الفهرس 
Abstract
Grain RefinementOnly 14 pages are availabe for public view
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Abstract
Preparation of zirconia (Zr02) was carried out via alkali fusion of zircon (ZrSi04) using sodium hydroxide to produce fused mass called frit. This frit was washed with tap water. The residue after water leaching was reacted with hydrochloric acid to fonn ZlrCOl1lUm oxychloride (ZrOCh”8H20). The white crystals of ZlrCOl1lUm oxychloride are used to produce zirconium oxide.
Aluminothem1ic reduction of the produced zirconium oxide in the presence of cryolite flux for the preparation of Al-Zr master alloy was investigated. AI-8.9Zr master alloy with 93.6 % yield was produced by soaking the charge for I h at 1150 DC using 20 % excess Al and 125% Na2F6AlofZr02.
The effect of Zr addition on the grain refinement of commercial pure aluminiurn (99.7%) and Al 6063 alloy has been in’lestigated in the present study. Addition of 0.3 wt.%Zr significantly decreases the average grain size of aluminium from 1100 !.un to 162 !-tm at holding time of 90 seconds and 740 Qc. Grains of AI 6063 alloy can be refined from 256 pm to 95 ~lm via addition of 0.2 wt. % Zr at holding time 90 seconds and 760 qc. The grain refinement effect of zirconium is found to be due to AI}Zr particles which are effective nucleating sites for the primary aluminium phase.
The effect of joint addition of Zr, Ti and Cr on the grain refinement of commercial pure aluminium and Al 6063 alloy has been investigated by optical microscopy and sca11l1ing electron microscopy (SEM) as well as EDS. It was found that, joint addition of 0.15 wt. % Zr and 0.025 wt. % Ti to Al can result in a remarkable refinement with average grain size about I 02 ~lm. Joint additions of 0.15 wt. % Zr, 0.025 wt. % Ti and 0.15 w C % Cr to Al facilitate better grain refinement and the average grain size is as fine as75 Ilm. Simultaneous additions of 0.25 wt. % Zr and 0.025 wt. % Ti to Al 6063 alloy results in an equiaxed grain structure and the average grain size is as fine as 55 I.UTI. Joint additions of .25 wt. % Zr, 0.025 wt. % Ti and 0.2 wt. % Cr to Al 6063 alloy drastically reduce the average grain size to 40 f..lm. The significant grain refining effect of joint addition of Z1’ and Ti containing master alloys is attributed to the formation of AI)(Tix, Zrl_x) particles. AI)(Ti” Zr1-x) particles reduces lattice parameter mismatch between a-AI and AI)Zr palticles and acting as potent heterogeneous nucleating sites for a-AI. The importance of Cr is mainly attributed to that, Cr acts as nucleate substrate for AI) Ti, AI)Zr andAl)(Ti”, Zrl_x) pmticles and improve the wettability between liquid aluminium and the grain refiners.
Cold rolled AI-Zr has a poor refining performance on the gram refinement of commercial pure aluminium. At a reduction level of 10% and addition level of 0.3 wt. % Zr, the average grain size was 400f..lm. Increasing the reduction level of AI-Zr master alloy to 20% accelerates fading of pmticles hence decreasing the grain refining efficiency. This is due to that, cold rolling provides a shOlter critical contact time compared to the as cast master alloy which result in fast dissociation of AI)Zr pmticles.