Study On The Hot Deformation Behavior And Hot Extrusion Forming Technology Of AZ80 And ZK60 Alloy

As the requirements of producing large structural components, structural diversity and lightweight, wrought magnesium alloy will play an irreplaceable role in future traffic, aeronautics and astronautics, appliances and other areas. However, plasticity forming technology of wrought magnesium alloy develops relatively slow which greatly restricts their applications in structural field. Therefore, further studying on deformation behavior, formability, plasticity forming technology of wrought magnesium alloy has important theoretical and practical application significance for overcoming the current limitations of less product category and narrow application range, and wrought magnesium alloy as ideal light materials can be applied in more domains.The purpose of this paper is through studying on thermo-compressive behavior, microstructure evolution, formability and hot processing map of casting AZ80 Mg alloy and as-extruded ZK60 Mg alloy under different deformation conditions, and based on constitutive model and the evolution model of dynamic recrystallization, the hot deformation process and the microstructure evolution during typical extrusion parts of casting AZ80 Mg alloy and as-extruded ZK60 Mg alloy, multilayer shells and pipe, will be simulated. With the studying on the deformation of multilayer shells and pipe using the device designed by the author, the regularity of flow and microstructure evolution will be researched and the best condition for casting AZ80 Mg alloy and as-extruded ZK60 Mg alloy deformation will be determined, then magnesium alloy will be solid solution and aging treatment to improve the usability. So we can provide more reliable basis for the development and widely using of wrought magnesium alloy forming. For this, the following work has been conducted.Using the casting AZ80 Mg alloy and as-extruded ZK60 Mg alloy, uniaxial compression tests were carried out at the temperature range 240-440? and the strain rate range 0.001-1s-1 to study their plastic deformation behavior, and the accurate curves of stress-strain of AZ80 and ZK60 Mg alloy at different conditions were obtained, constitutive model, the evolution model of dynamic recrystallization and grain growth model at isothermal conditions were built, after the errors of the equipment, frictional effects and the affection of temperature were corrected.It provided effective calculating method and mathematical model for numerical simulation, predict flow stress and microstructure evolution. Hot processing maps of AZ80 and ZK60 Mg alloy were built at the temperature range 240-440?, the strain rate range 0.001-1s-1 and the strain range 0.1-0.6. It is shown that the vast majority regions in hot processing maps of AZ80 Mg alloy was the plastic processing region, plastic instability region changes with different deformation, and plastic instability region mostly distributed in high strain rate and low temperatures regions or high temperature regions. The good processability region of ZK60 Mg alloy is the temperature range 280-420? and the strain rate range 0.003-0.2s-1.Different cavity profile was researched on forward extruded tubes through combination of corresponding numerical simulation and physical simulation. The results show that when a is 60°-70°, the extrusion force is less and the flowing velocity of metal is better. Cone die adopted, minimal force and highest utilization are obtained for actual production. The 70° cone die has the least extrusion force. Numerical simulation technique is employed to analyz and forecast the influence rules of process parameters on forming force and microstructure during hot extrusion process of AZ80 multilayer shells and ZK60 pipe. The results show that extrusion force is proportional to extrusion speed, extrusion ratio and coefficient of friction, and inversely proportion to billet temperatures; As deformation temperatures elevating, grains grow, the grains are refined as extrusion speed and deformation degree increase. So, higher extrusion speed and extrusion ratio, lower billet temperature should be selected in order to obtain fine and uniform microstructure.The extrusion forming experimental facilities of AZ80 multilayer shells and ZK60 pipe were designed and manufactured by analyzing the extrusions structure and shape features and calculating deforming forces, and extrusion forming experiments were carried out.The backward extrusion of AZ80 multilayer shells was researched and the results show that formability of AZ80 Mg alloy is.better when the billet diameter is about 75mm; But cracking defects occur near the shells'bottom because of too much extrusion force when the billet diameter is greater than 88mm, the deficiency of eccentricity occurs because of the larger deviation of center line when the billets diameter is less than 65mm; AZ80 multilayer shells parts can obtain better appearance quality when the die temperature is constant than 290? and the billet temperature is greater than 350?; Extruded alloy after deformation obviously refines grains, and mechanical property decreases firstly, then increases subsequently with increased billet temperatures; Formability and mechanical property of AZ80 multilayer shells parts gradually increase with the die temperature increases when the billet temperature is constant temperature. So, better comprehensive performance of extruded AZ80 multilayer shells could be obtained at the die temperature range 320-350? and the billet temperature range 350-410?.The hot extrusion of ZK60 pipes was researched and the results show that increasing extrusion speed is beneficial to the grain refinement,but increases extrusion force. It is found that the reasonable extrusion speed is about 1.5mm/s through analyzing pipe extrusion experiment and metallographic structures; Through the experiment during temperature range 270-390?, it is found that extrusion force will decrease while grain sizes will increase when billet temperatures rise;The extrusion temperature range 300-360? is conducive to isothermal-extrusion process of ZK60 pipes on taking extrusion force and grain sizes into consideration, and when the billet temperature is 330? and the die temperature is 300?, grains of ZK60 pipes are very fine and homogenous, and extrusion force is more reasonable.The microstructure property of hot deformation AZ80 Mg alloy was investigated after solution treatment and aging temperature. the results show that the coarse, hard and brittle second phase ?-Mg17Al12 at grain boundary uniformly dispersed in matrix with temperature increasing.and because the second phase has dissolved, strengthening phase of Mg alloy is decreased or even disappeared. So plasticity of Mg alloy is improved, but strength decreases; The number of ?-Mg17Al12 and the percentage of continuous precipitated phases gradually increases with prolonging of aging time which causes tensile strength of Mg alloy to gradually increase, and elongation of Mg alloy apparently decrease. The fracture morphology of AZ80 Mg alloy after solution treatment is characterized by a certain plastic deformational quasi-cleavag, but the fracture morphology after aging temperature is characterized by cleavage-based brittle rupture, and the rupture generally occurs in weak grain boundary and between lamellaes of ?-Mg17Al12 precipitated phases.