Multi-Pass MIG Welding Interface Behavior in Aluminum-Magnesium Alloy Plates

Overview and Research Significance

Jian Haigen and colleagues from Hunan University of Technology and Central South University (Rare Metal Materials and Engineering, 2016, Vol. 45, No. 2, pp. 415-420) investigate the welding interface behavior in multi-pass MIG welding of 8 mm thick aluminum-magnesium alloy plates using self-made filler wire. This study is particularly relevant for pipe and structural fabrication, where multi-pass welding is the standard approach for thick-section aluminum alloy components. The use of advanced characterization techniques — optical metallography, scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD) — provides comprehensive microstructural and crystallographic insights.

Multi-Pass Welding Thermal Cycle Characteristics

Multi-pass welding involves a complex thermal history where each subsequent pass subjects the previously solidified weld metal and heat-affected zone (HAZ) to additional thermal cycles. For an 8 mm thick plate welded in multiple passes, the thermal cycle experienced by the root pass is fundamentally different from that of the cap pass. The root pass undergoes a single solidification cycle followed by multiple reheat cycles from subsequent passes. The cap pass undergoes only a single solidification cycle with minimal subsequent reheating. This differential thermal history creates distinct microstructural zones along the weld cross-section.

The cumulative thermal input from multiple passes leads to significant grain coarsening in the reheat-affected zones. In aluminum alloys, the grain growth kinetics are particularly aggressive due to the low melting point and high self-diffusion rates. The reheat coarse-grain HAZ (CGHAZ) adjacent to inter-pass boundaries becomes a critical region of microstructural heterogeneity.

Microstructural Analysis Results

The study reveals that the weld metal exhibits a typical cast structure with dendritic morphology, characteristic of solidification from the liquid phase. Importantly, the weld metal does not exhibit significant preferred orientation (texture), indicating that the solidification process is dominated by random nucleation rather than epitaxial growth from the base metal.

However, the inter-pass regions tell a different story. The CGHAZ adjacent to inter-pass boundaries shows strong epitaxial growth tendencies and pronounced texture. This is attributed to two factors: first, the cumulative thermal cycles promote grain coarsening and preferential growth of grains favorably oriented for heat extraction; second, the microstructural similarity between adjacent passes creates favorable conditions for epitaxial growth across the inter-pass boundary.

The CGHAZ at inter-pass boundaries emerges as the weakest link in the welded joint. The combination of coarse grains, strong texture, low hardness, and high residual stress creates a microstructural configuration that is particularly susceptible to crack initiation and propagation. This is a critical finding for fatigue design and fracture mechanics assessment of multi-pass aluminum welds.

Crystallographic Texture Analysis

The EBSD analysis provides quantitative texture data that is not accessible through conventional metallographic techniques. The texture strength in the CGHAZ at inter-pass boundaries is significantly higher than in the FGHAZ and weld metal. This strong texture indicates that grain growth in the CGHAZ is not random but is preferentially driven by crystallographic orientation factors. Grains with orientations that facilitate epitaxial growth across the inter-pass boundary grow at the expense of less favorably oriented grains, resulting in a highly anisotropic microstructure.

The implications of strong texture extend beyond mechanical properties. Textured microstructures exhibit anisotropic thermal conductivity, which can affect the thermal cycling history of subsequent passes and create additional microstructural heterogeneity. Furthermore, texture influences the anisotropy of mechanical properties — yield strength, ductility, and fracture toughness may vary significantly with direction relative to the texture axes.

Residual Stress Distribution

The residual stress analysis reveals that the weld root exhibits the highest residual stress, followed by the CGHAZ at inter-pass boundaries. This distribution is consistent with the thermal contraction sequence: the weld root solidifies first and cools last, experiencing the greatest thermal contraction restraint. The inter-pass CGHAZ, being reheated to temperatures above the recrystallization temperature during subsequent passes, undergoes thermal cycling that generates additional residual stresses superimposed on the primary welding stresses.

For aluminum-magnesium alloys, which are susceptible to stress corrosion cracking (SCC) in chloride-containing environments, the high residual stresses in the CGHAZ at inter-pass boundaries represent a significant concern. The combination of high residual tensile stress, coarse grains, and potential precipitate-free zones in the CGHAZ creates conditions favorable for SCC initiation.

Implications for Pipe and Fitting Welding

For engineers welding aluminum alloy pipes and fittings, this study highlights the critical importance of inter-pass temperature control and pass sequencing in multi-pass welding. Excessive inter-pass temperatures promote grain coarsening in the CGHAZ, exacerbating the weak-link behavior identified in this study. The recommended inter-pass temperature for aluminum alloy welding is typically below 150°C (some specifications limit it to 100°C), and this limit should be strictly enforced for thick-section multi-pass welds.

The choice of filler wire is also critical. The use of self-made filler wire in this study suggests that the filler metal composition was tailored to the base metal. For aluminum-magnesium alloys, the filler metal should match the base metal's magnesium content to avoid hot cracking and ensure adequate solid solubility. The study does not provide detailed filler wire composition data, which limits the generalizability of the findings.

Process Optimization Recommendations

Based on the findings of this study, several process optimization strategies can be recommended for multi-pass aluminum alloy welding:

1. Strict inter-pass temperature control below 150°C to minimize CGHAZ grain coarsening.
2. Careful pass sequencing to minimize the number of thermal cycles experienced by any single region.
3. Consideration of pulsed MIG welding to reduce heat input per pass and minimize CGHAZ width.
4. Post-weld stress relief treatment (solution treatment and aging) to mitigate residual stresses and refine the microstructure.
5. Non-destructive testing (NDT) with particular attention to inter-pass boundaries, where the microstructural heterogeneity may affect NDT signal interpretation.

Summary

This study provides a comprehensive microstructural and crystallographic analysis of multi-pass MIG welded aluminum-magnesium alloy joints, identifying the CGHAZ at inter-pass boundaries as the weakest link in the welded joint. The combination of coarse grains, strong texture, low hardness, and high residual stress in this zone creates a configuration susceptible to crack initiation and stress corrosion cracking. For pipe and fitting fabrication involving thick-section aluminum alloys, the findings underscore the critical importance of inter-pass temperature control, pass sequencing optimization, and post-weld heat treatment. The use of EBSD for quantitative texture analysis represents a powerful tool for microstructural characterization that should be adopted in advanced welding research and qualification programs.