In modern automotive engines, the vast majority of light vehicle diesel (LVD) pistons are made from gravity die cast monolithic AlSi based alloy systems. Presently, the market drivers for reduced emissions, more efficient fuel consumption and increased specific power output are providing cyclic thermal and mechanical fatigue loading above the safe life durability threshold for the current AlSi based alloy systems. Peak temperatures in the diesel piston's fatigue critical combustion bowl region are presently 420 °C for the AlSi based alloys, which represents a homologous TH value in excess of 0.8. In combination with peak temperatures of 420 C, the pistons are subject to cylinder pressures up to 220 bar, inducing mechanical stress amplitudes 15-20% greater than the allowable component fatigue strength for 1x108 cycles, in some applications. This durability deficit naturally leads to a requirement for new material and process solutions aimed at improving thermal and mechanical fatigue resistance at temperatures in excess of 420 C.One solution to this problem is to locally reinforce the pistons combustion bowl edge with a metal matrix composite (MMC) system.In this study, an aluminium based metal matrix composite (AlMMC) has been investigated and has shown some promise with increases in iso-thermal high cycle (1x 107) fatigue strength of 50 % compared to standard monolithic piston alloys. The AlMMC consists of a premium AlSi based LVD piston alloy matrix reinforced with 0.15 Vf of an interconnected network of 2-4 mm long Fe based fibres. The AlMMC is manufactured by pressure assisted infiltration of a sintered metallic fibre preform with as cast materials having a pore density of 0.2 %. In contrast to the use of ceramic fibre reinforcement systems generally requiring high pressure infiltration techniques, preform infiltration is considered possible with a comparably inexpensive manufacturing route. The Fe based fibre preforms can be infiltrated at lower pressure due to the reactivity between the Fe based fibres and the AlSi based matrix alloy. Unfortunately, this increased reactivity, although an advantage for preform infiltration, can result in (FeAlXX)Si(+X) interfacial reaction products forming between the fibre and matrix at operating temperatures of greater than 440 °C. These interfacial reactions result in a 15-20 m interfacial intermetallic layer after prolonged periods of exposure (>500 hrs), resulting in depleted fibre Vf and void formations on the matrix side of the interface.
|Date of Award||1 Aug 2014|
- The University of Manchester
|Supervisor||Philip Withers (Supervisor)|