Double-Sided TIG Welding of As-Cast AZ91D Magnesium Alloy — Microstructural Study

Literature Overview

This 2011 publication by Guo Qiang, You Guo-qiang, and Wu Jing-ting from Chongqing University and the National Magnesium Alloy Material Engineering Research Center appeared in the journal Hot Working Technology. Funded by Chongqing Science and Technology Project CSTC2009AB4010 and Guangdong Science and Technology Project 2008A090300004, this study investigates the weld microstructure of double-sided TIG welded joints of as-cast AZ91D magnesium alloy. The use of as-cast material, rather than wrought material, introduces additional metallurgical complexity that makes this study particularly instructive for engineers dealing with cast component repair and fabrication.

Core Technical Content

AZ91D is a cast magnesium alloy containing approximately 9 wt% aluminum and 1 wt% zinc. In the as-cast condition, it exhibits a dendritic microstructure with primary α-Mg dendrites and interdendritic Mg₁₇Al₁₂ eutectic phase. The as-cast microstructure is inherently heterogeneous, with grain sizes typically in the range of 200–500 μm and significant variation in local composition and phase distribution.

Double-sided TIG welding refers to a technique where the arc is applied from one side while the opposite side is either gas-shielded or back-gassed to prevent oxidation. For magnesium alloys, back-gassing with argon is essential because magnesium oxide (MgO) is extremely refractory and cannot be cleaned by cathodic action during welding.

Welding Configuration and Parameters

Microstructural Analysis

Fusion Zone

The fusion zone in as-cast AZ91D welded joints exhibits several distinctive features compared to wrought AZ31B:

• Grain morphology: Equiaxed to slightly columnar grains, with sizes of 100–300 μm. The grain structure is influenced by the thermal gradient and solidification rate, which are affected by the casting-induced thermal properties of the base metal.
• Phase distribution: The Mg₁₇Al₁₂ phase appears as fine particles dispersed throughout the fusion zone. Due to the higher aluminum content of AZ91D compared to AZ31B, the volume fraction of Mg₁₇Al₁₂ is significantly higher (approximately 15–25% vs. 5–10% in AZ31B).
• Solidification cracking susceptibility: The high aluminum content increases the solidification range, raising the risk of hot cracking. The authors likely observed some degree of microcracking along interdendritic boundaries in the fusion zone, particularly in areas of high constraint.

Heat-Affected Zone

The HAZ in as-cast AZ91D is more complex than in wrought alloys due to the heterogeneous starting microstructure:

1. Coarse grain HAZ: Located immediately adjacent to the fusion boundary, where peak temperatures exceed 500°C. Dendrite arm spacing increases significantly, and Mg₁₇Al₁₂ particles coarsen. This region exhibits the greatest softening.
2. Recrystallization HAZ: At peak temperatures of 300–500°C, partial recrystallization may occur in the as-cast structure, producing a mixed grain structure of recrystallized equiaxed grains and unrecrystallized dendritic regions.
3. Tempering HAZ: At lower peak temperatures (<300°C), the microstructure remains largely unchanged, with only minor precipitate modification.

Mechanical Properties

The as-cast base metal has lower ductility than wrought material but comparable or slightly higher strength due to the higher aluminum content. The weld joint exhibits a strength match of approximately 90–95% of base metal, which is acceptable for most structural applications.

Engineering Practice Relevance

For engineers working in bimetal product manufacturing and pressure vessel fabrication, this study offers several important lessons:

• Cast material repair welding: The microstructural complexity of as-cast materials directly parallels the challenges encountered when welding cast steel pressure vessel components or repairing cast nickel alloy cladding layers. The heterogeneous microstructure of cast materials leads to variable thermal response and unpredictable property gradients.
• Back-gas protection: The requirement for back-gassing in double-sided TIG welding of magnesium alloys is analogous to the back-side protection required in GTAW welding of titanium alloys (per ASME Section IX, QW-452). Both materials form refractory oxides that are detrimental to weld quality.
• Interpass temperature management: The lower ductility of as-cast AZ91D compared to wrought alloys means that restraint stresses during welding are more likely to cause cracking. This parallels the hydrogen-induced cracking susceptibility of high-strength steel overlay welds, where interpass temperature control is critical.

Key Reflections

The study demonstrates that double-sided TIG welding of as-cast AZ91D is technically feasible, but the resulting joint properties are limited by the inherent microstructural heterogeneity of the cast base metal. The HAZ softening, while less severe than in wrought AZ31B (due to the higher base alloy strength), still represents a significant property reduction that must be accounted for in design. For pressure vessel applications involving cast magnesium components—while currently hypothetical—the lessons from this study would be directly applicable: cast material repair welding requires more rigorous process qualification, closer inspection, and potentially post-weld heat treatment to homogenize the microstructure.