Ultrasonic Stress Relief Treatment of 7020 Aluminum Alloy After MIG Welding

Literature Overview

This study by Liu Guodong and Guo Xiaohui (published in Materials Development and Application, Vol. 28, No. 5, 2013, pp. 25-28) investigates the application of ultrasonic impact treatment for residual stress relief in 16 mm thick 7020 aluminum alloy plates welded by GMAW. The work is particularly relevant to naval applications (the authors are affiliated with naval military representative offices and shipbuilding research institutes) where residual stress control is critical for fatigue performance and dimensional stability.

Material and Welding Process

7020 aluminum alloy is a high-strength Al-Zn-Mg-Cu alloy (equivalent to AA7075) with excellent specific strength properties. It is widely used in naval structures, pressure hulls, and high-performance marine equipment.

Residual Stress in Aluminum Alloy Welds

Welding of 7xxx series aluminum alloys produces significant residual stresses due to:

• Thermal expansion mismatch: The coefficient of thermal expansion of aluminum alloys (23-24 × 10⁻⁶/K) is approximately double that of steel (12 × 10⁻⁶/K), leading to greater thermal strains
• Large plastic deformation: The low yield strength at elevated temperatures allows extensive plastic flow during welding, which is locked in upon cooling
• Temper softening: The T6 temper strength is reduced in the HAZ, creating a weaker zone that bears disproportionate stress

Typical residual stress levels in 16 mm thick 7020 alloy MIG welds include:

• Longitudinal tensile stress: 200-350 MPa (approaching or exceeding the yield strength of the base metal)
• Transverse tensile stress: 100-200 MPa
• Through-thickness stress: Compressive at the surface, tensile in the interior

Ultrasonic Impact Treatment (UIT) Mechanism

Ultrasonic impact treatment applies cyclic plastic deformation to the weld surface using high-frequency (20 kHz) and high-amplitude (50-100 μm) impacts from hardened steel balls. The mechanism operates through:

1. Surface plastic deformation: Creates a compressive residual stress layer 0.5-2.0 mm deep
2. Stress redistribution: The compressive surface layer induces compensating stress changes in the interior, reducing peak tensile stresses
3. Work hardening: Increases surface hardness by 20-40%, creating a barrier to crack initiation
4. Microstructure refinement: Breaks up coarse grains and redistributes precipitates in the HAZ

Experimental Results and Analysis

The study demonstrates that UIT effectively reduces weld residual stresses in 7020 aluminum alloy. The key findings include:

• Longitudinal stress reduction: Peak tensile stresses reduced by 30-50% after treatment
• Surface compressive stress: A compressive layer of 50-150 MPa is established at the weld surface
• Depth of effect: The treatment affects material to a depth of approximately 1.0-1.5 mm (for 16 mm thick plate)
• Uniformity: Stress reduction is relatively uniform along the weld length when proper treatment parameters are used

Engineering Practice Considerations

For naval applications, residual stress control is critical for:

• Fatigue life: Residual tensile stresses reduce fatigue life by 20-50% in high-strength aluminum alloys
• Stress corrosion cracking (SCC): 7xxx series alloys are highly susceptible to SCC in marine environments; residual tensile stresses accelerate crack initiation
• Dimensional stability: Residual stresses cause post-weld distortion, affecting assembly and machining tolerances
• Weld integrity: Excessive residual stresses can cause delayed cracking or hydrogen-assisted cracking

The UIT treatment provides a practical post-weld solution that:

• Requires no specialized equipment beyond the ultrasonic impact tool
• Can be applied to as-welded joints without preheating or post-heating
• Does not alter the base metal composition or temper condition significantly
• Can be combined with other treatments (grinding, TIG dressing) for enhanced effectiveness

Limitations and Cautions

Engineers should be aware of:

• Surface damage: Excessive impact energy can cause surface indentation or cracking in thin sections
• Hardness increase: The work-hardened layer may affect subsequent machining or coating adhesion
• Limited depth: Only the near-surface region is treated; bulk residual stresses remain largely unchanged
• Parameter sensitivity: Treatment effectiveness is highly dependent on impact parameters; optimization is required for each application

Key Reflections

This study highlights an important practical approach to residual stress management in high-strength aluminum alloy structures. The ultrasonic impact treatment offers a cost-effective alternative to full stress-relief heat treatment, which is often impractical for large naval structures. The combination of compressive surface stress and microstructural refinement provides dual benefits for fatigue and corrosion resistance. For pipe and fitting applications involving aluminum alloys, UIT should be considered as a standard post-weld treatment, particularly for fatigue-critical joints in marine and aerospace environments. The work demonstrates that post-weld treatment is not an optional step but a necessary component of the welding process for high-performance applications.