High temperature tensile deformation behavior and failure mechanisms of an Al–Si–Cu–Mg cast alloy — The microstructural scale effect

Abstract

In this study the high temperature tensile deformation behavior of a commercial Al-Si-Cu-Mg cast alloy was investigated. The alloy was cast with two different cooling rates resulted in average secondary dendrite arm spacing of 10 and 25 μm, which are typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10 -4 s -1 to 10 -1 s -1 ) and over a wide temperature range from ambient temperature to 500 ºC. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 ºC; the fracture mode in the fine microstructure was fully ductile above 200 ºC. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 ºC. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 ºC, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu-and Mg-bearing phases were higher in the fine microstructure.

Optimisation of heat treatment of Al–Cu–(Mg–Ag) cast alloys

“…Han et al [12] suggested different dissolution rates for eutectic Al 2 Cu and block-like Al 2 Cu phase where the former dissolves by fragmentation to smaller parts and eventually dissolves by radial diffusion of Cu in the matrix, while the latter dissolves by spheroidisation and shrinkage. The author pointed out in another work that size and morphology of Al 2 Cu phase influence dissolution rate where fine particles with lower aspect ratio dissolve faster than coarse elongated one [13]. In the present study, the comparison between the dissolution rates of Al 2 Cu for different microstructural scales from their three microstructural scales is not straightforward where the fast solidification rate resulted in dendritic growth of a-Al, while two slower solidification rates resulted in cellular growth (Fig.…”

“…The melt was gravity-die-casted in a 250°C preheated copper die having six cylindrical fingers of 200 mm long and 10 mm diameter. The cast rods were then re-melted at 730°C for 30 min under Ar atmosphere in a Bridgman furnace and then directionally solidified [13]. The furnace was mounted on a motorised lifting device, while the rods are in a stationary position.…”

“…Solution temperature controls the diffusion-based dissolution rate of Al 2 Cu, while homogenisation time is primarily determined by diffusion distance [11]. Factors such as fraction, type and morphology of Al 2 Cu phases also determine their dissolution rate [12,13]. T6 heat treatment was commonly applied for Al-Cu-Mg-(Ag) alloys, consisting of SHT at a temperature range of 495-530°C for various times from 2 to 22 h, quenching in the water at ambient temperature followed by artificial ageing at 150-180°C for 8-24 h [14].…”

“…and component geometry affect the extraction of heat during solidification which leads to variations in the size of microstructural features [24]. The cooling rate varies with the local thickness of the cast component: a high cooling rate results in a refined microstructural features, while lower cooling rates yield a coarser microstructure [13]. It was shown that the microstructural scale (coarseness of microstructure) in Al-Si alloys influences solution treatment mechanism [10] and artificial ageing response [25].…”

Microstructure and High Temperature Deformation of Extruded Al-12Si-3Cu-Based Alloy

“…The addition of Cu and Mg to Al-Si alloys was reported to promote their mechanical properties through the precipitation of the Al 2 Cu, Mg 2 Si phases [5][6][7][8]. Furthermore, it was reported that Ni is a desirable element for improving the mechanical properties of Al-Si-based alloys at elevated temperatures by forming thermally stable Ni-containing intermetallics [9].…”