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Title: Direct selective laser sintering of aluminium alloy powders
Author: Olakanmi, Eyitayo Olatunde
ISNI:       0000 0004 2673 4379
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2008
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The SLS/SLM of aluminium powder had been investigated by studying the effect of powder properties and laser processing parameters on the microstructures and properties of both single layer and multiple layer builds. On the basis of experimental evidence, the SLS/SLM of aluminium powders could be categorised into full melting (SLM) which was found to have occurred in both pure and pre-alloyed aluminium powders, and binary liquid phase sintering (SLS) which occurred in blended bimodal or trimodal powders. That successful disruption of the oxide film is possible is a significant result, as is the constitutional effect on this. The spheroidisation and oxide disruption phenomena in SLS/SLM processed aluminium powders arc suggested to be mainly controlled by the amount of oxide on the as-received powder's surface, the degree of the uniformity of the distribution of the surface oxide film covering the aluminium particles as well as the nature of thermal mismatch existing between the oxide film and the parent aluminium particle which was dependent on the phase present in the oxide film (alumina, mullite, and spinel). It was discovered that the attainment of high sintered density and desirable microstructural properties in the blended aluminium powders is consequent upon the determination of the right processing conditions, appropriate choice of powders (in terms of particle size distribution, spherical particle shape, and component ratio). Moreover, it is now evident that chemical constitution of the blended aluminium powders only becomes influential in the determination of the properties of SLS processed parts when the choice of processing parameters and powder properties are correct. The choice of powder properties determines the thermal conductivity of the powder bed which in effect controls the sintered properties. This had been inferred from the relationship between powder tapping density on one hand, and selective laser sintered (SLS) density, dendrite spacing and fraction of primary phase on the other hand. In making smaller samples, it has been shown that the attainment of high sintered density (up to 90%) and a good microstructure are feasible. These arc accompanied by reasonable hardness values, comparable to those of cast Al-12wt%Si castings. In fabricating larger sized parts for mechanical testing, defects such as delamination became more noticeable leading to poor mechanical properties in those samples. Thus, it is now clear that physical limitations of the sintering machine hinder the production of SLS/SLM processed parts having excellent structural integrity. On the basis of this work, it is envisaged that the use of pre-alloyed Al-Si powders of uniform composition, but a wider particle size and size distribution, blended to optimise the bed density, offers the potential to produce light alloy components by SLS. In conclusion, the specific laser energy input, the component ratio, and the particle size and size distribution of the powder were found to have strongly influenced the densitication mechanism and the solidification process in a small sized aluminium powdered part fabricatedb y SLS/SLM process.
Supervisor: Cochrane, R. F. ; Dalgarno, K. W. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available