**1. Introduction**

328 Recent Trends in Processing and Degradation of Aluminium Alloys

Zhang, J., Alpas, T. A. (1997). Transition Between Mild and Severe Wear in Aluminium

Thermo-mechanical fatigue is generally due to a cyclic thermal load in conjunction with restrained thermal expansion. Because of the considerable amplitude of strain this load leads to local cyclic plastic deformation and thus to material fatigue. Usually several concurrent and complex damage mechanisms are involved in thermo-mechanical fatigue due to the temperatures and stresses attained. In addition to typical fatigue damage caused by plastic deformation, elevated temperature leads to an increase in corrosive effects (e.g. oxidation) and creep damage. The areas of application are manifold: besides thermomechanical fatigue of combustion engine components (e.g. cylinder heads, pistons, exhaust elbows) such effects are common with tanks used in the chemical industry, pipelines, braking systems, turbine blades, as well as all machine tool components and components subjected to elevated operating temperatures. All these applications show cyclic thermal load which, for example, is caused by start-up and shutdown procedures, as well as a mechanical load caused by either restrained thermal expansion (e.g. cylinder heads) or considerable centrifugal forces (e.g. turbine blades).

While in 1992 the maximum specific power for a diesel passenger car was 35 kW/l, the typical ignition pressure was about 130 bar, resulting in a maximum piston temperature of 330 °C. Owing to demands targeting reduction of costs, emissions and fuel consumption, an increase in efficiency by means of "Downsizing" is called for. This is realised by reducing the cubic capacity as well as the number of cylinders and along with additional charging, resulting in an increase in firing pressures and temperature the combustion chamber. The specific power thus obtained is in the region of 70 kW/l, together with ignition pressures of 200 bar and a maximum piston temperature of more than 400 °C, (Reichstein, 2005).

Modern cylinder head materials are typically produced out of aluminium-cast alloys, of which aluminium-silicon-magnesium (AlSiMg) and aluminium-silicon-copper (AlSiCu) alloys are most common. Aluminium and silicon form a eutectic at 577°C and 12 weight percent. The Al-solid solution, silicon and additional secondary phases have a eutectic solidification. The cooling rate influences the dendrite arm spacing (DAS) and the morphology of the eutectic silicon. A high cooling rate leads to a low DAS and finer secondary microstructure. The hypoeutectic alloys are used for cylinder heads and hypereutectic alloys are found in pistons and crankcases.

Comparison of Energy-Based and Damage-Related

propagation under an OP-TMF load.

σ

**hot cold**

Plastic deformation

Crack initiation and propagation

**time**

**time**

**time**

Plastic deformation

**TMF out-ofphase cycle**

> Cyclic ageing

TMF loading (on the right)

Coarsening effects

Creep effects

**Temperature**

**Strain**

**Stress**

Oxidation

Recovery process

Fatigue Life Models for Aluminium Components Under TMF Loading 331

dislocations at low temperatures, cyclic ageing at medium temperatures, and diffusion creep at high temperatures. However, both under IP (temperature and stress cycle are in phase) and OP (temperature and stress cycle are out of phase) TMF loading the microstructure evolution and the oxidation processes are more often than not dominated by the temperature range close to the maximum temperature. The maximum temperature occurs in the tensile stress region in case of an IP-TMF load, and in the compressive stress region in

On the other hand concerning OP-TMF, crack initiation and growth are linked directly with the processes in the temperature range closest to the minimum temperature where tensile stresses prevail, and are only linked indirectly with processes that occur at the maximum temperature. Nevertheless this indirect influence can be even more distinctive than it would be in isothermal experiments. For example a layer of scale might build up as a result of oxidation which takes effect predominantly at high temperatures. This layer of scale is very brittle at low temperatures and thus causes early crack initiation and accelerated crack

case of an OP-TMF load, see figure 1 (according to Löhe et al., 2004) and figure 2.

ε

Fig. 1. Active damaging mechanisms during an OP- and IP-TMF cycle (Löhe et al., 2004)

**Temperature**

**Strain**

**Stress**

Fig. 2. History of temperature, strain and time under OP-TMF loading (on the left) and IP-

**time**

**time**

**time**

Hardening process
