Effect of the casting process on microstructure and lifetime of the Al-piston-alloy AlSi12Cu4Ni3 under thermo-mechanical fatigue with superimposed high-cycle fatigue loading

Abstract

For the present research work, the well-known Al-piston-alloy AlSi12Cu4Ni3 was manufactured in three different processes (gravity die casting – GDC, low pressure die casting – LPDC, and high pressure die casting – HPDC) and T5 heat treated. The microstructure of the material from each process was analyzed, specimens were tested in OP-TMF loading with and without superimposed high-frequency fatigue and lifetimes were compared.The microstructure of GDC specimens shows a homogeneous distribution of primary Si and intermetallic phases. The LPDC material also shows a homogeneous microstructure over the whole sample. However, the Al-mixed-crystal formed bigger dendrite arms compared to the GDC material. The HPDC material shows a gradient in the microstructure getting finer from the center to the outer shape. In the very fine microstructure in the outer regions of the specimen no primary Si was formed and the Al-mixed-crystal built a globular–dendritic structure, surrounded by a eutectically solidified melt. Porosity was higher in the LPDC and HPDC compared to the GDC material.To simulate the thermally induced loading at the “hot side” of a piston during start–stop, strain controlled out-of-phase thermal–mechanical fatigue (TMF) tests with superimposed high-cycle fatigue (HCF) loading were performed. The TMF-cycles were carried out with a minimum temperature Tmin=200°C and a maximum temperature Tmax=440°C. The mechanical strain amplitude εa,tme of the TMF cycles was kept equal to 50% of the thermal strain amplitude εath and the amplitude of the superimposed HCF cycles εa,tHCF was varied between 0.03% and 0.05%.The lifetime of the specimens produced in the GDC-process represents the current state of the art. Compared to that, the specimens manufactured in LPDC and HPDC reached nearly the same TMF lifetimes as the GDC samples. The maximum stress of the LPDC specimens is approximately equal to that observed at the GDC material. However, during the very first TMF cycles, HPDC-specimens show higher maximum stress than the reference material. This is attributed to the very fine, nearly defect free microstructure at the outer shape of the HPDC material. Afterwards, the maximum stress of the HPDC samples is decreasing faster than that of the GDC material due to early formation of crack networks starting from fine pores in the HPDC microstructure. In TMF/HCF-testing the HPDC material shows the same effect. Furthermore, the HPDC samples show pronounced swelling during temperature cycling and TMF testing. The root cause was identified as the high internal pressure of air encased in the pores formed during HPDC.