Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of TIG Welds in 7055 Aluminum Alloy Produced by Spray Forming and Hot Extrusion
Literature Overview
This paper, published in Hot Working Technology (Vol. 51, Issue 23, 2022, pp. 44-47) by Yang Jiajia, Cao Hui, Wang Yifan, and Liu Zhancai, investigates the microstructure and mechanical properties of TIG welds produced in 3 mm thick 7055 aluminum alloy fabricated by spray forming and hot extrusion. The study was supported by the North China University of Water Resources and Electric Power high-level talent research funding project (No. 4001/40678). The 7055 alloy is a high-strength Al-Zn-Mg-Cu type alloy widely used in aerospace structural applications, and understanding its weldability is essential for structural fabrication.
Background and Material Context
7055 aluminum alloy belongs to the 7xxx series and contains approximately 5.6% Zn, 2.4% Mg, and 1.5% Cu. It offers high strength and good fatigue resistance but is susceptible to hot cracking during welding due to the low melting point of eutectic phases involving Zn and Mg. The material studied here was produced by spray forming (also known as gas atomized deposition or GAD) followed by hot extrusion, resulting in an annealed condition with a unique microstructure that differs from conventionally rolled or forged 7055.
Welding Process Parameters
The TIG welding was performed with ER5356 filler wire, which is a standard Al-Mg-Si filler alloy commonly used for welding 7xxx series aluminum alloys. The use of ER5356 provides a dilution effect that reduces the susceptibility to hot cracking by lowering the Zn content in the weld metal and introducing Si, which promotes the formation of Al-Mg-Si phases instead of low-melting-point Al-Zn-Mg eutectics.
Microstructural Analysis
The metallographic examination revealed a distinct microstructural gradient across the weld zone:
Weld Metal
The weld metal exhibited coarse dendritic crystals, which is typical for TIG welding of aluminum alloys where the solidification rate is relatively low and there is insufficient nucleation to produce fine equiaxed grains. The dendritic structure reflects the directional heat extraction from the weld pool.
Fusion Zone
The fusion zone displayed fine equiaxed grains, which is attributed to the remelting and resolidification of the spray-formed extruded microstructure at the fusion boundary. The fine equiaxed grains in this region suggest that the spray-formed microstructure provided a high density of nucleation sites (such as oxide particles or precipitates) that promoted heterogeneous nucleation during resolidification.
Heat-Affected Zone
The HAZ was notably wide at 22 mm, which is considerably broader than typical HAZ widths for conventionally processed 7xxx alloys welded by TIG. This extensive HAZ is likely related to the thermal conductivity and thermal capacity of the spray-formed extruded material. The HAZ microstructure showed a gradient:
- Near the weld: Elongated, layered grains with significant grain growth, indicating substantial thermal exposure and grain coarsening
- Far from the weld: The base material microstructure was retained, showing the spray deposition extruded structure
The wide HAZ is a concern for mechanical properties because the temper condition in the HAZ is significantly affected by the welding heat input. In 7xxx alloys, the HAZ typically experiences overaging or dissolution of strengthening precipitates, leading to a reduction in strength.
Mechanical Property Analysis
Microhardness Distribution
The microhardness profile across the weld zone exhibited an M-shaped curve, with two peaks located in the HAZ near the weld. This M-shaped profile is characteristic of 7xxx alloy welds where:
- The weld metal has lower hardness due to the dilution effect of ER5356 (lower Cu content than base metal) and the coarse dendritic structure
- The fusion zone may show variable hardness depending on the grain structure
- The HAZ near the weld shows two peaks corresponding to the peak aging condition on either side of the weld, where the welding thermal cycle has produced a temper that optimally precipitates the strengthening phases
- The base metal away from the weld shows the as-extruded annealed hardness
The M-shaped hardness profile indicates that the maximum hardness is not in the base metal but in the HAZ, which is a favorable characteristic for joint strength.
Tensile Properties
The average tensile strength of the weld joint was 190 MPa, and the average elongation after fracture was 6.76%. For context, the base metal 7055 alloy in the annealed condition typically has a tensile strength of approximately 250-300 MPa, meaning the joint retains about 63-76% of the base metal strength. This joint efficiency is moderate and would need to be evaluated against applicable design codes.
Fracture Mode
The fracture analysis revealed a mixed ductile-brittle fracture mode, characterized by a combination of dimpled rupture (indicative of ductile failure) and cleavage-like features (indicative of brittle failure). This mixed mode suggests that the joint has adequate ductility but also contains brittle regions, likely in the coarse-grained HAZ or in the coarse dendritic weld metal.
Comparative Summary of Weld Zone Characteristics
| Zone | Microstructure | Approximate Hardness Trend | Key Feature |
|---|---|---|---|
| Base Metal (far) | Spray-formed extruded annealed structure | Moderate | Retains as-extruded microstructure |
| HAZ (far) | Slightly affected extruded structure | Rising | Peak aging possible |
| HAZ (near) | Elongated, coarsened grains | Peak (M-shaped peaks) | Maximum hardness, grain growth |
| Fusion Zone | Fine equiaxed grains | Variable | Heterogeneous nucleation from spray-formed structure |
| Weld Metal | Coarse dendritic crystals | Lower | ER5356 dilution, low solidification rate |
Engineering Practice Implications
The findings of this study have several important implications for the welding of spray-formed aluminum alloy components:
- Wide HAZ concern: The 22 mm HAZ width is substantially larger than expected for 3 mm thick material, and this must be considered in design calculations for fatigue and creep resistance. The extensive HAZ represents a significant volume of material with modified mechanical properties.
- Filler metal selection: ER5356 was effective in producing sound welds without hot cracking, but the dilution effect reduced weld metal strength. Alternative filler metals such as ER4047 or ER5183 could be evaluated for improved joint strength.
- Post-weld heat treatment: Given the mixed fracture mode and moderate joint efficiency, post-weld aging (T6 or T73 treatment) could significantly improve joint properties by reprecipitating strengthening phases in both the weld metal and HAZ.
- Process optimization: The coarse dendritic weld metal structure suggests that techniques to promote grain refinement—such as ultrasonic vibration, magnetic field application, or pulsed TIG welding—could improve weld metal properties.
Key Questions and Reflections
The wide HAZ observed in this study raises questions about the thermal properties of the spray-formed extruded material. The spray forming process produces a material with a unique microstructure that may have different thermal conductivity or specific heat compared to conventionally processed 7055 alloy, which could explain the wider HAZ. Further investigation of the thermal properties of spray-formed extruded 7055 would be valuable.
Additionally, the study does not report on the effect of welding parameters (current, voltage, travel speed) on the microstructure and properties. A systematic parameter study would help identify optimal welding conditions for minimizing HAZ width and improving weld metal grain refinement.
Study Insights and Outlook
This study provides valuable baseline data on the weldability of spray-formed extruded 7055 aluminum alloy, a relatively novel material processing route. The M-shaped hardness profile with peaks in the HAZ is an interesting finding that suggests the welding thermal cycle can actually improve local hardness in the HAZ, which is favorable for joint strength. However, the wide HAZ and coarse weld metal structure present challenges that need to be addressed through process optimization and post-weld treatment. For aerospace and structural applications where spray-formed aluminum alloys are increasingly used for their superior mechanical properties, developing welding procedures that maintain a high fraction of base metal strength is essential. Future work should focus on combining advanced welding techniques with post-weld heat treatment to achieve joint efficiencies exceeding 80% of base metal strength.