Dynamical coarsening processes of microstructures in non dendritic AlSi_7Mg alloy remelted in semi solid state

The microstructural variation in the non dendritic AlSi 7Mg samples remelted in the semi solid state has been investigated. It is proposed that the primary α Al phases are mainly coarsened by connecting the secondary arms or fine primary α Al phases together at the stage of a small quantity of liquid and slowly coarsened through diffusion at the stage of a great quantity of liquid. The dynamical coarsening equation controlled by diffusion is in good agreement with the equation of d 3- d 3 0= kt and the effect of the starting microstructures on the coarsening of primary α Al phases is gradually decreased when the soaking time is long enough.

Effects of Compression Parameters on Deformation Behaviors of Semi-Solid ZA27 Alloys

An investigation was performed on the effects of semi solid compression parameters,such as strain rate,compression temperature and heating time at these temperatures on deformation behaviors of two kinds of ZA27 alloys,one was modified by Zr and the other was unmodified.The results indicate that with the increasing of the strain,the stress of the modified composite first sharply increases to a peak value,then dramatically decreases to a plateau value,and again increases till the end of deformation.But for the unmodified,after being up to a peak value,the stress only decreases slowly.As the compression temperature or the heating time decreases,or the strain rate increases,the stress level and the cracking degree of these two kinds of alloys increase.Under the same deformation conditions,the stress level and the cracking degree of the unmodified alloy are higher than those of the modified one.But there is an exception that the stress level of the unmodified alloy is minimum and smaller than that of the modified one when deformed at the low temperature of 450℃.These phenomena were mainly discussed through analyzing the microstructures under different conditions and the deformation mechanisms at different deformation stages.

Effect of Mg and semi solid processing on microstructure and impression creep properties of A356 alloy

The effects of Mg and semi solid processing on the creep properties ofA356 A1 alloy were investigated. The results show that the dislocation climb controlled creep is the dominant creep mechanism and it is not affected by the semi solid processing and further addition of Mg. Mg improves the alloy creep properties probably by forming large Chinese script Mg2Si compounds at the interdendritic regions. The semi solid processed specimens exhibit better creep properties in comparison with the as cast ones. It is attributed to the reduction in the stacking fault energy resulting from the significant dissolution of Mg in the a（A1） phase.

Effect of yttrium on the microstructure of a semi-solid A356 Al alloy

The semi-solid slurry of an A356 Al alloy, which was grain-fined by yttrium, was manufactured by low temperature pouring, The effects of grain-refining on the morphology and the grain size of the primary α phase in the semi-solid A356 Al alloy were researched. The results indicate that the semi-solid A356 AI alloy with particle-like and rosette-like primary α-Al can be prepared by low temperature pouring from a liquid grain-refined A356 alloy. The grain size and particle morphology of primary α-Al in the A356 Al alloy are markedly improved by the addition of 0.5 wt,% Y. The fining mechanism of Y on the morphology and grain size of the primary α-Al in the semi-solid A356 Al alloy was delved.

Preparation for Semi-Solid Aluminum Alloy Slurry under Weak Electromagnetic Stirring Conditions

The effects of pouring temperature, short electromagnetic stirring with low strength and then soaking treatment on the microstructure of AISi7Mg alloy were investigated. The results show that if AlSi7Mg alloy is poured at 630 or 650℃ and meanwhile stirred by an electromagnetic field at a low power for a short time, the pouring process can be easily controlled and most solidified primary α-Al grains become spherical and only a few of them are rosette-like. Weak electromagnetic stirring makes the temperature field more homogeneous and makes the primary α-Al grains disperse in a larger region, which leads to the spherical microstructures of primary α-Al grains. When the AISi7Mg alloy is soaked or reheated at the semisolid state, the primary α-Al grains ripen further and they become more spherical, which is favorable to the semi-solid forming of AlSi7Mg alloy.

Industrial development of gas induced semi-solid process

The gas induced semi-solid(GISS) is a rheocasting process that produces semi-solid slurry by applying fine gas bubble injection through a graphite diffuser.The process is developed to be used in the die casting industry.To apply the GISS process with a die casting process,a GISS maker unit is designed and attached to a conventional die casting machine with little modifications.The commercial parts are developed and produced by the GISS die casting process.The GISS die casting shows the feasibility to produce industrial parts with aluminum 7075 and A356 with lower porosity than liquid die casting.

Effects of holding temperature and time on semi-solid isothermal heat-treated microstructure of ZA84 magnesium alloy

The feasibility of fabricating ZA84 magnesium alloy with non-dendritic microstructure by a semi-solid isothermal heat treatment process and the effects of holding temperature and time on the semi-solid isothermal heat-treated microstructure of the alloy were investigated. The results indicate that it is possible to produce ZA84 alloy with non-dendritic microstructure by suitable semi-solid isothermal heat treatment. After being treated at 560-575 ℃ for 120 min, ZA84 magnesium alloy can obtain a non-dendritic microstructure with 14.2%-25.6% liquid fraction and an average size of 56-65 μm of the unmelted primary solid particles. With the increasing holding time from 30 to 120 min or holding temperature from 560 to 575 ℃, the average size of unmelted primary solid particles decreases and globular tendency becomes more obvious. Under the experimental condition, the microstructural evolution of ZA84 alloy during semi-solid isothermal treatment is mainly composed of three stages of initial coarsening, structural separation and spheroidization. The subsequent coarsening of spheroidal grains is not observed.

Reheating microstructure of refined AZ91D magnesium alloy in semi-solid state

By means of equal channel angular extrusion (ECAE) test, upsetting test and metalloscope, reheating microstructures of raw casting ingots, materials prepared by SIMA and materials extruded by ECAE in semi-solid state were investigated. The results show that compared with those of raw casting ingots and materials prepared by SIMA, reheating microstructure of materials extruded by ECAE is the best and the final grain size is the finest. With increasing holding time, a growing phenomenon occurs in reheating microstructure of materials extruded by ECAE, which can be described by Ostwald ripening law. The average grain size increases firstly, subsequently decreases and the shape factor of grains approaches to 1 as the reheating temperature increases. With increasing equivalent strain, the average grain size decreases. This demonstrates that reheating material extruded by ECAE technology is a good method to prepare AZ91D magnesium alloy semi-solid billets.

THE APPARENT VISCOSITY OF SEMI-SOLID AZ91D ALLOY AT STEADY STATE

The rheological behavior of semi-solid AZ91D at steady state is studied usinga Couette type viscometer in the present paper. The results show that the apparent viscosity ofsemi-solid AZ91D at the steady state increases with the solid fraction increasing, and goes upsharply when the solid fraction reaches a certain value, which is called critical fraction. Inaddition, the apparent viscosity of semi-solid AZ91D at the steady state takes on a distinctdowntrend with the shearing rate increasing, which indicates a strong shear thinning property. Inaddition, the critical solid fraction becomes higher under larger shearing rate, owing to the moreglobular shape of the solid particles. Based on the present experiment results, an empiricalequation is built as that, relating the steady state apparent viscosity of semi-solid AZ91D with thesolid fraction f_s and shearing rate gamma at the same time: eta_(app)=10.74 exp (6.95 f_s)gamma^(-0.86).

Research on semi-solid thixoforming process of AZ91D magnesium alloy brackets for generators in JH70-type motorbikes

The investment on semi-solid die casting processes of AZ91D magnesium alloy brackets for generators in JH70-type motorbikes is introduced. The processes of low super-heat and cooling slope for the preparation of billets with non-dendritic microstructure, the remelting of billets for thixoforming and the parameters in the process of semi-sohd thixoforming have been researched. The results show that primary billets with non-dendritical structures can be prepared by forming great amount of nuclei in melt via the process of low super heat. By optimizing the remelting process through adjusting the current of the induced equipment, semi-solid billets with a structure of spherical grains were obtained from the primary billets with non-dendritical structure. The range of 580℃ to 583℃ is the proper remelting temperatures by which the billets have an expected thixotropy and can be transferred to a die-casting machine. The optimized parameters of semi-solid forming in a die-casting machine are as follows： the area of the ingate in the die is 383.5 mm^2, the speed of the pierce of the machine 5 m/s, the shot pressure of the pierce 75 MPa, and the maintenance pressure of the pierce 350 MPa. The castings of brackets for supporting generators in JH70 type motorbikes were formed by adopting the optimized processes and parameters mentioned above.

Research on semi-solid slurry of a hypoeutectic Al-Si alloy prepared by low superheat pouring and weak electromagnetic stirring

The semi-solid slurry of a hypoeutectic Al-Si alloy was manufactured by low superheat pouring and weak electromagnetic stirring. The effects of pouting temperature and stirring power on the semi-solid slurry were investigated. The results indicated that the semi-solid slurry to satisfy rheocasting can be manufactured by low superheat pouring and weak electromagnetic stirring. The pouring temperature （or superheat） and the stirring power remarkably affected the morphology of primary α-Al and the size of primary α-Al, and there is no obvious effect of stirring time on primary α-Al. Compared with the samples made by low superheat pouring with no stirring, the nucleation rate, particle morphology and grain size of primary α-Al in A356 were markedly improved by low superheat pouring and weak electromagnetic stirring. On the condition of weak electromagnetic stirring, the pouring temperature with low superheat can be suitably raised to reach the effectiveness obtained from the lower pouring temperature without stirring.

New Technique of Casting-rolling Strips for Semi-solid Magnesium Alloys

The conjugation of semi-solid process technique and casting-rolling technique applied to produce the magnesium strips was studied. The semi-solid slurry has been prepared continuously by the mechanical method and its temperature was controlled strictly at the same time. AZ91D and AZ31 casting magnesium alloys were applied to the experiment. The casting-rolling strips with non-dendritic structure were obtained and its main mechanical property is better. The process ability of the casting-rolling strips was studied. It is significative to link the semi-solid process techniques and casting-rolling techniques, through which we can get high quality magnesium alloy strips with non-dendritic structure and improve the overall properties of the products.

Formation and Evolution of Non-dendritic Microstructures of Semisolid Alloys Prepared by Shearing/Cooling Roll Process

The shearing/cooling roll （SCR） process was adopted to prepare semi-solid A2017 alloy. The formation and evolution of non-dendritic microstructures in semi-solid A2017 alloy were studied. It is shown that the microstructures of semi-solid billets transform from coarse dendrites into fine equiaxed grains as the pouring temperature of molten alloy decreases o.r roll-shoe cavity height is reduced. From the inlet to the exit of roll-shoe cavity, microstructure of semi-solid slurry near the shoe surface is in the order of coarse dendrites, degenerated dendrites or equiaxed grains, but fine equiaxed grains are near the roll surface. Microstructural evolution of semi-solid slurry prepared by SCR process is that the molten alloy nucleates and grows into dendrite firstly on the roll and shoe＇s surface. Under the shearing and stirring given by the rotating roll, the dendrites crush off and disperse into the melt. Under the shearing and stirring on semi-solid slurry with high volume fraction of solid, the dendrite arms fracture and form equiaxed grain microstructures.

Effect of pouring temperature on fractal dimension of primary phase morphology in semi-solid A356 alloy

The fractal dimensions of primary phase morphology in semi-solid A356 alloy prepared by low superheat pouring and slightly electromagnetic stirring were calculated,and the effect of pouring temperature on fractal dimension of primary phase morphology in semi-solid A356 alloy was researched.The results indicate that it is feasible to prepare semisolid A356 alloy slurry by low superheat pouring and slightly electromagnetic stirring,and there is an important effect of pouring temperature on the morphology and the grain size of the primary phase in semi-solid A356 alloy,in which the reduction of pouring temperature can obviously improve grain size and shape factor of primary phase in semi-solid A356 alloy under the condition of a certain stirring power.The primary phase morphology of semi-solid A356 alloy prepared by low superheat pouring and slightly electromagnetic stirring can be characterized by fractal dimension,and the primary phase morphology obtained by the different processing parameters has the different fractal dimensions.Solidification of semi-solid alloy is a course of change in fractal dimension.

Effect of pouring temperature on semi-solid slurry of A356 Al alloy prepared by weak electromagnetic stirring

The semi-solid slurry of A356 Al alloy was manufactured by low superheat pouring and weak electromagnetic stirring. The effects of pouring temperature on the slurry manufactured by weak electromagnetic stirring were researched. The results indicate that it is feasible to manufacture the slurry with particle-like primary phases by low superheat pouring and weak electromagnetic stirring, and there is an important effect of the pouring temperature (superheat temperature) on the morphology and the size of primaryα-Al in A356 Al alloy. By the action of suitable weak electromagnetic stirring, increasing pouring temperature to put low superheat pouring in practice is capable of obtaining semi-solid slurry of A356 Al alloy with particle-like primary phase. Compared with the samples made only by low superheat pouring without stirring, raising pouring temperature by 15-35 ℃ above the liquidus temperature under condition of weak electromagnetic stirring can ensure the same grain size and morphology of the primary phase.

Effect of isothermal heat treatment on semi-solid microstructure of AZ91D magnesium alloy containing rare earth Gd

The AZ91 D magnesium alloy containing rare earth Gd was prepared in this study, and the effect of semi-solid isothermal heat treatment on the microstructure of the alloy was investigated to obtain an optimum semi-solid structure. Results show that Gd can refine the microstructure of AZ91 D magnesium alloy, and the optimum semi-solid AZ91 D microstructure can be achieved by adding 1.5wt.% Gd. After treated at 585 °C for 30 min, the well distributed rose-shaped and near-spherical semi-solid microstructures of AZ91D+1.5wt.%Gd alloy can be obtained. The liquid phase of the semi-solid alloy consists of three components, namely, the molten pool, the "entrapped liquid" pool and the liner liquid film which separates two neighbor particles. The solid phase is composed of two phases, the primary α-Mg particles and the α-Mg phase formed in the second stage of solidification. With the increase of holding time, melting which causes the decrease of the primary α-Mg particle size is the dominant mechanism in the initial stage while coalescence and Ostwald ripening tend to be the principles later.

Manufacture technique of semi-solid slurry of hypoeutectic Al-Si alloy by low superheat pouring and weak electromagnetic stirring

The semi-solid slurry of hypoeutectic Al-Si alloy was manufactured by low superheat pouring and weak electromagnetic stirring. The effects of pouring temperature and stirring power on the semi-solid slurry making process were investigated. The results indicate that the semi-solid slurry to satisfy rheocasting requirement can be made by a combination of low superheat pouring and weak electromagnetic stirring. The pouring temperature (or superheat) and the stirring power significantly affect the morphology and the size of primary α-Al, while there is no obvious effect of the stirring time on primary α-Al. Compared with the samples made by low superheat pouring without stirring, the nucleation rate, particle morphology and grain size of primary α-Al in A356 Al alloy are markedly improved by a process of applying both low superheat pouring and weak electromagnetic stirring. Under the condition of weak electromagnetic stirring applied, the pouring temperature with low superheat can be equivalently to reach the effectiveness obtained from the even lower pouring temperature without stirring.

Preparation of semi-solid slurry containing fine and globular particles for wrought aluminum alloy 2024

The semi-solid slurry of wrought aluminum alloy 2024 was prepared by a well developed rheocasting process, low superheat pouring with shearing field(LSPSF). The appreciate combination of pouring temperature and rotation speed of barrel, can give rise to a transition of the growth morphology of primary α(Al) from coarse-dendritic to coarse-particle-like and further to fine-globular. The combined effects of both localized rapid cooling and vigorous mixing during the initial stage of solidification can enhance wall nucleation and nuclei survival, which leads to the formation of fine-globular primary α(Al). By using semi-solid slurry prepared by LSPSF, direct squeeze cast cup-shaped component with improved mechanical properties such as yield strength of 198 MPa, ultimate tensile strength of 306 MPa and elongation of 10.4%, can be obtained.

Annulus electromagnetic stirring for preparing semisolid A357 aluminum alloy slurry

The effects of pouring temperature and annulus gap width on the microstructure of the semi-solid A357 aluminum alloy slurry prepared by annulus electromagnetic stirring(AEMS)technology were investigated.The results show that low pouring temperature and narrow annulus gap are advantageous to obtaining the small spherical primaryα(Al)phase.The lower the pouring temperature is and the smaller the annulus gap width is,the more uniform,the smaller and the more spherical the microstructure is. The microstructures obtained by the ordinary electromagnetic stirring and AEMS were compared.The results indicate that the primaryα(Al)particles are globular,small and distribute homogeneously in the slurry obtained by AEMS.But in the slurry obtained by the ordinary electromagnetic stirring,the primaryα(Al)particles are small dendrites in the edge of the slurry and they are large and rosette-like or dendritic in the inner of the slurry.

Microstructure,mechanical response and fractography of AZ91E/Al_(2)O_(3)_(p) nano composite fabricated by semi solid stir casting method

The present study confers to the fabrication and its characterization of magnesium alloy(AZ91E)based nano composites with nano Al_(2)O_(3) particulate reinforcements.A novel Semi Solid stir casting technique was adopted for the fabrication of the composite.An average particle size of 50 nm was used as reinforcement to disperse in matrix.The effects of change in weight fraction of reinforcements on the distribution of particles,particle–matrix interfacial reactions,physical as well as mechanical properties were reported.The SEM and EDS analysis has shown the uniform distribution of particles in the composite along with the presence of elements.The mechanical properties of reinforced and unreinforced composite were evaluated and presented.Fractography of tensile specimens was also discussed.