The development of high strength multi-component Al-based die-cast alloys

Abstract

High-pressure die casting (HPDC) has drawn great attention in recent decades, because of its good surface finish, high productivity and excellent mechanical properties. In recent years, many scientific investigations have been performed on multi-component alloys, including high entropy alloys, metallic glass or high-order eutectic alloys. These multi-component alloys have excellent mechanical properties as compared with many conventional alloys. Thus, in this thesis, aluminium die-cast alloy compositions were designed with the concept of multi-component eutectic systems (e.g. Al-Si-Mg and Al-Cu-Si-Mg). The CALPHAD modelling was used to design the alloys compositions with the required phase constitution (e.g. various volume fraction of eutectic mixture). The microstructural evolution during solidification for these newly developed aluminium alloys was studied using a combination of DSC, XRD, SEM, TKD, TEM. The mechanical properties were obtained from the standard tensile testing and Vickers hardness test. Experimental results confirmed that the new Al-Si-Mg-Mn alloys have bimodal microstructure. It consists of α-Al, α-AlFeMnSi, binary eutectic (Al+Mg2Si) and ultrafine quaternary eutectic (Si+α-Al+Mg2Si+π-AlFeMnSiMg). The high strength is induced by the formation of the multi-scale eutectic mixture and fine α-AlFeMnSi particles. The microstructure of quaternary Al-Cu-Si-Mg eutectic alloy consisted of four phases, including α-Al, Al2Cu, Si, and Al4Cu2Mg8Si7 coexisted together in a cellular microstructure with an ultrafine lamellar eutectic mixture as well as in the nano-scaled anomalous eutectic at the intercellular region. The Al-Cu-Si-Mg hypoeutectic alloys were investigated with Cu content ranging from 5wt% to 10wt%. The microstructure of these alloys comprised of the multi-scale eutectic mixture, from coarse binary to ultrafine quaternary eutectic. It was found that the yield strength and elongation can be tailored by controlling the volume fraction of the eutectic mixture. Optimised solution heat treatment conditions of newly developed aluminium die-cast alloys were developed. They are 540°C/10min for Al-Si-Mg-Mn alloys and 500°C/30(60) min for Al-Cu-Si-Mg alloys. The effects of solution time and temperature on the expansion of porosity were assessed. There is no apparent porosity expansion under short solution treatment or the prolonged solution time with lower temperatures. The short solution treatment in Al-Si-Mg-Mn alloys enables the dissolution of Mg and spheroidisation of eutectic Si and Mg2Si phases, thereby minimising the growth of α-Al grains. The π-AlFeMnSiMg in the final quaternary eutectic reaction transforms into nano-scale body-centred α-AlFeMnSi phase after the short solution treatment due to the reaction with the dissolved Si and Mg in the α-Al matrix. The peak ageing hardness was dominated by solution temperature, and the highest peak hardness was obtained under the short T6 heat treatment with the highest solution treatment temperature. The coherent β’’ precipitate with needle-like morphology was found after the short T6 heat treatment. The solution treatment in Al-Cu-Si-Mg alloys for 30/60 min at 500 °C dissolves sufficient quantity of Cu (~3wt%), and spheridises the Al2Cu and Al5Cu2Mg8Si7 phases. A large number of θ’ precipitates and Q’’ coexist inside the α-Al matrix after T6 heat treatment, contributing to the exceptional mechanical strength. In addition, the T5 heat treatment of Al-Cu-Si-Mg alloys were also studied. Due to the various Cu content in the α-Al matrix, the alloys were strengthened by β’’ in Al5Cu alloy, β’’ and θ’ in Al6.6Cu alloy, and θ’ and Q’’ in Al10.6Cu alloy. Depending on the alloy composition, these newly developed aluminium die-cast alloys exhibit mechanical properties that are comparable and even exceed those in existing aluminium die-cast alloys under similar T6 heat treatment.