Thermo-Mechanical Analysis on Thermal Deformation of Thin Stainless Steel in Laser Micro-Welding

Thermo-Mechanical Analysis on Thermal Deformation of Thin Stainless Steel in Laser Micro-Welding

Mohd Idris Shah Ismail (Universiti Putra Malaysia, Serdang, Malaysia), Yasuhiro Okamoto (Okayama University, Okayama City, Japan) and Akira Okada (Okayama University, Okayama City, Japan)
DOI: 10.4018/IJMMME.2016040104
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In the present study, a three-dimensional finite element model has been developed to simulate the temperature, stress and deformation fields in continuous wave (CW) laser micro-welding of thin stainless steel sheet. The welding deformation was experimentally evaluated using a single-mode fiber laser with a high-speed scanning system. Application of developed thermal model demonstrated that the laser parameters, such as laser power, scanning velocity and spot diameter have a significant effect on temperature field and the weld pool. In the case of welding deformation, numerical simulation was carried out by an uncoupled thermo-mechanical model. The welding stress and deformation are generated by plastic deformation during the heating and cooling periods. It was confirmed that the residual stress is higher than yield strength and has strongest effect upon the welding deformation. The numerical simulated results have proved that the developed finite element model is effective to predict thermal histories, thermally induced stresses and welding deformations in the thin material.
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1. Introduction

Laser welding has become a promising micro-joining process because there are many outstanding advantages compared with the conventional welding processes. It has significant advantage in welding of heat sensitive components with precision control of heat input, minimal thermal distortion, small heat-affected zone and excellent repeatability (Schlueter, 2007; Tsukamoto, 2003). One of the greatest challenges in the laser micro-welding process is to create a reliable and accurate joint. During the process, the rapid cooling and the associated material shrinkage of the weld zone often causes a thermal distortion. For a precision welding, the thermal deformation is very important factor, and it is essential to understand the distortion caused by laser micro-welding process. Correcting unacceptable weld distortion of thin sheet is extremely costly and impossible in some cases (Masubuchi, 1996).

Welding deformation has negative effects on the accuracy of assembly, external appearance in final product quality and mechanical properties of the weld joint (Radaj, 1992; Deng & Murakawa, 2008). It cannot be ignored especially for precision welding in thin sheet, where the accuracy is required. As the welding stresses have a strong influence on weld deformation, the evaluations of welding stresses have become imperative. The knowledge of stress distributions and deformations may lead to better control over undesirable aspects of the machining process, which includes the dimensional inaccuracies and distorted shapes. However, the measurement of thermal stresses associated to a weld joint is complicated and practically limited by either cost or accuracy (Radaj, 1992; Carmignani, Mares, & Toselli, 1999; Armentani, Esposito, & Sepe, 2007). With modern computing technologies, the numerical simulation has proven to be an effective tool and offers a comprehensive solution in predicting the welding induced stresses and deformations. Therefore, numerical simulations based on finite element method (FEM) can be used to solve this problem.

Numerical simulation on the welding process has been a major topic in welding research. The results of simulations can be used to explain complex phenomenon in the welding process and process optimization. In the past decades, many numerical analyses had been conducted for predicting welding deformation. Deng and Murakawa (2008) and Liang et al. (2006) have employed a three-dimensional FEM using iterative substructure method (ISM) in a long welded joint. Sulaiman et al. (2011) investigated the capability of linear thermal elastic numerical analysis to predict the welding distortion of butt and T-joints. Schenk et al. (2009) studied the influence of clamping on welding deformation using FEM. They found that the residual stresses and deformations depend strongly on the clamping conditions. However, these advances are mostly associated with conventional arc welding. Similar numerical models for laser forming also have been developed (Namba & Katayama, 1999; Terasaki, 2003). Namba and Katayama (1999) carried out to explore the thermal stresses generated during laser forming process using three-dimensional thermal elastic-plastic analysis. They reported that the thermal expansion and contraction occurred during the laser forming.

Prediction of welding stresses and deformations induced by laser welding is extremely difficult. There is very limited numerical data of the welding deformation in laser welding. Many numerical models were developed for laser welding in thick materials (Spina, Tricarico, Basile, & Sibillano, 2007; Moraitis & Labeas, 2008, 2009; Yilbas, Arif, & Abdul Aleem, 2010), and little attention has been made to welding stresses in laser micro-welding. In general, the three-dimensional thermal elastic-plastic analysis could be uncoupled into a thermal transient analysis and an elastic-plastic analysis (Zain-ul-Abdein, Nelias, Jullien, & Deloison, 2010). Spina et al. (2007) attempted to predict deformations in 3 mm aluminum alloy plate and suggested that an accurate thermal analysis is required to predict welding deformations in mechanical analysis. Moraitis and Labeas (2008, 2009) focussed on the thermo-mechanical numerical model, which is based on the keyhole theory to simulate the stress, strain and deformation fields.

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