The Effects Of Rare Earth And Boron On Microstructures And Properties Of AL-SI Casting Alloys

In this paper,the effects of lanthanum and boron additions on microstructures and tensile properties of Al-Si casting alloys were discussed by microstructure observation(OM?SEM?TEM etc.),thermal analysis during solidification and tensile test,respectively.The interactions among Ti,La,B and Sr elements in the Al-11Si alloy were investigated,whilst addition sequence of these elements was taken into consideration.The results indicate that the addition of 0.05 wt.%B induces a transformation of eutectic Si from finely fibrous to coarsely plate-like morphology in the 0.02wt.%Sr modified Al-11Si alloy,owing to the poisoning of IIT mechanism,and the eutectic Si grows only with TPRE mechanism.Titanium can neutralize the so-called co-poisoning effect between Sr and B in the Al-11Si alloy,and the eutectic Si grows with IIT mechanism again.Lanthanum can lead to the refinement of a-Al grains and neutralize the co-poisoning effect between Sr and B in the Al-11Si alloy,but the neutralizing effect of La is dependent on the addition sequence.In near-eutectic and hypereutectic Al-Si casting alloys,the addition of B can promote the nucleation of Si phases,leading to a higher density of unidirectioned Si twins,which causes the coupled eutectic zone to be broadened and eutectic Si phases to branch constantly into finer and more intense flakes.The close-packed plane({110})of LaB6 crystal has a good crystallographic relationship with the close-packed plane({111})of Si crystal,which means LaB6 may act as the heterogeneous nuclei to promote the nucleation of Si phase.There are also good crystallographic matching relationships for multiple lattice planes between LaB6 and the Fe-rich phase,which means that LaB6 may be taken as the nucleation substrates of the Fe-rich phase,and thus the Fe-rich phase acts as the heterogeneous nuclei to promote heterogeneous nucleation of Si phase.The Al-2La-1B master alloy was prepared by the in-situ reaction method taking industrial pure aluminum,Al-3B and Al-lOLa master alloy as raw materials.In permanent mold casting,except for the ZL101A alloy,the Al-7Si,Al-11Si and ZL101 alloys can be effectively refined with the addition of the Al-2La-1B master alloy.As for the sand mold,the addition of the Al-2La-1B master alloy can effectively refine the ZL104 alloy and improve the tensile properties.When sufficient Ti is added into Al-11Si-1.2La-0.6B alloy,LaB6 reacts with Ti to form TiB2,and then La precipitates in the form of La-rich compounds during the subsequent solidification.