Porosity formation mechanisms and fatigue response in Al-Si-Mg alloys made by selective laser melting

Defects in Al-7Si-Mg and Al-10Si-Mg alloys produced by selective laser melting are categorised into three types. The first type are large irregular-shaped defects with unmelted powder particles, formed due to a lack of fusion as a result of insufficient volumetric energy density. The second type are small round gas pores below 5 mu m in diameter, associated with high area energy density. These pores enlarge during solution heat treatment, but the enlargement is reduced significantly when the powder is pre-dried at 200 degrees C for 16 h under an argon atmosphere immediately before the build. The last type are large round keyhole type pores located at the base of melt pools. They can either form in contour scan regions, at the edges of core scans, or at island boundary overlap regions due to an excessive local energy density compared with the nominal energy density. Sub-surface porosity due to contour and core edge keyhole type defects can be more detrimental to the fatigue performance than net-shaped rough surfaces, but such sub-surface porosity can be minimised by either lowering the laser energy input for the contour scan and/or changing the way the laser turns between scan tracks.