Range-resolved interferometric layer-height measurement for laser-additive manufacturing

Laser additive manufacturing uses a high power laser to melt a metal feedstock into a desired shape, which is then allowed to solidify. Layers of deposition are built up a few millimetres at a time to manufacture the desired part. Monitoring of the deposited layers allows for adjustment to the deposition parameters in future layers which is critical for completing the build of an acceptable, quality-controlled part of near net-shape. The height of the deposited layer is one of the most important physical parameters. Developed at Cranfield University, Range resolved interferometry (RRI) is an approach that applies wavelength modulation of a low-cost diode light source, and interferometry between the metal surface of the newly built part and the optical fibre tip to measure layer height, whilst also rejecting the incoherent light scattered from the high power laser. Using RRI the distance between the metal surface and the fibre tip can be determined across a range of tens-of-centimetres. Compared to Optical Coherence Tomography systems this operating range is much greater and the instrument cost is substantially lower. Here initial results are presented using an RRI system integrated into a commercial laser processing head applied to laser wire directed energy deposition laser additive-manufacture wall-building.