Method for improving fluidity of weld metal in friction stir welding of dissimilar aluminum alloy

Abstract

The invention provides a method for improving fluidity of weld metal in friction stir welding of dissimilar aluminum alloy, and belongs to the technical field of friction stir welding of dissimilar metal. The method comprises the following steps that 7075 and 6061 aluminum alloy plates are in butt joint and fixed, and friction stir welding is conducted; and the ratio of the length of a stirring needle for friction stir welding to the thickness of the aluminum alloy plate is 0.675-0.725. The metal fluidity and hardness distribution uniformity of a welding joint of dissimilar aluminum alloy friction stir butt welding are influenced by regulating and controlling the needle length of the stirring needle. And when the ratio of the length of the stirring needle to the thickness of the plate is 0.7, the elliptical onion ring formed in the nugget zone of the welding joint is complete in structure, the number of circles is large, and the whole position is moderate, so that the weld metal fluidity of the welding joint is better, and the hardness distribution is more uniform.

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 fluidity of weld metal in friction stir welding of heterogeneous aluminum alloys. Background Art

[0002] With the rapid development of the national economy, people's demand for lightweight structures is increasing. Selecting reasonable lightweight materials is a very effective method. Aluminum alloy is a lightweight metal material with many advantages. It is widely used in aerospace, automobile, rail transportation and other fields, such as engine casings, turbine blades, EMU bodies, automobile aluminum alloy bodies, automobile cylinder heads, gearboxes and other parts.

[0003] As a medium-strength aluminum alloy, 6 series aluminum alloy has a certain degree of weldability and can be welded by traditional melting welding, such as TIG welding (argon arc welding), MIG welding (metal inert gas welding), etc. 7 series high-strength aluminum alloys are generally considered to be unweldable in traditional melting welding due to their poor welding performance, which limits the application of assemblies containing 7 series aluminum alloys. FSW (friction stir welding) is a solid-state welding process, and the temperature during the welding process is generally lower than the melting point of the material, so it can avoid thermal cracks, micropores and other solidification defects caused by traditional melting welding.

[0004] At present, the variables studied in friction stir welding are mainly rotation speed, welding speed and downward pressure, and the stirring head itself is rarely involved, such as the stirring needle length, shoulder size, tilt angle, etc. For the interface of the dissimilar aluminum alloy welding joint, there are two states of mutual mixing and simple extrusion, which reflect the problems of poor weld metal fluidity and uneven hardness distribution, which seriously affect the mechanical properties of the joint. During the friction stir welding process, the flow behavior of the plastic material around the stirring head has a significant impact on the joint organization and mechanical properties. The "elliptic onion ring" (Elliptic onion ring) that appears in the cross section of the joint is the most obvious manifestation of plastic metal flow. The hardness distribution of the 6061 / 7075 dissimilar aluminum alloy friction stir welding joint and the position and size of the "elliptic onion ring" in the weld core area are important characteristics that reflect the fluidity of the weld metal. Studying the stirring needle length is of great significance to improving the weld metal fluidity (the position and size of the "elliptic onion ring" in the weld core area) of the 6061 / 7075 dissimilar aluminum alloy friction stir welding joint. 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 fluidity of the weld metal of the stir friction welding of dissimilar aluminum alloys, aiming to solve the technical problems of poor weld metal fluidity and uneven hardness distribution in the stir friction welding joint of 6061 / 7075 dissimilar aluminum alloys.

[0006] In the first aspect, the present invention provides a method for improving the metal fluidity of the stir friction welding weld of dissimilar aluminum alloys, comprising the following steps: butting and fixing 7075 and 6061 aluminum alloy plates, and performing stir friction welding; the ratio of the stirring needle length of the stir friction welding to the thickness of the aluminum alloy plate is 0.675~0.725.

[0007] Preferably, the ratio of the length of the stirring needle of the friction stir welding to the thickness of the aluminum alloy plate is 0.7.

[0008] Preferably, the thickness of the aluminum alloy plate is 3-6 mm.

[0009] Preferably, the thickness of the aluminum alloy plate is 4 mm.

[0010] Preferably, the 7075 and 6061 aluminum alloy plates are ground, polished and dried before being butted and fixed.

[0011] Preferably, the friction stir welding is performed using a FSW-TS-S08 friction stir welding machine.

[0012] Preferably, the stirring needle is made of H13 steel and is shaped like a threaded cone.

[0013] Preferably, the stirring head rotation speed of the friction stir welding is 1000-1500 rpm, the welding speed is 80-120 mm / min, and the downward pressure is 0-0.2 mm.

[0014] Preferably, the stirring head rotation speed of the friction stir welding is 1200 rpm, the welding speed is 100 mm / min, and the downward pressure is 0 mm.

[0015] The principle of this technical solution is: the present invention improves the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys by adjusting the length of the stirring needle. The length of the stirring needle will affect the strength of the stirring effect and affect the mechanical properties of the welded joint. When the stirring needle is too short, the stirring effect on the lower part of the plate is weak, and the material cannot flow fully; as the length of the stirring needle increases, the stirring effect on the weld core area of ​​the welded joint becomes stronger. When the stirring needle is too long, the stirring effect on the lower surface will be more intense, while the stirring effect on the upper surface will be weaker. Therefore, the length of the stirring needle has a significant effect on the fluidity of the weld metal of the welded joint, and an "elliptical onion ring" structure is formed in the SZ (Stirring zone) of the joint, and its position and size change significantly with the length of the stirring needle; when the length of the stirring needle increases, the position of the "elliptical onion ring" structure will decrease significantly, but there is no obvious change in the size of the structure area; as the length of the stirring needle increases, the mean hardness of the entire weld metal area decreases first and then increases, and the tensile strength and elongation after fracture of the joint increase first and then decrease. When the stirring needle length is 2.8 mm, the area of ​​the “elliptical onion ring” structure formed by SZ is smaller, which is 7.68 mm 2 The overall position of the ring is moderate, and the average hardness value of the entire weld metal area reaches the minimum, which is 98.80HV. The tensile strength and elongation of the joint reach the maximum values, which are 207MPa and 2.49% respectively.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a method for improving the metal fluidity of a dissimilar aluminum alloy friction stir welding weld, wherein 7075 and 6061 aluminum alloy plates are butt-jointed and fixed, and friction stir welding is performed; the ratio of the stirring needle length of the friction stir welding to the thickness of the aluminum alloy plate is 0.675 to 0.725. The present invention improves the metal fluidity and uniformity of hardness distribution of a weld joint of a dissimilar aluminum alloy friction stir butt welding by regulating the needle length of the stirring needle. When the ratio of the stirring needle length to the plate thickness is 0.7, the "elliptical onion ring" formed in the weld core area of ​​the weld joint has a complete structure, a moderate position and a number of layers, a more sufficient stirring effect, better weld metal fluidity, more uniform mixing of metal elements, and a higher degree of mechanical bonding. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the friction stir butt welding of the present invention; Figure 2 The cross-sectional macroscopic morphology of the 7075 / 6061 dissimilar aluminum alloy friction stir butt welding joint under different stirring needle lengths of the present invention; Figure 3 The morphology of the weld core area of ​​7075 / 6061 dissimilar aluminum alloy friction stir butt welding joints with different stirring needle lengths of the present invention; Figure 4 The interface between the stirring zone and the thermomechanically affected zone of the welding joint of the 7075 aluminum alloy at the forward side under different stirring needle lengths of the present invention; Figure 5 It is a hardness cloud distribution diagram of 7075 / 6061 dissimilar aluminum alloy friction stir butt welding joints with different stirring needle lengths of the present invention; Figure 6 The microhardness distribution of the weld at the center of the 7075 / 6061 dissimilar aluminum alloy friction stir butt welding joint under different stirring needle lengths of the present invention; Figure 7 A schematic diagram of a stirring head for friction stir butt welding of the present invention; Figure 8 Schematic diagram of the friction stir welding stirring needle penetrating into the welding plate under different stirring needle lengths of the present invention; Fig. 9 Schematic diagram of sampling of tensile specimens and weld joint weld sections. DETAILED DESCRIPTION

[0018] 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.

[0019] The tensile strength of the two aluminum alloy base materials 7075-T6 and 6061-T6 used in the embodiment of the present invention is 309 and 591 MPa respectively, and the Vickers hardness (load is 50g) is 112 HV respectively. 0.05 and 196HV 0.05 The chemical composition of 7075-T6 aluminum alloy parent material is shown in Table 1, and the chemical composition of 6061-T6 aluminum alloy parent material is shown in Table 2.

[0020] Table 1 Chemical composition of 7075-T6 (mass fraction, %)

[0021] Table 2 Chemical composition of 6061-T6 (mass fraction, %)

[0022] All welding samples in the embodiments of the present invention were welded by a FSW-TS-S08 friction stir welding machine. The stirring head was made of H13 steel and was in the shape of a threaded round table. Figure 1 Schematic diagram of the friction stir butt welding of the present invention, the advancing side (AS) is 7075-T6 aluminum alloy, and the retreating side (RS) is 6061-T6 aluminum alloy; Figure 7 Schematic diagram of a stirring head for friction stir butt welding of the present invention, Figure 8This is a schematic diagram of the penetration of the stirring needle into the weld plate during friction stir welding under different stirring needle lengths of the present invention.

[0023] Example 1 Take one piece of 7075-T6 and one piece of 6061-T6 aluminum alloy plate with the size of 300×75×4mm, grind and polish the 7075-T6 and 6061-T6 aluminum alloy plates to remove the oxide film on their surface, and then place them in a vacuum drying oven for 24 hours.

[0024] The two treated test plates were butt-jointed, and the butt-jointed test plates were fixed horizontally on the welding platform of the friction stir welding machine for welding. The specific process parameters of welding were: the stirring head speed was 1200 rpm, the welding speed was 100 mm / min, the downward pressure was 0 mm, and the stirring needle length was 2.8 mm.

[0025] Comparative Example 1 Take one piece of 7075-T6 and one piece of 6061-T6 aluminum alloy plate with the size of 300×75×4mm, grind and polish the 7075-T6 and 6061-T6 aluminum alloy plates to remove the oxide film on their surface, and then place them in a vacuum drying oven for 24 hours.

[0026] The two treated test plates were butt-jointed, and the butt-jointed test plates were fixed horizontally on the welding platform of the friction stir welding machine for welding. The specific process parameters of welding were: the stirring head speed was 1200 rpm, the welding speed was 100 mm / min, the downward pressure was 0 mm, and the stirring needle length was 2.5 mm.

[0027] Comparative Example 2 Take one piece of 7075-T6 and one piece of 6061-T6 aluminum alloy plate with the size of 300×75×4mm, grind and polish the 7075-T6 and 6061-T6 aluminum alloy plates to remove the oxide film on their surface, and then place them in a vacuum drying oven for 24 hours.

[0028] The two treated test plates were butt-jointed, and the butt-jointed test plates were fixed horizontally on the welding platform of the friction stir welding machine for welding. The specific process parameters of welding were: the stirring head speed was 1200 rpm, the welding speed was 100 mm / min, the downward pressure was 0 mm, and the stirring needle length was 3.0 mm.

[0029] Comparative Example 3 Take one piece of 7075-T6 and one piece of 6061-T6 aluminum alloy plate with the size of 300×75×4mm, grind and polish the 7075-T6 and 6061-T6 aluminum alloy plates to remove the oxide film on their surface, and then place them in a vacuum drying oven for 24 hours.

[0030] The two test plates treated as above were butt-jointed, and the butt-jointed test plates were fixed horizontally on the welding platform of the friction stir welding machine for welding. The specific process parameters of welding were: the stirring head speed was 1200 rpm, the welding speed was 100 mm / min, the downward pressure was 0 mm, and the stirring needle length was 3.5 mm.

[0031] Test Case Three tensile specimens and one weld seam section of a welded joint were prepared by wire cutting for the four pairs of aluminum alloy plates welded in the embodiment and the comparative example. The specimen dimensions are as follows: Fig. 9 As shown, perform the following tests: (1) Mechanical properties test of tensile specimens The tensile test was carried out on a CMT universal material tensile testing machine. The test results of the mechanical properties of the tensile specimens of the welded joints at different stirring needle lengths are shown in Table 3.

[0032] Table 3 Mechanical properties of welded joints at different stirring needle lengths

[0033] The results in Table 3 show that when the stirring needle length is 2.8 mm, that is, the stirring needle length is 70% of the plate thickness, the tensile strength reaches a maximum value of 207 MPa, the elongation reaches a maximum value of 2.49%, and the hardness (taking the average value of the entire weld joint) reaches a minimum value of 98.80 HV 0.05 .

[0034] (2) Macroscopic and microscopic morphology test of welded joint cross section Optical microscopy (OM) and scanning electron microscopy (SEM) were used to observe the macroscopic and microscopic morphologies of the welded joint cross section.

[0035] The macroscopic morphology of the cross section of 7075-T6 / 6061-T6 dissimilar aluminum alloy friction stir butt welded joints under different stirring needle lengths is shown in Figure 2. Figure 2 As shown, Figure 2 (ab) stirring needle length 2.5mm, Figure 2 (cd) stirring needle length 2.8mm, Figure 2 (ef) stirring needle length 3.0mm, Figure 2 (gh) The stirring needle length is 3.5 mm. In addition, the size of the weld nugget area of ​​the weld joint under different stirring needle lengths was calculated based on the weld nugget area observed in the spectrum, and the results are shown in Table 4.

[0036] Table 4 The size of weld nugget zone of welded joints with different stirring needle lengths

[0037] Figure 2The results in Table 4 show that when the stirring needle length is 2.8 mm, the weld nugget zone (“onion ring” structure) of the weld joint is located in the middle of the joint and the formed area is small.

[0038] Microscopic morphology of weld nugget zone of 7075 / 6061 dissimilar aluminum alloy friction stir butt welding joint under different stirring needle lengths Figure 3 As shown, Figure 3 (a) The stirring needle is 2.5 mm long. Figure 3 (b) The stirring needle is 2.8 mm long. Figure 3 (c) The stirring needle is 3.0 mm long. Figure 3 (d) The stirring needle is 3.5 mm long. Figure 3 It can be seen that when the stirring needle length is 2.8 mm, the "onion ring" formed on the weld joint is complete and has many layers. Figure 3 It can be seen that the “elliptical onion ring” structures formed by SZ are not the same. Figure 3 (a) and Figure 3 (b) The central area of ​​the “elliptical onion ring” structure is composed of 7075 aluminum alloy, while Figure 3 (c) and Figure 3 (d) The center of the "elliptical onion ring" structure is composed of 6061 aluminum alloy. This is because as the length of the stirring needle increases, the stirring effect on the weld core area of ​​the weld joint becomes stronger, and the difference in fluidity between the two aluminum alloys no longer dominates. It can also be clearly seen that when the stirring needle is 2.5 mm long, the "elliptical onion ring" formed is discontinuous on the lower surface. This is because when the stirring needle is too short, the stirring effect on the lower part of the plate is weak, and the material cannot flow fully. When the stirring needle is 3.0 mm long, the material flow on the forward and backward sides is uneven. This is because when the stirring needle is slightly longer, the stirring effect on the forward and backward sides can be more intense. When the stirring needle is 3.5 mm long, the "elliptical onion ring" structure moves down significantly. This is because when the stirring needle is too long, the stirring effect on the lower surface will be more intense. When the stirring needle length is 2.8 mm, the number of well-closed elliptical rings is the largest and their positions are centered, which reflects that the degree of stirring and mixing of 7075 aluminum alloy and 6061 aluminum alloy is the best at this time.

[0039] (5) Element content test of weld nugget zone of welded joint Since 7075 aluminum alloy belongs to the Al-Zn-Mg-Cu system and 6061 aluminum alloy belongs to the Al-Mg-Si system, Zn and Cu elements can be used as characteristic elements of 7075 aluminum alloy to indicate the uniformity of mixing of the two materials in the weld nugget zone. The element content in the weld nugget zone of the welded joint under different stirring needle lengths was tested, and the test results are shown in Table 5.

[0040] Table 5 Element contents in the weld nugget zone of the weld joint at different stirring needle lengths

[0041] The results in Table 5 show that with the increase of the stirring needle length, the content of Al element first increases and then decreases, and the contents of Cu and Zn elements first decrease and then increase. This can reflect to a certain extent that when the stirring needle length is too short or too long, the amount of 6061 aluminum alloy on the RS side extruded to the AS is small; when the stirring needle length is moderate, the amount of 6061 aluminum alloy on the RS side extruded to the AS side is large, which indicates that a violent stirring action occurs and the two materials are fully mixed. This shows that when the stirring needle length is 2.8 mm, the weld metal fluidity is better during welding and the metal elements are mixed more evenly.

[0042] (6) Microstructure of welded joint cross section The microstructure of the cross section of the welded joint was observed using a GX51-OlyMpus metallographic microscope. The interface between the SZ (weld nugget zone) and TMAZ (thermomechanical affected zone) of the welded joint on the forward side of the 7075 aluminum alloy was as follows: Figure 4 As shown, Figure 4 (a) The stirring needle is 2.5 mm long. Figure 4 (b) The stirring needle is 2.8 mm long. Figure 4 (c) The stirring needle is 3.0 mm long. Figure 4 (d) The stirring needle is 3.5 mm long. Figure 4 It can be seen that with the increase of the stirring needle length, the deformation degree of 7075-TMAZ shows obvious differences. When the stirring needle length is 2.8 mm, the deformation degree of 7075-TMAZ is the largest, and the dual effects of mechanical stirring and heat input are the strongest, causing the shear deformation of the grains to be the largest, indicating that when the stirring needle length is 2.8 mm, the stirring effect is stronger and the metal fluidity is better.

[0043] (7) Hardness cloud distribution of the entire weld joint area The hardness cloud distribution of the entire weld joint area was tested using a Vickers hardness tester (UMT-VMHT VMH-104). The applied load was 50g, the loading time was 10 seconds, and the hardness of the entire weld joint section was measured with horizontal and vertical spacing of 0.5mm and 0.4mm respectively. Each sample was tested 351 times. The hardness cloud distribution of 7075-T6 / 6061-T6 dissimilar aluminum alloy friction stir butt welded joints under different stirring needle lengths is shown in the figure below. Figure 5 As shown, from left to right, they are the heat affected zone (7075-HAZ) on the forward side, the heat mechanical affected zone (7075-TMAZ) on the forward side, the weld nugget zone (SZ) on the forward and backward sides, the heat mechanical affected zone (6061-TMAZ) on the backward side, and the heat affected zone (6061-HAZ) on the backward side. Figure 5(a) The stirring needle is 2.5 mm long. Figure 5 (b) The stirring needle is 2.8 mm long. Figure 5 (c) The stirring needle is 3.0 mm long. Figure 5 (d) The stirring needle is 3.5 mm long. Figure 5 It can be seen that when the stirring needle is too short or too long, the TMAZ of 7075-AS has a higher hardness and no obvious softening. This is because when the stirring needle is too short, it receives less heat, and the hardness difference between the two materials is large, and the fluidity is poor. When the stirring needle is too long, the SZ will move down significantly, and under the strong stirring friction, the heating is uneven. The SZ under different stirring needle lengths is quite different. When the stirring needle length is 2.5mm, there is no obvious softening in the center of the SZ, which reflects that the 6061 aluminum alloy on the RS (backward side) is not stirred into the SZ in large quantities and is subjected to poor stirring. When the stirring needle length is 3.0 and 3.5mm, its SZ does not show the stratification corresponding to the "elliptical onion ring", while when the stirring needle length is 2.8mm, it shows obvious stratification. The HAZ on the AS side and the RS side are softened compared with the BM (parent material), and the area with the lowest hardness is located in the HAZ on the 6061-RS side. When the stirring needle is 2.8 mm long, SZ is located in the middle of the plate, the hardness distribution is most uniform, and the stratification of the two materials can be clearly seen, which corresponds to the previous "elliptical onion ring" structure, reflecting that the fluidity of the two materials is the best and a welding joint with good performance can be obtained.

[0044] (8) Hardness distribution at the center of the weld joint The Vickers hardness tester (UMT-VMHT VMH-104) was used to test the center of the welded joint ( Figure 6 (b) Hardness distribution of (20×10×4mm), the applied load is 50g, the loading time is 10 seconds, each sample is tested 39 times, and the lateral spacing is 0.5mm. The hardness distribution at the center of the weld joint is as follows Figure 6(a) shows that the negative value of the coordinate is the distance from the 7075-T6 aluminum alloy side to the weld center, and the positive value is the distance from the 6061-T6 aluminum alloy side to the weld center. The hardness of the joint is roughly distributed as high on the left and low on the right, with an obvious softening zone. This is mainly due to the large difference in hardness between the materials on both sides. The curve of the weld core zone is a ripple with large fluctuations. The 7075 aluminum alloy has a higher hardness value, and the 6061 aluminum alloy has a lower hardness value. On the 7075 aluminum alloy side, the TMAZ and HAZ hardnesses far away from the weld core center are higher, while the hardness value of the SZ is lower. On the 6061 aluminum alloy side, the TMAZ hardness far away from the weld core center is lower, while the hardness values ​​of the SZ and HAZ are higher. When the stirring needle needle length is 2.8mm, the hardness distribution at the center (weld core zone) forms a "ripple-like fluctuation" structure with high on the left and low on the right, and its amplitude is the most uniform, which shows that the two materials are mixed more evenly and show good fluidity. 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 fluidity of weld metal in friction stir welding of dissimilar aluminum alloys, characterized in that: The following steps are involved: The 7075 and 6061 aluminum alloy plates are butt-jointed and fixed, and stir friction welding is performed; the ratio of the stirring needle length of the stir friction welding to the thickness of the aluminum alloy plate is 0.675-0.

725.

2. A method for improving the fluidity of weld metal in friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The ratio of the length of the stirring needle of the friction stir welding to the thickness of the aluminum alloy plate is 0.

7.

3. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The thickness of the aluminum alloy plate is 3-6 mm.

4. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 3, characterized in that: The thickness of the aluminum alloy plate is 4 mm.

5. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The 7075 and 6061 aluminum alloy plates are ground, polished and dried before being butted and fixed.

6. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The friction stir welding is performed using a FSW-TS-S08 friction stir welding machine.

7. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The stirring needle is made of H13 steel and is in the shape of a threaded frustum.

8. The method for improving the fluidity of the weld metal of the friction stir welding of dissimilar aluminum alloys according to claim 1, characterized in that: The stirring head rotation speed of the friction stir welding is 1000-1500 rpm, the welding speed is 80-120 mm / min, and the downward pressure is 0-0.2 mm.

9. The method for improving the fluidity of the weld metal in the friction stir welding of dissimilar aluminum alloys according to claim 8, characterized in that: The stirring head rotation speed of the friction stir welding is 1200 rpm, the welding speed is 100 mm / min, and the downward pressure is 0 mm.

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