Thermal fatigue testing of laser powder bed fusion (L-PBF) processed AlSi7Mg alloy in presence of a quasi-static tensile load

The AlSi7Mg (A357) alloy is a suitable material to be used in different industrial applications. Even though the majority of parts is produced by casting, a large interest is raising on laser additive manufacturing of these alloys owing to their good processability. Service conditions of several applications, including cylinder heads and exhaust manifolds, cause the alloy to undergo fluctuating thermal and mechanical loads. The thermal fatigue of A357 alloy processed by Laser Powder Bed Fusion is here investigated using a Gleeble (R) 3800 equipment. The material is thermo-mechanically tested in the artificially aged condition. Microstructural and mechanical behavior of the alloy was investigated and it was found that by keeping fixed the maximum and minimum temperatures of thermal cycles between 100 and 280 degrees C, the fatigue life of the alloy deteriorates significantly by increasing the mechanical load from 90 to 120 MPa. Fractographic analyses showed the occurrence of ductile fracture nucleated from large process-induced pores and numerous fine dimples created due to plastic deformation. Secondary cracks were observed on the fracture surface of samples, which nucleated and propagated from large cavities and micro-dimples. Results based on the analysis of the elasto-plastic behavior of the material at high temperature showed that the inelastic strain of the broken samples was about 5 times higher than that of the run-out samples. Hardness drop occurred in all specimens after thermal fatigue experiments due to the coarsening of strengthening phases and modification of the Si-rich particles.