Microstructure and Mechanical Properties of 7075 and 5A06 Dissimilar Aluminum Alloy TIG Welded Joints

Literature Overview

The research conducted by Liao Chuanqing, Su Guoyou, Gao Yanfang, Song Wei, Bao Hongwei, and Gao Jiashuang, published in the Chinese Journal of Nonferrous Metals in 2015 (Volume 25, Issue 1, pp. 43-48), funded by a National International Cooperation Project (2013DFR504xx), investigates the TIG welding of 7075 and 5A06 aluminum alloys, which represents a challenging dissimilar aluminum alloy welding problem. The 7075 alloy is a high-strength Al-Zn-Mg-Cu alloy used in aerospace structural applications, while 5A06 is a medium-strength Al-Mg-Si alloy with good corrosion resistance. The study evaluates two filler metals (BJ380A and ER4043) and provides comprehensive characterization of the joint microstructure and mechanical properties.

Material Selection and Dissimilar Welding Challenges

The welding of 7075 to 5A06 presents several fundamental challenges that are inherent to dissimilar aluminum alloy welding:

• Significant strength mismatch: 7075 has a yield strength of approximately 500 MPa, while 5A06 has approximately 250 MPa, creating a joint where the weaker material governs overall strength
• Different alloying element systems: 7075 is strengthened by Zn, Mg, and Cu, while 5A06 relies on Mg and Si, leading to different precipitation behavior in the weld zone
• Thermal expansion coefficient differences: Although both are aluminum alloys, the different compositions result in slightly different thermal expansion behaviors
• Precipitation-free zone formation: The T6 temper of 7075 is lost in the heat-affected zone, creating a precipitate-free zone (PFZ) that is susceptible to stress corrosion cracking

Filler Metal Comparison

The study evaluates two commercially available filler metals for this dissimilar joint:

Both filler metals produce joints with approximately 220 MPa tensile strength, which represents about 70% of the 5A06 base metal strength. This value is significantly lower than the 7075 base metal strength, confirming that the joint strength is governed by the weaker 5A06 side. The use of either filler metal results in fracture occurring in the weld zone, which is the expected behavior for dissimilar aluminum alloy welds.

Microstructural Analysis

The metallographic examination reveals several important features:

Weld zone: Both filler metals produce crack-free welds, but scattered porosity is present in both cases. The BJ380A filler produces coarser weld grains, while ER4043 produces finer grains. The presence of porosity is attributed to hydrogen gas porosity, which is a common defect in aluminum alloy TIG welding due to the high hydrogen solubility in molten aluminum and the rapid solidification rate.

7075 heat-affected zone: The HAZ on the 7075 side exhibits significant grain coarsening, which is more pronounced than on the 5A06 side. This grain growth is attributed to the higher thermal exposure experienced by the 7075 side, possibly due to the higher thermal conductivity of the copper-containing alloy. The grain coarsening in the 7075 HAZ reduces the yield strength of this region and increases susceptibility to stress corrosion cracking.

5A06 heat-affected zone: The HAZ on the 5A06 side shows less grain coarsening, which is consistent with the lower strength and different alloy chemistry of this alloy.

Microhardness distribution: The microhardness profile reveals contrasting behavior between the two HAZ regions:

The softening of the 7075 HAZ is a critical concern because it creates a region of reduced strength adjacent to the weld, which may be susceptible to stress corrosion cracking in aggressive environments. The hardening of the 5A06 HAZ is attributed to the precipitation of Mg2Si during cooling, which is a normal feature of the T6 temper restoration in this alloy.

Engineering Implications and Practical Considerations

The findings of this study have important implications for engineering practice:

1. Joint design: The 70% strength retention relative to the 5A06 base metal means that the joint is significantly weaker than the 7075 material. Design calculations must account for this strength reduction, and the joint should not be subjected to loads exceeding the reduced strength.
2. Stress corrosion cracking risk: The softening of the 7075 HAZ creates a region susceptible to stress corrosion cracking. Post-weld heat treatment may be necessary to restore the precipitate distribution, but this would affect the entire joint and potentially reduce the weld strength further.
3. Porosity control: The presence of porosity in both welds indicates that gas cleanliness and shielding gas flow rate are critical process parameters. For critical applications, stricter gas purity requirements and possibly pulsed TIG welding may be necessary to minimize porosity.
4. Filler metal selection: While both filler metals produce similar joint strength, ER4043 produces finer weld grains and higher weld hardness, which may be preferable for applications where weld zone toughness is important. BJ380A may be more appropriate where lower hardness and better ductility are desired.

Summary

This study provides comprehensive characterization of 7075/5A06 dissimilar aluminum alloy TIG welded joints and demonstrates that while crack-free welds can be achieved with appropriate filler metal selection, the joint strength is inherently limited to approximately 70% of the weaker base metal. The contrasting HAZ behavior, with softening on the 7075 side and hardening on the 5A06 side, creates a complex mechanical property profile that must be carefully considered in joint design. Engineers working with dissimilar aluminum alloy joints should recognize that the fundamental strength mismatch cannot be overcome by filler metal selection alone, and that alternative joining methods or joint designs may be necessary for critical applications where full strength retention is required.