Microstructure Characteristics of Zinc-Based Alloy TIG Weld Joints

Literature Overview

This 2016 study by Li Xueshun, Sun Fuyang, Guo Qingchao, Hou Bin, and Zhang Mengfei from Xi'an Special Equipment Inspection and Testing Institute and Great Wall Motor Co., Ltd. investigates the microstructural evolution in zinc-based alloy weld joints produced by TIG welding. Published in Hot Working Technology, this work addresses a relatively uncommon but increasingly important welding application in the automotive and corrosion protection industries.

Zinc-based alloys, particularly those containing alloying elements such as Al, Cu, Mg, and Ti, are used for galvanizing, die-casting of automotive components, and corrosion-resistant coatings. The welding of zinc-containing materials presents unique challenges due to zinc's low boiling point (907°C), which can lead to zinc vaporization, porosity, and hot cracking during welding.

Microstructural Analysis

The researchers examined the weld metal, heat-affected zone (HAZ), and base metal microstructures using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Key findings include:

• Weld metal: Exhibits a dendritic microstructure with primary α-Zn dendrites and interdendritic β-phase (Cu-Zn intermetallic compound) in alloys containing copper. The grain structure is columnar near the fusion line and equiaxed in the center of the weld bead.
• HAZ: Shows partial melting with a mixed microstructure of α-Zn and β-phase. The width of the HAZ depends on heat input, typically ranging from 0.5 mm to 2.0 mm for the parameters used.
• Base metal: Consists of equiaxed α-Zn grains with uniform distribution of alloying element precipitates.

Welding Parameters and Process Optimization

The study evaluated TIG welding with the following parameter ranges:

• Welding current: 80–150 A (DC)
• Travel speed: 6–12 mm/min
• Shielding gas: Argon (99.99%) or Ar/He mixtures
• Wire diameter: 0.8–1.2 mm
• Polarity: DCEP (direct current electrode positive)

The optimal parameter combination identified was 100 A at 8 mm/min with pure argon shielding. Higher currents (>150 A) led to excessive zinc vaporization, resulting in surface spatter, porosity, and incomplete fusion. Lower travel speeds increased heat input, widening the HAZ and promoting excessive grain growth.

Defect Analysis and Countermeasures

The study identified several typical defects in zinc-based alloy TIG welds:

1. Zinc vapor porosity: Caused by zinc boiling and vaporization during welding. Countermeasures include reducing heat input, using higher travel speeds, and employing pulsed TIG welding to control instantaneous heat input.
2. Hot cracking: Occurs in the β-phase-rich interdendritic regions due to restricted solidification shrinkage. Mitigation involves adjusting filler metal composition to reduce β-phase fraction or adding small amounts of Mg to modify the solidification behavior.
3. Incomplete fusion: Resulting from insufficient heat input or improper joint preparation. Prevention requires adequate joint design (single-V or square butt with proper fit-up) and sufficient preheat for thicker sections.
4. Surface oxidation: Zinc oxide formation during welding can lead to surface roughness and reduced corrosion resistance. This is minimized by maintaining a clean, oxide-free surface prior to welding and using proper shielding gas coverage.

Engineering Practice Implications

For engineers working in the automotive industry, where zinc-based die-cast components are increasingly used for weight reduction, this study provides valuable guidance on:

• Weldability assessment: Zinc-based alloys are generally considered difficult to weld, but with proper parameter selection, acceptable weld quality can be achieved for thin sections (<3 mm).
• Filler metal selection: Matching filler metals (same composition as base metal) are preferred to minimize microsegregation and reduce the risk of hot cracking.
• Post-weld inspection: X-ray radiography is essential for detecting porosity, while ultrasonic testing can identify lack of fusion and hot cracks.

The study's findings are particularly relevant for engineers involved in repairing or joining zinc-coated steel components in pressure vessel applications, where galvanic corrosion resistance is critical. The understanding of microstructural evolution during welding helps in predicting long-term corrosion behavior and service life.

Study Insights

This research fills an important gap in the welding literature for zinc-based materials. The systematic microstructural characterization provides a foundation for developing welding procedure specifications (WPS) for zinc alloys. The emphasis on the relationship between welding parameters, microstructure, and defects is a model for welding procedure development in any alloy system.