بررسي تاثير نوع سيستم راهگاهي بر ايجاد اكسيدهاي دو لايه و استعداد به پارگي گرم آلياژ A206

چكيده انگليسي :

Study of the Effects of Gating System Type on Formation of Oxide Bifilms and Hot Tearing Susceptibility of A206 Alloy Hamed Bagherpoor Torghabe h bagherpoortorghabe@ma iut ac ir Date of Submission 7 30 2011 Department of Material Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisors Behzad Niroumand behzn@cc iut ac ir Maryam Karbasi m karbasi@cc iut ac irAbstractHot tearing is a common defect which occurs during solidification of cast alloys with large solidificationrange Alloy composition solidification mode casting design melt cleanness and resistance of the mold tofree solidification shrinkage are among the main factors affecting the hot tearing tendency of castcomponents Determination of the hot tearing tendency and the effective parameters are of great interest toresearchers in this field Structural defects such as oxide bifilms influence the hot tearing occurrence Poorlycontrolled molten metal transfer and poor gating system design are the main factors causing the formation ofthe oxide bifilms Bifilms are the result of enfolding of the liquid surface into the bulk melt due to meltsurface turbulence Since the melt surface is usually covered by an oxide film fragmented oxide films mayentrain in the melt under turbulent surface conditions In such case the dry surfaces of oxide films may comein contact hence the name bifilm and act as cracks for initiation of hot tears during solidification Bifilmscan remain suspended in melts for long periods of time and enter the mold upon pouring The reduction ofbifilm content of castings can be achieved by new gating system designs and possible use of filters In thepresent research the effects of three types of gating system design on occurrence of hot tearing in A206aluminum alloy was studied by computer simulation using PROCAST software and real casting experimentsin T shaped constrained and unconstrained sand moulds The tensile forces required for tearing andfracturing of the castings in constrained and unconstrained conditions as well as the torn and fracturedsurfaces of the constrained and unconstrained specimens were analyzed for each gating system design Theresults showed that the type of gating system influences the velocity and surface turbulence of the melt andconsequently influences the formation of oxide bifilms and hot tearing susceptibility of A206 aluminumalloy The results of the mould filling and solidification simulations of the castings showed that the naturallypressurized gating system with surge control had resulted in lower surface turbulence during mould filling decreased hot tearing susceptibility based on the Clyne and Davies criterion and increased cooling rate of themelt The experimental results of constrained mould castings confirmed the prediction of Clyne and Daviescriterion for hot tearing susceptibility as the most severe hot tearing occurred with the pressurized gatingsystem and the least severe hot tearing occurred with naturally pressurized gating system with surge control It was also shown that the naturally pressurized gating system with surge control resulted in less oxide bifilmformation and smaller grain size in the hot spots of the castings and consequently in improved tensilestrength of unconstrained castings improved resistance to hot tearing of constrained castings and the higherdensity and hardness in both castings conditions Keywords Hot tearing susceptibility Oxide bifilms Gating system design Casting simulation A206 alumimum alloy Mechanical properties

باقرپور طرقبه، حامد

بررسي تاثير نوع سيستم راهگاهي بر ايجاد اكسيدهاي دو لايه و استعداد به پارگي گرم آلياژ A206

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