After casting and solidification, Al-cast cylinder heads undergo a sequence of heat treatments to achieve high temperature high mechanical resistance. This sequence comprises solution heat treatment, quenching and artificial aging. During quenching to room temperature, it comes to the formation of internal stresses due to temperature gradients within the complex geometrical structure of the cylinder heads. These internal stresses affect machine components, provoking distortion and reducing permanent strength. However, during aging, these internal stresses decrease again, i.e. they relax. For the production of cylinder heads, it is essential to quantify and describe the process of relaxation to achieve the required performance during service.
Tensile relaxation tests are carried out on a Gleeble® 3800 using a self-designed specimen geometry, suitable for the machine’s resistive heating method. The specimens are artificially aged in the Gleeble machine at different aging temperatures after applying different starting stresses that simulate the quenched situation. The conditions of the samples before testing goes from solid solution up to different degrees of ageing. Then, the specimens are heated up to test-temperature followed by loading at the test stress keeping the strain constant. The relaxation of the applied stress is then recorded for 10h. Plastic strain and plastic strain rates are calculated as a function of the processing parameters and microstructure. To identify the precipitation kinetics during this time, Vickers micro hardness measurements, Differential Scanning Calorimetry (DSC), as well as Scanning Electron Microscopy (SEM) are conducted on heat treated samples in different conditions.
The results show that more than 50% of the total stress-relaxation decreases already within the first 500s. In all experiments it is observed that at a certain point a change in the relaxation rate happens. The relaxation process is dependent not only on the test temperature and the amount of applied stress, but also on the precipitation state. Two parallel effects, which influence the relaxation behaviour were found: a) the simultaneous precipitation strengthening of the material during relaxation, and b) a change in the volume due to Si containing precipitates that produce an additional strain.