Study of the Endurance Limit of AA7075 Aluminum Produced by High-Pressure Vacuum Die Casting Analyzed by Classical Whöler Curve

Abstract

One of the strongest aluminum alloy is AA7075; it has a yield strength of 350 MPa in the T73 heat-treated state, which is 50% stronger than construction steels. The alloy contains a high mass fraction of alloying elements, but no silicon; therefore it is not easily cast into complex shapes by traditional foundry processes since it is prone to shrinkage, hot tearing and has low fluidity. However, the high pressure vacuum die casting (HPVDC) process uses high metal velocities to fill the mold cavity and applies a pressure during solidification to feed shrinkage, which could facilitate the production defect free castings. Casting AA7075 alloy would be highly beneficial to produce complex shapes and reduce assembly steps. This work will focus on AA7075 production by HPDVC process and the effect of grain refiner on the casting defects and mechanical properties of cast AA7075. It is well known that castings endurance limit is highly affected by defects (porosity, oxides, cold shuts, etc.). In this work, the main objective is to study the impact of the HPVDC process on the endurance limit of AA7075, as well as the effect of grain refiner (TiB₂) in reducing the casting defects and enhancing the endurance limit of the cast alloy. The endurance limit will be studied through uniaxial tension-tension fatigue tests on T73 heat-treated AA7075 castings. The microstructures of AA7075 alloy cast with and without TiB₂ was observed by optical and electron microscopy. Stress controlled fatigue testing up to 10⁶ cycles will be used to trace the Whöler curves and study the effect of TiB₂ on the endurance limit.