Numerical Simulation Of Transient Physical Fields Of Semi-continuous Casting Of Large Scale Slab Of Magnesium Alloy

Wide sheet and strips are the main products of wrought magnesium alloys,bearing high value and broad prospect in large scale.The industrialized route including slab casing,hot rolling and coil rolling should be the optimum combining scale and cost.It is known to all that the DC casting is the main schedule to produce the large size and high quality magnesium slabs for the rolling route.To date,hot crack is one of the primary technical problems in wide magnesium slab manufacture.Stress and strain fields in DC casting are key points to optimize the casting crystallizer design and determine the processing widow.These stress and strain fields can be well investigated by numerical simulations,which are full of significance.AZ31B is the main magnesium alloy designation for rolling sheet and strips.This work firstly investigated the effects of Ca on the hot cracking sensitivity of AZ31B as well as those of Y on ZK60.On the basis above,the unsteady stress,strain behaviors and hot cracking tendency of a 400 mm×1450 mm magnesium slab during the base feeding and start-up stages.The main conclusions are as following:The predicting result by Cylne-Davies mode shows that AZ31B-3Ca and ZK60-0.5Y have the lowest hot cracking sensitivity.The results of thermal contraction experiment show that the hot cracking tendency of AZ31B rises firstly then declines with the increasing Ca addition,while that of ZK60 rises with the increasing Y addition.The predicted results of CSC agree well with the experimental results of thermal contraction.The numerical simulation result of the base feeding stage shows that a base holding time and liquid level height of 40 s and 50 mm,respectively,can keep the lowest hot cracking tendency for the ?160mm AZ31B billet.During the base feeding stage,the sequence of hot tearing tendency among the three investigated alloys is ZK60>AZ31B>AZ80.In the simulated alloy,AZ31B-3Ca and ZK60-0.5Y have the lowest hot cracking sensitivity The large scale magnesium slab(400 mm×1450 mm)has the lowest hot tearing tendency when the base holding time and liquid level height are 60 s and 150 mm,respectively.The numerical simulation result of start-up stage shows that the casting speed has a remarkable impact on the hot cracking of large scale 400 mm×1450 mm magnesium slabs.A fast casting speed can accelerate the flow velocity of melt,deepen the liquid cave,and enlarge the equivalent stress,strain and CDI.However,all the CDI of solidification risk point are lesser.Thus,the maximum speed for a safe start-up can reach as fast as 36 mm/min.Increasing casting temperature(around 930?970 K)can also deepen the liquid cave and have a weak influence on the equivalent stress and strain,but the maximum value of CDI rises firstly then reduces.Thus,the hot cracking tendency is lowest at 950 K.When the electromagnetic frequency increases(from 10 to 30 Hz),the magnetic induction intensity and Lorentz force distribution get uniform inside the liquid cave,despite of the enhanced skin efect The magnetically forced convection difference along the width direction also reduces correspondingly,as well as the convection intensity along the thickness direction.The equivalent stain and maximum value of CDI at slab center are significantly decreased at a higher frequency,so does the equival ent stress difference at the slab surface.The penetrating depth of magnetic induction intensity and the Lorentz force get increased after increasing the coil ampere-turns(from 12 to 36 kAt).Correspondingly,the maximum melt flow velocity get promoted,but the melt flow direction is hardly changed.However,the distribution of magnetic induction intensity,Lorentz force and melt flow get nonuniform.Meanwhile,the liquid cave depth increases,so do the equivalent strain and the maximum value of CDI at the slab center.When the coil ampere-turns is 12 kAt,the hot tearing tendency is lowest.