Analisis Pengaruh Proses Precipitation Hardening terhadap Nilai Kekerasan dan Struktur Mikro Paduan Piston Bekas dan ADC 12 yang Dicor Ulang pada Temperatur Penuangan 750 oC

Abstract

In the automotive industry, aluminum is often used as a piston material in automotive engine. Production of vehicles in Indonesia is growing rapidly, so that many of the pistons which is not feasible to use wasted. Hence the need for reprocessing wasted into recycling piston which is expected to be used in motor vehicles. Piston recycling does not guarantee good mechanical properties; therefore it is necessary to heat treatment process so as to improve its mechanical properties. These researches discuss the influence of temperature and holding time on the process of precipitation hardening heat treatment. Materials used in this research were 50%wt used piston and 50%wt ADC 12. They were sand-casted and poured on steel dies at 750oC. Variation of aging temperature and holding time are as followed: 100-200 oC and 2-5 hours, respectively. Characterization of the products includes testing of chemical composition, hardness and microstructure examination. The test results show cast product is a cast aluminum piston material with AA333.0 codification. The highest hardness values are at aging temperature of 155oC with holding time of 5 hours, amounting to 80.5 HRB. This caused the temperature and time detentions occurred θ'' (GP2) precipitates growth of alloying elements that form a new phase in the matrix of aluminum-α + eutectic Si, so these precipitates can hinder the movement of dislocations in the alloy. Over aging phenomenon occurs on specimens that have aging at a temperature of 200oC after 4 hours of detention. The growth of precipitates in over aging condition has crossed the line from the optimal condition so that decreased the mechanical properties of the alloy.