Research On Deformation Mechanism Of Aluminum-steel Bimetal Parts By Thixotropic-core Compound Forging

The application of bimetal parts is regarded as a promising method to reduce global weight of machines.Compound forging of aluminum alloys and steel is promising for fabricating bimetal parts with a high performance and a reduced weight.However,reliable bonding is hard to be obtained in bimetal parts due to the nonsynchronous deformation of two materials during forming.In this paper,a thixotropic-core compound forging process for fabricating aluminum-steel bimetal part was proposed.By the means of this,the synchronous deformation is possible to be realized due to the satisfactory fluidity of the thixotropic aluminum core.The deformation mechanism of a standard cylindrical spur gear with modulus of3,tooth number of 17,was investigated in this work.The dies with a floating cavity was designed.The synchronous deformation mechanism of two materials was simulated by using Deform-3D v11.0.The effect of initial temperature and thickness of steel shell on the synchronous deformation was discussed.The results suggested that the two materials can flow synchronously due to the good fluidity of thixotropic core,and the deformation is mainly dominated by the pre-buckling behavior of the steel shell.The buckling process of the steel shell is mainly affected by the ratio of thickness to radius,and the ratio of thickness to length.As increasing the thickness of the steel shell,the unstable deformation can be avoided,and meanwhile the risk of cracking is reduced.However,when the steel shell is over thick,the thickness variation trend on horizontal direction is sharp,causing the billet flowing inward and outward synchronously.The thickness distribution of deformed steel shell in the bimetal gear was seriously uneven that,from the point of horizontal view,the tooth tip was much thicker than the initial thickness,and the tooth root was much thinner.On the vertical direction,the both ends were thick and the middle was thin.Increasing the thickness of steel shell is helpful to reduce the risk of cracking during forming.However,it is unprofitable for the lightweight.Further on,increasing the thickness of the steel shell will cause the remarkable elevation on the forming load.The initial temperature of the steel shell mainly affects the status of the aluminum core after the contact of two materials.When the temperature of steel shell is over high,the thixotropic aluminum core will be molten after contacting the steel shell,thus resulting in the disordered flowing of the core.Increasing the temperature of steel shell is helpful to reduce the forming loads,but it has almost no effect on the thickness distribution of the steel layer in the bimetal gear.When using a steel shell with a thickness of 4 mm,the shell and the core flowed synchronously during forming,and the bimetal gear can be deformed with no crack emerged on the surface.To optimize the thickness distribution of the steel layer in the bimetal gear,we considered to use a non-uniform thickness steel shell as the outer layer billet.Based on the finite element method,the simulation for using a non-uniform thickness steel shell in thixotropic-core compound forging was performed.A thickness ratio factor was induced for designing the geometric characteristics of the steel shell from horizontal and vertical directions.The simulation results suggested that using a steel shell with optimized-design shape is helpful to promoting the thickness uniformity of the bimetal gear.The thickness uniformity at the tooth root and tip reached to 6 2%and 86%,respectively.Meanwhile,the thickness of steel layer near both end faces in the bimetal gear kept higher than that of other regions,which is helpful to the combination of the aluminum core and steel shell.Further on,a twice-optimizing method for the manufacturing feasibility and thickness distribution was proposed in this paper.