Heated Friction Stir Welding: An Investigation into how Preheating Aluminum 6061 Affects Process Forces

Abstract

Friction Stir Welding (FSW) is a recently-developed solid-state joining process in which a rotating cylindrical tool is passed through the work piece, heating the material to a plasticized state through friction and stirring the material together to create the joint. As this technique has expanded to welding higher strength materials, large process forces and extreme tool wear have become issues. One possible solution mentioned in literature is introducing an additional heating source in front of the FSW tool such that the pre-heating softens the material and reduces the tool loads. However, very little work has been published to accurately catalogue the precise benefits afforded by increasing the initial work piece temperature for FSW. The following research investigates in a controlled and detailed manner the advantages of elevating temperature. A raised anvil was constructed to cover an electric heating element while supporting the welding forces. Bead on plate welds were performed with a Trivex tool in aluminum alloy (AA 6061) heated to initial material temperatures up to 300 oC. Macrograph cross sections of the welds revealed a slight increase in material flow with increasing temperatures. More significant, the welding forces were recorded and analyzed to reveal up to a 43% reduction in the axial force with even moderate heating. An intriguing trend is observed that the process forces do not decrease steadily with increasing initial temperature, as might be expected, but rather exhibit a more complex polynomial shape which, for some heating intervals, actually increases.