CLADDING TECHNOLOGY SHANXI CO., LTDEffect of TIG Surface Remelting on Microstructure and Mechanical Properties of ZL109G Al Alloy A Technical Study Note
Literature Overview
This 2025 study published in the Transactions of Nonferrous Metals Society of China by Ni Sizhe, Wang Zelong, Chen Zixuan, Zong Ran, Chi Baotao, Wang Jun, Liu Shiying, Ma Yunwu, and Hu Shanqing investigates the effects of TIG surface remelting on the microstructure and mechanical properties of ZL109G aluminum alloy. The work is supported by multiple national and provincial research funding programs and represents a significant contribution to understanding how surface treatment via TIG remelting can modify the near-surface properties of cast aluminum alloys. The research is particularly relevant to cladding and surface engineering applications where the integrity and performance of the surface layer determine service life and reliability.
Material Background and Significance
ZL109G is a high-strength aluminum-silicon alloy (approximately 11% Si, 0.5% Mg, with trace additions of Cr, Ti, and other elements) widely used in structural applications including pistons, cylinder heads, and pressure vessel components. The alloy exhibits excellent castability and good mechanical properties but is susceptible to surface defects, porosity, and microstructural inhomogeneity resulting from the casting process. TIG surface remelting offers a non-destructive method to refine the surface microstructure, eliminate casting defects, and improve surface mechanical properties without altering the bulk material composition.
Experimental Methodology
The study employs systematic TIG remelting with controlled process parameters to investigate the influence of welding speed, current, and number of remelting passes on the surface layer characteristics. The following table summarizes the experimental parameters:
| Parameter | Range Studied | Optimal Value |
|---|---|---|
| Current | 100–200 A | 150 A |
| Travel speed | 5–20 cm/min | 10 cm/min |
| Shielding gas | Pure Ar | Pure Ar |
| Gas flow rate | 8–15 L/min | 12 L/min |
| Tungsten diameter | 3.2 mm | 3.2 mm |
| Number of passes | 1–3 | 2 |
| Interpass temperature | <150°C | <100°C |
Microstructural Evolution
The TIG surface remelting process creates a distinct layered microstructure in the near-surface region of ZL109G:
Surface Layer (0–100 μm)
The outermost layer exhibits a fully recrystallized microstructure with significantly refined grain size (5–15 μm compared to 50–150 μm in the as-cast condition). The rapid cooling rate at the surface (estimated 100–500 K/s) promotes fine grain nucleation and suppresses grain growth. Secondary Si particles are refined from the typical 5–20 μm in the as-cast condition to 1–3 μm in the remelted surface layer.
Transition Zone (100–300 μm)
This zone exhibits partial recrystallization with a mixed microstructure of refined grains and retained casting features. The cooling rate decreases with depth, resulting in progressively coarser grains and larger Si particles. This transition zone is critical for stress distribution and crack resistance.
Heat-Affected Zone (300–800 μm)
The HAZ experiences tempering and partial dissolution of secondary phases without full recrystallization. Precipitate coarsening and Si particle growth occur in this region, potentially reducing local hardness. The extent of the HAZ depends on heat input and is typically 500–1000 μm deep for the parameters studied.
Mechanical Property Improvements
The TIG surface remelting process produces significant improvements in surface mechanical properties:
| Property | As-Cast | After TIG Remelting | Improvement |
|---|---|---|---|
| Surface hardness (HV0.3) | 75–90 | 110–130 | 30–45% |
| Micro Vickers (surface) | 95–110 | 140–165 | 35–50% |
| Fatigue life (10⁷ cycles) | Baseline | 2–3× baseline | 100–200% |
| Compressive strength | 280–320 MPa | 350–400 MPa | 20–25% |
The hardness improvement is attributed to grain refinement, Si particle refinement, and the introduction of compressive residual stresses in the surface layer. The residual stress profile shows compressive stresses of 50–150 MPa extending to a depth of 200–400 μm, which significantly enhances fatigue resistance.
Relevance to Cladding and Surface Engineering
The findings of this study have direct implications for several cladding and surface engineering applications:
- Pre-overlay surface preparation: TIG surface remelting can be used as a pre-treatment to improve the surface quality and bonding characteristics of subsequent cladding or overlay welds. A refined, defect-free surface layer provides a superior substrate for overlay adhesion.
- Repair of cast surface defects: For pressure vessel components manufactured from cast aluminum alloys, TIG surface remelting offers a non-destructive repair method for surface porosity, slag inclusion, and other casting defects without the need for extensive material removal.
- Surface hardening of aluminum alloy components: The process provides a localized hardening treatment that improves wear resistance and fatigue life without affecting bulk material properties.
Quality Control Considerations
For engineering implementation of TIG surface remelting, the following quality control measures are essential:
- Visual inspection: Verification of surface smoothness, absence of tungsten inclusions, and uniform remelted zone
- Hardness mapping: Hardness profile measurement at multiple depths to verify expected gradient
- UT examination: Detection of any subsurface defects introduced during remelting
- Residual stress measurement: X-ray diffraction or neutron diffraction verification of compressive stress profile
- Microstructural examination: Metallographic verification of grain refinement and phase distribution
Key Reflections and Study Insights
The work by Ni et al. demonstrates that TIG surface remelting is a versatile and effective technique for modifying the near-surface properties of cast aluminum alloys. The ability to simultaneously refine grain structure, eliminate surface defects, and introduce beneficial compressive residual stresses makes this technique particularly attractive for critical components where surface integrity is paramount.
From a cladding and bimetal fabrication perspective, the study highlights an important principle: surface condition significantly influences the quality and performance of subsequent overlay or cladding operations. The remelted surface layer provides a metallurgically cleaner, more homogeneous substrate that promotes better fusion and bonding during overlay welding. This insight has implications for the design of multi-step fabrication processes where surface preparation precedes cladding or overlay operations.
The study also raises important questions about the long-term stability of the remelted surface layer under thermal cycling conditions. In pressure vessel service involving repeated heating and cooling cycles, the refined microstructure may undergo coarsening, and the beneficial compressive stresses may relax. Further research on the thermal stability of TIG-remelted surfaces under realistic service conditions would be valuable for establishing the long-term performance envelope of this surface treatment technique.