Role of microstructure in the static and fatigue properties of samples extracted from engine heads manufactured with recycled Al–Si–Cu–Mg alloy

The present study investigates the role of microstructure and defects on the static and fatigue behaviour of specimens extracted from engine heads manufactured using a recycled Al-Si-Cu-Mg alloy (recycling rate 97-100%). These components were manufactured by an Italian company leader in the production of high-values cars. The use of recycled aluminium alloys is essential for reducing the environmental impact in the automotive sector. However, they may contain impurities (mainly iron-based compounds) that can negatively affect their mechanical performance. The aim of this research was therefore to verify the possibility of achieving mechanical properties comparable to those of the primary (bauxite-derived) alloy. Several analytical techniques were employed, including tensile and fatigue testing, chemical composition analysis, microstructural characterization by optical microscopy for SDAS measurements, fractographic analyses by stereomicroscope and scanning electron microscope. The analyses carried out are the same as those performed for the characterisation of the primary alloy used in the past. However, it must be considered that the manufacturing process of the component has changed over the years. Moreover, defects that reduce mechanical performance cannot always be attributed to the use of the secondary alloy but are often related to handling and process control. Nevertheless, the higher iron content typically found in secondary recycled alloys, can induce lower mechanical properties. Tensile and fatigue tests revealed lower elongation of the secondary alloy compared with the primary one and a decrease in fatigue strength. Optical microscopy showed the presence of iron-rich intermetallic compounds, non-metallic inclusions and oxides. SEM fractography confirmed that most cracks, in specimens failing at low stress, originated from these defects.