Method for improving performance of 6061 / 316L friction stir lap joint welded joint

Abstract

The invention provides a method for improving the performance of a 6061 / 316L friction stir lap joint welding joint, and belongs to the technical field of heterogeneous metal friction stir welding. The method comprises the following steps that the microscopic structure and the mechanical property of 6061 aluminum alloy base metal are regulated and controlled, a 6061 aluminum alloy plate and a 316L stainless steel plate are fixed in a lap joint mode, and friction stir lap joint welding is conducted; the microscopic structure comprises a small-angle grain boundary proportion and an average grain size; the mechanical properties comprise tensile strength, ductility and Vickers hardness; the proportion of the small-angle grain boundary ranges from 10% to 25%, the average grain size ranges from 15 micrometers to 25 micrometers, the tensile strength ranges from 200 MPa to 230 MPa, the ductility ranges from 20% to 25%, and the Vickers hardness ranges from 60 HV to 80 HV. According to the method, the structure and performance of the 6061 aluminum alloy base material are firstly adjusted, and then proper friction stir welding process parameters are selected, so that the performance of a welding joint can be remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of friction stir welding of heterogeneous metals, and in particular to a method for improving the performance of a 6061 / 316L friction stir lap welding joint. Background Art

[0002] In recent years, with the development of social economy, energy crisis, environmental pollution and other problems have become increasingly severe, and the requirements for low-carbon development, energy conservation and emission reduction in the automotive industry have become more urgent. As an excellent lightweight material, aluminum alloy occupies a pivotal position in the lightweight manufacturing process of automobiles due to its light weight, high strength and good corrosion resistance. Among them, 6061 in the 6 series aluminum alloy is widely used in various industrial structural parts that require certain strength and high corrosion resistance because of its medium strength, good corrosion resistance, heat treatability, weldability and good oxidation effect. And 316L is a widely used stainless steel material on the market.

[0003] The aluminum / steel composite body structure can not only give full play to the characteristics of aluminum alloy and reduce the overall weight of the body, but also take into account the strength and safety of the body. Therefore, it is of great significance to achieve high-quality connection between 6061 aluminum alloy / 316L stainless steel dissimilar materials.

[0004] Friction stir welding is a new type of solid-phase connection technology, which mainly uses friction heat generation and plastic deformation to achieve the connection of plates. As a new type of solid-phase welding technology, it has the advantages of no need to add welding wire and no pollution, and is suitable for the welding of 6061 aluminum alloy / 316L stainless steel dissimilar materials. However, the current methods to improve the 6061 aluminum alloy / 316L stainless steel friction stir welding joint are to adjust the conventional 6061 / 316L welding process parameters, and the improvement effect is limited. Therefore, there is an urgent need for a method that significantly improves the performance of 6061 aluminum alloy / 316L stainless steel friction stir welding joints and is simple to operate. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present invention provides a method for improving the performance of 6061 / 316L friction stir lap weld joints, aiming to solve the technical problem of poor performance of 6061 aluminum alloy / 316L stainless steel friction stir lap weld joints in the existing welding process.

[0006] In a first aspect, the present invention provides a method for improving the performance of a 6061 / 316L friction stir lap welding joint, comprising the following steps: regulating the microstructure and mechanical properties of a 6061 aluminum alloy plate base material, overlapping and fixing the 6061 aluminum alloy plate and the 316L stainless steel plate, and performing friction stir lap welding; The microstructure includes the proportion of small-angle grain boundaries and the average grain size. The proportion of small-angle grain boundaries is 10%~25%, and the average grain size is 15~25μm. The mechanical properties include tensile strength, elongation and Vickers hardness. The tensile strength is 200~230MPa, the elongation is 20%~25%, and the Vickers hardness is 60~80HV.

[0007] Preferably, the proportion of low-angle grain boundaries is 10% to 21.3%, the average grain size is 15 to 22.4 μm, the tensile strength is 200 to 225 MPa, the elongation is 20% to 25%, and the Vickers hardness is 60 to 76.8 HV.

[0008] Preferably, the 6061 aluminum alloy plate has a low-angle grain boundary ratio of 11.5%, an average grain size of 22.4 μm, a tensile strength of 205 MPa, an elongation of 25%, and a Vickers hardness of 60.3 HV.

[0009] Preferably, the welding process parameters of stir friction lap welding are as follows: the stirring head speed is 600~1000rpm, the welding speed is 80~120mm / min, and the pressing amount is 0~0.4mm. When welding is performed at a lower speed, the fluidity of the welding material will be reduced due to insufficient heat input during the welding process, which will hinder the full flow and fusion of the material in the weld area, and ultimately affect the appearance and internal quality of the welded joint. If the speed during welding is too high, the heat input in the welding area will be excessive, which will promote the formation of brittle intermetallic compounds in the welded joint, thereby weakening the overall strength of the joint and reducing the mechanical properties of the welded joint. When the welding speed is slow, too much heat is obtained per unit length of the weld, so that the temperature of the weld zone is close to the melting temperature of the parent material, resulting in local overheating or even melting; when the welding speed is too fast, the heat obtained in the weld zone is reduced, and the heat input per unit area of ​​the weld is reduced, resulting in the formation of hole defects on the weld surface. Excessive pressing amount will lead to overheating and defects in the joint area, while too small pressing amount will lead to loose organization density in the weld, resulting in insufficient mechanical properties of the joint.

[0010] Preferably, the welding process parameters of the stir friction lap welding are as follows: the stirring head speed is 800 rpm, the welding speed is 100 mm / min, and the downward pressure is 0 mm.

[0011] Preferably, the 6061 aluminum alloy plate and the 316L stainless steel plate are surface treated to remove the oxide layer before being overlapped and fixed.

[0012] Preferably, the surface treatment includes sanding, cleaning and drying.

[0013] Preferably, the overlap fixing is that the 6061 aluminum alloy plate is placed on the 316L stainless steel plate.

[0014] Preferably, the overlap width is 30 mm.

[0015] Preferably, the friction stir lap welding is performed using a FSW-TS-S08 friction stir welding machine.

[0016] Preferably, the stirring pin for the friction stir lap welding is made of tungsten-rhenium alloy, and the shape of the stirring pin is a threaded frustum.

[0017] Preferably, the length of the stirring needle is 2.8 mm, the diameter of the stirring needle is 6 mm, the shoulder diameter of the stirring head is 15 mm, and the inclination angle of the stirring head is 2°.

[0018] The principle of this technical solution is: the hardness, tensile strength, elongation, average grain size and small-angle grain boundary ratio of 6061 aluminum alloy have a significant effect on the performance of aluminum-steel welded joints. The reduction in hardness and tensile strength means that the wear resistance of the welded joint is reduced, affecting its plasticity, making it easier for the parent material to flow and fill the weld, and the joint is more uniform and tighter during welding; the elongation of the parent material is relatively high, which will make it more plastic, and its welded joint will perform better under complex stress conditions; the average grain size has a significant effect on the mechanical properties of the aluminum alloy. During the welding process, the refinement of the grains helps to improve the strength of the welded joint, but may have an adverse effect on the elongation; the small-angle grain boundary ratio has an important influence on the plasticity and toughness of the material. In the friction stir welding (FSW) joint, there is a certain density of dislocations. The accumulation of dislocations may lead to the formation of non-equilibrium grain boundaries in the weld nugget zone, that is, the grain boundaries of subgrains. The existence of these substructures may be one of the reasons for the decrease in the elongation of the joint; the increase of small-angle grain boundaries may promote the movement of dislocations, thereby improving the plasticity of the material, but during the welding process, the formation of this structure may reduce the overall plasticity of the joint. Therefore, the hardness, strength, elongation, average grain size and small-angle grain boundary ratio of 6061 aluminum alloy have a significant effect on the mechanical properties of 6061 aluminum alloy / 316L stainless steel friction stir lap welding joints. In the actual welding process, these factors need to be considered comprehensively, and the mechanical properties of the welded joints should be balanced by optimizing the welding process parameters to meet specific application requirements.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for improving the performance of a 6061 / 316L friction stir lap welding joint. Compared with the traditional method of improving the performance of a 6061 aluminum alloy / 316L stainless steel joint by adjusting the friction stir welding process parameters, the present invention first adjusts the structure and performance of a 6061 aluminum alloy plate before welding, and then selects appropriate welding process parameters to perform the friction stir lap welding of 6061 / 316L, which can significantly improve the performance of a 6061 aluminum alloy / 316L stainless steel friction stir lap welding joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1This is a schematic diagram of the friction stir lap welding of 6061 aluminum alloy / 316L stainless steel of the present invention; Figure 2 This is the interface hardness distribution diagram of the friction stir lap weld of 6061 aluminum alloy and 316L of the present invention; Figure 2 (a) represents comparative example 2, Figure 2 (b) represents Example 3, Figure 2 (c) represents Example 2. DETAILED DESCRIPTION

[0021] The following embodiments of the technical solution of the present invention are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore only used as examples, and cannot be used to limit the protection scope of the present invention.

[0022] The chemical composition of the 6061 aluminum alloy base material used in the embodiment of the present invention is shown in Table 1, and the chemical composition of the 316L stainless steel base material is shown in Table 2.

[0023] Table 1 Chemical composition of 6061 aluminum alloy parent material (mass fraction, wt%)

[0024] Table 2 Chemical composition of 316L stainless steel base material (mass fraction, %)

[0025] All welding samples in the embodiments of the present invention are welded by a FSW-TS-S08 friction stir welding machine. The stirring head is made of tungsten-rhenium alloy and is in the shape of a threaded truncated cone. Figure 1 It is a schematic diagram of the friction stir lap welding of the present invention.

[0026] 1. Welding method Example 1 The microstructure characteristics and mechanical properties parameters of 6061 aluminum alloy parent material are adjusted as follows: The Vickers hardness is 60HV, the low angle grain boundary (LAGB) ratio is 10%, the average grain size is 15μm, the tensile strength is 200MPa, the elongation is 20%, and the tensile strength / elongation is 1000.

[0027] Take one piece of 6061 aluminum alloy plate and one piece of 316L stainless steel, both with dimensions of 300×75×2.5 mm, and perform surface treatment on the 6061 aluminum alloy plate and the 316L stainless steel plate: polish with sandpaper to remove the surface oxide layer, then clean with ethanol, and place the plate in a vacuum box for drying after air drying.

[0028] The 6061 aluminum alloy plate and the 316L stainless steel plate were stir-friction lap welded in the FSW-TS-S08 friction stir welding equipment. The overlap width was set to 30 mm. The 6061 aluminum plate was placed on the 316L stainless steel plate for welding (such as Figure 1 As shown). The stirring head is made of tungsten-rhenium alloy and is in the shape of a threaded truncated cone. The specific parameters of the stirring head are as follows: the stirring needle length is 2.8 mm, the diameter is 6 mm, the shoulder diameter of the stirring head is 15 mm, and the inclination angle of the stirring head is set to 2°; the welding process parameters are as follows: the welding rate is 100 mm / min, the downward pressure is 0 mm, and the stirring head rotation rate is 800 rpm.

[0029] Example 2 The difference between this embodiment and embodiment 1 is that: The microstructural characteristics and mechanical property parameters of the 6061 aluminum alloy are adjusted as follows: Vickers hardness is 60.3 HV, low angle grain boundary (LAGB) ratio is 11.5%, average grain size is 22.4 μm, tensile strength is 205 MPa, elongation is 25%, and tensile strength / elongation is 820. The remaining steps are the same as those in Example 1.

[0030] Example 3 The difference between this embodiment and embodiment 1 is that: The microstructural characteristics and mechanical property parameters of the 6061 aluminum alloy are adjusted as follows: Vickers hardness is 76.8 HV, low angle grain boundary (LAGB) ratio is 21.3%, average grain size is 19.3 μm, tensile strength is 225 MPa, elongation is 25%, and tensile strength / elongation is 900. The remaining steps are the same as those in Example 1.

[0031] Example 4 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 80 mm / min, the pressing amount is 0 mm, and the stirring head rotation rate is 800 rpm. The remaining steps are the same as those in embodiment 2.

[0032] Example 5 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 120 mm / min, the pressing amount is 0 mm, and the stirring head rotation rate is 800 rpm. The remaining steps are the same as those in embodiment 2.

[0033] Example 6 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 100 mm / min, the pressing amount is 0 mm, and the stirring head rotation rate is 600 rpm. The remaining steps are the same as those in embodiment 2.

[0034] Example 7 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 100 mm / min, the pressing amount is 0 mm, and the stirring head rotation rate is 1000 rpm. The remaining steps are the same as those in embodiment 2.

[0035] Example 8 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 100 mm / min, the pressing amount is 0.2 mm, and the stirring head rotation rate is 800 rpm. The remaining steps are the same as those in embodiment 2.

[0036] Example 9 The difference between this embodiment and embodiment 2 is that the welding process parameters are as follows: the welding rate is 100 mm / min, the pressing amount is 0.4 mm, and the stirring head rotation rate is 800 rpm. The remaining steps are the same as those in embodiment 2.

[0037] Comparative Example 1 The difference between this comparative example and Example 2 is: The microstructural characteristics and mechanical property parameters of the 6061 aluminum alloy are adjusted as follows: Vickers hardness is 80 HV, the low angle grain boundary (LAGB) ratio is 25%, the average grain size is 25 μm, the tensile strength is 230 MPa, the elongation is 13%, and the tensile strength / elongation is 1777. The remaining steps are the same as those in Example 2.

[0038] Comparative Example 2 The difference between this comparative example and Example 2 is: The microstructural characteristics and mechanical property parameters of the 6061 aluminum alloy are adjusted as follows: Vickers hardness is 94 HV, the low angle grain boundary (LAGB) ratio is 73.4%, the average grain size is 44.5 μm, the tensile strength is 270 MPa, the elongation is 12%, and the tensile strength / elongation is 2250. The remaining steps are the same as those in Example 2.

[0039] 2. Test Method (1) Mechanical properties test of welded joints Three tensile specimens were prepared by wire cutting for the welded aluminum alloy plates of the embodiment and the comparative example, and tensile tests were carried out on a CMT5015 universal material tensile testing machine at a tensile test rate of 2 mm / min. The average value of the three groups of tests was recorded as the final data.

[0040] (2) Hardness cloud distribution of welded joint section The hardness values ​​of the aluminum alloy plate samples welded in the embodiments and comparative examples were measured using a Vickers hardness tester (UMT-VMHT VMH-104). The samples for microstructure analysis were prepared by sandpaper grinding from No. 200 to No. 3000 and mechanical polishing with diamond compounds (W1.5, W2.5). The load applied for Vickers hardness measurement was 0.98N for 10 seconds. The hardness of the entire cross section of the welded joint was detected at 0.2 mm horizontal and vertical intervals, and a hardness diagram was drawn. Each sample was tested at least 720 times.

[0041] III. Analysis of test results of various embodiments and comparative examples (1) Mechanical properties of tensile specimens The performance test results of 6061 / 316L friction stir welding joints under different base material microstructures and mechanical properties are shown in Table 3.

[0042] Table 3 Properties of 6061 / 316L friction stir welding joints under different base material microstructures and mechanical properties

[0043] The results in Table 3 show that the microstructure and mechanical properties of the 6061 aluminum alloy parent material have a great influence on the mechanical properties of the 6061 aluminum alloy / 316L stainless steel friction stir welding joint. The mechanical properties of the 6061 aluminum alloy / 316L stainless steel friction stir welding joint can be significantly improved by adjusting the microstructure and mechanical properties of the 6061 aluminum alloy parent material. The shear resistance of the joint obtained by welding under the microstructure and mechanical properties of the 6061 aluminum alloy parent material shown in Example 2 is up to 8817N, which is 174% higher than the shear resistance (3220N) of the joint obtained by welding in Comparative Example 2.

[0044] The performance test results of 6061 / 316L friction stir welding joint under different friction stir welding process parameters are shown in Table 4.

[0045] Table 4 Performance of 6061 / 316L friction stir welding joint under different welding process parameters

[0046] The results in Table 4 show that the rotation rate of the stirring head, the welding rate and the amount of pressure have a certain influence on the performance of the friction stir welding joint. Too low or too high a rotation rate may have an adverse effect on the mechanical properties of the welded joint. When the welding speed is slow, too much heat is obtained per unit length of the weld, resulting in local overheating or even melting; when the welding speed is too fast, the heat input per unit area of ​​the weld is reduced, resulting in the formation of holes on the weld surface, thus affecting the mechanical properties of the welded joint. Too high a pressure will lead to overheating and defects in the joint area.

[0047] (2) Hardness cloud distribution of the entire weld joint area Figure 2 This is the interface hardness distribution diagram of the 6061 aluminum alloy and 316L friction stir lap weld joint of the present invention, wherein Figure 2 (a) represents comparative example 2, Figure 2 (b) represents Example 3, Figure 2 (c) represents Example 2. It can be seen from the figure that the overall hardness of the 6061 / 316L joint obtained in Comparative Example 2 in the weld core area is low, and the hardness distribution is uneven. The overall hardness of the 6061 / 316L joint obtained in Example 3 is higher in the weld core area, and the heterogeneity of the hardness distribution is greater, which may lead to stress concentration and reduced joint strength when subjected to external force. The 6061 / 316L joint obtained in Example 2 has a uniform high hardness, which can ensure the strength of the joint while reducing the tendency of stress concentration. The above phenomenon shows that the welding quality and overall performance of the 6061 / 316L joint can be improved by first adjusting the organization and properties of the 6061 aluminum alloy sheet.

[0048] It should be noted that the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present invention are all included in the technical scope of the present invention. In addition, without departing from the scope of the main purpose of the present invention, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present invention.

Claims

1. A method for improving the performance of 6061 / 316L friction stir lap welding joints, characterized in that: The following steps are involved: The microstructure and mechanical properties of the 6061 aluminum alloy base material are adjusted, and the 6061 aluminum alloy plate and the 316L stainless steel plate are overlapped and fixed to perform stir friction lap welding; The microstructure includes a small-angle grain boundary ratio and an average grain size, wherein the small-angle grain boundary ratio is 10% to 25%, and the average grain size is 15 to 25 μm; The mechanical properties include tensile strength, elongation and Vickers hardness. The tensile strength is 200-230 MPa, the elongation is 20%-25%, and the Vickers hardness is 60-80 HV.

2. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The low-angle grain boundary ratio is 10% to 21.3%, the average grain size is 15 to 22.4 μm, the tensile strength is 200 to 225 MPa, the elongation is 20% to 25%, and the Vickers hardness is 60 to 76.8 HV.

3. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The 6061 aluminum alloy plate base material has a low-angle grain boundary ratio of 11.5%, an average grain size of 22.4 μm, a tensile strength of 205 MPa, an elongation of 25%, and a Vickers hardness of 60.3 HV.

4. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The welding process parameters of the stir friction lap welding are as follows: the stirring head speed is 600-1000 rpm, the welding speed is 80-120 mm / min, and the downward pressure is 0-0.4 mm.

5. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 4, characterized in that: The welding process parameters of the friction stir lap welding are as follows: the stirring head rotation speed is 800 rpm, the welding speed is 100 mm / min, and the downward pressure is 0 mm.

6. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The 6061 aluminum alloy plate and the 316L stainless steel plate are subjected to surface treatment to remove the oxide layer before being overlapped and fixed.

7. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 6, characterized in that: The surface treatment includes grinding, cleaning and drying.

8. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The overlapping fixation is specifically that the 6061 aluminum alloy plate is placed on the 316L stainless steel plate.

9. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The stirring pin for the friction stir lap welding is made of tungsten-rhenium alloy, and the shape of the stirring pin is a threaded frustum.

10. The method for improving the performance of 6061 / 316L friction stir lap welding joint according to claim 1, characterized in that: The stirring needle length of the friction stir lap welding is 2.8 mm, the stirring needle diameter is 6 mm, the shaft shoulder diameter of the stirring head is 15 mm, and the inclination angle of the stirring head is 2°.

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