Microstructural, microscratch and nanohardness mechanical characterization of secondary commercial HPDC AlSi9Cu3-type alloy

A key issue in producing high quality aluminum automotive components using High-Pressure Die Casting (HPDC) process is minimizing the amount of imperfections. In the present research work, by means of optical microscopy (OM), SEM, TEM and AFM characterization techniques, microstructural evolution of secondary commercial HPDC AlSi9Cu3-type alloy was studied. For this purpose, one Al-alloy called C1 containing nominal composition and a second one called C2 going out of specification were employed. In addition to metallographic characterization, Vickers and nanohardness and microscratch mechanical testing techniques were applied to analyze material behavior. Furthermore, complementary thermodynamic calculations were done to estimate alloy phases precipitation as function of temperature. Results indicate that globular and ramified dendrites are developed in C1 and C2 Al-alloys, respectively, due the major solidification time between them. In fact, gas porosity and shrinkage microporosity were detected. α-Fe particle increased in size but not in volume fraction, as Al2Cu and β-Fe intermetallics do. Mostly, it was found that α-Fe particle growth following a coring behavior, formed by nanospherical particles. Thus, main shrinkage microporosity is associated with Al2Cu precipitate, where shrinkage pores in alloy C2 present several nanoparticles located between dendrite arms, identify by SEM and TEM analysis. Finally, hardness of alloys differs in 30 HV values due intermetallics population, whereas nanohardness allowed measuring single value of each phase. Because no major nanohardness differences were observed, microscratch resistance behaves similar in both alloys. Hence, present microstructural/mechanical characterization results can be taken into account for redesign and improving commercial HPDC Al-components.