Effect of TIG Surface Remelting on Microstructure and Mechanical Properties of ZL109G Al Alloy

Literature Overview

This study, authored by Si-zhe Niu, Ze-long Wang, Zi-xuan Chen, Ran Zong, Bao-tao Chi, Jun Wang, Shi-ying Liu, Yun-wu Ma, and Shan-qing Hu from Shandong University of Technology, Shanghai Jiao Tong University, Binzhou Bohai Piston Co., and the Institute of Materials at China Academy of Engineering Physics, was published in Transactions of Nonferrous Metals Society of China in 2025. The work investigates the effects of TIG surface remelting on the microstructure and mechanical properties of ZL109G aluminum alloy, a high-strength cast aluminum alloy widely used in automotive piston applications.

Technical Background

ZL109G (also known as Al-7Si-1.5Mg-0.5Ni-0.5Cu-0.2Mn in Chinese classification) is a high-strength cast aluminum alloy designed for piston applications where thermal fatigue resistance and strength at elevated temperatures are critical. The as-cast microstructure of ZL109G typically consists of a dendritic α-Al matrix with primary Si particles, Al5FeSi intermetallics, and eutectic Si phases. This microstructure, while providing good strength, is characterized by coarse grain boundaries, uneven distribution of reinforcing phases, and potential hot spots that serve as crack initiation sites.

TIG surface remelting is a solid-state or semi-solid processing technique that uses the TIG arc to remelt the surface layer of a component without adding filler metal. The rapid solidification that follows remelting refines the microstructure, homogenizes the phase distribution, and can significantly improve surface mechanical properties.

Process Parameters and Microstructural Evolution

The following table summarizes the key process parameters and their effects on the remelted microstructure:

The remelting depth is typically in the range of 0.5–2.0 mm, depending on the process parameters. The cooling rate in the remelted layer can reach 10^2–10^3 K/s, which is significantly higher than the cooling rates in the as-cast material (typically 1–10 K/s). This rapid cooling promotes:

• Grain refinement: The grain size in the remelted layer can be reduced by 50–80% compared to the as-cast microstructure.
• Phase homogenization: Coarse intermetallic particles are dissolved and re-precipitated as finer, more uniformly distributed phases.
• Dendrite arm spacing reduction: The secondary dendrite arm spacing (SDAS) is significantly reduced, improving the strength-ductility balance.
• Elimination of casting defects: Surface porosity, shrinkage cavities, and segregation bands are healed during remelting.

Mechanical Property Improvements

The TIG surface remelting process produces measurable improvements in the surface mechanical properties of ZL109G:

• Hardness: Surface hardness can increase by 15–30% due to grain refinement and phase homogenization. The hardness profile shows a gradual transition from the hardened remelted layer to the unchanged as-cast substrate.
• Microhardness distribution: The remelted layer exhibits a more uniform hardness distribution compared to the as-cast material, which typically shows significant hardness variation due to dendritic segregation.
• Wear resistance: The refined microstructure and improved phase distribution enhance wear resistance, which is particularly important for piston ring groove applications.
• Fatigue resistance: The elimination of surface casting defects and the refinement of the microstructure can improve fatigue life by reducing crack initiation sites.

Defect Analysis and Quality Control

The TIG surface remelting process introduces specific quality considerations:

• Cracking: The high cooling rates can promote solidification cracking in the remelted layer, particularly if the alloy composition is susceptible to hot cracking. ZL109G, with its Mg and Si content, has moderate hot cracking susceptibility.
• Porosity: Gas porosity can form if the shielding gas coverage is inadequate or if hydrogen dissolved in the aluminum is not adequately controlled.
• Surface oxidation: Aluminum's strong affinity for oxygen can lead to oxide inclusions in the remelted layer if the shielding is compromised.
• Thermal distortion: The localized heating and cooling can cause residual stresses and minor surface distortion, which may require post-processing.

Engineering Applications and Practice

For piston manufacturing, TIG surface remelting offers several practical advantages:

• Surface property improvement without material addition: The remelting process improves surface properties without introducing filler metal, avoiding potential composition changes.
• Selective treatment: Only the critical surface layers are treated, leaving the bulk material properties unchanged.
• Compatibility with existing equipment: TIG welding equipment is widely available in foundries and machining shops, making the process accessible.
• Scalability: The process can be automated for high-volume production using robotic TIG systems.

Study Insights and Reflections

This study demonstrates the practical potential of TIG surface remelting as a post-casting treatment for high-strength aluminum alloys used in demanding applications such as automotive pistons. The microstructural refinement achieved through rapid solidification is well-documented in the literature, but the specific application to ZL109G and the quantification of property improvements provide valuable engineering data. The key insight is that TIG surface remelting can serve as a cost-effective alternative to more complex surface treatment processes such as laser remelting or electron beam surface melting, particularly where the equipment investment must be minimized. However, the process must be carefully controlled to avoid introducing new defects (cracking, porosity) that could compromise the component's integrity. The collaboration between academia (Shandong University of Technology, Shanghai Jiao Tong University) and industry (Binzhou Bohai Piston Co.) exemplifies the productive partnership needed to translate fundamental research into practical manufacturing solutions. Future work should focus on optimizing process parameters for specific piston geometries and validating the long-term performance of remelted surfaces under thermal cycling conditions representative of engine operation.

The five studies reviewed collectively illustrate the breadth and depth of current research in TIG-based welding and surface engineering technologies. From additive manufacturing of bimetallic composites to deep penetration welding of thick titanium sections, from high deposition rate enhancements to surface remelting of cast aluminum alloys, the TIG process continues to evolve as a versatile and adaptable manufacturing technology. The common thread across all five studies is the pursuit of improved efficiency, quality, and applicability through process innovation, and the recognition that the fundamental physics of arc welding provides a rich foundation for technological advancement. Engineers working in cladding, bimetallic product manufacturing, and pressure vessel fabrication should closely monitor these developments, as they represent practical pathways to enhanced productivity and improved component performance in demanding industrial applications.