Lars Cederqvist

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.

Software for PID design: benefits and pitfalls

The most common PID design methods in industry are based on formulas. This article will present some major advantages of instead using the power of computer based softwares for PID controller design. The Matlab based software used in this work was developed in 2007 and derives robust, IAE minimizing, PID controllers. The experiences of using this software are collected in this article and include control signal activity limitation due to measurement noise, controller design on an industrial Friction Stir Welding process and fast controller design for large batches of processes. It is shown that the properties of the software make it suitable for design of PID controllers and in PID research. There are, however, some possible design pitfalls that the user needs to be aware of. Some of these are presented as well.

Cascaded control of power input and welding temperature during sealing of spent nuclear fuel canisters

The friction stir welding procedure to seal copper canisters requires variable power input throughout the 45 minute long weld cycle to keep the welding temperature within the process window. This is due to variable thermal boundary conditions throughout the weld cycle which, together with fast disturbances in the spindle torque, requires control of both the power input and the welding temperature to achieve a stable and robust process. By using a cascaded loop that determines the power input requirement, the regulator will not be dependent on repeatability in the necessary power input between weld cycles. As a result, a more accurate and reliable closed-loop control of the welding temperature is acquired.

Depth and Temperature Control During Friction Stir Welding of 5 cm Thick Copper Canisters

Depth control is needed to repeatedly produce welds with minimum flash formation and hook defect, which disturb the temperature control and reduce the corrosion barrier, respectively. The need for depth control is mainly caused by different manufacturing techniques and heat treatments of the lids and tubes that lead to varying properties. The depth is measured using four different sensors; a laser sensor, two linear variable differential transformers (LVDT), and an axial position sensor. The actual depth is estimated from measurements of the shoulder footprint, and can then be compared with the depth sensors. The depth controller uses the axial force to manipulate the shoulder depth during the first two weld sequences. Thirteen welds were carried out in three different lids/rings (twelve short and one full circumferential) with an active depth controller. The laser sensor was used as feedback signal to the controller, and the desired shoulder depth was set to 2.2 mm. For comparison, eighteen short welds (also in three different lids/rings) were performed without any depth control. For the thirteen welds with active depth control, the shoulder depth measured by the laser sensor varied between 2.19 and 2.40 mm (0.21 mm span) at a point two degrees into the joint line, i.e. with a maximum control error of 0.20 mm. The shoulder footprint depth ranged between 2.52–2.86 mm (0.34 mm span). For the eighteen uncontrolled welds, the laser varied between 1.74 and 2.49 mm (0.75 mm span). The corresponding span for the footprint depth was 0.60 mm. Furthermore, macro samples from the thirteen depth controlled welds showed no signs of hook defect nor of joint line remains. The flash formations from the same welds were also small (0–1 mm). It was concluded that the LVDT sensor placed in the lid is best suited for feedback to the depth controller, partly because it is best at modelling the shoulder footprint depth out of the four depth sensors, but also since it does not suffer from extensive measurement noise like the laser sensor.