Multiple beam laser diode additive manufacturing for metal parts
Creators: name: Payne, Andrew Timothy
Publisher: name: Apollo - University of Cambridge Repository
Year: 2018
Subjects: Additive manufacturing
Laser diode
3D pinting
Metal

description: MULTIPLE BEAM LASER DIODE ADDITIVE MANUFACTURING FOR METAL PARTS Andrew Payne Abstract Fibre laser based powder bed fusion has become the dominant commercial additive manufacturing technology for producing fully dense metal parts. This technology is difficult to scale up for build volume and build rate. Continuing advances in semiconductor technology have produced single emitter laser diodes with outputs of many tens of Watts and diode stacks that can output hundreds of Watts. These powers now make laser diodes effective for metal based additive manufacturing which when configured in a raster scanned multi beam array allow for scaling of build volume and rate. A multiple laser diode powder bed fusion additive manufacturing platform has been developed along with the firmware and software to enable the automated building of multiple layer objects. An ‘expose-move-expose’ strategy was employed for energy delivery; the multiple beam array is stationary whilst the diodes are on. The influence of powder layer thickness, exposure power, exposure time and the temporal and spatial positioning of exposures were investigated by the use of custom software analysis tools. Five scanning strategies were developed and characterised for the creation of layered objects. The higher absorptivity and lower thermal conductivity of powders when compared to bulk material produced an independence of melt volume from power for a given pulse energy in deep powder. Conversely the melt volume produced for a given pulse energy in a substrate without powder was proportional to the power used. The melt volume produced from a thin layer of powder on a substrate was also found to be proportional to power but the extent of powder denudation, whilst proportional to pulse energy, was independent from power. For a given pulse energy per exposure when creating lines of melt balls the greatest line sharpness and a lowest surface roughness were achieved with a small powder layer thickness, a high exposure power, a low spot pitch, a temporal delay of 400 ms and a square exposure profile. Single layers build in deep powder using the vertical scanning strategies (numbers one and two) showed the greatest consolidation but as layer thicknesses were between 400 and 1000 microns the relative density was no better than that of the un-fused powder. It is concluded that the intensity of the diode lasers used in this research, 2400 W/mm2 from 42 W focussed into a 150 μm diameter spot, is insufficient for building high density parts. This intensity did not produce sufficient melting of the substrate to enable efficient wetting of the melted powder. Consequently, the tendency for balling within the melt produced features within the re-solidified material that were hundreds of microns high. These features required commensurate powder layer thickness to allow uninterrupted passage of the powder wiper. The density of parts was compromised by the large layer thickness needed with 61% being the highest density achieved. Further study would benefit from using laser diodes with an output power greater than 40W.
description: EPSRC