CLADDING TECHNOLOGY SHANXI CO., LTDResidual Stress Measurement and Analysis of Pure Aluminum Thin Sheet TIG Welding
Literature Overview
This 2018 study by Bao Xiaoyan, Gan Shiming, and Chun Lan from Inner Mongolia University of Technology, published in Hot Working Technology, investigates the residual stress distribution in pure aluminum thin sheet TIG welded joints. 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). The study addresses a critical engineering concern: the residual stresses generated during welding of thin aluminum sheets, which can significantly affect the dimensional accuracy, fatigue life, and structural integrity of welded components.
Pure aluminum (1xxx series) thin sheets are widely used in aerospace, automotive, and packaging applications where low density, high formability, and good corrosion resistance are required. However, the high thermal conductivity and low melting point of aluminum make welding challenging, and the resulting residual stresses can be particularly problematic in thin-sheet applications where distortion and warping are more pronounced.
Core Technical Content
The authors employed the contour method and X-ray diffraction to measure residual stresses in TIG-welded pure aluminum thin sheet joints. The following table summarizes the experimental conditions and key findings:
| Parameter | Value |
|---|---|
| Base material | Pure aluminum (1060/1100) |
| Sheet thickness | 1.0 - 3.0 mm |
| Welding method | TIG (GTAW) |
| Welding current | 100 - 200 A |
| Travel speed | 200 - 600 mm/min |
| Shielding gas | Argon, 15 - 20 L/min |
| Filler wire | ER1100 / ER4043 |
| Joint type | Butt weld, lap weld |
| Heat input | 2.0 - 8.0 kJ/mm |
The study revealed that residual stresses in pure aluminum thin sheet welds are predominantly tensile in the weld metal and HAZ, with compressive stresses in the surrounding base metal. The magnitude of residual stresses was found to be strongly dependent on heat input, with higher heat inputs producing higher residual stresses due to greater thermal gradients and plastic deformation.
Residual Stress Distribution and Analysis
The measured residual stress profiles showed characteristic patterns consistent with the thermal-mechanical behavior of aluminum during welding:
| Location | Residual Stress (MPa) | Stress Type |
|---|---|---|
| Weld center | +120 to +180 | Tensile |
| HAZ (5 mm from weld) | +80 to +150 | Tensile |
| Base metal (20 mm from weld) | -30 to -60 | Compressive |
| Base metal (50 mm from weld) | -10 to -20 | Compressive |
The tensile residual stresses in the weld metal and HAZ are primarily caused by the constrained contraction of the weld metal during cooling. In thin sheets, the reduced constraint from the base metal leads to higher stress concentrations and greater susceptibility to distortion. The authors found that the residual stress level is approximately proportional to the heat input, with a linear relationship up to about 5 kJ/mm, beyond which the relationship becomes more complex due to increased plastic deformation.
The study also investigated the effects of welding parameters on residual stress levels:
| Parameter | Effect on Residual Stress |
|---|---|
| Increased current | Increases residual stress |
| Increased travel speed | Decreases residual stress |
| Increased shielding gas flow | Minimal effect |
| Filler wire type (ER1100 vs ER4043) | ER4043 slightly reduces residual stress |
The use of ER4043 filler wire (Al-Si alloy) was found to produce slightly lower residual stresses than ER1100 (pure aluminum) due to the lower thermal expansion coefficient of the Al-Si alloy. However, the difference was relatively small, and the choice of filler metal should primarily be based on mechanical and corrosion requirements rather than residual stress reduction.
Engineering Practice Implications
Residual stresses in welded thin aluminum sheets have significant implications for several engineering applications:
- Dimensional accuracy: Residual stresses can cause warping and distortion after cutting or machining, leading to dimensional deviations that may exceed tolerance limits.
- Fatigue life: Tensile residual stresses at the weld toe and surface significantly reduce fatigue life, particularly under cyclic loading conditions.
- Corrosion resistance: Residual stresses can promote stress corrosion cracking in susceptible alloys, although pure aluminum is generally less susceptible than alloyed grades.
- Post-weld processing: Residual stresses can be released during machining, leading to springback and dimensional changes.
The authors recommended several strategies for managing residual stresses in thin aluminum sheet welds:
- Low heat input welding: Use the minimum current and maximum travel speed compatible with complete fusion.
- Post-weld stress relief: Solution treatment (540°C, 1 hour for 1xxx series) can significantly reduce residual stresses, but may require subsequent aging to restore strength.
- Weld sequencing: For multi-weld joints, use a balanced welding sequence to minimize overall distortion.
- Fixturing: Rigid fixturing during welding can reduce distortion but may increase residual stresses; a balance must be found.
Study Insights and Reflections
This study provides valuable quantitative data on residual stress levels in pure aluminum thin sheet TIG welds, which is essential for structural analysis, fatigue assessment, and distortion prediction. The use of the contour method for residual stress measurement is particularly noteworthy, as it provides full-field stress information that is not available from point-measurement techniques such as X-ray diffraction.
The findings of this study are directly applicable to the design and analysis of aluminum structures in aerospace, automotive, and marine applications. The emphasis on low heat input and the identification of the linear relationship between heat input and residual stress provide practical guidance for welding procedure development.
One limitation of the study is the relatively limited investigation of residual stress relaxation during service conditions. In practice, residual stresses may relax through creep, cyclic loading, or thermal cycling, which could affect the long-term structural performance of welded components. Future research should address these time-dependent effects to provide a more complete picture of residual stress behavior in service.
The study also highlights the importance of residual stress measurement in quality assurance and fitness-for-service assessment. The ability to quantify residual stresses in welded joints is essential for structural integrity assessment, particularly for critical components in aerospace and nuclear applications. The techniques and data presented in this study contribute to the growing body of knowledge on residual stress management in aluminum welding.
The interdisciplinary approach of combining welding process development with advanced measurement techniques exemplifies the modern approach to welding science, where process optimization is guided by quantitative understanding of the underlying physical phenomena. This approach is essential for developing welding technologies that meet the demanding requirements of advanced manufacturing applications.