Optimization of Thermo-Mechanical Fatigue Life for Eutectic Al-Si Alloy by the Ultrasonic Melt Treatment

The eutectic cast Al-Si alloys with excellent high-temperature and casting performance are widely used in engine pistons. During frequent starts and stops, the thermo-mechanical fatigue (TMF) is the most important failure cause. Ultrasonic melt treatment (UT) was chosen to compare and investigate the influence of micro-structures on fatigue life and damage mechanisms of as-cast (AC) eutectic Al-Si alloys under TMF loading. After UT, the grain size, primary Si, and intermetallic particles are reduced significantly in the alloy; fatigue life increases obviously. As a result of pilling-up of dislocations, the competitive effects of the critical strain/stress for fatigue crack nucleation can be found. There are two different crack initiation mechanisms under TMF: one is primary Si fracture for AC alloys with limited critical strain/stress for fatigue crack nucleation at fractured Si particles, and the other is primary Si debonding for UT alloys with increasing critical fracture strain/stress. After the crack initiation, the fractured or debonded primary phases provide the advantages for the further development of main cracks for both alloys. The UT alloy (805 +/- 253 cycles) has about twice the TMF life of the AC alloy (403 +/- 98 cycles). The refinement of micro-structures is instrumental in improving the fatigue resistance and life of TMF for the UT alloy.

Automated summary

The microstructures of eutectic Al-Si alloys have a significant influence on the fatigue life and damage mechanisms under thermo-mechanical fatigue (TMF) loading. Ultrasonic melt treatment (UT) can effectively reduce the grain size and primary Si, resulting in an increased fatigue life. There are two different crack initiation mechanisms under TMF, which are fracture of Si particles and debonding of primary Si. The UT alloy exhibits a longer TMF life due to the refinement of microstructures, which plays an important role in improving fatigue resistance and life.