Effect of Fe, Sr and Mn on the Microstructure and Mechanical Performance of Secondary Cast Al-Si-Cu-Mg Alloys for Cylinder Head Applications

Abstract

Cast aluminium alloys have many uses such as in the automotive industry as well as for manufacturing purposes like in construction industry to fabricate windows and door frames. These alloys are important because they are light and as such reduce vehicle weight; they have good fluidity and are corrosion resistant. However, the microstructure and mechanical properties of recycled aluminum alloy are still unpredictable. This is mainly due to problems associated with controlling the chemical composition of the recycled aluminium alloys since the chemical compositions of the various scrap inputs are different. Solidification after casting defines the mechanical properties of cast alloy through the resulting microstructure. The main objective of this research is to develop a high performance recycled aluminium alloy for automotive cylinder head applications. Previous study has mainly focused on the development of recycle-friendly alloys for automotive cylinder head applications. The mechanical properties that were carried out recommended reusing directly the cylinder head scrap. The present research focuses on the effect of minor elements additions such as Sr, Fe and Mn on the microstructure and fatigue performance of recycled Al-Si alloy for cylinder head applications. The alloy used in this study was obtained by melting different scrap cylinder heads from small vehicles and then was tested to predict their fatigue performance after element additions in heat treated and as cast conditions. The specimens for the fatigue tests and the microstructural analysis as well as tensile tests were machined from the resulting alloys. An Optical Microscopy (OM) and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDS) were used to investigate the microstructural characterization. The tensile test specimens were machined according to the ASTM B557M standard. Fatigue life performance was investigated at room temperature using S-N fatigue data. Constant amplitude, fully reversed fatigue tests under a stress ratio of R = -1 was carried out using a rotating bending machine. The microstructure features of base alloy consisted mainly of a structure with primary Al-matrix, coarse acicular Si particles and intermetallic phases such as Al2Cu and AlCuNi. When 0.02%Sr was added to the base alloy, coarse acicular Si particles were modified to a fine fibrous form. With addition of 0.38%Fe, results in the formation of large eutectic silicon particles and Fe rich intermetallic. Moreover, when 0.45%Mn was added in combination with 0.9%Fe, the Al2Cu, and α-AlFeMnSi with Chinese script morphology were identified. It is noticed that after T6 heat treatment, the Si particles are seen to spheroidize and fragment while the Al2Cu phases dissolve completely. These changes lead to improved mechanical performance of the alloy. The results also showed that the strontium addition has the highest average UTS in both as cast and after T6 heat treatment condition. The ductility of the alloys decreased after T6 heat treatment. The highest fatigue lives in the as cast alloy were observed in the base alloy with strontium. After T6 heat treatment, the fatigue performance of base alloy with iron and manganese increased while the fatigue life of base alloy and with strontium decrease. These results obtained from this study can be used by automotive powertrain component manufacturers to produce high quality engine parts using secondary Al-Si alloys.