تاثير پارامترهاي ريخته گري نيمه جامد و كوبشي بر خصوصيات ساختاري و مكانيكي آلياژ AZ91

چكيده انگليسي :

Effects of Semisolid and Squeeze Casting Parameters on Microstructure and Mechanical Properties of AZ91 Alloy Hashem Azarpira h azarpira@ma iut ac ir Date of submission 7 27 2011 Department of Materials Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisor Behzad Niroumand behzn@cc iut ac irAbstractSSR Semi Solid Rheocasting is a relatively new simple and low cost semisolid process for producingsemisolid slurries and ingots which rely on formation of a high nuclei density at the early stages ofsolidification Due to lack of research on this process especially for magnesium alloys in this study thefeasibility of SSR processing of AZ91 magnesium alloy was investigated The experimental procedure can bedivided into two stages In the first stage AZ91 alloy was subjected to three different melt treatmentprocedures before being cast into a steel die at three different temperatures i e a temperature above theliquidus temperature and two temperatures within the semisolid temperature range of the alloy The maintreatments included normal cooling to the casting temperature immersing a stationary cold copper rod intothe melt during cooling of the alloy ICR and inserting a rotating copper rod during cooling of the melt SSR The microstructures of the cast specimens were then characterized In the second stage the effect ofapplied pressure during secondary solidification of the semisolid slurries on the microstructure andmechanical properties of the specimens were studied and compared with those of the conventionally squeezecast specimens The result showed that non dendritic microstructures could be obtained with both ICR andSSR processes but SSR process generated smaller and more spherical primary particles In each method sphericity and grain size of primary particles increased with decreasing the casting temperature increasingthe solid fraction Formation of non dendritic primary particles in the microstructures decreased the hardnessof the alloys In the SSR process increasing the applied pressure had a little effect on the morphology andgrain size of the primary particles but it had a considerable effect on the intermetallic phases and eutecticstructure The results showed that strength of conventionally squeeze cast specimens increased withincreasing the applied pressure but similar trend was not observed for the SSR specimens Study of thefracture surfaces suggested this to be associated with the higher possibility of formation and entrapment ofoxide bifilms in the SSR processed slurries The effect of applied pressure on the hardness of conventionallysqueeze cast specimens was more than on SSR specimens Density of SSR specimens cast under 30MPapressure was more than that of conventionally squeeze cast specimens at this pressure Considering theresults it was recognized that SSR processing of flux protected magnesium melts was not a suitable methoddue to the high possibility of formation of oxide layers and entrapment of inclusions and flux in the meltwhich would result in loss of mechanical properties For utilization of full benefits of SSR process the entireprocess should be performed under a suitable protective gas Keywords SSR process AZ91 magnesium alloy Semisolid casting Squeeze casting Non dendriticmicrostructure Mechanical properties

آذرپيرا، هاشم

تاثير پارامترهاي ريخته گري نيمه جامد و كوبشي بر خصوصيات ساختاري و مكانيكي آلياژ AZ91

فرايند SSR , ريز ساختار غير دندريتي