Zhuojin Pipe Fitting Co., LtdNumerical Simulation of Residual Stress in Titanium Alloy Thin Plate Laser Welding and TIG Welding
Literature Overview
This paper by He Xiaodong et al. from Xi'an Jiaotong University and the Beijing Institute of Aeronautical Manufacturing Engineering, published in Materials in Mechanical Engineering (Vol. 29, No. 3, 2005, pp. 25–28), presents a comparative finite element analysis of welding residual stress in titanium alloy thin plates subjected to laser welding and TIG (GTAW) welding. The work was supported by the National Key Laboratory of High Energy Beam Processing Technology, reflecting the strategic importance of titanium alloy joining in aerospace applications at that time. The study employs ANSYS for coupled thermal-mechanical analysis, incorporating temperature-dependent material properties, and validates the numerical predictions against experimental data obtained via the hole-drilling strain gauge method.
Core Technical Methodology
The numerical simulation framework follows a well-established coupled thermal-mechanical approach. The temperature field is solved first using transient heat conduction equations, considering the moving heat source characteristic of both welding processes. The laser heat source is typically modeled as a Gaussian or double-ellipsoidal distribution, while the TIG arc heat source is commonly represented by a conical or double-conical model. The key innovation in this work lies in the incorporation of temperature-dependent material parameters throughout the simulation, which is critical for titanium alloys whose thermal conductivity, specific heat, Young's modulus, and yield strength all exhibit significant variation with temperature.
| Parameter | Laser Welding | TIG Welding |
|---|---|---|
| Heat source type | Gaussian / double-ellipsoidal | Conical / double-conical |
| Power density | 10⁶–10⁷ W/cm² | 10⁴–10⁵ W/cm² |
| Typical welding speed | 100–500 mm/min | 20–100 mm/min |
| HAZ width | Narrow (0.5–2 mm) | Wider (2–5 mm) |
| Heat input | Low | Moderate |
| Residual stress peak | Higher localized stress | More distributed stress |
The mechanical analysis is performed incrementally, with each time step of the thermal solution driving the mechanical deformation and stress state. Plastic strain accumulation during heating and subsequent elastic-plastic behavior during cooling are captured through the elastic-plastic constitutive model. The temperature-dependent yield surface evolution is essential for accurately predicting the residual stress magnitude and distribution pattern.
Residual Stress Mechanism Analysis
The paper provides valuable insight into the fundamental mechanisms governing residual stress development in titanium alloys. During welding, the central weld zone experiences rapid heating above the melting point, followed by rapid cooling. The constrained thermal contraction during cooling generates tensile residual stresses in the weld and HAZ regions, balanced by compressive stresses in the far-field material. Titanium alloys, particularly Ti-6Al-4V, exhibit several characteristics that influence residual stress: relatively low thermal conductivity compared to aluminum alloys, high specific heat, and a phase transformation from beta to alpha-plus-beta during cooling, which introduces additional transformation strains.
The comparative analysis reveals that laser welding, despite its lower total heat input, produces higher peak residual stresses due to the extremely high power density and rapid cooling rates. The narrow weld zone constrains plastic deformation to a small region, resulting in steep stress gradients. In contrast, TIG welding distributes the heat input over a larger area, producing more gradual stress transitions but potentially lower peak values. The temperature-dependent material properties play a crucial role: as the yield strength of titanium alloy drops significantly at elevated temperatures, plastic flow is facilitated during heating, which partially relaxes the stresses that would otherwise develop. However, upon cooling, the material regains its strength while still under thermal contraction, leading to the final residual stress state.
Validation and Accuracy Assessment
The hole-drilling method used for experimental validation is a well-recognized standard technique (ASTM E837 / EN ISO 8517) for measuring residual stresses. The method involves incrementally drilling a small hole in the surface of the component and measuring the resulting elastic relief strain using strain gauges. The agreement between the numerical predictions and experimental measurements, as reported by the authors, confirms the adequacy of the coupled thermal-mechanical model and the temperature-dependent material property database. However, it is worth noting that the hole-drilling method measures near-surface stresses, while the numerical model provides through-thickness information. For thin plates, this is less of a concern, but for thicker sections, the comparison would need more careful consideration.
Engineering Implications and Reflections
From a practical engineering standpoint, the findings have several important implications for titanium alloy fabrication. The higher localized residual stresses in laser welds suggest a greater susceptibility to stress corrosion cracking (SCC) in aggressive environments, particularly in aerospace applications where titanium components may be exposed to chlorides or other corrosive media. The residual stress distribution also affects fatigue performance, as tensile residual stresses at the weld surface can accelerate crack initiation under cyclic loading. Post-weld treatments such as stress relief annealing, shot peening, or laser shock peening should be considered for critical applications.
The study also highlights the trade-offs between process selection and residual stress control. While laser welding offers superior geometric accuracy and minimal HAZ, its residual stress profile may require additional mitigation measures. TIG welding, with its more moderate thermal cycle, may be preferable for components where residual stress is a critical design consideration. For thin plate applications specifically, the choice of welding process should be guided not only by productivity and cost but also by the residual stress implications for the intended service environment.
This paper, while published in 2005, remains relevant as a foundational reference for understanding the residual stress behavior of titanium alloys under different welding processes. The methodology described is still widely applied in modern numerical studies, and the comparative framework established here continues to inform process selection decisions in aerospace and biomedical titanium alloy manufacturing.