Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of AZ91B Magnesium Alloy TIG Welded Joints
Literature Overview
This study by Xu Jinfeng and Zhai Qiuya from Xi'an University of Technology, published in Special Casting and Nonferrous Alloys in 2004 (Volume 24, Issue 4, pages 23-25), investigates the microstructure and mechanical properties of TIG welded joints of AZ91B magnesium alloy using two different filler wires: AZ61 and AZ91. The research addresses a critical practical challenge in magnesium alloy welding: the selection of filler metal that balances weldability, joint integrity, and mechanical compatibility with the base metal.
Core Technical Content
AZ91B is a widely used cast magnesium alloy containing approximately 9% aluminum and 1% zinc, with the balance being magnesium and minor impurities. The alloy is valued for its good castability and moderate mechanical properties, making it suitable for automotive and aerospace applications. However, magnesium alloys present significant welding challenges due to their high thermal conductivity, low melting point, tendency to oxidize rapidly, and susceptibility to hot cracking during solidification.
The study systematically examines the microstructural characteristics of three distinct regions of the welded joint: the weld metal, the fusion zone (also referred to as the partially melted zone), and the heat-affected zone (HAZ). Each region exhibits a characteristic microstructure that reflects its unique thermal history during the welding process.
Microstructural Analysis
| Region | Microstructure Description | Key Features |
|---|---|---|
| Weld Metal | Well-developed dendritic α-Mg with snowflake morphology; interdendritic (α + Mg17Al12) low-melting-point eutectoid | High eutectoid content due to Mg evaporation; increased Al content |
| Fusion Zone | Fine α-Mg equiaxed grains with interdendritic eutectoid | Rapid solidification; fine grain structure |
| HAZ | Coarse α-Mg dendrites with α + Mg17Al12 eutectoid distributed between dendrites | Similar to base metal but with coarser grain size due to recrystallization |
The weld metal microstructure is dominated by well-developed dendritic α-Mg crystals with a distinctive snowflake morphology, reflecting the rapid solidification conditions and the influence of the welding thermal cycle. The interdendritic regions contain the low-melting-point (α + Mg17Al12) eutectoid, which forms during the final stages of solidification. The high volume fraction of this eutectoid phase in the weld metal is attributed to the evaporation of magnesium during the welding process, which effectively increases the aluminum content of the weld metal relative to the base metal composition.
Filler Wire Selection and Weldability
A critical finding of this study is the comparison between AZ61 and AZ91 filler wires for welding AZ91B base metal. Both filler wires produce sound welds with good fusion and no major defects, but AZ61 filler wire offers superior performance in terms of hot cracking resistance and microstructural compatibility with the base metal.
The hot cracking susceptibility of magnesium alloy welds is primarily governed by the solidification range and the volume fraction of low-melting-point eutectoid at the interdendritic regions. The Mg17Al12 eutectoid has a melting point of approximately 449°C, which is significantly lower than the solidus temperature of the AZ91B alloy. During solidification, these low-melting-point phases remain liquid while the dendritic α-Mg skeleton has already formed, creating a network of brittle phases that are susceptible to cracking under the tensile stresses induced by solidification shrinkage and thermal contraction.
The use of AZ61 filler wire, which contains approximately 6% aluminum compared to 9% in AZ91B, results in a weld metal with a composition that is closer to the eutectic composition. This produces a weld microstructure that is more consistent with the base metal in terms of phase distribution and reduces the volume fraction of the low-melting-point eutectoid relative to what would be produced with an AZ91 filler wire. The lower aluminum content in the AZ61 weld metal also reduces the thermal cracking tendency by narrowing the solidification range and promoting more uniform solidification.
Engineering Practice Implications
For practical welding of AZ91B magnesium alloy components, the following recommendations emerge from this study:
- Filler wire selection: AZ61 filler wire is preferred over AZ91 for welding AZ91B base metal, as it provides better hot cracking resistance and more compatible microstructure.
- Shielding gas protection: Given the high oxidation susceptibility of magnesium, inert gas shielding (argon or helium) must be maintained continuously throughout the welding process, including preheating and post-weld cooling.
- Heat input control: The welding parameters should be selected to minimize the time spent in the temperature range where hot cracking is most likely to occur (approximately 400-500°C for AZ91B).
- Post-weld inspection: The weld joints should be carefully inspected for hot cracks, particularly at the weld surface and at the weld toes, using non-destructive testing methods such as penetrant testing or ultrasonic testing.
Key Questions and Reflections
One significant limitation of this study is the absence of quantitative mechanical property data. While the study describes the microstructural differences between the weld regions, it does not provide tensile strength, elongation, or hardness measurements that would allow for a more comprehensive evaluation of joint performance. For engineering qualification purposes, these mechanical data are essential for establishing acceptance criteria and welding procedure specifications.
Another reflection concerns the role of preheating. The study does not address the effect of preheating temperature on the weld microstructure and cracking tendency. In practice, preheating is often used to reduce the cooling rate and minimize the thermal gradient in the weld region, which can significantly influence the hot cracking susceptibility of magnesium alloy welds.
Study Insights and Implications
This research provides a clear understanding of the microstructural evolution in AZ91B magnesium alloy TIG welded joints and offers practical guidance for filler wire selection. The finding that AZ61 filler wire produces welds with better hot cracking resistance than AZ91 filler wire is consistent with the general principle that filler metals with lower aluminum content should be used for welding high-aluminum magnesium alloys to reduce the volume fraction of low-melting-point eutectoid in the weld metal.
The snowflake morphology of the α-Mg dendrites in the weld metal is a distinctive feature that can be used as a qualitative indicator of weld quality during metallographic examination. Welds that exhibit this characteristic morphology are likely to have solidified under conditions that promote uniform dendritic growth, while deviations from this morphology may indicate process abnormalities such as excessive heat input or inadequate shielding.