Development of Recycle-friendly Secondary Cast Aluminium Alloy for Cylinder Head Applications

Abstract

Automotive components are currently manufactured from di erent aluminium alloys. The production of these components from primary aluminium alloys has become a huge burden in terms of economic bene ts and environmental aspects. Hence, recycling alu- minium alloys is adopted as a strategy in foundry industries for production of various components in the transport and other structural applications. However, the quality of secondary casting alloys is always in question in most recycling practices since alloy chemistry control is a major issue during aluminium recycling and majority of the scrap alloys have di erences in chemical compositions. This in turn a ects microstructure, the overall mechanical performance of the alloy and castability. It is therefore important to seek ways of maximising alloy chemistry control during aluminium recycling so as to im- prove the e ciency of the process as well as to utilize the scrap metals in to high quality products. The aim of this study was to identify the alloys used for cylinder head and develop an alloy that allows direct recycling of post-consumer cylinder heads for reuse in the same application and investigate its microstructure, mechanical performance and castability. An assessment was conducted in the literature and on scrap samples to inves- tigate the chemical composition of alloys used for cylinder heads. Recycle-friendly alloy that accommodates di erent cylinder head scraps was identi ed based on the standard alloys used for cylinder head application. An alloy (base alloy (SI)) was developed from scrap cylinder heads which fall within the identi ed alloy chemical composition range by melting di erent scrap cylinder heads. Its microstructure, mechanical properties and

uidity characteristics were investigated. Further, the e ect of Sr, Fe, Mn and di erent heat treatment parameters were investigated on the alloy and a satisfactory mechanical properties and uidity characteristics of the alloy was obtained. The common alloys used in cylinder heads were found to be type 319.0, 356.0, 355.0, 354.0, A380.0 and their equivalent alloys in other countries. The chemical composition analysis of the scrap samples showed that most of them were in the range of JIS AC2B alloy which is equiv- alent to 319.0 alloy. The microstructure of the alloy in the as-cast reveals phases such as coarse acicular eutectic silicon particles and intermetallic phases of iron and copper. Some Fe-intermetallic phases were also seen to be modi ed by Sr. After heat treatment, necking, fragmentation and spheroidisation of eutectic silicon particles were observed. Moreover, Al 2 Cu was completely dissolved in the Al-matrix. The size and distribution of porosity was increased by the addition of 0.02% Sr as modi er and with the increase of Fe to 0.38%. The pore sizes were large with these alloys, especially with the modi- ed one which had pore sizes above 100 m. The addition of strontium as a eutectic modi er and iron as an impurity were observed to decrease the tensile strength (UTS) and ductility due to associated porosity and intermetallic phases. The average UTS in the as-cast alloy was 209.5 MPa, 203.6 MPa, 195.4 MPa and 201.5 MPa in the base, Sr, Fe and Mn added alloys respectively. After T6 heat treatment; the UTS, impact toughness and hardness increased while ductility decreased. The highest micro-hardness value was registered by the alloy containing 0.38% Fe after 2 h aging time and at 170 aging temperature. The suitable aging time of the alloy for obtaining high values of hardness was found to be between 2 and 6 hours at 170 o C and T4 treatment was also found convenient than aging for long hours. The uidity of the base alloy was found to o Cpossess the highest ow length in a curved channel while addition of 0.02% Sr to the base alloy was observed to reduce the uidity by 5.2%. The increase of Fe content of the base alloy to 0.38% reduced the uidity by 21.9%. However, the combined addition of 0.9% Fe and 0.45% Mn was observed to reduce the uidity by 12.1%.