Effect of Heat Input on Microstructure and Mechanical Properties of AZ91D Magnesium Alloy TIG Weld Joints

Literature Overview

This 2017 paper published in Heat Processing Technology (热加工工艺) by Li Caixia from Chongqing Technology and Business University and Wang Jing from Chongqing University examines the influence of heat input on the microstructure and mechanical properties of TIG-welded joints in AZ91D magnesium alloy. The research is supported by the Chongqing Municipal Education Commission Science and Technology Research Project (KJ1503906). AZ91D is a widely used cast magnesium alloy in automotive and aerospace applications, known for its high specific strength but limited corrosion resistance and weldability.

Core Technical Content

AZ91D magnesium alloy has a nominal composition of 9.0 wt% Al, 1.0 wt% Zn, and 0.2 wt% Mn, with the balance being Mg. The alloy is typically supplied in the as-cast condition with a microstructure consisting of α-Mg matrix grains, α+β eutectic interdendritic regions (where β is Mg₁₇Al₁₂), and occasional AlMg₅ and MgZn₂ phases. The welding of AZ91D presents significant challenges due to the low melting point of magnesium (650°C), high thermal conductivity, and susceptibility to hot cracking and oxidation.

The heat input in TIG welding is defined as Q = (V × I × η) / v, where V is the arc voltage, I is the welding current, η is the heat transfer efficiency (typically 0.6–0.8 for TIG welding), and v is the travel speed. The heat input determines the thermal cycle experienced by the weld zone, which in turn governs the microstructure evolution and mechanical properties.

Microstructural Evolution with Heat Input

At low heat input, the rapid cooling rate promotes a fine dendritic solidification structure in the fusion zone with a high volume fraction of the β (Mg₁₇Al₁₂) phase at the interdendritic regions. The β phase is brittle and can act as a crack initiation site, but the fine spacing of the dendrites and the small size of the β particles limit the extent of cracking. The HAZ is narrow, with minimal exposure to temperatures above the recrystallization temperature, resulting in limited softening.

At medium heat input, the cooling rate is moderate, and the fusion zone develops a coarser dendritic structure with a more equiaxed morphology of the β phase. The HAZ widens, and the thermal exposure is sufficient to cause partial dissolution and coarsening of the β phase in the HAZ, leading to localized softening. The mechanical properties are generally at their optimum in this range, as the balance between strength (from the β phase) and ductility (from the matrix) is favorable.

At high heat input, the slow cooling rate produces a coarse dendritic structure with large, interconnected β phase networks. This microstructure is highly susceptible to cracking, both during solidification (hot cracking) and during service (intergranular fracture). The HAZ is wide, with significant exposure to temperatures that cause complete dissolution and coarsening of the β phase, resulting in severe softening. The tensile strength and elongation both decrease markedly.

Mechanical Property Analysis

The tensile properties of the TIG weld joint are dominated by the weakest region, which is typically the HAZ at high heat inputs or the fusion zone at low heat inputs. The base metal AZ91D typically exhibits a tensile strength of 220–240 MPa and an elongation of 3–4% in the as-cast condition. The weld joint strength is generally lower due to the microstructural changes introduced by the welding thermal cycle.

The hardness profile across the weld cross-section reveals a characteristic pattern:

• The fusion zone hardness is typically 40–60 HV, reflecting the relatively soft α-Mg matrix with dispersed β phase.
• The HAZ hardness decreases to 30–45 HV in the region of maximum softening, where the β phase has been partially dissolved.
• The base metal hardness is 50–70 HV, reflecting the fine β phase distribution in the as-cast microstructure.

The hardness minimum in the HAZ is a critical parameter for assessing the weld joint quality, as it indicates the region most susceptible to plastic deformation and crack initiation under load.

Engineering Practice Considerations

The welding of AZ91D magnesium alloy requires careful attention to several process parameters beyond heat input:

• Shielding gas: A mixture of argon and helium (typically 75% Ar / 25% He) is recommended to improve arc stability and heat input efficiency for magnesium alloys.
• Filler metal: A filler with a composition similar to the base metal (AZ91 or AZ92) is typically used. The filler composition should be adjusted to minimize hot cracking susceptibility — for example, by adding a small amount of rare earth elements (Ce, La) to refine the grain structure.
• Preheating: Preheating to 150–200°C is often necessary to reduce the cooling rate in the HAZ and minimize the risk of cold cracking due to hydrogen embrittlement.
• Interpass temperature: For multi-pass welds, the interpass temperature should be maintained below 250°C to avoid excessive grain growth and softening.

From a quality control perspective, the following inspections are recommended:

1. Visual inspection for surface defects, porosity, and undercut.
2. Radiographic testing (RT) or ultrasonic testing (UT) for internal defects such as porosity and lack of fusion.
3. Metallographic examination of the weld cross-section to assess the microstructure and identify any cracking.
4. Hardness traverse across the weld to map the softening zone.
5. Tensile testing of the weld joint to verify that the strength meets the acceptance criteria.

Key Reflections

This study provides valuable guidance for the optimization of TIG welding parameters in AZ91D magnesium alloy, which is increasingly used in lightweight structural applications. The key finding — that there exists an optimal heat input range that balances strength and ductility — is directly applicable to welding procedure qualification and process control in manufacturing.

The microstructural analysis reveals that the β (Mg₁₇Al₁₂) phase is the critical microstructural feature governing the mechanical properties of the weld joint. Its morphology, distribution, and volume fraction are all functions of the heat input, and understanding these relationships is essential for predicting and controlling weld joint performance.

From a broader perspective, the welding of magnesium alloys remains a challenging area of welding engineering due to the material's low melting point, high reactivity, and limited ductility. The TIG process, with its precise control over heat input and shielding gas coverage, is one of the most suitable processes for magnesium alloy welding, but even TIG welding requires careful parameter selection and process control to produce acceptable weld joints. The findings of this study contribute to the development of improved welding procedures and the qualification of welding processes for magnesium alloy components in automotive and aerospace applications.