Effect of building direction on porosity and fatigue life of selective laser melted AlSi12Mg alloy

Gas porosity is one of the most common defects in aluminum alloy parts manufactured by solidification processing, and can have a strong influence on fatigue properties. This study shows that gas pores with a fraction of 0.2-1.6% and an average size of 20-55 µm are present in the Al-Si alloy parts manufactured by Selective Laser Melting (SLM). Failure after fatigue testing was found to initiate from surface or subsurface gas pores and fatigue life prediction equations were developed considering the influence of pores. The building direction did not have a statistically verifiable effect on the average gas porosity fraction, size and distribution, although the scatter in porosity fraction was greater in the vertically built specimens. At the same applied stress, the fatigue life of SLM manufactured specimens decreased with an increase in pore size, and specimens built horizontally exhibited a greater fatigue life than those built vertically. The cause is attributed to greater propensity of cracks to propagate along lower strength melt pool boundary layers in vertically built specimens.