An Experimental and Numerical Study of Friction Stir Processing using Novel Tool Designs and its Application to Damage Repair.

Abstract

Friction Stir Welding (FSW) is a solid-state joining technique developed by The Welding Institute, UK. The tool employed in FSW consists of a spinning pin with a stepped shoulder. The pin is inserted into the seam line between two sheets or plates to be welded. As the pin rotates and traverses along the joint line, it heats the surrounding material, plasticizes it severely, and stirs it, thus creating a weld. Friction Stir Processing (FSP), which is a derivative of FSW, uses the stirring action of the pin to produce fine-grained material. Traditional estimates of the strain-rate during FSP have relied on the relationship between the measured grain-size and Zener-Hollomon parameter of the process. In this thesis a soft and ductile aluminum foil was embedded within a work-piece undergoing FSP to observe the deformation zone around the pin. The logarithmic thickness strain was experimentally measured by observing the change in thickness of the foil. Additionally the strain-rate was numerically evaluated from the FSP process parameters and the deformed foil geometry using finite-element analysis. In the same vein research was carried out to enhance shear-rate in FSP by devising a novel joining technique using multiple pins. Using a patented concept (U.S. Patent No. 7597237) a multi-pin Friction Driven Stitch Welding (FDSW) apparatus was designed and tested. In addition the joining efficacy of the FDSW apparatus was numerically estimated via finite element analysis. FSW has traditionally been employed in joining low melting-temperature metals e.g. aluminum and magnesium. In this thesis FSP was used in conjunction with laser-assisted Direct Metal Deposition (DMD) to repair and restore marine components which suffer degradation due to corrosion. Towards that end FSP of copper-nickel 70-30 and 4340 steel was explored. Repair was also attempted on a marine component consisting of a thin cylindrical shell. This particular part which is made out of 7475 aluminum doesn’t lend itself to deposition by DMD. Direct plug-based repair of this part by FSP was attempted. The challenges encountered and the means of circumventing them are described in this thesis.