A welding method for improving mechanical properties of aluminum-lithium alloy welded joints

Abstract

The application belongs to the technical field of welding, and particularly relates to a welding method for improving the mechanical properties of an aluminum-lithium alloy welded joint, which comprises the preparation of a welding plate, pretreatment before welding and welding; the preparation of the welding plate comprises the following steps: after homogenization treatment of an aluminum-lithium alloy ingot, rolling deformation is carried out at 440-460 DEG C, and the deformation amount is 60-80%, to obtain a rolled plate; the rolled plate is subjected to solid solution and aging treatment at 480-510 DEG C, to obtain a solid solution plate; the solid solution plate is subjected to low-temperature rolling deformation at -196 DEG C to -120 DEG C, and the deformation amount is 50-60%; finally, overaging treatment is carried out at 150-170 DEG C, to obtain a welding plate; the application improves the microstructure of the aluminum-lithium alloy welded joint, avoids the generation of fine equiaxed zone (EQZ), and improves the mechanical properties of the welded joint.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a welding method for improving the mechanical properties of aluminum-lithium alloy welded joints. Background Art

[0002] Aluminum-lithium alloys are formed by adding lithium to aluminum and aluminum alloys. In recent years, third-generation aluminum-lithium alloys, represented by 2195, have gained widespread attention in the manufacture of aerospace structural components due to their low density and high performance. However, the addition of lithium makes them extremely active at high temperatures, making them more difficult to weld than conventional aluminum alloys. This leads to increased sensitivity to hot cracking, the generation of porosity defects, and significantly reduced mechanical properties of welded joints.

[0003] The 2195 sheet currently used for TIG welding is generally an aluminum-lithium alloy sheet in the T8 state. The main purpose of using the T8 state for 2195 aluminum-lithium alloy sheet is to improve its mechanical properties, without considering its welding performance. The weld joint can be roughly divided into three parts: the fusion zone, the heat-affected zone, and the base metal zone. When TIG welding is performed on aluminum-lithium alloy plates in the T8 state, the welding heat input will cause the main strengthening phase T1 phase in the heat-affected zone of the 2195 alloy plate to dissolve or coarsen, thereby reducing its mechanical properties. In addition, during the welding process of the 2195 aluminum-lithium alloy plate, fine equiaxed crystal zones (EQZs) with an average grain size of about 2-10 μm and a width ranging from tens to hundreds of microns are easily generated near the fusion line at the junction of the heat-affected zone and the fusion zone. The grain size of the fine equiaxed crystal zone (EQZ) is significantly smaller than that of the fusion zone and the heat-affected zone, resulting in stress concentration at the fine equiaxed crystal zone (EQZ) during the stress process of the weld joint. The weld joint is prone to fracture at this position, which greatly reduces the mechanical properties of the weld joint. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a welding method for improving the mechanical properties of aluminum-lithium alloy welded joints, improving the microstructure of aluminum-lithium alloy welded joints, avoiding the generation of fine equiaxed crystal bands (EQZ), and improving the mechanical properties of welded joints.

[0005] The embodiment of the present invention provides a welding method for improving the mechanical properties of aluminum-lithium alloy welded joints, comprising preparation of welding plates, pretreatment before welding, and welding;

[0006] The preparation of the welding plate comprises the following steps: after homogenizing the aluminum-lithium alloy ingot, rolling and deforming it at 440-460°C with a deformation of 60-80% to obtain a rolled plate; the rolled plate is subjected to a solid solution aging treatment at 480-510°C to obtain a solid solution plate; the solid solution plate is subjected to low-temperature rolling and deformation at -196--120°C with a deformation of 50-60% (at this time, the thickness of the plate is preferably 4-7 mm); and finally, overaging treatment is performed at 150-170°C to obtain the welding plate.

[0007] Optionally, the ingot is rolled and deformed at 450° C. with a deformation amount of 80% to obtain a rolled plate.

[0008] Optionally, the rolled plate is subjected to a solution aging treatment at 505° C. for 1-2 hours to obtain a solution aging plate.

[0009] Optionally, before the low-temperature rolling deformation, the solid solution plate is immersed at -196-120°C to make the temperature of the solid solution plate be -196-120°C.

[0010] Optionally, the solid solution plate is subjected to low temperature rolling deformation at -150°C, with a deformation amount of 50%.

[0011] Optionally, the over-aging treatment is performed at a temperature of 160° C. for 30-35 hours.

[0012] Optionally, the aluminum-lithium alloy is 2195 aluminum-lithium alloy.

[0013] Optionally, the pretreatment before welding is to bevel the welding plate, with a single-side bevel angle of 35-40° and a blunt edge thickness of 1-1.2 mm; then remove oil stains on the surface of the welding plate, and then keep the welding plate warm.

[0014] Optionally, the insulation temperature of the welding plate is 100°C.

[0015] Optionally, the welding is a single-sided double-layer welding method, and the back and front of the weld are protected by inert gas. The welding wire is 2319 welding wire, and the welding parameters are: the base layer welding current is 180-220A, and the cover layer welding current is 160-180A; the argon flow rate of the TIG welding gun is controlled at 15-20L / min, and the argon flow rate of the back inert gas protection is 5-8L / min; the welding speed is controlled at 100-120mm / min.

[0016] The present invention has the beneficial effect of conventionally using TIG welding for 2195 aluminum-lithium alloy using aluminum-lithium alloy sheet in the T8 state. The primary steps in preparing the T8 state aluminum-lithium alloy sheet are to pre-deform the sheet by 3-6% at room temperature after solutionizing to introduce dislocations and lattice defects, promoting uniform nucleation and precipitation of strengthening phases (such as T1 phase Al2CuLi and θ' phase Al2Cu) during aging. Finally, artificial aging is performed. Generally, during pre-deformation after solutionizing, the tensile deformation cannot exceed the uniform elongation of the sheet (generally less than 15-20%). A pre-deformation of 3% is typically used, followed by aging to produce the peak-aged 2195 sheet. The primary purpose of pre-stretching is to introduce high-density dislocations, providing nucleation sites for the T1 phase, the primary precipitate phase, during subsequent aging, promoting the precipitation of fine, dispersed strengthening phases and improving the sheet's mechanical properties. TIG welding using this process often produces fine equiaxed crystal bands near the fusion line. These bands have grain sizes significantly smaller than those in the partial melt and fusion zones, leading to stress concentrations and cracking, which in turn reduces the strength and elongation of the welded joint. The grain size of the heat-affected zone (HAZ) remains unchanged despite the welding thermal cycle, and the fine, dense T1 phase dissolves or coarsens under the influence of the welding thermal cycle, resulting in reduced mechanical properties in the HAZ.

[0017] The 2195 rolled plate of the present invention is subjected to a large rolling deformation (50-60%) in an ultra-low temperature environment (-196--120°C) after solution treatment, resulting in a greater degree of deformation than tensile pre-deformation. The large deformation introduces a certain amount of strain energy into the plate, and the rolling deformation in the ultra-low temperature environment can make the deformation of the plate more uniform, uniformly generating a large number of dislocations at the grain boundaries and within the grains. However, the deformation cannot be too large. Under this solution, excessive ultra-low temperature rolling deformation can cause edge cracking during the plate preparation process, resulting in material waste. The plate that has completed the rolling deformation is then subjected to an overaging treatment for 30-35 hours. The purpose of the overaging treatment is to slightly reduce the strength of the plate and obtain a coarsened T1 phase. The coarse T1 phase has a larger size and smaller interfacial energy, so it requires a higher temperature or longer time to dissolve. Compared with the plate peak-aged in the T83 state, under the same welding thermal cycle, the coarse T1 phase will dissolve less, and the performance degradation of the welded plate in the heat-affected zone is smaller than that of the T83 state, which is also conducive to improving the mechanical properties of the welded joint. The welded joints obtained by TIG welding of this plate, under the action of the welding thermal cycle, use the strain energy introduced by the large deformation during ultra-low temperature rolling as the driving force for recrystallization, forming a uniform and fine recrystallized structure near the fusion line. No fine equiaxed zones (EQZs) are generated near the fusion line. Compared to the microstructure near the fusion line of the T83 plate welded joint, the grain size distribution is more uniform, resulting in a more uniform stress distribution there, avoiding stress concentration and thus improving the strength and elongation of the welded joint. In the heat-affected zone, the coarse T1 phase, due to its larger size and smaller interfacial energy, requires higher temperatures or longer times to dissolve. Compared to the T83 plate, under the same welding thermal cycle, the coarse T1 phase dissolves less, resulting in less performance degradation in the heat-affected zone than the T83 plate, which is also beneficial for improving the mechanical properties of the welded joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a metallographic photograph of the base material structure of the welded plate in Example 1 of the present invention.

[0019] Figure 2 This is the surface morphology of the weld joint in Example 1 of the present invention.

[0020] Figure 3 This is a metallographic photograph near the fusion line of the weld joint in Example 1 of the present invention.

[0021] Figure 4 This is a metallographic photograph of the base material structure of the welded plate in Comparative Example 1 of the present invention.

[0022] Figure 5This is a metallographic photograph near the fusion line of the weld joint of Comparative Example 1 of the present invention.

[0023] Figure 6 This is a photo of the rolled cracked plate of Comparative Example 2 of the present invention.

[0024] Figure 7 This is a metallographic photograph of the base material structure of the welded plate in Comparative Example 3 of the present invention.

[0025] Figure 8 This is a metallographic photograph near the fusion line of the weld joint of Comparative Example 3 of the present invention.

[0026] Figure 9 1 is a comparison chart of the yield strength, tensile strength and elongation of Example 1 of the present invention and Comparative Example 1.

[0027] Figure 10 This is a comparison chart of the hardness distribution of the heat-affected zone of Example 1 of the present invention and Comparative Example 4. DETAILED DESCRIPTION Example 1

[0028] A welding method for improving the mechanical properties of a 2195 aluminum-lithium alloy TIG welded joint. The 2195 alloy used comprises the following components by mass: Cu: 4.23%, Li: 1.16%, Mg: 0.42%, Zr: 0.08%, Ag: 0.45%, Mn: 0.02%, Ti: 0.02%, Zn: 0.02%, Si: 0.01%, Fe: 0.03%, and the remainder is Al; the solidus temperature is approximately 545°C. The method comprises preparation of a 2195 aluminum-lithium alloy welding plate, pretreatment of the 2195 aluminum-lithium alloy plate before welding, and a welding process of the 2195 aluminum-lithium alloy plate.

[0029] Preparation of 2195 aluminum-lithium alloy welding plate, including:

[0030] 1. The homogenized 2195 aluminum-lithium alloy ingot is rolled at 450°C with a total deformation of 80%, and then air-cooled to room temperature.

[0031] 2. The rolled plate is then solution treated at 505°C for 1.5 hours and then quickly quenched in cold water (0-5°C);

[0032] 3. Then, the plate after solution treatment and quenching is rolled and deformed by 50% at ultra-low temperature of -150℃. In order to ensure that the plate temperature is lower than -150℃, the plate after solution treatment and quenching is immediately immersed in liquid nitrogen for 1 hour and then quickly rolled and deformed. The thickness of the plate is 5 mm and air-cooled to room temperature.

[0033] 4. Finally, the plate is subjected to an over-aging treatment at 160°C for 35 hours and then air-cooled to room temperature.

[0034] Figure 1 This is the metallographic structure of the RD-ND surface of the 2195 aluminum-lithium alloy plate with large deformation.

[0035] Pre-weld pretreatment of 2195 aluminum-lithium alloy welding plates includes:

[0036] After aging treatment, the welded plates were beveled. The bevel angle was 35° on one side, and the blunt edge thickness was 1 mm. The welded plates were then acid- and alkali-washed to remove surface oil. The washed plates were then held at 100°C for 30 minutes.

[0037] The welding process of 2195 aluminum-lithium alloy plate includes:

[0038] The 2195 plate, which had been kept at 100°C for 30 minutes, was quickly taken out for welding. The single-sided double-layer welding method was adopted. After the base layer welding was completed, the cover layer welding was carried out quickly. The back and front sides of the weld were protected by inert gas. The welding wire used was 2319 welding wire. The base layer welding current was 200A, and the cover layer welding current was 170A. The argon flow rate of the TIG welding gun was 20L / min, and the argon flow rate of the inert gas protection on the back was 5L / min. The welding speed was controlled at 110mm / min.

[0039] Figure 2 This is a diagram showing the frontal formation of the welded joint in Example 1. Figure 3 The metallographic structure near the fusion line of the welded joint section of Example 1 is shown in FIG. Figure 1 The strain energy stored in the ultra-low temperature and large deformation plate of the fibrous crystal structure of the base material is released under the action of the welding thermal cycle, causing the plate structure near the fusion line to recrystallize. The average grain size is 27.9μm, and no fine equiaxed crystal band (EQZ) is found near the fusion line.

[0040] The mechanical properties of the welded joint were tested. According to the tensile test method for welded joints (GB / T 2651-2008), three parallel samples were taken for testing. The test results are as follows: Figure 9 As shown in the figure, the average tensile strength of the welded joint is 352 MPa and the average elongation is 4.3%.

[0041] Comparative Example 1

[0042] Comparative Example 1 is different from Example 1 in that the preparation of the 2195 aluminum-lithium alloy welded plate is different, and the rest is the same as Example 1. The welded plate of Comparative Example 1 is a 2195 aluminum-lithium alloy plate in the T83 peak aging state, and the specific steps are as follows:

[0043] 1. The homogenized 2195 aluminum-lithium alloy ingot is rolled at 450°C with a total deformation of 80%.

[0044] 2. The rolled plate was then solution treated at 505°C for 1.5 hours;

[0045] 3. Then the plate after solution treatment is subjected to 3% tensile pre-deformation at room temperature, and the thickness of the plate is 5mm;

[0046] 4. Finally, the plate is aged at 160°C for 26 hours.

[0047] Figure 4 The metallographic structure of the RD-ND surface of the T83 peak aged plate is shown in FIG. The plate is welded using the same pre-welding pretreatment and welding process as in Example 1, and the welding parameters are the same as in Example 1. Figure 5 The metallographic structure near the fusion line of the T83 state plate weld joint can be found. It can be found that the grain size of the parent material near the fusion line has not changed significantly, while a small equiaxed zone (EQZ) is produced near the fusion line in the fusion zone. The same mechanical properties test method was used for the test, and the results are as follows. Figure 9 As shown in Figure 1, the average tensile strength of the welded joint is 319 MPa, and the average elongation is 2.5%. Due to the presence of fine equiaxed crystal bands near the fusion line of the welded joint, stress concentration occurs at this location during the room temperature tensile test, resulting in lower tensile strength and elongation in Comparative Example 1 than in Example 1.

[0048] Comparative Example 2

[0049] Comparative Example 2 is compared with Example 1, except that step 3 of preparing the 2195 aluminum-lithium alloy welding plate is:

[0050] The plates that have completed solution treatment and quenching are then rolled and deformed by 80% at an ultra-low temperature of -150°C. In order to ensure that the plate temperature is below -150°C, the plates that have completed solution treatment and quenching are immediately immersed in liquid nitrogen for 1 hour before being quickly rolled and deformed. The thickness of the plates is 5 mm and they are air-cooled to room temperature.

[0051] The other steps are the same as those in Example 1.

[0052] Figure 6 The photo of the rolled plate of Comparative Example 2 shows that when the deformation of the rolled plate is too large, the plate is very likely to crack, resulting in a waste of material utilization.

[0053] Comparative Example 3

[0054] Comparative Example 3 is different from Example 1 in that step 3 of preparing the 2195 aluminum-lithium alloy welding plate is as follows:

[0055] The plates that have completed solution treatment and quenching are then rolled and deformed by 20% at an ultra-low temperature of -150°C. In order to ensure that the plate temperature is below -150°C, the plates that have completed solution treatment and quenching are immediately immersed in liquid nitrogen for 1 hour and then quickly rolled and deformed. The thickness of the plates is 5 mm and they are air-cooled to room temperature.

[0056] The other steps are the same as those in Example 1.

[0057] Figure 7 and Figure 8 These are metallographic photographs of the cold-rolled plate base material and the welded joint with a deformation of 20%. It can be seen that the deformation of ultra-low temperature rolling needs to be large enough to introduce sufficient strain energy so that recrystallization can occur under the action of the welding thermal cycle and the EQZ fine equiaxed crystal bands can be eliminated.

[0058] Comparative Example 4

[0059] Comparative Example 4 is different from Example 1 in that step 4 of the aging treatment of the 2195 aluminum-lithium alloy welded plate is:

[0060] Finally, the plate was subjected to peak aging treatment at 160°C for 26 hours and then air-cooled to room temperature.

[0061] The other steps are the same as those in Example 1.

[0062] pass Figure 10 Comparing the hardness distribution of the welded joints of the 26-hour peak-aged plate and the 35-hour over-aged plate, it can be found that the hardness value of the over-aged plate decays less, and the hardness distribution of the heat-affected zone of the welded joint is relatively more uniform, which is conducive to reducing the degree of stress concentration.

[0063] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of protection of the present application is limited to these examples. In line with the present application, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of different aspects of one or more embodiments of the present application as described above, which are not provided in detail for the sake of simplicity.

[0064] The one or more embodiments of this application are intended to encompass all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of one or more embodiments of this application should be included in the scope of protection of this application.

Claims

1. A welding method for improving the mechanical properties of aluminum-lithium alloy welded joints, characterized in that: Including preparation of welding plates, pretreatment before welding and welding; The preparation of the welding plate comprises the following steps: after homogenizing the aluminum-lithium alloy ingot, rolling and deforming it at 440-460°C with a deformation of 60-80% to obtain a rolled plate; solution treating the rolled plate at 480-510°C to obtain a solid solution plate; low-temperature rolling and deforming the solid solution plate at -196--120°C with a deformation of 50-60%; and finally overaging treatment at 150-170°C to obtain a welding plate. The aluminum-lithium alloy is 2195 aluminum-lithium alloy.

2. The welding method according to claim 1, wherein: The ingot is rolled and deformed at 450° C. with a deformation amount of 80% to obtain a rolled plate.

3. The welding method according to claim 1, wherein: The rolled plate is subjected to a solution treatment at 505° C. for 1 to 2 hours to obtain a solution plate.

4. The welding method according to claim 1, wherein: Before the solid solution plate is subjected to low temperature rolling deformation, the solid solution plate is immersed at -196 to -120°C to make the temperature of the solid solution plate be -196 to -120°C.

5. The welding method according to claim 1, wherein: The solid solution plate is subjected to low temperature rolling deformation at -150°C, with a deformation amount of 50%.

6. The welding method according to claim 1, wherein: The overaging treatment is performed at a temperature of 160° C. for 30 to 35 hours.

7. The welding method according to any one of claims 1 to 6, wherein: The pre-welding pretreatment is to bevel the welded plate, with a single-side bevel angle of 35-40° and a blunt edge thickness of 1-1.2 mm; Then remove the oil stains on the surface of the welded plates, and then keep the welded plates warm.

8. The welding method according to claim 7, wherein: The insulation temperature of the welding plate is 100℃.

9. The welding method according to any one of claims 1 to 6, wherein: The welding is a single-sided double-layer welding method, and the back and front sides of the weld are protected by inert gas. The welding wire used is 2319 welding wire. The welding parameters are: the base layer welding current is 180~220A, and the cover layer welding current is 160~180A; the argon flow rate of the TIG welding gun is controlled at 15~20L / min, and the argon flow rate of the back inert gas protection is 5~8L / min; the welding speed is controlled at 100~120mm / min.

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