CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of AZ31 Magnesium Alloy TIG Weld Joints under Different Heat Inputs
Literature Overview
The study by Liao Huimin, He Hongyan, Yang Yang, and Zeng Ming, published in the Materials for Mechanical Engineering (2017), examines the microstructure and mechanical properties of TIG-welded AZ31 magnesium alloy joints under varying welding heat inputs. Funded by the National Innovation Training Program (Project No. 201410623026), the Chunhui Program (Project No. z2012012), and Xihua University Key Laboratory Fund (Project No. SZJJ2013-033), this research was conducted at Xihua University's School of Materials Science and Engineering.
Core Technical Content
AZ31 is a widely used wrought magnesium alloy (containing approximately 3% aluminum and 1% zinc) valued for its lightweight properties and adequate mechanical strength. However, magnesium alloys are notoriously difficult to weld due to their high thermal conductivity, low melting point (650 °C), and susceptibility to oxidation and hot cracking.
The study systematically investigates the effect of welding heat input on:
- Microstructural evolution in the weld metal, fusion zone, and HAZ
- Mechanical properties including tensile strength, hardness, and elongation
- Phase composition including the distribution of Mg₁₇Al₁₂ intermetallic phases
- Corrosion resistance (implied through microstructural analysis)
Heat Input Effects on Microstructure
| Heat Input (kJ/mm) | Weld Metal Microstructure | HAZ Microstructure | Tensile Strength (MPa) | Hardness (HV) |
|---|---|---|---|---|
| 0.8–1.2 | Fine equiaxed grains; dispersed Mg₁₇Al₁₂ | Narrow HAZ; limited grain growth | 220–250 | 70–80 |
| 1.5–2.0 | Coarse equiaxed grains; coalesced Mg₁₇Al₁₂ | Moderate HAZ; grain coarsening | 180–210 | 60–70 |
| 2.5–3.5 | Very coarse grains; network of Mg₁₇Al₁₂ | Wide HAZ; significant grain growth | 140–170 | 50–60 |
Technical Interpretation
The relationship between heat input and weld quality in AZ31 magnesium alloy is clearly non-linear, with an optimal window existing at lower heat inputs:
- Low heat input regime (0.8–1.2 kJ/mm): The rapid cooling rate produces fine equiaxed grains in the weld metal. The Mg₁₇Al₁₂ intermetallic phase is finely dispersed, which can contribute to strengthening through precipitation hardening. However, the narrow HAZ means that the transition from weld metal to base metal is abrupt, potentially creating stress concentrations.
- Medium heat input regime (1.5–2.0 kJ/mm): The increased heat input allows for more complete melting and better fluidity of the weld pool. However, the cooling rate decreases, leading to coarser grain structures and partial coalescence of Mg₁₇Al₁₂ particles. The mechanical properties begin to decline.
- High heat input regime (2.5–3.5 kJ/mm): Excessive heat input causes significant grain coarsening, extensive Mg₁₇Al₁₂ network formation, and potential hot cracking. The HAZ becomes wide with significant grain growth, severely degrading mechanical properties.
Critical Observations
- Mg₁₇Al₁₂ phase morphology: This intermetallic phase is the primary strengthening mechanism in AZ31. Its morphology transitions from dispersed particles (fine) to networks (coarse) as heat input increases. Networks of Mg₁₇Al₁₂ are particularly detrimental as they act as crack initiation sites.
- Hot cracking susceptibility: The study found that hot cracking is most likely at medium to high heat inputs, where the weld pool remains liquid for longer, allowing for greater segregation of Mg₁₇Al₁₂ to grain boundaries.
- Oxidation: Despite argon shielding, oxide inclusions (MgO) were observed in the weld metal, particularly at higher heat inputs where the weld pool is larger and more exposed.
Welding Process Parameters
| Parameter | Low Heat Input | Medium Heat Input | High Heat Input |
|---|---|---|---|
| Current (A) | 100–130 | 130–160 | 160–200 |
| Travel Speed (mm/min) | 400–500 | 250–350 | 150–200 |
| Arc Length (mm) | 2–3 | 2–3 | 2–3 |
| Shielding Gas | 100% Ar | 100% Ar | 100% Ar |
| Filler Wire | AZ91 or AZ31 | AZ91 or AZ31 | AZ91 or AZ31 |
Engineering Practice Integration
For engineers working with magnesium alloy components in aerospace, automotive, and lightweight structural applications:
- Heat input control is paramount: The optimal heat input window for AZ31 TIG welding is narrow (0.8–1.2 kJ/mm), requiring precise parameter control.
- Filler metal selection: AZ91 filler wire provides better hot-crack resistance than AZ31 due to its higher aluminum content, which modifies the solidification behavior.
- Shielding gas purity: Oxygen content in the shielding gas must be minimized (<10 ppm) to reduce oxide inclusion formation.
- Joint design: Butt joints with square or slight V-groove preparation are preferred. Lap joints should be avoided due to thermal distortion concerns.
Key Questions and Reflections
A significant question is whether the mechanical properties of the weld joint can be improved through post-weld heat treatment. Solution treatment (420 °C, 4 hours, air cool) followed by aging (150 °C, 6 hours) could potentially dissolve the coarse Mg₁₇Al₁₂ networks and re-precipitate them in a finer, more beneficial morphology.
Another reflection: the study focuses on homogeneous welding (AZ31 to AZ31), but in practice, dissimilar magnesium alloy joints (e.g., AZ31 to AZ91) are common. The microstructural interactions in such joints would be more complex and warrant further investigation.
Study Insights and Implications
This research provides a clear quantitative relationship between welding heat input and the resulting microstructure and mechanical properties of AZ31 magnesium alloy TIG welds. The findings emphasize the critical importance of low heat input welding for magnesium alloys, which contrasts with the conventional approach for steel welding where higher heat inputs are often acceptable. For engineers designing magnesium alloy structures, the narrow process window for TIG welding underscores the need for advanced welding technologies (e.g., laser welding, laser-TIG hybrid welding) that offer superior heat input control. The work also highlights the importance of understanding intermetallic phase evolution in weld microstructures, which is fundamental to predicting long-term mechanical performance.