1. Introduction
Magnesium alloys are not only lightweight and strong, but also come with advantages of good vibration reduction, castability, electrical conductivity, electromagnetic shielding, and heat dissipation, and are becoming a preferred metal material for many industrial products.
At present, magnesium alloys are widely used in the aerospace industry for small load-bearing components such as cabin frames, equipment brackets, and wheel hubs .
As modern large-scale manufacturing equipment transforms and upgrades, the demand for lightweight magnesium alloy structural components has become extremely urgent.
However, there are many defects in magnesium alloy welding, making it difficult to obtain high-forming quality and high-performance welded joints.
Through the analysis of the causes of welding defects in magnesium alloys and proposing prevention and control measures, this article can help promote the application of magnesium alloy materials and has practical significance for the manufacturing equipment field.
2. Welding Processes for Magnesium Alloys
The common welding processes for magnesium alloys are fusion welding and solid-phase welding. Fusion welding methods mainly include tungsten inert gas welding, metal inert gas welding, electron beam welding, and laser welding.
Solid-phase welding mainly includes friction stir welding. Among them, friction stir welding, with its advantages of less pre-weld preparation work, no need for protective gas and welding materials, the ability to achieve full position welding, good mechanical properties of welded joints, and low post-welding stress deformation, has become a preferential welding method.
However, friction stir welding has disadvantages such as the need for rigid fixation of the welded parts, low welding speed, rapid wear of the stirring head, and easy formation of keyholes at the end of the weld, which make fusion welding a common welding method.
3. Analysis of Welding Defects in Magnesium Alloys
Magnesium alloys have disadvantages such as easy evaporation, oxidation, nitridation, and high thermal stress, often exhibiting various welding defects during welding.
This section focuses on the causes and prevention measures of common defects such as porosity, hot cracks, and deformation.
3.1 Porosity
(1) causes: Porosity often appears in the welds of fusion-welded joints.
For example, Fig. 1 shows the morphology of porosity in the weld of a tungsten inert gas welded joint of ordinary pressure-cast AZ91D magnesium alloy, which includes two types of micro-pores dominated by hydrogen and macro-pores dominated by nitrogen.
Figure 1. Internal morphologies of porosity in the weld of an AZ91D magnesium alloy tungsten inert gas welded joint.
The formation of porosity is mainly attributed to two reasons: one is that insoluble gases generated by the metallurgical reaction of the welding pool accumulates between solidified dendrites, making it difficult to be discharged, thus forming porosity; the other is that the welding pool absorbs and dissolves some external gases.
During the solidification stage, the gas solubility decreases rapidly with the steep drop of the welding pool temperature, and the gas easily gathers at the dendrite front during growth to form porosity along the crystallization layer.
During the fusion welding of magnesium alloys, porosity mainly comes from dissolved hydrogen. The hydrogen gas in the welding pool mainly comes from the moisture around the base material, welding wire, or arc column atmosphere.
Magnesium alloys have strong thermal conductivity, and the welding pool solidifies quickly, causing hydrogen to not escape in time, thus forming porosity. At the same time, the surface of magnesium alloys easily forms a MgO film, and the higher the Mg content, the more MgO is generated.
Compared with oxides such as Al2O3, MgO is relatively loose and more prone to adsorbing moisture, leading to the formation of porosity.
Currently, the porosity rate of fusion MIG welding joints is the highest because MIG welding relies on continuous melting of the welding wire. The oxide film on the surface of the welding wire causes the attached moisture to dissolve strongly in the molten droplet, leading to an increase in hydrogen in the welding pool.
The porosity rates in the welds of electron beam welding and laser welding are also relatively high, which is due to the low welding heat input and fast cooling rate of these two methods, causing the hydrogen in the welding pool to not escape in time.
(2) Prevention measures:
Pre-weld treatment: combination of mechanical cleaning and chemical cleaning to remove oxide films and oil stains from the surface of the base material and the welding wire as much as possible.
Use drying methods to remove moisture from the surface of the base material and welding wire as much as possible. Avoid welding in a humid environment as much as possible.
Optimize welding parameters:
Welding parameters can affect the conditions of gas outflow and dissolution in the welding pool. Only when the conditions for outflow are more favorable than those for dissolution can the porosity rate be reduced.
Fig. 2 shows the relationship between the tendency of gas porosity and welding parameters of LF6 aluminum-magnesium alloy. It is beneficial to reduce porosity when both welding current and speed are high.
Use protective gas with appropriate oxidizing properties:
From the perspective of preventing hydrogen dissolution, adding a small amount of CO2 or O2 to inert gases such as Ar and He used for welding protection can help reduce the porosity rate.
3.2 Hot Cracks
(1) Causes:
The most common hot cracks are solidification cracks and liquation cracks. Solidification cracks are cracks caused by the separation of residual liquid film between dendrites when the welding metal of the weld cools to near the solidus line.
Liquation cracks occur when the intergranular phase melts into the liquid phase under overheated conditions in the near-weld zone, and then cracks occur due to the separation of the liquid film.
For example, Fig. 3 shows the solidification crack conditions in the welds corresponding to different welding speeds of ZK60 magnesium alloy in laser welding .
During welding, the main alloying element magnesium easily reacts with trace elements such as aluminum, copper, and nickel to form low-melting eutectic compounds.
During solidification, these un-solidified low-melting eutectics will be distributed between the dendrites in the form of a liquid film in the brittle temperature range, significantly reducing the intergranular bonding force.
Magnesium alloy has a large thermal expansion coefficient, which generates significant thermal deformation during welding. When solidifying, it is subjected to large shrinkage stress that the liquid film between the grains is difficult to resist, making it prone to cracking and forming solidification cracks.
Similarly, magnesium alloy has a high thermal conductivity and strain rate, and the fast welding thermal cycle causes the intergranular phase in the near-weld zone to melt and the mechanical properties at the grain boundaries to decrease, making it easy to crack under stress.
(2) Prevention measures:
Adjust the element content in the base material and welding wire: Limit the content of elements that are prone to segregation and harmful impurities in the base material and welding wire, and minimize the macrosegregation and aggregation of low-melting second phases in the weld.
Optimize welding parameters: Reasonable welding speed selection can be achieved through optimizing welding parameters. Fig. 4 shows the relationship between the shape of the weld pool and welding speed.
When the welding speed is low, the weld pool shows a oval shape, columnar crystals grow towards the center of the weld with a herringbone pattern, it’s difficult to form a segregation weak surface, and there is a small tendency of hot cracking.
But, at high welding speed, the weld pool shows a teardrop shape, columnar crystals grow nearly vertically towards the weld axis, and a segregation weak surface is easy to form at the joint surface, which increases the tendency of hot cracks.
It is also possible to reduce the grain size and intermetallic size by properly reducing the welding heat input, and slowing down the cooling rate to reduce the strain degree of the weld shrinkage, which can all help reduce the occurrence of hot cracks.
Control the restraint reasonably: Minimize the joint strain by controlling the restraint. For example, select a suitable welding sequence. When the welding sequence is improper, the last few welded joints may be in a state of large restraint, difficult to shrink freely, and the strain increases significantly, which increases the risk of hot cracks.
Figure 4. Relationship between the weld pool shape and welding speed.
3.3 Deformation
(1) Causes:
Magnesium alloy has a high thermal conductivity and a large thermal expansion coefficient.
Therefore, the welding seam cools quickly, and the near-weld zone and base material are easily affected by shrinkage stress, leading to significant deformation and changes in final shape and size.
For example, Fig. 5 shows the concave deformation of an aluminum-magnesium alloy due to the angle weld being too close to the ring weld of the cylinder.
(2) Prevention measures:
Optimize weld structure: Reasonably layout the position of the weld and ensure that there is sufficient heat dissipation space for each weld to avoid the over-concentration of the weld in the area. Choose the appropriate weld shape and size [6].
Increase the rigidity fixation: When welding magnesium alloy plates, use special fixtures, supporting rods, and other devices to fix the magnesium alloy plates on the workbench. After welding cools to room temperature, use a hammering method to release some of the welding stress, and then remove the rigid fixation.
Preheating before welding: Preheating before welding can increase the base material temperature and ensure that the temperature difference between the welding metal and the surrounding base material is reduced. This can reduce the welding shrinkage internal stress.
Select a reasonable welding sequence: Divide the structure into several small units, first weld each small unit separately, and then weld the small units as a whole to allow asymmetric or large-shrinkage welds to shrink more freely without affecting the overall structure.
Anti-deformation control: Estimate the size and direction of welding deformation in advance, and then set an opposite direction and equal artificial deformation during welding assembly to offset the welding deformation with the preset anti-deformation.
3.4 Other Defects
(1) Pores
Pores often appear in the welds of friction stir welded joints. For example, Fig. 6 shows the pore defects in the weld of AZ31 magnesium alloy friction stir welding.
When welding magnesium alloy, insufficient plastic deformation of the weld metal and poor material flow in the weld can result in the incomplete closure of the internal region of the weld, forming pores when the welding heat input is not sufficient.
Excessive welding heat input can cause the expansion overflow of the welding material in the forward side of the stirring head, insufficient backfilling, and the formation of pores.
Using a cylindrical or conical stirring head without a thread can also cause incomplete plastic deformation of the welding area, which is prone to form pores.
Properly controlling the welding speed and the rotational speed of the stirring head to adjust the welding heat input or selecting a suitable stirring head geometry can help prevent the occurrence of pore defects.
Figure 6. Pore defects in AZ31 magnesium alloy friction stir welding joint (AS for advancing side, RS for retreating side) .
(2) Burn-through
Burn-through often occurs in the welding seam of a molten welding joint. Due to the high melting point of magnesium oxide and the low melting point of magnesium alloy, it is difficult to fuse them together. When welding thin sheets of magnesium alloy, the melted welding seam is difficult to observe.
Once the heat input increases to an unreasonable range, the color of the molten pool does not change significantly, but the unmelted metal beneath the molten pool cannot resist the stress it receives, which leads to burn-through.
The cleaning of the magnesium alloy surface should be done before welding, and welding should be done as soon as possible after cleaning to prevent burn-through defects.
In addition, burn-through can be avoided by optimizing welding parameters to limit the penetration depth.
4. Typical case analysis of welding defects in magnesium alloys
6 mm thick GW63K magnesium alloy was welded using laser welding and electron beam welding, and the macroscopic morphology of the welds obtained is shown in Figures 7 and 8, respectively.
Both types of molten welding seams exhibited obvious defects such as spatter and undercut, which were caused by the low melting point and high thermal expansion coefficient of the magnesium alloy, as well as the high welding heat input.
The process can be optimized by reducing the welding heat input or using other methods to eliminate these defects.
5. Conclusion
When welding magnesium alloys, it is common to encounter welding defects such as porosity, hot cracking, deformation, pores, and burn-through.
Welding process operators can refer to the prevention and control measures proposed above, optimize welding parameters, adjust structural designs, and take other measures to prevent welding defects.
