Abstract
The optimisation of heat treatment parameters for Al–Cu–(Mg–Ag) cast alloys (2xxx) having different microstructural scales was investigated. Thermo-Calc software was used to design optimal alloy compositions. Differential scanning calorimetry (DSC), scanning electron microscopy and wavelength-dispersive spectroscopy technique were employed to determine proper solution heat treatment temperature and homogenisation time as well as incidence of incipient melting. Proper artificial ageing temperature for each alloy was identified using DSC analysis and hardness measurement. Microstructural scale had a pronounced influence on time and temperature required for complete dissolution of Al2Cu and homogenisation of Cu solute atoms in the α-Al matrix. Refined microstructure required only one-step solution treatment and relatively short solution treatment of 10 h to achieve dissolution and homogenisation, while coarser microstructures desired longer time. Addition of Mg to Al–Cu alloys promoted the formation of phases having a rather low melting temperature which demands multi-step solution treatment. Presence of Ag decreases the melting temperature of intermetallics (beside Al2Cu) and improvement in age-hardening response. Peak-aged temperature is primarily affected by the chemical composition rather than the microstructural scale.
Highlights
The optimisation of heat treatment parameters for Al–Cu–(Mg–Ag) cast alloys (2xxx) having different microstructural scales was investigated
Addition of Mg to Al–Cu alloys promoted the formation of phases having a rather low melting temperature which demands multi-step solution treatment
T6 heat treatment was commonly applied for Al– Cu–Mg–(Ag) alloys, consisting of solution heat treatment (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]
Summary
The optimisation of heat treatment parameters for Al–Cu–(Mg–Ag) cast alloys (2xxx) having different microstructural scales was investigated. 2xxx family of aluminium casting alloys are age hardenable, based on Al–Cu system, which offers high strength and hardness at ambient and elevated temperatures [1, 2]. The highest rate of precipitation occurs when all secondary phases (Al2Cu, Al2CuMg, etc.) which are formed during solidification are entirely dissolved and atoms of alloying elements (e.g. Cu and Mg) are homogenised in the a-Al matrix during the solution heat treatment (SHT) [10]. Moller et al [16] instead proposed a two-step homogenisation/solution heat treatment: homogenisation at 490 °C for 24 h, followed by a SHT at 520 °C for 2 h, which successfully prevents the formation of incipient melting pores to a great extent in 2139 alloy. Daswa et al [17] suggested a mix of three-step SHT coupled with controlled heating for 2139 alloy: controlled heating from 400 to 513 °C and holding at 513 °C for 2 h and eventually heating to 525 °C and holding for 16 h
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