CLADDING TECHNOLOGY SHANXI CO., LTDApplication of MIG Welding in Magnesium Alloy Welding Technology
Overview and Technical Significance
This 2015 review article from the North University of China provides a comprehensive overview of the application of MIG (GMAW) welding in the fabrication and repair of magnesium alloy components. The authors, Tian Runa and Hou Jibo, synthesize the state of the art in magnesium alloy welding, emphasizing the unique challenges posed by the high reactivity of magnesium, its low melting point, and its susceptibility to oxidation and porosity during welding.
Magnesium alloys are the lightest structural metals in engineering use, with a density of approximately 1.74 g/cm3, which is about 75% lighter than aluminum alloys and 60% lighter than steel. This makes them ideal for applications where weight reduction is critical, such as aerospace structures, automotive body panels, and portable electronic device housings. However, the welding of magnesium alloys is notoriously difficult due to several inherent material characteristics that must be addressed through careful process selection and parameter optimization.
Fundamental Challenges in Magnesium Alloy Welding
The primary challenges in welding magnesium alloys include the following:
| Challenge | Description | Impact on Weld Quality |
|---|---|---|
| High reactivity | Mg reacts vigorously with oxygen and nitrogen at elevated temperatures | Formation of MgO and Mg3N2 inclusions; poor surface quality |
| Low melting point | Melting point of pure Mg is 650°C, significantly lower than Al (660°C) and Fe (1538°C) | Excessive heat input leads to burn-through and distortion |
| High thermal conductivity | Thermal conductivity of Mg is approximately 150 W/(m·K) | Rapid heat dissipation requires high heat input for adequate fusion |
| Hydrogen solubility | Hydrogen is highly soluble in liquid Mg but has very low solubility in solid Mg | Massive hydrogen porosity during solidification |
| Low surface tension | Molten Mg has a low surface tension of approximately 0.53 N/m | Poor wetting and bead profile control |
| Oxide formation | MgO has a melting point of 2852°C, far above the melting point of Mg | Oxide inclusions in the weld metal; poor fusion |
The MIG welding process, with its controllable heat input and high deposition rate, is one of the most commonly used processes for welding magnesium alloys. However, the process must be carefully optimized to mitigate the challenges listed above.
MIG Welding Parameters for Magnesium Alloys
The selection of welding parameters for magnesium alloy MIG welding requires careful consideration of the material properties and the desired weld quality. The following table summarizes the recommended parameter ranges for common magnesium alloy grades.
| Parameter | AZ31 | AZ91 | ZK60 | WE43 |
|---|---|---|---|---|
| Filler wire | AZ91 or AZ31 | AZ91 | ZK60 | WE43 |
| Current (A) | 120 - 200 | 150 - 250 | 180 - 300 | 200 - 350 |
| Voltage (V) | 16 - 22 | 18 - 24 | 20 - 28 | 22 - 30 |
| Travel speed (mm/min) | 300 - 600 | 400 - 800 | 500 - 1000 | 600 - 1200 |
| Wire feed speed (m/min) | 3 - 5 | 4 - 7 | 5 - 8 | 6 - 10 |
| Shielding gas | Pure Ar | Pure Ar | Pure Ar | Pure Ar |
| Gas flow rate (L/min) | 15 - 25 | 20 - 30 | 25 - 35 | 30 - 40 |
| Preheat temperature (°C) | 0 - 100 | 0 - 150 | 0 - 150 | 0 - 200 |
| Interpass temperature (°C) | Below 150 | Below 150 | Below 150 | Below 200 |
The shielding gas selection is critical for magnesium alloy welding. Pure argon is the preferred shielding gas because it provides excellent protection against oxidation without introducing hydrogen. The addition of hydrogen or helium to the shielding gas mixture should be avoided, as hydrogen increases porosity and helium increases the arc temperature, which can exacerbate oxidation. The gas flow rate should be sufficient to create a protective envelope around the weld pool, but excessive flow can cause turbulence and entrainment of atmospheric contaminants.
Microstructural Evolution and Mechanical Properties
The microstructure of MIG-welded magnesium alloy joints is characterized by a complex arrangement of phases and precipitates. In AZ31 and AZ91 welds, the primary alpha-Mg phase is accompanied by Mg17Al12 intermetallic particles, which form at grain boundaries and within the dendritic cells. The morphology and distribution of these intermetallics are strongly influenced by the cooling rate and the composition of the weld metal.
The mechanical properties of the weld metal are typically lower than those of the base metal, primarily due to the coarser grain structure and the presence of intermetallic phases. The tensile strength of AZ91 welds is typically in the range of 180 to 250 MPa, compared to 260 to 320 MPa for the base metal in the as-cast condition. The elongation of the weld metal is generally higher than that of the base metal, reflecting the more ductile microstructure of the weld.
Post-weld heat treatment, such as solution treatment at 415°C for 4 hours followed by aging at 175°C for 6 hours, can significantly improve the mechanical properties of the weld metal by dissolving the coarse intermetallic phases and promoting the formation of fine precipitates. This treatment can increase the tensile strength of the weld by 20 to 30% and improve the elongation by 15 to 25%.
Defect Analysis and Countermeasures
The following table summarizes the common defects observed in MIG-welded magnesium alloy joints and the recommended countermeasures.
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hydrogen porosity | Hydrogen pickup from shielding gas or moisture | Use pure argon; dry filler wire; increase gas flow |
| Oxide inclusions | Inadequate shielding or oxide on filler wire | Use flux-coated wire; increase gas flow; pre-clean surfaces |
| Hot cracking | High Mg content; excessive heat input | Reduce heat input; use filler with lower Mg; preheat |
| Burn-through | Excessive current or low travel speed | Reduce current; increase travel speed; use backing plate |
| Poor fusion | Low arc force; inadequate penetration | Increase current; reduce travel speed; adjust nozzle distance |
| Distortion | Excessive heat input; asymmetric joint | Use back-up plate; clamp joint; use intermittent welding |
Engineering Practice and Process Selection
For engineering applications involving magnesium alloy components, the MIG welding process should be selected based on the following criteria:
- Component thickness: For thin sheets below 2 mm, pulsed MIG welding is preferred to minimize heat input and prevent burn-through. For thicker sections above 4 mm, short-circuit transfer MIG welding may be used to achieve adequate penetration.
- Joint configuration: Butt joints are the most straightforward to weld, while T-joints and fillet joints require careful parameter optimization to ensure adequate fusion at the root.
- Production volume: For high-volume production, automated MIG welding with robotic wire feeding and motion control is recommended to ensure consistency and repeatability.
- Quality requirements: For critical applications such as aerospace structures, the welding procedure should be qualified in accordance with applicable standards, and the welds should be inspected using non-destructive testing methods such as ultrasonic testing (UT) and radiographic testing (RT).
Study Insights and Future Directions
This review article provides a valuable summary of the current state of MIG welding technology for magnesium alloys, highlighting both the achievements and the remaining challenges. The emphasis on shielding gas selection and the avoidance of hydrogen-containing gases is particularly important, as hydrogen porosity remains the most common defect in magnesium alloy welds. The recommendation to use post-weld heat treatment to improve mechanical properties is also practical and widely applicable.
Future research should focus on the development of advanced MIG welding variants, such as cold metal transfer (CMT) and hot-wire MIG, which offer even lower heat input and may further reduce the formation of intermetallic phases and porosity. Additionally, the integration of real-time monitoring and control systems, such as arc sensing and wire feeding speed regulation, can improve the process stability and weld quality in automated welding applications. The continued development of high-performance magnesium alloys with improved weldability will also expand the range of applications for MIG-welded magnesium components in structural engineering.