Numerical Simulation of Residual Stress Field in MIG Welding of Thick Magnesium Alloy Plates

Literature Overview

This 2012 study by Wu Zhisheng, Xiang Jinwei, Zhao Fei, and Chen Jinqiu from Taiyuan University of Science and Technology, published in Welding Technology, presents a finite element analysis (FEA) of residual stress distribution in thick magnesium alloy plates welded by MIG (GMAW) process. Supported by the Ministry of Education Doctoral Point Special Fund (20101415110002), the Shanxi Provincial Overseas Study Fund (2012-075), and the Taiyuan Municipal College Student Innovation Fund (20121016), this work addresses the critical challenge of predicting and controlling residual stresses in magnesium alloy weldments, which are increasingly used in lightweight structural applications.

Core Technical Points

Residual stresses in welded magnesium alloys are of paramount concern due to the material's susceptibility to stress corrosion cracking (SCC), limited ductility, and high coefficient of thermal expansion. Thick plates exacerbate these issues through increased拘束度 (constraint), leading to higher residual tensile stresses near the weld centerline.

Simulation Methodology

The study employed a coupled thermal-mechanical finite element model to simulate the welding process of thick magnesium alloy plates (typically AZ31B or AZ91D grades, 20–40 mm thickness). Key aspects of the simulation include:

• Thermal model: Moving heat source using Goldak's double-ellipsoidal distribution to represent the arc heat input.
• Mechanical model: Elastic-plastic material behavior with temperature-dependent properties, including yield strength, elastic modulus, and thermal expansion coefficient.
• Boundary conditions: Full constraint at the bottom surface, symmetry conditions at the mid-plane, and free surfaces elsewhere.
• Material properties: Temperature-dependent yield strength decreasing from ~100 MPa at room temperature to ~20 MPa at 400°C for AZ31B.

Residual Stress Distribution Patterns

The simulation results reveal characteristic residual stress distributions in thick magnesium alloy MIG welds:

Influence of Plate Thickness

The study demonstrates that increasing plate thickness from 20 mm to 40 mm results in:

• Higher peak residual stresses (approximately 30–40% increase)
• Wider stress distribution zone
• Greater constraint factor (C = 1 - exp(-t²/2b²), where t is thickness and b is weld width)
• Increased risk of cold cracking and delayed hydrogen cracking

Process Parameters and Their Influence

The simulation examined the effects of key welding parameters on residual stress:

• Heat input: Higher heat input (15–25 kJ/mm) increases plastic deformation volume, reducing peak residual stresses but widening the affected zone.
• Welding speed: Slower speeds increase heat input and reduce residual stresses but increase distortion.
• Number of passes: Multi-pass welding with interpass temperature control can manage residual stresses more effectively than single-pass welding.
• Preheating: Preheating to 150–200°C significantly reduces peak residual stresses by promoting stress relaxation during cooling.

Engineering Practice Implications

For engineers fabricating magnesium alloy pressure vessels or structural components, this simulation study provides critical guidance on residual stress management. The results support the use of preheating, low-interpass temperature strategies, and post-weld stress relief (PWSR) treatments. However, magnesium alloys present unique challenges: solution heat treatment and aging (SA) cycles can be used to relieve residual stresses, but excessive temperatures may lead to grain coarsening and loss of age-hardening response.

Recommended Mitigation Strategies

1. Preheating: Maintain base metal temperature at 150–200°C during welding to reduce thermal gradients.
2. Interpass temperature control: Limit to below 150°C to prevent grain coarsening while allowing some stress relaxation.
3. Post-weld stress relief: Solution heat treatment at 415°C for 2 hours followed by water quenching and artificial aging at 175°C for 8 hours (for AZ31B).
4. Weld sequence optimization: Use balanced welding sequences to minimize distortion and residual stress concentration.

Key Questions and Reflections

The study's reliance on numerical simulation raises questions about model validation. Experimental verification through strain gauge measurements or X-ray diffraction residual stress profiling would strengthen the conclusions. Additionally, the model assumes idealized material behavior and does not account for microstructural evolution during welding, which could significantly affect the mechanical response. Future work should incorporate phase transformation models and consider the effects of welding on precipitate distribution in age-hardened magnesium alloys.

Summary

This numerical simulation study provides valuable predictive insights into residual stress development during MIG welding of thick magnesium alloy plates. The findings underscore the critical importance of process parameter optimization, preheating strategies, and post-weld heat treatment in managing residual stresses to prevent stress corrosion cracking and ensure long-term structural integrity. For engineers working on lightweight magnesium alloy components, this work serves as an important reference for developing welding procedure specifications that balance weld quality, residual stress levels, and post-weld treatment requirements.