CLADDING TECHNOLOGY SHANXI CO., LTDEffects of MIG Welding Parameters on AZ31 Magnesium Alloy
Literature Overview
This study by Liu Peiye, Hou Jibo, and Liu Yanhui from the School of Materials Science and Engineering at North University of China, published in Special Casting and Nonferrous Alloys in 2017, investigates how MIG (Melted Inert Gas) welding parameters influence the weld quality of AZ31 magnesium alloy. AZ31 is an age-hardenable magnesium alloy containing approximately 3 wt% aluminum and 1 wt% zinc, widely used in lightweight structural applications where specific strength is critical. The research examines the interplay between welding current, voltage, travel speed, shielding gas composition, and preheating temperature on weld bead geometry, microstructure, and mechanical properties.
Core Technical Findings
The study establishes several critical parameter windows for successful MIG welding of AZ31:
| Parameter | Optimal Range | Effect on Weld Quality |
|---|---|---|
| Welding current | 120-180 A | Controls penetration depth and bead width |
| Voltage | 16-22 V | Affects arc stability and spatter rate |
| Travel speed | 400-700 mm/min | Determines heat input and dilution |
| Shielding gas | 99.99% Ar or Ar+5% CO2 | Prevents oxidation and porosity |
| Preheat temperature | 100-200 °C | Reduces hot cracking susceptibility |
| Wire diameter | 1.0-1.2 mm | Influences deposition rate and arc characteristics |
The researchers found that the heat input (calculated as current × voltage / travel speed) is the dominant factor governing weld morphology. Excessive heat input leads to excessive grain growth in the heat-affected zone (HAZ), while insufficient heat input causes incomplete fusion and undercutting. The optimal heat input range was identified as approximately 0.6-1.2 kJ/mm for typical AZ31 sheet thicknesses of 2-4 mm.
Microstructural Analysis and Defect Mechanisms
The microstructure of the weld zone in AZ31 MIG welds exhibits distinct characteristics that directly impact mechanical performance. The weld metal typically shows a dendritic structure with interdendritic phases of Mg17Al12 (β-phase), while the HAZ displays a mixture of recrystallized and partially recrystallized grains. The study highlights that the eutectic temperature of the Mg-Al system (approximately 449 °C) creates a narrow solidification range that makes the alloy particularly susceptible to hot cracking during welding.
The primary defects identified include:
- Hot cracking — Occurs at the grain boundaries in the weld centerline when the solidification rate is too slow, allowing the Mg17Al12 eutectic to form continuous networks that facilitate crack propagation under residual stress.
- Pitting corrosion pits — Result from insufficient shielding gas coverage or excessive spatter, leading to localized aluminum oxide inclusion formation.
- Porosity — Arises from hydrogen absorption during welding, as magnesium alloys readily pick up hydrogen from moisture in the atmosphere or contaminated filler wire.
- Undercut — Manifests at the weld toe when travel speed exceeds the optimal range, creating stress concentration sites.
Engineering Practice Implications
From my perspective in bimetal product manufacturing and pressure vessel fabrication, the findings of this study carry significant relevance for applications involving magnesium alloy components in composite structures. While magnesium alloys are not commonly used as cladding materials in traditional pressure vessel construction, they appear increasingly in lightweight aerospace and automotive structural applications where hybrid joining strategies are employed.
The key engineering lesson is that magnesium alloy welding demands much tighter parameter control than carbon steel or stainless steel welding. The narrow solidification range (approximately 200 °C) compared to carbon steel (which can exceed 300-400 °C depending on composition) means that process windows are narrower and deviations more consequential. In practice, I have observed that manufacturers of AZ31 welded structures invest heavily in automated welding systems with real-time monitoring of arc voltage and current to maintain parameters within the identified optimal ranges.
Preheating to 100-200 °C is particularly important for thicker sections (above 4 mm), as it reduces the cooling rate at the weld centerline and allows more time for interdendritic feeding, thereby mitigating hot cracking. Post-weld heat treatment (solution treatment at 415 °C for 2-4 hours followed by aging at 175 °C) is essential to restore mechanical properties in the weld and HAZ regions, as the as-welded microstructure typically shows 15-25% reduction in ultimate tensile strength compared to the base metal.
Study Insights and Reflections
This literature provides a systematic foundation for understanding the fundamental challenges of magnesium alloy MIG welding. The emphasis on shielding gas purity and preheating temperature reflects a broader principle in reactive metal welding: controlling the thermal cycle and atmospheric environment are equally important as the electrical parameters themselves. For engineers working in the pressure vessel and clad equipment industry, the methodology of systematically varying one parameter while holding others constant provides a transferable approach to welding procedure qualification under standards such as NB/T 47014 or ASME IX, even though those standards primarily address ferrous materials. The recognition that AZ31 welds require post-weld heat treatment to achieve acceptable mechanical properties parallels the requirements for certain austenitic stainless steel welds in pressure vessel fabrication, where solution heat treatment or stabilization annealing is mandatory.