Research On Hardening And Toughing Heat Treatment Of Magnesium Alloys

Due to the low density,superior specific strength,cutting,damping and being able to be recycled of magnesium alloys,magnesium alloys are increaselingly applied in automobile structures to reduce weight and energe consumpsion under the pressure of environment protection.Nowadays,the main way to manufacture magnesium alloy parts is casting due to the possibility of producing parts with sophisticated shape and a higher production rate. However,materials made through casting are loose in microstructure and exist pores normally, thus deficiency of fatigue strength made it unsuitable for safety structures.Compared with casting alloys,improved mechanical properties can be achieved by extrusion,forging and rolling.Moreover,subsequent heat treatment can further improve mechanical properties to satisfy the needs of structural materials.In this paper,AZ31 magnesium alloy is manufactured by electromagnetic continuous casting(EMC) and strengthened by extrusion and heat treatment,the relationship between microstructures and mechanical properties was studied to get the optimized combination techanics of hardening and toughing.An electromagnetic continuous casting system and a protection system applied for magnesium alloys were designed and altered in this study.After several experimental analyses,proper technical parameters were obtained.Such as power-supply system(2500Hz, 10 kW),pouring temperature of 700～710℃,liquid level equal to upper margin of induction coil±2mm and the flow rate of cooling water from 1.0 m~3/h are usually used.Under the conditions mentioned above,AZ31 magnesium alloy billets with good quality are obtained. Soft-contact EMC billets have more fine and uniform grain structure compared with the direct chill casting(DC) ones.Moreover,in the conditions of Soft-contact EMC,precipitation ofβ-phase particles decreases and disperses.Mechanical properties of Soft-contact EMC billet are greatly improved.Hardness,tentile strength,impact absorbed energy and elongation of Soft-contact EMC billets increased by 11%,14.8%,24.5%and 71.5%,respectively, compared with DC ones.The fracture morphologies of Soft-contact EMC magnesium alloys have more characters of gliding fracture.Heat treatments were conducted to billets without extrusion,and the results show that: the hardness and ductility increases after solution treatment;the ductility decreases after ageing treatment while hardness increases;the optimized heat treatment technic is 420℃12h, impact absorbed energy and harness of billet under this technic increase to 3.75J and 51.60HB.Extrusion was conducted to electromagnetic continuous casting billets,the dimensions of extruded bars are 110×φ25mm,and subsequently heat treatments were conducted to extruded bars,results show that:hardness and tensile strength of extruded bars are 16.2%and 35.0% higher that those without extrusion,while elongation shows a 3.7%decrease;hardness of bars after T4 treatment increases at first and decreases gradually as holding time increases,while it decreases with holding temperature increases;hardness of bars after T5 treatment increases at first,then decreases gradually to a stable value as holding time increases;under T6 treatment, hardness decreases with holding temperature increases.With the same solution treatment temperature,hardness increases with ageing treatment temperature increases.Mechanical properties of bars after 360℃12h,170℃6h treatment decrease with tensile properties, elongation drop by 2.1%,7.6%and 47.3%,45.8%,respectively;The fracture morphologies of extruded bars show more characters of brittle fracture.