MICROSTRUCTURAL AND MICROHARDNESS CHARACTERIZATION OF PRIMARY AND RECYCLED CAST AL-SI PISTON ALLOYS PROCESSED BY HIGH-PRESSURE TORSION

Abstract

This research has characterized the microstructure and microhardness properties of primary (virgin material) and recycled (herein referred to as secondary alloys) cast Al-Si piston alloys processed by high-pressure torsion. Three primary Al-Si piston all alloys (Al-12%Si, unmodi ed Al-7%Si and modi ed Al-7%Si) and one secondary Al-Si piston alloy (10.6%Si piston alloy) were investigated. Modi cation of Al-7%Si alloy was achieved through addition of silicon modi er (strontium). All the alloys were processed by high-pressure torsion at room temperature for con- stant speed of 1 revolution per minute and at a pressure of 3.0 GPa. The samples were processed up to 10 high-pressure turns. It was found that both primary and secondary alloys behaved similarly during high-pressure torsion. Microhardness for all the alloys increased with number of turns along the diameter as revealed by the microhardness line pro les. Further- more, the microhardness for most of the alloys were symmetrical about the centre; with lowest microhardness at centre. The microhardness along the diameter of all the samples tended towards homogeneity although a homogenous microhard- ness distribution was not achieved after 10 turns. However, it was found that Al-12%Si alloy exhibited a very high microhardness gradient even after 10 turns. The microhardness in primary modi ed silicon (modi ed Al-7%Si) alloy increased slowly compared to the other alloys. This is because there was only breakdownof the Si structures rather than Si particles as observed for the other alloys. The microhardness-equivalent strain relationship revealed that all the alloys studied in this work undergo strain hardening with slow recovery during high-pressure tor- sion. Microstructural analysis through Scanning Electron Microscopy, optical micro- scope and ImageJ software, revealed that ne structures can be achieved through high-pressure torsion of both primary and secondary Al-Si piston alloys. The microstructural analysis and Weibull distribution plots of particle sizes revealed smaller phases at the edges than at the centre for all the alloys even after 10 turns. This indicates that Al-Si piston alloys do not breakdown fully after 10 turns un- like simple Al-Si alloys and pure aluminium. After 10 turns, for all the alloys, it was observed that nearly all the intermetallic phases except the Si-rich phases had broken down and redistributed within the Al-matrix. It was also observed that there was not much break down of the Si particles in the modi ed Al-Si piston alloy since most of the break down was observed on the network of modi ed Si structures after 10 turns. These results indicate the possibility of processing Al-Si piston alloys to ultra ne grain structures through high-pressure torsion for improved performance in their application as engine materials. However, future work is recommended on process- ing the samples for more than 10 turns to evaluate the achievement of homogenousmicrostructure. To further understand the deformation mechanisms, grain bound- ary and dislocation motions should be studied.There is also need to characterize the high-pressure torsion processed Al-Si piston alloys for other properties such as thermal stability, corrosion and wear.