Zhuojin Pipe Fitting Co., LtdMIG Welding of AZ91D Magnesium Alloy Using Pure Aluminum Filler Wire
Literature Overview
This research by Mao Jin, Hou Jibo, Xu Kai, and Liu Yaxin from the North University of China's School of Materials Science and Engineering examines the weldability of AZ91D magnesium alloy using pure aluminum filler wire in a MIG welding process. Published in "Foundry Technology" (Vol. 39, Issue 2, 2018, pp. 400-403), the study addresses a practical challenge in magnesium alloy fabrication: the limited availability and high cost of magnesium-based filler wires, prompting investigation of aluminum wire as an alternative consumable. The research evaluates both the microstructural characteristics and the practical process challenges encountered during welding.
Core Technical Findings
The most significant finding is the formation of extensive α-Mg + β-Mg₁₇Al₁₂ eutectic microstructure in the weld metal. This eutectic phase, which forms at the Mg-Al binary eutectic composition (approximately 29 wt% Al), is fundamentally different from the microstructure of the AZ91D base metal, which contains dispersed β-Mg₁₇Al₁₂ particles within an α-Mg matrix. The presence of large volumes of eutectic structure in the weld is identified as the primary cause of degraded mechanical properties and poor joint performance.
Process Defect Analysis
| Defect Category | Description | Severity | Root Cause |
|---|---|---|---|
| Narrow Process Window | Limited range of acceptable welding parameters | High | High sensitivity of Mg-Al system to heat input |
| Excessive Spatter | Significant wire and molten metal ejection | High | Arc instability with Al wire in Mg base |
| Sagging (Drooping) | Loss of weld bead geometry in horizontal/overhead positions | Moderate | Low surface tension of Mg-Al liquid |
| No Reinforcement | Weld bead flush or concave with no convex profile | Moderate | Excessive melting of base metal relative to filler |
The narrow process window is particularly problematic for production environments where consistent quality must be maintained across varying joint geometries and positions. The excessive spatter not only wastes consumable but also creates porosity in the weld as spatter particles become entrapped in the solidifying melt.
Metallurgical Interpretation
The AZ91D base metal composition is approximately 9% Al and 1% Zn in a magnesium matrix. When pure aluminum wire is used as filler, the dilution ratio determines the final weld composition. If the aluminum dilution exceeds approximately 30%, the weld composition shifts toward the Mg-Al eutectic, resulting in the observed α-Mg + β-Mg₁₇Al₁₂ eutectic structure. This eutectic structure is inherently weaker than the base metal microstructure because:
- The eutectic lamellar spacing is coarse, reducing dislocation density and strengthening effects.
- The β-Mg₁₇Al₁₂ phase is brittle and acts as crack initiation sites under tensile loading.
- The lack of Zn in the weld composition eliminates the grain refinement effect that Zn provides in AZ91D.
Recommended Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Welding Current | 120-160 A | Lower end preferred for thin sections |
| Welding Voltage | 18-22 V | Maintain spray transfer mode |
| Travel Speed | 400-700 mm/min | Higher speed reduces heat input |
| Shielding Gas | Pure Ar | Minimum 15 L/min flow rate |
| Wire Diameter | 1.0 mm | Pure Al ER1100 equivalent |
| Joint Preparation | V-groove or square butt | Fit-up tolerance ±0.5 mm |
Engineering Practice Implications
The study's conclusion that pure aluminum wire is technically feasible but produces inferior joints compared to Mg-based filler wires (such as ER51A or ZL102) has significant implications for manufacturing decisions. While the cost savings of using aluminum wire may be attractive, the resulting joint weakness and process difficulties likely result in higher overall production costs due to:
- Increased rework rates from weld defects.
- Reduced service life requiring earlier replacement.
- Inability to meet design strength requirements without joint redesign.
For applications where magnesium alloy welding is required, the use of proper Mg-Al-Zn filler wire matching the base composition is strongly recommended. The study serves as a cautionary reference, demonstrating that filler wire selection cannot be compromised without accepting significant performance penalties.
Study Insights and Reflections
The research provides valuable insight into the metallurgical consequences of filler/base metal mismatch in magnesium alloy welding. The identification of the eutectic structure as the root cause of poor performance is technically sound and aligns with the phase diagram predictions for the Mg-Al binary system. However, the study would benefit from quantitative mechanical property data (tensile strength, elongation, hardness profiles) to fully characterize the performance gap between the eutectic weld and the base metal. Additionally, the study does not explore intermediate compositions that might offer a compromise between cost and performance, such as using a Mg-Al-Zn wire with reduced Zn content. The practical process difficulties identified—particularly the narrow window and spatter—suggest that even with proper filler wire, AZ91D MIG welding requires careful parameter control and skilled operators.