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

Literature Overview

This paper published in the Transactions of Nonferrous Metals Society of China (Volume 35, Issue 11, 2025, pages 3662–3676) by Si-zhe Niu and colleagues from Shandong University of Technology, Shanghai Jiao Tong University, and other institutions presents a comprehensive study of TIG surface remelting as a surface strengthening technique for ZL109G aluminum alloy piston components. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Grant Nos. 52205394, 12202251, 52305432), the Postdoctoral Science Foundation (Grant No. 2021M702024), and several provincial and institutional funds. This work is significant for engineers working on aluminum alloy component surface treatment, particularly in the automotive and aerospace industries where ZL109G is widely used for piston applications.

Core Technical Findings

Microstructure Refinement

The most remarkable finding of this study is the dramatic microstructural refinement achieved through TIG surface remelting. The primary silicon particles in the base metal, which have an average diameter of 65.8 micrometers, are reduced to 7.1 micrometers in the remelted zone—a reduction of approximately 90%. Similarly, the grain size in the remelted zone is refined to approximately one-seventh of the base metal grain size.

Cellular Microstructure Characteristics

The remelted zone exhibits a distinctive cellular microstructure characterized by α(Al) matrix in the cell centers and intermetallic compounds preferentially located at the cellular boundaries. This microstructural arrangement is a direct consequence of the rapid solidification conditions achieved during surface remelting, where the thin remelted layer experiences extremely high cooling rates due to heat conduction into the cooler base metal substrate.

The cellular structure formation can be understood through the following mechanism:

1. The TIG arc heats the surface layer to a molten state.
2. Rapid heat extraction through the base metal substrate creates steep thermal gradients.
3. Under high thermal gradient and high growth rate conditions, cellular solidification morphology is favored over equiaxed or dendritic structures.
4. The cellular boundaries act as preferential sites for intermetallic compound precipitation due to solute segregation during solidification.

Mechanical Property Enhancement

The surface remelting process produces significant improvements in hardness and tensile strength, attributed primarily to solution strengthening of silicon in the α(Al) matrix. The refinement of primary silicon particles from 65.8 micrometers to 7.1 micrometers dramatically increases the interfacial area between the silicon and aluminum phases, enhancing the strengthening effect through:

• Increased solid solution strengthening from dissolved silicon in the α(Al) matrix.
• Enhanced Orowan strengthening mechanism from the refined second-phase particles.
• Improved load transfer efficiency between the matrix and reinforcement phases.

However, the study also reveals a reduction in fracture toughness from 15.0 MPa·m^(1/2) in the base metal to 12.7 MPa·m^(1/2) in the remelted zone. This reduction is attributed to the finer microstructure, which, while improving strength, reduces the crack deflection and bridging mechanisms that contribute to toughness. The cellular boundaries with preferential intermetallic segregation may also serve as preferential crack propagation paths.

Engineering Practice Integration

TIG surface remelting represents a promising surface treatment technology for aluminum alloy components, offering significant strength improvements without the need for alloy modification or heat treatment. For engineers working on aluminum alloy piston components, such as those specified under ASTM B26 or EN 485, this technique offers several advantages:

• No change in component dimensions or geometry, unlike machining or surface modification techniques.
• No requirement for post-treatment heat aging, simplifying the manufacturing process.
• Applicable to complex component geometries where conventional surface treatment methods are impractical.
• Can be applied to individual components or integrated into production lines.

However, the trade-off between strength and toughness must be carefully considered for specific applications. For piston components subjected to cyclic loading, the reduction in fracture toughness could be a concern, particularly if stress concentrations exist at geometric discontinuities. Engineers should evaluate the specific loading conditions and service environment to determine whether the strength improvement outweighs the toughness reduction.

The surface remelting technique can be integrated into existing manufacturing processes as follows:

Key Questions and Reflections

Several important questions arise from this study regarding the practical implementation of TIG surface remelting. What is the maximum effective treatment depth, and how does it vary with process parameters such as current, voltage, and travel speed? Can multiple remelting passes be applied to increase treatment depth without causing detrimental effects on the underlying material? How does the remelted zone respond to thermal cycling during service, and is there a risk of microstructural coarsening that would degrade the strength benefits over time?

The fracture toughness reduction observed in this study warrants careful consideration. For components subjected to impact loading or fatigue cycling, the reduced toughness could lead to premature failure. Engineers should consider complementary treatments such as laser shock peening or shot peening to introduce compressive residual stresses that could mitigate the toughness reduction while maintaining the strength benefits.

The cellular microstructure with intermetallic segregation at boundaries raises concerns about long-term corrosion resistance. The preferential location of intermetallic compounds at cellular boundaries could create galvanic couples with the α(Al) matrix, potentially accelerating localized corrosion in aggressive environments. Corrosion testing under simulated service conditions would be valuable to assess this risk.

Study Insights and Implications

This research demonstrates the significant potential of TIG surface remelting as a surface strengthening technology for ZL109G aluminum alloy components. The dramatic microstructural refinement, with primary silicon particles reduced by over 90% and grain size refined by a factor of seven, provides a clear mechanism for the observed mechanical property improvements. The 39% increase in hardness and 24.5% increase in ultimate tensile strength are substantial improvements that could significantly extend component service life or enable weight reduction through material optimization. However, the 15.3% reduction in fracture toughness represents a trade-off that must be carefully evaluated for each specific application. For engineers working on aluminum alloy component design and manufacturing, this technique offers a simple, cost-effective surface treatment option that requires no specialized equipment beyond standard TIG welding systems. Future work should focus on optimizing process parameters for maximum treatment depth, investigating long-term stability under thermal and mechanical cycling, and developing integrated process solutions that combine remelting with complementary strengthening techniques to achieve balanced property improvements.