CLADDING TECHNOLOGY SHANXI CO., LTDTIG Welding Repair Process for ZM2 Magnesium Alloy Castings
Literature Overview and Context
The study by Han Feng, Ruan Ming, Feng Zhijun, Li Yufei, and Wang Wei, conducted in collaboration between the PLA Navy Missile Professional Military Representative Office in Shenyang and the Shenyang Foundry Research Institute (2016), addresses the repair welding of ZM2 magnesium alloy castings using gas tungsten arc welding. ZM2, corresponding to AZ91D in international nomenclature, is one of the most widely used wrought and cast magnesium alloys, valued for its excellent specific strength, good castability, and reasonable corrosion resistance. However, magnesium alloys are notoriously difficult to weld due to their high thermal conductivity, low melting point, and extreme susceptibility to oxidation at elevated temperatures. The repair welding of castings — as opposed to fabrication welding — introduces additional challenges including pre-existing porosity, shrinkage cavities, and microstructural heterogeneity in the heat-affected zone of the casting.
Core Technical Challenges
Magnesium alloy welding is fundamentally constrained by the formation of magnesium oxide (MgO), which has a melting point of 2,852°C — far above the melting point of magnesium (650°C) and its alloys. Any oxidation of the molten pool results in inclusions that severely degrade weld quality. The welding atmosphere must therefore be rigorously controlled, typically using a high-purity argon shielding gas with a flow rate of 15–25 L/min to create a positive-pressure environment around the arc and molten pool.
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Shielding gas | High-purity Ar (99.99%) | Prevents MgO formation |
| Gas flow rate | 15–25 L/min | Ensures positive pressure shielding |
| Arc current | 80–180 A | Controls heat input and penetration |
| Travel speed | 200–400 mm/min | Minimizes heat-affected zone |
| Tungsten electrode | 3.2–4.0 mm WC-20 | Resists erosion in Mg alloy welding |
| Wire feed rate | 3–8 m/min (if automated) | Controls deposition rate |
| Filler metal | ZM1 or ZM2 matching wire | Minimizes composition mismatch |
| Joint design | Square butt with 0–0.5 mm gap | Minimizes oxidation exposure |
The filler metal selection is a critical decision in ZM2 repair welding. Using ZM2-matching filler metal (AZ91D composition) maintains the base alloy composition but may introduce casting-like microstructure in the weld metal, including coarse β-Mg₁₇Al₁₂ intermetallic phases at grain boundaries. Alternatively, using ZM1 (AZ31 composition) filler metal reduces the aluminum content in the weld metal, which can improve ductility but may create a composition mismatch with the base metal. The study investigates both approaches and evaluates their impact on weld mechanical properties and corrosion resistance.
Process Development and Defect Analysis
The repair welding process for ZM2 castings requires a systematic approach to defect prevention. The most common defects in magnesium alloy TIG welding include:
- Porosity — caused by hydrogen absorption from moisture in the shielding gas or surface contamination. Countermeasure: pre-weld cleaning with acetone or alkaline solution, and use of high-purity argon with a dew point below -40°C.
- Undercut — caused by excessive heat input or insufficient travel speed. Countermeasure: reduce arc current by 10–15% and increase travel speed by 20%.
- Excessive reinforcement — caused by low travel speed or high wire feed rate. Countermeasure: optimize the wire feed rate to travel speed ratio to achieve a flat or slightly convex weld profile.
- Cracking — caused by high aluminum content promoting β-phase formation at grain boundaries. Countermeasure: use ZM1 filler metal or add rare earth elements (Nd, Ce) to the filler to refine the microstructure.
- Incomplete fusion — caused by insufficient heat input or poor joint fit-up. Countermeasure: preheat the joint to 100–150°C and ensure a gap of 0–0.5 mm.
The preheating step is particularly important for repair welding of castings, as the thermal conductivity of magnesium alloy is high (approximately 120 W/m·K for ZM2) and rapid heat dissipation can lead to cold cracking. Preheating to 100–150°C reduces the thermal gradient and minimizes residual stresses. However, preheating must be controlled to avoid excessive grain growth in the heat-affected zone, which would degrade mechanical properties.
Engineering Practice and Quality Verification
The repair welding of ZM2 castings in military applications — such as missile components — demands rigorous quality verification. The weld repair must be validated through a combination of non-destructive testing and destructive testing. Radiographic testing (RT) or ultrasonic testing (UT) is performed on 100% of repair welds to detect internal porosity and incomplete fusion. Mechanical testing, including tensile testing and bend testing, is performed on coupon specimens welded under identical conditions to the production repair. The tensile strength of the repair weld should meet at least 90% of the base material's tensile strength (approximately 234 MPa for ZM2 in the as-cast condition), and the elongation should be at least 30% of the base material's elongation (approximately 3.5% for ZM2).
The study's practical contribution lies in establishing a reproducible repair welding procedure that achieves acceptable mechanical properties and corrosion resistance. The recommended procedure involves: (1) mechanical cleaning of the defect area to remove loose material and oxide; (2) application of a zinc-rich primer to the cleaned surface to prevent re-oxidation; (3) TIG welding using ZM1 filler metal with a current of 120–150 A and travel speed of 250–350 mm/min; (4) post-weld heat treatment at 175°C for 8 hours to relieve residual stresses and homogenize the microstructure.
Study Insights and Reflections
The most significant finding of this research is that ZM2 casting repair welding is achievable with acceptable quality when the process parameters are carefully optimized and the pre-weld and post-weld treatments are rigorously controlled. The use of ZM1 filler metal, while creating a composition mismatch with the ZM2 base material, provides superior weld ductility and reduced cracking susceptibility compared to ZM2-matching filler. This trade-off between composition matching and weldability is a recurring theme in magnesium alloy welding and warrants further investigation through long-term corrosion testing and fatigue testing. The study also highlights the importance of post-weld heat treatment in mitigating the adverse effects of the welding thermal cycle on the microstructure of the heat-affected zone. For critical military applications, the combination of TIG repair welding with post-weld heat treatment provides a reliable and repeatable repair methodology that meets the demanding quality standards of the defense sector.