Reducing Fracture Tendencies in PCBN FSW Tools

摘要

The overall goal of this program is to increase the life of PCBN tools in friction-stir welding of high temperature metals through understanding of mechanisms of damage development in the tool materials, identifying microstructural characteristics that mitigate this damage, and developing methods for measuring relevant material properties. Some key results from the first year of study are; (i) demonstration that cracking by indentation with a Vickers indenter can be used to obtain valid fracture toughness measurements in this class of materials ; (ii) some of the best performing materials were found to contain well dispersed diamond grains, a result that was unexpected by the manufacturers, (iii) An SEM imaging method was discovered that reveals internal deformation structures in BN grains, which are not normally detectable by SEM (useful for assessing damage caused by grain-to-grain contact during high pressure consolidation), (iv) observations from tools fractured during FSW indicate that crack growth occurs incrementally, likely over many thermal and/or mechanical cycles, rather than by unstable growth at a critical condition.