Study On Li Modification In A Hypoeutectic Aluminium Silicon Alloy

Hypoeutectic Al-Si alloys are widely used in industry.The properties of the alloys depend on its size and morphology.The modification effects of Li in Al-7Si alloy were studied in this paper.Effects of Li addition on the morphology,size of eutectic Si and properties have been worked out and the modification mechanism was discussed by means of the change in microstructures.The morphology,size of eutectic Si and were improved by solid solution treatment and aging,which can also improved properties of the alloys.By adding Li the size of eutectic Si decreased and the morphology of eutectic silicon can be improved,which is transformed from flake to coralline with plenty of branches.The phase AlLiSi emerge when Li cotent is 0.2wt%and 0.4wt%,which causes a decline of tensile strength and elongation.The strength and elongation of the alloy increased when Li was added.The highest strength,178MPa,and elongation,28.8%,of the alloy were obtained in 0.1wt%Li addition.More Li addition decreased.strength and elongation.Higher cooling rate can reduce size of Eutectic Si effectively,can not change morphology of eutectic silicon.Li addition can change morphology of eutectic silicon and reduce the size of eutectic silicon with increase in cooling rate.Eutectic Al in the alloy nucleated on primary ?-Al in no modification.Hovewer,Eutectic Al in the alloy nucleated by itself,grain size decreased and number of low angle boundary increased in eutectic area by Li modification.There is little twins in the eutectic Si in the alloy without modification.A huge amount of twins can be observed inside the modified eutectic silicon,which resulted in a change in preferential growth direction of eutectic silicon and formation of dendritics.The morphology of eutectic silicon can be improved by solid solution treatment.The eutectic silicons dissolved and broke,becomed sphere shape with increase in temperature of solid solution and time,which will be accelerated by Li modification.The yield strength of the alloy increased remarkably after solid solution at 540? for 3 hours and aged 200? for 6 hours.The yield strength of the alloy with 0.1 wt%Li was increased by 31%,that of the alloy with 0.2wt%Li was increased by 14%and that of the alloy with 0.2wt%Li was increased by 14%.