TIG Welding Repair Process Research for ZM2 Magnesium Alloy

Literature Overview

This 2016 publication by Han Feng, Ruan Ming, Feng Zhijun, Li Yufei, and Wang Wei from the PLA Navy Missile Professional Military Representative Office in Shenyang and the Shenyang Institute of Foundry Research investigates the TIG welding repair process for ZM2 magnesium alloy. Published in Casting, this work addresses a critical challenge in magnesium alloy component manufacturing: the repair of casting defects in components that may serve in demanding defense applications. The research is particularly relevant given the increasing use of magnesium alloys in lightweight structural applications where density reduction is paramount.

Core Technical Points

ZM2 is an AZ91-type magnesium alloy (approximately 9% Al, 1% Zn, 1% Mn balance Mg) that is widely used in automotive and aerospace applications due to its excellent specific strength. However, welding repair of magnesium alloy castings presents unique challenges:

High vapor pressure of magnesium: At temperatures above 650 °C, magnesium exhibits significant vapor pressure, leading to severe evaporation losses during welding
Oxidation sensitivity: Magnesium forms a tenacious MgO layer that is difficult to remove mechanically and interferes with arc stability
Cracking susceptibility: The wide freezing range of ZM2 (approximately 180 °C) promotes hot cracking and hot shortness
Hydrogen pickup: Magnesium readily absorbs hydrogen from moisture, leading to porosity and delayed cracking

Welding Process Parameters

Parameter	Recommended Range	Critical Consideration
Shielding Gas	Argon (99.99%) or Ar + 2% H₂	Hydrogen addition reduces arc voltage but increases porosity risk
Arc Current	80–150 A (DCEN)	Higher current increases Mg vaporization
Travel Speed	150–400 mm/min	Must balance penetration with reduced thermal input
Wire Diameter	1.0–2.0 mm	Smaller diameter for thinner sections
Wire Composition	ZM1 or AZ91 match	Slightly higher Al content in filler to compensate for burn-off
Preheat Temperature	150–200 °C	Reduces thermal gradient and cracking tendency
Interpass Temperature	< 150 °C	Prevents excessive heat accumulation

Process Development Approach

The research employs a systematic approach to developing a viable repair welding procedure:

Surface preparation: Mechanical removal of the MgO layer using stainless steel brushes or chemical cleaning with dilute acid solutions followed by immediate welding to prevent re-oxidation. The oxide layer, if not removed, creates cold cracks and poor fusion at the weld interface.
Filler metal selection: The choice between ZM1 and ZM2 filler metals involves a trade-off between crack resistance and mechanical property matching. ZM1 (higher Al content, approximately 11%) provides better hot crack resistance due to its narrower freezing range, while ZM2 filler maintains better microstructural compatibility with the base metal.
Shielding strategy: Dual shielding with trailing gas is often necessary for repair welds in difficult geometries. The trailing gas protects the hot weld metal during solidification, preventing oxidation that would otherwise compromise the repair quality.

Defect Analysis and Countermeasures

Defect	Mechanism	Prevention Strategy
Hot cracking	Wide freezing range + Al interdendritic segregation	Use ZM1 filler; control cooling rate with preheat
Porosity	Mg vaporization + hydrogen pickup	Increase shielding coverage; dry filler thoroughly
Excessive burn-through	Low melting point of Mg	Reduce current; increase travel speed; use backing
Surface oxidation	Rapid MgO formation	Dual shielding; minimize arc exposure time
Lack of fusion	MgO interference at interface	Aggressive surface cleaning; increase current

Engineering Practice Integration

In defense manufacturing, magnesium alloy components such as missile housings, weapon mountings, and structural brackets must meet stringent quality requirements. Casting defects such as shrinkage porosity, inclusions, and cold shuts are common in complex geometries and require welding repair. The repair process must maintain the structural integrity of the component while addressing the local defect.

Key engineering considerations include:

Heat affected zone management: The repair weld creates a local HAZ that may have reduced mechanical properties compared to the cast microstructure. Post-weld heat treatment (solution treatment at 400 °C followed by aging) is typically required to restore properties in the HAZ.
Residual stress: Localized welding heat input creates residual stresses that may promote stress corrosion cracking in the presence of chlorides. Stress relief at 250–300 °C for 1–2 hours is recommended after repair welding.
Dimensional control: Magnesium alloy has a high thermal expansion coefficient (approximately 26 × 10⁻⁶/°C), making distortion control challenging during repair welding. Fixturing and sequential welding patterns are essential.

Study Insights and Implications

This research contributes valuable process knowledge for magnesium alloy repair welding in defense applications. The key insight is that successful ZM2 repair welding requires not just proper parameter selection but also rigorous attention to surface preparation, environmental control, and post-weld treatment. The MgO problem alone can render otherwise sound welding parameters ineffective. For engineers involved in magnesium alloy component qualification, this work underscores the necessity of dedicated WPS development for repair welding, as casting repair welds experience different thermal histories than production welds due to the surrounding sound material acting as a heat sink. The systematic approach to parameter optimization provides a foundation for extending repair welding capabilities to other magnesium alloy systems including AZ31, AZ61, and WE43.