Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of TIG Welds in Gas-Atomized Powder Metallurgy 7055 Aluminum Alloy
Literature Overview
This study 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 in 3 mm thick 7055 aluminum alloy produced via gas atomized powder metallurgy (GAPM) hot extrusion. The base material was in the annealed condition, and ER5356 filler wire was used for the welding trials. The research was supported by the North China University of Water Resources and Electric Power high-level talent research startup fund (Grant No. 4001/40678). The 7055 alloy is a high-strength Al-Zn-Mg-Cu alloy widely used in aerospace structures, and its powder metallurgy variant offers enhanced mechanical properties and isotropy compared to conventional wrought alloys.
Core Technical Findings
Weld Microstructure Characterization
The study identified distinct microstructural zones in the weld joint:
| Zone | Microstructure Description | Grain Characteristics |
|---|---|---|
| Weld metal (WM) | Coarse dendritic structure | Large columnar dendrites |
| Fusion zone (FZ) | Fine equiaxed grains | Small, uniform grains |
| HAZ (near weld) | Elongated, layered structure | Significant grain growth |
| HAZ (far from weld) | Retained GAPM extruded structure | Original processed microstructure |
| Base metal | GAPM hot extruded structure | Fine, uniform grains |
The HAZ width extends to approximately 22 mm, which is notably large and indicates significant thermal exposure during welding. The near-weld HAZ exhibits pronounced grain coarsening with an elongated, layered morphology, which is characteristic of the recrystallization and grain growth occurring in the partially recrystallized region.
Mechanical Properties
The mechanical test results reveal a classic M-shaped hardness profile with two peaks located in the near-weld HAZ:
| Property | Value | Assessment |
|---|---|---|
| Average tensile strength | 190 MPa | Moderate for 7055 alloy |
| Average elongation | 6.76% | Acceptable ductility |
| Fracture mode | Mixed ductile-brittle | Indicates localized embrittlement |
The M-shaped hardness curve is a hathe writing systemark of aluminum alloy welds, where the two hardness peaks correspond to the zones of peak aging and over-aging in the HAZ. The weld metal itself exhibits lower hardness due to the dissolution of precipitates during welding and incomplete re-precipitation during cooling.
FMEA Analysis of Weld Joint Defects
Applying a Failure Mode and Effects Analysis (FMEA) framework to the identified characteristics:
| Failure Mode | Root Cause | Severity | Occurrence | Detection | RPN |
|---|---|---|---|---|---|
| Low weld metal strength | Precipitate dissolution, coarse dendrites | 8 | 8 | 6 | 384 |
| HAZ embrittlement | Grain coarsening, over-aging | 9 | 7 | 6 | 378 |
| Reduced ductility | Mixed fracture mode, precipitate coarsening | 7 | 7 | 7 | 343 |
| Large HAZ width | Excessive heat input | 6 | 8 | 5 | 240 |
The high RPN values for weld metal strength and HAZ embrittlement indicate that these are the primary quality concerns requiring mitigation.
Microstructural Mechanisms
The coarse dendritic structure in the weld metal results from the rapid solidification of the aluminum-rich ER5356 filler alloy, which lacks the alloying additions present in the base 7055 material. The fine equiaxed grains in the fusion zone are attributed to the interaction between the solidification front and the pre-existing fine grains in the GAPM base material, which act as heterogeneous nucleation sites.
The large HAZ width (22 mm) is a direct consequence of the relatively low thermal conductivity of the 7055 alloy and the heat input characteristics of TIG welding. The elongated, layered grain structure in the near-weld HAZ results from directional grain growth along the thermal gradient during the welding thermal cycle. The retained GAPM structure in the far HAZ and base metal indicates that the thermal influence of welding is confined to a relatively narrow band near the weld.
Engineering Practice Integration
For aerospace and high-performance structural applications where 7055 alloy is used:
| Requirement | Current Weld Performance | Gap Analysis |
|---|---|---|
| Tensile strength | 190 MPa | Well below base metal (typically >350 MPa) |
| Ductility | 6.76% | Acceptable but below base metal |
| Fracture toughness | Mixed mode | Needs improvement for critical applications |
| Fatigue resistance | Not reported | Coarse HAZ grains likely detrimental |
The significant strength reduction in the weld joint (approximately 50% of base metal strength) is a major concern for structural applications. The M-shaped hardness profile and mixed fracture mode indicate that the joint is susceptible to localized failure under cyclic loading, which is particularly problematic for aerospace structural components subject to fatigue.
Key Questions and Reflections
Several important questions emerge from this study:
- Would the use of a matching filler alloy (ER7055 or similar Al-Zn-Mg-Cu composition) improve weld metal strength and reduce the M-shaped hardness profile?
- Could post-weld heat treatment (PWHT) effectively restore precipitate strength in the weld metal and HAZ?
- How does the GAPM microstructure of the base metal influence weldability compared to conventional wrought 7055?
- What are the effects of alternative welding processes (MIG, laser, friction stir welding) on the same material?
- Can welding parameter optimization (lower heat input, multi-pass welding) reduce the HAZ width and improve joint properties?
Study Insights and Implications
This research provides valuable baseline data on the weldability of GAPM 7055 aluminum alloy using conventional TIG welding with ER5356 filler. The significant strength reduction and large HAZ width highlight the challenges of welding high-strength aluminum alloys, particularly those produced via powder metallurgy. The coarse dendritic weld metal and M-shaped hardness profile are characteristic of aluminum alloy welds where the filler alloy composition does not match the base metal. For engineering applications, this work underscores the need for either filler alloy matching, post-weld heat treatment, or alternative joining processes to achieve acceptable joint performance in high-strength aluminum alloys. The study serves as an important reference for process development in the aerospace and high-performance structural sectors where 7055 alloy is increasingly being adopted for its superior mechanical properties.