In situ monitoring of build height during powder-based laser metal deposition

Abstract

A powder-based laser metal deposition (LMD) system can fabricate customised three-dimensional (3D) parts, layer by layer, based upon a computer-aided design (CAD) model. However, the deposition will not always feature the expected geometry due to excessive heat input and inconsistent powder flow. Due to the layer-by-layer nature of LMD, geometrical error in one layer is compounded in all following layers and may result in a build failure. Thus, it is critical to monitor online the track and layer height. This study developed an in situ monitoring system integrating a webcam and a narrow bandpass filter. The laser/powder defocus distance was extracted from the melt pool images, and the track/layer height was calculated from the laser/powder defocusing distance and preprogrammed layer spacing. The presented approach does not need additional illumination sources and is a nonintrusive online method. Therefore, it is a potential precursor to a feedback build height control system. It also can be used for measuring omnidirectional height, i.e. height in different build directions relative to the substrate, which has been tested by fabricating two thin-wall structures with customised shapes. These online-measured height data were successfully validated against dimensional measurements from an offline 3D scanner, thus demonstrating the online system’s potential utility in a feedback control system for ensuring acceptable part geometrical accuracy.