Microstructure and Properties of TIG Hardfacing on Thick ZL205A Cast Aluminum Alloy Plates

Literature Overview

This research article published in Precision Forming Engineering (2023, Vol. 15, No. 7, pp. 136–145) by Liu Hao, Wen Quan, Wu Xuemeng, Chen Qian, Zhao Jing, and Xiang Huiyao from State-Owned Sida Machinery Manufacturing Company presents a comprehensive study on the microstructure and mechanical properties of TIG (Tungsten Inert Gas) hardfacing welds on 25 mm thick ZL205A cast aluminum alloy plates. The study addresses the challenge of achieving high-quality welds in multi-layer, multi-pass TIG welding of thick aluminum alloy sections, where porosity and lack of fusion are common defects.

Research Objectives and Methodology

The primary objective of this study was to solve the problems of porosity and lack of fusion (virtual welding) that frequently occur during multi-layer, multi-pass TIG welding of 25 mm thick ZL205A cast aluminum alloy. The research systematically investigated the effects of inter-pass welding current on weld microstructure and mechanical properties.

The experimental approach involved welding multiple test coupons with different inter-pass current levels, followed by metallographic examination, mechanical testing, and fractographic analysis.

Microstructural Analysis

The study identified four distinct zones in the weld joint based on microstructural characteristics:

A key finding was that the weld core grain size was approximately 20 μm, which is only about 1/5 of the base metal grain size. This significant grain refinement in the weld zone is attributed to the rapid solidification rates and high nucleation density during TIG welding.

The weld core contained a high density of intermetallic particles distributed along grain boundaries, including:

• Al₂Cu — Hard, brittle particle phase contributing to strength but reducing ductility.
• AlCu — Another brittle intermetallic phase that promotes intergranular fracture.
• Al₁₂CuMn₂ — Minor particle phase with complex chemistry.

Mechanical Properties and Current Effects

The study revealed that both tensile strength and elongation exhibited a non-monotonic relationship with inter-pass welding current:

The optimal welding current produced a joint with tensile strength reaching 94% of the base metal strength, which is a commendable result for aluminum alloy welding. However, the elongation was limited to only 41.72% of the base metal value, indicating that the weld joint remained relatively brittle.

Defect Analysis and Failure Mechanisms

The study identified porosity as the dominant defect in the weld joints, particularly at higher welding currents. The porosity formation mechanism involves:

1. Hydrogen porosity — Aluminum's ability to dissolve hydrogen during melting and reject it during solidification creates gas bubbles. Higher currents increase the volume of molten metal and the dissolution capacity, leading to more porosity.
2. Nitrogen porosity — Inadequate shielding gas coverage can allow nitrogen dissolution, particularly at higher heat inputs.

Fractographic analysis revealed two distinct fracture locations:

• Weld fracture — Caused by porosity defects acting as crack initiation sites.
• Base metal fracture — Caused by the presence of brittle AlCu and intermetallic particles along grain boundaries that promote intergranular cracking.

Process Optimization Recommendations

Based on the study findings, the following recommendations emerge for practical TIG welding of thick ZL205A plates:

1. Current control — The inter-pass current must be carefully optimized to balance penetration depth, heat input, and porosity formation. The optimal range is narrow, requiring precise process control.
2. Preheating — Preheating the base metal to 200–300 °C can reduce thermal gradients and minimize hydrogen porosity by allowing dissolved gases to escape before solidification.
3. Shielding gas management — Maintaining consistent argon flow (typically 15–20 L/min) and ensuring proper gas coverage of the weld pool is critical for porosity prevention.
4. Filler wire selection — Using an aluminum-silicon filler wire with appropriate Si content (5–7%) can improve fluidity and reduce porosity tendency.
5. Interpass grinding — Grinding between passes to remove surface oxides and porosity can improve subsequent pass quality.

Study Insights and Engineering Implications

This study provides valuable quantitative data on the microstructure-property relationships in TIG hardfacing of thick aluminum alloy plates. The finding that the optimal welding current produces joints with 94% of base metal strength demonstrates that high-quality aluminum alloy welds are achievable through careful process control. However, the limited ductility (41.72% of base metal) highlights the inherent challenge of maintaining toughness in aluminum alloy welds due to the formation of brittle intermetallic phases.

The study also underscores the critical role of porosity control in aluminum alloy welding quality. For industrial applications involving thick ZL205A plates, the welding process must be carefully calibrated to minimize porosity while maintaining adequate penetration. This requires a systematic approach to process parameter optimization, potentially involving statistical methods such as Taguchi design of experiments or response surface methodology.

Conclusion

The TIG hardfacing study on 25 mm thick ZL205A cast aluminum alloy plates demonstrates that high-strength weld joints are achievable through careful control of inter-pass welding current, with optimal results reaching 94% of base metal tensile strength. The microstructural analysis reveals that grain refinement in the weld zone and the formation of dispersed intermetallic particles are key features influencing mechanical performance. Porosity remains the primary defect mechanism limiting weld quality, particularly at higher heat inputs. The study provides a solid foundation for developing optimized welding procedures for thick aluminum alloy structures in industrial applications.