CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Characteristics of Zinc-Based Alloy TIG Welds
Literature Overview
This 2016 study published in Hot Working Technology by Li Xueshun, Sun Fuyang, Guo Qingchao, Hou Bin, and Zhang Mengfei from the Xi'an Special Equipment Inspection and Testing Institute and Great Wall Motor Co., Ltd. investigates the microstructure characteristics of zinc-based alloy TIG welds. Zinc-based alloys are increasingly used in automotive applications for lightweighting and corrosion resistance, and understanding their welding behavior is essential for developing reliable manufacturing processes. The study provides valuable insights into the microstructural evolution during TIG welding of zinc-based alloys and the factors that influence weld quality.
Core Technical Content
Zinc-based alloys, such as Zn-Al and Zn-Mg alloys, offer excellent corrosion resistance, good formability, and low density, making them attractive for automotive applications. However, their low melting point (approximately 419°C for pure zinc) and high vapor pressure present unique challenges for TIG welding. The study examines the microstructure of the weld metal, heat-affected zone (HAZ), and base metal, as well as the effects of welding parameters on the weld quality.
Typical Zinc-Based Alloy Compositions
| Alloy System | Composition (wt%) | Melting Point (°C) | Density (g/cm³) | Tensile Strength (MPa) |
|---|---|---|---|---|
| Zn-22Al-2.5Cu | Zn-22Al-2.5Cu | 420–440 | 6.7 | 250–300 |
| Zn-27Al-2Cu | Zn-27Al-2Cu | 430–450 | 6.5 | 280–320 |
| Zn-8Mg-2Al | Zn-8Mg-2Al | 350–380 | 6.5 | 200–250 |
Welding Process Parameters
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current (I) | 50–120 A | Higher current increases grain size |
| Welding speed (v) | 100–300 mm/min | Higher speed decreases grain size |
| Shielding gas | Argon (99.99%) | Prevents oxidation and Zn evaporation |
| Preheat temperature | 100–200°C | Reduces thermal stress and cracking |
| Interpass temperature | <200°C | Prevents excessive grain growth |
Microstructural Features of Zinc-Based Alloy Welds
The study identified the following microstructural features:
- Weld metal: The weld metal exhibits a dendritic microstructure with primary Zn-rich dendrites and interdendritic phases (Al-rich and Cu-rich phases). The grain size is typically 50–100 µm, depending on the welding parameters.
- HAZ: The HAZ exhibits a mixed microstructure with partially melted dendrites and solid-state transformed grains. The grain size in the HAZ is larger than in the base metal due to the thermal cycling during welding.
- Base metal: The base metal microstructure remains largely unchanged, except for a narrow region near the weld where some grain growth may occur.
Interpretation of Technical Points
Challenges of TIG Welding Zinc-Based Alloys
- Zinc evaporation: Zinc has a high vapor pressure at welding temperatures, which can lead to significant zinc loss from the weld pool. This results in compositional changes in the weld metal and the formation of zinc-rich phases that are susceptible to cracking.
- Low melting point: The low melting point of zinc-based alloys means that the weld pool is small and the heat-affected zone is narrow. This requires precise control of the welding parameters to avoid excessive heat input.
- Cracking susceptibility: Zinc-based alloys are susceptible to hot cracking due to the formation of low-melting-point eutectics at the grain boundaries. The addition of copper and aluminum helps to reduce cracking susceptibility by modifying the solidification behavior.
- Porosity: Zinc evaporation during welding can lead to the formation of porosity in the weld metal. This is particularly problematic in thick sections where the cooling rate is lower and the zinc evaporation is more pronounced.
Effect of Welding Current on Microstructure
The study found that the welding current has a significant effect on the microstructure of zinc-based alloy welds:
- Low current (50–70 A): Produces a fine dendritic microstructure with small interdendritic phases. The weld metal has high strength but low ductility.
- Medium current (70–100 A): Produces a moderate dendritic microstructure with a good balance of strength and ductility. This is the recommended current range for most applications.
- High current (100–120 A): Produces a coarse dendritic microstructure with large interdendritic phases. The weld metal has lower strength and higher susceptibility to cracking.
Effect of Welding Speed on Microstructure
The welding speed also has a significant effect on the microstructure:
- Low speed (100–150 mm/min): Produces a coarse microstructure with large grains and interdendritic phases. The weld metal has lower strength and higher susceptibility to cracking.
- Medium speed (150–250 mm/min): Produces a fine microstructure with small grains and interdendritic phases. The weld metal has high strength and good ductility.
- High speed (250–300 mm/min): Produces a very fine microstructure but may lead to incomplete fusion and lack of penetration.
Engineering Practice Integration
Welding Procedure Specification for Zinc-Based Alloys
| Parameter | Specification | Rationale |
|---|---|---|
| Process | TIG (GTAW) | Low heat input, minimal dilution |
| Filler metal | Zn-22Al-2.5Cu wire (matching base metal) | Match base metal composition |
| Welding current | 70–100 A | Balance of strength and ductility |
| Welding speed | 150–250 mm/min | Fine microstructure, good penetration |
| Shielding gas | Argon (99.99%) | Prevent oxidation and Zn evaporation |
| Preheat temperature | 100–200°C | Reduce thermal stress and cracking |
| Interpass temperature | <200°C | Prevent excessive grain growth |
| Post-weld cooling | Air cooling | Avoid quenching effects |
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Low-melting-point eutectics at grain boundaries | Add Cu and Al to filler metal, control interpass temperature |
| Porosity | Zinc evaporation during welding | Use high-purity argon shielding gas, reduce welding current |
| Lack of fusion | Insufficient heat input | Increase welding current, decrease welding speed |
| Excessive zinc loss | High welding temperature | Use lower welding current, increase welding speed |
| Distortion | Asymmetric heat input | Use symmetric weld sequence, fix the part appropriately |
Key Questions and Reflections
The study provides valuable insights into the microstructural characteristics of zinc-based alloy TIG welds and the factors that influence weld quality. One of the key findings is that the welding current and welding speed must be carefully balanced to achieve a fine microstructure with good mechanical properties. The use of matching filler metal is essential to minimize compositional changes and cracking susceptibility.
The study also highlights the importance of shielding gas purity in preventing zinc evaporation and porosity formation. High-purity argon (99.99%) is recommended to minimize oxidation and zinc loss from the weld pool. Additionally, the preheat temperature and interpass temperature must be controlled to prevent excessive grain growth and cracking.
Study Insights and Implications
The microstructural characteristics of zinc-based alloy TIG welds are governed by the welding parameters and the composition of the base metal and filler metal. The study provides a framework for optimizing the welding process to achieve a fine microstructure with good mechanical properties. For engineers working on automotive applications, the key takeaway is that zinc-based alloys can be successfully welded using TIG, provided that the welding parameters are carefully controlled. Future work should focus on developing welding procedures for thicker sections and higher-strength zinc-based alloys, which are increasingly being used in automotive lightweighting applications.