TIG Welding Thermal Cycle Effects on AZ91D Magnesium Alloy Hardness

Literature Overview

This 2005 study published in Welding by Xiao Feng, Fu Ya, Li Chuntian, Zhao Weilin, Yang Hui, and Xu Xiaoling from Chongqing Institute of Technology investigates the effects of TIG welding thermal cycles on the hardness of AZ91D magnesium alloy. The research was supported by multiple funding sources including the Ministry of Education Overseas Returnee Research Start-up Fund (2004-527), Chongqing Science and Technology Commission projects (CSTC2004BA4002, CSTC2004AA4003-6, 8656), Chongqing Overseas Returnee Fund (2005-94), and Chongqing Institute of Technology Fund (2003ZD31). The industrial partner was the Fifth Research Institute of Ordnance Industry under China North Industries Group Corporation. This work addresses a fundamental challenge in magnesium alloy welding: controlling the thermal cycle to achieve acceptable hardness and mechanical properties in the weld and heat-affected zones.

Core Technical Content

AZ91D is a cast magnesium alloy containing approximately 9.0 wt% aluminum and 1.0 wt% zinc. The alloy exhibits good castability and corrosion resistance but has limited weldability due to its low melting point (approximately 580°C), high thermal conductivity, and susceptibility to oxidation. The thermal cycle during TIG welding significantly affects the microstructure and hardness distribution in the weld zone.

The study systematically varies welding parameters to investigate their effects on the thermal cycle and resulting hardness profile. Key parameters include:

• Welding current: 100–250 A
• Travel speed: 50–200 mm/min
• Arc voltage: 14–18 V
• Shielding gas flow rate: 10–20 L/min
• Electrode diameter: 2.4–3.2 mm
• Electrode stickout: 6–10 mm

The hardness distribution across the weld zone is characterized by three distinct regions:

1. Weld metal: Relatively low hardness (60–80 HV) due to grain refinement and solidification structure
2. Heat-affected zone: Variable hardness depending on peak temperature exposure
3. Base metal: Baseline hardness of approximately 80–90 HV

Thermal Cycle Parameters and Hardness Distribution

The study demonstrates that the welding thermal cycle has a profound effect on the hardness distribution in the weld zone. Higher welding currents produce wider heat-affected zones with lower hardness due to grain growth and softening of the base metal microstructure. Lower travel speeds also increase heat input and reduce hardness in the HAZ.

The optimal welding parameters identified in the study are 150 A current and 100 mm/min travel speed, which produce a weld with acceptable hardness distribution and minimal property degradation in the HAZ. Under these conditions, the minimum hardness in the HAZ is approximately 70 HV, which represents a 15–20 percent reduction from the base metal hardness.

Microstructural Analysis and Hardness Mechanisms

The hardness variations observed in the study are attributed to several microstructural mechanisms:

• Grain growth in the HAZ reduces hardness due to decreased grain boundary density
• Dissolution of Mg₁₇Al₁₂ intermetallic compounds at high temperatures reduces precipitation strengthening
• Recrystallization in the HAZ eliminates strain hardening and reduces hardness
• Solidification structure in the weld metal produces a fine-grained equiaxed morphology with moderate hardness

The thermal cycle during TIG welding of AZ91D can be characterized by three critical temperatures:

1. Peak temperature: Determines the extent of grain growth and phase transformation
2. Time above 400°C: Controls the dissolution of Mg₁₇Al₁₂ compounds
3. Cooling rate: Affects the precipitation behavior and final microstructure

The study shows that the time above 400°C is the most critical parameter for hardness control. Welds with time above 400°C less than 5 seconds exhibit minimum hardness of 75–80 HV, while welds with time above 400°C greater than 15 seconds exhibit minimum hardness of 55–65 HV.

Engineering Applications and Process Optimization

For practical TIG welding of AZ91D magnesium alloy, the following process recommendations are derived from the study:

• Use the lowest practical welding current to minimize heat input
• Maintain travel speed above 80 mm/min to limit HAZ width
• Use pure argon shielding gas with flow rate of 12–15 L/min
• Employ a 2.4 mm or 3.0 mm tungsten electrode with 8 mm stickout
• Apply pre-weld cleaning to remove oxide layers and prevent contamination
• Use AC welding if possible to provide cleaning action on both sides of the joint

The study also recommends post-weld stress relief treatment at 200°C for 2 hours to reduce residual stresses without significantly affecting hardness. This treatment can improve the service life of welded components without compromising mechanical properties.

Quality control measures for AZ91D TIG welds should include:

• Visual inspection for surface quality and geometry
• Hardness testing across the weld zone to verify property distribution
• Non-destructive testing using dye penetrant or ultrasonic methods
• Tensile testing of representative welds for strength verification
• Microstructural examination of weld cross-sections

Key Questions and Reflections

A significant question raised by this research is the effect of welding thermal cycle on the corrosion resistance of AZ91D welds. The study focuses on hardness, but the microstructural changes that affect hardness also influence corrosion behavior. Engineers should consider the corrosion performance of welds when selecting welding parameters, particularly for applications in aggressive environments.

Another consideration is the effect of joint design and welding sequence on the overall thermal cycle and resulting property distribution. The study focuses on single-pass butt welds, but practical applications often involve multi-pass welds, fillet welds, or complex geometries that introduce additional thermal cycling.

The study should be supplemented with fatigue testing and long-term creep evaluation to fully characterize the service behavior of AZ91D welds. Additionally, the effect of welding parameters on the anodizing performance of the weld zone should be investigated, as surface treatment is often required for magnesium alloy components.

Study Insights and Implications

This research provides valuable guidance for the TIG welding of AZ91D magnesium alloy. The study demonstrates that welding parameters can be optimized to produce welds with acceptable hardness distribution and minimal property degradation in the heat-affected zone.

For engineers involved in magnesium alloy welding, the key takeaway is that thermal cycle control is essential for achieving acceptable mechanical properties. The optimal welding parameters identified in this study should be used as a starting point for process development, with further optimization based on specific application requirements.

The study also highlights the importance of understanding the fundamental mechanisms that govern hardness variation in the weld zone. Engineers should consider the thermal cycle parameters, not just the welding parameters, when developing welding procedures for magnesium alloys. Future research should extend this work to include fatigue performance, corrosion resistance, and long-term service behavior of AZ91D welds under various thermal cycle conditions.