Finite Element Modelling of Temperature Distribution in Friction Stir Welding Process and Its Influence on Distortion of
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Finite Element Modelling of Temperature Distribution in Friction Stir Welding Process and Its Influence on Distortion of Copper Canisters Therese Källgren*, Lai-Zhe Jin, and Rolf Sandström, Department of Materials Science and Engineering, Royal Institute of Technology, Brinellvägen 23, S-100 44 Stockholm, Sweden ABSTRACT In an effort to enhance safety for long time disposal of waste nuclear fuel, friction stir welding has been developed as one alternative to seal copper canisters. To avoid the formation of voids and cracks during the welding process, an understanding of the heat and material flow and thereby the evolution of the microstructure, is of great importance. Finite element modelling has been used to simulate the heat and material flow as well as thermal expansion during the friction stir welding process. A model involving heat transfer, material flow, and continuum mechanics has been developed. The steady state solutions have been compared with experimental temperature observations as well as analytical solutions, showing good agreement. Temperature distribution is affected by the welding speed. For a given reference point perpendicular to the welding direction, a lower welding speed corresponds to a higher peak temperature. The plunging position of welding tool influences the temperature distribution and therefore the displacement distribution of the weldment. Keywords: friction stir welding, FSW, copper, finite element modelling INTRODUCTION In Sweden it is planned to use copper canisters for depositing nuclear fuel waste at a depth of 500 metres in the bedrock. The canister will be produced by welding a lid and possibly a bottom cover to a cylinder. As a corrosion barrier, the copper canister including the sealing must be of very high quality and remain intact for 100,000-year. The Swedish Nuclear Fuel and Waste Management Corporation (SKB) have studied different welding methods in cooperation with The Welding Institute (TWI) [1]. The Friction stir welding (FSW) process is one alternative to seal the canister. The first years of the collaboration between TWI and SKB were focused on the experimental work on copper plates with thickness of 30-50 mm [2], see Figure 1 and 2. During that period, some thermal data were collected [3, 4]. When the development had come so far that tools and parameters were sufficiently correct, short rings with a thickness of 50 mm and a diameter of 1050 mm were used for simulating the canister and the lid. Later a stationary welding machine was developed at the canister laboratory in Oskarshamn. Nowadays full-scale experiments can be performed [5]. The FSW joints are a solid-state weld with a complex microstructure. They can be characterised as possessing different zones with changes in microstructure and crystallographic order. Recrystallisation takes place in the nugget due to deformation and high temperature, see Figure 1. Some recrystallisation also occurs in the surrounding area, the thermo mechanically * Author for correspondence (E-mail: [email protected]).
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