Investigation On Deformation Technology, Microstructure And Mechanical Properties Of Twin Roll Cast Magnesium Alloy

Magnesium alloys are considered to have the most promising development outlook as metallic structural materials in 21^st century,for their high specific strength and specific stiffness,good damping capacity,excellent machinability and high recycling rate.Magnesium alloy has become one of the research hotspots in materials research field.The principal direction to improve the integrated properties of magnesium alloy is to satisfy the requirements of some structural materials,such as aluminum,or even steel.Researches which aim to increase the elongation,decrease the anisotropy and improve the plastic deformation capability,simplify fabrication technics,reduce manufacturing cost and expand the applications of magnesium alloy are also received global attention.It is difficult to fabricate magnesium alloy sheet for its poor technologic stability, low elongation,severe anisotropy and poor plastic deformation capability.Twin roll casting(designated as TRC in short) technology has fast cooling rate and plastic deformation effect under semi-solid states.It combines casting and hot rolling into a single step,which can refine grain and second-phase size,reduce segregation, improve plastic deformation capability,and also shorten the fabrication process of magnesium alloy.This paper aims to improve the integrated properties,reduce anisotropy of magnesium alloy by optimizing the rolling and heat treatment parameters.The forming mechanism of TRC magnesium alloy's microstructure during strip casting process,the microstructure evolution of ZK60 alloy sheet during rolling and heat treatment process,and factors which influence the microstructure were studied systematically.In addition,the strengthening mechanism of warm rolled magnesium alloy was also discussed in this paper.With the above research,the experimental and academic foundations of helping improve the formability, microstructural uniformity and integrated properties of magnesium alloy sheet were obtained.ZK60 and Mg-4.5Al-1.0Zn(designated as AZ41 in short) strip with 3～3.5mm thickness were fabricated by TRC technology in this paper,and the fabrication parameters of TRC temperature,rolling speed and roller gap were also determined (those of ZK60 and AZ41 strip are 640～680℃,5～6rpm,2mm and 630～650℃, 5～6rpm,2mm,respectively).The forming mechanism of TRC magnesium alloy's microstructure during strip casting process was studied.The results showed that, chilled grain structure appeared just below the surface of the TRC strip,and dendrite structure developed from surface to the center of TRC strip along the thermal gradient. Fine eutectics and intermetallic compounds were seen in the interdendritic region.The grain size of TRC alloy was much smaller than that of CC alloy due to higher cooling rate and the plastic deformation effect under semi-solid states during TRC process. The intermetallic compounds were also refined and distributed uniformly,which could improve the deformation capability of magnesium alloy.The optimized rolling parameters were obtained by discussing the effect of rolling parameters on microstructure and mechanical properties of ZK60 alloy sheet. It should be separated into two steps to get ZK60 alloy sheet with 0.5mm thickness which possessed good properties.The TRC strip was rolled at 350℃to reduce the thickness from 3.5mm to 1mm in the first step.And then the warm rolled sheet was annealed at 350℃for 30min,after that changed the rolling temperature to 300℃to reduce the sheet thickness to 0.5mm.The rolling reduction ratio of 30%per pass was made for warm rolling.The sheet was annealed at the rolling temperature for 5min between the two passes of the rolling process.Warm rolling changed the shape of grains from dentrite to fibrous structure, which consisted of elongated grains along the rolling direction.High density shear bands,dislocation and twinning were formed during the rolling process.Dynamic recrystallization took place during the rolling process at and above 350℃.With the decreasing of the rolling temperature,increasing of the per pass rolling reduction and total rolling reduction,the grain size decreased,while the density of shear bands increased,strength and hardness increased.The tensile strength,yield strength and elongation of as rolled ZK60 alloy sheet produced by the optimized rolling parameters are 510MPa,441MPa and 11.3%,respectively.The optimized heat treatment parameters were also determined by analyzing the effect of annealing,solid solution and artificial aging heat treatment on microstructures and mechanical properties of warm rolled ZK60 alloy sheets.The optimized annealing parameter is annealed at 375℃for 10³s.And the optimized T6 treatment parameter is solid solution treatment at 375℃for 3hrs,and then artificial aging at 175℃for 10hrs.Static recrystallization was observed during the annealing process of ZK60 alloy sheet at and above 300℃.Fine equiaxed structure was obtained after annealing treatment.Fraction of recrystallized grain increased with the increasing of annealing temperature and holding time,but too high annealing temperature and too long holding time would induce grain growth.The strength and hardness of ZK60 alloy sheet decreased a little after annealing heat treatment,but the elongation increased obviously.The tensile strength,yield strength and elongation of ZK60 alloy sheet annealed at optimized annealing parameter are 388MPa,301MPa and 22.9%, respectively.It can be noted that proper T6 treatment can obtain fine equiaxed structure(the mean grain size is 6.7μm),so the anisotropy of ZK60 alloy sheet's tensile properties was obviously decreased.The deviation of tensile strength,yield strength and elongation along different angle to rolling direction are 23MPa,10MPa and 1.0%,respectively.Warm rolled ZK60 alloy sheet exhibited high intensity of(0001) basal pole texture,and the formation of shear bands tends to cause the basal pole texture tilt slightly to the transverse direction.The maximum pole intensity of(0001) pole figure increased with the decreasing of rolling temperature,increasing of per pass rolling and total rolling reduction.ZK60 alloy was strengthened by(0001) basal pole texture.Disc-shaped precipitates could be observed after annealed at different temperatures,and both disc-shaped and rod-like precipitates could be found after proper T6 treatment in the warm rolled ZK60 alloy sheet.The existence of the dispersed strengthening particles improved the uniformity of structure and the resistance to softening at elevated temperatures of ZK60 alloy sheet.Zn could be found both in disc-shaped particles and rod-like particles.Zr was found in disc-shaped or irregular shaped particles,but was not found in the rod-like precipitates.In addition, the uniformly distributed Zr in the matrix plays a role of refining the grain size.Three strengthening mechanisms existed during the rolling process of ZK60 alloy—grain refining strengthening,second-phase particle strengthening and dislocation strengthening.The three strengthening mechanisms dominate the strength effect at different moment during rolling process separately,and the strength of ZK60 alloy was improved by their corporated effects.In addition,the formation of high density(0001) basal pole texture can also strengthen ZK60 alloy.