Characterization and optimization of direct drive friction welding parameters in small stainless steel tube welds

Abstract

Rotational friction welding is a common joining process used to join cylindrical metal components. Typically, one piece is rotated and a secondary piece is held rigid. The two samples are then forced together in a controlled manner, and the resulting friction generates enough heat to weld the two pieces. This process was characterized and optimized for 304 Stainless Steel tubes with a .317 cm (.125 in) outer diameter and .14 cm (.055 in) inner diameter. The goal was to characterize and optimize parameters around a weld with no leak, strong ultimate tensile strength, and proper through-hole integrity. Also, solid bars were welded to some tubes to analyze a capped system. Key parameters to the process that were monitored and/or controlled include rotational speed, applied force, temperature, duration, and material upset. Often times the applied force is divided into two steps. A lower force is applied during heating (friction force), and a larger force is applied once rotation stops (forging force). The material upset, maximum temperature, and forging fore were the primary controlling variables in this study. Other parameters were held constant. A testing setup was built to analyze these factors. Modifications were made to a three axis mill to perform friction welding in a controlled environment. Then, tests were run to understand the effects each parameter had on weld quality. Welds with an upset greater than .1 cm held a pressure at a much higher success rate than welds with lower upsets. In general, the forging force was shown to have a large positive impact on ultimate tensile force. The integrity of the through-hole was compromised in many of the tube to tube tests. Several welds were post-drilled to recreate the through-hole. Tests with this done held a pressure 66.67% of the time. It was found that successful welding can be accomplished with this process, and different adjustments to testing procedures can maximize different qualities in the weld.