| Magnesium alloys are the highest structure materials at present, which have a lot of excellent properties. The utilization of magnesium has penetrated the fields of engineering components in the automotive, spaceflight, computer industry, etc. Magnesium alloys were preferable for the reduction of automobile weight, energy saving and environment protecting. So they are referred to as the 21st century green engineering materials.Semi-solid forming (SSF) are being a potential technology for metal forming for its many advantages. The key feature that permits the shaping of alloys in the semisolid state is the absence of dendritic characteristics from the morphology of the solid phase. The microstructures reveal very small flow resistance and finer molding capability. Therefore, the preparation of eligible semisolid microstructure is the basic for semisolid processing. Strain induced melt activation (SIMA) method omits the procedure of molten metal treatment and the metal is partial remelted so that the inflammation of the magnesium is avoided. SIMA is considered as a net-shape process in which the basic extruded or rolled bars are subjected to additional deformation to accumulate a large quantity of deformation energy to induce sufficient strain, then heated to the semisolid state to transform the dendritic structure to develop a fine, uniform and spheroidal microstructure.The semisolid forming (SSF) has special technical and economic advantages. Therefore, SSF can produce diversiform parts that involve automotive and aeronautic parts and so on. But, magnesium alloys currently used in SSM processing are restricted to a few commercial alloys such as AZ91, AM50 and AM60. Now, the traditional casting or forging alloys cannot show all advantages of semisolid metal processing. We should design new SSF alloys that can meet process ability requirements of different parts, sequentially the all advantages of semisolid forming can exert adequately. Al and Zn element is low-cost and effective alloying element of magnesium applies that to develop most magnesium alloys.Firstly, through observation of microstructural evolution of semisolid alloy to founded the evolution model of semisolid microstructure in this paper. The microstructural evolution of semisolid magnesium alloy as follow: after compression form fibroid structure and dendrite rupture→fibroid structure has disappeared, take placed recovery and recrystallization, eutectic and second phase gets dissolved inα-Mg matrix partially below the solidus temperature→the eutectic and agglomerate Mg17Al12 phase melted firstly when the isothermal temperature is slightly higher than the solidus temperature→the liquid phase saturated into high-energy recrystallization grain boundaries, isolated solid particle appeared→solid particle rounded→solid particle coarsing. The semisolid microstructure consists of primary solid particle, liquid drop within primary solid particle and liquid phase that consists of eutectic and re-solidificationα-Mg. The liquid drop consists of eutectic and secondly phase that has acicular and globular morphology. The microstructural evolution of semisolid combined two mechanisms that are dendrite rupture and recrystallization. When contain many compound, recrystallization and dendrite rupture influence microstructural evolution of semisolid together. When contain lesser compound, recrystallization is primary mechanism.Strain induced melt activation (SIMA) process was used to manufacture the semisolid billets of magnesium alloys in this paper. Firstly, the effect of compression ratio, asymmetrical deformation, isothermal holding temperature, isothermal holding time and original as-cast microstructure on the microstructure evolution of semisolid AZ91D magnesium alloy was investigated. Based above investigations, we found appropriate parameter range for favorable semisolid microstructure. The investigation indicates that the effect of mould material on the as-cast microstructure is very great, but the effect on the semisolid microstructure falls evidently. The plastic deformation is asymmetrical in different area of specimen that separate into three areas which are prone to deform area, difficult to deform area and uncontrolled area. The wrought causation of semisolid microstructure is different in the three areas. Liquid formed in different place of specimen because of compression. The liquid appear along the new recrystallization grain boundaries in middle and drum where deformation degree is large and it primary dendrite boundaries and solute-rich area in grains mostly, along the new recrystallization grain boundaries no other than a little in the areas of upper and nether surface. The stress and strain affect the nucleation of liquid together. The nucleation of liquid is much more sufficient in drum area that experiences the tension stress than in middle area that experiences the compression stress. During the same isothermal holding times, the solid fraction is lower slightly in drum area than in middle area. As a whole, after isothermal holding for a certain times (20 min in this paper), the asymmetric deformation by compression induced that affect the holistic performance of semisolid billets on the small side. Consequently, it can avoid the effect of asymmetric deformation on the microstructure of semisolid in SIMA process. Compression ratio is an important parameter for semisolid forming. With increasing compression ratio, the recrystallization grain size decreases and the recrystallization grain quantity increases, simultaneity the semisolid microstructure is much more uniform and fine. When the compression ratio less than 15%, with increasing compression ratio, the solid fraction and solid particle size decreased rapidly, the alterant extent is biggish. When the compression ratio bigness than 15%, with increasing compression ratio, the decreasing speed of solid fraction and solid particle size slower besides solid particle size occurred coarseness at critical compression ratio. The critical compression ratio is about 40% for favorable semisolid billet in this experimental condition. The isothermal holding temperature and isothermal holding time are interrelated two parameters. Isothermal holding long times and low temperature and isothermal holding short times and high temperature can obtain same purpose, but the isothermal holding time and temperature all have a critical value. At overly low temperature, the kinetic capability of atom is not strong, consequently even if isothermal holding long times also cannot obtain favorable semisolid billet. At exorbitant temperature, the diffusivity of atom increased, in isothermal holding short times the semisolid microstructure can obtain, but solid particle coarseness very severe that fall short of need of favorable semisolid billet. The appropriate isothermal holding temperature is 550℃～590℃and corresponding isothermal holding time is (45-60min)～(6-20min) in this experimental condition.The Mg-Al-Zn alloys prepared that have different Al and Zn element content. Semisolid microstructural evolution rule was investigated at different compression ratio, isothermal holding temperature and isothermal holding time. The rheological parameter of semisolid billet was analyzed and via that estimate usability of semisolid billet. The experiment demonstrated that with increasing Al content, the temperature sensitivity of alloys decreased that is propitious to control quality variety of semisolid billet. At equal Al content, with increasing temperature, the variational speed of solid fraction increased. The Zn element reduced final freezing point, and promoted appearance ofβphase along grain boundary and it enrichment in theβphases that cause melting point of compound drop. The increase of Al and Zn content favor of the transformation of semisolid that come down to two causes. With increasing Al and Zn content, the compound quantity increased that melt fleetly at isothermal holding, as a result can fleetly obtain fine and globular semisolid microstructure. On the other hand, with increasing element content the recrystallization grain decreased that bring small solid particle size.The effect factors of semisolid microstructure were educed that involve conglomeration deposition of elements, compression ratio, distribution of stress and strain and alloy component etc. The many alloy elements result in severe conglomeration deposition that turn into nucleus of liquid phase at semisolid temperature isothermal holding, which in favor of form the semisolid microstructure. On the other hand, many alloy elements increase the volume fraction of second phase that bring fine recrystallization grain, sequentially can bring fine semisolid microstructure. The compression ratio also influenced recrystallization consequently and the semisolid microstructure. Stress state influenced nucleation of liquid phase. The tension stress reduces nucleation temperature of liquid phase that accelerate nucleation, and the compressive stress increase nucleation temperature of liquid phase that restrain nucleation.The corrosion behaviour of semisolid AZ91D magnesium alloy was investigated for the first time by electrochemical test, immersion test and salt spray test. The investigation demonstrates that the corrosion resistance of semisolid specimen is higher than as-cast specimen by three different testing methods. A severe substantive desquamation and downthrows appeared in as-cast specimen, whereas the semisolid specimen reveals flat seeming breach. Galvanic corrosion is a major reason for both kinds of specimen, and the microstructural difference is result in a grave difference of corrosion behavior. For as-cast specimens, theβphase discontinuously distribute in the grain boundaries that accelerate the corrosion process. Contrarily, the solid particles surround byβphase that inhibit the overall corrosion of the semisolid specimen. There are three kinds of galvanic corrosion in semisolid specimen that first is between solid phase and liquid phase, second is within liquid phase, and final is within solid phase. The primary breakage by the corrosion along the interphase of solid phase and liquid phase bring on. Theα-Mg around liquid drop is also a place of corrosion nucleation easily in the solid particles that accelerate whole mass loss. The effect of corrosion is low in liquid phase, owing to the barrier effect ofβphase.The effect of morphological parameter of semisolid microstructure is foremost on corrosion performance of semisolid specimens. At same solid fraction, small solid particle size and large numbers of solid particle amount induce speedy corrosion at the initiatory step of corrosion, whereas big solid particle size and a small quantity of solid particle amount induce big and deep corrosion pit and large numbers of corrosion amount at last step. When Al content achieves 8%, the Mg-Al alloy can form continuousβphase in semisolid alloy that have upper solid fraction and biggish solid particle size. When Al content achieves 11.05%, the Mg-Al alloy can form continuousβphase in experimental all condition. The continuousβphase can restrain corrosion of semisolid alloy effectively that detailed research can provide a new way for antisepsis of magnesium alloys.
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