Welding process for improving mechanical property of AZ series magnesium alloy MIG welding joint

Abstract

The invention discloses a welding process for improving the mechanical property of an AZ-series magnesium alloy MIG welding joint, which aims at the welding of a thin plate surface part of an AZ-series magnesium alloy hollow thin-wall profile, and the thickness of a thin plate weldment is 3mm. The welding process comprises the following steps: when the weldment is AZ31B magnesium alloy, the welding heat input quantity is controlled to be 330J. Mm <-1 >-480J. Mm <-1 >, and the welding heat input quantity is controlled to be 330J. Mm <-1 >-480J. Mm <-1 >; the tensile strength of a welding joint is not lower than 85% of the tensile strength of base metal; when the weldment is AZ61A magnesium alloy, the welding heat input quantity is controlled to be 440 J.mm <-1 >-650 J.mm <-1 >, and the tensile strength of a welded joint is not lower than 85% of the tensile strength of base metal; and when the weldment is AZ80A magnesium alloy, the welding heat input quantity is controlled to be 710 J.mm <-1 >-1180 J.mm <-1 >, and the tensile strength of a welding joint is not lower than 85% of the tensile strength of base metal. The heat input range suitable for the thin plate surface part of the AZ series magnesium alloy hollow thin-wall profile researched by the invention is used for controlling the welding process aiming at the base metal with different Al contents and structure characteristics and controlling the structure change of a heat affected zone, so that excellent welding strength can be obtained.

Description

Technical Field

[0001] This invention relates to the field of welding technology, specifically to a welding process for improving the mechanical properties of MIG welded joints of AZ-based magnesium alloys. Background Technology

[0002] Magnesium alloys, as lightweight engineering materials, exhibit significant application potential in aerospace vehicle frames, rail transit equipment, new energy vehicle battery casings, and 3C electronic product components due to their excellent specific strength, electromagnetic shielding, and vibration damping properties. In particular, AZ-series magnesium-aluminum alloys, through the Mg-Al-Zn ternary strengthening system formed by adjusting Al and Zn alloying elements, can achieve the strength and ductility required for engineering applications. However, AZ-series magnesium alloys are prone to metallurgical defects such as grain boundary oxidation, hot cracking sensitivity, and joint softening during gas inert gas (MIG) welding, restricting their widespread application in load-bearing structures. Especially with Mg-Al-Zn ternary strengthening system alloys, different grades such as AZ31, AZ61, and AZ80 exhibit varying microstructures, resulting in differences in the microstructure changes and final weld strength coefficients of the welded joints during MIG welding. Therefore, finding ways to make the strength of the welded joint as close as possible to the strength of the base material is of paramount engineering significance.

[0003] Currently, magnesium alloy profiles have a wide range of industrial applications. Among them, wide-width hollow thin-walled profiles used in aerospace, aviation, and rail transportation face challenges in extrusion manufacturing, particularly in forming the localized stiffening surfaces of the 3mm thick plates. The connection difficulties in engineering applications also lie in achieving good appearance and sufficiently high weld strength after welding these approximately 3mm thick plates. Compared to the thicker sections of wide-width hollow thin-walled profiles and plates less than 10mm thick, welding 3mm thick plates presents significant challenges in achieving reliable mechanical properties, especially given the prevalence of long welds in large components with demanding weld shape requirements. The inherent physicochemical properties of magnesium alloys (such as high thermal conductivity, low melting point, high oxidation tendency, and large coefficient of thermal expansion) have a significantly amplified adverse effect during thin-plate welding. Thin plates have low heat capacity, making them highly susceptible to burn-through, collapse, and severe deformation due to improper heat input control. Furthermore, the inherent hot cracking sensitivity and porosity tendency of magnesium alloys are more difficult to control under thin-plate conditions. Furthermore, in the welding of AZ-series magnesium alloy profiles, welding is actually mostly carried out on thin-walled plates. The problems such as uneven heat accumulation and distribution, overall structural deformation control, and stability of continuous welding of long weld joints are further highlighted. Therefore, how to achieve high mechanical properties of welded joints of thin-walled plates in such magnesium alloy hollow profiles is a technical problem that needs to be solved by those skilled in the art.

[0004] MIG (Metal Inert Gas Welding) is an arc welding technique that uses an inert gas (such as argon) as the shielding gas, primarily for welding metallic materials. Unlike TIG welding, MIG welding uses a molten welding wire as the electrode. Under the protection of an inert gas, an electric arc is generated between the workpiece and the welding wire, melting both the wire and the base metal. Compared to other welding methods for magnesium alloys, MIG welding offers advantages such as high welding efficiency, strong filler wire capacity, deep weld penetration, and excellent bridging ability. It can also compensate for surface collapse caused by magnesium alloy evaporation and the loss of alloying elements. However, the heat input in MIG welding is difficult to control, leading to unstable droplet transfer. This is particularly problematic for large, hollow, thin-walled magnesium alloy plates with localized 3mm thick wall surfaces and stiffening surfaces, making welding difficult and significantly limiting the industrial application of magnesium alloys. Summary of the Invention

[0005] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide a welding process to improve the mechanical properties of MIG welded joints of AZ-based magnesium alloys, focusing on solving the problem of thin-walled plates of AZ-based magnesium alloy profiles with a thickness of 3mm, so that the welded joints can obtain higher mechanical properties.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A welding process for improving the mechanical properties of MIG welded joints of AZ-based magnesium alloys, characterized in that: for welding the thin plate portion of hollow thin-walled AZ-based magnesium alloy profiles, where the thickness of the thin plate is 3mm, the welding process includes:

[0008] When the workpiece is an AZ31B magnesium alloy sheet, the welding heat input is controlled at 330 J·mm. -1 -480J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base metal;

[0009] When the workpiece is an AZ61A magnesium alloy sheet, the welding heat input is controlled at 440 J·mm. -1 -650J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base metal;

[0010] When the workpiece is an AZ80A magnesium alloy sheet, the welding heat input is controlled at 710 J·mm. -1 -1180J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base material.

[0011] Furthermore, the optimized welding process mainly includes:

[0012] For AZ31B magnesium alloy, the welding heat input is controlled at 346.9 J·mm. -1 -413.6 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base metal;

[0013] For AZ61A magnesium alloy, the welding heat input is controlled at 482.4 J·mm. -1 -584.5 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base metal;

[0014] For AZ80A magnesium alloy, the welding heat input should be controlled at 996.3 J·mm. -1 -1026.5 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base material.

[0015] Furthermore, for welding the thin-plate portion of AZ-series magnesium alloy hollow thin-walled profiles, with a plate thickness of 3mm, the optimized welding process includes:

[0016] The welding current for AZ31B magnesium alloy is 100-120A, and the arc voltage is 17.7-19.7V.

[0017] The welding current for AZ61A magnesium alloy welding is 110-155A, and the arc voltage is 15.7-17.7V.

[0018] The welding current for AZ80A magnesium alloy is 165-170A, and the arc voltage is 15.7V.

[0019] Furthermore, optimized welding processes include:

[0020] The welding current of AZ31B magnesium alloy is 115A, the arc voltage is 18.7V, the welding speed is 5.6mm / s, the joint strength reaches 241.1MPa, the welding coefficient is 98.0% of the base material, the yield strength and elongation are 147.5MPa and 9.6%, respectively.

[0021] The welding current of AZ61A magnesium alloy is 140A, the arc voltage is 16.7V, the welding speed is 4.5mm / s, the joint strength reaches 286.6MPa, the welding coefficient is 96.8% of the base material, the yield strength and elongation are 153.6MPa and 12.8%, respectively.

[0022] The welding current of AZ80A magnesium alloy is 165A, the arc voltage is 15.7V, the welding speed is 2.6mm / s, the joint strength reaches 295.4MPa, the welding coefficient is 91.2% of the base material, and the yield strength and elongation are 165.5MPa and 7.2%, respectively.

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

[0024] 1. This invention addresses the MIG welding of thin-walled plates from AZ-based magnesium alloy hollow thin-walled profiles with a thickness of 3mm. After systematically studying the MIG welding process of three AZ-based magnesium alloys with different Al contents and microstructures, a non-monotonic variation in the influence of welding heat input on grain size and compound phases was discovered. Traditionally, it is believed that increasing welding heat input usually leads to more severe grain coarsening. The influence of heat input obtained from different combinations of process parameters on the grain size of the heat-affected zone (HAZ) shows a unique trend of "first decreasing and then increasing" as the welding heat input increases. This range of heat input that results in a "decreasing" grain size is a crucial and unexpected discovery, indicating the existence of an optimized moderate heat input range that prevents excessive grain growth in the HAZ, thus avoiding a severe reduction in mechanical properties. This has significant guiding significance for overcoming the problem of grain coarsening in the HAZ during magnesium alloy MIG welding, which restricts the mechanical properties of welded joints.

[0025] 2. This invention also discovered that the welding heat input per unit length has a continuous positive impact on the volume fraction of the second phase, especially in the weld heat-affected zone (HAZ), where the volume fraction of the second phase shows a continuous increasing trend with increasing welding heat input. This contrasts with the non-monotonic change in grain size. This indicates that the diffusion and precipitation behavior of elements changes under specific thermal cycling conditions, meaning that appropriately increasing the heat input may be beneficial to increasing the number of strengthening phases. This pattern has different degrees of influence with heat input for different alloy types, and coupled with the aforementioned influence of grain size, it forms a complex influence law. This invention also quantitatively analyzes the contribution values ​​of various strengthening mechanisms constituting the strength of the HAZ of the welded joint, providing a new perspective for optimizing the HAZ microstructure and controlling its performance by regulating heat input.

[0026] 3. This invention provides highly practical optimized process parameters and reveals a complex, non-monotonic dependence between welding heat input and the microstructure (grain size, second phase) and properties (hardness) of magnesium alloy MIG welded joints. This is an unexpected discovery that transcends traditional understanding and has significant theoretical and application value, pointing out key control directions for the future development of higher-performance magnesium alloy welding processes. Attached Figure Description

[0027] Figure 1This is a schematic diagram of an AZ-based magnesium alloy profile according to an embodiment of the present invention; wherein, the thickness of the thin-walled plate is 3mm, (a) schematic diagram of the cross-section of the magnesium alloy profile; (b) actual picture of the profile;

[0028] Figure 2 The diagram shows the MIG welding apparatus used in this invention; (a) is the welding equipment; (b) is the MIG welding machine; and (c) is a diagram of the longitudinal seam special machine and clamping device.

[0029] Figure 3 A schematic diagram illustrating the specific implementation of MIG welding joints;

[0030] Figure 4 Metallographic microstructures of the heat-affected zone of AZ-based alloys under different welding heat inputs: where (ac) are AZ31B alloy samples 3#, 7#, and 11#; (df) are AZ61A alloy samples 16#, 20#, and 24#; and (gi) are AZ80A alloy samples 29#, 33#, and 37#.

[0031] Figure 5 The GOS diagrams (Grain Orientation Spread, used to quantitatively characterize the recrystallization degree of each region) of the heat-affected zone of AZ-based alloys under different welding heat inputs are shown below: (ac) represents AZ31B alloy samples 3#, 7#, and 11#; (df) represents AZ61A alloy samples 16#, 20#, and 24#; and (gi) represents AZ80A alloy samples 29#, 33#, and 37#.

[0032] Figure 6 Scanning electron microscope (SEM) images of the heat-affected zone (HAZ) of AZ-based alloys under different welding heat inputs: (ac) represents AZ31B alloy samples 3#, 7#, and 11#; (df) represents AZ61A alloy samples 16#, 20#, and 24#; and (gi) represents AZ80A alloy samples 29#, 33#, and 37#.

[0033] Figure 7 Pole diagrams of the heat-affected zones of AZ-based alloys under different welding heat inputs: where (ac) represents AZ31B alloy samples 3#, 7#, and 11#; (df) represents AZ61A alloy samples 16#, 20#, and 24#; and (gi) represents AZ80A alloy samples 29#, 33#, and 37#.

[0034] Figure 8 SF diagrams (basal slip Schmidt factor, used to quantitatively characterize the effect of texture strength on plastic deformation behavior) of the heat-affected zone of AZ alloys under different welding heat inputs: (ac) AZ31B alloy samples 3#, 7#, and 11#; (df) AZ61A alloy samples 16#, 20#, and 24#; (gi) AZ80A alloy samples 29#, 33#, and 37#.

[0035] in, Figure 4 - Figure 8 The welding heat input conditions were as follows: (ac) AZ31B alloy samples: 346.9 J / mm, 384.0 J / mm, 413.6 J / mm; (df) AZ61A alloy samples: 449.6 J / mm, 519.6 J / mm, 581.8 J / mm; (gi) AZ80A alloy samples: 835.6 J / mm, 996.3 J / mm, 1123.3 J / mm.

[0036] Figure 9 To investigate the influence of AZ31B welding process parameters on joint weld width, weld depth, and reinforcement height, the following diagrams are used: (a) Welding current variation (V = 5.6 mm / s, U = 18.7 V); (b) Welding speed variation (I = 115 A, U = 18.7 V); (c) Arc voltage variation (I = 115 A, V = 5.6 mm / s).

[0037] Figure 10 To investigate the influence of AZ61B welding process parameters on joint weld width, weld depth, and reinforcement height, the following diagrams are used: (a) Welding current variation (V = 4.5 mm / s, U = 16.7 V); (b) Welding speed variation (I = 140 A, U = 16.7 V); (c) Arc voltage variation (I = 140 A, V = 4.5 mm / s).

[0038] Figure 11 To investigate the influence of AZ80A welding process parameters on joint weld width, weld depth, and reinforcement height, the following diagrams are used: (a) Welding current variation (V = 2.6 mm / s, U = 15.7 V); (b) Welding speed variation (I = 165 A, U = 15.7 V); (c) Arc voltage variation (I = 165 A, V = 2.6 mm / s). Detailed Implementation

[0039] The technical solutions of the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the present invention are within the scope of protection of the present invention.

[0040] Unless otherwise specified in the specific circumstances, the numerical ranges listed herein include upper and lower limits, as well as all values ​​within that range, but are not limited to the specific values ​​listed when the range is defined.

[0041] I. A welding process for improving the mechanical properties of MIG welded joints of AZ-based magnesium alloys

[0042] For welding 3mm thick thin-walled plates of AZ-series magnesium alloy profiles, MIG welding is used to optimize welding process parameters by controlling the welding heat input. The main welding process includes: when the AZ-series magnesium alloy is AZ31B magnesium alloy, the welding heat input per unit length is controlled at 330 J·mm. -1 -480J·mm -1 The tensile strength of the weld joint shall not be less than 85% of the tensile strength of the base metal; when the AZ-series magnesium alloy plate is AZ61A magnesium alloy, the welding heat input per unit length shall be controlled at 440 J·mm. -1 -650J·mm -1 The tensile strength of the weld joint shall not be less than 85% of the tensile strength of the base metal; when the AZ-series magnesium alloy plate is AZ80A magnesium alloy, the welding heat input per unit length shall be controlled at 710 J·mm. -1 -1180J·mm -1 The tensile strength of the weld joint shall not be less than 85% of the tensile strength of the base material. Here, unit length refers to the joint length when performing MIG welding on magnesium alloy sheets.

[0043] This invention does not merely employ simple trial-and-error process optimization methods, but rather comprehensively studies the thermal effects of the MIG welding process on the microstructure and compound phases of the base metal in AZ series magnesium alloys. In its research on MIG welding of commonly used AZ-series magnesium alloys AZ31B, AZ61A, and AZ80A, this invention discovered that the influence of welding heat input on the performance and microstructure of welded joints differs from conventional understandings in existing technologies. The underlying mechanisms for these differences were investigated. Compared to traditional empirical parameter selection, this invention effectively suppresses weld joint defects (such as lack of fusion) while avoiding excessive heat input that leads to grain coarsening, thereby improving mechanical properties. This systematic parameter matching strategy provides a new perspective for optimizing the MIG welding process of AZ series magnesium alloys.

[0044] This invention reveals that the AZ31B alloy MIG welded joint consists of a base metal region (BM), a heat-affected zone (HAZ), and a weld joint region (FZ). The grain size exhibits a gradient distribution characteristic of HAZ (20.2 μm) > BM (18.6 μm) > FZ (11.1 μm). The volume fraction and average diameter of the second phase in the HAZ show an increasing trend after welding, while the texture strength follows the pattern of HAZ (23.57) > BM (17.53) > FZ (8.09). The HAZ is a weak area, and joint fracture always occurs within it, characterized by quasi-cleavage fracture. AZ61A and AZ80A joints exhibit the same pattern. The performance and microstructure analysis of the HAZ in AZ-based magnesium alloy welded joints is a key focus of research and a major basis for process optimization.

[0045] Table 1 shows the weld joint performance of AZ31B, AZ61A, and AZ80A alloys using typical welding processes with different heat inputs. The performance of the weld joints for these three alloys initially decreases and then increases with increasing heat input, exhibiting high strength at the midpoint of the heat input range.

[0046] Table 1 Mechanical properties of joints under different welding heat inputs

[0047]

[0048] Table 2 shows the changes in the microstructure of the heat-affected zone (HAZ) of the three alloy welded joints under different welding heat inputs. With increasing welding heat input, the average grain size and texture intensity of the HAZ of the three magnesium alloy MIG welded joints showed a trend of first decreasing and then increasing, while the volume fraction and average diameter of the second phase both showed an increasing trend with increasing welding heat input. Analysis of various strengthening mechanisms of the HAZ revealed that grain size had the greatest impact. The mechanical properties of the joint were affected by the process, mainly by grain size and the second phase. This finding breaks through the traditional linear correlation assumption that "higher heat input leads to coarser grains," providing new theoretical principles and innovative ideas for the precise control of the microstructure and properties of welded joints.

[0049] Table 2. Microstructure differences in the heat-affected zone of AZ-series magnesium alloys under different welding heat inputs.

[0050]

[0051] II. Specific Implementation Methods

[0053] 1. Experimental Materials

[0054] See Figure 1 The experimental materials included AZ31B, AZ61A, and AZ80A extruded profiles and their homogeneous welding wire. The profiles were selected from those manufactured by Shanxi Yinguang Huasheng Magnesium Industry Co., Ltd., after comparing data from samples of products from multiple manufacturers. The profiles had a rectangular cross-section of 110mm × 80mm × 3mm. The thin plates used for welding were sampled from the prepared hollow thin-walled profiles. The profiles were laser-cut to obtain 80mm × 100mm plates. Before welding, the oxide layer on the laser-cut surface was milled off to avoid affecting the welding process and performance results. The welding wire was manufactured by Henan Weiketo Magnesium Alloy Technology Co., Ltd., with a diameter of Φ1.2mm. Argon gas with a purity of 99.9% was used as the welding shielding gas.

[0055] The chemical composition and mechanical properties of the extruded profiles are shown in Table 3.

[0056] Table 3 Chemical composition and properties of the base material

[0057]

[0058] 2. Welding methods and equipment

[0059] MIG welding is a welding method that uses an electric arc burning between a continuously fed welding wire and the workpiece as a heat source. An inert gas is used to protect the arc, the molten pool, and the surrounding high-temperature metal, preventing them from reacting with oxygen, nitrogen, and water vapor in the air. The MIG welding equipment used in this invention is as follows: Figure 2 As shown, the wire feeding and welding power system of the EWM-MIG welding machine is used. The welding torch is mounted on the torch holder of the CNC straight seam welding platform, maintaining an inclination angle of 45°. A clamping device is used to fix the workpiece on the welding platform. In this way, the traditional handheld welding torch operation of MIG welding is transformed into automated welding that can strictly repeat parameters including welding speed, so as to ensure the stability and repeatability of the welding process and provide a solid foundation for the reliability of experimental data.

[0060] See Figure 3 In practice, the thin plate joints are welded using a 60° bevel, with a 30° bevel angle on each side. Before MIG welding, the oxide film on the plate surface is removed by mechanical grinding to avoid welding defects due to the presence of oxide film during the welding process.

[0061] 3. Analysis of the intensification mechanism in the heat-affected zone

[0062] Several major strengthening mechanisms that determine the strength of metals include grain refinement strengthening, solid solution strengthening, second-phase strengthening, dispersion strengthening, and texture strengthening. In MIG welding, the heat-affected zone (HAZ) of the weld joint experiences a significant impact from the heating and cooling process caused by the welding heat input, primarily due to the increase or decrease in grain refinement strengthening, second-phase strengthening, and texture strengthening. The degree to which the strength of the HAZ relative to the base metal is reduced during the heating process can be attributed mainly to the increase or decrease in grain refinement strengthening, second-phase strengthening, and texture strengthening.

[0063] (1) Refine grain strengthening

[0064] Metallographic structure of the heat-affected zone of MIG welded joints of AZ-based magnesium alloys under different welding heat inputs, such as Figure 4As shown in (ai), the grain size of all three alloys initially decreased and then increased with increasing welding heat input. This is due to differences in the degree of recrystallization in the heat-affected zone (HAZ). The highest temperature in the HAZ exceeded the recrystallization temperature of the AZ-based magnesium alloys, prompting dynamic recovery and incomplete recrystallization in the HAZ. As the welding heat input increased, the recrystallization nucleation rate in the HAZ increased, resulting in grain refinement. When the welding heat input exceeded the optimal welding heat input, although the cooling rate remained high, the excessively high welding heat input led to a prolonged high-temperature residence time in the HAZ, significantly increasing the grain boundary migration rate and causing abnormal grain growth.

[0065] To further explore the microstructure evolution mechanism under the above-mentioned thermal cycling, this invention uses a quantitative analysis method based on grain orientation difference to characterize the heat-affected zone of MIG welded joints of AZ-based magnesium alloys. Figure 5 (ai) Corresponding to each sample Figure 4 (ai) Each sample shows the GOS distribution characteristics of the heat-affected zone of the joint under different welding heat input conditions. The blue area represents a smaller GOS value, and the red area represents a larger GOS value. As the welding heat input increases, the degree of recrystallization of the joint increases. After reaching the optimal welding heat input, the degree of recrystallization of the joint does not change much. This trend is consistent with the metallographic analysis.

[0066] According to Hall-Petch formula (1): σ y =σ0+kd -1 / 2 (1)

[0067] Where σ0 is the lattice friction resistance (approximately 11 MPa), and k is a constant (approximately 220 MPa·μm). -1 / 2 ), where d is the average grain size.

[0068] Substituting the grain size of the heat-affected zone (HAZ) of the joint under different heat input conditions from Table 4 into the calculations, the fine-grain strengthening contribution values ​​of the joint under different welding heat input conditions were obtained, as shown in Table 4. Analysis shows that with the increase of welding heat input, the fine-grain strengthening contribution value of the joint exhibits a trend of first increasing and then decreasing, consistent with the experimentally measured value variation of the joint. This confirms that welding heat input affects the joint strength by regulating the grain size of the HAZ.

[0069] Table 4. Average grain size and fine grain strengthening contribution of AZ-based magnesium alloys under different welding heat inputs.

[0070]

[0071]

[0072] (2) Second phase reinforcement

[0073] Figure 6 Scanning electron microscope (SEM) images of the heat-affected zone (HAZ) of AZ-based magnesium alloy MIG welded joints under different welding heat inputs, from... Figure 6 It can be seen that the weld joint area of ​​AZ31B is mainly distributed in a dotted pattern in the weld joint area, while the second phase of the weld joint area of ​​AZ61A and AZ80A is mainly distributed in a blocky pattern at the grain boundaries, with a small amount of dotted second phase precipitated in the grain.

[0074] The volume fraction of the second phase and the average diameter of the second phase were statistically analyzed using ImageJ software. Figure 6 The characteristic data of the second phase corresponding to each sample in (ai) are shown in Table 5. The results show that with the increase of welding heat input, the volume fraction of the second phase and the average diameter of the second phase in the heat-affected zone of the weld joint both show an increasing trend. This is because with the increase of welding heat input, the small-sized second phase dissolves and diffuses to the large-sized particles, accelerating the coarsening process of the large-sized particles and promoting the increase of the average size of the second phase.

[0075] The average size of the second phase in the heat-affected zone of the joint was substituted into formula (2) to calculate the strengthening contribution value. The results are shown in Table 5.

[0076]

[0077] The strengthening contribution of the second phase in different magnesium alloy joints exhibits a differentiated evolution pattern with the change of welding heat input: the strengthening contribution of the AZ31B joint first increases and then decreases with the increase of welding heat input, while the AZ61A and AZ80A joints show a continuous decreasing trend.

[0078] Table 5. Volume fraction of the second phase and strengthening contribution value of the heat-affected zone in AZ-based magnesium alloys under different welding heat inputs.

[0079]

[0080] (3) Texture enhancement

[0081] Pole diagrams of the heat-affected zone of MIG welded joints of AZ-based magnesium alloys under different welding heat inputs are shown below. Figure 7 As shown, the basal texture strength weakens with increasing welding heat input. This weakening can be attributed to the recrystallization evolution caused by increased welding heat input: at higher welding heat input, both the recrystallization nucleation rate and grain boundary migration rate increase, promoting the random orientation growth of equiaxed recrystallized grains, which reduces the degree of preferred orientation, thus weakening the texture strength.

[0082] Figure 8The SF values ​​of MIG welded joints of AZ-based magnesium alloys under different welding heat inputs are shown in the figures. Red areas represent relatively low SF values, and blue areas represent relatively high SF values. The SF values ​​for Figure (ac) are 0.46, 0.44, and 0.43; for Figure (df) are 0.45, 0.42, and 0.41; and for Figure (gi) are 0.42, 0.39, and 0.38. The results indicate that the SF value decreases with increasing welding heat input, but the overall trend is not significant.

[0083] Table 6 shows the SF values ​​of the heat-affected zone for AZ31B, AZ61A, and AZ80A alloys under different welding heat inputs. Substituting the SF values ​​into formula (3):

[0084] σ=t0 / m (3)

[0085] Where t0 is the critical shear stress for slip on the basal plane, which is 5.24 MPa, and m is the Schmidt factor coefficient.

[0086] The calculated contribution values ​​of the texture strength of AZ-based magnesium alloys are shown in Table 6. Analysis indicates that the texture strengthening value of the heat-affected zone (HAZ) of AZ-based magnesium alloys does not change significantly under the influence of welding heat input. This is because although welding heat input increases the recrystallization degree of the HAZ, some small-angle grain boundaries still maintain a certain degree of orientation consistency, thus the texture strength does not change much.

[0087] Table 6. SF values ​​and texture strengthening contributions in the heat-affected zone of AZ-based alloys under different welding heat inputs.

[0088]

[0089] Table 7 shows the contribution values ​​of various strengthening mechanisms in the heat-affected zone (HAZ) of AZ-based magnesium alloy MIG welded joints under different welding heat inputs. Analysis of the various strengthening mechanisms of the HAZ reveals that grain size has the greatest impact. Under different welding heat inputs, fine-grain strengthening and the second strengthening mechanism consistently dominate, while texture strengthening contributes between 13.3% and 16.5%, showing no significant sensitivity to welding heat input.

[0090] Table 7 Actual tensile strength and calculated values ​​of various strengthening methods of AZ series alloys under different welding heat inputs.

[0091]

[0092] In summary, this invention systematically studied the evolution of the microstructure of the heat-affected zone (HAZ) of AZ-based magnesium alloy MIG welded joints using microstructural characterization techniques such as OM, SEM, TEM, and EBSD. It also explored the mechanism by which welding heat input affects the mechanical properties of the HAZ. The main conclusions are as follows:

[0093] (1) With changes in welding heat input, the tensile strength of the three magnesium alloy joints showed a significant correlation with the degree of grain coarsening in the heat-affected zone. Specifically, the joint strength of AZ31B magnesium alloy under low, medium, and high welding heat input conditions were 233.0 MPa, 241.1 MPa, and 232.8 MPa, respectively, with corresponding HAZ grain coarsening rates of 24.7%, 3.7%, and 38.1%; the strength of AZ61A magnesium alloy was 265.7 MPa, 286.8 MPa, and 274.1 MPa, respectively, with corresponding HAZ grain coarsening rates of 36.3%, 9.8%, and 32.9%; and the strength of AZ80A magnesium alloy was 287.4 MPa, 295.4 MPa, and 279.6 MPa, respectively, with corresponding HAZ grain coarsening rates of 50.0%, 16.7%, and 63.7%. The grain coarsening degree of AZ80A alloy (up to 63.7%) was significantly higher than that of AZ31B (38.1%) and AZ61A (36.3%), resulting in a significant increase in the joint strength deterioration rate (up to 13.7%).

[0094] (2) With increasing welding heat input, the volume fraction and average diameter of the second phase in the heat-affected zone (HAZ) both increased significantly: the volume fraction of the second phase in AZ31B, AZ61A, and AZ80A increased by 1.73%, 2.46%, and 2.70%, respectively, and the average diameter increased by 0.5 μm, 0.8 μm, and 1.1 μm, respectively. Among them, the AZ80A alloy, due to its highest aluminum content, easily promoted the formation of the second phase. After the heat input process, its second phase volume fraction (up to 4.91%) and average diameter (up to 3.2 μm) showed the largest increase. The study shows that the increase in Al content in the alloy and the increase in welding heat input jointly lead to the intensification of the coarsening of the second phase, thereby weakening the mechanical properties of the joint. The coarsening of the second phase can be suppressed by controlling the heat input, thereby improving the mechanical properties of the joint.

[0095] Furthermore, the principle of how heat input and process affect the mechanical properties and microstructure of welded joints for alloys with different compositions and microstructures, as described in this invention, is suitable for welding the thin plate and rib surfaces of AZ-series magnesium alloy hollow thin-walled profiles with a thickness of 3 mm. It also provides reference and guidance for the formulation of welding processes for other series of magnesium alloys with similar microstructures to AZ31, AZ61, or AZ80 in the AZ-series magnesium alloys.

[0096] More importantly, the aforementioned principle is also applicable to other parts of hollow thin-walled profiles with different thicknesses, including plates obtained by other deformation methods, such as magnesium alloy plates of 3mm-10mm. When the plate thickness changes, the required heat input and process can be adjusted accordingly by referring to the above principle of this invention.

[0097] III. Practical Optimized Process Parameters

[0098] The formula for calculating the welding heat input of MIG welding described in this invention is shown in equation (4).

[0099] E=UI÷V (4)

[0100] Where E is the welding heat input (J·mm⁻¹), U is the arc voltage (V), I is the welding current (A), and V is the welding speed (mm·s⁻¹).

[0101] Increased heat input may lead to more complete melting of the weld joint, potentially increasing the weld penetration and width. However, high heat input can also have negative effects, such as exceeding the boiling point of magnesium alloys, causing excessive spatter, easily entraining large amounts of gas leading to weld porosity, and potentially causing coarsening of the heat-affected zone and second phase, affecting the mechanical properties of the weld joint. In production practice, producing thin plates around 3mm thick presents the challenge of controlling process parameters and heat input while ensuring good weld joint shape and heat-affected zone strength.

[0102] Therefore, when studying the MIG welding of AZ-based magnesium alloy hollow thin-walled profiles, a 3mm thick thin plate wall was selected for the welding experiment. The relationship between the mechanical properties of the welded joint and the welding process parameters was obtained as follows:

[0103] The welding process of AZ31B was studied with welding current of 90-125A, arc voltage of 17.7-19.7V, and welding speed of 3.6-8.5mm / s. The optimal combination of welding process parameters (115A, 18.7V, 5.6mm / s) was obtained, and the joint strength reached 241.1MPa (welding coefficient 98.0%).

[0104] Welding process research was conducted on AZ61A with welding current of 95-150A, arc voltage of 15.2-19.7V, and welding speed of 3.6-7.4mm / s. The optimal combination of welding process parameters (140A, 16.7V, 4.5mm / s) was obtained, and the joint strength reached 286.6MPa (welding coefficient 96.8%).

[0105] Through welding process research on AZ80A with welding current of 120-175A, arc voltage of 14.7-21.7V, and welding speed of 1.8-3.6mm / s, the optimal combination of welding process parameters (165A, 15.7V, 2.6mm / s) was obtained, and the joint strength reached 295.4MPa (welding coefficient 91.2%).

[0106] The AZ31B alloy underwent process tests at different speeds of 5.2, 5.6, and 6.2 mm / s. At a welding speed of 5.6 mm / s, the widest process window was available, ranging from 110-120A to 17.7-19.7V.

[0107] The AZ61A alloy underwent process tests at different speeds of 4.0, 4.5, and 5.2 mm / s. At a welding speed of 4.5 mm / s, the widest process window was available, ranging from 110 to 155 A and 15.7 to 19.7 V.

[0108] The AZ80A alloy underwent process tests at different speeds of 2.2, 2.6, and 3.1 mm / s. At a welding speed of 2.6 mm / s, the widest process window was available, ranging from 110-120 A to 17.7-19.7 V.

[0109] In particular, the present invention takes into account the influence of the macroscopic morphology of the weld zone, including the impact of its geometry and volume on the overall performance of the welded joint. This is specifically reflected in three aspects: weld width, weld depth, and weld reinforcement height.

[0110] The influence of AZ31B welding process parameters on joint weld width, weld depth, and reinforcement height, such as... Figure 9 As shown in the figure, (a) shows the effect of welding current variation when V = 5.6 mm / s and U = 18.7 V is fixed; (b) shows the effect of welding speed variation when I = 115 A and U = 18.7 V is fixed; and (c) shows the effect of arc voltage variation when I = 115 A and V = 5.6 mm / s is fixed.

[0111] The influence of AZ61B welding process parameters on joint weld width, weld depth, and reinforcement height, such as... Figure 10 As shown in the figure, (a) shows the effect of welding current variation when V = 4.5 mm / s and U = 16.7 V is fixed; (b) shows the effect of welding speed variation when I = 140 A and U = 16.7 V is fixed; and (c) shows the effect of arc voltage variation when I = 140 A and V = 4.5 mm / s is fixed.

[0112] The influence of AZ80A welding process parameters on joint weld width, weld depth, and reinforcement height, such as... Figure 11 As shown in the figure, (a) shows the effect of welding current variation when V = 2.6 mm / s and U = 15.7 V is fixed; (b) shows the effect of welding speed variation when I = 165 A and U = 15.7 V is fixed; and (c) shows the effect of arc voltage variation when I = 165 A and V = 2.6 mm / s is fixed.

[0113] Furthermore, when optimizing the range of welding process parameters, the influence of the above-mentioned microstructure and macroscopic morphology of the welded joint on the mechanical properties was analyzed. Among them, the experimental results of the influence of different welding processes on three alloys with different compositions and microstructures are as follows.

[0114] The influence of different process parameters on the joint morphology and mechanical properties of AZ31B alloy is shown in Table 8.

[0115] Table 8. Experimental Results of AZ31B Regularity

[0116]

[0117] The influence of different process parameters on the joint morphology and mechanical properties of AZ61A alloy is shown in Table 9.

[0118] Table 9. Experimental Results and Patterns of AZ61A

[0119]

[0120] The influence of different process parameters on the joint morphology and mechanical properties of AZ80A alloy is shown in Table 10.

[0121] Table 10. Experimental Results of AZ80A Regularity

[0122]

[0123] As shown in Table 8-10, when using the MIG welding process for welding the thin-plate portion of AZ-series magnesium alloy hollow thin-walled profiles, with a plate thickness of 3mm, the welding process includes:

[0124] When the workpiece is an AZ31B magnesium alloy sheet, the welding heat input is controlled at 330 J·mm. -1 -480J·mm -1 ;

[0125] When the workpiece is an AZ61A magnesium alloy sheet, the welding heat input is controlled at 440 J·mm. -1 -650J·mm -1 ;

[0126] When the workpiece is an AZ80A magnesium alloy sheet, the welding heat input is controlled at 710 J·mm. -1 -1180J·mm -1 ;

[0127] By controlling the welding heat input as described above, the tensile strength of the welded joint can be ensured to be no less than 85% of the tensile strength of the base material.

[0128] Furthermore, this invention further optimizes the welding process, and the optimized welding process mainly includes:

[0129] For AZ31B magnesium alloy, the welding heat input is controlled at 346.9 J·mm. -1 -413.6 J·mm -1 ;

[0130] For AZ61A magnesium alloy, the welding heat input is controlled at 482.4 J·mm. -1 -584.5 J·mm -1 ;

[0131] For AZ80A magnesium alloy, the welding heat input should be controlled at 996.3 J·mm. -1 -1026.5 J·mm -1 ;

[0132] By further precise control of the welding heat input, it can be ensured that the tensile strength of the weld joint is not less than 90% of the tensile strength of the base material.

[0133] In summary, this invention systematically studies the MIG welding of thin plates of AZ-series (AZ31B, AZ61A, and AZ80A) magnesium alloy profiles with a thickness of 3mm. It explores the influence of welding process parameters on the macroscopic morphology and tensile properties of MIG welded joints, summarizes the general influence of welding heat input on the microstructure and properties of MIG welded joints, and provides theoretical support for the application of MIG welding of AZ-series magnesium alloys.

[0134] The process rules discovered in this invention have reference significance for the selection of process parameters for extruded profiles of AZ-series (AZ31B, AZ61A and AZ80A) magnesium alloys, as well as welded objects such as extruded plates and rolled plates, when the thickness is between 3mm and 10mm. It is only necessary to increase the heat input accordingly as the thickness increases, based on the control range of process parameters for 3mm thin plates of different alloys.

[0135] The process principles discovered in this invention target three alloys—AZ31B, AZ61A, and AZ80A—which possess different compositional and microstructural characteristics. AZ31B has a low alloying content and is not heat-treatable; its base metal's strengthening mechanisms primarily include solid solution strengthening, grain refinement strengthening, and deformation strengthening, resulting in moderate strength and excellent ductility. AZ80A is a high-alloying alloy with extremely high second-phase compound content, exhibiting extremely high strength but low ductility; its base metal's strengthening mechanisms mainly include second-phase strengthening, solid solution strengthening, and grain refinement strengthening. AZ61A alloy has a moderate alloying content, balancing strength and ductility; its base metal's strengthening mechanisms primarily include second-phase strengthening, solid solution strengthening, grain refinement strengthening, and fine second-phase dispersion strengthening obtained through heat treatment. When developing MIG welding processes for other series of magnesium alloys, this invention provides valuable reference for the control techniques using different combinations of heat input and process parameters for alloys with varying alloying compositions and microstructural characteristics.

[0136] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A welding process for improving the mechanical properties of MIG welded joints of AZ-based magnesium alloys, characterized in that: When using the MIG welding process, for welding the thin-plate portion of AZ-based magnesium alloy hollow thin-walled profiles, with a plate thickness of 3mm, the welding process includes: When the workpiece is an AZ31B magnesium alloy sheet, the welding heat input is controlled at 330 J·mm. -1 -480J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base metal; When the workpiece is an AZ61A magnesium alloy sheet, the welding heat input is controlled at 440 J·mm. -1 -650J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base metal; When the workpiece is an AZ80A magnesium alloy sheet, the welding heat input is controlled at 710 J·mm. -1 -1180J·mm -1 The tensile strength of the welded joint shall not be less than 85% of the tensile strength of the base material.

2. The welding process according to claim 1, characterized in that: For welding the thin-plate sections of AZ-series magnesium alloy hollow thin-walled profiles, the welding process mainly includes: For AZ31B magnesium alloy, the welding heat input is controlled at 346.9 J·mm. -1 -413.6 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base metal; For AZ61A magnesium alloy, the welding heat input is controlled at 482.4 J·mm. -1 -584.5 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base material; For AZ80A magnesium alloy, the welding heat input should be controlled at 996.3 J·mm. -1 -1026.5 J·mm -1 The tensile strength of the welded joint shall not be less than 90% of the tensile strength of the base material.

3. The welding process according to claim 1 or 2, characterized in that: Its welding process includes: When the welding speed of AZ31B magnesium alloy is 5.6 mm / s, the welding current is 100-120A and the arc voltage is 17.7-19.7V. When welding AZ61A magnesium alloy at a speed of 4.5 mm / s, the welding current is 110-155 A and the arc voltage is 15.7-17.7 V. When welding AZ80A magnesium alloy at a speed of 2.6 mm / s, the welding current is 165-170 A and the arc voltage is 15.7 V.

4. The welding process according to claim 3, characterized in that: Its optimized welding process includes: The welding current of AZ31B magnesium alloy is 115A, the arc voltage is 18.7V, the welding speed is 5.6mm / s, the joint strength reaches 241.1MPa, the welding coefficient is 98.0% of the base material, the yield strength and elongation are 147.5MPa and 9.6%, respectively. The welding current of AZ61A magnesium alloy is 140A, the arc voltage is 16.7V, the welding speed is 4.5mm / s, the joint strength reaches 286.6MPa, the welding coefficient is 96.8% of the base material, the yield strength and elongation are 153.6MPa and 12.8%, respectively. The welding current of AZ80A magnesium alloy is 165A, the arc voltage is 15.7V, the welding speed is 2.6mm / s, the joint strength reaches 295.4MPa, the welding coefficient is 91.2% of the base material, and the yield strength and elongation are 165.5MPa and 7.2%, respectively.

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