Selective laser melting of Ti-6Al-4V: Comparing μCT with in-situ process monitoring data

Abstract

Additive Manufacturing (AM) is increasingly used for the fabrication of metallic components used in the medical device, aerospace and automotive industries. With the wider adoption of AM in these sectors, there is an increased demand for the in-situ process monitoring of the build process. This study investigates the performance of a photodiode based, co-axial in-situ process monitoring (PM) system, during the Selective laser melting (SLM) of Ti6Al4V alloy parts. The PM system measures the optical and thermal emissions created by the meltpool, as well as the intensity and stability of the laser during the SLM process. The process monitoring software then creates a 2D or 3D representation of the part, based on the signal intensity recorded.The Ti6Al4V alloy parts were manufactured containing internal cavities, with diameters/width’s in the range of 200–600μm, while varying the input energy between 32.9 and 131.6J/mm3. A close correlation was established between the laser monitoring photodiode signal intensity and the laser energy input. Along with this, an increase in signal intensity recorded, by the meltpool monitoring photodiode, was observed when the first capping layer above a cavity, was processed by the laser. Further to this, it was shown that only the first layer was influence by the overhang, with the signal generated by the layers directly above this remaining unaffected. In addition to providing data on the laser energy during the build process, the PM system also provided valuable information regarding the intensity of the meltpool. A comparison was made between the dimensional measurements obtained using PM software, with those obtained through CT scanning of the parts, post build. It was found that for the 600μm cavities that the measurements were, at best, within 1.7% of each other. This closeness of such measurements however decreased very significantly as the size of the cavities decreased, with a variation for example, of up to 32% been obtained, for 400μm cavities.