Magnesium alloy welding is a common process in the welding industry. When magnesium alloys are welded, the process is actually very complicated. At present, there are ten process options for magnesium alloy welding. Next, let's take a look at the ten processes, as follows:

1. TIG welding

TIG welding is the most commonly used welding method for welding magnesium alloys. It uses arc heat to melt base metal and filler metal under the maintenance of inert gas. The reverse polarity connection method should be used when the DC power supply is welded, so as to use the cathode atomization effect to damage, remove the oxide film on the surface of the base metal, and reduce or prevent the inclusion of oxides in the weld. The size and deformation of the heat-affected zone of argon arc welding are relatively small, and the mechanical function and corrosion resistance of the weld are also relatively high.

The TIG welding method can weld magnesium alloys with or without filler metal. Since the electrode and the filler wire are independent, it can overcome the shortcomings of the narrow standard welding scale of the MIG method, and can perform stable welding under wider process conditions. TIG welding is more widely used in the welding of magnesium alloys than MIG, and is especially suitable for the welding of magnesium alloy sheets. However, due to the large thermal expansion coefficient of magnesium alloys, it is prone to welding cracks, post-weld deformation and other shortcomings. Therefore, it is necessary to use fixture fixing, groove treatment, and pre-weld and post-weld heat treatment methods to ensure good welded joints. The research found that after using the communication TIG method to weld AZ31B magnesium alloy sheet, there are mainly defects such as wave deformation, post-welding misalignment, welding bulge, surface "pocket" phenomenon and arc crater cracks. After adjusting the welding sequence, use high current and fast welding And rigid fixing and other methods can obtain better welded joints, and the joint strength can reach more than 80% of the base metal.

Regarding the welding of magnesium alloy thick plates, in order to obtain a larger penetration depth, many studies have focused on active tungsten argon arc welding (A 2 TIG). This method is to apply a single active agent TiO 2 or chloride (LiCl, CaCl 2, CdCl 2, PbCl 2, CeCl 3 ) on the surface of the material to be welded before welding, and then apply welding, which can make the weld penetration deeper than conventional TIG When the welding is added twice, the microstructure of the joint is not significantly different from that when it is not coated, the weld fusion is excellent, and there are no defects such as cracks, pores, and slag inclusions. The principle is that adding an active agent can increase the arc voltage and arc temperature, and increase the arc width in the welding direction, so that the increase in heat input during the welding process is accompanied by re-distribution of the warm current.

Magnesium alloy TIG welding generally uses a communication welder or a DC welder with continuously adjustable amperage. When welding thin plates, AC or DCEP power sources can be selected; when welding magnesium alloys with a thickness greater than 418mm, the AC welding machine has an advantage due to its larger penetration depth. In addition, when using AC welding, it is generally necessary to superimpose a high-frequency pulse current to stabilize the arc, but if a square wave AC is used, there is no need to superimpose a high-frequency current, and a strong cathode atomization effect can be produced.

The selection of electrodes mainly depends on the type of power source used and the size of the welding current. Generally speaking, pure tungsten electrodes, zirconium tungsten electrodes and thorium tungsten electrodes with a diameter of 0.25 mm to 6.35 mm are often used for TIG welding.

2. Melting electrode argon arc welding

The MIG welding method for welding magnesium alloys has the following characteristics:

① Compared with TIG welding, the welding speed is fast, the productivity is high, and the automatic welding speed is as high as about 1 m/min;

② Due to the use of welding wire as the electrode, the suitable welding standard is narrow;

③ Due to the small surface tension of the molten magnesium, the droplets at the front end of the electrode wire are difficult to detach, and when the welding current is too high, the droplets explode and evaporate to form splashes;

④ Due to the soft electrode wire and poor wire feeding stability, special wire feeding equipment with push-pull method should be selected during the welding process;

⑤ Welding wires with a diameter of less than 1.6mm are rarely available in the market, and it is difficult to find suitable welding wires for workpieces with a welding thickness of less than 2mm.

There are three types of droplet transfer during MIG welding of magnesium alloys: short-circuit transfer, pulse eruption transfer and eruption transfer.

Which transition form occurs during welding depends on many factors, including the melting speed of the wire, the welding current, the wire feed speed, and the diameter of the wire. Meanwhile, the pulse eruption transition is between the short-circuit transition and the eruption transition, and a pulse current is required to complete it. Otherwise, under the conditions of a specific current range, wire feeding speed and spherical end face of the welding wire, a coarse drop transition form is obtained, the arc is unstable, and spatter is likely to occur.

If the line energy required for pulse eruption transition is less than that of successive eruption transition, it is suitable for medium thickness plates; short-circuit transition is suitable for thin plate welding; eruption thickness is suitable for thick plate welding. MIG arc welding of magnesium alloys generally uses DCEP power source, constant voltage source can be used for short-circuit transition and most eruption transitions; constant current source is used for eruption transition, which is beneficial to reduce spatter. For pulse MIG arc welding, it is necessary to use a special pulse current constant voltage source.

Studies have shown that with appropriate welding power and heat input, the static load strength of magnesium alloy joints can be approximately equal to the strength of the base metal. After removing the excess weld height, the fatigue strength is 75% higher than that of the base metal.

Melting electrode and non-melting electrode TIG welding wire selection:

WE-33M magnesium alloy welding wire was developed by American R&D Industrial Company in 1987. It is used to process various deformed magnesium alloys and cast magnesium alloys in repair. , mainly used for welding common magnesium alloys such as AZ31, AZ61, ZA91, AZ93, etc., and are mostly used in kitchen utensils, auto parts, bicycles, aerospace and other fields.

WE-33M magnesium alloy welding wire is suitable for gas welding and TIG arc welding of various forged magnesium alloys and casting magnesium alloys. It has good crack resistance for common magnesium alloys, and the welding layer can be suitable for heat treatment.

WE-33M is used for various forged magnesium alloys and cast magnesium alloys. It is widely used in the welding of optical instruments, aerospace, auto parts and civil magnesium products and handicrafts. Magnesium alloy welding wire.

Safety regulations for argon arc welding of magnesium alloys:

1) It is necessary to have fire prevention equipment at the welding site, such as sand boxes, fire extinguishers, fire hydrants, water buckets, etc. The distance between flammable materials and welding places shall not be less than 5m. If the regular interval cannot be met, it can be properly covered with asbestos board, asbestos cloth, etc. to prevent sparks from falling into flammable objects. The interval between explosives and welding places shall not be less than 10m. The argon arc welding site should have excellent natural ventilation and fixed mechanical ventilation equipment to reduce the damage of argon arc welding harmful gases and metal dust.

2) The manual tungsten argon arc welding machine should be placed in a dry and ventilated place, and operate strictly in accordance with the operating instructions. A comprehensive inspection of the welding machine should be carried out before use. Assuming that there is no danger, turn on the power again. Welding can be performed only after the no-load operation is normal. Make sure the welding machine is wired correctly, it is necessary to have a good, solid ground to ensure safety. The on and off of the power supply of the welding machine is controlled by the switch on the power board. It is forbidden for the load to move the switch to prevent the shape contact from burning.

3) The operation of the cooling water system of the argon arc welding torch should be checked frequently, and if any blockage or leakage is found, it should be dealt with immediately to prevent the torch from being burned and the welding quality being affected.

4) When the welder leaves the workplace or the welding machine is not in use, it is necessary to cut off the power supply. If the welding machine breaks down, it should be repaired by professionals, and safety measures such as preventing electric shock should be taken during maintenance. Welders should be dusted and cleaned at least once a year.

5) The high-frequency electromagnetic field generated by the high-frequency oscillator of the tungsten argon arc welding machine will cause certain dizziness and fatigue. Therefore, the moment of high frequency electromagnetic field effect should be minimized during welding, and the high frequency power supply should be cut off immediately after the arc is ignited. The welding torch and welding cable are shielded with soft metal braided wire (one end of the hose is connected to the welding torch, the other end is grounded, and the outside is not insulated). If possible, use crystal pulse arc ignition instead of high frequency arc ignition as much as possible.

6) During argon arc welding, the intensity of ultraviolet rays is very high, which can easily cause electro-optic ophthalmia and arc burns, and together with ozone and nitrogen oxides, affect the respiratory tract. Therefore, welders should wear white canvas work clothes, masks, face shields, protective gloves, foot covers, etc. when operating. In order to prevent electric shock, insulating rubber should be covered on the ground adjacent to the workbench, and workers should wear insulating rubber shoes.

3. Plasma arc welding

Plasma arc is a bound non-free arc, also known as compression arc. Its temperature and energy density are significantly higher than those of ordinary arcs, and its penetrating power is strong, which is suitable for thick plates and large arc length requirements. When using plasma arc welding to weld magnesium alloys, one-time full penetration of the thick plate butt joint can be completed without a backing plate on the reverse side, and the surface of the weld seam is lubricated, showing excellent fatigue mechanical properties. Studies have shown that the adjustable welding parameter range of variable polarity plasma arc welding of magnesium alloys is relatively narrow, and the influence of parameter changes is greater. Changing the time-to-width ratio of positive and negative polarities will change the cathode finishing effect of the workpiece, which will have a certain impact on the tensile strength of the joint. After reasonable selection of welding parameters, the ideal welding effect can be obtained, and the joint strength can reach more than 90% of the base metal.

4. Gas welding

The heat source of gas welding is flame (composed of mixed combustion of oxygen and gas), the heat is not concentrated, the heated area of ​​the weldment is wide, and it simply causes a large shortening stress in the joint area, forming cracks and other shortcomings. At the same time, the flux remaining in the weld is prone to slag inclusion and corrosion, so gas welding is mainly used for welding repairs on sites without suitable fusion welding equipment or less important thin-plate components and castings. Magnesium and magnesium alloy gas welding can use QJ401 flux. The experiment shows that the flux is good in technology, but it is highly corrosive to magnesium. It should be thoroughly cleaned after welding. When welding magnesium alloy parts with a thickness of less than 3 mm, the gas welding torch and welding wire should move longitudinally, and lateral swing should not be used. When the thickness of the weldment is large, the gas welding torch and welding wire are allowed to sway slightly laterally. For weldments with a thickness greater than 5 mm, the whole or part of the weld should be preheated to 300 ℃ ~ 400 ℃ before welding; when the thickness is greater than 12 mm, multi-layer welding can be used. Generally, fine brass should be used before welding the next layer. Wire brush to remove welding slag. During the welding process, the welding wire can be used to continuously stir the molten pool to damage the oxide film on the surface of the molten pool and lead the welding slag out of the molten pool.

5. Electron beam welding

The electron beam has high energy density, strong penetrating power, fast welding speed, low heat input, narrow weld bead width and heat affected zone, large weld bead penetration, small deformation, and high weld purity. When welding magnesium alloys, magnesium vapor will be generated immediately under the electron beam, and the molten metal will then enter the small holes generated. Due to the low melting point and high vapor pressure of magnesium alloys, the pores generated are also larger than other metals, and the pores are simply formed at the root of the weld, so a set of accurate operation processes are required to prevent pores and overheating. During the welding process, the circumferential swing of the electron beam and the adjustment of the focus point orientation are beneficial to eliminate pores and obtain high-quality welds. In addition, pre-placing a homogeneous filler metal around the weld and using a tightly fitted gasket on the reverse side can reduce porosity. Welding by adding wire method can easily obtain welds without defects such as shrinkage porosity, shrinkage holes and pores. The static load strength of the joint can be appropriate to that of the base metal, and the corrosion resistance of the joint is even better than that of the base metal.

Electron beam welding is generally carried out in a vacuum chamber, but the evaporation of metal when welding magnesium alloys pollutes the vacuum chamber greatly, which limits its application. For example, there are few practical applications. There are examples of research on AZ3l magnesium materials, indicating that Excellent welding effect. Studies have shown that non-vacuum electron beams can be used for the welding of magnesium alloys. For AZ31 wrought magnesium alloys, AM50A and AZ91D cast magnesium alloys, excellent joints can be obtained under appropriate welding processes. The relatively high energy density allows welding speeds of up to 15 m/min, low heat input and high welding power. Non-vacuum electron beam welding can obtain excellent weld formation, which is beneficial to improve the fatigue strength of the joint. The high-speed, high-efficiency and highly automated non-vacuum electron beam welding method is expected to ensure the wide application of magnesium alloy structural parts.

The shape of the electron beam welding seam is greatly affected by the welding parameters, especially the size of the current. As the current increases, the width of the weld and heat affected zone also increases. Studies have shown that different welding methods are used for AZ91D alloy, and it is found that the mechanical properties of electron beam welded joints are the highest and higher than those of the base metal, which is mainly related to the very fine grains in the weld zone and the narrow heat affected zone.

6. Laser welding

Laser welding is an efficient and fine processing method that uses a high energy density laser beam as a heat source for welding. Compared with other fusion welding methods, laser welding has the advantages of high energy density, less heat input, small residual stress and deformation in the joint area, narrow melting zone and heat-affected zone, large penetration depth, fine welding seam arrangement, and good joint performance. In addition, laser welding does not require vacuum conditions, and the type of maintenance gas and pressure range can be easily selected. The deflecting prism or optical fiber can guide the laser beam to inaccessible parts for welding, the operation is flexible, and it can pass through transparent materials to gather welding and so on. All are difficult to have in electron beam welding. The laser beam can be controlled sensitively, and it is easy to complete the three-dimensional automatic welding of workpieces. The research shows that the strength of the laser welding seam of deformed magnesium alloy can be close to that of the base metal, and the occurrence of pores and undercuts can be prevented by selecting appropriate process parameters.

7. Laser-TIG hybrid welding

Laser-TIG hybrid heat source welding was proposed in 1970, but the real application did not appear until recent years, mainly due to the development of laser technology and arc welding equipment, especially the improvement of laser power and current control technology. Laser arc recombination has a very obvious improvement in welding power. This is mainly based on two effects: one is that the higher energy density leads to a higher welding speed and the convective loss of the workpiece is reduced; the other is the superposition effect of the mutual effects of the two heat sources. During welding, the laser-induced plasma makes the arc more stable, and at the same time, the arc can enter the small hole in the molten pool, reducing energy loss.

Laser-TIG hybrid welding can significantly increase the welding speed, which is about 2 times that of TIG welding, and the tungsten electrode burning loss is greatly reduced, and the life is increased; the groove angle is also reduced, and the weld width is close to the laser welding time. The Welding Technology Research Institute of Dalian University of Technology in China has developed a laser-TIG hybrid welding equipment with independent intellectual property rights, which can effectively combine laser welding and argon arc welding, give full play to their respective strengths, and further improve their generalization functions. High-speed welding. Using the laser argon arc composite heat source welding process, high-quality welded joints can be obtained. The tensile strength, fatigue strength and impact toughness of the joints are all appropriate to the base metal. Compared with the currently selected argon arc welding method, the joint performance (especially the fatigue strength) , impact toughness) has improved significantly.

8. Resistance spot welding

Both magnesium alloy sheets and extruded parts can be welded by conventional resistance welding, such as seam welding, spot welding and bright butt welding, of which spot welding is the most common. The resistance welding performance of Mg2 Al and Mg2 Zn alloys is better. Resistance spot welding is generally used for welding workpieces under low load. For example, some magnesium alloy frames, instrument cabins, partitions, etc. are often used for resistance spot welding. As long as the power of the welding machine can ensure instantaneous and rapid heating, the DC pulse spot welding machine and the general communication spot welding machine can be suitable for spot welding of magnesium alloys.

The process characteristics of magnesium alloy resistance spot welding are as follows:

(1) Magnesium alloys have excellent electrical and thermal conductivity. When spot welding, a large current must be passed in a short time;

(2) The surface of magnesium is easy to be oxidized, and the touch resistance between the welded surfaces is relatively large. When a large welding current is passed, spatter often occurs;

(3) Due to good thermal conductivity and large coefficient of linear expansion, the nugget cools down quickly after power failure, which is easy to cause shrinkage holes and cracks.

9. Friction welding

Now, cast magnesium alloys, especially die-cast magnesium alloys, are widely used. However, many remaining micro-pores are a fatal problem in die-casting alloy products. These pores are aggregated and grown due to heating, which seriously affects the mechanical properties of the alloy. Therefore, it is generally difficult to obtain ideal welds for fusion welding of such magnesium alloys. Therefore, the conflict welding of magnesium alloys has become one of the hottest topics.

Friction mixing welding is to use a mechanical rotating mixing rod to transform the metal from a solid state to a plastic state through rotating friction and mixing effect, and then join the materials together with the kneading effect. This method of using mixing rods to form metal plastic activities can be used for butt joints and lap joints of plate-shaped components, especially for welding of low-melting-point metals such as aluminum and magnesium.

At present, some researchers have successfully completed the connection of magnesium alloy sheets by stir friction welding. After the joint is formed, there is almost no deformation, the upper and lower surfaces of the joint are lubricated, there is no build-up, and there are no cracks, pores, and imperfections on the reverse side. In addition, the mix conflict welding has been successfully used for the same-material welding and dissimilar-material welding of AZ61A and AM60 magnesium alloys. Preliminary research shows that friction stir welding can also be used for the connection between magnesium and aluminum dissimilar materials.

10. Brazing

The brazing process of magnesium alloys is similar to that of aluminum alloys. Flame brazing, furnace brazing and dip brazing can be used, among which dip brazing is the most widely used. The brazing filler metal used in brazing is generally composed of magnesium-based alloys, such as Mg 2 Al 2 Zn filler metal, and the suitable flux is the mixed powder of chloride and fluoride. At present, the brazing process of uncoated magnesium alloys is generally limited to brazing, because no suitable flux for film removal and interfacial activation has been found. Therefore, flux-free soldering of uncoated magnesium alloys is limited to welding fillet joints and filling surface defects on non-critical surfaces of deformed parts and castings before spraying. For magnesium alloys with coatings, commonly used soldering techniques can be used.