Comparative thermal fatigue behavior of AlSi7Mg alloy produced by L-PBF and sand casting

By increasing the demand for additively manufactured Al parts for industrial applications, the knowledge about their thermal and mechanical behavior in service environments becomes highly required. Since Al alloys are widely used for the production of lightweight structures in transportation industry including several engine components, their thermo-mechanical behavior deserves special attention, considering the thermal and mechanical load fluctuations experienced in parts like cylinder heads, pistons, brake disks and calipers. In the present study, a comparative research has been carried out on the thermal fatigue (ThF) behavior of an AlSi7Mg (A357) alloy processed either by sand casting or laser powder bed fusion (L-PBF). Both alloy conditions were cycled within three temperature intervals with a lower temperature of 100 °C and upper limits of 200, 240 and 280 °C, in presence of a constant uniaxial tensile load. Three tensile loads of 110, 120 and 150 MPa were applied for each temperature range in order to explore the effect of both thermal cycling and concurrent tensile load on thermal fatigue resistance. Both alloys showed a similar ThF lifetime when exposed to temperature cycles from 100 to 200 °C under all the three tensile loads investigated, while the L-PBF A357 alloy tested to the highest temperature limits of 240 °C and 280 °C comparatively revealed an improved ThF resistance than the cast counterpart. Microstructural analyses on the cross-sections of both samples revealed that a large amount of strain was accumulated close to the fracture regions and several micropores and microcracks were developed in these areas. Microcracks preferentially nucleated at eutectic Si and Fe-bearing coarse intermetallics in the ThF tested cast alloy, while micropores nucleated from the fragmented silicon network in L-PBF samples on exposure to thermal cycling.