alumina film jumps from stable folding and entrainment to breakage or drift. Using the hydraulic diameter d of the guide channel and the ratio L / d as an intermediary, the stirring frequency and the pull-down speed are coupled synergistically through the intermediary to form a continuous sheet-like folding and roll-in near the inlet of the guide channel.

2. The in-situ directional solidification device for observing induced entrapment of alumina thin films according to claim 1, characterized in that, The hydraulic diameter d of the flow guide channel is 5-8 mm, and the ratio of length L to hydraulic diameter d, L / d, is 1.0-2.

0. The contraction ratio A1 / A2 between the inlet cross-sectional area A1 of the gradually narrowing guide section and the minimum cross-sectional area A2 of the guide channel is 1.5-4.

0.

3. The in-situ directional solidification device for observing induced entrapment of alumina thin films according to claim 1, characterized in that, The observation unit includes a radiation source and a detector, and the device also includes an observation window, which is located on the side wall of the crucible or the side wall of the furnace. An X-ray transmission membrane is provided on the outside of the observation window. The periphery of the X-ray transmission membrane is sealed to the mounting surface of the observation window through a sealing layer. A clamping frame is provided on the outside of the X-ray transmission membrane to clamp and fix the X-ray transmission membrane to the observation window.

4. The in-situ directional solidification apparatus for observing induced entrapment of alumina thin films according to claim 3, characterized in that, An insulation layer is also provided on the outside of the observation window to keep the X-ray transmission film within a preset temperature range. The sealing layer is continuously coated along the circumference of the X-ray transmission membrane to form a closed ring-shaped sealing area; The clamping frame is connected to the observation window or furnace body by fasteners, which are evenly distributed along the circumference of the clamping frame.

5. A method for observation using the in-situ directional solidification apparatus for observing induced entrapment of alumina thin films as described in any one of claims 1 to 4, characterized in that, include: The aluminum alloy in the molten zone is heated to form a melt, and a temperature gradient is established in the solidification zone; The observation unit is activated to image the solidification zone; The melt is driven to move in the direction from the molten zone to the solidification zone, and controlled flow is induced near the inlet of the guide channel, so that a local velocity gradient is generated at the inlet edge when the melt flows through the inlet of the guide channel. The alumina film on the melt surface is folded and rolled up by a local velocity gradient instead of breaking, and the roll-up process is recorded by an observation unit.

6. The method according to claim 5, characterized in that, Also includes: Before heating, a crucible with a hydraulic diameter d of 5-8 mm and a length L to hydraulic diameter d ratio of 0.8-3.0 is selected in advance, based on the thickness of the alumina film to be observed. The steps of driving the melt to move in a direction from the molten zone to the solidification zone and inducing controlled flow near the inlet of the guide channel include: The frequency locking window is determined based on the hydraulic diameter d of the guide channel and the ratio L / d. The frequency locking window is the natural frequency range of vortex shedding at the inlet of the guide channel. Adjust the stirring frequency of the electromagnetic stirring unit and the pull-down rate of the pull-down mechanism to maintain the parameter combination of stirring frequency and pull-down rate on the stable folding and winding branch within the frequency locking window, thereby forming a continuous sheet-like folding and winding of the alumina film near the inlet of the flow channel.

7. The method according to claim 6, characterized in that, The steps for adjusting the stirring frequency of the electromagnetic stirring unit and the pulling speed of the pulling mechanism include: In response to the tendency of alumina films to break, the stirring frequency was reduced; In response to the drift of the alumina film on the melt surface, the stirring frequency is increased; When the combination of stirring frequency and pull-down rate approaches the bifurcation boundary, the bifurcation boundary is the critical region where the deformation state of the alumina film jumps from stable folding and incorporation to breakage or drift. Increasing the adjustment range allows the parameter combination to cross the bifurcation boundary and return to the stable folding and incorporation branch.

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