Study Of Strontium And Manganese On Neutralizing The Detrimental Effect Of Iron Phase In Casting Al-Si Alloys

Aluminium-silicon casting alloys are widely used in such industrial fields as aerospace engineering and automotive manufacture.Iron is the most common impurity in Al-Si alloys,and the fragile iron intermetallic phase platelets deteriorate the mechanical properties.Now,the purification effect is unperfect with iron removal method in industry,so the key of eliminating the harmful effect is the morphology of iron phase could be transformed.In the thesis,addition of manganese and strontium were made to improve the morphology of iron phase in Al-Si casting alloys.The effects of these elements additions on the microstructure and mechanical properties in the alloys were investigated comprehensivly by XRD,OM,EPMA and INSTRON5882 EUMTS,etc. The mechanism of Sr,Mn and their interaction on iron phase were analyzed.The influencing factors of precipitation and morphology of iron phases and the effect ofβ-Fe platelet and Sr compound on the formation of porosity are also discussed.The results showed that Sr addition can't transform iron phase from needle-likeβto Chinese script-likeα,but cause a phenomena of bend,crack and dissolution on iron phases.The addition of Mn favors the formation ofα-Fe intermetallics,but also increases the total volume percentage of iron-rich intermetallic phases.The number of script-likeαincreases at the expense of needle-likeβwith increasing Mn level.When Sr plus Mn addition,the role of Mn on iron phse is more obvious than Sr and the effect of fragmentation and dissolution of Sr is likely suppressed by Mn.Fe content is the most influential factor of the mechanical properties in Al-12Si alloys,Mn takes the second place and Sr is the least.When the alloy is added with Sr plus Mn,the detrimental influence of iron is counteracted,as a result,the mechanical properties are improved.The optimized addition amount isω(Mn)/ω(Fe)=1.1～1.3 andω(Sr)= 0.12%.In the microstructure of Al-Si alloys,the common types of iron phases include long needle,short needle,coarse Chinese script,fine Chinese script and fishbone-like. Long and short needle-likeβ-Fe phases are formed by binary eutectic Al-βand ternary eutectic Al-Si-βrespectively.The latter three iron phases belong toα-Fe, which are formed by binary eutectic Al-αand ternary eutectic Al-α-β.Theα-Fe phase, precipitated in alloy with Mn addition,actually is AlSiFeMn intermetallics,which due to solid dissolution and growth of Mn inβ-Fe needle.Blocky Al₂Si₂Sr compounds were observed to have frequent physical contact withβ-Al₅FeSi needles in the microstructure.The majority of pores are distributed in accumulation regions ofβ-Fe needles,but the regions containing Chinese scriptα-Fe particles are almost free of porosity.Theβ-Fe needles are potential sites for porosity formation regardless of the alloy composition.The Sr compounds particles were seen inside or around with the pores,which indicates that the Sr compounds play a role in pore formation,and that this is probably one of the reasons for the widely distributed porosity in Sr-modified Al-Si alloys.