| Aluminium alloys with copper are more and more widely used in the industrial manufacture due to high specific strength, good decay resistance and formability, especially applied as significant material for cylinder blocks and cylinder cover in the automobile industry. Although copper benefits strength improving greatly for aluminium alloys, meanwhile it can severly reduce the alloy’s hot tearing resistance, leads to higher rejection rate as well as the cost. Correspondingly, numerous studies prove that copper acts as "tearing element" by enlarging solidifying range of the alloy. Amass of reports put forward theories to elaborate the hot tearing forming mechanism and explore the solving methods. It has been widely agreed that refinement is one of the best ways to reduce the hot tear tendency, and especially the refinement of primary a-Al structure can effectively release the shrinkage stress and ease the hot tearing by delaying the point of dendritic crystals overlap and increasing the liquid feeding. With continuous studying, researchers recognize hot tears probably develop within the eutectic reaction, since the eutectic grains appear in the last solidifying stage, which means eutectic structures are closely related to alloy’s hot tearing susceptibility. It is obvious that eutectic cells occupy the majority volume of the solidified structure in aluminium alloys with composition near eutectic point(12.5wt.%Si),which shifts the hot tears totally below the primary α-Al solidifying temperature range. As a result, the eutectic phase exploring is more critical to remove the hot cracks defects than primary a-Al dendrites with regards to alloys near Al-Si eutectic composition. However, due to difficulties in observing eutectic cells and characterizing solidifying phase, little information about eutectic cells size or morphology has been reported currently.Considering Al-Si-Cu casting alloys tend to hot tears which limits their extensive use, we choose Al-10 wt.% Si-2 wt.% Cu(Al-10Si-2Cu) as target alloy(to obtain eutectic cells easily) and add 1 wt.% new type master alloy(Al-10 wt.% Si-2 wt.% Fe) inoculant to refine the eutectic cells. By contracting, we are aimed to explore the hot tearing alleviating mechanism which is focused on the impact of eutectic refinement, so as to find methods to remove hot cracking defects of Al-Si-Cu castings. Contrasting alloys are Al-10 Si-2Cu base alloy, exclusively Sr-modified alloy, further inoculated alloys after Sr modification, respectively. We obtain eutectic cells with flatbed scanner and metalloscope, characterize solidifying behavior by thermal analysis, observe AI2Cu, β-Al5 FeSi microstructure with SEM and qualitatively evaluate hot tear susceptibility by Constrained Rod Casting(CRC)mould.The experiment results indicate eutectic cells size with Sr and refiner treatment is reduced to 1/5 compared to exclusively Sr modified ones, as well as the macroscopic casting porosity size and density. The cooling curves show the lift of eutectic platform, which certifies the effect of refiners in promoting eutectic nucleation. Contrasting HTS(Hot Tearing Susceptibility) of the above alloys displays the rule that hot tears are greatly reduced with eutectic cells refining. Because intermetallic compounds also act as the vital hot tearing factor, AI2Cu size is observed remaining unchanged, while β-Al5FeSi phase is getting samller after inoculation, helping reducing hot tear tendency. In addition, thermal analysis reveals the master alloy refiner will lose its function in aluminium alloys containing copper with time passing. With the help of SEM, we find that AI2Cu is likely to "capture" rich-Fe phase, causing the reduction of (AlFeSi) clusters forming and the recession of refinement. It is noticeable that there is no impact deteriorating mechanical performance with master alloy addition containing Fe since the alloy’s tensile strength is enhanced as well.This paper develops mechanism about impacts of eutectic refinement on hot tearing, including two parts as following:1.Liquid feeding mechanism. Intermetallic compounds, such as Al2Cu and β-Al5FeSi, can easily block the liquid flowing path and reduce the permeability of semi-solid metal, as a result increase the background porosity of the alloy. Smaller eutectic cells play a key role in improving the liquid feeding by pushing off the overlap of grains and suppressing pores nucleation. Meanwhile that β-Al5FeSi size is also reduced with eutectic cells refinement helps ease hot cracking.2.Semi-solid metal plastic forming mechanism. Smaller semi-solid eutectic cells can rotate and slide more easily to partially release shrinkage stress with the help of more liquid films and less obstruction by primary a-Al dendrites. However, when the solid fraction is too high, liquid pools are totally isolated and liquid feeding can no longer work, more liquid films in unit cell due to eutectic refinement will not only assist eutectic cells rotating, but also benefit in dispersing solidifying stress concentration. |
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