MICROSTRUCTUE AND MECHANICAL PROPERTY CONTROL DURING ADDITIVE MANUFACTURING
Abstract
The microstructure of Inconel superalloy thin walls has been characterised. The thin walls were built using direct laser deposition (DLD) with variable parameters: a wide range of continuous laser powers with different pulsing types, frequencies, powder feed rates, scanning speeds and strategies. The walls were analysed to assess the role of the DLD parameters on the microstructure, elemental segregation, mechanical properties, discontinuities, cracks and build geometry, in order to identify the sensitivity of the grain and precipitate morphology to the various process parameters. The builds were examined using optical microscopy, scanning electron microscopy, energy dispersive X-ray analysis and electron backscattered diffraction. An analytical thermal camera and thermocouples were used to study the influence of deposit temperature on grain size and hardness. The power mode was used to control the heat input and cooling rates, which resulted in significant modifications in the microstructure and properties of the deposits. In this work, DLD was performed on IN718 and CM247LC Ni-superalloy using continuous wave laser power modes, and different pulsed wave modes to understand the impact on the product using advanced material characterisation techniques. The study showed that increasing the laser power gave rise to a columnar grain structure, and the grains became coarser as the laser power increased. Furthermore, the deposition path appeared to affect the orientation of the dendrites; this change can be attributed to the variation in the heat flux within the melt pool. Power pulsing increased the cooling rate and reduced the average heat input, and most importantly this broke the dendrites and disturbed the segregation process, thereby reducing the formation of Laves phase. The microstructure could be tailored to a specific size using pulsing, which could also reduce Nb segregation and so reduce the time needed for heat treatment after deposition. Furthermore, the pulse duration played a significant role in reducing segregation and eliminating cracking. A significant variation was observed in the grain size distribution and morphology while the porosity volume fraction was limited, and this varied marginally with the process parameters.