<p>In the last forty years, most of researches in casting fields especially in semi-solid metal state were dedicated to find new ways to enable near net shaped casting of Al alloys to improve the product properties and decreases the product cost. The thixoforming and rheocasting processes are presented as a ways by which the microstructure of the alloys can be changed to non-dendritic microstructure leading to improve the mechanical properties by mitigating the defect associated with the dendritic microstructure. Unfortunately, these processes have proved to be capital cost prohibitive and complicated for commercial production. Further, near net shaped casting of Al wrought alloys along with the superior properties and performance of these alloys have been a challenge for conventional casting routes due to the main disadvantage of hot tearing or hot cracking during solidification, which renders the cast component ineffective. To overcome the disadvantages of thixoforming and rheocasting processes, Controlled diffusion solidification (CDS) process was innovated to enable casting aluminum 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 cast the resultant mixture of the desired solute composition and temperature as a near net shaped cast product. The process lends itself to easy commercialization 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.</p><p>The CDS process has been proven to yield a cast product with a non-dendritic Al phase morphology and this dissertation presents the in-depth details and analysis of the various events occurring during the process to obtain a successful cast part. The process involves various inter-related events such as mixing, re-distribution of thermal field, redistribution of solute field, three types of nucleation events and growth of these different nuclei. Further the dissertation aims to present a study of the critical parameters such as temperatures of the two pre-cursor alloys, initial mass ratio of these alloys and the rate of mixing them on the effectiveness of the CDS process.</p> <p>The results from this study shows that mixing two precursor alloys to form the final desired alloy presents a natural environment for copious nucleation events aided by distribution of these nuclei by forced convection followed by the formation of unique cells in the resultant mixture (micro-scale) with significant thermal and solute gradients. The solidification in the CDS process is unique and different from conventional casting process in that initial growth of the nuclei takes place with the solute diffusing towards and temperature diffusing away from the solid/liquid interface which presents a favorable environment for a stable unperturbed growth of the solid/liquid interface resulting in a non-dendritic morphology of the primary AI phase.</p><p>The proposed events in the CDS process has been verified with a few Al based wrought
alloys and organic alloy systems.</p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/18982 |
| Date | 11 1900 |
| Creators | Abdul Amer Khalaf, Abbas |
| Contributors | Shankar, Sumanth, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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