Welding method for improving grain boundary segregation and joint softening of 700MPa-grade aluminum alloy

Abstract

The invention provides a welding method for improving grain boundary segregation and joint softening of 700MPa-grade aluminum alloy, relates to the technical field of aluminum alloy welding, and adopts a friction stir tool to weld an aluminum alloy part to be welded; wherein the aluminum alloy part is an Al-Zn-Mg-Cu alloy with the zinc content of more than 10 wt.% and the strength of more than 700 MPa; in the welding treatment process, the shaft shoulder is controlled to rotate at the first rotating speed, and the stirring needle rotates at the second rotating speed; in order to solve the industrial problem that the joint strength coefficient in traditional welding is only 60%, the heat input is reduced by controlling the first rotating speed to be 50-500 revolutions per minute, and joint softening and a grain boundary continuous segregation zone are inhibited; and meanwhile, the continuous segregation zone is further crushed at a second rotating speed which is 3-4.5 times of the first rotating speed, and the defects of incomplete root penetration and weak bonding are eliminated. On the basis of the method, discretization regulation and control of a weld grain boundary segregation band and synchronous inhibition of joint softening are successfully achieved, the strength retention rate of a welded joint reaches 75% or above of a base material, and the high-zinc-aluminum alloy welding problem is solved in a breakthrough mode.

Description

Technical Field

[0001] The invention belongs to the technical field of aluminum alloy welding, and in particular relates to a welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy. Background Art

[0002] 7xxx series (Al-Zn-Mg-Cu) ultra-high strength aluminum alloys are the most typical lightweight materials in metal materials. They have the advantages of high specific strength, good processing performance and low cost. They are widely used in fields such as aerospace and military industries. High strength is the eternal development direction of aluminum alloys and the key to achieving lightweight. In order to make the strength of ultra-high strength aluminum alloys reach above 700MPa, the zinc content in the alloying elements needs to exceed 10wt.%. However, due to the substantial increase in the content of alloying elements, this type of high zinc aluminum alloy has extremely poor weldability when melt welding, a large tendency to hot cracks, and severe defects such as alloying element burnout and hydrogen pores, and is generally considered to be an unweldable aluminum alloy.

[0003] Friction stir welding is a solid phase connection technology. Since no molten pool is formed during the welding process, a series of fusion welding defects such as element burnout, solidification cracks and pores are avoided, and it has outstanding advantages in welding ultra-high strength aluminum alloys. Among them, the weld formation of friction stir welding is highly dependent on the plastic flow ability of the material. When the zinc content in the ultra-high strength aluminum alloy exceeds 10wt.%, the hardness of the alloy increases significantly, but the plasticity and toughness decrease, which reduces the plastic flow ability of the material. When the existing friction stir welding method is used, the weld formation is difficult, the welding process window is narrow, and defects are easily generated. In order to improve the weld formability, it is necessary to increase the tool speed or reduce the welding speed, which greatly increases the welding heat input, resulting in severe softening of the joint under the action of the welding heat cycle. The most typical feature is the formation of a low hardness zone in the heat affected zone, which greatly reduces the joint strength and makes this type of material lose its high strength advantage. In addition, during the friction stir welding process, the original grain structure in the parent material is stirred and rotated until the weld core zone is formed, and the second phase also undergoes solid solution under the strong thermal coupling, and then precipitates and coarsens the structure transformation. Since the alloying element content of 700MPa grade ultra-high strength aluminum alloy is very high, when using the existing stir friction welding, the second phase will precipitate in large quantities at the grain boundary and form a continuous grain boundary segregation zone in the weld core area. This segregation zone is periodically distributed in the weld core area (such as Figure 1 As shown in Figure 2, the tensile strength of the joint after friction stir welding is generally lower than 500 MPa, even if the tensile strength of the parent material is as high as 726 MPa.

[0004] The prior art discloses a method and auxiliary device for underwater friction stir welding of 7 series aluminum alloys. The tensile strength of the joint of 7A52 aluminum alloy (wherein the Zn content is less than 4.8%) produced by the method is 475MPa, and the joint strength is increased by 15% compared with the joint produced by the conventional friction stir welding method. However, for 700MPa-level ultra-high strength aluminum alloys with a zinc content of more than 10wt.%, the cooling water increases the heat dissipation, which easily leads to insufficient heat input during underwater welding, thereby forming defects such as incomplete penetration or weak bonding at the bottom of the weld. In extreme cases, the phenomenon of "needle breakage" may occur due to insufficient local heat input.

[0005] In summary, when the existing friction stir welding method is used to weld aluminum alloys with a zinc content exceeding 10 wt.%, it is easy to cause problems such as poor weld formation quality and poor joint strength after welding. Summary of the invention

[0006] Therefore, the present invention provides a welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy. The main purpose is to provide a method suitable for stir friction welding of aluminum alloys with high zinc content and ensure the weld forming quality of the aluminum alloy and the strength of the welded joint.

[0007] In order to solve the above problems, the present invention provides a welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy, using a friction stir tool to perform welding treatment on the aluminum alloy parts to be welded;

[0008] Wherein, the aluminum alloy part is an Al-Zn-Mg-Cu alloy; the zinc content in the Al-Zn-Mg-Cu alloy is greater than 10wt.%;

[0009] Among them, the stirring friction tool includes a shoulder and a stirring needle; during the welding process, the shoulder is controlled to rotate at a first speed and the stirring needle is controlled to rotate at a second speed; the first speed is 50 to 500 rpm; the ratio of the first speed to the second speed is 1:3 to 4.5.

[0010] Furthermore, a cooling process is performed during the welding process so that the welding temperature is within a set temperature range;

[0011] Preferably, the set temperature is 300-475°C.

[0012] Furthermore, the cooling treatment is carried out by water cooling; preferably, the water cooling treatment includes placing the aluminum alloy part to be welded in water for welding; and / or

[0013] The cooling rate is greater than or equal to 15°C / s.

[0014] Furthermore, the second rotation speed is 150-1000 rpm.

[0015] Furthermore, the welding speed of the friction stir tool is 100 to 600 mm / min.

[0016] Furthermore, the diameter of the shoulder is 18-38 mm; and / or

[0017] A spiral line is arranged on the surface of the shoulder.

[0018] Furthermore, the tensile strength of the aluminum alloy part is greater than or equal to 700 MPa.

[0019] Furthermore, in terms of mass percentage, the chemical composition of the Al-Zn-Mg-Cu alloy includes: Zn 10-14wt.%, Mg 2-6wt.%, Cu 1-3wt.%; preferably, the mass percentage ratio of Zn to Mg in the Al-Zn-Mg-Cu alloy, Zn / Mg, is 3.2-3.6.

[0020] On the other hand, the present invention provides a weld joint in which no segregation band is observed in the weld core area of ​​the weld joint; the weld core area includes equiaxed crystals, and the size of the equiaxed crystals is 1-2 μm; η-MgZn is precipitated at the grain boundaries and in the crystals of the weld core area 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 20-50nm; the η-MgZn precipitated at the grain boundary 2 Phase size 50-300nm;

[0021] The heat-affected zone of the welded joint has precipitated η-MgZn 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 10-30nm; the η-MgZn precipitated at the grain boundary 2 The size of the phase is 200-600nm.

[0022] Furthermore, the tensile strength of the welded joint is greater than 500 MPa, and the elongation at break is greater than 5.5%. Preferably, the welded joint is obtained by any of the welding methods described above.

[0023] Compared with the prior art, the present invention has at least the following beneficial effects:

[0024] 1. On the one hand, the present invention provides a welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy, wherein a friction stir tool is used to perform welding treatment on an aluminum alloy part to be welded; wherein the aluminum alloy part is an Al-Zn-Mg-Cu alloy; the zinc content in the Al-Zn-Mg-Cu alloy is greater than 10wt.%; the friction stir tool comprises a shoulder and a stirring needle; during the welding treatment, the shoulder is controlled to rotate at a first speed and the stirring needle is controlled to rotate at a second speed; wherein the first speed is 50 to 500 rpm; and the ratio of the first speed to the second speed is 1:3 to 4. 5. Based on the above method, for aluminum alloy parts with a zinc content greater than 10wt.%, the present application reduces the heat input during welding by a lower shoulder speed, avoids severe softening of the joint under the action of the welding heat cycle, thereby ensuring the strength of the joint, and improves the plastic flow ability of the material by a higher stirring needle speed, avoiding the formation of defects such as incomplete penetration or weak bonding at the bottom of the weld, thereby ensuring good weld formation. Therefore, the present application avoids the formation of grain boundary segregation zones in the weld core area through the optimal matching of low shoulder speed and high stirring needle speed, ensuring good weld formation, and ensuring that the weld joint has high strength.

[0025] 2. Furthermore, the present invention performs cooling treatment during the welding process, and preferably places the aluminum alloy parts to be welded in water for stir friction treatment to ensure that the welding temperature is 300-475°C. Within this temperature range, the grain boundaries will not form continuous segregation bands, and at the same time, the size coarsening of the precipitated phase in the heat-affected zone is suppressed to avoid the appearance of a low hardness zone due to excessively high temperatures in the heat-affected zone, thereby ensuring the strength of the welded joint; the cooling rate is preferably greater than or equal to 15°C / s, the faster the cooling, the less the microstructure in the low hardness zone is affected by heat, the more the organizational characteristics are maintained similar to those of the parent material, and the higher the joint strength.

[0026] 3. On the other hand, the present invention provides a weld joint obtained by the above welding method, wherein no segregation band is observed in the weld nugget zone of the weld joint; the weld nugget zone includes equiaxed crystals, and the size of the equiaxed crystals is 1-2 μm; η-MgZn is precipitated at the grain boundaries and in the grains of the weld nugget zone 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 20-50nm; the η-MgZn precipitated at the grain boundary 2 The size of the phase is 50-300nm; η-MgZn is precipitated both at the grain boundary and within the grain in the heat affected zone of the weld joint. 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 10-30nm; the η-MgZn precipitated at the grain boundary 2 The size of the phase is 200-600nm; the characteristics of the precipitation phase in the heat affected zone are the key to determining the performance of the joint. The η-MgZn2 The smaller phase size ensures the performance of the joint, so the obtained welded joint has a tensile strength greater than 500 MPa and an elongation at break greater than 5.5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0028] Figure 1 The microstructure and properties of the welded joint obtained by the new underwater differential method; (a) is the macroscopic metallographic microstructure diagram of the new underwater differential joint, (b) is the scanning and magnified microstructure diagram of the new underwater differential joint; (c) is the cross-sectional hardness distribution diagram, (d) is the macroscopic segregation band of the traditional friction stir welding joint, and (e) is the scanning and magnified diagram of the segregation band of the traditional friction stir welding joint.

[0029] Figure 2 This is the morphology and distribution diagram of the precipitation phase in the low hardness area of ​​the welded joint obtained in Example 1 of the present invention;

[0030] Figure 3 is the metallographic structure diagram of the welded joint obtained in Comparative Example 1 of the present invention;

[0031] Figure 4 is a hardness distribution diagram of the welded joint obtained in Comparative Example 1 of the present invention;

[0032] Figure 5 This is the morphology and distribution diagram of the precipitation phase in the low hardness zone of the welded joint obtained in Comparative Example 1 of the present invention;

[0033] Figure 6 is the metallographic structure diagram of the welded joint obtained in Comparative Example 2 of the present invention;

[0034] Figure 7 is a hardness distribution diagram of the welded joint obtained in Comparative Example 2 of the present invention;

[0035] Figure 8 This is the morphology and distribution diagram of the precipitation phase in the low hardness area of ​​the welded joint obtained in Comparative Example 2 of the present invention;

[0036] Fig. 9 It is the metallographic structure diagram of the welded joint obtained in Comparative Example 3 of the present invention.

[0037] Fig.10 is a microstructure diagram of a welded joint obtained in Comparative Example 4 of the present invention;

[0038] Fig.11is a hardness distribution diagram of the welded joint obtained in Comparative Example 4 of the present invention;

[0039] Fig.12 This is the morphology and distribution diagram of the precipitation phase in the low hardness zone of the weld joint obtained in Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0040] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0041] The present invention provides a welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy, and adopts a friction stir tool to perform welding treatment on the aluminum alloy parts to be welded;

[0042] Wherein, the zinc content in the aluminum alloy part is greater than 10wt.%, and the tensile strength of the aluminum alloy part is greater than or equal to 700MPa;

[0043] The friction stir tool includes a shaft shoulder and a stirring needle. During the welding process, the shaft shoulder is controlled to rotate at a first speed and the stirring needle is controlled to rotate at a second speed. The first speed is 50 to 500 rpm, the second speed is 150 to 1000 rpm, the ratio of the first speed to the second speed is 1:3 to 4.5, and the welding speed of the friction stir tool is 100 to 600 mm / min. The diameter of the shaft shoulder is 18-38 mm. Three Archimedean spirals are processed on the surface of the shaft shoulder. During welding, the material can be driven by the spirals, thereby enhancing the fluidity of the material.

[0044] It should be noted that for aluminum alloy parts with a zinc content greater than 10wt.%, in order to reduce the welding heat input, a low shoulder speed is used, because 70-80% of the heat generated during the friction stir welding process comes from the rotating shoulder. However, when the shoulder speed is reduced, the plastic flow ability of the material will inevitably decrease due to the reduced heat input. Therefore, the speed of the stirring needle must be increased to enhance the plastic flow ability of the material. In order to effectively reduce the heat input, the shoulder speed does not exceed 500rpm, and the shoulder speed cannot be too low, otherwise even if the stirring needle speed is high, it will cause defects or even broken needles due to insufficient heat input; the stirring needle speed is matched according to the shoulder speed, and the speed ratio of the two is limited to 1:3~4.5. This is because when the shoulder speed and the stirring needle speed are close, the heat input is not significantly reduced, and when the ratio is large, it will cause the material flow to be unbalanced between the shoulder and the stirring needle, the needle is too fast, the material flow is high, the weld surface quality is poor, and the needle speed increase will also increase the heat input to a certain extent. The application sets the stirring needle speed to 150~1000 rpm to ensure welding forming. Therefore, the present application reduces the heat input during welding through a lower shoulder speed, avoids severe softening of the joint under the action of the welding heat cycle, thereby ensuring the strength of the joint, and improves the plastic flow ability of the material through a higher stirring needle speed, avoiding the formation of defects such as incomplete penetration or weak bonding at the bottom of the weld, thereby ensuring good weld formation. Therefore, the present application ensures good weld formation and ensures that the weld joint has high strength through the optimal matching of low shoulder speed and high stirring needle speed.

[0045] In some embodiments, the aluminum alloy parts to be welded are placed in water for welding treatment so as to be cooled during the welding process so that the welding temperature is within the set temperature range; wherein the set temperature is 300-475°C and the cooling rate is greater than or equal to 15°C / s. The welding temperature is maintained at 300-475°C to avoid the appearance of a low hardness zone due to excessively high temperature in the heat-affected zone, thereby reducing the strength of the joint; the cooling rate is preferably greater than or equal to 15°C / s. The faster the cooling, the less the microstructure in the low hardness zone is affected by heat, the more the microstructure characteristics similar to the parent material are maintained, and the higher the joint strength.

[0046] Before welding, the surface and joint surface of the aluminum alloy parts to be welded are mechanically polished, and the oil and water stains on the surface of the workpieces to be welded are cleaned with alcohol or acetone. Then the welded aluminum alloy parts are rigidly fixed, and then the water tank is filled with water. When the water level in the water tank rises to the preset height, the water outlet is opened to keep the water flowing.

[0047] In some embodiments, the chemical composition of the Al-Zn-Mg-Cu alloy (aluminum alloy to be welded) includes, by mass percentage, Zn 10-14wt.%, Mg 2-6wt.%, Cu 1-3wt.%; preferably, the mass percentage ratio of Zn to Mg in the Al-Zn-Mg-Cu alloy is 3.2-3.6. At this time, η-MgZn is precipitated in the weld joint. 2 phase, which can improve the strength of the alloy. Further increasing the content of Mg and Cu can precipitate S-Al 2 CuMg phase, this precipitation phase can also improve the strength of the alloy, and has better thermal stability. Among them, for Al-11Zn-3.2Mg-2.6Cu alloy, its strength is 720-750MPa. When welding a 6mm plate, the speed of the shoulder is 200-500rpm, the speed of the stirring needle is 600-1000rpm, and the welding speed is 200-400mm / min; for Al-10Zn-5.8Mg-3.2Cu alloy, it can precipitate η-MgZn 2 Phase and S-Al 2 CuMg phase, strength is 750-780MPa. As the alloy element content of this material increases, the tendency of grain boundary segregation increases, and the tool speed needs to be appropriately reduced. At this time, the speed of the shoulder is 200-400rpm, and the speed of the stirring needle is 400-800rpm. In order to avoid tool breakage during high-strength material welding, the welding speed is appropriately reduced to 100-300mm / min.

[0048] On the other hand, the present invention provides a welded joint obtained by the friction stir welding method of any one of the aluminum alloy parts described above, wherein no segregation band is observed in the weld nugget zone of the welded joint; the weld nugget zone includes equiaxed crystals, and the size of the equiaxed crystals is 1-2 μm; η-MgZn is precipitated at the grain boundaries and in the grains of the weld nugget zone 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 20-50nm; the η-MgZn precipitated at the grain boundary 2 The size of the phase is 50-300nm; η-MgZn is precipitated both at the grain boundary and within the grain in the heat affected zone of the weld joint. 2 phase, among which η-MgZn precipitated at the grain boundary 2 The size of the phase is 10-30nm; the η-MgZn precipitated at the grain boundary 2 The size of the phase is 200-600nm; the characteristics of the precipitation phase in the heat affected zone are the key to determining the performance of the joint. The η-MgZn 2 The phase grain size is small, which can ensure the performance of the joint. Therefore, the tensile strength of the obtained welded joint is greater than 500MPa and the elongation at break is greater than 5.5%.

[0049] The present invention is further described below with reference to specific embodiments and comparative examples.

[0050] Example 1

[0051] This embodiment provides a welding method for improving grain boundary segregation and joint softening of 700 MPa grade aluminum alloy, using a 6 mm thick Al-11.4Zn-2.6Mg-1.1Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate as an aluminum alloy part to be welded (tensile strength 745 MPa), specifically comprising the following steps:

[0052] After cleaning the water stains and oil stains on the surface of the aluminum alloy workpiece to be welded, the aluminum alloy workpiece to be welded is rigidly fixed, water is poured into the water tank until the water level is 2 cm higher than the surface of the workpiece to be welded, and then the aluminum alloy workpiece to be welded is welded using a stir friction tool to obtain a welded joint;

[0053] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 22 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 5.7 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the shoulder speed is 200 rpm (first speed), the stirring needle speed is 600 rpm (second speed), and the welding speed is 250 mm / min.

[0054] The microstructure of the welded joint obtained in this embodiment is as follows: Figure 1 As shown in Figure 2, it can be seen that the grain structure in the weld core area is significantly refined and the structure is uniform. Figure 1 As shown in a, there is no continuous grain boundary segregation band structure in the traditional friction stir welding joint ( Figure 1 c); Compared with the traditional friction stir welding joint, the microhardness of the new underwater differential friction stir welding joint is greatly improved ( Figure 1 b); Figure 2 As shown, a large amount of nano-scale η strengthening phases are precipitated in the low hardness area, with a size of about 10-30nm and a very high density. The welded joint obtained in this embodiment is subjected to a room temperature tensile test, and the fracture position of the joint is the weld nugget area, the tensile strength is 543MPa, and the elongation at break is 6.3%.

[0055] Example 2

[0056] This embodiment provides a welding method for improving grain boundary segregation and joint softening of 700 MPa grade aluminum alloy, using an 8 mm thick Al-10.9Zn-2.4Mg-1.3Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate (tensile strength 724 MPa), specifically comprising the following steps:

[0057] After cleaning the water stains and oil stains on the surface of the aluminum alloy workpiece to be welded, the aluminum alloy workpiece to be welded is rigidly fixed, water is poured into the water tank until the water level is 2 cm higher than the surface of the workpiece to be welded, and then the aluminum alloy workpiece to be welded is welded using a stir friction tool to obtain a welded joint;

[0058] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 28 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 7.7 mm, the downward pressure is 0.25 mm, the welding inclination angle is 3°, the shoulder speed is 300 rpm (first speed), the stirring needle speed is 1000 rpm (second speed), and the welding speed is 300 mm / min.

[0059] The microstructure of the welded joint obtained in this embodiment shows that there is no welding defect and the microstructure is dense. The room temperature tensile test shows that the fracture position of the joint is the thermomechanical affected zone, the tensile strength is 514 MPa, and the elongation at break is 6.6%.

[0060] Example 3

[0061] This embodiment provides a welding method for improving grain boundary segregation and joint softening of 700 MPa grade aluminum alloy, using a 7.2 mm thick Al-12.3Zn-2.8Mg-1.5Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate (tensile strength 763 MPa), specifically comprising the following steps:

[0062] After cleaning the water stains and oil stains on the surface of the aluminum alloy workpiece to be welded, the aluminum alloy workpiece to be welded is rigidly fixed, water is poured into the water tank until the water level is 2 cm higher than the surface of the workpiece to be welded, and then the aluminum alloy workpiece to be welded is welded using a stir friction tool to obtain a welded joint;

[0063] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 26 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 6.9 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the shoulder speed is 250 rpm (first speed), the stirring needle speed is 850 rpm (second speed), and the welding speed is 250 mm / min.

[0064] The welded joint obtained in this embodiment has no welding defects and a dense microstructure. The room temperature tensile test shows that the fracture position of the joint is the weld nugget area, the tensile strength is 568 MPa, and the elongation at break is 5.9%.

[0065] Comparative Example 1

[0066] This comparative example provides a welding method for a 700 MPa grade aluminum alloy, using a 6 mm thick Al-11.4Zn-2.6Mg-1.1Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate as an aluminum alloy part to be welded (tensile strength 745 MPa), specifically comprising the following steps:

[0067] After cleaning the water stains and oil stains on the surface of the aluminum alloy parts to be welded, the aluminum alloy parts to be welded are rigidly fixed, and then the aluminum alloy parts to be welded are welded using a stir friction tool to obtain a welded joint;

[0068] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 22 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 5.7 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the rotation speed of the stirring tool is 800 rpm, and the welding speed is 250 mm / min.

[0069] The microstructure of the welded joint obtained in this comparative example is as follows Figure 3 As shown, it can be found that the grain size is coarse and there are a large number of continuous segregation bands in the weld core area, such as Figure 4 There are two obvious low hardness areas in the joint shown, such as Figure 5 As shown, the precipitate phase in the low hardness zone is severely coarsened, the size of the precipitate phase becomes larger, up to 100nm, and the density of the precipitate phase is greatly reduced. The room temperature tensile results show that the fracture position of the joint is the weld core zone, the tensile strength is 458MPa, and the elongation at break is 1.2%. This comparative example adopts the existing stir friction welding method, and the rotation speed of the shoulder and the stirring needle cannot be adjusted. Since the shoulder rotation speed is too fast, the friction heat generation is large, so there is "overheating". A large number of segregation bands are formed in the weld core zone, forming a weak connection zone. At the same time, the greater heat input causes the precipitation phase in the heat-affected zone to be severely coarsened, the size increases, the precipitation density decreases, a low hardness zone is formed, and the joint performance is reduced. Therefore, during the tensile process, the yield strength of the material is significantly reduced. Due to the appearance of the segregation band, severe stress concentration is formed near the segregation band during the plastic deformation stage, and the fracture occurs in the weld core zone with low elongation.

[0070] Comparative Example 2

[0071] This comparative example provides a welding method for a 700 MPa grade aluminum alloy, using a 6 mm thick Al-11.4Zn-2.6Mg-1.1Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate as an aluminum alloy part to be welded (tensile strength 745 MPa), specifically comprising the following steps:

[0072] After cleaning the water stains and oil stains on the surface of the aluminum alloy workpiece to be welded, the aluminum alloy workpiece to be welded is rigidly fixed, water is poured into the water tank until the water level is 2 cm higher than the surface of the workpiece to be welded, and then the aluminum alloy workpiece to be welded is welded using a stir friction tool to obtain a welded joint;

[0073] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 22 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 5.7 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the rotation speed of the stirring tool is 800 rpm, and the welding speed is 250 mm / min.

[0074] The microstructure of the welded joint obtained in this comparative example is as follows Figure 6 As shown, it can be found that the grain size is slightly refined compared with conventional friction stir welding, and the segregation band still exists, such as Figure 7 As shown in Figure 2, the hardness of the joint is improved compared with that of conventional friction stir welding, but there are still two low hardness areas, such as Figure 8 As shown, the size of the precipitated phase in the low hardness zone is finer and the density is increased compared with the traditional stir friction welding, but the size is still coarse compared with the embodiment. The room temperature tensile results show that the joint fracture position is the weld core area, the tensile strength is 483MPa, and the elongation at break is 4.8%. Due to the high speed of the shaft shoulder in this comparative example, the segregation band problem still exists, the density of the precipitated phase is small, the size is coarse, the joint hardness is improved, but the low hardness zone is still formed in the heat affected zone. The coarser organization and the formation of the segregation band result in the strength and elongation of the joint being lower than the welded joint of this embodiment.

[0075] Comparative Example 3

[0076] This comparative example provides a welding method for a 700 MPa grade aluminum alloy, using a 7.2 mm thick Al-12.3Zn-2.8Mg-1.5Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate (tensile strength 763 MPa), specifically comprising the following steps:

[0077] After cleaning the water stains and oil stains on the surface of the aluminum alloy workpiece to be welded, the aluminum alloy workpiece to be welded is rigidly fixed, water is poured into the water tank until the water level is 2 cm higher than the surface of the workpiece to be welded, and then the aluminum alloy workpiece to be welded is welded using a stir friction tool to obtain a welded joint;

[0078] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 26 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 6.9 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the shoulder speed is 200 rpm (first speed), the stirring needle speed is 1200 rpm (second speed), and the welding speed is 250 mm / min.

[0079] The microstructure of the welded joint obtained in this comparative example is as follows Fig. 9 As shown, it can be seen that a tunnel defect appears in the weld nugget area, which is due to the low ratio of the shaft shoulder and the stirring needle speed, resulting in insufficient heat generation of the shaft shoulder. The room temperature tensile test shows that the joint fracture position is the weld nugget area, the tensile strength is 459MPa, and the elongation at break is 0.1%.

[0080] Comparative Example 4

[0081] This embodiment provides a welding method for 700 MPa grade aluminum alloy, using a 6 mm thick Al-11.4Zn-2.6Mg-1.1Cu (mass percentage) 7034 ultra-high strength aluminum alloy plate as the aluminum alloy part to be welded (tensile strength 745 MPa), specifically comprising the following steps:

[0082] After cleaning the water stains and oil stains on the surface of the aluminum alloy parts to be welded, the aluminum alloy parts to be welded are rigidly fixed, and then the aluminum alloy parts to be welded are welded using a friction stir tool to obtain a welded joint;

[0083] Among them, the stirring friction tool is made of cemented carbide material, the shoulder diameter is 22 mm, the surface of the shoulder is provided with a spiral line, the stirring needle is a tapered threaded needle, the needle length is 5.7 mm, the downward pressure is 0.2 mm, the welding inclination angle is 3°, the shoulder speed is 200 rpm (first speed), the stirring needle speed is 600 rpm (second speed), and the welding speed is 250 mm / min.

[0084] This comparative example can still obtain a defect-free welded joint, and the number of grains in the weld nugget zone is increased compared with Example 1. However, due to the lack of auxiliary water cooling, the weld nugget material is accumulated layer by layer as the stirring needle rotates, and a small amount of continuous grain boundary segregation bands exist between layers due to insufficient cooling rate, such as Fig.10 As shown in the figure, the microhardness of the joint is W-shaped, and there are two heat-affected zones with low hardness, such as Fig.11 As shown; the η strengthening phase precipitates in the low hardness area, the size can reach ~80nm, and the density is reduced, such as Fig.12 The welded joint obtained in this comparative example was subjected to a room temperature tensile test, and the fracture position of the joint was the weld nugget area, the tensile strength was 475 MPa, and the elongation at break was 6.1%.

[0085] It is easy for those skilled in the art to understand that, without conflict, the advantageous technical features of the above-mentioned methods can be freely combined and superimposed.

[0086] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present invention. These improvements and variations should also be regarded as the protection scope of the present invention.

Claims

1. A welding method for improving grain boundary segregation and joint softening of 700MPa grade aluminum alloy, characterized in that: The aluminum alloy parts to be welded are welded using a friction stir tool; Wherein, the aluminum alloy part is an Al-Zn-Mg-Cu alloy; the zinc content in the Al-Zn-Mg-Cu alloy is greater than 10wt.%; Among them, the stirring friction tool includes a shoulder and a stirring needle; during the welding process, the shoulder is controlled to rotate at a first speed and the stirring needle is controlled to rotate at a second speed; the first speed is 50 to 500 rpm; the ratio of the first speed to the second speed is 1:3 to 4.

5.

2. The welding method according to claim 1, characterized in that: During the welding process, the aluminum alloy parts to be welded are cooled so that the welding temperature is within the set temperature range; Preferably, the set temperature is 300-475°C.

3. The welding method according to claim 2, characterized in that: The cooling treatment is carried out by water cooling; preferably, the water cooling treatment includes placing the aluminum alloy part to be welded in water for welding; and / or The cooling rate is greater than or equal to 15°C / s.

4. The welding method according to claim 1, characterized in that: The second rotation speed is 150-1000 rpm.

5. The welding method according to any one of claims 1 to 4, characterized in that: The welding speed of the friction stir tool is 100 to 600 mm / min.

6. The welding method according to claim 1, characterized in that: The diameter of the shoulder is 18-38 mm; and / or A spiral line is arranged on the surface of the shoulder.

7. The welding method according to claim 1, characterized in that: The tensile strength of the aluminum alloy part is greater than or equal to 700 MPa.

8. The welding method according to claim 1, characterized in that: In terms of mass percentage, the chemical composition of the Al-Zn-Mg-Cu alloy includes: Zn 10-14wt.%, Mg 2-6wt.%, Cu 1-3wt.%; preferably, the mass percentage ratio of Zn to Mg in the Al-Zn-Mg-Cu alloy, Zn / Mg, is 3.2-3.

6.

9. A welding joint, characterized in that: No segregation band is observed in the weld nugget zone of the weld joint; the weld nugget zone includes equiaxed crystals, and the size of the equiaxed crystals is 1-2 μm; η-MgZn2 phases are precipitated at both the grain boundaries and the grains in the weld nugget zone, wherein the size of the η-MgZn2 phase precipitated at the grain boundaries is 20-50 nm; the size of the η-MgZn2 phase precipitated at the grain boundaries is 50-300 nm; The η-MgZn2 phase is precipitated both at the grain boundary and in the grain of the heat affected zone of the weld joint, wherein the size of the η-MgZn2 phase precipitated at the grain boundary is 10-30nm; the size of the η-MgZn2 phase precipitated at the grain boundary is 200-600nm.

10. The welded joint according to claim 9, characterized in that The tensile strength of the welded joint is greater than 500 MPa, and the elongation at break is greater than 5.5%; Preferably, the welded joint is obtained by the welding method described in any one of claims 1-8.

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