PHD THESIS: CONTROLLED DIFFUSION SOLIDIFICATION PROCESS (CDS) OF AL-7XXX WROUGHT ALLOYS: HEAT TREATMENT,MICROSTRUCTURE, AND MECHANICAL PROPERTIES

Abstract

Over the past decades, researchers in casting fields, especially in semi-solid metal state, have endeavored to find new ways to enable the Al wrought alloys of casting using the conventional casting processes; mainly in order to improve the product properties and decrease the product cost. The thixoforming and rheocasting processes have been presented as ways by which the microstructure of Al-base wrought alloys can be changed into non-dendritic, which in turn can lead to improvement to the mechanical properties. This can be because the effect of the non-dendritic microstructure on the mechanical properties of the material. Unfortunately, these processes have proved to be cost prohibitive and be a bit complicated for commercial applications. Further, conventional casting of Al-base wrought alloys along with their superior properties and performance have been a challenge for foundry industry due to the main disadvantage of hot tearing or hot cracking during solidification process. This can render the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled Diffusion Solidification (CDS) process was innovated mainly to enable casting of aluminum wrought alloys with a non-dendritic morphology of the primary Al phase in the resultant cast microstructure and thus alleviating the problem of hot tearing and obtaining a cost effective product with improved mechanical properties. The CDS is a simple process involving mixing of two precursor alloys of different thermal masses (temperature and solute) and subsequently casting the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process can easily be commercialized with a marginal capital cost required for set up such as the addition of an extra holding furnace. Further, the CDS process would prove itself to be unique in its ability to cast Al-based wrought alloys into near net shaped components without additional processes and cost. The originality of this study is to present a viable casting process for the Al-7xxx wrought alloys (Al-Zn-Mg-Cu); by which the Al-7xxx family alloys are presented in cast condition to have an acceptable uniaxial property range that is comparable with their wrought counterparts. This study presents the process and alloy parameters necessary for the casting of Al-7xxx wrought alloys (Al-Zn-Mg-Cu), by using the CDS process coupling with tilt pour gravity casting (TPGC) machine. The uniaxial tensile mechanical properties of several Al-7xxx CDS castings under various heat treatment conditions, namely, solutionizing (T4), peak aged (T6) and annealing (O), necessary for development of an ageing process on the material were investigated and presented. The tilt pour gravity casting process coupled with the CDS technology was employed to demonstrate the ability to cast Al-7xxx wrought alloys into high integrity components with high strength and ductility. The microstructure characterization was carried out by Electron Microscopy (TEM, SEM and EDS) and DSC test experiments for all the as cast (F), T4, T6 and anneals (O) conditions of the CDS cast components. Also, the predictive capabilities for the yield strength of Al 7xxx alloys CDS cast components was investigated using structural-properties modeling for the various strengthening effects that are recently proposed specifically for the Al-7xxx wrought counterparts. The study has successfully led to a more in-depth understanding of the innovative CDS casting process by applying it to several compositions of Al-7xxx wrought alloys in an industrial scale CDS casting experiments, using tilt pour gravity casting (TPGC) machine. This will hopefully lead us to a clearer path towards commercializing the CDS process and obtaining a viable casting process for Al-base wrought alloys into near net shape components without much change to economics of the casting process.