Residual Stress Measurement of 7A52 Aluminum Alloy VPPA-MIG Hybrid Welding Based on Elastic Modulus Variation

Literature Overview

This study by Gan Shiming and colleagues from Inner Mongolia University, funded by the National Natural Science Foundation of China (Grant 51665044), was published in the journal Transactions of the China Welding Institution, Volume 40, Issue 5, pages 13–17, in 2019. The research focuses on residual stress measurement in 7A52 aluminum alloy welded joints using variable polarity plasma arc (VPPA) combined with MIG welding, with a particular emphasis on correcting elastic modulus errors that affect measurement accuracy. The work addresses a significant challenge in residual stress analysis: the assumption of constant elastic modulus across the weld, HAZ, and base metal regions, which is invalid for aluminum alloys where the elastic modulus varies significantly across the weldment.

Core Technical Findings

The authors developed a residual stress measurement system based on the hole-drilling method, using virtual instruments and a National Instruments data acquisition card. A key innovation is the measurement of elastic modulus at different locations across the hybrid-welded joint and the fitting of a curve to describe the elastic modulus variation with position. This correction is applied to the residual stress calculations to eliminate systematic errors that arise from assuming a uniform elastic modulus.

For 10 mm thick 7A52 aluminum alloy plates, the VPPA-MIG hybrid welding residual stress distribution shows several important characteristics. The residual stress distribution is approximately symmetric about the weld centerline on both sides. The maximum tensile residual stress occurs in the fusion zone, with maximum transverse residual stress σy of 118 MPa and maximum longitudinal residual stress σx of 223 MPa. From the fusion zone to the heat-affected zone, residual stresses remain tensile, gradually decreasing but remaining higher than the residual stress at the weld center.

Residual Stress Distribution and Analysis

The comparison between VPPA-MIG hybrid welding and single MIG welding reveals that the hybrid process produces higher maximum residual stresses in both transverse and longitudinal directions. However, the high-stress zone in the hybrid weld is narrower than in the MIG weld. This is consistent with the higher heat input and deeper penetration of the VPPA component, which creates a more concentrated thermal cycle and thus more localized residual stress. The narrower high-stress zone may be advantageous in certain applications because the affected region is smaller, potentially reducing the likelihood of stress corrosion cracking or fatigue crack initiation in a broader area.

Elastic Modulus Correction Methodology

The elastic modulus correction methodology employed in this study is technically significant. The hole-drilling method for residual stress measurement relies on the assumption that the elastic modulus is known and uniform in the region where stress relief occurs. In aluminum alloys, the elastic modulus can vary by 5–15% across the weld, HAZ, and base metal due to differences in microstructure, precipitate distribution, and thermal history. Without correction, this variation introduces systematic errors in the calculated residual stresses that can be comparable to or even exceed the actual stress differences between regions.

The approach of measuring elastic modulus at multiple points across the weld and fitting a continuous curve is a practical and effective solution. This method accounts for the non-linear variation of elastic modulus and provides a more accurate representation of the stress-strain relationship used in the residual stress calculations. The use of virtual instruments and data acquisition cards enables precise and reproducible measurements of both elastic modulus and residual stress.

Engineering Practice and Implications

For engineers working on aluminum alloy structural components, particularly in aerospace and high-speed rail applications where 7A52 aluminum alloy is commonly used, accurate residual stress measurement is essential for predicting component performance. Residual stresses can significantly affect fatigue life, stress corrosion cracking susceptibility, dimensional stability, and dimensional accuracy of machined components. The residual stress values reported in this study—up to 223 MPa longitudinal and 118 MPa transverse—are substantial and must be considered in the design and analysis of welded structures.

The finding that VPPA-MIG hybrid welding produces higher but more localized residual stresses has important implications for process selection. In applications where the overall magnitude of residual stress is less critical than the affected area, the hybrid process may be preferred because the narrower high-stress zone reduces the volume of material susceptible to stress-related degradation. Conversely, in applications where maximum stress levels are critical, the lower peak stresses from single MIG welding may be advantageous.

The elastic modulus correction methodology developed in this study has broader applicability beyond VPPA-MIG hybrid welding. Any residual stress measurement of aluminum alloy weldments using the hole-drilling method should consider the variation in elastic modulus across the weldment. This study provides a practical framework for implementing such corrections, which can improve the accuracy of residual stress data used in structural analysis and quality control.

In summary, this study provides a rigorous methodology for measuring residual stresses in 7A52 aluminum alloy VPPA-MIG hybrid welded joints, incorporating elastic modulus correction to improve measurement accuracy. The results demonstrate that hybrid welding produces higher but more localized residual stresses compared to single MIG welding, with maximum longitudinal stress of 223 MPa and transverse stress of 118 MPa. The elastic modulus correction approach is a valuable contribution to the field of residual stress measurement and should be adopted in any hole-drilling-based residual stress analysis of aluminum alloy weldments.