Method for strengthening stainless steel and titanium alloy welded joint based on CFD simulation

Abstract

According to the stainless steel and titanium alloy welded joint strengthening method based on CFD simulation, a stainless steel component is a cylindrical tube shell, one end of the cylindrical tube shell is completely open, and a through hole is formed in the other end of the cylindrical tube shell; the titanium component is a thin plate, and the thin plate is arranged in the cylindrical tube shell and is fastened by applying pressure; a FLOW-3D simulation platform is adopted, boundary conditions applied to six surfaces of a computational domain are all constant-pressure boundary conditions, and laser power, pulse frequency, defocusing amount, welding speed and material attributes serve as input to predict temporal and spatial changes of temperature, fluid flow and element distribution in a molten pool; and based on the prediction result of the FLOW-3D simulation platform, the nanometer laser is incident from one side of the thin plate at the predicted laser power, pulse frequency, defocusing amount and welding speed, the laser scanning track is a circular path, cooling is conducted to the room temperature after welding is completed, a supersaturated solid solution in a welding seam is reserved, and the situation that intermetallic compounds are separated out again is avoided.

Description

Technical Field

[0001] The invention relates to the technical field of welding, and in particular to a stainless steel and titanium alloy weld joint strengthening method based on CFD simulation. Background Art

[0002] Titanium and stainless steel welding has always been difficult. One of the main reasons is that Fe-Ti binary alloys have two types of intermetallic compounds, namely, FeTi with a B2-type CsCl structure with a narrow homogeneity range (47.5-50.3at.%Fe) and a hexagonal C14MgZn2-type Laves_Fe2Ti containing (64.5-72.4at.%Fe). These brittle intermetallic compounds lead to a significant decrease in the mechanical properties of the weld. In addition, the physical properties of Ti and Fe, such as the thermal expansion coefficient (Ti, 7.6mm -1 K -1 ; SS, 17-18mm -1 K -1 ) leads to excessive deformation and residual stress. In order to solve the above problems, the prior art suppresses the generation of brittle intermetallic compounds by adding additional alloying elements at the Ti-Fe interface. By adding medium-entropy or high-entropy alloys containing Fe or Ti elements at the steel / titanium interface, it is expected to form a high-entropy alloy in the weld to avoid the generation of brittle intermetallic compounds. Unfortunately, FeTi or Fe2Ti intermetallic compounds will still be produced in the weld. In addition, the prior art makes a Ti / steel composite interlayer by explosion welding, and then places it between the stainless steel and Ti plates to be welded, and laser welds the steel / steel and Ti / Ti interfaces respectively, avoiding the mixing of Ti and steel. However, this welding process increases the complexity of the welding process, increases the production cost, and when the wall thickness of the welded component is very thin, the addition of the intermediate layer will become extremely difficult.

[0003] According to the Ti-Fe binary phase diagram, the maximum solid solubility of Ti in α-Fe is about 10at.%. If the ratio of Fe to Ti in the molten pool is precisely controlled to not exceed 9:1 during the welding process and the cooling speed is fast enough, the supersaturated α-Fe of Ti can be kept at room temperature to avoid the formation of Fe2Ti intermetallic compounds. Traditional millisecond pulse lasers or continuous fiber lasers are prone to problems such as weld penetration, large deformation, and low welding strength during the welding of dissimilar metal sheets with a thickness of less than 0.2mm, and can no longer meet the welding requirements of dissimilar metal sheets.

[0004] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the invention

[0005] The present invention provides a stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, which accurately controls the ratio of Ti and Fe elements in the molten pool, obtains a faster cooling rate, inhibits the generation of brittle intermetallic compounds, clarifies the mechanism of the effect of the titanium content in the weld on the joint strength, and provides a new welding strategy for thin-walled titanium-stainless steel dissimilar metal welding.

[0006] A method for strengthening stainless steel and titanium alloy welded joints based on CFD simulation includes: The stainless steel component is a cylindrical shell, one end of which is completely open and the other end is provided with a through hole; the titanium component is a thin plate, which is placed in the cylindrical shell and tightened by applying pressure; The FLOW-3D simulation platform was used, and the boundary conditions imposed on the six surfaces of the computational domain were all constant pressure boundary conditions. The laser power, pulse frequency, defocus, welding speed, and material properties were used as inputs to predict the spatiotemporal changes in temperature, fluid flow, and element distribution in the molten pool. Based on the prediction results of the FLOW-3D simulation platform, the nanolaser is incident from one side of the thin plate with the predicted laser power, pulse frequency, defocus, and welding speed. The laser scanning trajectory is a circular path. After welding is completed, it is cooled to room temperature so that the supersaturated solid solution in the weld can be retained to prevent the precipitation of intermetallic compounds again.

[0007] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the diameter of the cylindrical shell is 4 mm, the height is 4 mm, the shell wall thickness is 0.25 mm, and the diameter of the titanium component is 4 mm and the thickness is 0.1 mm.

[0008] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the minimum grid size of the calculation domain is 10 μm and the maximum grid size is 100 μm.

[0009] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the bottom of the molten pool is located at the center of the titanium component, and the bottom of the molten pool penetrates the component but does not penetrate the titanium component. The simulated titanium content in the weld is 9.99at.%.

[0010] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the wavelength of the nano laser is 1059 nm, the maximum average power is 200 W, and the maximum peak power is 10000 W.

[0011] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the laser power is 0.16kW-0.2kW, the defocus is 3mm, the welding speed is 110mm / s, the pulse width is 0.000002s, and the shielding gas flow rate is 5L / min.

[0012] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the circular path diameter is 3 mm.

[0013] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the predicted result is that the titanium content in the weld is less than 10at.%, and the predicted laser power, pulse frequency, defocus amount, and welding speed are determined based on the predicted result.

[0014] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, when the molten pool penetrates the titanium component, the average content of titanium in the weld exceeds 23at.%, and when the bottom of the molten pool is located at the center of the titanium component, the average content of titanium in the weld is 9.99at.%.

[0015] In the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, laser welding is performed under an inert gas protective atmosphere.

[0016] Compared with the prior art, the present invention has the following advantages: the CFD model is used to predict the content and distribution of the Ti element in the weld under different welding parameter conditions, and the welding process parameters are preferably such that the titanium content in the weld is less than 10at.%. The welding heat input is precisely controlled by a nanosecond laser, thereby adjusting the depth of the molten pool, so that the degree of mixing with the Fe element and the Ti element entering the molten pool is reduced, thereby suppressing the generation of brittle intermetallic compounds. Secondly, the rapid cooling conditions provided by nanosecond laser welding allow the supersaturated solid solution in the weld to be retained, thereby preventing the precipitation of intermetallic compounds again. Finally, the method has the advantages of simple welding process, low cost, and easy promotion, and is suitable for the connection of thin-walled titanium alloy and stainless steel lap joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] By reading the detailed description of the preferred specific embodiments below, various other advantages and benefits of the present invention will become clear to those of ordinary skill in the art. The drawings in the specification are only for the purpose of illustrating the preferred embodiments and are not considered to be limitations of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative work. Moreover, the same reference numerals are used to represent the same components throughout the drawings.

[0018] In the attached picture: Figure 1 It is the CFD calculation model and the schematic diagram of the pulse laser waveform; Figure 2 Schematic diagram of pressure fastening fixture for lap joint of titanium alloy and stainless steel; Figure 3This is a schematic diagram of the laser welding structure and mechanical properties test of titanium-steel dissimilar metal lap joint; Figure 4 Schematic diagram of the weld appearance of laser welding of titanium-steel dissimilar metal lap joints obtained by CFD simulation prediction under four different welding process parameters; Figure 5 Schematic diagram of the appearance of the laser weld of titanium-steel dissimilar metal lap joints obtained under four different welding process parameters; Figure 6 Schematic diagram of the cross-sectional morphology of the laser welding weld of titanium-steel dissimilar metal lap joints obtained by CFD simulation prediction under four different welding process parameters; Figure 7 Schematic diagram of the cross-sectional morphology of the laser welding weld of titanium-steel dissimilar metal lap joints obtained under four different welding process parameters; Figure 8 Schematic diagram of load-displacement curves of laser welded titanium-steel dissimilar metal lap joints obtained under four different welding process parameters; Fig. 9 The figure is a bar graph showing the Ti content in the weld obtained under four different welding process parameters and the maximum load when the weld fails.

[0019] The present invention is further explained below in conjunction with the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0020] The specific embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although the specific embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art.

[0021] It should be noted that certain words are used in the specification and claims to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component. This specification and claims do not use the difference in nouns as a way to distinguish components, but use the difference in the functions of the components as the criterion for distinction. As mentioned throughout the specification and claims, "including" or "comprising" is an open term, so it should be interpreted as "including but not limited to". The subsequent description of the specification is a preferred embodiment of the present invention, but the description is based on the general principles of the specification and is not intended to limit the scope of the present invention. The scope of protection of the present invention shall be determined by the attached claims.

[0022] To facilitate understanding of the embodiments of the present invention, further explanation will be given below by taking specific embodiments as examples in conjunction with the accompanying drawings, and each of the accompanying drawings does not constitute a limitation on the embodiments of the present invention.

[0023] like Figures 1 to 9 As shown in the figure, the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation includes the following steps: The stainless steel component is a cylindrical shell, one end of which is completely open and the other end is provided with a through hole; the titanium component is a thin plate, which is placed in the cylindrical shell and tightened by applying pressure; The FLOW-3D simulation platform was used, and the boundary conditions imposed on the six surfaces of the computational domain were all constant pressure boundary conditions. The laser power, pulse frequency, defocus, welding speed, and material properties were used as inputs to predict the spatiotemporal changes in temperature, fluid flow, and element distribution in the molten pool. Based on the prediction results of the FLOW-3D simulation platform, the nanolaser is incident from one side of the thin plate with the predicted laser power, pulse frequency, defocus, and welding speed. The laser scanning trajectory is a circular path. After welding is completed, it is cooled to room temperature so that the supersaturated solid solution in the weld can be retained to prevent the precipitation of intermetallic compounds again.

[0024] In the preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the diameter of the cylindrical shell is 4 mm, the height is 4 mm, the shell wall thickness is 0.25 mm, and the diameter of the titanium component is 4 mm and the thickness is 0.1 mm.

[0025] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the minimum grid size of the calculation domain is 10 μm and the maximum grid size is 100 μm.

[0026] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the bottom of the molten pool is located at the center of the titanium component, and the bottom of the molten pool penetrates the component but does not penetrate the titanium component. The simulated titanium content in the weld is 9.99at.%.

[0027] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the wavelength of the nanolaser is 1059nm, the maximum average power is 200W, and the maximum peak power is 10000W.

[0028] In the preferred implementation of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the laser power is 0.16kW-0.2kW, the defocus is 3mm, the welding speed is 110mm / s, the pulse width is 0.000002s, and the shielding gas flow rate is 5L / min.

[0029] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the circular path diameter is 3 mm.

[0030] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, the predicted result is that the titanium content in the weld is less than 10at.%, and the predicted laser power, pulse frequency, defocus amount, and welding speed are determined based on the predicted result.

[0031] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, when the molten pool penetrates the titanium component, the average content of titanium in the weld exceeds 23 at.%, and when the bottom of the molten pool is located at the center of the titanium component, the average content of titanium in the weld is 9.99 at.%.

[0032] In a preferred embodiment of the stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, laser welding is performed under an inert gas protective atmosphere.

[0033] In one embodiment, a stainless steel and titanium alloy weld joint strengthening method based on CFD simulation includes: (1) Welding specimens: The stainless steel component is a cylindrical tube shell with a wall thickness of 0.25 mm and an inner diameter of 4 mm. One end of the tube shell is completely open, and there is a through hole with a diameter of 2 mm at the other end; the titanium component is a thin plate with a wall thickness of 0.1 mm and a diameter of 4 mm.

[0034] (2) Simulation method: The simulation platform uses FLOW-3D, and the minimum grid size of the computational domain is 10 μm and the maximum grid size is 100 μm. The boundary conditions applied to the six surfaces of the computational domain are all constant pressure (atmospheric pressure) boundary conditions. The model uses the welding parameters (laser power, pulse frequency, defocus and welding speed) and material properties as input to predict the spatiotemporal changes in temperature, fluid flow and element distribution in the molten pool.

[0035] (3) Welding equipment: The welding heat source uses TrumpfTruPulse laser with a wavelength of 1059nm, a maximum average power of 200W, and a maximum peak power of 10000W. The model and pulse laser waveform are as follows: Figure 1 shown.

[0036] (4) Welding process: laser power 0.16kW-0.2kW, defocus 3mm, welding speed 110mm / s, pulse width 0.000002s, shielding gas flow 5L / min. Table 1 shows the four different welding process parameters implemented.

[0037] Table 1 Process parameters

[0038] (5) Place the titanium sheet in the stainless steel tube shell and apply pressure to tighten it. The pressure tightening device is as follows: Figure 2 shown.

[0039] (6) The laser is incident from one side of the stainless steel sheet, and the laser scanning trajectory is a circular path with a diameter of about 3 mm. After welding is completed, it is cooled to room temperature.

[0040] (7) After welding, the weld is inspected to measure the forming quality and microstructure. The mechanical properties of the joint are tested by the following methods: Figure 3 shown.

[0041] The weld appearance morphology predicted by CFD simulation under four different welding process parameters is shown in the figure below. Figure 4 As shown, Figure 4 (a), (b), (c) and (d) are the weld appearance morphologies predicted by CFD simulation when the process parameters are 1#-4# respectively.

[0042] The weld appearance obtained under four different welding process parameters is as follows: Figure 5 As shown, Figure 5 (a), (b), (c) and (d) are the weld appearance morphologies obtained from the test with process parameters 1#-4# respectively.

[0043] Figure 6 (a), (b), (c) and (d) are the cross-sectional morphology and Ti element concentration field of the weld predicted by CFD simulation when the process parameters are 1#-4#, respectively. Figure 7 (a), (b), (c) and (d) are the cross-sectional morphologies of the welds obtained from the tests with process parameters 1#-4# respectively. The dotted line is the cross-sectional profile of the weld. Figure 7 (e), (f), (g) and (h) are the EDS mapping results of Fe element in the weld cross section obtained from the 1#-4# process parameter tests. Figure 8 is the load-displacement curve of the welded joint with different welding process parameters, Figure 8 (a), (b), (c) and (d) are the joints obtained by welding with process parameters 1#-4# respectively. Fig. 9The bar charts of the Ti content in the weld obtained for different welding process parameters and the maximum load when the weld fails are compared. From the statistical results, it can be seen that when welding with process 1#, the average Ti content in the weld is about 9.99%, and the maximum average load when the weld fails is about 40.87N; when welding with process 2#, the average Ti content in the weld is about 23.41%, and the maximum average load when the weld fails is about 20.6N; when welding with process 3#, the average Ti content in the weld is about 25.67%, and the maximum average load when the weld fails is about 17.6N; when welding with process 4#, the average Ti content in the weld is about 27.54%, and the maximum average load when the weld fails is about 17.24N. When the Ti content in the weld is 9.99%, the maximum average load when the weld fails increases by about 2 times.

[0044] In one embodiment, the CFD model is used to predict the content and distribution of Ti elements in the weld under different welding parameters, and the welding process parameters that make the titanium content in the weld less than 10at.% are preferably selected. The mixing degree of Ti and stainless steel is accurately controlled to inhibit the formation of intermetallic compounds in the weld, thereby improving the strength of the welded joint. Nanosecond laser is used as a welding heat source to accurately control the welding heat input, and the titanium thin plate is placed under the stainless steel thin plate, and laser welding is performed under an inert gas protective atmosphere. Regardless of whether the molten pool penetrates the titanium thin wall, the titanium element is easy to form an enrichment zone at the edge of the molten pool on one side of the stainless steel. When the molten pool penetrates the titanium thin plate, the average content of titanium in the weld exceeds 23at.%, and when the bottom position of the molten pool is located at the center of the titanium thin wall, the average content of titanium in the weld is about 9.99at.%, and the strength of the weld can be doubled. Compared with other methods of adding alloying elements for strengthening, this method has the advantages of simple welding process, low cost, and easy promotion, and is suitable for the connection of thin-walled titanium alloy and stainless steel lap joints.

[0045] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the field should understand that as mentioned above, similar technical methods can be derived from the scheme contents given in combination with the drawings and embodiments. For heat sources such as arcs and electron beams, both Ti and stainless steel lap welding can be performed. The CFD model is used to predict the content and distribution state of the Ti element in the weld under different welding parameters, preferably causing the welding process parameters with a titanium content of less than 10at.%. The degree of mixing with the Fe element and the Ti element entering the molten pool is reduced, and the generation of brittle intermetallic compounds is suppressed. Secondly, by rapid cooling, the supersaturated solid solution in the weld is retained to avoid the precipitation of intermetallic compounds again, thereby improving the strength of the weld. Any simple modifications, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention are still within the scope of the technical solution of the present invention.

[0046] Although the embodiments of the present invention are described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields, and the above specific embodiments are only illustrative and instructive, rather than restrictive. A person of ordinary skill in the art can also make many forms under the guidance of this specification and without departing from the scope of protection of the claims of the present invention, all of which belong to the protection of the present invention.

Claims

1. A stainless steel and titanium alloy weld joint strengthening method based on CFD simulation, characterized in that: The steps include: The stainless steel component is a cylindrical shell, one end of which is completely open and the other end is provided with a through hole; the titanium component is a thin plate, which is placed in the cylindrical shell and tightened by applying pressure; The FLOW-3D simulation platform was used, and the boundary conditions imposed on the six surfaces of the computational domain were all constant pressure boundary conditions. The laser power, pulse frequency, defocus, welding speed, and material properties were used as inputs to predict the spatiotemporal changes in temperature, fluid flow, and element distribution in the molten pool. Based on the prediction results of the FLOW-3D simulation platform, the nanolaser is incident from one side of the thin plate with the predicted laser power, pulse frequency, defocus, and welding speed. The laser scanning trajectory is a circular path. After welding is completed, it is cooled to room temperature so that the supersaturated solid solution in the weld can be retained to prevent the precipitation of intermetallic compounds again.

2. A stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 1, characterized in that: Preferably, the diameter of the cylindrical shell is 4 mm, the height is 4 mm, the wall thickness of the shell is 0.25 mm, and the diameter of the titanium component is 4 mm, and the thickness is 0.1 mm.

3. The stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 1 is characterized in that: The minimum grid size of the computational domain is 10 μm, and the maximum grid size is 100 μm.

4. The stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 1 is characterized in that: The bottom of the molten pool is located at the center of the titanium component.

5. A stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 4, characterized in that: The bottom of the molten pool penetrates the component but not the titanium component. The simulated titanium content in the weld is 9.99at.%.

6. The stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 1 is characterized in that: The circular path diameter is 3 mm.

7. The stainless steel and titanium alloy weld joint strengthening method based on CFD simulation according to claim 1 is characterized in that: Laser welding is performed under an inert gas protective atmosphere.

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