Corrective Measures to Restore Corrosion Resistance Following Friction Stir Welding

摘要

Friction stir welding (FSW) is a solid state joining process invented at TWI in 1991. This technology makes it possible to join aluminum alloys, which are difficult or impossible to weld by conventional techniques.(1- 7) A schematic illustration of the FSW process is shown in Figure 1. To friction stir weld either a butt or lap joint, a specially designed cylindrical tool is rotated and plunged into the joint line. The tool has a small diameter entry probe with a concentric larger diameter shoulder. When descended to the part, the rotating entry probe contacts the surface and rapidly friction heats and softens a small column of metal. As the probe penetrates beneath the surface, part of this metal column is extruded above the surface. The tool shoulder and length of probe control the depth of penetration. The present program (N00014-02-C-0212) has investigated five different approaches to corrective measures that either produce less sensitization or restore the corrosion resistance: (1) Active cooling during FSW, where cold water is circulated through the anvil and a mist of water is sprayed on the tool. This is a process change, which lowers the maximum temperature and decreases the time at elevated temperature. (2) Laser surface melting where surface melting and rapid quenching alters the microstructure, (3) low plasticity burnishing, where a tool puts compressive stresses in a surface layer, (4) pre AND post weld heat treatments which homogenizes the grain boundary chemistry, and (5) change in tool design which alters the temperature/time profile and microstructure during FSW. Only tool design and heat treatment had an effect on the corrosion properties. (11-13).