Semi-solid State Recycling Process Of AZ91D Magnesium Alloy Chips And Microstructure Evolution Mechanism

Magnesium alloy is the lightest metal structural materials in industry application.It has become the preferred material for weight-reduction,energy saving and environmental protection.At present,magnesium alloy has been more and more widely used in the fields of communication and transportation,aerospace,electronic communications,national defense and military and other fields.Semi-solid forming technology can realize near-net shaping and non-turbulence filling by using non-dendritic semi-solid slurry,because of which the probability of gas entrapment reduces and product performances improve.Meanwhile,the development and application range of magnesium alloy broadens.Along with the wide application of the magnesium alloy,large amount of wastes are produced during machining and using.Thereby,waste recycling of magnesium alloy becomes the strategic focus of the industrial development.In this paper,the concept of semi-solid state recycling technique was proposed by the reference of magnesium alloy waste recycling combined with semi-solid forming technology.According to solid state recycling and SIMA method,chip recycled pressing process(CRP for short)was put forward to prepare semi-solid billet.This new method could promote the sustainable development of magnesium industry.In this paper.AZ91D magnesium alloy was used to prepare chips with different size by dry cutting.Billets were produced by CRP process.Analysis and test means of microstructure were carried out by hydrostatic device,optical microscope,scanning eletron microscope and transmission electron microscope.Compactness and microstructure of CRP billet were investigated.Mechanism of preparation of billets by CRP process was discussed and CRP process was optimized via orthogonal test.The optimum technological parameters were preheated temperature 350?,pressure 500MPa,holding time 10min and dwell time 30s.Under the optimum parameters,the density of CRP billet was 1.76g/cm3 and the relative density was as high as 98%.In the pressing process,the grains with obvious deformation characteristics were remarkably refined and uniformly distributed.Compared with other traditional asymmetric deformation process,such as compression,upsetting and extrusion,CRP process could induce homogeneous strain in billet.Effects of holding time and temperature on semi-solid microstructure,semi-solid microstructure and its uniformity of AZ91D original casting billet,extruded billet and CRP billet during semi-solid isothermal treatment were studied via quantitative metallography analysis of solid fraction,particle average diameter and average shape factor.Technological parameters of semi-solid isothermal treatment were optimized.Through semi-solid isothermal treatment,semi-solid microstructure transformation trended to occur in AZ91D original casting billet,but solid particles were coarsing above 145?m and irregular,some even kept the morphology of dendrite.Therefore,the spheroidizing result of AZ91D original casting billet was undesirability.When heated to 580? for 30min,the semi-solid microstructure of extruded billet was perfect in the center,particle average diameter was 75.6?m and average shape factor was 1.45.However,semi-soid microstructure of surface along the extrusion direction was not so good,average shape factor was 4.2 and solid particles also distributed along the extrusion direction.Thus,semi-solid microstructure of extruded billet had orientation and inhomogeneity.Under the same conditions,in the semi-solid microstructure of CRP billet,particle average diameter was 98.6?m and average shape factor was 1.45.Although the size of solid particle increased,comparedwith other non-uniform deformation process,the semi-solid microstructureuniformity of CRP billet was excellent.Effects of chips with different sizes on compactness,microstructure,semi-solid microstructure,oxidation and distribution of oxygen were investigated.The results showed that with the increase of chip size,grain refinement effect weakened,deformation uniformity aggravated and oxygen content of CRP billet increased.Under the same CRP process conditions,the relative densities of four kind of CRP billet were 98.27%,98.13%,98.05%and 97.92%,respectively.Meanwhile,there was no significant difference in compactness and microstructure between them.Normally,oxygen content was not more than 1.6%and uniformly dispersed in billet.Hence,the influence of size and deformation inhomogeneity of chip on compactness and microstructure of CRP billet eliminated via CRP process.In CRP billet,microstructure and strain were uniform and this inherited during semi-solid isothermal treatment.Semi-solid microstructural evolution of AZ91D original casting billet,extruded billet and CRP billet were studied.The spheroidizing mechanism and coarsening mechanism of solid particle were analyzed from the viewpoint of thermodynamics.The formation process of spherical a-Mg solid particle mainly included three stages,i.e.,refining,spheroidizing and coarsening.During semi-solid isothermal treatment,the equilibrium melting temperature drop and chemical potential drop caused by interfacial tension and interfacial curvature were the fundamental cause of the spheroidizing and coarsening of solid particle.The coarsening of solid particle was by means of Ostwald riping and merging growth.In fact,semi-silid microstructural evolution is the course of atom diffusion.