Study On The Technology And Theory Of A390 Alloy Hollow Billet Farbricated By Direct Chill Casting

The purpose of this study is to fabricate A390 alloy hollow billets with high quality by direct chill(DC)casting,which can be used in the automobiles' engine after hot extrusion and heat treatment to replace the traditional cast iron liner.In order to obtain A390 alloy hollow billet with the fine primary Si particles uniformly distributed,the stability of casting process and solidification structure of hollow billet were studied.The numerical simulation and casting experiments are combined to study the flow field,temperature field,solidification behavior,thermal contraction and effects of low-frequency electromagnetic field during the DC casting process of A390 alloy hollow billet with the cross-section of ?164 mm/?60 mm.The effects of core taper angle,relative height between the core and mold,feeding scheme,casting temperature,casting speed and low-frequency electromagnetic field on the physical fields mentioned above and solidification structure of hollow billets were discussed,finally the optimal technical conditions for DC casting of A3 90 alloy hollow billet were obtained.The numerical simulation results in DC casting process of A390 alloy hollow billets showed that the melt flow and the initial formation position of inner and outer solidification shell were not influenced by the change of core taper angle which,however,has effects on the temperature field and contact state between the inner solidification shell and core.With the decrease of core taper angle,the sump depth decreased,while the thickness of inner solidification shell increased significantly.In order to avoid the possibility of "hanging"because of the excessive thermal contraction of inner solidification shell,core taper angle should be set to no less than 1.5 degree.The contact height between the core and melt decreased with lowering the position of core,but the initial formation position of inner solidification shell was not influenced.The optimal uniformity of melt flow can be obtained when the upper edge of core and mold kept at the same level.The temperature field and thermal contraction were not affected,but the melt flow was obviously affected by the change of feeding scheme.The flow symmetry and uniformity of melt in the melt pool can be improved by the feeding scheme of modified four inlets.With increase in casting temperature,the melt flow velocity slightly increased,while the initial formation position of inner solidification shell and bottom of sump depth slightly moved down.With increase in casting speed,the melt flow velocity and sump depth increased,while the initial formation position of inner and outer solidification shell gradually decreased.When low frequency electromagnetic field was applied during the DC casting,a forced convection was produced in the melt,thus the melt temperature in the whole melt pool decreased below the liquidus,and the sump depth increased,the initial formation position of inner solidification shell obviously shift upward but its thickness became thin.The experimental results in DC casting process of A3 90 alloy hollow billets showed that the roughness of inner wall was improved with the decrease of core taper angle.The core taper angle should be larger than 1.5 degree but less than 5 degree to avoid the happening of"hanging" problem and melt leaking at the inner wall.Hot crack generated during the DC casting process when the upper edge of core was 15 mm lower than that of mold.The hollow billet can be smoothly cast when the upper edge of core and mold kept at the same level.The average size of primary Si particles was not affected by the changing of feeding scheme.Only by the feeding scheme of modified four inlets the primary Si particles were uniformly distributed in the hollow billet.The distribution homogeneity of primary Si particles in the hollow billet was improved and their size decreased when increasing the casting temperature.The roundness inner wall got worse because of partially remelting with the increase in casting speed.When the casting temperature kept at 1000 K,the homogeneity of primary Si particles in the hollow billet was improved,and their size decreased with increase in casting speed.When the casting temperature kept at 1050 K,primary Si particle size was not affected while the thickness of Si-depleted layer at the inner wall increased with the increase in casting speed.With the application of low-frequency electromagnetic field,the distribution homogeneity of primary Si particles deteriored and their size increased in the hollow billet.The optimal DC casting parameters of hollow billet were obtained based on the numerical simulation and experimental results as follows:the copper core taper angle is 3 degree,the upper edge of core and mold keep at the same level,the feeding scheme is the modified four inlets,the casting temperature is 1050 K and the casting speed is 100-110 mm/min.In these conditions,primary Si particles are uniformly distributed and the average size is no larger than 27.6 ?m in as-cast hollow billet.During homogenization and T6 heat treatment,the sharp edges and corners of primary Si particles became smooth,the eutectic Si particles size increased and the morphology became more rounded,Al5Cu2Mg8Si6(Q)and Al2Cu(?)phase precipitated out around the dissolved metastable phase Al8Mg3FeSi6(?).A few coarse primary Si particles fractured or became into smaller ones,while intermetallic phases and fine eutectic Si particles with more rounded morphology were uniformly distributed in the matrix after the extrusion process.The average tensile strength and elongation of A3 90 alloy pipes extruded&T6 heat treated are 386.9 MPa and 1.47%at room temperature(25 ?),and 186.7 MPa and 2.07%at high temperature(250 ?),respectively.The mechanical properties of A390 alloy pipes meet the technology requirements of a domestic automotive company.