Numerical Simulation And Experimental Study On Thixo-Rolling Process Of7050Aluminum Alloy

As an advanced metal forming method, Semi-solid metal forming technology (SSM) notonly has the good formability as liquid metal processing but also has good mechanicalproperties as solid metal processing. In order to study the coordinative deformationmechanism of solid and liquid in thixo-rolling process and obtain the rolled strips withexcellent performance, the thixo-rolling process of7050aluminum alloy was studiedsystematically in this work. The semi-solid billets were prepared by Strain Induced MeltActivation method (SIMA) and tested at Gleeble-3500isothermal simulation testing machineby using cylindrical specimens which were cut into certain size from the billets. Athixo-forming constitutive model of7050aluminum alloy was established by analyzingrheological properties and microstructures evolution during compression. Then, thethixo-rolling process of7050aluminum alloy was studied by using finite element simulationwhich based on the platform of MSC.MARC software. Finally, the processes of single-passand multi-pass thixo-rolling were studied.The main results are as follows.Ideal semi-solid7050aluminum billets were prepared by SIMA method with hotextrusion as pre-deformation. The optimum parameters are as follows: the extrusion ratio isabout11.11, isothermal temperature is about585℃, and isothermal time is about25min. Inthe middle of isothermal procedure, different curvatures lead to partial melting andsolidification finally resulting in the same curvature of the solid particles, and thedisappearance of small particles and the growth of large particles. During this procedure, theaverage diameter of solid particles and the isothermal time approximately obey the equation:d2.5d2.50=Kt, where K is68.65um4/s at585℃and89.948um4/s at595℃respectively,which also shows that as temperature increases, the coarsening rate of solid particlesincreases.The compression test of semi-solid7050aluminum alloy was conducted at Gleeble-3500thermal simulation testing machine, and the thixo-deformation true stress-strain curves wereobtained. By using temperature jumping experiments, strain rate jumping experiments anddifferent deformation experiments, it proves that stress of semi-solid7050aluminum alloy has a close relationship with temperature, strain rate and strain.A viscoplastic constitutivemodel during thixo-deformation was established through regression analysis In addition, theresults for liquid and solid phase evolution, which compares semi-solid with regular casting7050aluminum alloy, shows that the deformation, temperature and strain rate have significanteffects on microstructures and liquid distribution. Furthermore, due to the remarkablemicrostructural difference between semi-solid alloy and casting alloy, the crack propagationtypes are also different, which corresponding to solid-liquid separation process andsolid-liquid, solid-solid mixture separation process, respectively.Numerical simulation results of thixo-rolling process show that the simulated rollingforce agrees with experimental one very well, which indicates the established constitutivemodel is reasonable and correct. At last, an analysis of orthogonal experiment results wasconducted. The results show the roller temperature, billet temperature, compression ratio,roller diameter is the main factors that affect the temperature gradient and rolling forceremarkably.During single-pass thixo-rolling process, the material deformation strongly depends onthe liquid fraction. Proper preheating of rollers can significantly reduce the rolling force.Increasing rolling temperature results in increasing amount of liquid phase and decreasedeformation of solid phase. The deformation at the center of a strip increases with inceasingrolling temperature and liquid obviously flows to the edge. When roller temperature is350℃rolling temperature is540℃, and the deformation is50%, the maximum Vickersmicro-hardness of the strip is about324.5HV.During multi-pass thixo-rolling process, spherical particles become flat due to the plasticdeformation. When thixo-rolling temperature is540℃, the tensile strength of strip is472MPaand the elongation is9.8%. The fracture surface of the tensile samples shows that its fracturemechanism is microvoid accumulation fracture.