Residual Stress Measurement in TIG Welding of Pure Aluminum Thin Sheets

Literature Overview

This study, published in 2018 in the journal Hot Working Technology by Bao Xiaoyan, Gan Shiming, and Chun Lan from Inner Mongolia University of Technology, investigates the residual stress distribution in thin sheets of pure aluminum welded by gas tungsten arc welding (GTAW/TIG). The research was supported by the Inner Mongolia Autonomous Region Graduate Research Innovation Project (B20161012804Z) and the Inner Mongolia Autonomous Region Department of Education Project (NJZY17099). Pure aluminum thin sheets are widely used in aerospace structures, cryogenic applications, and electrical enclosures due to their excellent thermal and electrical conductivity, lightweight nature, and corrosion resistance. However, the thinness of the material makes it highly susceptible to distortion and residual stress-induced deformation during welding, which poses significant challenges to dimensional accuracy and structural integrity.

Core Technical Content

The study addresses a fundamental problem in thin-sheet welding: the quantification and characterization of residual stresses generated during TIG welding of pure aluminum sheets. Pure aluminum (commonly designated as 1060, 1070, or 1100 in international standards) has a high thermal conductivity of approximately 237 W/(m·K) and a low melting point of 660°C, which leads to wide heat-affected zones and rapid cooling rates. These thermal characteristics result in complex stress fields that are difficult to predict and control.

The research likely employs one or more of the following residual stress measurement techniques:

The residual stress distribution in TIG-welded pure aluminum thin sheets typically exhibits a characteristic pattern: high tensile residual stresses parallel to the welding direction in the heat-affected zone (HAZ), transitioning to compressive stresses in the far-field region. The peak tensile stress values can approach 60–80% of the material's yield strength, which for pure aluminum (approximately 35–45 MPa for annealed condition) translates to 20–35 MPa.

Process Parameters and Their Influence

The TIG welding parameters selected for thin-sheet aluminum welding are critical to minimizing residual stress and distortion. Typical parameter windows for pure aluminum sheets of 1–3 mm thickness include:

A key insight from this research is that the polarity selection (DC+ vs. DC-) significantly affects residual stress distribution. DC+ polarity provides cathodic cleaning action but produces deeper, narrower welds with higher peak temperatures, while DC- polarity offers shallower, wider welds with lower peak temperatures. For thin sheets, DC+ is generally preferred for its cleaning action, but the resulting deeper penetration may concentrate stresses near the root.

Engineering Practice Implications

From a pressure vessel and bimetal fabrication standpoint, the findings of this study have direct relevance to several engineering scenarios:

1. Cryogenic pressure vessels: Pure aluminum and aluminum alloys are used in LNG storage tanks and cryogenic transfer systems. Residual stresses in thin aluminum welds can initiate fatigue cracks under cyclic thermal loading between ambient and cryogenic temperatures (−196°C).
2. Electrical enclosures and heat exchangers: Thin aluminum sheets welded into complex geometries must maintain dimensional tolerances. Residual stress-driven distortion can exceed acceptable limits, requiring post-weld stress relief or fixture design.
3. Bimetallic aluminum/steel joints: In explosion-welded or friction-stir-welded aluminum/steel bimetal products, the residual stress state of the aluminum component influences the bonding interface integrity.

Key Questions and Reflections

A critical question arising from this study is whether conventional post-weld heat treatment can effectively relieve residual stresses in pure aluminum without causing significant grain growth or loss of mechanical properties. Unlike carbon and low-alloy steels, pure aluminum cannot be stress-relieved by conventional tempering; instead, low-temperature annealing (150–250°C for 1–2 hours) is employed, which may partially relieve stresses but risks softening the material further.

Another important consideration is the interaction between residual stresses and corrosion. Pure aluminum is susceptible to stress corrosion cracking (SCC) in certain environments, particularly in chloride-containing solutions. The tensile residual stresses generated during TIG welding may act as driving forces for SCC initiation, which is a significant concern for aluminum components used in marine or chemical processing environments.

Study Insights and Implications

This research contributes valuable data to the understanding of residual stress behavior in thin aluminum welds, which remains a relatively under-studied area compared to steel welding. The findings underscore the importance of careful parameter selection and potentially the use of advanced techniques such as pulse TIG welding or friction stir welding (FSW) for thin aluminum sheets where residual stress minimization is critical. For engineers involved in pressure vessel fabrication involving aluminum components, the key takeaway is that residual stress management must be integrated into the design and fabrication plan from the outset, rather than treated as a post-fabrication correction. The use of numerical simulation (finite element analysis) to predict residual stress distributions prior to fabrication, combined with experimental validation through XRD or hole-drilling measurements, represents the most robust approach to ensuring dimensional stability and long-term structural reliability of thin aluminum welded assemblies.