Effect of Preheat on TIG Welding of AZ61 Magnesium Alloy

Literature Overview

This 2012 study published in the International Journal of Minerals, Metallurgy and Materials by Jun Shen and Nan Xu from the College of Material Science and Engineering investigates the influence of preheating temperature on the TIG welding behavior of AZ61 magnesium alloy. Magnesium alloys, particularly the AZ-series, are gaining significant attention in aerospace and automotive applications due to their exceptional specific strength-to-weight ratio. However, their welding remains challenging owing to high thermal conductivity, rapid cooling rates, and susceptibility to solidification cracking and porosity. The research was supported by the Key Scientific and Technological Project of Chongqing and the Natural Science Foundation, reflecting the strategic importance of lightweight structural materials in China's manufacturing sector.

Core Technical Findings

The study systematically examines how preheat temperature affects weld microstructure, mechanical properties, and defect formation in AZ61 alloy. Key findings include:

Preheating significantly reduces the cooling rate at the weld center, thereby suppressing the formation of coarse columnar dendrites that are prone to hot cracking.
Without preheat, the weld zone exhibits severe solidification cracking due to the narrow solidification temperature range of AZ61 and the low ductility of the Mg-Al solid solution during solidification.
Optimal preheat temperatures in the range of 150–250°C substantially improve weld integrity by allowing more uniform solidification and reducing thermal gradients.
Excessive preheat (>300°C) leads to grain coarsening in the heat-affected zone (HAZ) and potential reduction in base metal strength.

Weld Microstructure Evolution

Preheat Temperature	Weld Zone Microstructure	HAZ Characteristics	Defect Status
25°C (no preheat)	Coarse columnar dendrites, high porosity	Severe grain growth	Cracking and porosity
100°C	Reduced columnar growth, moderate equiaxed fraction	Moderate grain refinement	Minor porosity
150–200°C	Predominantly equiaxed grains, fine eutectic	Fine, uniform grain structure	No visible defects
250°C	Fine equiaxed grains, slightly coarsened	Moderate grain growth	Acceptable quality
300°C	Coarsened equiaxed grains	Significant grain coarsening	Reduced HAZ strength

Process Parameters and Engineering Practice

The typical TIG welding parameters for AZ61 alloy in this study involved:

Welding current: 80–120 A
Travel speed: 80–120 mm/min
Shielding gas: 100% Argon, flow rate 15–20 L/min
Filler wire: AZ91 or AZ61 matching wire

Defect Analysis and Countermeasures

The primary defects encountered in AZ61 TIG welding without adequate preheat include:

Solidification cracking: Caused by the combination of low solute diffusion rate in the Mg matrix, high thermal conductivity leading to rapid cooling, and the low solidification range of the Mg-Al system. Countermeasure: preheat to 150–200°C and use pure Mg or Mg-1%Zn filler wire to modify the solidification path.
Hydrogen porosity: Magnesium has an extremely high affinity for hydrogen, which is introduced from moisture in the shielding gas or oxide layer. Countermeasure: strict gas purity control, preheating to 200°C to drive off surface moisture, and ensuring proper gas coverage with a trailing shield.
Undercut: Resulting from excessive heat input causing melt pool depression. Countermeasure: optimization of current and travel speed ratio, and ensuring proper torch angle.

Connection to Cladding and Bimetal Applications

While this study focuses on structural welding of magnesium alloy, the findings have direct relevance to cladding and bimetal applications. Magnesium alloys are increasingly used as cladding materials for corrosion resistance in specific chemical environments. The preheat strategy is equally critical in weld-overlay cladding of magnesium alloys onto steel substrates, where the thermal mismatch is even more pronounced. In practice, when considering Mg-alloy overlay on carbon steel, the preheat temperature must be carefully balanced — high enough to prevent cracking in the overlay but low enough to avoid excessive softening of the steel substrate.

Study Insights and Reflections

This research underscores a fundamental principle in reactive alloy welding: thermal management through preheat is often the single most effective lever for defect control. The narrow effective preheat window (150–250°C) reflects the delicate balance between suppressing solidification cracking and preserving base metal properties. For engineers designing welding procedures for magnesium alloy components — particularly in aerospace applications where weight savings are paramount — this work provides a clear procedural framework. The methodology of systematically varying preheat while holding other parameters constant represents good experimental design practice that should be adopted in welding procedure qualification per ASME IX or ISO 15614.