Ultrasonic Stress Relief Treatment After MIG Welding of 7020 Aluminum Alloy

Literature Overview

The paper by Liu Guodong and Guo Xiaohui, published in Materials Development and Application (Vol. 28, Issue 5, 2013), investigates the effect of ultrasonic impact treatment (UIT) on residual stresses in MIG-welded 7020 aluminum alloy plates. The study was conducted in collaboration between the Navy Military Representative Office for Ship Gun Systems in the Shanghai region and the 725th Research Institute of China Shipbuilding Industry Corporation. This work is directly relevant to naval engineering applications, where aluminum alloy structures must withstand severe loading conditions with minimal residual stress to avoid fatigue failure and stress corrosion cracking.

Material and Welding Process

7020 aluminum alloy is a high-strength Al-Zn-Mg-Cu alloy belonging to the 7000 series, known for its excellent strength-to-weight ratio. It is widely used in naval applications, particularly in ship structures and weapons systems, where high strength and low weight are critical requirements. The T6 temper provides a yield strength of approximately 500 MPa and a tensile strength of approximately 570 MPa.

The welding process used in this study is MIG (Metal Inert Gas) welding of 16 mm thick plates. MIG welding is a common process for aluminum alloy fabrication due to its high deposition rate and good weld quality. However, welding of thick-section aluminum alloy plates generates significant residual stresses, which can be detrimental to the fatigue and stress corrosion resistance of the structure.

7020 Aluminum Alloy Properties

Ultrasonic Impact Treatment (UIT) Principles

Ultrasonic impact treatment is a mechanical surface treatment technique that uses focused ultrasonic vibrations to plastically deform the surface layer of a metal component. The treatment is performed by impacting the surface with a hardened tool, typically a tungsten carbide rod, while applying ultrasonic vibrations at frequencies of 20 kHz.

The mechanism of residual stress modification by UIT involves several processes:

1. Plastic deformation of the surface layer: The impact causes localized plastic deformation of the material at and near the surface, creating a layer of cold-worked material.
2. Compressive residual stress introduction: The plastic deformation of the surface layer, constrained by the elastic substrate beneath, introduces compressive residual stresses in the treated region.
3. Work hardening: The cold work increases the hardness and strength of the surface layer, which can improve wear resistance and fatigue performance.
4. Surface roughness modification: The treatment modifies the surface roughness, which can affect stress concentration at the surface.

UIT Process Parameters

Residual Stress Analysis

The paper reports that ultrasonic impact treatment after MIG welding of 7020 aluminum alloy effectively reduces the residual stresses in the weld and HAZ regions. The key findings include:

• Reduction of tensile residual stresses: The as-welded condition typically exhibits high tensile residual stresses in the weld and HAZ regions, which can reach values approaching the yield strength of the material. UIT reduces these tensile stresses and introduces compressive residual stresses.
• Depth of stress relief: The depth of the compressive stress zone depends on the UIT parameters, with typical depths of 0.5-2.0 mm for standard treatment conditions.
• Uniformity of treatment: The uniformity of the residual stress distribution depends on the treatment parameters, particularly the impact speed and overlap. Insufficient overlap can result in non-uniform stress distributions.

Engineering Significance for Naval Applications

For naval structures, the reduction of residual stresses through UIT is particularly important for several reasons:

1. Fatigue resistance: Compressive residual stresses at the surface inhibit fatigue crack initiation and slow crack propagation, significantly improving the fatigue life of the structure. In naval applications, where structures are subjected to cyclic loading from wave actions and machinery vibration, fatigue is a primary failure mode.
2. Stress corrosion cracking resistance: Residual tensile stresses are a major contributor to stress corrosion cracking (SCC) in aluminum alloys, particularly in marine environments. Reducing residual stresses through UIT can significantly improve SCC resistance.
3. Dimensional stability: Residual stresses can cause distortion and dimensional instability during subsequent machining operations. UIT can reduce these stresses, improving dimensional accuracy of machined surfaces.
4. Compliance with naval standards: Naval standards such as MIL-HDBK-5 and various naval welding specifications require residual stress relief for critical structures. UIT provides an alternative to thermal stress relief, which is not always practical for aluminum alloy structures due to the risk of over-aging and loss of mechanical properties.

Comparison with Other Stress Relief Methods

Summary

This study demonstrates the effectiveness of ultrasonic impact treatment in reducing residual stresses in MIG-welded 7020 aluminum alloy plates. The technique offers a practical alternative to thermal stress relief for aluminum alloy structures, particularly in naval applications where over-aging must be avoided. The introduction of compressive residual stresses through UIT can significantly improve fatigue and stress corrosion resistance, which are critical performance requirements for naval structures. For engineers involved in the fabrication of aluminum alloy naval structures, this work highlights the value of post-weld mechanical stress relief techniques as a means of improving structural integrity and service life. The findings also suggest that further optimization of UIT parameters for specific welding conditions and geometries could yield even better results in residual stress reduction and fatigue performance improvement.