Effect of Mn and heat treatment on improvements in static strength and low-cycle fatigue life of an Al–Si–Cu–Mg alloy

Abstract

Tensile and low cyclic fatigue properties were used to assess the influence of Mn additions and heat treatment on the performance of the Al–7Si–1Cu–0.5Mg (wt%) alloy. As a reference, the Mn-free base and the currently used automotive alloys A380 and A319 were used. In the as-cast state, the Mn-modified alloy consisted of the α-Al, eutectic Si, and Cu- and Mg-based phases including Al2.8Cu, Al14.9SiCu4.9 and Al13.8Si4.5CuMg4. In addition, the Mn-rich phases of Al59.2Si9.2CuFe3.4Mn6.3, and Al26Si4.1Cu1.1MgFe1.4Mn2.6 were present after casting. During solution treatment, Cu-based phases were completely dissolved, while the eutectic Si and Mg-rich phases experienced partial dissolution. Some of the Mn-rich phases were partially dissolved while the Al59.2Si9.2CuFe3.4Mn6.3 compound transformed to Al15(FeMn)3(Si)2. The Mn-modified alloy after T6 heat treatment achieved an elongation of 13.5% with an ultimate tensile strength of 355MPa, which is ~18MPa higher than the Mn-free base and ~30% higher than the A319 alloy after similar heat treatment. The fatigue life of the Mn-modified alloy was also substantially longer than that of the reference base and the A319 grade. After failure, the Mn-modified alloy exhibited ductile fracture with major cracks propagated through inter-dendritic regions of the primary Al-phase with fragmented intermetallics. It is believed that the Mn-rich dispersoid precipitates, formed as a result of T6 heat treatment, improved performance of the Mn-modified alloy.