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.

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.

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angular extrusion in semi-solid isothermal treatment

Microstructure evolution of processed Mg-Al-Zn alloy by equal channel angular extrusion(ECAE)in semi-solid isothermal treatment was investigated.The results show that with increasing semi-solid isothermal treatment temperature,the α phase solid grain size of processed Mg-Al-Zn alloy by ECAE increases firstly due to coarsening of α phase solid grains,then decreases due to melting of α phase solid grains.With the increase of extrusion passes during ECAE,the α phase solid grain size in the following semi-solid isothermal treatment decreases.The α phase solid grain size of processed Mg-Al-Zn alloy by ECAE under route BC is the smallest,while the α phase solid grain size of processed material by ECAE under route A is the largest.The primary mechanism of spheroid formation depends on the melting of recrystallizing boundaries and diffusion of solute atoms in the semi-solid state.

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.

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.

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.

EFFECT OF GRAIN REFINING ON PRIMARYαPHASE IN SEMI-SOLID A356 ALLOY PREPARED BY LOW SUPERHEAT POURING AND SLIGHT ELECTROMAGNETIC STIRRING

The semi-solid slurry of A356 alloy, which is grain-refined by Al-Ti-B master alloy, is prepared by low superheat pouring and slight electromagnetic stirring. The effects of grain refining on the morphology and the grain size of the primary α phase in the slurry manufactured are researched. The results indicate that the slurry with particle-like and rosette-like primary α phases can be prepared by low superheat pouring and slight electromagnetic stirring from liquid A356 alloy grain-refined, in which the pouring temperature can be suitably raised. Compared with the A356 samples without grain refining, the grain size and particle morphology of primary α phase as well as the distribution of the grain with particle-like or rosette-like along radial in the ingot in A356 are markedly improved by grain refining.

Effects of compressing and remelting in SIMA processing on semi-solid structure evolution of an Al-Zn wrought alloy

Structure evolution of an Al-Zn wrought alloy in remelting processing in thestrain induced melt activated (SIMA) serai-solid procedure was observed, and effects of factors, theremelting temperature, the holding time, and the compression strain, on structures and grain sizesof the alloy were investigated. The results show that (1) the proper temperature of remelting is inthe range of 610 to 615℃; (2) the grain size in specimen with greater compression strain is smallerthan that with smaller compression strain in condition of the same remelting temperature andholding time, and the grain size in local area with great local equivalent strain is smaller thanthat with small one; (3) liquid occurs in form of cluster in matrix during remelting and itsquantity increases with remelting time increasing; liquid in specimen with great compression strainoccurs earlier than that with small one, and quantity of liquid in the center of specimen withgreater local equivalent strain is greater than that in the two ends of it; (4) distortion energyafter deforming in matrix of the alloy is the significant factor to activate melting of matrix atlocal area with great local equivalent strain.

Preparation of semi-solid aluminum alloy slurry poured through a water-cooled serpentine channel

A water-cooled serpentine channel pouring process was invented to produce semi-solid A356 aluminum alloy slurry for rheocast ing, and the effects of pouring temperature and circulating cooling water flux on the microstmcture of the slurry were investigated. The results show that at the pouring temperature of 640-680~C and the circulating cooling water flux of 0.9 m3/h, the semi-solid A356 aluminum alloy slurry with spherical primary a（A1） grains can be obtained, whose shape factors are between 0.78 and 0.86 and the grain diameter can reach 48-68 ~am. When the pouring temperatures are at 660-680~C, only a very thin solidified shell remains inside the serpentine channel and can be removed easily. When the serpentine channel is cooled with circulating water, the microstructure of the semi-solid slurry can be improved, and the serpentine channel is quickly cooled to room temperature after the completion of one pouring. In terms of the productivity of the special equipment, the water-cooled serpentine channel is economical and efficient.

Effect of Holding Time on Thixotropic Fluidity of Semi-solid AZ91D Magnesium Alloy

To investigate the thixotropic fluidities, microstructures and mechanical properties of semi-solid AZ91D magnesium alloy during reheating, a self-made die set with channels of different sizes were used. The results show that critical forming forces and maximal forming forces could be obtained and related to the holding times in the semi-solid forming process. In the holding time of 0-2700 s, with increasing the holding time, maximal forming force decreased sharply and critical forming force decreased slowly. In the whole thixotropic flowing process, the filling-in was steady and the surface fineness was good. The forming force increased when the slurry changed the flowing direction or flowed from the big-diameter chamber to the small-diameter one. The tensile strength and elongation to failure of the sample after holding time for 2700 s, compared to as-cast sample, are increased by approximately 42.7% and 180%, respectively, and the fractured surfaces presented dimple-like pattern.

Investigation of rheo-diecasting mold filling of semi-solid A380 aluminum alloy slurry

The rheo-diecasting mold filling capacity and the microstructure of the semi-solid A380 aluminum alloy slurry were investigated. The results show that the mold filling capacity was strengthened with increasing pouring temperature or increasing injection pressure. Under certain process parameters, the mold cavity was fully filled. However, the mold filling capacity decreased with increasing holding time. The mold filling capacity was improved with increasing shape factor of primary α(Al) grains; however, the solid fraction and the grain size significantly increased at the same time. In addition, the microstructures along the route of the spiral samples obviously differed. The grain size decreased gradually from the near-end to the far-end, whereas the shape factor increased gradually.

Non-dendritic structural 7075 aluminum alloy by liquidus cast and its semi-solid compression behavior

Fine, equiaxed, non dendritic structure needed by semi solid processing was obtained by liquidus cast, i.e. 7075 wrought aluminum alloy cast from liquidus temperature. The microstructures after heat treatment at different temperatures and time in the semi solid range were observed, and the compression deformation behavior at different temperatures (490～600 ℃) and strain rates (5×10 -3 ～5s -1 ) was investigated by means of Gleeble 1500 thermal mechanical simulator. The results show that the deformation resistance of the non dendritic structure attained by liquidus cast in semi solid is remarkably lower than that of conventional dendritic structure. The formability of non dendritic structure is better than that of dendritic structure.

Degassing effect of ultrasonic vibration in molten melt and semi-solid slurry of Al-Si alloys

In the process of semi-solid slurry preparation with direct ultrasonic vibration (UV) by dipping the horn into the melt, one of the questions is whether the gas content in the melt would be increased or not by the cavitation effect of ultrasonic vibration. By application of quantitative gas content measurement technique, this paper investigated the effect of the ultrasonic vibration on the gas content of both the melt and the semi-solid slurry of Al-Si alloys, and the variations of the gas contents in two kinds of aluminum alloys, i.e., A356 alloy and Al-20Si-2Cu-1Ni-0.6RE alloy (Al-20Si for short). The results show that ultrasonic vibration has an obvious degassing effect on the molten melt, especially on the semi-solid slurry of Al-Si alloy which is below the liquidus temperature by less than 20 ℃. The ultrasonic degassing efficiency of the A356 alloy decreases with the reduction of the initial gas content in the melt, and it is nearly unchanged for the Al-20Si alloy. The gas content of both alloys decreases when the ultrasonic vibration time is increased. The best vibration time for Al-20Si alloy at the liquid temperature of 710 ℃ and semi-solid temperature of 680 ℃ is 60 s and 90 s, respectively; and the degassing efficiency is 48% and 35%, respectively. The mechanism of ultrasonic degassing effect is discussed.

Effects of Cr and Cr/Mn Combined Additions on Semi-Solid Microstructures of Al-Mg-Si Alloys Produced by D-SSF Process

The effects of Cr and Cr/Mn combined additions on the semi-solid microstructure of wrought Al-Mg-Si alloys are investigated.In the Cr-added alloy,the Al-7Cr compound is formed with homogeneously distributed in theα-Al matrix after homogenization.Both of the Al-7Cr andα-Al-（12）Mn3-Si-2 dispersiod particles are found in the homogenized microstructure of the Cr/Mn-added alloy.In the semi-solid microstructures,the smallestα-Al grains are obtained in the 60% cold-rolled alloys.After prolonged holding time,theα-Al grain size of the Cr/Mn-added alloy is smaller than that of the Cr-added alloy.Heavy deformation by cold-rolling accelerates spheroidization of theα-Al grains.The D-SSF process is found to be useful to modify the microstructures of both the Cr-added and Cr/Mn-added Al-Mg-Si alloys.

Study on Semi-solid Rheocasting Forming for Magnesium Alloy by Mechanical Stirring

In this paper the effect of pouring temperature of magnesium melt, preheating temperature of the barrel of screw mixer and shear rate on the solidified microstructures of semi-solid slurry was investigated by mechanical stirring method. The appropriate processing parameters of slurry preparation were obtained. The mold filling for thin walled casting was examined. Results indicated that the solid volume fraction of non-dendritic structure increased with decrease in pouring temperature of magnesium melt and the barrel preheating temperature of screw mixer. Also the grain size of primary α phase was reduced. Furthermore, the solid volume fraction of semi-solid non-dendritic structure decreased with the increase in shear rate. In the same time, grain size of primary α phase slightly changed. The fine-round granular solidified structure with 30～50 μm of prepared AZ91D magnesium semi-solid slurry was presented. Finally, it is successful to fabricate 1.0 mm extremely thin walled casting with clear contour and good soundness.

Effects of solution treatment and aging process on microstructure refining of semi-solid slurry of wrought aluminum alloy 7A09

A new method was exploited using solution treatment and aging process as a pretreatment in preparing semi-solid slurry with fine microstructure before isothermal treatment of wrought aluminum alloy 7A09.Parameters of pretreatment were optimized by orthogonal experiment design and proper precursor was prepared.The evolution of microstructure of semi-solid slurry during isothermal treatment was analyzed and the mechanism of microstructure refining was discussed.The result of orthogonal experiment design shows that the optimum parameters are 462 ℃for solution temperature,40min for solution time,132 ℃for aging temperature and 14 h for aging time.Microstructure of isothermal treatment is fine,homogenous,with globular solid grains and a solid fraction between 50%and 70%,which is qualified for later semi-solid forming process.Mechanism of microstructure evolution includes the agglomeration ofα-phase and Ostwald ripening.Precipitations prepared by solution and aging treatment prevent the grains from coarsening and promote the grain ripening to globular shape.

Rheological Behavior of Semi-Solid AZ91D Alloy

The rheological behavior and microstructure of semi-solid AZ91D was studied using a Couette type viscometer. The results show that the apparent viscosity of semi-solid AZ91D increases with the cooling rate increasing when it is continuously cooled and mechanically stirred and the empiric relation of the apparent viscosity with the cooling rate and solid fraction at shearing rate 93.7 s-1 is expressed as η=[0.66-0.63e(-ε/3.01)]e(fa/0.14), and with the shearing rate and solid fraction at cooling rate 4℃/min is shown as η=0.55e(fa/0.11-γ/87.59), as well as the cooling rate and shearing rate have an important effect on the microstructure of the semi-solid AZ91D alloy slurry, and decreasing the cooling rate and increasing the shearing rate are favorable to the nondendritic evolution of the primary grains.

Research on fractal characteristics of primary phase morphology in semi-solid A356 alloy

Semi-solid slurry of A356 alloy was prepared by low superheat pouring and slightly electromagnetic stirring, and the fractal characteristics of morphology in semi-solid primary phase was researched. The fractal dimensions of primary phase morphology in the semi-solid A356 alloy were calculated by the program written to calculate the fractal dimensions of box-counting in the image of solid phase morphology in semi-solid metal slurry. The results indicated that the morphology of primary phase in semi-solid A356 prepared by low superheat pouring and slightly electromagnetic stirring is characterized by fractal dimension, and the primary phase morphology obtained by the different processing parameters has different fractal dimensions. The morphology of primary phase at the different position of ingot has different fractal dimensions, which reflected the effect of solidified conditions at different positions in the same ingot on the morphology of semi-solid primary phase. Solidification of semi-solid alloy is a course of change in fractal dimension.