Measuring the onset of sintering using laser ultrasonics

Abstract A noncontact laser ultrasonics method for determining the onset temperature and the early stage sintering state is studied. Because this technique measures properties near the surface in selected regions on the sample, it is particularly well‐suited to parts produced by additive manufacturing routes such as selective laser flash sintering, where local variations in sintering state along one dimension result from the laser scan pattern. We demonstrate the ability to measure very small changes in interparticle neck size by measuring Rayleigh wave speeds. The changes in wave speed result initially from rapid changes in Young's modulus that occur at the onset and in the early stages of sintering, before significant densification is observed. This measurement method is demonstrated using alumina pellets that were partially sintered at different temperatures to produce parts with a range of neck sizes and relative densities. Using the laser ultrasonics technique, the onset of sintering is detected at temperatures between 520°C and 650°C, which is significantly below the onset sintering temperatures detectable using traditional methods.