Effect Of Solidfication Conditions On Microstructureand Mechanical Properties Of Zl208 Alloys

In the paper, by means of gravity casting, I uses resin sand molding casting to make slab castings. Then systematic study of the effect of wall thickness, chills, pouring temperature on microstructure and mechanical properties of ZL208 alloys that are in the as-cast or heat treatment state is done.Microcosnic organization of ZL208 alloys under different solidification conditions is observed with Olympus optical microscope and FEI Sirion scanning electron microscope. It is found that, the microstructure of ZL208 alloys is dendritic crystal,and the second phase is a network or a skeleton structure. After the castings are treated with T6, their issue become bulky,and the distribution of their second phase is more uniform.Reducing the wall thickness, applying cold irons and decreasing pouring temperature will reduce the grain size of the casting, the secondary dendrite arm spacing and the size of the second phase in the cast state. Especially when the wall thickness of the castings decreases from 20 mm to 10 mm, the ZL208 castings that are poured by the application of cold irons at 720℃ makes the secondary dendrite arm spacing decreased by 37.88%.After the heat-treatment of T6, by decreasing the wall thickness, applying cold iron and reducing pouring temperature, the average diameter of the dendrite is decreased, and the volμme fraction of the second phase is increased. Especially after the castings whose thickness is 20 mm are applyed with cold irons at 720℃, their second phase volμme fraction is increases by 102.92%.The research shows that, the decrease of the wall thickness, the application of cold irons and the decreasing pouring temperature can make the tensile strength, elongation and yield strength of the castings that are in the as-cast or heat treatment state(T6) gotten a degree of improvement at room temperature. In particular, the extension rate of the casting at the room temperature is improved. When the wall thickness of the castings that are in the as-cast state decreases from 20 mm to 10 mm, the room temperature elongation of the ZL208 castings that are poured by the application of cold irons at 720℃ is increased by 105.4%. after the castings whose thickness is 20 mm in the heat treatment state are applyed with cold irons at 720℃,their room temperature elongation increased by 183.70%After the castings are treated with T6,decreasing the wall thickness of the castings increases the high temperature tensile strength of the casting at 300℃.When the wall thickness of the castings decreases from 20 mm to 10 mm, the high temperature tensile strength of the ZL208 castings that are poured by the application of cold irons at 720℃ is increased by 10.30%, and the high temperature break elongation is also greatly improved. Applying cold iron has a lesser influence on the high temperature tensile strength and yield strength of the castings.Tensile fracture mechanism of the ZL208 castings at room temperature is cleavage fracture when the castings are in the as-cast or T6 state. However, the tensile fracture mechanism of the castings at 300℃ is ductile fracture when the castings are in the T6 state.