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Aluminium cast alloys for high performance pistons

In the course of engine "downsizing", the automotive industry develops more efficient engines with higher power densities. Consequently, the requirements on the used materials increase.

Separately cast HPDC specimen in the TMF/HCF testing deviceSeparately cast HPDC specimen in the TMF/HCF testing device

The currently used aluminum pistons in modern diesel engines can stand maximum combustion pressures up to 180bar and maximum operating temperatures up to 400°C. This is, with a value of 0.9, already the highest applied homologous material temperature occurring in technical application (Thomolog = 0.9 ≡ 90% TS).

As part of a BMBF-funded WING project, new cast aluminum piston alloys are developed for production by two, for this application, new casting processes. Low pressure die casting (LPDC) and high pressure die casting (HPDC) offer a smoother melt flow and a significantly higher rate of solidification, respectively, which are advantageous compared to the gravity die casting (GDC) process used in mass production today. This advantages, together with appropriate alloy development, is expected to lead to a further increase of the possible operating parameters to combustion pressures up to 200bar maximum temperatures up to 440°C (Thomolog = 0.95).

Temperature and mechanical strain profile of a TMF/HCF testTemperature and mechanical strain profile of a TMF/HCF test

Casting defect as crack initiation of a TMF/HCF testCasting defect as crack initiation of a TMF/HCF test

The research work of the department of materials mechanics at IEK-2 include the characterization of the microstructure using LM, SEM and TEM, as well as thermo-mechanical fatigue (TMF) experiments with and without superimposed high-cycle fatigue (HCF) loading. In the TMF tests, due to the different manufacturing processes, two different sample geometries are used. The thinner samples with only 4.5 mm have a very small cross section in the gauge length. In order to avoid buckling of these samples, a special glass holder was invented which minimizes bending force caused by attaching the extensometer. The basic TMF tests are performed with a minimum temperature of Tmin= 200°C, a maximum temperature Tmax = 440°C and a dwell time at Tmax of tdwell = 60s. The mechanical strain amplitude is 50% of the thermal strain amplitude (εa, t me = 50% εa, therm). The superimposed HCF loading with different amplitudes (εa, t HCF = 0.03 to 0.05% εmech) is applied at a frequency of fHCF=10Hz. By using this test method, the thermally induced stresses at the combustion chamber side of the piston during start-stop operation as well as the superimposed fatigue loading caused by ignition pressure and inertial forces are simulated. From the resulting lifetime, the cyclic deformation curve, the fracture mode and metallographic cross sections, conclusions can be drawn about the damage mechanisms serving as an important contribution to alloy and process development.