STUDIES ON ELECTRO-MAGNETICALLY STIR CAST Al-Si ALLOYS

Abstract

Hypoeutectic and hypereutectic Al-Si alloys are widely used in automobile, aerospace and military applications due to their light weight, resistance to corrosion, fair strength to weight ratio and good wear resistance. The mechanical and tribological performances of hypoeutectic alloys are primarily governed by morphology of eutectic Si, a- Al and other intermetallic compounds depending upon the composition. In case of hypoeutectic Al-Si alloys primary silicon particles, /3 intermetallic compounds and eutectic matrix significantly affect the mechanical and tribological properties. Dendritic structure of a- Al, eutectic silicon and large size primary silicon particles along with /3 intermetallic compounds are known to deteriorate mechanical, metallurgical and tribological properties of hypoeutectic and hypereutectic Al-Si alloys besides lowering of the machinability; therefore refinement of primary silicon particles becomes mandatory to produce acceptable castings. Traditionally, phosphorous (P) based compounds have been used for refining the primary silicon particles; however this method suffers with high volatility of P. Therefore, attempts are being made to explore new methods for refining the dendritic structure and primary silicon particle (PSPs) in these alloys. Semi-solid metal processing has been reported to spheroidize and refine the alpha aluminium grains and literature survey revealed that not many studies have been reported on the application of semi-solid processing of hypoeutectic and hypereutectic Al-Si alloys using electro-magnetic stirring approach. Therefore, in this investigation attempts were made to study the effect of electromagnetic stirring on microstructure, mechanical properties and tribological properties of Mn modified hypoeutectic Al-Si alloy and RE (Lanthanum, Cerium) and Mn modified hypereutectic Al-Si alloy. Attempts have been made to improve mechanical performance and wear behaviour of hypoeutectic and hypereutectic Al-Si alloys by refining a) coarse polyhedral shape primary silicon particles b) needle shape 13-intermetallic compounds using semisolid processing technique and controlled alloying of Mn and RE (La203 and CeO2). An electromagnetic stirring setup was developed for semisolid metal processing of alloys. Development of cast hypoeutectic and hypereutectic Al-Si alloys was done for investigation of mechanical, metallurgical and tribological properties. Mn was added to the hypoeutectic Al-Si alloy for modification and rare earths CeO2 and La203 were added to the hypereutectic Al-Si alloy separately for refinement of primary silicon particles. Post heat treatment (T6) was carried out. Samples were made for hardness, tensile, fatigue and iv Moreover, reduction in wear rate of the EMS processed Ce02-Mn modified alloy was more than other alloys. EMS processing of unmodified, RE modified and RE-Mn modified alloy hypereutectic Al-16%Si alloy decreased the wear rate irrespective of sliding conditions used in this work. The extent of reduction in wear rate due to EMS processing of Ce02-Mn modified hypereutectic Al-16%Si alloy was more than unmodified, RE modified and RE-Mn modified alloy. Further, influence of EMS processing on reduction in wear rate of alloy systems under study was more at low normal load and high reciprocating velocity conditions used in this study. Addition of alloying elements namely Mn in hypoeutectic Al-6.5%Si and RE & Mn in hypereutectic Al-16%Si alloys, in general, decreased the wear rate of alloys both conventional cast and EMS processed conditions. Addition of Mn in hypoeutectic Al-6.5%Si alloy decreased wear rate both in conventional cast and EMS processed conditions. Addition of CeO2 and Ce02-Mn in hypereutectic A1-16%Si alloy decreased wear rate both in conventional cast and EMS processed conditions. Moreover, extent of reduction in wear rate with the addition of Ce02-Mn in unmodified hypereutectic Al-16%Si alloy was more than CeO2 modified alloy. Similarly, addition of La203 and La203-Mn in hypereutectic Al-16%Si alloy also decreased wear rate both in conventional cast and EMS processed conditions. Extent of reduction in wear rate with the addition of La203-Mn in unmodified hypereutectic Al-16%Si alloy was more than La203 modified alloy. Further, influence of alloying (RE and Mn addition) on reduction in wear rate of alloy systems under study was more at low normal load and high reciprocating velocity conditions used in this study. In general, heat treatment (T6) of both hypoeutectic and hypereutectic Al-Si alloys reduced the wear rate irrespective of processing method, alloying and sliding conditions used in this work. Heat treatment of unmodified and Mn modified hypoeutectic Al-6.5%Si alloy decreased wear rate both in conventional cast and EMS processed conditions. Heat treatment of unmodified, CeO2 and Ce02-Mn modified hypereutectic Al-16%Si alloy also decreased wear rate both in conventional cast and EMS processed conditions. Similarly, heat treatment of unmodified, La203 and La203-Mn modified hypereutectic Al-16%Si alloy also decreased wear rate irrespective processing conditions. Extent of reduction in wear rate with heat treatment of La203-Mn modified hypereutectic Al-16%Si alloy was more than unmodified, La203 modified alloy. Further, influence of heat treatment on reduction in wear rate of alloy systems under study was more at low normal load and high reciprocating velocity conditions used in this study.