Grain Refinement Of Aluminum Alloys And TC4 Titanium Alloy By A Melting Control Technique

Based on previous research work, this thesis further investigates the feasibility and the validity of grain refining by a melting control technique and sums up the series of the research on this technique systemically. Using aluminum alloys to carry out the sand mold casting experiments, different alloy melt treatment techniques are adopted in the casting experiments for grain refining. One is the direct-pouring after remelting, the pouring temperature of which is～50oC above the alloy liquidus. Another is high temperature melt-holding prior to the pouring, in which the holding-temperature is～150 oC above the alloy liquidus for 1h, and then pouring at the same temperature as that in the former treatments. Through the compare of macro-structure examinations on sections of the tested castings, it can evaluate the effectiveness of the present grain-refining method and confirmed that the mechanism of the present grain refining is to take advantage of the possible"dendrite fragments"that may survive in the alloy melt for a period of heating up. To evaluate the sensitivity of grain refining to cooling rates, step-castings in sand molds with helps of chill irons and thermal insulation materials to make as large difference in cooling rates in the same casting as possible.Based on the experiments of the aluminum alloys, TC4 alloy is tested using the similar melting control technique. The TC4 alloy casting experiments were carried out in a vacuum ISM furnace with a step iron mold: comparing to a standard vacuum gravity casting; another one is carried out with some slices of the same alloy compositions added to the TC4 alloy melt before the gravity pouring. Through the observations of the macrostructure, the validity of the grain-refinements can be evaluated.In the present experiments, using a multi-channel computer data acquisition system, the cooling curves for the casting parts of different thicknesses are recorded and analyzed for the corresponding cooling rates. Through observations on the microstructures of the solidified step-castings in the different section parts, the relationship between secondary dendrite arm spacing and cooling rate are determined for both melt-treated castings. Through the tensile tests and the Bush-hardness tests, the influences of the remelting grain-refining on the casting mechanical properties are evaluated. The experimental results indicate that using the present remelting technique, the effect of grain refining is obvious and the corresponding mechanical properties can be enhanced to some extents...