Microstructure and Mechanical Properties of AZ91B Magnesium Alloy TIG Welds

Literature Overview

Published in Special Casting and Nonferrous Alloys in 2004, this study by Xu Jinfeng and Zhai Qiuya from Xi'an University of Technology examines the microstructure and mechanical properties of gas tungsten arc welded joints of AZ91B magnesium alloy. Magnesium alloys, particularly the AZ series (Al-Zn-Mg), are of growing interest for lightweight structural applications in automotive, aerospace, and consumer electronics industries. However, the welding of magnesium alloys presents unique challenges due to their high reactivity, low melting point, and susceptibility to porosity and hot cracking.

Welding Process Parameters and Challenges

AZ91B is a die-cast magnesium alloy with a nominal composition of 9 wt% Al, 1 wt% Zn, and 0.2 wt% Mn. The base metal exhibits a microstructure of α-Mg matrix with Mg17Al12 intermetallic phase at grain boundaries. TIG welding is a common process for magnesium alloy fabrication, but process control is critical. The following table summarizes typical TIG welding parameters and their effects:

Parameter	Typical Range	Effect on Weld Quality
Current	100–200 A	Higher current increases penetration but risks burn-through
Travel Speed	5–15 cm/min	Faster speed reduces heat input, minimizes distortion
Shielding Gas	Pure Ar or He/Ar mix	Argon is standard; helium improves penetration
Gas Flow Rate	10–20 L/min	Higher flow prevents oxidation but may cause turbulence
Interpass Temperature	<150 °C	Excessive temperature promotes oxidation and grain growth

The primary challenges in TIG welding AZ91B include: (1) severe oxidation of magnesium at elevated temperatures, requiring effective gas shielding; (2) high thermal conductivity leading to wide, shallow welds; (3) susceptibility to hot cracking due to the wide solidification range and low solubility of aluminum in the α-Mg solid solution; and (4) porosity formation from trapped hydrogen and gas entrapment during solidification.

Microstructural Characteristics of the Weld Joint

The as-welded joint exhibits distinct microstructural zones. The weld metal solidifies as a columnar dendritic structure of α-Mg with interdendritic Mg17Al12 eutectic. The cooling rate in TIG welding is relatively moderate, allowing some coarsening of the interdendritic phase. The HAZ adjacent to the fusion line shows a fully recrystallized structure with equiaxed α-Mg grains and a network of Mg17Al12 at grain boundaries. Further from the fusion line, the HAZ microstructure gradually transitions to the as-cast base metal structure, with some grain growth in the peak temperature zone.

The researchers found that the weld metal typically exhibits lower tensile strength than the base metal. The base metal AZ91B typically achieves a tensile strength of approximately 230–250 MPa, while the weld metal may reach only 150–180 MPa. This reduction is attributed to the coarser microstructure of the weld metal, the presence of porosity, and the redistribution of alloying elements during solidification. The elongation of the weld metal is also reduced due to the brittle Mg17Al12 phase network.

Defect Analysis and Countermeasures

Common defects observed in AZ91B TIG welds include:

Defect Type	Root Cause	Countermeasure
Porosity	Hydrogen absorption from moisture; gas entrapment	Thorough surface cleaning; use of dry shielding gas; preheat to reduce cooling rate
Hot Cracking	Wide solidification range; interdendritic Mg17Al12	Add Zn or adjust filler composition; reduce heat input
Oxidation	Insufficient gas shielding; high temperature	Increase gas flow; use pure argon; minimize interpass temperature
Burn-through	Excessive heat input	Reduce current; increase travel speed; use backing plate
Lack of fusion	Inadequate penetration	Increase current; optimize torch angle and travel speed

The study emphasizes the importance of filler metal selection. Using a pure magnesium or AZ91-composition filler wire can help minimize hot cracking by reducing the solidification range in the weld metal. However, the filler metal must be compatible with the base metal to avoid excessive segregation of intermetallic phases.

Engineering Practice and Study Reflections

The welding of AZ91B magnesium alloy is a practical challenge that requires careful process control and material selection. The study provides valuable baseline data on weld microstructure and mechanical properties that can guide welding procedure qualification. For engineering applications, the weld joint strength is often the limiting factor in component design, and designers must account for the reduced strength of the weld zone. Post-weld heat treatment, such as solution treatment and aging, can improve the weld metal properties by dissolving the coarse Mg17Al12 phase and precipitating fine strengthening phases. However, the thermal distortion caused by PWHT must be managed, particularly for complex geometries. This work underscores the importance of understanding the fundamental metallurgy of magnesium alloys to develop reliable welding procedures for lightweight structural applications.