Microstructure and Mechanical Properties of TIG Welded Joints of AZ91D Magnesium Alloy

Literature Overview

This 2009 paper by Liu Jun and colleagues from Inner Mongolia University of Technology, published in Light Alloy Fabrication Technology (Vol. 37, No. 2, pp. 42–44), investigates the microstructure and mechanical properties of TIG welded joints of AZ91D magnesium alloy. The authors used homogenous filler wire (matching the base metal composition) for TIG welding and employed metallographic microscopy, X-ray diffraction (XRD), tensile testing, and Brinell hardness testing to characterize the weld joint. The study provides fundamental data on the weldability of AZ91D, which is one of the most widely used die-cast magnesium alloys in automotive and aerospace applications.

Welding Process Parameters and Experimental Setup

The study identifies optimal welding parameters of 140 A current and 12 L/min argon gas flow rate. These parameters represent a balance between adequate heat input for complete fusion and avoidance of excessive melting that would lead to burn-through or excessive dilution.

The use of homogenous filler wire is a deliberate choice that simplifies the metallurgical analysis by eliminating composition mismatch between weld metal and base metal. However, this also means that the weld metal has similar susceptibility to solidification cracking as the base metal, which is a known concern for Al-containing magnesium alloys.

Microstructural Analysis

Weld Metal Microstructure

The weld metal exhibits fine, equiaxed grains that are finer and more uniform than the base metal grains. This is a characteristic feature of weld metal microstructure, resulting from the rapid cooling rates that occur during welding. The finer grain structure in the weld metal provides several benefits:

1. Higher strength: According to the Hall-Petch relationship, finer grains increase yield strength.
2. Better toughness: Fine grains improve fracture toughness by providing more crack deflection sites.
3. More uniform properties: Equiaxed grains avoid the anisotropy associated with columnar grain structures.

Phase Composition

XRD analysis reveals that the weld metal consists of α-Mg solid solution and Mg₁₇Al₁₂ intermetallic phase. This is consistent with the equilibrium phase diagram for the Mg-Al system and reflects the solidification behavior of the AZ91D alloy during welding.

The distribution and morphology of Mg₁₇Al₁₂ particles are critical for mechanical properties. In the base metal, these particles are typically distributed along grain boundaries and dendrite arms. In the weld metal, the rapid solidification may produce finer and more uniformly distributed particles, which can be beneficial for strength but may also create more potential crack initiation sites.

Heat-Affected Zone Microstructure

The HAZ typically shows a gradient of microstructural changes from the base metal to the weld metal. The peak temperature in the HAZ determines the extent of phase transformations and grain growth. For AZ91D, the HAZ may exhibit:

• Partial melting and resolidification near the fusion line
• Grain growth due to high temperatures
• Dissolution and reprecipitation of Mg₁₇Al₁₂ particles
• Potential formation of new phases due to non-equilibrium cooling

Mechanical Properties

Tensile Properties

The tensile strength of the weld joint is approximately 80% of the base metal strength. This is a typical value for magnesium alloy welds and reflects the combined effects of:

• Microstructural differences between weld metal and base metal
• Residual stresses in the HAZ
• Potential defects such as porosity or incomplete fusion
• The inherent weakness of the HAZ due to grain coarsening

The fracture occurring in the HAZ is a significant finding. In many welded joints, fracture initiates in the weld metal due to its coarser microstructure and higher susceptibility to defects. However, in AZ91D TIG welds, the HAZ is the weakest region, likely due to:

1. Grain coarsening: The high temperatures in the HAZ promote grain growth, reducing strength according to the Hall-Petch relationship.
2. Phase coarsening: Mg₁₇Al₁₂ particles may coarsen or dissolve and reprecipitate in a less favorable morphology.
3. Residual stresses: The HAZ experiences significant residual stresses due to differential thermal expansion during welding and cooling.

Hardness Distribution

The Brinell hardness profile across the weld joint typically shows:

The hardness dip in the HAZ is consistent with the tensile fracture occurring in this region. For applications requiring high joint strength, post-weld heat treatment (PWHT) may be necessary to restore HAZ properties through precipitation hardening.

Engineering Practice Implications

Welding Process Optimization

Based on this study, the following recommendations can be made for TIG welding of AZ91D:

1. Current control: Maintain welding current at or below 140 A to avoid excessive heat input that would further degrade HAZ properties.
2. Gas shielding: Ensure adequate argon flow (≥12 L/min) to prevent oxidation, which is particularly critical for magnesium alloys that form thick, non-protective oxide layers.
3. Filler wire selection: Homogenous filler wire provides composition match but may not be optimal for all applications. For higher strength requirements, consider filler wires with modified composition.
4. Joint design: Design joints to minimize HAZ exposure to critical loads. Consider using overlap joints or other configurations that reduce the proportion of HAZ in the load-bearing cross-section.

Post-Weld Heat Treatment

Given that the HAZ is the weakest region, post-weld heat treatment should be considered for applications requiring high joint strength. Common PWHT options for AZ91D include:

The combination of solution treatment and aging (T6 temper) can significantly improve HAZ strength and overall joint properties. However, PWHT introduces additional costs and potential distortion, which must be weighed against the strength benefits.

Key Reflections

This study provides a solid foundation for understanding the TIG weldability of AZ91D magnesium alloy. The finding that the HAZ is the weakest region is particularly important for engineering design, as it means that joint strength is limited not by the weld metal but by the thermal effects on the base metal. This has implications for fatigue design, as the HAZ may be susceptible to fatigue cracking under cyclic loading.

The use of homogenous filler wire simplifies the metallurgical analysis but may not represent the optimal approach for all applications. In practice, engineers should consider the trade-offs between composition matching and property optimization when selecting filler wires for magnesium alloy welding.

I reflect on the fact that magnesium alloy welding remains a challenging field due to the inherent reactivity of magnesium, the susceptibility to porosity, and the sensitivity of the HAZ to thermal effects. The 80% joint strength achieved in this study is respectable for a simple TIG process without PWHT, but further improvements are likely necessary for demanding structural applications. The combination of process optimization, filler wire selection, and post-weld treatment represents the most promising path toward higher-strength AZ91D welds.