Study On The Master Alloys Prepared By Sub-rapidly Solidified Technology

The nice comprehensive mechanical property of metals and aluminium alloyscan be achieved by the grain refinement. Due to the remarkable grain refining effectby few additions of master alloys, the easy operating chemical refining method hasbeen widely used in industry. However, subject to the conventional preparationmethod, the widely used master alloys, Al-10T and Al-5Ti-1B, contain coarse Al₃Tiparticles in strip-like. All of these resulted in a poor grain refining efficiency, andthese kinds of master alloys can not meet the current demand of grain refining ofaluminium alloys. At the same time, a certain grain refining gap of master alloys withthe same elemental composition is existed between inland and abroad. This can beattributed to the microstructural characteristic of nucleate particles. Therefore, it ismeaningful to investigate and improve the nucleate particles Al₃Ti in master alloys.In this thesis, Al-Ti master alloys have been improved by the copper molddie-casting sub-rapid solidification （SRS） method, and remelting experiments hasbeen conducted with the SRS master alloys. The microstructural characteristics ofSRS and remelted Al-Ti master alloy were examined by scanning electronmicroscopy （SEM） and optical microscopy （OM）. Then grain refining experimentshave been conducted with the SRS master alloys. The performance and mechanismsof grain refinement of Al-Ti master alloy were investigated. The results are showed asfollows:1. SRS technology is a valid method to improve the microstructure of Al-Ti masteralloys. Generally, the size of Al₃Ti particles can be reduced to less than10μm, and theparticle shape changed from strip-like to short rod-like, blocky and petaloid. Above1200℃,with the increased holding time and melting temperature, the particle sizereduced, while the value of increased. Appropriate processing parameters weredetermined as: for Al-5Ti-1B master alloy, melting temperature of1250℃andholding time of30-35min, corresponding size and value of are2.91².95μm and0.70⁰.71, respectively; for Al-10Ti master alloy, melting temperature of1300℃andholding time of30-35min, corresponding size and value of are6.15⁷.09μm and0.55⁰.60, respectively.2. During the remelting process of SRS Al-10Ti master alloy, with the increasedholding time and remelting temperature, average size and the value of Al₃Ti particles increased. At the solid heating stage, Al₃Ti particles grow up by means ofexsolution precipitation and Ostwald ripening, and the growth trend is fading with theincreasing of holding time. At the beginning of solid-liquid heating stage, Al₃Tiparticles grow up by means of exsolution precipitation and Ostwald ripening.However, at later stage, Al₃Ti particles grow up by means of combination. During theremelting process of SRS Al-5Ti-1B master alloy, the shape change of Al₃Ti particlesis not very obvious, and no spheroidizing was found in Al-5Ti-1B. There can beexplained as follows, the moving of Al₃Ti particles is blocked by TiB2particles. Inaddition, the Ti come from the dissolve Al₃Ti particles could precipitated on the TiB2surface to form a thin Al₃Ti film. Therefore, the growth and shape change of Al₃Tiparticles are restricted due to the lack of Ti in the melt of Al-5Ti-1B master alloy.3. The grain refining efficiency of Al-5Ti-1B and Al-10Ti master alloys on Al andaluminium alloys can be improved by SRS method. Compared with common masteralloy, Al-7Si alloy refined with SRS Al-10Ti master alloy has an increase inconversion rate of columnar to equiaxed grain structure. The enhanced grain refiningeffect of SRS master alloys are owing to the fine Al₃Ti particles with high value of during the SRS process. These nucleate particles can dissolve rapidly and increase thegrowth-restriction factor Q to refine the grain size. The pure Al and Al-7Si alloywhich were refined by SRS master alloys showed higher microhardness than that ofby common master alloy. Compared to unrefined pure and Al-7Si alloy, themicrohardness of pure and Al-7Si alloy refined by SRS master alloys have beenraised about29.69%and35.17%, respectively. The Hall-Pecth （H-P） plot has beendrawn according to the determined grain size and microhardness of refined pure Al bycommon and SRS Al-5Ti-1B master alloy, result shows a basically fit with H-Pformula.