Residual Stresses In Aluminium Automotive Blocks

The working conditions within the combustion chamber of engine blocks forces the use of liners able to resist the pressure and wear caused by reciprocating pistons. Most liners are made from iron alloys and can be either inserted after the block has solidified, or can be cast-in the block. Cast-in liners promote residual stresses in regions such as cylinder bridges. These stresses arise from the differences in the thermal expansion coefficient of iron and aluminium and may promote the failure of the block if high enough. This work describes the procedure followed to measure the residual stresses in aluminium engine blocks with cast-in grey iron liners by means of extensometric measurements. The trials were conducted in blocks of I-4 and V-8 configuration that were either in the as-cast or heat treated condition. It was found that the stresses can be higher than 150 MPa in tension, depending on the geometry and size of the piece. Similar analyses were carried out in blocks that have pressed-in liners, which resulted to have compressive stresses. It is concluded that these analyses can provide for an early advice when changes take place in design, processing or material.