Method for improving joint strength by using composite plasma arc welding in dissimilar metal mig welding

Abstract

The method for improving joint strength by using composite plasma arc welding of dissimilar metal MIG welding is characterized in that a V-shaped groove is prepared on one side of a stainless steel welding plate, a U-shaped groove is formed on one side of a magnesium alloy welding plate, and a groove is formed in the middle section of the copper backing plate close to the stainless steel backing plate; the stainless steel welding plate is placed on the stainless steel backing plate, the magnesium alloy welding plate is placed on the copper backing plate to perform welding plate assembly, the program of robot control MIG welding gun and plasma arc welding gun is prepared before welding; then the stainless steel welding plate is heated at both ends of the stainless steel welding plate, the robot is started to perform MIG welding, the robot is started again to perform plasma arc welding on the weld for once, after the surface welding layer is melted and burned, the back of the weld is burned again, and the welding is finished; the welding process method realizes the connection of the magnesium alloy / stainless steel dissimilar metal, has low cost, high welding efficiency, small welding deformation and high welding strength.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of welding, and particularly relates to a method for improving joint strength by MIG welding of dissimilar metals and composite plasma arc welding. BACKGROUND

[0002] Magnesium alloy has the characteristics of low density, light weight, high specific strength and specific stiffness, and good shock absorption performance, and has significant effects on product structure lightweight, reduction of energy consumption, and reduction of environmental pollution, and its application on automobile body and chassis has become the goal of researchers. Stainless steel has the characteristics of high strength, good formability and corrosion resistance, and is widely used in modern manufacturing industry. In order to reduce the weight of the automobile and increase the strength, the thick carbon steel parts on the automobile can be replaced by magnesium alloy and stainless steel. The composite structure of magnesium alloy / stainless steel dissimilar metals can fully exert the excellent performance of the two metals, and has important practical value and practical significance. However, there are great differences in physical and metallurgical properties between the two kinds of metal materials, and the solubility of iron and magnesium is approximately equal to zero, so it is difficult to connect the two kinds of metals by using conventional melting welding process.

[0003] In recent years, magnesium alloy / stainless steel dissimilar metal connection is mostly achieved by laser-TIG composite brazing, friction stir welding, continuous drive friction welding and diffusion welding. However, in these welding methods, the laser-TIG composite brazing is inconvenient to add filler material, the welding process is complex, the cost is high, and the joint strength after welding is only 50%-85% of that of the magnesium alloy base material, which is difficult to be applied to automobile parts. The friction stir welding, continuous drive friction welding and diffusion welding are solid phase welding, and during the welding, the two kinds of metals are solid-solved or diffused, and since the solubility of magnesium and stainless steel is approximately equal to zero, no metallurgical reaction occurs at the interface, and only mechanical engagement occurs, so the joint strength after welding is low. Therefore, it is urgent to find an effective welding process to improve the strength of magnesium alloy / stainless steel dissimilar metal welded joint to meet the strength requirements of automobile parts. SUMMARY

[0004] In order to overcome the defects or deficiencies of the prior art, the purpose of the present application is to provide a method for improving joint strength by MIG welding of dissimilar metals and composite plasma arc welding, which has low cost, high welding efficiency, small welding deformation, and the thickness of intermetallic compound is easy to control, and can effectively improve the strength of the welded joint.

[0005] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0006] The method for improving joint strength by MIG welding of dissimilar metals and composite plasma arc welding comprises the following steps:

[0007] (1) Welding plate groove preparation: a notch is processed along the welding direction on the side of the stainless steel welding plate of the magnesium alloy / stainless steel dissimilar metal by wire cutting, and the edge thickness on both sides of the notch is 0.45mm-0.5mm; then, 40-60 mesh quartz sand is selected, the notch is cleaned by using a sandblasting machine, and residual sand particles are blown away by using a high-pressure air gun; then, the notch is filled with brass by using a flame brazing method; finally, a single V-shaped groove is processed on the side of the brass-filled surface along the thickness direction by using a milling machine, and the single side of the V-shaped groove is 25°±2°; a single U-shaped groove is opened on the side of the magnesium alloy welding plate in the magnesium alloy / stainless steel dissimilar metal, the single side of the single U-shaped groove is 15°±2°, the blunt edge length is 1.5-2.5mm, the blunt edge thickness is 0.5-0.8mm, the groove and the blunt edge are smoothly transitioned, and the transition arc radius is 0.2-0.3mm;

[0008] (2) Welding plate cleaning: 80-100 mesh brown corundum sand is selected, sandblasting treatment is performed on the 15mm-20mm range near the groove of the stainless steel and magnesium alloy welding plate by using a sandblasting machine, then residual sand particles are blown away by using a high-pressure air gun, and a homogeneous metal appearance with Ra0.8-3.2μm roughness is obtained;

[0009] (3) Welding plate assembly and clamping: first, two welding backing plates are placed close to each other along the length direction on the welding workbench, the left backing plate is made of 316 austenitic stainless steel, and the right backing plate is made of red copper; a groove is opened on the side of the red copper backing plate close to the stainless steel backing plate, and the red copper backing plate is provided with an air inlet and an air outlet at two ends respectively, the lengths of the air inlet and the air outlet are 25mm-26mm respectively, the air inlet and the air outlet are communicated with the groove, and are provided with threads; then, the two welding plates are placed on the welding backing plates, the stainless steel welding plate is placed on the stainless steel backing plate, and the magnesium alloy welding plate is placed on the red copper backing plate, the gap between the two welding plate grooves is 1.5mm-2.8mm, the center line of the gap between the two welding plates is coincided with the center line of the groove of the red copper backing plate, and finally, the two welding plates are clamped on the welding workbench by using a clamp;

[0010] The size specification of the two welding backing plates is: length×width×thickness=(150±1)mm×(60±1)mm×(16±1)mm;

[0011] The groove of the red copper backing plate is a semicircle with a diameter of 11mm-12mm, and the groove length is 98mm-100mm;

[0012] (4) Welding process parameter setting:

[0013] (1) Turn on the switches of the robot and the MIG welder, program the walking path program of the MIG welding gun carried by the welding robot, and then program the walking path program of the plasma arc welding gun;

[0014] (2) Welding robot parameter setting; the robot carries MIG welding gun swing mode: Z-shaped; welding gun angle: along the welding direction included angle 100°-110°, and the magnesium alloy welding plate included angle 80°-85°; welding gun swing width: 2.5mm-5.5mm; welding gun walking speed: 360mm-450mm / min; welding gun stays on both sides of the weld time: stainless steel side 1.0s-2s, magnesium alloy side 0-0.5s; the robot carries the plasma arc welding torch swing mode: V-shaped; welding gun angle: along the welding direction included angle 90°-95°, and the magnesium alloy welding plate included angle 70°-80°; welding gun swing width: 2.5mm-6mm; welding gun walking speed: 240mm-280mm / min; welding gun stays on both sides of the weld time: stainless steel side 1.0s-2s, magnesium alloy side: 0s;

[0015] (3) MIG welding parameter setting: the welding plate thickness is 3mm-4mm, single-layer welding is adopted, and the welding plate thickness is greater than or equal to 4mm, multi-layer welding is adopted; power type and polarity: direct current reverse connection; droplet transfer form: pulse jet transfer; pulse current (wire feed speed): 85A-145A (5m-8.6m / min); welding voltage: 23V-27V; nozzle distance 9mm-15mm; argon flow rate: 30L-60L / min; weld back protection argon flow rate: 5L-6L / min; welding wire: magnesium-aluminum alloy welding wire;

[0016] (4) Plasma arc welding parameter setting: power type and polarity: alternating current; melting current: 100A-120A; melting voltage: 28V-29V; nozzle distance 2.8mm-3mm; argon ion gas flow rate: 1.5L-2L / min; argon protection gas flow rate: 11L-13L / min;

[0017] (5) Start the robot for simulation welding: before formal welding, in order to test whether the robot welding program is normal, first carry out simulation welding of two welding methods, and then formally weld after observation; MIG welding machine and robot have communication between them, and when the robot welding is started, the MIG welding machine can automatically arc and collect arc; there is no communication between the plasma arc welding machine and the robot, and when the robot welding is started, the plasma arc welding gun button must be turned on at the same time, and after the welding is finished, the plasma arc welding gun button must be turned off;

[0018] (6) Preheating of the stainless steel welding plate: MZ-1000 submerged arc welding machine is selected as the heating power supply, and the positive and negative electrodes are connected to the two ends of the stainless steel welding plate in the length direction. The current of the submerged arc welding machine can be adjusted to control the heating temperature by adjusting the current and the on-time. The submerged arc welding power supply is turned on to heat the stainless steel welding plate before welding. The current adjustment range is 300A-1000A, and the heating time is 0.8min-1.1min. The purpose of preheating the stainless steel welding plate is to cause the pre-prepared brass surface at the side of the stainless steel welding plate to be slightly melted during welding;

[0019] (7) Starting the robot to perform automatic MIG welding and plasma arc welding: After the stainless steel welding plate is heated, the argon gas for welding and the argon shielding gas are turned on, and the robot is started to perform MIG welding. After each layer of MIG welding is completed, the interlayer temperature is 80℃-100℃, and the plasma arc welding program controlled by the robot is called to perform plasma arc welding. After the plasma arc welding is completed, if it is multi-layer welding, steps (5) and (7) are repeated immediately. In addition, the welding direction of the lower layer of the MIG welding is opposite to that of the upper layer.

[0020] (8) After the cover welding layer of the MIG welding in step (7) is completed, the plasma arc welding is used to melt and burn the back of the weld for one time, and the welding is completed.

[0021] In step (1), when the single-sided V-shaped groove is machined, the machining depth is 0.2mm-0.3mm less than the notch depth, so that a single-sided V-shaped brass surface groove can be prepared on one side of the stainless steel welding plate along the welding direction. The thickness of the single-sided V-shaped brass surface along the notch depth direction is 0.2mm-0.3mm, and the thickness along the plate thickness direction is the plate thickness minus 2 times the notch side edge thickness, i.e. the upper and lower surfaces of the stainless steel welding plate in the plate thickness direction are not prepared with brass surface, which is 0.45mm-0.5mm.

[0022] In step (1), the materials of the magnesium alloy welding plate and the stainless steel welding plate are AZ31 magnesium alloy and 316 austenitic stainless steel respectively. The size specification of the magnesium alloy welding plate is length×width×thickness=150mm×60mm×(≥3mm), and the size specification of the stainless steel welding plate is length×width×thickness=190mm×40mm×(≥3mm). The length of the stainless steel welding plate is longer than that of the magnesium alloy, and the width is narrower than that of the magnesium alloy. The purpose is to facilitate the connection of the electrode and the generation of larger resistance heat in the subsequent preheating step of the stainless steel welding plate.

[0023] In step (1), the notch is a slope in the plate thickness direction, with a depth of 2.5mm-3.0mm on one side of the plate thickness direction and a depth of 1.5mm-2.0mm on the other side. The notch is filled with brass, and the single-sided V-shaped groove angle is formed by machining the brass surface, while ensuring that the thickness of the brass surface along the notch depth direction is between 0.2-0.3mm.

[0024] The magnesium-aluminum alloy welding wire has a diameter of 1.2 mm, and the mass percentage of each element is as follows: aluminum 3.0%-3.9%, copper 2.5%-3.5%, zinc 1.8%-1.9%, manganese 0.5%-1.0%, silicon 0.3%-0.8%, nickel 2.9%-3.5%, and the balance is magnesium.

[0025] The single-layer welding is one layer of welding, and the multi-layer welding is base welding and cover welding, or base welding, filling welding and cover welding; and the welding parameters of different layers are selected from the set range parameters.

[0026] The plasma arc welding is to melt and burn each layer of welding seam after MIG welding without adding welding wire by using a fusion-type plasma arc.

[0027] The advantages of the present application are as follows:

[0028] The process method for improving joint strength of the magnesium alloy / stainless steel dissimilar metal MIG welding composite plasma arc welding obtained by the method of the present application is suitable for AZ31 magnesium alloy and any austenitic stainless steel, and the plate thickness is 3mm-12mm, and good connection of the magnesium alloy / stainless steel can be achieved. In the MIG welding process, metallurgical reaction occurs between the brass surface prepared on one side of the stainless steel welding plate groove and the magnesium element in the magnesium-aluminum welding wire, mainly generating Mg2Cu, CuMgZn intermetallic compound and Mg(Al,Zn) solid solution, and a nanoscale amorphous transition layer is also generated. The thickness of the generated intermetallic compound is between 1.2μm-1.6μm, and the thickness of the nanoscale amorphous transition layer is less than 100nm; in addition, in the plasma arc welding melting and burning process, the Fe element in the upper surface and the lower surface of the stainless steel welding plate reacts with the Ni and Al elements in the welding seam, mainly generating Fe 0.94 Ni 0.06 and Fe3Al intermetallic compounds. By selecting reasonable melting and burning parameters of the plasma arc welding, the thickness of the intermetallic compound is 1.0μm-1.2μm, and the hardness and brittleness are small. Therefore, the joint has a small tendency of thermal cracking, so that the strength of the joint is significantly improved. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a partial enlarged schematic view of the assembly welding of the present application.

[0030] Figure 2 It is a schematic view of the overall assembly welding of the present application. DETAILED DESCRIPTION

[0031] The present application will be described in detail below in combination with the drawings and examples.

[0032] The method for improving joint strength of dissimilar metal MIG welding composite plasma arc welding includes the following steps:

[0033] Referring to Figure 1 and Figure 2 , (I) the welding plate bevel preparation. The dissimilar metal of the present application is magnesium alloy and stainless steel, a notch is processed on one side of the thickness of the stainless steel welding plate 3 along the welding direction by wire cutting, the thickness of the edges on both sides of the notch is 0.45mm-0.5mm; then, 40-60 mesh quartz sand is selected, the notch is cleaned by a sandblasting machine, and the residual sand particles are blown away by a high-pressure air gun; then, the notch is filled with brass by flame brazing; finally, a single-sided V-shaped bevel 2 is processed on one side of the brass surface along the thickness direction by a milling machine, and the single-sided V-shaped bevel is 25°±2°. When the single-sided V-shaped bevel is processed, the processing depth is 0.2mm-0.3mm less than the depth of the notch. That is, a single-sided V-shaped brass surface bevel is prepared on one side of the stainless steel welding plate 3 along the welding direction, the thickness of the single-sided V-shaped bevel brass surface along the depth direction of the notch is 0.2mm-0.3mm, and the thickness along the plate thickness direction is the plate thickness-2×the thickness of the edges on both sides of the notch, that is, the stainless steel welding plate has no brass surface prepared on the upper and lower surfaces in the plate thickness direction, and the thickness is 0.45mm-0.5mm. A single-sided U-shaped bevel 4 is provided on one side of the magnesium alloy welding plate 5, the single-sided U-shaped bevel has a single-sided inclined surface angle of 15°±2°, a blunt edge length of 1.5-2.5mm, and a blunt edge thickness of 0.5-0.8mm, and the bevel and the blunt edge are smoothly transitioned, and the radius of the transition arc is 0.2-0.3mm.

[0034] The magnesium alloy welding plate 5 and the stainless steel welding plate 3 are respectively made of AZ31 magnesium alloy and 316 austenitic stainless steel, the size specification of the magnesium alloy welding plate 5 is length×width×thickness=150mm×60mm×(≥3mm), and the size specification of the stainless steel welding plate 3 is length×width×thickness=190mm×40mm×(≥3mm). The length of the stainless steel welding plate 3 is longer than that of the magnesium alloy, and the width is narrower than that of the magnesium alloy, so that the electrode is convenient to connect and the larger resistance heat is generated in the subsequent preheating step of the stainless steel welding plate 3.

[0035] The notch is a slope in the plate thickness direction, the depth of one side of the slope in the plate thickness direction is 2.5mm-3.0mm, and the depth of the other side is 1.5mm-2.0mm, the notch is filled with brass, and the single-sided V-shaped bevel angle is formed by processing the brass surface, while ensuring that the thickness of the brass surface along the depth direction of the notch is between 0.2mm-0.3mm.

[0036] (II) Welding plate cleaning. 80-100 mesh brown corundum sand is selected, and the sandblasting machine is used to sandblast the range of 15-20mm near the bevel of the stainless steel and magnesium alloy welding plate, and then the residual sand particles are blown away by a high-pressure air gun to obtain a homogeneous metal appearance with a roughness of Ra0.8-3.2μm.

[0037] (Three) welding plate group and clamping. First, two pieces of welding backing plate are placed on the welding workbench 16 along the length direction, the left backing plate is 316 austenitic stainless steel, and the right backing plate is a red copper backing plate 6. The red copper backing plate 6 is provided with a groove 8 on the side close to the stainless steel backing plate 7. The red copper backing plate is provided with an air inlet and an air outlet at two ends respectively. The length of the air inlet and the air outlet is 25mm-26mm respectively. The air inlet and the air outlet are communicated with the groove and are provided with threads. The threads are used to install the air inlet nozzle 9 and the air outlet valve 10. The air inlet nozzle is tapered and stepped. The air outlet valve is a needle valve, which controls the size of the air outlet. Then, the two welding plates are placed on the welding backing plate 6. The stainless steel welding plate 3 is placed on the stainless steel backing plate 7, and the magnesium alloy welding plate 5 is placed on the red copper backing plate. The gap between the two welding plate bevels is 1.5mm-2.8mm. The center line of the gap between the two welding plates coincides with the center line of the groove of the red copper backing plate. Finally, the two welding plates are clamped on the welding workbench 16 by using a caliper.

[0038] The size of the two welding backing plates is: length x width x thickness = (150±1)mm x (60±1)mm x (16±1)mm.

[0039] The groove of the red copper backing plate 6 is provided on the middle section of one side edge and is a semicircle with a diameter of 11mm-12mm. The length of the groove 8 is 98mm-100mm. The red copper groove has two functions. One is to provide a back surface for shielding gas, which ensures good back forming. The other is that the welding spatter on the red copper material is easy to remove.

[0040] The stainless steel welding plate 3 is placed on the stainless steel backing plate 7. Compared with magnesium alloy, the melting point of the stainless steel welding plate and the prepared brass surface on one side is higher. Therefore, more heat is needed for subsequent welding. The thermal conductivity of stainless steel material is small, and the heat dissipation is slow. Therefore, stainless steel material is selected as the backing plate of the stainless steel welding plate.

[0041] The magnesium alloy welding plate is placed on the red copper backing plate. When welding the magnesium alloy / stainless steel dissimilar materials, the stainless steel welding plate needs a larger heat input, and the magnesium alloy welding plate needs a relatively smaller heat input. However, in order to meet the needs of both, a larger welding current is used to obtain a high heat input of the stainless steel welding plate. Then, the fast heat dissipation of the red copper material is used to obtain a low heat input of the magnesium alloy welding plate. This setting has the function of adjusting the heat input of the two materials.

[0042] (Four) setting of welding process parameters.

[0043] (1) Turn on the switches of the robot 12 and the MIG welding machine 13, and program the walking path program of the MIG welding gun 13 carried by the welding robot 12. After programming, the walking path program of the plasma arc welding 11 welding gun is programmed separately.

[0044] (2) Welding robot parameter setting. The MIG welding gun carried by the robot 12 swings in the following manner: Z-shaped; the welding gun angle: 100°-110° along the welding direction, 80°-85° with the magnesium alloy welding plate; the welding gun swing width: 2.5mm-5.5mm; the welding gun walking speed: 360mm-450mm / min; the welding gun stays on both sides of the weld for 1.0s-2s on the stainless steel side and 0-0.5s on the magnesium alloy side. The plasma arc welding gun carried by the robot 12 swings in the following manner: V-shaped; the welding gun angle: 90°-95° along the welding direction, 70°-80° with the magnesium alloy welding plate; the welding gun swing width: 2.5mm-6mm; the welding gun walking speed: 240mm-280mm / min; the welding gun stays on both sides of the weld for 1.0s-2s on the stainless steel side and 0s on the magnesium alloy side.

[0045] (3) MIG welding parameter setting. For welding plates with a thickness of 3mm-4mm, single-layer welding is adopted, and for welding plates with a thickness of ≥4mm, multi-layer welding is adopted. Power type and polarity: direct current reverse connection; droplet transfer form: pulsed jet transfer; pulsed current (wire feed speed): 85A-145A (5m-8.6m / min); welding voltage: 23V-27V; nozzle distance: 9mm-15mm; argon flow rate: 30L-60L / min; back weld protection argon flow rate: 5L-6L / min; welding wire: magnesium-aluminum alloy welding wire.

[0046] (4) Plasma arc welding parameter setting. Power type and polarity: alternating current; arc current: 100A-120A; arc voltage: 28V-29V; nozzle distance: 2.8mm-3mm; argon ion gas flow rate: 1.5L-2L / min; argon protection gas flow rate: 11L-13L / min.

[0047] (5) Start the robot 12 for simulation welding. Before formal welding, in order to verify whether the welding program of the robot 12 is normal, simulation welding of the two welding methods is first carried out, and after observation, formal welding is carried out. The MIG welding machine 13 and the robot 12 have communication between them, and when the robot 12 is started for welding, the MIG welding machine 13 can automatically start and stop the arc. There is no communication between the plasma arc welding machine and the robot 12, and when the robot is started for welding, the plasma arc welding gun button must be turned on at the same time, and after welding is completed, the plasma arc welding gun button must be turned off.

[0048] (vi) Preheating of the stainless steel welding plate. A MZ-1000 submerged arc welding machine 15 is selected as the heating power source, and the positive and negative electrodes are connected to the two ends of the stainless steel welding plate in the length direction. The current of the submerged arc welding machine 15 is adjustable, and the heating temperature is controlled by adjusting the current and the on-time. The submerged arc welding power switch is turned on to heat the stainless steel welding plate before welding, the current is adjusted in the range of 300A-1000A, and the heating time is 0.8min-1.1min. The purpose of preheating the stainless steel welding plate is to cause the pre-prepared brass surface at the side groove of the stainless steel welding plate to be slightly melted during welding.

[0049] (vii) Starting the robot 12 to perform automatic MIG welding and plasma arc welding. After the stainless steel welding plate 3 is heated, the argon gas for welding and the argon shielding gas are turned on, and the robot 12 is started to perform MIG welding. After each layer of MIG welding is completed in single-layer welding or multi-layer welding, the interlayer temperature is 80°C-100°C, and the plasma arc welding program controlled by the robot is called to perform plasma arc welding. After the plasma arc welding is completed, if it is multi-layer welding, steps (v) and (vii) are immediately repeated. In addition, the welding direction of the lower layer of the MIG welding is opposite to that of the upper layer.

[0050] (viii) After the cover welding of the MIG welding and the plasma arc welding in step (vii) are completed, the back of the weld is welded again by plasma arc welding, and the welding is completed.

[0051] (ix) Post-weld inspection. The appearance inspection, penetration testing, and X-ray testing methods are used to inspect whether there are welding cracks and pores and other defects on the surface and inside of the weld. If no defects are detected, it indicates that the welding process method is feasible, the welding process parameters are reasonably selected, and the welding quality is good.

[0052] The magnesium-aluminum alloy welding wire is a specially customized welding wire 14 with a diameter of 1.2mm, and the mass percentage of each element is: aluminum 3.0%-3.9%, copper 2.5%-3.5%, zinc 1.8%-1.9%, manganese 0.5%-1.0%, silicon 0.3%-0.8%, nickel 2.9%-3.5%, and the balance is magnesium.

[0053] The single-layer welding is one layer of welding, and the multi-layer welding is base welding and cover welding, or base welding, filling welding, and cover welding. The welding parameters of different layers are selected from the range of the set parameters.

[0054] The MIG welding mainly melts the magnesium alloy welding plate and the welding wire, and at the same time, the brass at the side groove of the stainless steel welding plate is slightly melted under the auxiliary heating temperature of the stainless steel, and Mg2Cu, CuMgZn intermetallic compounds and Mg(Al,Zn) solid solution are generated, forming a pre-joining.

[0055] The plasma arc welding is to melt and burn the each layer of weld joint after MIG welding by using the fusion type plasma arc without adding welding wire. The purpose of melting and burning has three aspects: first, eliminating the pores and micro cracks formed by MIG welding filling wire; second, remelting the stainless steel welding plate side which is not completely melted by MIG welding to generate stable Mg2Cu, CuMgZn intermetallic compound and Mg(Al, Zn) solid solution, and generating nanoscale amorphous transition layer. The thickness of the generated intermetallic compound is between 1.2 μm-1.6 μm, and the thickness of the nanoscale amorphous transition layer is less than 100 nm; third, when the cover layer and the back welding layer are melted and burned, the upper surface and the lower surface of the stainless steel welding plate side are micro-melted. Since the brass is not prepared at the thickness of 0.45 mm-0.5 mm of the upper surface and the lower surface of the stainless steel during the preparation of the bevel, when the plasma arc melting and burning is carried out, the Fe in the stainless steel and the Ni and Al elements in the weld joint occur metallurgical reaction to form micron-scale Fe0.94Ni0.06 and FeAl3 intermetallic compounds on the upper and lower surfaces. In addition, by controlling the plasma arc penetration parameters during the melting and burning, the thickness of the intermetallic compound can be controlled, the thickness of the intermetallic compound is controlled between 1.0 μm-1.2 μm, and the brittleness is significantly reduced, thereby realizing the beneficial metallurgical bonding of the joint and significantly improving the joint strength.

[0056] The magnesium alloy / stainless steel dissimilar metal is completed by using the MIG welding composite plasma arc welding method. When MIG welding, the molten welding wire and the micro-melted brass on one side of the stainless steel bevel occur metallurgical reaction to form pre-bonding. Then, the plasma arc welding is used to melt and burn the weld joint, which not only eliminates the pores and micro cracks formed by MIG welding, but also makes the microstructure formed by MIG welding uniform and the metallurgical bonding more reliable. In addition, the plasma arc welding also makes the upper and lower surfaces of the stainless steel which are not prepared with brass occur metallurgical reaction, the thickness of the intermetallic compound formed by the metallurgical reaction is effectively controlled, and thus a high-quality weld joint is obtained. The process method has low cost, high welding efficiency, small welding deformation and ideal welding effect.

[0057] Example One

[0058] In this example, the thickness of the magnesium alloy and the stainless steel welding plate is 3 mm, according to the method of the present application, the two welding plates are not beveled, and the gap between the two plates is 1.5 mm. In addition, no brass surface is prepared on one side of the stainless steel welding plate, the stainless steel welding plate is not heated before welding, and the plasma arc welding is not carried out after MIG welding. The welding process parameters are shown in Tables 1 and 2.

[0059] Table 1 Welding robot process parameters

[0060]

[0061] Table 2 Welding process parameters

[0062]

[0063] Post-weld appearance inspection: the weld appearance is well formed, but the magnesium alloy and stainless steel welding plates are not effectively connected.

[0064] The joint tensile property is 0 MPa.

[0065] Comparative Example 1

[0066] The thickness of the magnesium alloy and the stainless steel welding plate in this example is 3 mm, and the two welding plates are not beched according to the method of the present application, the joint gap is 1.5 mm, and the welding process parameters are shown in Table 3 and Table 4. Different from Example 1, the brass surface is prepared on one side of the stainless steel welding plate, and the stainless steel welding plate is heated before welding, the heating current is 700 A, the heating time is 0.8 min, and the MIG welding is followed by plasma arc welding.

[0067] Table 3 Welding robot process parameters

[0068]

[0069] Table 4 Welding process parameters

[0070]

[0071] Post-weld appearance inspection: the weld appearance is well formed, and there is no obvious defect, and the connection is good.

[0072] According to the GB / T5097-2005 inspection standard, the welded joint is subjected to X-ray flaw detection nondestructive testing, the negative evaluation level is grade II, the weld internal quality is qualified, and the result meets the standard.

[0073] The performance of the welded joint of the example is as follows:

[0074] Tensile test:

[0075] Tensile test results (gauge length 25 mm)

[0076]

[0077]

[0078] Bend test: the bend test of the welded joint adopts the (GB / T2653-1989) standard, the bend radius is 10 mm, and the cold bend is 150°, and no cracks are found in the welded joint after bending.

[0079] Example 2

[0080] The thickness of the magnesium alloy and the stainless steel welding plate in this example is 6 mm. According to the method of the present application, a brass surface is prepared on one side of the stainless steel welding plate, and a groove is processed. The V-shaped groove of the stainless steel welding plate has a brass surface with a single-side angle of 23°, and the thickness of the brass surface is 0.2 mm-0.22 mm. The U-shaped groove of the magnesium alloy welding plate has a single-side bevel angle of 15°, a blunt edge length of 2.0 mm, a blunt edge thickness of 0.5 mm, and a groove-blunt edge transition arc radius of 0.2 mm. The gap between the two welding plate groups is 2.8 mm. The stainless steel welding plate is heated before welding at a temperature of 980℃±20℃ for 1.1 min. After MIG welding, no plasma arc welding is performed. The welding process parameters are shown in Tables 5 and 6.

[0081] Table 5 Welding robot process parameters

[0082]

[0083] Table 6 Welding process parameters

[0084]

[0085] Post-weld appearance inspection: The weld appearance is well formed, has no obvious defects, and is well connected.

[0086] According to the GB / T5097-2005 inspection standard, the welded joint is subjected to X-ray flaw detection non-destructive testing. Obvious pores and incomplete fusion are visible inside the weld, and the internal quality of the weld is unqualified.

[0087] The performance of the welded joint of the example is as follows:

[0088] Tensile test:

[0089] Tensile test results (gauge length 25 mm)

[0090] Specimen b (MPa) ​ δ (%) Fracture condition Magnesium alloy base material 238±4 5.2±0.3 Base material Welded joint specimen 137±7 1.1±0.6 Fracture in weld

[0091] Bend test: The bend test of the welded joint is performed according to the (GB / T2653-1989) standard, with a bend radius of 10 mm and a 150° cold bend. After bending, cracks appear in the welded joint.

[0092] Comparative Example Two

[0093] The thickness of the magnesium alloy and the stainless steel welding plate in this example is 6mm. According to the method of the present application, a brass surface is prepared on one side of the stainless steel welding plate, and a groove is processed. The V-shaped groove of the stainless steel welding plate has a single-side angle of 23°, the thickness of the brass surface is 0.2mm-0.22mm, the U-shaped groove of the magnesium alloy welding plate has a single-side angle of 15°, the length of the blunt edge is 2.0mm, the thickness of the blunt edge is 0.5mm, and the transition radius of the groove and the blunt edge is 0.5mm. The gap between the two welding plates is 2.8mm. The stainless steel welding plate is heated before welding at a temperature of 980℃±20℃ for 1.1min. Different from example two, MIG welding is followed by plasma arc welding, and the welding robot and welding process parameters are shown in Table 7 and Table 8.

[0094] Table 7 Welding robot process parameters

[0095]

[0096] Table 8 Welding process parameters

[0097]

[0098] Post-weld inspection: the weld appearance is well formed, has no obvious defects, and the connection is good.

[0099] According to the X-ray flaw detection non-destructive testing standard of GB / T5097-2005, the bottom plate evaluation level of the welded joint is grade II, the internal quality of the weld is qualified, and the result meets the standard.

[0100] The performance of the welded joint of the example is as follows:

[0101] Tensile test:

[0102] Tensile test results (gauge length 25mm)

[0103] Specimen b (MPa)]]> ​ δ (%) Fracture condition Magnesium alloy base material 238±4 5.2±0.3 Base material Welded joint specimen 231±8 3.5±0.5 Fracture in magnesium alloy base material

[0104] Bend test: the bend test of the welded joint adopts the standard of (GB / T2653-1989), the bend radius is 10mm, and the cold bend is 150°. After bending, no cracks are found in the welded joint.

Claims

1. A method of improving joint strength in dissimilar metal MIG welding composite plasma arc welding, characterized by, The method comprises the following steps: (1) Welding plate bevel preparation: a notch is processed on the side of the stainless steel welding plate of the magnesium alloy / stainless steel dissimilar metal along the welding direction by using wire cutting, and the thickness of the notch on both sides is 0.45mm-0.5mm; then, 40-60 mesh quartz sand is selected, the notch is cleaned by using a sand blasting machine, and residual sand particles are blown away by using a high-pressure air gun; then, the notch is filled with brass by using flame brazing; finally, a single V-shaped groove is processed on the side of the brass-filled surface along the thickness direction by using a milling machine, and the single V-shaped groove is 25°±2°; a single U-shaped groove is formed on the side of the magnesium alloy welding plate of the magnesium alloy / stainless steel dissimilar metal, the single U-shaped groove is 15°±2°, the blunt edge length is 1.5mm-2.5mm, the blunt edge thickness is 0.5mm-0.8mm, the groove and the blunt edge are smoothly connected, and the radius of the transition arc is 0.2mm-0.3mm; (2) Welding plate cleaning: 80-100 mesh brown corundum sand is selected, the sand blasting machine is used to perform sand blasting treatment on the range of 15mm-20mm near the bevel of the stainless steel and magnesium alloy welding plate, and then the residual sand particles are blown away by using a high-pressure air gun to obtain a homogeneous metal appearance with a roughness of Ra0.8-3.2μm; (3) Welding plate assembly and clamping: two welding backing plates are placed close to each other along the length direction on the welding workbench, the left backing plate is made of 316 austenitic stainless steel, the right backing plate is made of red copper, a groove is formed on the side of the red copper backing plate close to the stainless steel backing plate, the red copper backing plate is provided with an air inlet and an air outlet at two ends respectively, the lengths of the air inlet and the air outlet are 25mm-26mm, the air inlet and the air outlet are communicated with the groove and are provided with threads; then, the two welding plates are placed on the welding backing plates, the stainless steel welding plate is placed on the stainless steel backing plate, the magnesium alloy welding plate is placed on the red copper backing plate, the gap between the two welding plate bevels is 1.5mm-2.8mm, the center line of the gap between the two welding plates is coincided with the center line of the groove of the red copper backing plate, and finally, the two welding plates are clamped on the welding workbench by using a clamp; (4) Welding process parameter setting: (1) Turn on the switches of the robot and the MIG welder, program the walking path program of the MIG welding gun carried by the welding robot, and then program the walking path program of the plasma arc welding gun; (2) Welding robot parameter setting; the robot carries MIG welding gun swing mode: Z-shaped; welding gun angle: along the welding direction included angle 100°-110°, and the magnesium alloy welding plate included angle 80°-85°; welding gun swing width: 2.5mm-5.5mm; welding gun walking speed: 360mm / min-450mm / min; welding gun stays on both sides of the weld time: stainless steel side 1.0s-2s, magnesium alloy side 0-0.5s; the robot carries the plasma arc welding gun swing mode: V-shaped; welding gun angle: along the welding direction included angle 90°-95°, and the magnesium alloy welding plate included angle 70°-80°; welding gun swing width: 2.5mm-6mm; welding gun walking speed: 240mm / min-280mm / min; welding gun stays on both sides of the weld time: stainless steel side 1.0s-2s, magnesium alloy side: 0s; (3), MIG welding parameter setting: thickness of 3mm-4mm welding plate, using single-layer welding, thickness ≥4mm welding plate, using multi-layer welding; power type and polarity: DC reverse connection; droplet transfer form: pulse jet transition; pulse current (wire feed speed): 85A-145A (5m-8.6m / min); welding voltage: 23V-27V; nozzle distance 9mm-15mm; argon flow: 30L / min-60L / min; weld back protection argon flow: 5L / min-6L / min; Welding wire: magnesium-aluminum alloy welding wire; (4), plasma arc welding parameter setting: power type and polarity: AC; melting current: 100A-120A; melting voltage: 28V-29V; nozzle distance 2.8mm-3mm; argon ion gas flow: 1.5L / min-2L / min; argon protection gas flow: 11L / min-13L / min; (Five), start the robot for simulation welding: before formal welding, in order to test whether the robot welding program is normal, first carry out simulation welding of two kinds of welding methods, after observation, formal welding; MIG welding machine and robot between the establishment of communication, start the robot welding, MIG welding machine can automatically arc and arc; plasma arc welding machine and robot between the no communication, start the robot welding, must simultaneously open the plasma arc welding gun button, after welding, must turn off the plasma arc welding gun button; (Six), stainless steel welding plate before welding heating: choose MZ-1000 submerged arc welding machine as heating power, the positive and negative are connected to the two ends of the stainless steel welding plate length direction respectively; submerged arc welding machine current can be adjusted, by adjusting the current and on time control heating temperature; before welding open submerged arc welding power switch heating stainless steel welding plate, current adjustment range is 300A-1000A, heating time 0.8min-1.1min; the purpose of stainless steel welding plate before welding heating: make the brass surface prepared at the slope of stainless steel welding plate side melt slightly during welding; (Seven), start the robot for automatic MIG welding and plasma arc welding: stainless steel welding plate heating, immediately open the welding argon and argon shielding gas, and start the robot for MIG welding; single layer welding or multi-layer welding after each layer MIG welding, when the interlayer temperature is 80-100 ℃, call the robot control plasma arc welding program to carry out plasma arc melting; after the completion of plasma arc melting, if it is multi-layer welding, then immediately repeat (five) and (seven) steps; in addition, the MIG welding direction of the lower layer weld is opposite to that of the upper layer weld. (Eight), after the completion of the MIG welding of the cover surface in step (seven) and the plasma arc welding of the cover surface layer, the plasma arc welding is used to melt and burn the back of the weld for one time, and the welding is completed.

2. The method of hybrid MIG welding composite plasma arc welding to improve joint strength of dissimilar metals of claim 1, wherein, In step (one), when the single-sided V-shaped groove is processed, the processing depth is 0.2-0.3 mm smaller than the depth of the gap, so that a single-sided V-shaped brass surface groove can be prepared on one side of the stainless steel welding plate along the welding direction. The thickness of the single-sided V-shaped brass surface along the depth of the gap is 0.2-0.3 mm, and the thickness along the plate thickness direction is the plate thickness minus 2 times the thickness of the gap on both sides, that is, the upper and lower surfaces of the stainless steel welding plate in the plate thickness direction are 0.45-0.5 mm without brass surface.

3. The method of hybrid metal MIG welding composite plasma arc welding to improve joint strength according to claim 1, characterized in that, In step (one), the magnesium alloy welding plate and the stainless steel welding plate are respectively made of AZ31 magnesium alloy and 316 austenitic stainless steel. The size specification of the magnesium alloy welding plate is length x width x thickness = 150 mm x 60 mm x (≥3 mm), and the size specification of the stainless steel welding plate is length x width x thickness = 190 mm x 40 mm x (≥3 mm). The length of the stainless steel welding plate is longer than that of the magnesium alloy, and the width is narrower than that of the magnesium alloy, so that the electrode can be easily connected and a larger resistance heat can be generated in the subsequent preheating step of the stainless steel welding plate.

4. The method of hybrid MIG welding composite plasma arc welding to improve joint strength of dissimilar metals of claim 1, wherein, In step (one), the gap is a slope in the plate thickness direction, the depth of one side of the slope in the plate thickness direction is 2.5-3.0 mm, and the depth of the other side is 1.5-2.0 mm. The gap is filled with brass, and the single-sided V-shaped groove angle is formed by processing the brass surface, while ensuring that the thickness of the brass surface along the depth of the gap is between 0.2 mm and 0.3 mm.

5. The method of hybrid MIG welding composite plasma arc welding to improve joint strength of dissimilar metals of claim 1, wherein, The diameter of the magnesium-aluminum alloy welding wire is 1.2 mm, and the mass percentage of each element is: aluminum 3.0%-3.9%, copper 2.5%-3.5%, zinc 1.8%-1.9%, manganese 0.5%-1.0%, silicon 0.3%-0.8%, nickel 2.9%-3.5%, and the balance is magnesium.

6. The method of hybrid metal MIG welding composite plasma arc welding to improve joint strength according to claim 1, wherein, The single layer welding is one layer welding, and the multi-layer welding is base welding and cover welding, or base welding, filling welding and cover welding. Different layer welding parameters are selected from the set range parameters.

7. The method of hybrid metal MIG welding composite plasma arc welding to improve joint strength according to claim 1, wherein, The plasma arc welding is a melting type plasma arc without adding welding wire for each layer of MIG welded weld.

8. The method of hybrid metal MIG welding composite plasma arc welding to improve joint strength according to claim 1, wherein, The size specification of the two welding backing plates is length x width x thickness = (150±1) mm x (60±1) mm x (16±1) mm. The copper backing plate groove is located in the middle section of one side edge, which is a semicircle with a diameter of 11-12 mm, and the groove length is 98-100 mm.

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