Previous work has shown that the processing of aluminium alloys by Selective Laser Melting (SLM) is challenging, with fully dense components only being produced at high laser powers (minimum 330 W). The high laser power requirement for these parts is a problem as it is higher than that available in many SLM machines. Attempts to produce dense components by combining lower power and slow laser scanning speeds create a large melt pool that is difficult to control, leading to balling of the melted material and possible damage to the powder distribution system. Even when processing is carried out successfully, the laser scan speed, which has to be slower than 150 mm/s, increases build time and hence manufacturing costs. This thesis considers the changes that can be made to the SLM process to reduce the laser power required and to increase the laser scanning rates, while still producing components with a relative density approaching 100%. It also considers why aluminium and its alloys are much more difficult to process than stainless steels and commercially pure titanium. One reason for the difficulties in SLM of aluminium was the flowability of aluminium, which led to the redesign of the powder deposition mechanism of the SLM machines that were used. The major challenge for processing aluminium and its alloys was found to be oxidation due to the presence of oxygen within the build chamber. This formed thin oxide films on both the solid and molten materials. Examination of manufactured parts showed that when processing at laser power of 100 W the oxide films were broken between successive build layers due to Marangoni forces stirring the melt pool, but that walls of oxide were present between tracks within the same layer. As these films occurred in pairs one from each track, pores were formed in between creating open porosity.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:632653 |
| Date | January 2012 |
| Creators | Louvis, Eleftherios |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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