CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Properties of AZ31B Magnesium Alloy MIG Welds
Literature Overview
This 2020 study from Fujian University of Technology, supported by the Fujian Province University-Industry Cooperation Project (2019H6021), investigates how welding current parameters influence the microstructure and mechanical properties of MIG welds in AZ31B magnesium alloy. The research addresses a critical materials challenge: magnesium alloys offer exceptional lightweighting potential but exhibit significant challenges in welding due to their low melting point, high thermal conductivity, and susceptibility to porosity and cracking.
AZ31B Magnesium Alloy Characteristics
AZ31B is a wrought magnesium alloy containing approximately 3% aluminum and 1% zinc, with the remainder being magnesium. Its key characteristics relevant to welding include:
- Low melting point (481°C), requiring careful thermal management
- High thermal conductivity, which promotes heat dissipation but also causes rapid cooling
- Limited solid solubility of aluminum in magnesium, affecting precipitate behavior
- Susceptibility to hot cracking during solidification
- Oxidation resistance challenges requiring effective shielding
For pressure vessel and structural applications, AZ31B welds must achieve adequate strength, ductility, and corrosion resistance while maintaining the weight advantages of the magnesium base metal.
Welding Current Effects
The study systematically varied welding current from 60 A to 180 A and examined the resulting microstructural and mechanical property changes:
| Current (A) | Heat Input (kJ/cm) | Weld Width (mm) | Penetration (mm) | HAZ Width (mm) |
|---|---|---|---|---|
| 60 | 0.8 | 3.5 | 0.8 | 1.2 |
| 90 | 1.2 | 5.0 | 1.5 | 2.0 |
| 120 | 1.8 | 7.0 | 2.2 | 3.0 |
| 150 | 2.5 | 9.5 | 3.0 | 4.5 |
| 180 | 3.5 | 12.0 | 3.8 | 6.0 |
Microstructural Evolution
The microstructure of the weld metal evolves significantly with current level:
Low current (60-90 A): The weld metal exhibits a fine-grained equiaxed structure with limited columnar grain development. The rapid solidification rate promotes a high density of precipitates, which can improve strength but may reduce ductility.
Medium current (120-150 A): A mixed grain structure develops with equiaxed grains near the fusion boundary transitioning to columnar grains toward the weld centerline. The columnar grain width increases with current, and the grain orientation aligns with the thermal gradient direction.
High current (180 A): Coarse columnar grains dominate the weld metal, with grain widths exceeding 50 μm. The high heat input promotes significant grain growth and coarsening of precipitates, reducing the strengthening effect of second-phase particles.
Mechanical Property Response
| Current (A) | Tensile Strength (MPa) | Elongation (%) | Hardness (HV) |
|---|---|---|---|
| 60 | 165 | 12 | 52 |
| 90 | 175 | 14 | 55 |
| 120 | 185 | 16 | 58 |
| 150 | 170 | 13 | 54 |
| 180 | 155 | 10 | 48 |
The results demonstrate a clear optimum in the medium current range (120-150 A), where both strength and ductility are maximized. At lower currents, incomplete fusion and inadequate penetration limit joint strength. At higher currents, excessive grain coarsening and potential hot cracking reduce both strength and ductility.
Defect Analysis
The study identified several defect types associated with different current levels:
- Porosity: Most prevalent at low currents due to incomplete gas shielding and inadequate arc stability; reduced at medium currents but can reappear at high currents due to increased turbulence
- Hot cracking: Absent at low and medium currents but appears at 180 A due to high peak temperatures and restricted solidification shrinkage
- Undercut: More common at high currents where excessive arc force erodes the base metal near the fusion boundary
- Incomplete fusion: Occurs at low currents when insufficient heat input fails to achieve complete coalescence
Engineering Practice Considerations
For magnesium alloy pressure vessels and structural components, the welding current selection must balance multiple competing requirements:
- Joint strength: Medium currents (120-150 A) provide the best combination of strength and ductility
- Distortion control: Lower currents minimize thermal distortion but may require multiple passes
- Productivity: Higher currents increase deposition rates but compromise weld quality
- Corrosion resistance: The microstructure at medium currents provides better resistance to stress corrosion cracking
The optimal welding current for AZ31B MIG welding is approximately 120-130 A for single-pass welds on sheets up to 3 mm thick. For thicker sections, multi-pass welding with interpass temperature control is recommended, with each pass maintained within the optimal current window.
Study Insights
This research underscores the critical importance of welding parameter optimization for magnesium alloys, where the narrow process window leaves little margin for error. The current-induced microstructural changes directly affect the long-term performance of welded joints, particularly under cyclic loading or corrosive environments.
For engineers involved in lightweight pressure vessel design, the findings suggest that magnesium alloy welding requires process qualification protocols more rigorous than those typically applied to steel or aluminum. The sensitivity of weld properties to current variations means that process control must be maintained throughout production, with regular verification of welding parameters and periodic mechanical property testing.