Effect Of Rare Earth Element Diffusion On The Solidification Structure Of Semi - Solid A356 Aluminum Alloy By Electromagnetic Field

Semi-Solid Metal Processing, for short SSM, is a new a forming technology that in the range of the solid liquid phase, using lots of methods to produce slurry, the dendritic grain was smashed which was formed during metal solidification, then direct casting or before remelting thixotropic casting to acquire granular organization which the shape is globular. Using this method has many advantages compared with traditional casting, such as casting internal microstructure denseness, low pore, low segregation, which can manufacture complex parts and can improve mechanical property and so on.Using A356 as the matrix raw material, adding rare earth element and applying electromagnetic stirring the effect of rare earth supplementation on the primary α phase of the semi-solid A356 aluminum alloy was researched. The effect of electromagnetic stirring and frequency on the rare earth distribution regular in the transgranular and grain boundary and at different radius was also studied.In this paper, 0.2%、0.4%、0.6% and 0.8%Ce as well as 0.3%、0.5%、0.7% and 0.9%Yb was added in the A356 to research the best rare earth addition. The refinement mechanism of refiner is whether it can form coherent boundary with matrix grain. The reason of the most appropriate addition amount of rare earth is related to composition of supercooling degree of the interface of the solid-liquid. Making a conclusion that under the circumstance of low degree of superheat and the frequency of 30 Hz and 15 s stirring time, the most suitable rare earth addition was 0.4%Ce and 0.7%Yb.Four stirring frequencies(10Hz、20Hz、30Hz and 40Hz) was selected to discuss the effect of the frequency on the primary α phase of the semi-solid A356 aluminum alloy and the rare earth distribution in the transgranular and grain boundary. The conclusion indicated that when the stirring frequency was 30 Hz, the average shape factor and average equal-area circle diameter was 0.8 and 76.1μm respectively which means the primary α phase was the best. The effect of frequency on the rare earth addition was also analyzed. If the frequency was too low or too high, the melt flow effectiveness and solute atom diffusion was bad, including stirring effectiveness decreased and existing micro-segregation.Under the circumstance of the stirring frequency of the 30 Hz and adding 1.0%、1.4%、1.8% and 2.2%Ce rare earth addition, not only the effect of whether bring electromagnetic stirring to melt on the rare earth Ce distribution at different ingot radius was compared, but also the rare earth Ce morphology was observed. The conclusion indicated that not bring electromagnetic stirring to alloy melt, the rare earth Ce morphology was needle-like and disorded which was worse than bringing electromagnetic stirring. Having electromagnetic stirring, the rare earth Ce addition presented regularity which was that from center to ingot edge the content of Ce was increased gradually until 60% ingot radius, then decreased severely. On the contrary, not bringing electromagnetic stirring, the rare earth Ce addition was unregular and appeared segregation in the part.