1 preamble
Magnesium alloys are not only light, high-strength, and low in price, but also have good vibration damping, castability, electrical conductivity, electromagnetic shielding, and heat dissipation, and have become the preferred metal materials for many industrial products. At present, magnesium alloys are widely used in components with small bearing capacity such as cockpit frames, equipment brackets, and wheel hubs in the aviation industry [1].
With the transformation and upgrading of modern large-scale manufacturing equipment, the demand for lightweight magnesium alloy structural parts has become very urgent. However, there are many defects in the welding of magnesium alloys, and it is difficult to obtain welded joints with high forming quality and high comprehensive performance. This paper analyzes the causes of magnesium alloy welding defects and proposes preventive measures, which can help the popularization and application of magnesium alloy materials, and has practical significance for the field of manufacturing equipment.
2 Welding process of magnesium alloy
Common welding processes for magnesium alloys include fusion welding and solid phase welding. Fusion welding mainly includes tungsten argon arc welding, metal argon arc welding, electron beam welding, laser welding and other methods, and solid phase welding is mainly friction stir welding. Among them, friction stir welding has become a preferred welding method due to its advantages of less pre-weld preparation work, no need for shielding gas and welding materials, all-position welding, good mechanical properties of weldments, and small post-weld stress deformation. However, friction stir welding has the disadvantages that the weldment must be rigidly fixed, the welding speed is low, the stirring head wears quickly, and the keyhole is easy to form at the end of the weld, which makes fusion welding a common welding method.
3 Analysis of welding defects of magnesium alloy
Magnesium alloys have disadvantages such as easy evaporation, easy oxidation, easy nitriding, and large thermal stress, and often show a variety of welding defects during welding. The causes and preventive measures of common defects such as pores, thermal cracks, and deformation are sorted out.
3.1 Stomata
(1) Causes of formation Pores often appear in the weld of the fusion welding joint. For example, Figure 1 shows the pore morphology of the welding seam of an ordinary die-casting AZ91D magnesium alloy argon tungsten arc welding joint. There are two types of microscopic pores dominated by hydrogen gas and entangled macroscopic pores dominated by nitrogen [2 ].
The formation of pores is mainly attributed to two reasons: one is that the insoluble gas generated by the metallurgical reaction in the welding pool gathers between the solidified dendrite crystals and is not easy to discharge to form pores; the other is because the welding pool absorbs and dissolves some In the solidification stage, the gas solubility decreases rapidly with the sharp drop of the molten pool temperature, and the gas is easy to gather at the front of the growing dendrites, forming pores along the crystal layer.
During fusion welding of magnesium alloys, the pores mainly come from dissolved hydrogen, while the hydrogen in the molten pool mainly comes from the moisture around the base metal, welding wire or arc column atmosphere. Magnesium alloys have strong thermal conductivity, and the solidification speed of the molten pool is very fast, causing hydrogen to escape and form pores. At the same time, MgO film is easy to form on the surface of magnesium alloy. The more Mg content leads to more MgO, MgO is looser than Al2O3 and other oxides, and it is easier to absorb water and form pores.
At present, the porosity of molten inert gas shielded welding (MIG) welds is the highest. This is because MIG welding relies on the continuous melting of the welding wire, and the oxide film in the welding wire will strongly dissolve the attached water into the droplet, resulting in hydrogenation of the molten pool. . Electron beam welding and laser welding also have more porosity in the weld, which is due to the less welding heat input of these two methods, the faster cooling rate of the molten pool, and the hydrogen in the molten pool has no time to escape.
(2) Preventive measures Pre-welding treatment: combine mechanical cleaning and chemical cleaning to remove the oxide film and oil stains on the surface of the base metal and welding wire as much as possible; use drying methods to remove the moisture on the surface of the base metal and welding wire as much as possible; try to avoid Welding in environment.
Optimizing welding parameters: Welding parameters can affect the conditions of gas escaping and melting in the molten pool. When the escaping conditions are more favorable than the melting conditions, it is possible to reduce the porosity. Figure 2 shows the relationship between LF6 aluminum-magnesium alloy porosity tendency and welding parameters [3]. Larger welding current and welding speed are conducive to the reduction of porosity.
The protective atmosphere has appropriate oxidative properties: From the perspective of preventing hydrogen dissolution, adding a small amount of CO2 or O2 to the inert gas used for welding protection such as Ar and He can help reduce porosity.
3.2 Thermal cracks
(1) Causes of formation The most common thermal cracks are solidification cracks and liquefaction cracks. Solidification cracks are cracks caused by the separation of the remaining liquid film between the weld metal when the solidification temperature drops to near the solidus line. The liquefaction crack is that the intercrystalline phase melts into the liquid phase when the near-slit area is overheated, and the liquid film separates and cracks. For example, Fig. 3 shows the state of solidification cracks in the weld corresponding to different welding speeds during laser welding of ZK60 magnesium alloy [4].
During the welding process, the main alloying element magnesium easily reacts with trace elements such as aluminum, copper, nickel, etc. to form a low melting point eutectic compound. During solidification, in the brittle temperature range, these unsolidified eutectics will be distributed among the grains in the form of a liquid film, which seriously reduces the intergranular bonding force. Magnesium alloy has a large thermal expansion coefficient, which causes large thermal deformation during welding, and will be subject to large shrinkage stress during solidification. The intergranular liquid film is difficult to resist this shrinkage stress, and it is easy to crack and form solidification cracks. In the same way, the thermal conductivity and strain rate of magnesium alloy are relatively large, and the welding heat cycle will quickly melt the intergranular phase near the seam, and the mechanical properties of the grain boundary will decrease, which is easy to crack under stress.
(2) Preventive measures Adjust the content of elements in the base metal and welding wire: limit the content of easily segregated elements and harmful impurities in the base metal and welding wire, and minimize the macro-segregation and low-melting second phases that occur in the weld.
Optimizing welding parameters: by selecting a reasonable welding speed, Figure 4 shows the relationship between the shape of the molten pool and the welding speed [3]. When welding at low speed, the molten pool is elliptical, and the columnar crystals grow to the middle of the weld in a herringbone pattern, which is not easy to form segregated weak surfaces, and the tendency of thermal cracks is small; but when welding at high speed, the molten pool is teardrop-shaped, and the columnar crystals are similar to It grows vertically to the axis of the weld, and it is easy to form a segregation weak surface at the meeting surface, and the tendency of thermal cracking is large. It is also possible to refine the grain size and reduce the intergranular phase size by appropriately reducing the welding heat input, and slow down the strain of the solidification and shrinkage of the weld by reducing the cooling rate, all of which can reduce the occurrence of thermal cracks.
Reasonable control of restraint: By controlling restraint, the strain on the joint is reduced as much as possible. For example, choosing an appropriate welding sequence. When the welding sequence is improper, the last few welds may be in a state of large restraint, it is difficult to shrink freely, the amount of strain increases significantly, and cracks are prone to occur.
3.3 Deformation
(1) Causes of formation Magnesium alloys have high thermal conductivity and large thermal expansion coefficient, so the cooling rate of the weld seam is fast, and the near seam area and the base metal are easily deformed by shrinkage stress, and the final shape and size change. For example, Figure 5 shows that an aluminum-magnesium alloy has concave deformation because the fillet weld of the nozzle is too close to the girth weld of the cylinder [5].
(2) Preventive measures Optimize the weld structure: rationally arrange the position of the welds, ensure that each weld has sufficient heat dissipation space, and avoid excessive concentration of welds in the area; select the appropriate shape and size of the welds [6].
Increase rigidity and fixation: When welding magnesium alloy plates, use special fixtures, support rods and other devices to fix the magnesium alloy plates on the workbench. After cooling to room temperature after welding, the hammering method is used to release part of the welding stress, and then the rigid fixation is removed.
Preheating before welding: Preheating before welding increases the temperature of the base metal to ensure that the temperature difference between the weld metal and the surrounding base metal during welding is reduced, thereby reducing the internal stress of welding shrinkage.
Choose a reasonable welding sequence: Divide the component into several small units appropriately, weld each small unit separately, and then weld the small units as a whole, so that asymmetric welds or welds with large shrinkage can shrink more freely without shrinkage. affect the entire structure [7].
Anti-deformation control: Estimate the size and direction of welding deformation, and then set artificial deformations with opposite directions and equal sizes during welding assembly, so that the deformation generated by welding can be offset by the preset anti-deformation.
3.4 Other defects
(1) Holes Holes often appear in the weld of friction stir welded joints. For example, Fig. 6 shows the void defect in the friction stir welding seam of AZ31 magnesium alloy [8]. When welding magnesium alloys, when the welding heat input is insufficient, the plastic deformation of the deposited metal will be insufficient, the material fluidity will be poor, and the inside of the weld will not be completely closed, forming holes; when the welding heat input is too large, the stirring head will be caused The weld material on the forward side expands and overflows, and the backfill is insufficient, forming holes; when a columnar or conical stirring head without thread is used, the plastic deformation of the material in the weld area is insufficient, and holes are easily formed. The occurrence of hole defects can be avoided by reasonably controlling the welding speed and the rotation speed of the stirring head to adjust the welding heat input, or choosing the appropriate geometry of the stirring head.
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Fig.6 Pore defect of friction stir welded joint of AZ31 magnesium alloy (AS is the forward side, RS is the backward side)[8]
(2) Burn-through Burn-through often occurs in the weld seam of the fusion welding joint. Due to the high melting point of magnesium oxide and the low melting point of magnesium alloy, it is difficult to fuse the two when they are attached together. When the magnesium alloy sheet is welded, it is difficult to observe the weld melting. Once the heat input increases to an unreasonable range, the color of the molten pool does not change significantly, but the unmelted metal below the molten pool cannot resist the stress it receives, and burn-through occurs at this time. Do a good job of cleaning the surface of the magnesium alloy before welding, and weld as soon as possible after cleaning to avoid the occurrence of burn-through defects. In addition, by optimizing the welding parameters to limit the depth of penetration, burn-through can also be avoided.
4 Typical Case Analysis of Welding Defects in Magnesium Alloys
The 6mm thick GW63K magnesium alloy was welded by laser welding and electron beam welding respectively, and the macroscopic appearance of the weld seam is shown in Fig. 7 and Fig. 8 respectively. The two kinds of fusion welding seams have obvious defects such as spatter and undercut, which are caused by the low melting point of magnesium alloy, large thermal expansion coefficient, and large welding heat input. Subsequent methods can be used to reduce welding heat input. Process Optimization.
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Fig.7 Macroscopic morphology of laser welded seam of GW63K magnesium alloy
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Fig.8 Macroscopic morphology of electron beam welded seam of GW63K magnesium alloy
