Research Of Refining Primary α Phase By Multiple Al-RE Eutectic Reactions In Semi-solid A356Al Alloy

SSM that referred to semi-solid forming of metal, is a new shaping technology that by castingsemi-solid metal slurry or the other ways. It’s one of the most promising processing technologies ofmetal materials in21stcentury. To produce the qualified semi-solid metal slurry with non-dendriticmicrostructure is the key of SSM, specifically with tiny, uniform and spherical grain. In thiscontext, A356aluminum alloy, a hypoeutectic Al-Si alloy, was selected as the object of thisresearch. The effect of add rare earth to refine A356Al alloy and its refining mechanism werestudied. Under the main current of energy saving and green manufacturing, used self-madeelectromagnetic stirring device to stir the alloy, which frequency below50Hz. In order to preparethe semi-solid A356Al alloy under the condition of multiple RE and electromagnetic stirring, theeffectofelectromagneticstirringtothecasting ingotwasdiscussedin detail,as well.Three rare earth elements Ce, Y and Gd were selected as the A356alloy’s refiner and holdingthe liquid metal up and below the eutectic temperature10℃for2minutes in the melting alloyprocess, then process the pouring process. Optical microscopy (OM), X-ray diffraction (XRD),energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM) were used toanalyze the refining mechanism of rare earth, and Bramfitt’s planar disregistries theory was used toexplain the refining effect of Al-RE eutectic reaction, which induced by RE, to the A356Al alloy.The results indicate that: both α-Al and compounds, products of eutectic reaction between rareearth and aluminum, can be the site of heterogeneous nucleation for primary α phase. The increaseof heterogeneous nucleation’s core is the main reason of alloy getting refined. The proper additionamount of Ce is0.5wt%, and the average grain size of primary α phase in A356alloy is28.46μm,the average shape factor is0.75; The proper addition amount of rare earth Y is0.5wt%, and thegrain size of primary α phase in A356alloy is33.60μm, the average shape factor is0.74; The mostproper addition amount of Gd is0.8wt%, and the average equal-area circle diameter of primary αphaseis37.90μm,theaverageshapefactoris0.79.Based on the abovestudy, the self-madeelectromagnetic stirring device, which was controlledby changing the frequency, had been used to slightly stir the A356-0.5Y alloy for a short time.Then the low superheat casting technology and proper insulation process have been carried on. Theeffect of stirring frequency on morphology, grain size and distribution of the primary α phase in thesolidification microstructure had been researched. The result indicates that: due to the skin effect and effect of frequency on the power or speed of stirring, the frequency has a significant impact onmicrostructure of primary α phase, which along the radial direction of the cast ingot. The primary αphase’s grain size and morphology of edge area are better than the core area’s, and Y tends togather in the periphery of ingot. Liquid alloy that poured at620℃and slightly stirred which underthe condition of30Hz and15s, then holding at590℃for5min, thus the discrepancy ofnonuniformity along the radial direction become smaller. The average equal-area circle diameter ofgrain in the core area is64.95μm and the average shape factor is0.80; On the contrary, the averageequal-areacirclediameterof grainintheedgeareais58.98μm andtheaverage shapefactoris0.83.In the condition of above-mentioned electromagnetic casting process, multiple rare earth Ce, Yand Gd were added as refiner. Three different Al-RE eutectic reactions could be carried out alongthe cooling process before casting, and then the multiple effects of refining alloy were completed.The proper addition proportion of multiple RE and effect of different RE on the morphology andgrain size of primary α phase in semi-solid A356alloy were researched by orthogonal experiments.Results show that the most proper addition of multiple RE is0.2wt%Ce+0.4wt%Y+0.4wt%Gd, thusthe average equal-area circle diameter of semi-solid A356alloy is50.52μm, and the average shapefactor is0.83. The planar disregistries between primary α phase and compound of RE andaluminum, which is the product of eutectic reaction induced by rare earth, and its density have acertain degree of influence to the effect of refining Al alloy. The smaller planar disregistriesbetween them, the better refining result is got, and the bigger difference between densities ofcompound and molten aluminum alloy, the worse aggregation or sedimentation it is, which willhavenegativeeffectonrefiningalloy.The innovation of this article are using the theory of planar disregistries to systematicallyexplain the mechanism of RE refining A356alloy and presenting some issues of selected rareearth as refiner. Then after detailly studied the electromagnetic stirring process, the refining effectof three different RE on semi-solid A356alloy under the condition of electromagnetic stirring andmultiple RE was researched. A solid theory foundation is prepared to explore the new refinementmechanismofrareearth andcreate themultipleRE refinerwith multiplerefiningeffects.