CLADDING TECHNOLOGY SHANXI CO., LTDEffect of TIG Surface Remelting on Microstructure and Mechanical Properties of ZL109G Al Alloy
Literature Overview
The study examines the influence of gas tungsten arc (GTAW) surface remelting on the microstructure, mechanical properties, and surface integrity of ZL109G (Al-Mg-Si) casting alloy. Surface remelting represents a non-additive thermal process in which the substrate surface is selectively melted and rapidly solidified without introducing exogenous material, thereby refining grain structure and redistributing second-phase particles. This technique is particularly relevant in cladding and weld overlay contexts where surface performance must be enhanced without altering bulk geometry or composition.
Core Technical Points
Microstructural Transformation
The as-cast ZL109G alloy typically exhibits a coarse dendritic microstructure with large Mg₂Si precipitates and Al₂Cu particles distributed along grain boundaries. After TIG remelting, the following transformations are observed:
| Parameter | As-Cast Condition | Post-Remelting Condition |
|---|---|---|
| Grain size | 80–150 μm (dendritic) | 20–45 μm (columnar to equiaxed) |
| Mg₂Si particle size | 5–12 μm (coarse, irregular) | 0.5–2 μm (fine, dispersed) |
| Al₂Cu particle density | Low, segregated | Moderate, more uniformly distributed |
| Porosity | Moderate (shrinkage) | Reduced (re-melt and re-solidify) |
| Surface hardness (HV0.5) | 55–65 | 75–90 |
The rapid solidification rate achieved during remelting promotes nucleation of fine Mg₂Si particles and suppresses the coarsening tendency observed in the original casting. Columnar grains form perpendicular to the remelted surface, transitioning to equiaxed grains deeper in the heat-affected zone.
Mechanical Property Enhancement
The remelting process produces measurable improvements in surface hardness and wear resistance while maintaining acceptable ductility. The hardness increase of approximately 20–35% is attributed to grain refinement and the redistribution of strengthening precipitates. Micro-Vickers testing conducted at varying depths below the remelted surface reveals a gradient profile, with peak hardness at the surface and gradual decrease toward the unaffected substrate.
Process Parameter Optimization
The study highlights critical remelting parameters:
- Current: 120–160 A DC
- Travel speed: 6–12 mm/min
- Shielding gas: Argon at 12–18 L/min
- Arc voltage: 14–18 V
- Remelted layer thickness: 0.3–0.8 mm
Insufficient current or excessive travel speed leads to incomplete remelting, while excessive energy input causes excessive penetration and potential distortion.
Connection with Engineering Practice
In pressure vessel fabrication, surface remelting can serve as a pre-treatment before applying a weld overlay cladding layer. A refined and defect-free substrate surface provides a more uniform bonding interface for subsequent cladding passes. Additionally, in repair applications for aluminum alloy pressure components, remelting can eliminate surface porosity and micro-cracks without requiring material removal followed by re-welding.
Key Reflections and Study Insights
The fundamental insight from this study is that thermal cycling alone, without material addition, can significantly improve surface properties through solidification microstructure control. This has direct implications for cladding technology, where the quality of the base metal surface influences dilution behavior and bond integrity in subsequent overlay passes. Engineers should consider remelting as a complementary process step when the objective is to enhance surface performance while preserving dimensional accuracy. The technique also offers a low-cost alternative to more complex surface modification methods such as laser remelting or shot peening for certain applications where surface hardness improvement is the primary requirement.