A magnesium-aluminum dissimilar alloy friction stir welding method based on Ca-La synergistic modification of magnesium alloy and application thereof

Abstract

A magnesium-aluminum dissimilar alloy friction stir welding method based on Ca-La synergistic modified magnesium alloy and application thereof belong to the technical field of lightweight metal materials, dissimilar metal solid-phase connection and new energy automobile structure manufacturing. The method solves the problems of the existing Ca modified AZ31 magnesium alloy, such as the increase of the second phase, the uneven structure and the decrease of plasticity, and the problems of the magnesium-aluminum dissimilar welding process, such as the material flow mismatch and the high tendency of Mg-Al brittle intermetallic compound formation. The method comprises the following steps: I. weighing magnesium alloy, MgCa 15 alloy and AlLa 10 alloy; II. melting and pouring; III. homogenization treatment and hot extrusion; IV. friction stir welding. Application, it is used for preparing an electric vehicle battery tray.

Description

Technical Field

[0001] This invention belongs to the technical fields of lightweight metal materials, solid-phase bonding of dissimilar metals, and manufacturing of structural components for new energy vehicles. Background Technology

[0002] With the rapid development of the new energy vehicle industry, power battery systems are placing increasingly higher demands on the lightweighting, safety, load-bearing capacity, and connection reliability of structural components. As a crucial load-bearing and protective structure for the power battery system of new energy vehicles, the battery tray's quality directly impacts the vehicle's energy utilization efficiency and range performance. Currently, most new energy vehicle battery trays are made of aluminum alloy and formed into a single structure through welding and other methods. Aluminum alloys possess good strength, corrosion resistance, and mature processing technology, but their density is relatively high. Therefore, partially replacing traditional aluminum alloys with low-density magnesium alloys is expected to further reduce the weight of battery trays and achieve further lightweighting of structural components in new energy vehicles.

[0003] AZ31 magnesium alloy possesses good plastic deformation capacity and processing performance, making it well-suited for lightweight structural components. However, the flammability of magnesium alloys limits their application in structures with high safety requirements, such as battery trays for new energy vehicles. Therefore, improving the flame-retardant properties of magnesium alloys while maintaining their good strength and plasticity is a crucial technical challenge for promoting their application in new energy vehicle structural components.

[0004] Existing research indicates that adding calcium (Ca) to AZ31 magnesium alloys can improve their flame retardant properties and enhance their safety and stability at high temperatures. Therefore, Ca is an important alloying element for improving the application potential of magnesium alloys in the new energy vehicle field. However, after Ca is added, it interacts with elements such as Al in the alloy to form a Ca-containing second phase, such as the Al2Ca phase. With the addition of Ca, the amount of the second phase increases. While improving the high-temperature stability and flame retardant properties of the material, it may also cause an increase in localized hard and brittle phases, affecting the material's plasticity and ductility. Furthermore, Ca-modified magnesium alloys are prone to exhibiting a non-uniform microstructure with both coarse and fine grains, reducing the deformation coordination between different regions and hindering subsequent plastic processing and dissimilar metal welding.

[0005] On the other hand, joining magnesium alloys and aluminum alloys can further leverage the performance advantages of both materials. Magnesium alloys have low density, which can be used to achieve lightweight structures; 6061 aluminum alloys have good strength, corrosion resistance, and a solid foundation for engineering applications. Using magnesium alloys and 6061 aluminum alloys to form dissimilar metal structures can reduce the overall structural mass while ensuring structural performance. However, magnesium alloys and aluminum alloys differ significantly in melting point, crystal structure, thermophysical properties, and plastic flow behavior. When using traditional fusion welding methods to join magnesium and aluminum dissimilar metals, welding defects are prone to occur, and Mg and Al are easily reacted to form brittle Mg-Al intermetallic compounds, thereby reducing the plasticity and reliability of the joint.

[0006] Friction stir welding is a solid-state joining technique in which the materials do not melt as a whole during the welding process. Instead, the frictional heat generated by the rotation of the stirring head and the mechanical stirring action bring the materials to a thermoplastic state, causing them to flow and mix plastically. Therefore, friction stir welding can reduce the defects caused by melting and solidification in traditional fusion welding and is suitable for joining dissimilar metals such as magnesium alloys and aluminum alloys.

[0007] However, significant material flow mismatch and interfacial reactions still exist in magnesium-aluminum dissimilar metal friction stir welding. Due to the differences in thermoplasticity and flow behavior between magnesium and aluminum alloys, uneven material flow, localized defects, and interfacial reactions are prone to occur during welding. Furthermore, Mg and Al undergo elemental diffusion and reaction under welding thermal cycling and mechanical stirring, easily forming Mg... 17 Al 12 Brittle Mg-Al intermetallic compounds such as Mg2Al3. When these brittle phases form a concentrated distribution in the interface region, they reduce the load-bearing capacity and ductility of the welded joint.

[0008] Current research on magnesium-aluminum dissimilar metal friction stir welding mainly focuses on optimizing welding parameters, changing the stir head structure, adjusting the plate overlap form, or introducing an intermediate layer. However, these methods primarily control the welding process itself, with insufficient consideration given to the influence of the magnesium alloy base metal's microstructure, second phase, and plastic flowability on the welding behavior. Summary of the Invention

[0009] This invention aims to address the problems of increased second phase, uneven microstructure, and decreased plasticity in existing Ca-modified AZ31 magnesium alloys, as well as the mismatch in material flow and high tendency for the formation of brittle Mg-Al intermetallic compounds during magnesium-aluminum dissimilar alloy welding. It further provides a method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys and its application.

[0010] A method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys, comprising the following steps:

[0011] 1. Weigh out magnesium alloy, MgCa 15 Alloys and AlLa 10 Alloy, to obtain raw materials;

[0012] 2. The raw materials are melted and poured to obtain ingots;

[0013] 3. The ingot is homogenized, then the surface oxide is removed, and finally hot extrusion is performed to obtain Ca-La synergistic modified magnesium alloy sheet.

[0014] Fourth, assemble aluminum alloy sheet and Ca-La synergistic modified magnesium alloy sheet to form a welded joint, and then perform friction stir welding to obtain magnesium-aluminum dissimilar alloy, thus completing the friction stir welding method for magnesium-aluminum dissimilar alloy based on Ca-La synergistic modified magnesium alloy.

[0015] The prepared magnesium-aluminum dissimilar alloy is used in the manufacture of battery trays for electric vehicles.

[0016] The beneficial effects of this invention are:

[0017] This invention first utilizes Ca to improve the flame retardant properties of AZ31 magnesium alloy, and regulates the microstructure by forming Ca-containing second phases such as Al2Ca. Based on the Ca-modified AZ31 magnesium alloy, La is added. This is not alloying with La alone, but rather through the synergistic effect of Ca and La, further regulating the presence state of Al, the formation behavior of the second phase, and the grain structure of the magnesium alloy. The addition of La promotes more complete formation of the second phase in the alloy and changes the distribution state of Al, thereby reducing the tendency to form some brittle Mg-Al intermetallic compounds. Simultaneously, the addition of La can improve the heterogeneous microstructure of the Ca-modified magnesium alloy, where coarse and fine grains coexist, making the grain size and microstructure distribution more uniform, thus improving the material's deformation compatibility and ductility. Specifically:

[0018] (1) Adding Ca to AZ31 can improve the flame retardant properties of the alloy, providing a material basis for the application of magnesium alloys in safety-sensitive structures such as battery trays for new energy vehicles.

[0019] (2) Adding La to Ca modification can improve the unevenness of the microstructure and the loss of plasticity caused by Ca, making the grains and the second phase more uniformly distributed, which is conducive to obtaining a better strength and toughness match.

[0020] (3) Ca-La synergistic modification improves the plastic flow coordination ability of magnesium alloys in the friction stir welding process and weakens the adverse effect of the flow difference between magnesium and aluminum on joint formation.

[0021] (4) By controlling the welding heat input and stirring action, the tendency of Mg-Al brittle intermetallic compounds to form a continuous or thick reaction layer is reduced, which is beneficial to improving the integrity of the joint and the reliability of service.

[0022] (5) Friction stir welding is a solid-state bonding process. Compared with traditional fusion welding, it can avoid large-scale melting and solidification processes, which is beneficial to reducing problems such as porosity and solidification cracks in dissimilar metal fusion welding.

[0023] (6) The battery tray is made of magnesium alloy and 6061 aluminum alloy, which can reduce the amount of high-density aluminum alloy material while meeting the requirements of local load-bearing, connection and safety, thus achieving lightweighting from the structural level.

[0024] (7) The alloy preparation, hot extrusion and friction stir welding processes of the present invention all have relatively clear process windows, which facilitates further engineering scale-up.

[0025] Therefore, this invention, through Ca-La synergistic alloying, further improves the microstructure uniformity, second-phase distribution, and ductility and toughness of Ca-modified magnesium alloys while retaining the flame-retardant effect of Ca. Furthermore, the obtained Ca-La synergistic modified magnesium alloy is used to perform friction stir welding with aluminum alloys, thereby improving material flow and interfacial reaction during the magnesium-aluminum dissimilar metal welding process, enhancing the comprehensive mechanical properties of the magnesium-aluminum dissimilar weld joint, and further realizing its application in magnesium-aluminum hybrid battery trays for new energy vehicles. Attached Figure Description

[0026] Figure 1 This is a process flow diagram of the magnesium-aluminum dissimilar alloy friction stir welding method based on Ca-La synergistic modification magnesium alloy in Example 1.

[0027] Figure 2 Combustion experiment of AZ31-1Ca-1La prepared for Example 1 at 1100℃;

[0028] Figure 3 Optical morphology and grain size diagrams of three magnesium alloy plates: AZ31, AZ31-1Ca prepared in Comparative Experiment 1, and AZ31-1Ca-1La prepared in Example 1. (a) AZ31, (b) AZ31-1Ca, (c) AZ31-1Ca-1La, (d) grain size.

[0029] Figure 4 SEM and EDS images of AZ31, AZ31-1Ca prepared in Comparative Experiment 1, and AZ31-1Ca-1La prepared in Example 1, (a) AZ31, (b) AZ31-1Ca, (c) AZ31-1Ca-1La;

[0030] Figure 5XRD patterns of magnesium-aluminum dissimilar alloy friction stir welded joints in Example 1, Comparative Experiment 1, and 2, under the condition that the stirring head speed is 900 r / min;

[0031] Figure 6 The following are schematic diagrams of the cross-sectional structures of the magnesium-aluminum dissimilar alloy friction stir welding joints in Example 1, Comparative Experiments 1 and 2: (a) AZ31 / 6061 at a stirring head speed of 900 r / min, (b) AZ31-1Ca / 6061 at a stirring head speed of 900 r / min, (c) AZ31-1Ca-1La / 6061 at a stirring head speed of 900 r / min, and (d) AZ31-1Ca-1La / 6061 at a stirring head speed of 1000 r / min.

[0032] Figure 7 This is an enlarged schematic diagram of a partial weld node of a battery tray prepared using the magnesium-aluminum dissimilar alloy of Example 1. Detailed Implementation

[0033] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments.

[0034] Specific Implementation Method 1: This implementation method describes a magnesium-aluminum dissimilar alloy friction stir welding method based on Ca-La synergistic modification of magnesium alloys, which is carried out according to the following steps:

[0035] 1. Weigh out magnesium alloy, MgCa 15 Alloys and AlLa 10 Alloy, to obtain raw materials;

[0036] 2. The raw materials are melted and poured to obtain ingots;

[0037] 3. The ingot is homogenized, then the surface oxide is removed, and finally hot extrusion is performed to obtain Ca-La synergistic modified magnesium alloy sheet.

[0038] Fourth, assemble aluminum alloy sheet and Ca-La synergistic modified magnesium alloy sheet to form a welded joint, and then perform friction stir welding to obtain magnesium-aluminum dissimilar alloy, thus completing the friction stir welding method for magnesium-aluminum dissimilar alloy based on Ca-La synergistic modified magnesium alloy.

[0039] This specific embodiment does not simply use Ca and La as alloying elements separately, but rather utilizes a synergistic approach of "Ca first modifying, then La regulating". The addition of Ca to AZ31 improves flame retardant properties and promotes the formation of Ca-containing second phases such as Al2Ca, potentially leading to a non-uniform microstructure with both coarse and fine equiaxed grains. Taking the microstructure used in this embodiment as an example, the average grain sizes of AZ31, AZ31-1Ca, and AZ31-1Ca-1La can reach approximately 21.35 μm, 10.66 μm, and 20.10 μm, respectively. However, comparing only the average grain size is insufficient to reflect its weld plasticity; microstructure uniformity and grain morphology are equally important.

[0040] Adding La to Ca-modified alloys allows for further control over elemental distribution and second-phase formation during solidification, homogenization, and extrusion. This results in more complete and rationally distributed second-phase formation, while simultaneously transforming the grain structure from the significantly heterogeneous state achieved with Ca alone to a more uniform equiaxed grain structure. This reduces strain concentration caused by localized microstructure differences and improves the plasticity of magnesium alloys under the thermo-mechanical coupling effect of friction stir welding.

[0041] In the magnesium-aluminum dissimilar friction stir welding process, the better microstructure uniformity and plastic coordination of the Ca-La synergistic modified magnesium alloy are beneficial to improving the plastic flow of the magnesium-side material, enabling a more stable material mixture between the magnesium side and the 6061 aluminum alloy side under the action of the stirring head. Simultaneously, by controlling the stirring head rotation speed and welding speed, the welding heat input is kept within an appropriate range, avoiding excessive interfacial reaction, thereby reducing the tendency of brittle Mg-Al intermetallic compounds to form a continuous, thick reaction layer. It should be noted that "reducing the formation tendency" in this specific embodiment does not mean completely eliminating Mg-Al intermetallic compounds, but rather controlling their formation degree and spatial distribution through the synergistic control of composition, microstructure, and welding parameters.

[0042] Therefore, this specific implementation method forms the following technical logic: Ca improves the flame retardant safety of AZ31, but may lead to uneven microstructure and loss of plasticity; La further regulates the second phase and grain structure on the basis of Ca modification, making the microstructure more uniform and improving the strength and toughness matching; the magnesium alloy with optimized microstructure is then subjected to friction stir welding with 6061 aluminum alloy, and the material flow and interface reaction are improved through the synergistic regulation of composition-microstructure-welding parameters, and finally a magnesium-aluminum hybrid structure suitable for lightweight battery trays is obtained.

[0043] The beneficial effects of this embodiment are:

[0044] This embodiment first utilizes Ca to improve the flame retardant properties of AZ31 magnesium alloy, and regulates the microstructure by forming Ca-containing second phases such as Al2Ca. Based on the Ca-modified AZ31 magnesium alloy, La is added. This is not alloying with La alone, but rather through the synergistic effect of Ca and La, further regulating the presence state of Al, the formation behavior of the second phase, and the grain structure of the magnesium alloy. The addition of La promotes more complete formation of the second phase in the alloy and changes the distribution state of Al, thereby reducing the tendency to form some brittle Mg-Al intermetallic compounds. Simultaneously, the addition of La can improve the heterogeneous microstructure of the Ca-modified magnesium alloy, where coarse and fine grains coexist, making the grain size and microstructure distribution more uniform, thus improving the material's deformation compatibility and ductility. Specifically:

[0045] (1) Adding Ca to AZ31 can improve the flame retardant properties of the alloy, providing a material basis for the application of magnesium alloys in safety-sensitive structures such as battery trays for new energy vehicles.

[0046] (2) Adding La to Ca modification can improve the unevenness of the microstructure and the loss of plasticity caused by Ca, making the grains and the second phase more uniformly distributed, which is conducive to obtaining a better strength and toughness match.

[0047] (3) Ca-La synergistic modification improves the plastic flow coordination ability of magnesium alloys in the friction stir welding process and weakens the adverse effect of the flow difference between magnesium and aluminum on joint formation.

[0048] (4) By controlling the welding heat input and stirring action, the tendency of Mg-Al brittle intermetallic compounds to form a continuous or thick reaction layer is reduced, which is beneficial to improving the integrity of the joint and the reliability of service.

[0049] (5) Friction stir welding is a solid-state bonding process. Compared with traditional fusion welding, it can avoid large-scale melting and solidification processes, which is beneficial to reducing problems such as porosity and solidification cracks in dissimilar metal fusion welding.

[0050] (6) The battery tray is made of magnesium alloy and 6061 aluminum alloy, which can reduce the amount of high-density aluminum alloy material while meeting the requirements of local load-bearing, connection and safety, thus achieving lightweighting from the structural level.

[0051] (7) The alloy preparation, hot extrusion and friction stir welding processes of this embodiment all have relatively clear process windows, which facilitates further engineering scale-up.

[0052] Therefore, this embodiment, through Ca-La synergistic alloying, further improves the microstructure uniformity, second phase distribution, and ductility and toughness of Ca-modified magnesium alloy while retaining the flame-retardant effect of Ca. The obtained Ca-La synergistic modified magnesium alloy is then used to perform friction stir welding with aluminum alloy, thereby improving the material flow and interfacial reaction during the magnesium-aluminum dissimilar metal welding process, enhancing the comprehensive mechanical properties of the magnesium-aluminum dissimilar weld joint, and further realizing its application in magnesium-aluminum hybrid battery trays for new energy vehicles.

[0053] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the magnesium alloy mentioned in step one is AZ31 magnesium alloy. Everything else is the same as in Specific Implementation Method One.

[0054] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the mass percentage of Ca in the ingot described in step two is 0.5%~1%, and the mass percentage of La is 0.5%~1%. Everything else is the same as in Specific Implementation Method One or Two.

[0055] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: in step two, the raw materials are melted under the conditions of a mixed atmosphere of Ar and SF6, a temperature of 650℃~710℃, and a stirring speed of 50r / min~100r / min. The melted material is then poured into a steel mold preheated to 100℃~200℃ and finally allowed to cool naturally to room temperature to obtain an ingot. The flow ratio of Ar to SF6 in the mixed atmosphere is 1:(0.3~0.5). The diameter of the ingot is 88mm~95mm. Everything else is the same as in Specific Implementation Methods One to Three.

[0056] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the homogenization treatment in step three is specifically carried out at a temperature of 400℃~410℃ for 20h~24h. Everything else is the same as in Specific Implementation Methods One to Four.

[0057] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that: in step three, surface oxides are removed to obtain an ingot with a diameter of 80mm~85mm to be extruded; the hot extrusion described in step three is specifically carried out at a temperature of 360℃~380℃ and an extrusion ratio of (25~31):1 to obtain a Ca-La synergistic modified magnesium alloy sheet; the cross-sectional dimensions of the Ca-La synergistic modified magnesium alloy sheet are (40~50)mm×(5~8)mm. Everything else is the same as in Specific Implementation Methods One to Five.

[0058] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that: the aluminum alloy sheet mentioned in step four is a 6061 aluminum alloy sheet or a 5052 aluminum alloy sheet; the aluminum alloy sheet mentioned in step four has the same thickness as the Ca-La synergistic modified magnesium alloy sheet. Everything else is the same as in Specific Implementation Methods One to Six.

[0059] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that, in step four, under the conditions of an air atmosphere, a stirring head rotation speed of 600 r / min to 1200 r / min, and a welding speed of 40 mm / min to 200 mm / min, the stirring head moves along the weld seam to perform friction stir welding. Everything else is the same as in Specific Implementation Methods One to Seven.

[0060] In this specific embodiment, during the welding process, the shoulder of the stirring head maintains stable contact with the surface of the plate, and the stirring needle extends into the welding area so that the action area of ​​the stirring head covers the interface area of ​​the two materials. By controlling the heat input and stirring action, the plastic flow and interface reaction of the materials are regulated. By rotating the stirring head and moving it along the weld direction, the materials on both sides undergo plastic flow and solid-state metallurgical bonding.

[0061] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: the welding joint described in step four is one or a combination of butt joint and lap joint; the weld seam described in step four is a straight weld seam, a curved weld seam, or a closed circumferential weld seam. Everything else is the same as in Specific Implementation Methods One to Eight.

[0062] Specific Implementation Method 10: This implementation method describes the application of magnesium-aluminum dissimilar alloys, which are used to prepare battery trays for electric vehicles.

[0063] This specific embodiment utilizes a magnesium-aluminum dissimilar alloy based on Ca-La synergistically modified magnesium alloy to construct an electric vehicle battery tray. According to the structural stress characteristics of the battery pack, the Ca-La modified magnesium alloy is used for the tray's bottom plate, side plates, or other areas suitable for weight reduction, while the aluminum alloy is used for the side beams, reinforcing areas, or connecting load-bearing areas. The magnesium alloy components and aluminum alloy components are connected by friction stir welding, and a continuous weld, segmented weld, or multi-pass weld is used to form an integral frame, depending on the tray dimensions.

[0064] The tray has an internal battery module mounting area, and is surrounded by side beams and reinforcing ribs. For areas under high stress, the structural load-bearing capacity can be increased by adding aluminum alloy reinforcements or increasing the welding length in certain areas; for areas that mainly bear the functions of the panel and protection, the proportion of magnesium alloy can be increased to further reduce the overall weight.

[0065] The beneficial effects of the present invention are verified using the following embodiments:

[0066] Example 1, combined with Figure 1 Detailed explanation:

[0067] A method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys, comprising the following steps:

[0068] 1. Weigh out magnesium alloy, MgCa 15 Alloys and AlLa 10 Alloy, to obtain raw materials;

[0069] The magnesium alloy mentioned is AZ31 magnesium alloy;

[0070] 2. Under the conditions of a mixed atmosphere of Ar and SF6, a temperature of 650℃~710℃ and a stirring speed of 100r / min, the raw materials are melted, then poured into a steel mold preheated to 200℃, and finally cooled naturally to room temperature to obtain an ingot.

[0071] The ingot contains 1% Ca and 1% La by mass.

[0072] The flow ratio of Ar to SF6 in the Ar and SF6 mixed atmosphere is 1:0.5; the ingot diameter is 90 mm.

[0073] 3. Under the condition of 410℃, the ingot is homogenized for 24 hours, and then the surface oxide is removed to obtain an ingot with a diameter of 85mm. Finally, the ingot is hot extruded at a temperature of 375℃ and an extrusion ratio of 28:1. Samples are taken along the extrusion direction to obtain Ca-La synergistic modified magnesium alloy plate, namely AZ31-1Ca-1La.

[0074] The cross-sectional dimensions of the Ca-La synergistic modified magnesium alloy plate are 40mm × 5mm;

[0075] 4. The aluminum alloy sheet and the Ca-La synergistic modified magnesium alloy sheet are butt-welded to form a welded joint. Then, under the conditions of air atmosphere, stirring head speed of 800r / min~1200r / min and welding speed of 100mm / min, the stirring head is moved along the straight weld seam to perform stirring friction welding to obtain magnesium-aluminum dissimilar alloy, namely AZ31-1Ca-1La / 6061.

[0076] The aluminum alloy sheet and the Ca-La synergistic modified magnesium alloy sheet have the same thickness.

[0077] The aluminum alloy sheet is a 6061 aluminum alloy sheet.

[0078] Comparative Experiment 1: This comparative experiment differs from Example 1 in that: in step one, magnesium alloy and MgCa were weighed. 15The alloy was obtained as raw material; the Ca content in the ingot mentioned in step two was 1% by mass; step three prepared Ca-modified magnesium alloy sheet, namely AZ31-1Ca; step four prepared magnesium-aluminum dissimilar alloy, namely AZ31-1Ca / 6061. Other steps were the same as in Example 1.

[0079] Comparative Experiment 2: This comparative experiment differs from Example 1 in that the Ca-La synergistic modified magnesium alloy sheet was replaced with AZ31 magnesium alloy in step four; the magnesium-aluminum dissimilar alloy, AZ31 / 6061, was prepared in step four. Everything else was the same as in Example 1.

[0080] Figure 2 The figure shows the combustion experiment of AZ31-1Ca-1La prepared in Example 1 at 1100℃. As can be seen from the figure, when the magnesium alloy is subjected to flame combustion at 1100℃, the resulting magnesium alloy does not exhibit combustion phenomena like traditional magnesium alloys. Therefore, the magnesium alloy obtained in the experiment has flame-retardant properties and can be used in battery tray applications.

[0081] Figure 3 Optical morphology and grain size diagrams of three magnesium alloy plates—AZ31, AZ31-1Ca prepared in Comparative Experiment 1, and AZ31-1Ca-1La prepared in Example 1—are shown. (a) AZ31, (b) AZ31-1Ca, (c) AZ31-1Ca-1La, (d) grain size. As can be seen from the figures, the average grain sizes of the AZ31, AZ31-1Ca, and AZ31-1Ca-1La alloys are 21.35 μm, 10.66 μm, and 20.10 μm, respectively. For AZ31 (… Figure 3 (a) Its grain structure is uneven, and flow marks parallel to the extrusion direction (ED) are visible; after adding 1 wt.% Ca ( Figure 3 (b) The grain structure of AZ31-1Ca is refined, consisting of a mixture of non-equiaxed grains and fine equiaxed grains; the number density of the second phase distributed along the ED direction is significantly increased; with the addition of Ca and La ( Figure 3 (c) The microstructure of the extruded AZ31-1Ca-1La mainly transforms into an equiaxed grain structure, the second phase density along the ED direction further increases, and the average grain size is slightly larger than that of AZ31-1Ca.

[0082] Figure 4 SEM and EDS images of AZ31, AZ31-1Ca prepared in Comparative Experiment 1, and AZ31-1Ca-1La prepared in Example 1 are shown. (a) AZ31, (b) AZ31-1Ca, (c) AZ31-1Ca-1La. As can be seen from the figures, the Ca-containing second phase is represented by Al2Ca, and the La-containing second phase is represented by Al... 11La3 is the representative, and the Al-Mn phase is represented by Al8Mn5, in the AZ31 alloy ( Figure 4 (a) shows the presence of Al- and Mn-rich particles, indicating the presence of an Al-Mn intermetallic compound, which was verified to be the Al8Mn5 phase. The AZ31 alloy contains only a small amount of sparsely distributed second-phase particles. After the addition of Ca ( Figure 4 (b) A novel Al₂Ca phase, rich in both Al and Ca, was observed. When Ca and La were added simultaneously ( Figure 4 (c) In addition to Al2Ca and Al8Mn5 particles, Al was also observed. 11 La3 particles.

[0083] Figure 5 The XRD patterns of the magnesium-aluminum dissimilar alloy friction stir welded joints in Example 1, Comparative Experiment 1, and Experiment 2 are shown under the condition that the stirring head speed is 900 r / min. The figures show that when Ca-La is added in a coordinated manner, the composition of the second phase changes, transforming into a multi-component interfacial reaction, which regulates the formation of the interfacial phase at the joint.

[0084] Figure 6 The figures show the cross-sectional structures of the magnesium-aluminum dissimilar alloy friction stir welded joints in Examples 1 and Comparative Experiments 1 and 2. (a) shows AZ31 / 6061 at a stirring head speed of 900 r / min, (b) shows AZ31-1Ca / 6061 at a stirring head speed of 900 r / min, (c) shows AZ31-1Ca-1La / 6061 at a stirring head speed of 900 r / min, and (d) shows AZ31-1Ca-1La / 6061 at a stirring head speed of 1000 r / min. As can be seen from the figures, the matrix materials at the joint between the AZ31 and AZ31-1Ca magnesium alloys and the aluminum alloy are not fully mixed. In the mixing zone, a continuous and thick IMCs (intermetallic compounds) layer is formed, which easily causes stress concentration and reduces the strength of the joint. After adding lanthanum, under the lanthanum-calcium synergy, the material is mixed more uniformly, and the thickness of the IMCs layer decreases, but the continuity is reduced.

[0085] Table 1 shows the mechanical properties of the magnesium-aluminum dissimilar alloy friction stir welded joints from Example 1 and Comparative Experiment 1. As can be seen from the table, when calcium is added alone, the tensile strength and elongation are extremely low, insufficient to achieve performance suitable for application. When calcium and lanthanum are added synergistically, the tensile strength and elongation increase, the strength meets the usage standards, and the elongation is greatly improved, enhancing the material's deformation coordination ability, making it suitable for areas requiring weight reduction and flame retardancy.

[0086] Table 1

[0087]

[0088] Figure 7This is an enlarged schematic diagram of a partial weld joint of the battery tray prepared using the magnesium-aluminum dissimilar alloy of Example 1. As shown in the figure, the Ca-La synergistically modified AZ31 magnesium alloy and 6061 aluminum alloy constitute a hybrid tray structure. 6061 aluminum alloy is used in areas with high load-bearing and connection requirements, while Ca-La modified magnesium alloy is used in areas requiring weight reduction and flame retardancy, thus reducing the weight of the traditional all-aluminum structure. After welding, the surface is ground smooth, with no obvious gaps.

Claims

1. A method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistically modified magnesium alloys, characterized in that... It is done in the following steps:

1. Weigh out magnesium alloy, MgCa 15 Alloys and AlLa 10 Alloy, to obtain raw materials; 2. The raw materials are melted and poured to obtain ingots; 3. The ingot is homogenized, then the surface oxide is removed, and finally hot extrusion is performed to obtain Ca-La synergistic modified magnesium alloy sheet. Fourth, assemble aluminum alloy sheet and Ca-La synergistic modified magnesium alloy sheet to form a welded joint, and then perform friction stir welding to obtain magnesium-aluminum dissimilar alloy, thus completing the friction stir welding method for magnesium-aluminum dissimilar alloy based on Ca-La synergistic modified magnesium alloy.

2. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... The magnesium alloy mentioned in step one is AZ31 magnesium alloy.

3. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... The mass percentage of Ca in the ingot mentioned in step two is 0.5%~1%, and the mass percentage of La is 0.5%~1%.

4. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... In step two, the raw materials are melted in a mixed atmosphere of Ar and SF6 at a temperature of 650℃~710℃ and a stirring speed of 50r / min~100r / min. The melted material is then poured into a steel mold preheated to 100℃~200℃ and finally cooled naturally to room temperature to obtain an ingot. The flow ratio of Ar to SF6 in the mixed atmosphere is 1:(0.3~0.5). The diameter of the ingot is 88mm~95mm.

5. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... The homogenization process described in step three is specifically carried out at a temperature of 400℃~410℃ for 20h~24h.

6. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... In step three, surface oxides are removed to obtain an ingot with a diameter of 80mm~85mm to be extruded; the hot extrusion in step three is specifically carried out at a temperature of 360℃~380℃ and an extrusion ratio of (25~31):1 to obtain Ca-La synergistic modified magnesium alloy sheet; the cross-sectional dimensions of the Ca-La synergistic modified magnesium alloy sheet are (40~50)mm×(5~8)mm.

7. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... The aluminum alloy sheet mentioned in step four is either 6061 aluminum alloy sheet or 5052 aluminum alloy sheet; the aluminum alloy sheet mentioned in step four has the same thickness as the Ca-La synergistic modified magnesium alloy sheet.

8. The method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 1, characterized in that... In step four, under the conditions of air atmosphere, stirring head rotation speed of 600 r / min to 1200 r / min and welding speed of 40 mm / min to 200 mm / min, the stirring head moves along the weld seam to perform friction stir welding.

9. A method for friction stir welding of magnesium-aluminum dissimilar alloys based on Ca-La synergistic modification of magnesium alloys according to claim 8, characterized in that... The welding joint mentioned in step four is one or a combination of butt joint and lap joint; the weld mentioned in step four is a straight weld, a curved weld, or a closed circumferential weld.

10. The application of the magnesium-aluminum dissimilar alloy prepared according to claim 1, characterized in that... It is used to manufacture battery trays for electric vehicles.

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