Effect Of Grain Refinement On Microstructure And Properties Of Sand Mold Casting Al-Si Alloys

In this thesis,the effects of refinement on hypoeutectic/near-eutectic Al-Si alloys(commercial alloys ZL101/ZL101A/ZL104)cast in sand mold have been investigated.The changes of microstructure and properties of the well-refined alloys were studied.The La-B corefined samples were made by composition selection,sand mold casting,melt treatment,sample preparation and heat treatment.They were analyzed by microstructure observation,hardness test and tensile test,respectively.La-B co-refinement was applied to national standard sand mold casting Al-Si alloys,which changes ?-Al dendrites into smaller size of equiaxed ?-Al dendrites.It shows a limited effect of properties improvement in ZL101/ZL104 alloys due to the existence of casting defects.Also,the national standard heat treatment parameters are not suitable for the well-refined alloys.To avoid the effect of casting defects,the plaster mold with the same cooling rate of sand mold was used.It turns out that in as-cast condition,UTS of ZL101 increases to 154.7MPa and EI increases to 8.8%while UTS of ZL104 increases to 160.1MPa and EI increases to 8.2%respectively.The best heat treatment parameters for well-refined Al-Si alloys was determined as 535?×5h solution treatment and 175?×6h aging treatment which can make UTS of sand mold casting ZL104 increases to 278.6MPa and EI changes to 4.5%.Also,it can make UTS of plaster mold casting ZL104 increases to 280.1MPa and EI changes to 6.5%.The calculated Avrami exponent n decreases from 2.5 to 2.1,which demonstrates the major precipitation mechanism transforms from the diffusion-controlled one in ZL101A alloy to a combination of grainboundary precipitation and diffusion mechanism in ZL104 alloy.Also,the activation energy of the precipitation decreases from 16.6kJ/mol in unrefined ZL104 alloys to 11.6kJ/mol in refined ZL104 alloys.All of these are related to microstructure refinement including small grain and equiaxed ?-Al phase.E2EM model is used to calculate and predict the orientation relationships between LaB6 and ?-Al.And it turns out to be three possible relationships:(11l)Al//(l1l)LaB6,[011]Al//[011]LaB6;(l00)Al//(l00)LaB6,[0l0]Al//[0l0]LaB6;(110)Al//(110)LaB6,[00l]Al//[00l]LaB6.And the semi-coherent relationship is observed in the two-phase interface,which declares that LaB6 has the potency to act as effective heterogeneous nuclei for ?-Al grain to significantly refine the microstructure of Al-Si alloys.