DEVELOPMENT OF CERAMIC TOOL FOR FRICTION STIR WELDING

Abstract

The tool material for friction stir welding/ processing determine the quality of the weld and also the ease and feasibility of the welding/ processing. This welding process was initially developed for welding of soft materials like aluminium, magnesium and their alloys. Friction stir welding of high melting point materials like steel and titanium alloys is difficult. For welding of such materials, tools made of material having high hardness at elevated temperature and good thermal conductivity is required. For such requirements, ceramic material is a good option. Boron carbide (B4C) is the third hardest substance existing in nature after diamond and pcBN. It has low density, high melting point, chemical inertness and high hardness at elevated temperature. These properties of B4C can be useful for developing a friction stir welding tool. Primary aim of present work is to study the feasibility of ceramic as a tool material for FSP/W. B4C ceramic was sintered with Al2O3 as sintering aid and B4C-TiB2-SiC (BTS) was also sintered by spark plasma sintering. After this tool pin was sintered using B4C-Ti-SS combination but bonding at the interfaces was not good enough. The characterization of the disc shaped samples was done by X-Ray diffraction and SEM to analyze the interfacial bonding between B4C-Ti and Ti-SS interfaces of the pin. Performance of the fabricated tool was studied by performing FSP on magnesium alloy AZ91 at 1025 rpm and 10mm/min traverse speed and then on mild steel plate was performed with 1216 rpm and 26mm/min traverse speed. Form the SEM images and EDS analysis of the cross-section of the sintered pin it was found that bonding at the Ti-SS interface was strong but that at B4C-Ti interface was not good. With the BTS-Ti-SS pin, FSP on magnesium alloy AZ91 plate was successfully performed. But failure of the tool pin was there while performing FSP on mild steel plate. Failure of the tool pin can be attributed to the poor bonding at BTS-Ti interface which was not able to withstand the stresses generated during friction stir processing.