Method for obtaining casting defects of double-layer film

Abstract

A method for obtaining a double-layer film casting defect comprises the processes of melting and refining, mold cleaning and preheating, confluence, confluence sample treatment and the like, molten metal is poured in a static end liquid storage pool and a flowing end liquid storage pool respectively, and the double-layer film casting defect is formed at the confluence position of two molten metal flows in a horizontal section. The die comprises a static end liquid storage pool, a flowing end liquid storage pool and a horizontal section, the depth of the static end liquid storage pool and the depth of the flowing end liquid storage pool are smaller than or equal to 25 mm, the depth of the horizontal section of the die is smaller than or equal to 12.5 mm, and the flowing speed of molten metal in the horizontal section of the die is smaller than or equal to 0.5 m / s. According to the method, the casting defects of the double-layer film can be simply and quickly obtained at low cost, and the problem of randomness of the generation position of the double-layer film is effectively solved. Meanwhile, the characteristics of the double-layer film can be regulated and controlled through parameters such as the molten metal temperature, the static end liquid flow heat preservation time and components of molten metal poured into the static end liquid storage pool and the moving end liquid storage pool, and the influence of the double-layer film on the mechanical property is researched in an auxiliary mode.

Description

Technical Field

[0001] This invention belongs to the field of metal material casting, and specifically relates to a method for obtaining double-layer casting defects. Background Technology

[0002] Driven by the demands for lightweight automotive, high-efficiency rail transportation, and high safety, high reliability, and lightweight aerospace applications, aluminum / magnesium alloy castings have become core structural materials due to their superior specific strength. However, their casting process has long been plagued by double-film casting defects caused by melt oxidation and surface turbulence entrainment. The solid surface oxide film that forms instantaneously on the surface of the molten aluminum / magnesium alloy is entrained into the molten metal by surface turbulence during melting, transfer pouring, and mold filling, forming a "sandwich" structure consisting of two oxide films encasing gas. This double-film not only directly constitutes crack defects but also becomes the optimal initiation site for precipitation porosity, shrinkage cavities, and hot cracks, significantly reducing the tensile strength, toughness, fatigue resistance, corrosion resistance, airtightness, and performance consistency of castings. It is a highly significant type of casting defect.

[0003] The formation of bilayer films exhibits strong randomness, posing significant challenges to the detection and study of this type of casting defect. Researchers have attempted to simulate or reproduce bilayer film casting defects in castings through artificial introduction. Aryafa et al. (Aryafar M, Raiszadeh R, Shalbafzadeh A. Journal of materials science, 2010(45): 3041-3051.) used two 19mm diameter A356 aluminum alloy rods as raw materials. Using a specially designed steel pipe mold, they joined and melted the aluminum alloy rods to superimpose two surface oxide films, creating a bilayer film. However, the bilayer film prepared by this method differs fundamentally from the bilayer film generated by surface turbulence in terms of size, thickness, and microstructure. Divandari et al. (Divandari M, Campbell J. Aluminum Transaction, 2000(2): 233-238) continuously introduced bubbles into the alloy melt at the bottom of the melting crucible, obtaining a sandwich-like structure of oxide film-metal-oxide film overlapping at the collision points of the bubbles. This method can obtain oxide films in the alloy melt, facilitating the study of the oxide film's structure and morphology. However, this method cannot obtain the essential characteristics of a double-layer film, namely the oxide film-air gap-oxide film structure, and cannot completely equate the oxide film-metal-oxide film to a double-layer film casting defect for study. Currently, the casting field lacks an effective method to obtain double-layer film casting defects, which would facilitate a deeper understanding of double-layer film casting defects. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings in current research on double-layer film casting defects by proposing a method that can effectively obtain double-layer film casting defects, and on this basis, conduct in-depth research on double-layer film casting defects.

[0005] This invention is achieved through the following technical solutions.

[0006] The present invention provides a method for obtaining double-layer film casting defects, comprising the following steps.

[0007] Step 1: Melting and Refining. Add standard grade alloy ingots or pure metals and intermediate alloys to the melting furnace, and heat the furnace to 60-115°C above the alloy liquidus temperature. After complete melting, let it stand for 15-30 minutes. Purify the molten metal using a standard liquid alloy refining process (degassing and impurity removal). After skimming off the slag, adjust the temperature of the molten metal to 90-105°C above the alloy liquidus temperature and hold it at that temperature for 15-25 minutes.

[0008] Step 2: Mold cleaning and preheating: Clean the inner cavity of the mold, spray a high-temperature inert coating that does not wet the molten metal and does not produce a chemical reaction, preheat the mold to 10-100°C above the alloy liquidus temperature and keep it at that temperature for 2-3 hours to fully remove moisture.

[0009] The mold comprises three parts: a stationary end liquid storage tank, a flowing end liquid storage tank, and a horizontal section. The stationary end liquid storage tank and the flowing end liquid storage tank are located at opposite ends of the horizontal section.

[0010] Step 3: Scoop up the molten metal and slowly pour it into the stationary end storage tank until the leading edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it at a temperature of 0~40 min.

[0011] Step 4: After the stationary end of the liquid flow has been kept at a specific temperature for a certain period of time, scoop up the molten metal and slowly pour it into the reservoir at the flowing end until the leading edge of the moving molten metal merges with the stationary end of the liquid flow. A double-film casting defect is formed at the point where the two molten metal flows merge.

[0012] Step 5: Remove the mold from the heat preservation environment and cool it to room temperature in the air to obtain a confluence sample containing double-layer film casting defects.

[0013] Step 6: Process the manifold specimen. Break the manifold specimen on a tensile testing machine and observe the tensile fracture surface under a scanning electron microscope to obtain the characteristics of casting defects in the double-layer film.

[0014] Furthermore, the depth of the mold, the static end liquid storage tank and the flowing end liquid storage tank are ≤25mm, and the depth of the horizontal section of the mold is ≤12.5mm.

[0015] Furthermore, in steps 3-4, the chemical composition of the molten metal poured into the stationary end storage tank and the flowing end storage tank can be the same or different.

[0016] Furthermore, the flow velocity of the molten metal in the horizontal section of the mold is ≤0.5m / s.

[0017] The beneficial effects of this invention are as follows.

[0018] (1) In actual casting, the phenomenon of molten metal flowing into the mold cavity is very common, and the resulting double-film casting defects are also common. This invention utilizes the confluence of two molten metal flows to obtain double-film casting defects. The obtained double-film casting defects have their essential characteristics and are simple, fast, and low-cost, effectively solving the problem of the randomness of the location of double-film casting defects.

[0019] (2) The method of this invention provides rich information on casting defects in the double-layer film. The characteristics of the double-layer film (including film thickness, film composition and phase composition, film morphology, film area, etc.) can be controlled by adjusting parameters such as the temperature of the molten metal, the holding time of the liquid flow at the stationary end, and the composition of the molten metal poured into the storage tank at the stationary end and the storage tank at the flowing end. This is beneficial for studying the influence of the above parameters on the double-layer film. In addition, the tensile testing of the confluence sample can help study the influence of casting defects in the double-layer film on its tensile properties.

[0020] (3) This invention is applicable to cast aluminum alloys and cast magnesium alloys. Attached Figure Description

[0021] Figure 1 The images show the macroscopic morphology of the sluice sample and tensile fracture surface obtained by the method of this invention. Among them, (a) sluice sample; (bc) actual sluice sample and processing; (d) sluice sample with heat treatment for 0 min; (e) sluice sample with heat treatment for 30 min; (fh) macroscopic fracture surface of sluice sample with heat treatment for 0, 15 and 20 min.

[0022] Figure 2 The tensile fracture morphology of the stationary end liquid flow after holding for 20 min, 25 min, and 30 min in Example 1 is shown. Among them, (a) 0 min holding; (b) 20 min holding; (c) 30 min holding; (df) energy dispersive spectroscopy analysis of selected areas 1-3.

[0023] Figure 3 The tensile strength of the 7 busbar specimens obtained in Example 1.

[0024] Figure 4The tensile fracture morphology of the stationary end liquid flow after holding for 0 min, 20 min, and 30 min in Example 2 is shown. Among them, (a) 0 min holding; (b) 20 min holding; (c) 30 min holding; (di) energy dispersive spectroscopy analysis of selected areas 4-9.

[0025] Figure 5 The tensile fracture morphology of the stationary end liquid flow after 0 min, 20 min, and 30 min of heat preservation in Example 3 is shown. Among them, (a) without heat preservation; (b) 20 min; (c) 30 min; (di) energy dispersive spectroscopy analysis of selected areas 10-15.

[0026] Figure 6 The tensile strength of the busbar specimens obtained in Examples 2 and 3.

[0027] Figure 7 The tensile fracture characteristics of the busbar sample obtained in Example 4 are shown. Among them, (ac) is the tensile fracture with A356 on both sides of the busbar; (dg) is the tensile fracture with 0.5% Bi added on the stationary side; (h) is the EDS surface scan result at (g); and (i) is the chemical composition at each point in (f, g).

[0028] Figure 8 This is a schematic diagram of the mold used in this invention. Wherein, 1-stationary end liquid storage tank; 2-moving end liquid storage tank; 3-horizontal section; 4-stationary end liquid flow front; 5-moving end liquid flow front; 6-stationary end liquid flow surface oxide film; 7-moving end liquid flow surface oxide film. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the drawings and specific embodiments are explanations of the present invention and not limitations thereof. Example 1

[0030] A method for obtaining double-layer casting defects includes the following steps.

[0031] (1) The alloy composition (mass percentage %) of A356 is as follows: Si 7.10, Mg 0.32, Fe≤0.12, Mn≤0.05, Zn≤0.01, with the balance being aluminum. The alloy with the above proportions is heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal is kept at 720℃ and allowed to stand for 30 min.

[0032] (2) Cleaning and preheating the graphite mold: Clean the inner cavity of the mold, spray BN coating, place the mold in a heat preservation furnace, preheat to 650℃ and keep it at that temperature for 2 hours to remove moisture, etc. The depth of the liquid storage tank at the stationary end and the liquid storage tank at the flowing end is 15mm, and the depth of the horizontal section of the mold is 8mm.

[0033] (3) Scoop up the A356 molten metal and slowly pour it into the stationary end storage tank until the front edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it warm for 0 min.

[0034] (4) Scoop up the A356 molten metal and slowly pour it into the flowing end storage tank until the moving front of the molten metal merges with the stationary end liquid flow.

[0035] (5) Remove the mold from the heat preservation furnace and cool it to room temperature in the air to obtain a busbar sample containing double film defects.

[0036] (6) Repeat steps (3) to (5), setting the static end liquid flow holding time in step (3) to 5 min, 10 min, 15 min, 20 min, 25 min and 30 min. Obtain 7 manifold samples.

[0037] (7) Processing the busbar sample. The busbar sample was broken on an E45.105 universal testing machine at a tensile speed of 1 mm / min to obtain tensile properties; the tensile fracture surface was observed on a Quanta 200FEG field emission environmental scanning electron microscope to observe the casting defect characteristics of the double-layer film. Example 2

[0038] A method for obtaining double-layer casting defects includes the following steps.

[0039] (1) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe≤0.12, Mn≤0.05, Zn≤0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 min to obtain molten metal 1.

[0040] (2) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe≤0.12, Mn≤0.05, Zn≤0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 min. 0.5Bi was then added to obtain molten metal 2.

[0041] (2) Cleaning and preheating the graphite mold: Clean the inner cavity of the mold, spray BN coating, place the mold in a heat preservation furnace, preheat to 650℃ and keep it at that temperature for 2 hours to remove moisture, etc. The depth of the liquid storage tank at the stationary end and the liquid storage tank at the flowing end is 15mm, and the depth of the horizontal section of the mold is 8mm.

[0042] (3) Scoop up the molten metal 1 and slowly pour it into the stationary end storage tank until the front edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it warm for 0 min.

[0043] (4) Scoop up the molten metal 2 and slowly pour it into the liquid storage tank at the flowing end until the front of the molten metal flows into the liquid flow at the stationary end.

[0044] (5) Remove the mold from the heat preservation furnace and cool it to room temperature in the air to obtain a confluence sample containing double-layer film casting defects.

[0045] (6) Repeat steps (3) to (5), setting the static end liquid flow holding time in step (3) to 5 min, 10 min, 15 min, 20 min, 25 min and 30 min. Obtain 7 manifold samples.

[0046] (7) Processing the busbar sample. The busbar sample was broken on an E45.105 universal testing machine at a tensile speed of 1 mm / min to obtain tensile properties; the tensile fracture surface was observed on a Quanta 200FEG field emission environmental scanning electron microscope to observe the casting defect characteristics of the double-layer film. Example 3

[0047] A method for obtaining double-layer casting defects includes the following steps.

[0048] (1) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe≤0.12, Mn≤0.05, Zn≤0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 min to obtain molten metal 1.

[0049] (2) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe ≤ 0.12, Mn ≤ 0.05, Zn ≤ 0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 min. 1.0 Bi was added to obtain molten metal 2.

[0050] (2) Cleaning and preheating the graphite mold: Clean the inner cavity of the mold, spray BN coating, place the mold in a heat preservation furnace, preheat to 650℃ and keep it at that temperature for 2 hours to remove moisture, etc. The depth of the liquid storage tank at the stationary end and the liquid storage tank at the flowing end is 15mm, and the depth of the horizontal section of the mold is 8mm.

[0051] (3) Scoop up the molten metal 1 and slowly pour it into the stationary end storage tank until the front edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it warm for 0 min.

[0052] (4) Scoop up the molten metal 2 and slowly pour it into the liquid storage tank at the flowing end until the front of the molten metal flows into the liquid flow at the stationary end.

[0053] (5) Remove the mold from the heat preservation furnace and cool it to room temperature in the air to obtain a confluence sample containing double-layer film casting defects.

[0054] (6) Repeat steps (3) to (5), setting the static end liquid flow holding time in step (3) to 5 min, 10 min, 15 min, 20 min, 25 min and 30 min. Obtain 7 manifold samples.

[0055] (7) Processing the busbar sample. The busbar sample was broken on an E45.105 universal testing machine at a tensile speed of 1 mm / min to obtain tensile properties; the tensile fracture surface was observed on a Quanta 200FEG field emission environmental scanning electron microscope to observe the casting defect characteristics of the double-layer film. Example 4

[0056] A method for obtaining double-layer casting defects includes the following steps.

[0057] (1) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe≤0.12, Mn≤0.05, Zn≤0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 min. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 min. 0.5Bi was then added to obtain molten metal 1.

[0058] (2) The alloy composition (mass percentage %) of A356 was as follows: Si 7.10, Mg 0.32, Fe ≤ 0.12, Mn ≤ 0.05, Zn ≤ 0.01, with the balance being aluminum. The alloy with the above proportions was heated and melted at 720℃ and then allowed to stand for 20 minutes. After degassing, removing impurities, and removing slag, the molten metal was kept at 720℃ and allowed to stand for 30 minutes to obtain molten metal 2.

[0059] (2) Cleaning and preheating the graphite mold: Clean the inner cavity of the mold, spray BN coating, place the mold in a heat preservation furnace, preheat to 650℃ and keep it at that temperature for 2 hours to remove moisture, etc. The depth of the liquid storage tank at the stationary end and the liquid storage tank at the flowing end is 15mm, and the depth of the horizontal section of the mold is 8mm.

[0060] (3) Scoop up the molten metal 2 and slowly pour it into the stationary end storage tank until the front edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it warm for 10 minutes.

[0061] (4) Scoop up the molten metal 2 and slowly pour it into the liquid storage tank at the flowing end until the front of the molten metal flows into the liquid flow at the stationary end.

[0062] (5) Remove the mold from the heat preservation furnace and cool it to room temperature in the air to obtain a confluence sample containing double-layer film casting defects.

[0063] (6) Scoop up molten metal 1 and slowly pour it into the stationary end storage tank until the front edge of the molten metal reaches the middle of the horizontal section of the mold and stop pouring to obtain the stationary end liquid flow, and keep it warm for 10 minutes.

[0064] (7) Scoop up the molten metal 2 and slowly pour it into the liquid storage tank at the flowing end until the front of the molten metal flows into the liquid flow at the stationary end.

[0065] (8) Remove the mold from the heat preservation furnace and cool it to room temperature in the air to obtain a confluence sample containing double-layer film casting defects.

[0066] (9) Processing the busbar sample. The busbar sample was broken on an E45.105 universal testing machine at a tensile speed of 1 mm / min. The tensile fracture surface was observed on a Quanta 200FEG field emission environmental scanning electron microscope to observe the casting defect characteristics of the double-layer film.

[0067] Figure 1 The images show the macroscopic morphology of the molten metal sample and the tensile fracture surface obtained by the method of the present invention. It can be seen that the method of the present invention can effectively obtain double-layer casting defects by controlling the molten metal flow.

[0068] Figure 2 The microstructures of the tensile fracture surfaces after 0 min, 20 min, and 30 min of static end liquid flow holding in Example 1 are shown. Figure 3 The tensile strength of the seven busbar specimens obtained in Example 1. The fracture surface of the tensile specimen after a holding time of 0 min. Figure 2 (a) A double-layer casting defect with a size of approximately 20 micrometers was found on the cross-section. Energy dispersive spectroscopy (EDS) analysis showed that it was mainly composed of 55.85% O, 20.87% Mg, and 21.78% Al, indicating that the oxide film was likely composed of MgO and MgAl₂O₄. At holding times of 20 min and 30 min, Figure 2 (bc) A distinctly symmetrical double-layer casting defect appeared on the fracture surface. Compared with the sample that was held at 0 min, Figure 2 The oxide film shown in (bc) is more continuous and denser. Figure 3 This indicates that the tensile strength of the manifold specimen decreases with increasing heat preservation time. (Comparison) Figure 2 (ac) The size of casting defects in the double-layer film ranges from the micrometer level to the millimeter level. The increase in the area of ​​the double-layer film can explain why the tensile strength of the manifold samples after holding for 20 min and 30 min decreased significantly. Example 1 shows that the method of the present invention can be used to study the composition, phase composition, morphology of the double-layer film and its influence on tensile properties, and to study the effect of holding time on casting defects in the double-layer film.

[0069] Figure 4 The tensile fracture morphology is shown for 0 min, 20 min and 30 min after the static end liquid flow was kept at a constant temperature in Example 2. Figure 5 The images show the tensile fracture morphology of the stationary end liquid flow after holding for 0 min, 20 min, and 30 min in Example 3. Compared with Example 1, the addition of Bi altered the casting defects of the double-layer film in Examples 2 and 3. With the addition of 0.5% Bi, granular Bi-rich phases appeared on the casting defects of the double-layer film, with Mg atoms enriched in these phases. With the addition of 1% Bi, banded Bi-rich phases appeared on the casting defects of the double-layer film. Compared with the granular Bi-rich phases, the banded Bi-rich phases had higher O concentrations and lower Mg concentrations. Both the granular and banded Bi-rich phases fragmented the double-layer film matrix, disrupting the continuity of the casting defects, with the banded Bi-rich phases exhibiting a more pronounced fragmentation effect. No double-layer casting defects were observed on the tensile fracture surfaces of the samples with 0 min holding time (adding 0.5% Bi and 1% Bi), indicating that the addition of Bi can reduce double-layer casting defects in castings. Without Bi, a large area of ​​continuous double-layer film casting defects can be found on the tensile fracture surface after a holding time of 15 minutes; after adding Bi, a large area of ​​overlapping film can only be found on the fracture surface after a holding time of 20 minutes. The addition of Bi disrupts the continuity of the double-layer film and delays the formation time of the continuous oxide film. Figure 6 The tensile strength of the manifold specimens obtained in Examples 2 and 3 shows that the tensile strength gradually decreases with increasing holding time. Figure 6 and Figure 3 Bi can mitigate the decrease in tensile strength as the holding time increases. Examples 2 and 3 show that the method of the present invention can be used to study the effects of alloy composition on the composition, phase composition, and morphology of the bilayer film, and further investigate the influence of the bilayer film on tensile properties.

[0070] Figure 7 The tensile fracture characteristics of the busbar specimen obtained in Example 4 are shown. Figure 7 (ac) shows the tensile fracture surface of the busbar sample without Bi addition; the bilayer membrane size is large and continuous. After adding 0.5% Bi to the stationary side, the bilayer membrane size on the fracture surface is also large. Figure 7 Compared to the bilayer membrane shown in (ac), Figure 7(e) shows that the microscopic continuity of the bilayer film is disrupted, and the oxide film has obvious stress characteristics. Figure 7 (f, g) shows the microstructure of the fractured region. According to... Figure 7 (f) EDS analysis results at each point show that an oxide film and an aluminum alloy matrix coexist at the fracture site, and Bi-rich phases that have been pulled apart were also found around the fracture area (point 36). Figure 7 (g) The features shown are similar to Figure 7 (f) Similarly, the fracture region contains both the matrix and the oxide film, both of which exhibit tensile deformation characteristics. Therefore, adding Bi can transform the bilayer film from a continuous film to an overlapping film, and the overlapping oxide films can hinder the occurrence of fracture. Example 4 shows that the method of the present invention can be used to study the influence mechanism of added elements in molten metal on casting defects in bilayer films.

[0071] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any modifications to the equivalent structure or process described in the present invention, or direct or indirect applications to other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method of obtaining a cast defect in a bilayer film, characterized by The method comprises the following steps: Step 1: melting and refining: standard brand alloy ingots or pure metals and intermediate alloy raw materials are added into a melting furnace, and the furnace is heated to 60-115 DEG C above the liquidus temperature of the alloy, and after the raw materials are completely melted, the metal liquid is purified by degassing and impurity removal, and after slagging, the temperature of the metal liquid is adjusted to 90-105 DEG C above the liquidus temperature of the alloy, and the metal liquid is kept for 15-25 min; Step 2: mold cleaning and preheating: clean the inner cavity of the mold, spray high-temperature inert coating which does not wet the metal liquid and does not produce chemical reaction, preheat the mold to 10-100 DEG C above the liquidus temperature of the alloy and keep for 2-3 h, and remove water; The mold comprises a static end liquid pool, a flowing end liquid pool and a horizontal section, and the static end liquid pool and the flowing end liquid pool are respectively located at two ends of the horizontal section; Step 3: the metal liquid is scooped and slowly poured into the static end liquid pool until the metal liquid motion front reaches the middle of the mold horizontal section to stop pouring, and the static end liquid flow is obtained, and the temperature is kept for 0-60 min; Step 4: after the static end liquid flow is kept for a certain time, the metal liquid is scooped and slowly poured into the flowing end liquid pool until the metal liquid motion front meets the static end liquid flow; a double-layer film casting defect is formed at the meeting place of the two metal liquid flows; Step 5: the mold is taken out from the keeping environment and cooled to room temperature in air to obtain a confluence sample containing a double-layer film casting defect; Step 6: processing the confluence sample: the confluence sample is pulled apart on a tensile testing machine, and the tensile fracture is observed on a scanning electron microscope to obtain the characteristics of the double-layer film casting defect.

2. A method of obtaining casting defects of a bilayer film according to claim 1, characterized in that The depth of the static end liquid pool and the flowing end liquid pool is ≤25 mm, and the depth of the mold horizontal section is ≤12.5 mm.

3. The method of claim 1, wherein the method further comprises In steps 3-4, the chemical composition of the metal liquid poured into the static end liquid pool and the flowing end liquid pool is the same or different.

4. The method of claim 1, wherein the method further comprises The flow velocity of the metal liquid in the mold horizontal section is ≤0.5 m / s.

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