Fundamental Research And Numerical Simulation On Spray Rolling Of7050Aluminum Strips

As a novel metallic semi-solid near-net-shape forming technology, spray rolling has manycompelling advantages over the traditional manufacturing processes, such as the energy andmaterial saving, the short production run and high-performance process for the metal industry.A comprehensive study on the technology of spray rolling and its process principles will helpto manufacture plates and strips with a fine grain size and excellent mechanical propertiesfrom liquid metal within one step.The process of spray rolling aluminum strips were dividedinto three different regions: atomization stage, deposition stage and semi-solid rolling stage.In order to provide a theoretical basis for achieving continue production with high productionrate and producing a high quality materials, the relative density model, shape evolution modeland heat transfer model during spray rolling have been established in this paper. Thecorresponding experimental discussions and analyses are conducted.The thermal history and dynamic behavior of droplets during atomization stage in sprayrolling have been established. A nonlinear quadratic is used to describe the relationshipbetween the gas/droplet slip velocity and the processing parameters. The qualitativeinfluences of the key processing parameters on the history of dynamic behavior and enthalpyof the gas and atomized droplets in spray rolling of7050aluminum alloy were investigated.Results show that the processing parameters such as spray distance, gas initial velocity,droplet diameter, metal mass flow rate have remarkable influence on process performanceincluding the heat transfer coefficient, droplet velocity, temperature and solid fraction.During deposition stage in spray rolling, the relative density of materials at the deposit/rollsurface during spray rolling was estimated and analyzed theoretically in detail based on themodel of dynamic behavior of droplets and solidification. This simulation study reveals therelationship among the processing parameters of spray rolling, average solid fraction, relativedensity of the deposit. The deposit shape evolution during spray rolling of7050alluminumalloy was investigated using mathematical models, tracing the coordinates of a growingdeposit with respect to time. Some important realistic scenarios, such as droplet reboundingand redeposition, the geometrical features of deposition surface were considered. Then thecalculated results of the shape evolution were utilized to predict the deposit thickness and thecorresponding entry angle. The effects of spray-rolling parameters on the deposit thicknessand drag in angle were studied and discussed. The heat transfer model was coupled with theshape model during deposition stage. By means of modeling, evolution of temperature filedunder stead state for the deposition stage during spray rolling have been analyzed and discussed. The simulated results are in good agreement with the experimental results. Theeffects of spray-rolling parameters on the evolution of temperature filed were studied anddiscussed. The porosity mechanism during spray rolling process corresponds to the coldporosity dominant regime. The solid fraction at the range of60～70%corresponds with ahigh density level of strips. Results show that the deposit shape and production rate aresensitive to processing parameters such as the roll gap, the roller diameter and rotationalspeed, the spray distance and the mass flux distribution in the spray cone. The optimumspray-rolling processing parameters were determined and presented by using an orthogonaldesign method. Results also show that the temperature of deposited materials decreases morequickly at the radial direction of the roll than that at the tangential direction of the roll. Thedeposit thickness and the enthalpy input from the spray of droplets are found to be thedominant influencing factors on the thermal profiles of the deposit. A suitable depositthickness, initial droplet temperature and preheating the roller surface will produce ahomogeneous deposit with uniform cooling and solidification condition.During semi-solid rolling stage in spray rolling, the relationship among the average solidfraction, relative density of the strip and thickness reduction ratio was investigated. A heattransfer model for the rolling stage during spray rolling has been established. This modelconcentrates on the effects of temperature on material properties and the effect of materialsdeformation on temperature. In addition, generic algorithms of coupling of temperature fieldand strain field based on updated Lagrangian model and mesh adaptivity methods weredeveloped. Strips have been prepared form gas atomized powders and spray depositedaluminum alloy by semi-solid rolling also taking account of materials system, droplet massmedia diameter, solid fraction and rolling condition. The feasibility of semi-solid rolling wasevaluated. The effect of heating temperature and holding time on the microstructuralevolution and mechanical properties of the rolled strips was investigated, as well as themicrostructural evolution of strips after sintering or post treating. The simulations agree withexperimental results well. The average solid fraction at the range of60～70%correspondswith a low thickness reduction rate and a high density level of strips. The densificationmechanism during spray rolling process corresponds to the removal of porosity dominantregime or the removal of prior droplet boundaries dominant regime. The main processingparameters, such as metal mass max flux, substrate pre-heating temperature and initial droplettemperature have a great influence on the thermal profiles during spray rolling. A suitabledeposit thickness, droplet temperature and preheating of the rollers before spray rolling will produce a homogeneous strip with uniform cooling and solidification condition. On reheatingthe gas atomized powders and spray deposited aluminum alloy into semisolid state, the phasechemistry and quantity of liquid were typically changed. As the heating temperature wasincreased, the disappearance of primary powder boundary and isolated pores, inter-diffusionof species and grain coarsening were accelerated during semi-solid powder rolling. Moreover,the amount of MgZn2phase decreases with increasing of heating temperature. During thesemi-solid powder rolling process, the mechanism of microstructure evolution changes fromthe densification dominant regime to the coarsening dominant regime with the increasingheating temperature. By means of―pre-sintering—cold deformation—terminal sintering‖, therelative density of the strip is92.6%and the microhardness is310HV. The activation energyof grain growth for the spray deposited7050aluminum alloy is about70.5KJ/mol. Spraydeposited7075aluminum alloy is resistant to coarsen due to the presence of particles. Theoptimum semi-solid heating temperature for spray deposited7050aluminum alloy is590.The rolling deformation has great influences on the elimination of pores, broken of grain anddynamic recrystallization. The elongation of the alloy semisolid rolled at580was thehighest up to13.31%. The tensile property of spray deposited alloy semi-solid rolled at550after solution treated for15h and annealed was the best with yield strength of555.6MPa, ultimate tensile strength of533.1MPa.