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

Abstract

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.