Improved process stability during friction stir welding of 5 cm thick copper canisters through shoulder geometry and parameter studies

Abstract

The spent nuclear fuel from Swedish power plants will be placed in copper canisters that are sealed with friction stir welding and the stability and robustness of this process is now being optimised in three steps: first, the shoulder geometry was identified that produced the most stable weld cycle, then the welding parameters were optimised for that geometry with regards to stability, and finally, the chosen geometry and welding parameters were verified and evaluated during multiple weld cycles. The shoulder study showed that stable welds could be produced repeatedly with a convex scroll geometry which proved more stable than various concave and flat scroll geometries. In the subsequent parameter study, not only were the most stable values for the welding parameters derived, but a clear relationship was shown between power input and tool temperature. This relationship can be used to more accurately control the process within the parameter windows, not only for this application but for other applications where the welding temperature needs to be kept within a specified range. Similarly, the potential of the convex scroll shoulder geometry for use in applications with other metals and thicknesses is evident.

Influence of tool shoulder geometry on properties of friction stir welds in thin copper sheets

“…As mentioned by McNelley et al (2007) and Cederqvist et al (2009), they are used for components in a wide range of marine systems and, for example, for the encapsulation of nuclear waste material. However, copper alloys are difficult to weld by fusion because adherent oxides inhibit welding, or volatile and toxic elements, such as zinc, may be present in the alloys, requiring adequate ventilation.…”

Effect of tool rotation rate on microstructure and mechanical properties of friction stir welded copper

“…1,2 The rapid development of the FSW process in aluminium alloys and its successful implementation into commercial applications have motivated its application to other non-ferrous materials, such as copper, magnesium and titanium. [3][4][5][6][7][8] Recently, some industrial applications of FSW on copper have been released, such as the fabrication of the copper containment canister for nuclear waste 3,9 and the backing plate for sputtering equipment. 5 Copper, which has much higher thermal diffusivity than steel, is very difficult to be welded by conventional fusion welding techniques.…”

Simulation based evaluation of a nonlinear model predictive controller for friction stir welding of nuclear waste canisters

“…The tool used in SKB's application consists of a probe that is approximately 50 mm long, and a convex scroll shoulder with a diameter of 70 mm. The choice of a convex shoulder instead of the commonly used concave shoulder was investigated by Cederqvist et al [4]. This choice gives smaller variations in plunge depth and spindle torque, which is desirable from a control point of view.…”