Cubic Boron Nitride Claddings for Friction Stir Tooling

Friction stir welding (FSW) is an attractive joining method where high strength low porosity welds can be attained. However, the high temperatures and forces required for welding high strength materials like steel require the use of exotic tools. Cubic boron nitride (cBN)-based tools offer attractive tool wear characteristics in steels, but are very costly. Conversely, refractory metal alloys are moderately less expensive, but offer poorer wear properties. A potential economical solution is to apply cBN-based onto affordable metallics tools. Claddings containing cBN+binders deposited by plasma spray onto the wear surface of tools have the opportunity to reduce cBN volume thereby reducing tool cost while maintaining the wear rate of monolithic cBN-based tools. The proposed Phase I STTR will investigate plasma spray deposition of cBN-based materials. Liquid forming phases will be incorporated with cBN to avoid cBN sublimation. Thermodynamic computations by STTR partner, University of Alabama, will guide selection of these liquid forming phases. Promising deposits will be exposed to typical FSW temperatures. The University of Alabama will characterize claddings pre- and post-thermal exposures to identify phase stability. Follow-on efforts will encompass deposition trials onto a FSW tool and welding trials at the University of Alabama.