Numerical Simulation of IN738 Superalloy Thin Plate Laser and TIG Welding

Literature Overview

This paper by Nan Qing, Xiao Junfeng, Tang Wenshu, Gao Song, Gao Sifeng, Li Yongjun, and Zhang Jiong, published in Special Casting and Nonferrous Alloys (2019, Vol. 39, No. 9, pp. 960-963), presents a finite element numerical simulation of laser welding and TIG welding of IN738 nickel-based superalloy thin plates. Conducted at Xi'an Thermal Power Research Institute Co., Ltd. and supported by the Huaneng Group Science and Technology Program (HNKJ18-H11), the study compares residual stress and plastic strain distributions between the two welding processes. The research is highly relevant to the fabrication of superalloy components for power generation and aerospace applications.

Core Technical Content

Using MSC.Marc finite element analysis software, the authors performed numerical simulations of both laser welding and TIG welding processes on IN738 superalloy thin plates. The simulations were based on previously calculated temperature field results and included thermo-mechanical analysis to predict residual stress and plastic strain distributions. Experimental welding tests were conducted to validate the numerical predictions, with temperature fields and residual stress fields measured and compared with simulation results.

Comparison of Laser and TIG Welding Results

Interpretation of Technical Points

The simulation results reveal fundamental differences in the thermal and mechanical behavior of laser welding versus TIG welding for IN738 superalloy. Laser welding, with its highly concentrated energy input and rapid cooling rates, produces higher peak residual stresses but confined to a narrower zone. TIG welding, with its more diffuse heat input and slower cooling rates, produces lower peak stresses but distributed over a wider area.

The higher peak residual stresses in laser welding are attributed to:

• Rapid heating and cooling creating steep temperature gradients
• High thermal strains from large temperature differentials between weld and base metal
• Constrained plastic deformation in a narrow zone leading to stress concentration

The wider tensile stress zone in TIG welding results from:

• More extensive heating of the base metal surrounding the weld
• Greater thermal distortion and plastic deformation in the HAZ
• Slower cooling allowing more time for stress relaxation but over a larger area

The lower plastic strain values in laser welding indicate that while the stresses are higher, the total plastic deformation is less. This is consistent with the confined nature of laser welding heat input, where plastic deformation occurs primarily in the immediate weld region rather than extending far into the base metal.

Validation and Discrepancies

The study reports good agreement between calculated and measured temperature fields and residual stress fields, with the notable exception of transverse residual stress at the weld, where significant differences exist. This discrepancy in transverse residual stress may be attributed to:

• Simplified boundary conditions in the numerical model
• Inaccurate representation of arc or laser heat source parameters
• Neglected effects such as phase transformations or anisotropic material behavior
• Measurement uncertainties in experimental residual stress determination

Process and Standards Analysis

IN738 is a precipitation-hardened nickel-based superalloy widely used in high-temperature applications such as gas turbine components, aerospace engines, and power generation equipment. The welding of IN738 presents significant challenges due to:

• High susceptibility to solidification cracking
• Sensitivity to hot cracking from low-melting-point eutectics
• Limited weldability requiring careful heat input control
• Requirement for post-weld heat treatment to restore properties

For superalloy component fabrication, welding procedures must comply with relevant codes and standards, including:

• AWS C3.7 for nickel and nickel alloy welding
• AMS 2774 for welding procedures for nickel-base alloys
• EN ISO 13919 for welding of nickel and nickel alloys
• Manufacturer specifications for specific applications

The comparison of laser and TIG welding residual stress distributions provides valuable information for process selection. For thin plate applications where distortion control is critical, laser welding may be preferred despite higher peak stresses, as the confined stress zone reduces overall distortion. For applications requiring lower peak stresses, TIG welding may be more suitable, accepting the trade-off of wider stress distribution.

Engineering Practice Considerations

For IN738 superalloy thin plate fabrication, the following considerations apply:

• Preheating requirements to prevent cracking (typically 200-400°C depending on thickness)
• Interpass temperature control to prevent excessive grain growth
• Post-weld heat treatment for solution treatment and aging
• Distortion control through fixture design and welding sequence optimization
• Residual stress management through stress relief procedures

The numerical simulation approach presented in this paper can support process development by predicting residual stress distributions for different welding parameters and sequences, enabling optimization before physical testing.

Key Questions and Reflections

Several aspects of the study warrant further consideration. First, the discrepancy in transverse residual stress predictions suggests that the numerical model may require refinement in boundary conditions or material property definitions. Second, the study focuses on residual stresses but does not address the microstructural evolution and mechanical properties of the welds, which are critical for IN738 superalloy applications. Third, the effect of welding sequence and multi-pass welding on residual stress accumulation is not explored, which is important for practical fabrication.

From my experience with superalloy welding, the residual stress distributions predicted in this study are consistent with general observations. Laser welding typically produces higher peak stresses but less overall distortion, while TIG welding produces more uniform stress distributions. The choice between processes depends on the specific application requirements, including allowable distortion, required mechanical properties, and post-weld treatment options.

Study Insights and Implications

This paper provides a useful comparison of laser and TIG welding residual stress distributions for IN738 superalloy thin plates, offering guidance for process selection in superalloy component fabrication. The numerical simulation approach, validated through experimental testing, demonstrates the capability of finite element analysis to predict welding-induced residual stresses. For engineers working with superalloy components, the findings suggest that laser welding may be preferred for thin plate applications where distortion control is paramount, while TIG welding may be more suitable where lower peak stresses are required. The identified discrepancy in transverse residual stress predictions highlights the need for continued refinement of numerical models to achieve complete predictive accuracy for complex welding scenarios.