Abstract
Metallic powder bed additive manufacturing is capable of producing complex, functional parts by repeatedly depositing thin layers of powder particles atop of each other whilst selectively melting the corresponding part cross-section into each layer. A weakness with this approach arises when melting overhanging features, which have no prior melted material directly beneath them. This is due to the lower thermal conductivity of the powder relative to solid material, which as a result leads to an accumulation of heat and thus distortion. The Electron Beam Melting (EBM) process alleviates this to some extent as the powder must first be sintered (by the beam itself) before it is melted, which results in the added benefit of increasing the thermal conductivity. This study thus sought to investigate to what extent the thermal conductivity of local regions in a titanium Ti-6Al-4V powder bed could be varied by imparting more energy from the beam. Thermal diffusivity and density measurements were taken of the resulting sintered samples, which ranged from being loosely to very well consolidated. It was found that the calculated thermal conductivity at two temperatures, 40 and 730 °C, was more than doubled over the range of input energies explored.
Highlights
The need to sinter the powder bed prior to melting can be perceived as a disadvantage compared with the SLM process it does introduce two major benefits
The increased connectivity of the sintered powder relative to loose powder will improve the thermal conductivity of the powder bed
There appears to be some inconsistencies with these studies as one reports a higher relative density and thermal conductivity of the loose powder than that of sintered powder
Summary
The need to sinter the powder bed prior to melting can be perceived as a disadvantage compared with the SLM process it does introduce two major benefits. The increased connectivity of the sintered powder relative to loose powder will improve the thermal conductivity of the powder bed This becomes a desirable trait when melting over-hanging features (or negative surfaces) which, as the name suggests, have no prior material melted beneath them. The resulting measurements are related to the beam parameters and allow the energy/beam settings used to sinter the powder to be directly correlated to the thermal properties of the powder, which is not currently possible Will this enable more accurate inputs to models of melt pool behaviour but could allow the level of thermal conductivity to be tailored for local requirements
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