Microstructure and Mechanical Properties of TIG-Welded Thin 5B70 Aluminum Alloy Joints

Literature Overview

Published in Aeronautical Manufacturing Technology (2025, Vol. 68, No. 13, pp. 72–76), this study by Tian Zhijie, Sun Yuming, Du Han, Mi Haiming, and Hao Shuangxi from Capital Aerospace Machinery Co., Ltd. investigates the weldability of 5B70 aluminum alloy in thin sheet form (1.5 mm thickness) using TIG welding with 5B71 filler wire. The work is funded by the National Defense Science and Technology Program and Hunan Provincial Science and Technology Major Project, indicating its relevance to aerospace structural applications.

Material Background and Welding Challenges

5B70 is a high-strength Al-Zn-Mg-Cu alloy widely used in aerospace primary structures. Its composition (typically 5.0–6.5% Zn, 2.2–2.8% Mg, 1.2–1.8% Cu, with Fe and Ti as minor additions) provides excellent specific strength but presents significant welding challenges:

• Low melting point and high thermal conductivity promote wide HAZ and excessive heat input effects
• Susceptibility to hot cracking due to the Zn-Mg-Cu system
• Sensitivity to solidification cracking in the weld metal
• Potential for porosity due to hydrogen pickup from moisture in the atmosphere

For thin sheet applications (1.5 mm), the challenge is compounded by the risk of burn-through and distortion, requiring precise heat input control.

Welding Process Parameters and Results

The joint strength ratio of 0.82–0.84 indicates that the welded joint retains approximately 82–84% of the base metal tensile strength. For aerospace applications, this is generally acceptable for secondary structures but may require additional consideration for primary load-bearing components where higher strength ratios are desired.

Microstructural Analysis

Weld Metal Zone

The fusion zone exhibits refined equiaxed grain structure with non-uniform grain sizes. This is characteristic of TIG welding of aluminum alloys, where the relatively low heat input and slow cooling rate allow for some grain refinement through nucleation at the fusion boundary. However, the non-uniformity suggests incomplete grain refinement and possible columnar grain penetration from the fusion boundary.

Fusion Line

The fusion line is clearly defined, indicating a sharp boundary between the weld metal and the HAZ. The minimum hardness at this location (87 HV) is consistent with the formation of a thin zone of coarse equiaxed grains or a partial melt zone where precipitate dissolution occurs without subsequent re-precipitation during the relatively short cooling period.

Hardness Distribution

The double-V hardness profile is a classic signature of aluminum alloy welds:

• The weld center shows moderate hardness due to the as-cast microstructure
• Hardness drops at the fusion line due to precipitate dissolution in the partial melt zone
• Hardness rises in the HAZ due to precipitate coarsening and redistribution
• The base metal retains its peak-aged hardness

Fracture Surface Analysis

The fracture morphology reveals a 45° shear mode with mixed features of dimples and cleavage:

• Dimple formation indicates ductile microvoid coalescence
• Cleavage features suggest some brittle fracture contribution
• Strengthening phase particles found within some dimples indicate particle-induced void nucleation

Engineering Practice Considerations

For aerospace applications using 5B70 in thin sheet form, several practical considerations emerge:

1. Heat input control: The 1.5 mm thickness requires careful control of welding current, travel speed, and arc length to avoid burn-through while achieving complete fusion. TIG welding offers precise control but is relatively slow compared to MIG or laser welding.
2. Post-weld aging: The as-welded joint properties (342–350 MPa) may be improved through post-weld aging treatment. However, for thin sheet applications, distortion during aging may be a concern.
3. Filler wire selection: The use of 5B71 (which is essentially a 5xxx series alloy with slightly modified composition) provides good metallurgical compatibility. Alternative fillers such as 4043 or 5183 could be evaluated for different performance trade-offs.
4. Surface preparation: For thin sheet TIG welding, meticulous surface cleaning is essential to prevent porosity and ensure consistent arc stability.

Comparative Analysis with Similar Alloys

The 5B70 alloy demonstrates competitive strength performance with moderate elongation, positioning it as a viable material for thin-sheet aerospace structures where weight savings are critical.

Key Questions and Reflections

• How does the welding procedure affect the distribution of intermetallic phases (Al₂Cu, MgZn₂) in the weld metal and HAZ?
• What is the long-term stability of the joint properties under thermal cycling conditions typical of aerospace service?
• Can the non-uniform grain structure in the weld zone be improved through oscillation welding or pulsed TIG techniques?
• How does the joint performance compare under fatigue loading conditions, which are critical for aerospace structures?

Study Insights and Implications

This study provides valuable baseline data for the TIG welding of thin 5B70 aluminum alloy sheets. The achieved strength ratio of 0.82–0.84 with 9.5–13% elongation represents a reasonable performance envelope for this alloy in thin-section applications. The double-V hardness profile and mixed ductile-brittle fracture morphology highlight areas for process optimization, particularly in the fusion line region where precipitate dissolution creates a weak zone. For production engineering, the work confirms that TIG welding is a viable process for 5B70 thin sheets when parameters are carefully controlled, but also indicates that post-weld treatment and possibly advanced welding techniques (pulsed TIG, oscillation welding) may be necessary for critical aerospace applications requiring higher joint strength ratios and fatigue resistance.