Zhuojin Pipe Fitting Co., LtdResidual Stress Measurement in Pure Aluminum Thin Sheet TIG Welded Joints
Literature Overview
The study published in Hot Working Technology (2018, Vol. 47, No. 23) by Bao Xiaoyan, Gan Shiming, and Chun Lan from Inner Mongolia University investigates the welding residual stress distribution in 3 mm thick pure aluminum thin sheets joined by TIG welding. This research, supported by the Inner Mongolia Autonomous Region Graduate Research Innovation Project (B20161012804Z) and the Inner Mongolia Provincial Department of Education Project (NJZY17099), addresses an important but often underappreciated aspect of aluminum welding: the quantification and characterization of residual stress fields. The work employs the hole-drilling method combined with high-speed arc morphology analysis and wireless strain data acquisition, representing a methodologically rigorous approach to residual stress measurement.
Methodology and Technical Approach
The authors adopted a multi-faceted experimental approach to characterize both the welding process and the resulting residual stress state. High-speed camera analysis of the TIG arc morphology provided insights into the welding process stability and heat input characteristics. The arc shape and stability directly influence the thermal cycle, which in turn governs the residual stress development.
The hole-drilling method (also known as the small hole method or incremental hole-drilling method) was selected as the primary residual stress measurement technique. This method is widely accepted in the welding community for its practicality and relatively non-destructive nature. The technique involves drilling a series of concentric holes of increasing diameter at the measurement location, with each increment releasing a portion of the residual stress. Strain gauges arranged around the hole measure the elastic recovery, and the residual stress is calculated from the strain-stress relationship using the compliance factors appropriate for the material and geometry.
The use of wireless communication for strain data transmission represents a practical innovation that eliminates the interference and signal degradation associated with long wire runs during hole-drilling operations. The Matlab software was employed for residual stress calculation, providing flexibility in data processing and analysis.
Residual Stress Distribution Results
The residual stress distribution results reveal characteristic patterns that are consistent with established welding residual stress theory but provide specific quantitative data for 3 mm pure aluminum TIG weldments.
Transverse Direction (Perpendicular to Weld)
| Region | Residual Stress State | Trend Description |
|---|---|---|
| Weld center | High tensile stress | Maximum tensile stress concentration |
| Fusion zone vicinity | Peak residual stress | Maximum stress value achieved |
| Base metal region | Compressive stress | Tensile stress transitions to compressive |
| Constraint region | Gradually decreasing compressive | Compressive stress reduces toward far field |
| Far field | Slight tensile stress | Compressive stress transforms back to tensile |
The transverse residual stress profile follows the classic pattern observed in constrained weldments: high tensile stress near the weld that transitions to compressive stress in the surrounding base metal, eventually returning to near-zero or slightly tensile stress at the far field. The maximum residual stress occurs in the vicinity of the fusion zone, which is the region experiencing the most severe thermal gradients during welding.
Longitudinal Direction (Along Weld)
Along the weld direction, the longitudinal residual tensile stress gradually decreases from the weld ends toward the center. This pattern is characteristic of the progressive thermal cycling during continuous welding, where the weld start and end regions experience different thermal histories compared to the steady-state welding section.
Engineering Practice Implications
Understanding residual stress distribution in aluminum thin sheet weldments is critical for several engineering reasons. First, residual tensile stresses near the weld can reduce fatigue life, which is a primary concern for aerospace and automotive aluminum structures. Second, residual stresses can cause distortion and warping, which affects dimensional accuracy in precision fabrication. Third, in pressure-vessel applications, residual stresses can interact with service stresses to accelerate crack initiation and propagation.
The finding that the maximum residual stress occurs in the fusion zone vicinity rather than at the weld centerline itself is an important practical observation. This suggests that stress-relief treatments or post-weld machining should focus on the fusion zone region to achieve maximum residual stress reduction. The transition from tensile to compressive stress in the base metal region is also significant, as compressive residual stresses can be beneficial for fatigue performance by reducing the effective tensile stress range at the weld toe.
For pure aluminum specifically, the relatively low yield strength compared to aluminum alloys means that residual stresses can reach a significant fraction of the yield strength, potentially causing plastic deformation during welding. The 3 mm thickness is in the range where thin-sheet welding challenges are most pronounced, including susceptibility to burn-through, excessive distortion, and difficult arc stability.
Key Technical Points and Reflections
The residual stress measurement methodology employed in this study is well-suited for thin aluminum sheet applications. The hole-drilling method, when properly calibrated with appropriate compliance factors for aluminum, provides reliable stress values. However, engineers should be aware that the hole-drilling method has limitations in thin sheets, where the measurement depth and the stress gradient through the thickness can affect accuracy. The use of wireless strain data transmission is a practical improvement that should be adopted in field applications where cable management is challenging.
The arc morphology analysis provides complementary process information that helps correlate welding parameters with residual stress outcomes. Understanding the arc shape and stability allows engineers to optimize welding parameters not only for weld quality but also for minimizing residual stress. This holistic approach to welding process characterization is commendable and should be encouraged in future research.
The residual stress distribution patterns reported in this study are consistent with theoretical predictions and previous experimental results, providing confidence in the measurement methodology. However, the study would benefit from additional information on the specific welding parameters used (current, voltage, travel speed, shielding gas flow rate) and the welding sequence, as these factors significantly influence the residual stress state. Future work should also investigate the effectiveness of stress-relief measures, such as post-weld heating, vibration stress relief, or controlled cold working, in reducing the measured residual stresses.
This study contributes valuable quantitative data to the body of knowledge on aluminum welding residual stresses and demonstrates the practical applicability of the hole-drilling method in thin-sheet applications. The findings should be considered when designing fatigue-critical aluminum structures where residual stress management is a design requirement.