Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of AZ91D Magnesium Alloy Q345 Steel Dissimilar MIG Welded Joints
Literature Overview and Dissimilar Welding Challenges
The paper by Wen Ruili from the Department of Materials and Welding Technology, Hebei Locomotive Technician College, and the School of Management, Hebei Agricultural University, published in Hot Working Technology (2018, Vol. 47, No. 11, pp. 225-228), investigates the microstructure and mechanical properties of AZ91D magnesium alloy/Q345 steel dissimilar MIG welded joints. This research was supported by the National Natural Science Foundation of China (Grant No. 61475110).
The welding of dissimilar metals, particularly magnesium alloys to steels, presents unique challenges due to the significant differences in physical and metallurgical properties between the two materials. AZ91D is a widely used cast magnesium alloy with excellent specific strength and good castability, while Q345 is a common low-carbon structural steel with good weldability and mechanical properties. The combination of these two materials may be encountered in lightweight vehicle structures, where magnesium alloy components are joined to steel frames.
The fundamental challenges in dissimilar welding of magnesium alloy to steel include:
- Large difference in thermal conductivity (magnesium: ~70 W/m·K; steel: ~50 W/m·K)
- Large difference in thermal expansion coefficient (magnesium: ~26 × 10⁻⁶/K; steel: ~12 × 10⁻⁶/K)
- Formation of brittle intermetallic compounds at the interface
- Different melting points (magnesium: 650°C; steel: 1500°C)
- Different solidification characteristics and shrinkage behavior
Microstructural Characterization of the Weld Joint
The microstructural examination of the AZ91D/Q345 dissimilar welded joints reveals distinct regions with different microstructural characteristics. The weld zone consists of equiaxed alpha-Mg crystals, indicating that the weld metal composition is dominated by the magnesium alloy side. This is expected because the magnesium alloy has a much lower melting point and is more readily melted and transferred to the weld pool.
| Region | Microstructure | Grain Size | Phase Composition |
|---|---|---|---|
| Weld zone | Equiaxed α-Mg | Fine to moderate | α-Mg (Mg₃Al₅ may be present) |
| Fusion zone (AZ91D side) | Uneven α-Mg grains | Relatively fine overall | α-Mg with possible Al-rich phases |
| Transition layer (interface) | Mixed phases | Variable | Intermetallic compounds (Mg-Fe, Mg-Al-Fe) |
| HAZ (AZ91D side) | Coarse grains | Large | α-Mg with possible grain boundary precipitation |
| HAZ (Q345 side) | Ferrite-pearlite | Coarse | Ferrite + pearlite (tempered) |
The fusion zone on the AZ91D side exhibits α-Mg grains with uneven sizes, but the overall grain size is relatively fine. This is attributed to the rapid solidification rate at the fusion boundary, which promotes nucleation and limits grain growth. The heat-affected zone (HAZ) on the AZ91D side shows coarser grains due to the thermal cycle experienced during welding, which promotes grain growth.
A transition layer of variable thickness forms at the AZ91D/Q345 interface. This layer consists of intermetallic compounds formed by the reaction between magnesium and iron/aluminum from the steel side. The thickness and composition of this transition layer are influenced by welding parameters, particularly welding current and travel speed. The presence of this intermetallic layer is a critical factor affecting the mechanical properties of the joint, as intermetallic compounds are typically brittle and can act as crack initiation sites.
Mechanical Properties and Parameter Sensitivity Analysis
The mechanical properties of the AZ91D/Q345 dissimilar welded joints were evaluated through tensile testing. The results show that the tensile strength of the joint is influenced by both welding current and travel speed.
| Welding Current (A) | Welding Speed (mm/min) | Grain Size Trend | Tensile Strength Trend |
|---|---|---|---|