CLADDING TECHNOLOGY SHANXI CO., LTDComposition and Microstructure Characteristics of Fusion Zone in Hypereutectic ZA Alloy TIG Welding
Literature Overview
This paper, published in the Welding Journal in 2001 by researchers from Shandong University and Shandong University of Science and Technology, investigates the composition and microstructure characteristics of the fusion zone in TIG welding of hypereutectic ZA alloy. The work was supported by the Shandong Provincial Science and Technology Development Plan (993175101). ZA alloys are zinc-aluminum casting alloys that have gained increasing importance in automotive and industrial applications due to their excellent castability, corrosion resistance, and mechanical properties. The study provides fundamental metallurgical insights that are directly applicable to the repair welding and cladding of ZA alloy components.
Core Technical Viewpoints
Hypereutectic ZA alloys contain aluminum content above the eutectic composition (approximately 26% Al), with typical compositions of ZA-27 (27% Al) and ZA-32 (32% Al). The hypereutectic composition results in a microstructure consisting of primary alpha-phase (Al-rich solid solution) particles dispersed in an alpha-beta eutectic matrix. During TIG welding, the fusion zone undergoes complete melting and resolidification, leading to significant microstructural changes that directly affect the weld's mechanical properties and corrosion resistance.
The paper demonstrates that the fusion zone microstructure is characterized by:
- Coarsened primary alpha-phase particles with irregular morphology
- Modified eutectic structure with reduced beta-phase volume fraction
- Segregation of intermetallic phases at grain boundaries
- Non-equilibrium solidification features including microsegregation and banding
Composition Analysis of Fusion Zone
| Zone | Zn (%) | Al (%) | Fe (%) | Cu (%) | Mg (%) | Phase Composition |
|---|---|---|---|---|---|---|
| Base metal (ZA-27) | 72.5 | 27.0 | 0.2 | 0.1 | 0.05 | α + eutectic (α+β) |
| Fusion zone center | 71.0–73.0 | 26.0–28.0 | 0.1–0.3 | 0.05–0.15 | 0.02–0.08 | Coarse α + modified eutectic |
| Heat-affected zone (outer) | 72.0–72.5 | 27.0–27.5 | 0.2 | 0.1 | 0.05 | Recrystallized α + eutectic |
| Heat-affected zone (inner) | 71.5–72.0 | 27.0–27.5 | 0.2 | 0.1 | 0.05 | Partially dissolved primary α |
| Weld metal (with filler) | 70.0–72.0 | 28.0–30.0 | 0.1–0.2 | 0.05–0.1 | 0.02–0.05 | Fine α + eutectic + possible intermetallics |
Microstructural Analysis and Metallurgical Behavior
The fusion zone microstructure of hypereutectic ZA alloy TIG welds exhibits several distinctive features that require careful interpretation for engineering applications:
- Primary alpha-phase coarsening: The primary alpha-phase particles in the base metal typically range from 20–50 μm in size. In the fusion zone, these particles undergo partial dissolution during heating and regrow during solidification, resulting in coarsened particles of 80–200 μm. The coarsened particles have irregular, dendritic morphology rather than the equiaxed morphology of the base metal.
- Eutectic modification: The eutectic structure in the fusion zone shows reduced interlamellar spacing compared to the base metal, indicating faster solidification rates. However, the beta-phase (Zn-rich) lamellae are thinner and more discontinuous, resulting in a modified eutectic morphology that affects both mechanical properties and corrosion behavior.
- Intermetallic phase formation: The fusion zone contains small quantities of intermetallic phases including ZnAl2 (eta phase) and Zn5Al8 (epsilon phase) at grain boundaries and within the eutectic structure. These phases are harder and more brittle than the matrix phases and can serve as initiation sites for crack propagation.
- Microsegregation patterns: The non-equilibrium solidification conditions in the fusion zone produce microsegregation of zinc and aluminum. The center of the solidifying dendrites is enriched in zinc while the interdendritic regions are enriched in aluminum, creating compositional gradients that affect local corrosion resistance.
Welding Parameters and Their Effect on Microstructure
| Parameter | Low Value | High Value | Microstructural Effect |
|---|---|---|---|
| Welding current (A) | 80–100 | 150–200 | Higher current produces coarser grains and larger primary particles |
| Travel speed (mm/min) | 30–50 | 100–150 | Higher speed produces finer grains but may cause lack of fusion |
| Heat input (J/mm) | 200–400 | 600–1000 | Higher heat input promotes grain coarsening and phase coarsening |
| Filler wire composition | ZA-27 match | ZA-32 (higher Al) | Higher Al filler promotes finer microstructure and reduced cracking |
| Preheat temperature (°C) | 0–50 | 100–150 | Higher preheat reduces thermal gradients but may promote grain coarsening |
Engineering Practice and Defect Analysis
For engineers working with ZA alloy components in repair welding or cladding applications, understanding the fusion zone metallurgy is essential for predicting weld performance and identifying potential failure modes.
Common Defects and Their Metallurgical Origins
| Defect Type | Root Cause | Detection Method | Prevention |
|---|---|---|---|
| Hot cracking | Low-melting-point eutectic films at grain boundaries | Visual inspection, RT | Use higher-Al filler wire; control heat input |
| Cold cracking | Hydrogen embrittlement in hydrogen-sensitive microstructure | MT, PT | Preheat to 100–150°C; use low-hydrogen consumables |
| Porosity | Gas evolution from Zn evaporation during welding | RT, UT | Reduce welding current; increase shielding gas coverage |
| Lack of fusion | Insufficient heat input at weld toe | UT, MT | Increase current or reduce travel speed |
| Excessive spatter | Zn evaporation and spatter formation | Visual inspection | Reduce arc voltage; use short arc length |
The zinc evaporation issue is particularly significant in TIG welding of ZA alloys. Zinc has a boiling point of 907°C, which is below the melting point of the alloy (approximately 390–420°C for ZA-27). During welding, zinc preferentially evaporates from the weld pool surface, leading to compositional changes in the weld metal and potential spatter formation. The evaporation rate is strongly dependent on welding current, with higher currents producing significantly more zinc loss.
Mechanical Properties of Fusion Zone
| Property | Base Metal (ZA-27) | Fusion Zone | Heat-Affected Zone |
|---|---|---|---|
| Tensile strength (MPa) | 280–320 | 220–260 | 250–290 |
| Yield strength (MPa) | 1 |