Here we analyse the relationship between the monotonic and cyclic behaviour of cylindrical AlSi10Mg (CL31 AL) samples fabricated by Selective Laser Melting (SLM) to the build direaction, the presence of manufacturing defects (pores, voids, oxides, etc.) and the beneficial effect of post-processing - T6 and hot isostatic pressing (HIP) treatments. Correlative Computed Tomography (X-ray tomography, optical microscopy, electron back scatter diffraction, SEM and TEM) is used to characterise the microstructure and the three-dimensional (3D) structure of fatigue samples to shed light on the role of defects on the experimental fatigue behaviour. Pancake shaped pores are observed in the plane of the deposited layers having a 130% higher volume fraction for the vertical layering deposition (VL) than for horizontal layered (HL) orientations, and being larger and flatter. Further, while T6 treatment had relatively little effect on reducing porosity, the HIPping reduced the pore fraction by 44% and 65% for VL and HL samples, respectively. T6 + Hipping decreased the yield stress and the ultimate tensile strength considerably while increasing elongation and reduction of area accordingly. Our results suggest that the fatigue life seems to be dominated by the presence of these crack-like (pancake-like) defects perpendicular to the loading direction such that it is better to build samples transverse to the highest fatigue loads. In this case, both T6 heat treatment and HIPping appear to reduce the fatigue strength of the material regardless of the AM deposition scheme as they tend to enlarge and collapse pores/voids to flat crack-like defects.

The study looks into the impact of thermal post-processing using Hot Isostatic Pressing (HIPping) and/or T6-peak aging treatment, post-process machining, as well as the build orientation on the microstructural and mechanical properties development in AlSi10Mg alloy fabricated using Selective Laser Melting (SLM). The builds contained fine columnar grains, with a fine Si-enriched cellular dendritic network, resulting in tensile strengths exceeding the castings. To elucidate the as-fabricated microstructure and strength, thermal modelling was employed, predicting cooling rates of 10-10 °C/s. Voids, mostly due to oxide films, were observed using Micro-CT in the as-fabricated condition. HIPping collapsed most voids, showing virtually no trace even after a further T6 treatment. Generally, the tensile properties of the majority of conditions were significantly better than in the cast+T6 equivalent alloy. Post-process machining was also found to improve the strength (compared to the as-fabricated surface). However, HIPping+T6 allowed the builds to achieve the required tensile properties, without surface machining. By assessing the influence of powder recycling, it was found that the void content linked to oxide layer formation increased following powder recycling, resulting in a drop in tensile properties. The interaction between the microstructure, surface condition, thermal post-processing, and fracture mode is discussed.

The study looks into the impact of thermal post-processing using Hot Isostatic Pressing (HIPping) and/or T6-peak aging treatment, post-process machining, as well as the build orientation on the microstructural and mechanical properties development in AlSi10Mg alloy fabricated using Selective Laser Melting (SLM). The builds contained fine columnar grains, with a fine Si-enriched cellular dendritic network, resulting in tensile strengths exceeding the castings. To elucidate the as-fabricated microstructure and strength, thermal modelling was employed, predicting cooling rates of 10(5)-10(6) degrees C/s.Voids, mostly due to oxide films, were observed using Micro-CT in the as-fabricated condition. HIPping collapsed most voids, showing virtually no trace even after a further T6 treatment. Generally, the tensile properties of the majority of conditions were significantly better than in the cast + T6 equivalent alloy. Post-process machining was also found to improve the strength (compared to the as-fabricated surface). However, HIPping + T6 allowed the builds to achieve the required tensile properties, without surface machining. By assessing the influence of powder recycling, it was found that the void content linked to oxide layer formation increased following powder recycling, resulting in a drop in tensile properties. The interaction between the microstructure, surface condition, thermal post-processing, and fracture mode is discussed. (C) 2016 Elsevier Ltd. All rights reserved.

The main challenge associated with the application of selective laser melting (SLM) to Ni based superalloys is the performance of process optimisation to maximise the mechanical properties. The energy density parameter has typically been used as a semiquantitative approach to identify the energy threshold beyond which the material achieves virtually full consolidation. Nonetheless, some Ni superalloys are susceptible to crack formation during SLM, which cannot be avoided via process optimisation. In the present report, a comparative study is presented showing the utility of the energy density parameter in process optimisation for γ′ and γ′/γ″ strengthened Ni based superalloys. For both classes, it was found that the build density increases [i.e. void area () decreases] with the increase in the energy density. Nonetheless, no direct correlation can be found between the energy density parameter and the cracking density.