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 rheocasting, and the effects of pouring temperature and circulating cooling water flux on the microstructure of the slurry were investigated. The results show that at the pouring temperature of 640-680℃ and the circulating cooling water flux of 0.9 m3/h, the semi-solid A356 aluminum alloy slurry with spherical primary α(Al) grains can be obtained, whose shape factors are between 0.78 and 0.86 and the grain diameter can reach 48-68 μm. When the pouring temperatures are at 660-680℃, 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.     

Microstructural formation of semi-solid AZ91D alloy stirred by electromagnetic field

With the help of an electromagnetic stirring device, alloy melt quenching and EBSD (electron back scatter diffraction)analysis technology, the microstructure of the semi-solid AZ91D magnesium alloy slurry stirred by rotational electromagnetic field under different stirring power conditions has been studied. The results show that the size of primary α-Mg phase is reduced obviously when the solidifying alloy melt is stirred by rotational electromagnetic field, moreover, the primary α-Mg grains are changed to fine rosette grains or spherical grains which are proved to belong to the different grains in three-dimension by the EBSD analysis technology. The results also show that the stirring power is an important processing parameter in the preparation of the semi-solid AZ91D magnesium alloy slurry. The larger the stirring power, the finer the primary α-Mg grains, the less the rosette primary α-Mg grains, and the more the spherical primary α-Mg grains. Theoretical analysis indicates that a stronger flow motion leads to a more even temperature field and solute field and a stronger man-made temperature fluctuation in the alloy melt so that the specially fine rosette and/or spherical primary α-Mg grains are formed in the semi-solid AZ91D magnesium alloy slurry.     

Microstructure Evolution of Semi-solid Processed Hyper-eutectic Al-30%Si Alloy

An investigation was made on the influences of mechanical stirring on microstructure of hyper-eutectic Al-30%Si alloy (in mass fraction) during solidification. The primary Si crystals formed in the alloy melt were gradually changed from elongated platelets to near-spherical shapes by mechanical stirring. The spheroidization of primary St crystals occurs by the mechanism of bending and fracture of Si platelets, wear and collision between Si crystals, and coalescence of small Si particles. The influence of under-cooling and cooling rate of the alloy melt on primary Si crystals of semi-solid processed alloys is investigated as well. The increase of under-cooling and cooling rate decreases the size of primary Si crystals.     

Preparation of semisolid AlSi7Mg alloy slurry through weak traveling-wave electromagnetic stirring

The semisolid AlSi7Mg alloy slurry with large capacity was prepared by low superheat pouring and week traveling-wave electromagnetic stirring. The effects of electromagnetic stirring power and frequency on the shape and distribution of primary α-A1 grains in the AlSi7Mg alloy slurry were discussed. The experimental results show that the AlSi7Mg alloy slurry with fine and spherical primary α-A1 grains distributed homogeneously can be obtained. Under the condition of low superheat pouring and week traveling-wave electromagnetic stirring, when the pouring temperature is 630℃, raising the stirring power or frequency appropriately can gain a better shape of primary α-Al grains; but if the stirring power or frequency is increased to a certain value (1.72 kW or 10 Hz), the shape of primary α-A1 grains cannot be obviously improved and spherical primary α-Al grains distributed homogeneously can be still obtained.     

New method for the preparation of semi-solid AlSi7Mg alloy slurry

The effects of pouring temperature and short electromagnetic stirring with low strength on the microstructures of AlSi7Mg alloy were investigated. The results show that if liquid AlSi7Mg alloy is poured at 630 or 650℃, many primary α-Al grains in the solidified melt are rosette-like, and only a small number of them are spherical. However, if liquid AlSi7Mg alloy is poured at a lower superheat and meanwhile is stirred by an electromagnetic field at a low power for a short time, then most primary α-Al grains in the solidified melt become spherical, and only a few are rosette-like. The theoretical analysis indicates that the strengthened melt flow motion induced by the short electromagnetic stirring makes the temperature field more homogeneous in the melt, which is poured at a lower superheat, and makes the primary α-Al grains deposit in a larger region at the same time, so this new solidification kinetic condition leads to the microstructure of spherical primary α-Al grains. The experiments also demonstrate that pouring at an appropriate superheat and stirring by an electromagnetic field at a low power for a short time is a good new method for preparing the semi-solid slurry or billet of AlSi7Mg alloy.     

Semisolid A356 alloy feedstock poured through a serpentine channel

Fine spheroidal and equiaxed nondendritic microstructures required for semisolid casting can be formed through a serpentine pouting channel. Effects of the pouring temperature and the vertical section length of the serpentine pouring channel were studied. The results indicate that both favorable semisolid microstructures of A356 aluminum alloy can be generated by a serpentine pouring channel, and the solid shell inside the channel can be avoided when the pouting temperature is in the range from 660 to 680℃. It is also shown that the vertical section length of the serpentine pouting channel has a significant influence on the microstructure of the feedstock, namely both the microstructure of the feedstock and the microstructure uniformity in the radial direction get worse when the vertical section length is shortened; moreover, the pouring temperature for obtaining the feedstock with suitable microstructure decreases. The relevant mechanisms were discussed.     

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.     

Effect of rheo-diecast process on the mechanical properties of A390 alloy by serpentine channel

In this paper, the effects of rheo-diecast process parameters and T6 heat treatment on the microstructure and mechanical properties of the rheo-diecasting (RDC) semi-solid A390 alloy prepared through pure copper serpentine channel were investigated. The results indicate that the mechanical properties of the RDC samples change with the pouring temperature and injection pressure. In this case, a lower pouring temperature results in better tensile strength and elongation of the RDC A390 alloy; however, the tensile strength and elongation decrease when the pouring temperature decreases to 660°C. Higher injection pressures result in the improved mechanical properties of the RDC A390 alloy. To some extent, T6 heat treatment improves the tensile strength and ductility of the RDC A390 alloy compared to those of the non-heat treated alloy. However, when the pouring temperature and injection pressure are greater than 670°C and 70 MPa, respectively, the mechanical properties are sharply diminished.     

Effect of pouring height on the solidified microstructure of AlSi7Mg alloy

The effect of different pouring heights and evenly soaking process in the liquidus and solidus range on the solidified microstructure of AlSi7Mg alloy has been studied. The results show that if the pouring temperature is 630 or 650°C and the pouring height is 40 mm, the microstructure of the solidified melt is not homogeneous and there are many rosette-like primary α-Al grains. But if the pouring height is increased to 400 mm, the solidified microstructure becomes more homogeneous and favorable to obtain spherical primary α-Al grains in the solidified melt. With further being evenly soaked in the liquidus and solidus range for some time, the temperature difference between the melt center and the melt periphery can be controlled within ±2°C and the primary α-Al grains will evolve into more spherical grains. The theoretical analysis indicates that the higher pouring height promotes the melt flow motion and makes the temperature field in the melt more homogeneous and restrains the large rosette primary α-Al grains. This flow motion can also promote the ripening effect and the primary α-Al grains in the melt are gradually changed into spherical grains. It can be concluded from the experiments that pouring at an appropriate superheat and from a proper height is a good new method for preparing the semisolid slurry of AlSi7Mg alloy, its process control is easy and the preparation cost is lower.     

Semi-solid slurry preparation and rolling of 1Cr18Ni9Ti stainless steel

The preparation and rolling of the semi-solid slurry 1Cr18Ni9Ti stainless steel were researched. The experimental results show that when stirred for 2-3 min under the given test condition, the semi-solid slurry with about 50%-60% (volume fraction) solid and the spherical primary austenitic grains in the size of 100-200 μm can be obtained, and it is easy to be discharged from the bottom little hole of the stirring chamber. The semi-solid slurry of 1Cr18Ni9Ti stainless steel can be rolled into the given plate successfully.However, the solid phase and liquid phase are easily separated in the rolling process, so that the solid primary austenitic grains are concentrated in the center and the liquid phase is near the edge of the rolled plate. The strengths of the plate rolled in the semi-solid state are higher than that of the traditionally repeated hot-rolled plate of 1Cr18Ni9Ti stainless steel.     

Microstructure Characteristics and Apparent Viscosity of Hypereutectic Al-24%Si Alloy Melt During Semi-solid State Stirring

The microstructural evolution and apparent viscosity of hypereutectic Al-24%Si alloy during semi-solid state shearing were studied with a Searte type viscometer. When the alloy melt was continuously stirred from 720 ℃ to eutectic temperature, the primary Si crystals were gradually changed from elongated platelets to near-spherical shapes. It was found that some nondendritic α-phase formed when the melt was stirred below 585 ℃. The experiment showed that the semi-solid stirring had strong effect on inhibiting the anisotropic growth of Si crystals during solidification. The apparent viscosity of the alloy melt increased slowly with the decreasing of temperature before the formation of nondendritic α-phase, which caused the dramatic increase of apparent viscosity.     

Microstructure Study on Semi-solid 60Si2Mn during Compressing

The microstructure behavior and deformation mechanism of semi-solid 60Si2Mn fabricated by electromagnetic stirring under different deformation conditions during thixoforming are studied by means of Gleeble-1500 thermal-strain-stress simulator. The micro-structure of deformed 60Si2Mn and that of the non-deformed are compared. The results show that the grain size within deformation zone decreases as the amount of deformation increases or deformation temperature decreases, but deformation fate has small effect on grain size when the rate is very low. Besides, there is a critical amount of deformation to reduce grain size. These results offer theoretical and experimental basis for further producing semi-solid material with high melting temperature and direct forming of semi-solid slurry.     

Two-dimensional simulation on the rolling process of semi-solid 60Si2Mn by finite element method

The effect of various process variables on the law of metal flow for semi-solid rolling 60Si2Mn was studied by finite element method. Semi-solid 60Si2Mn can be described as compressible rigid visco-plastic porous material saturated with liquid. In terms of thermo-mechanical coupling condition, the distributions of stress, velocity and temperature were studied using software MARC. The simulation results showthat the rigid visco-plastic model can accurately describe the semi-solid 60Si2Mn rolling process.The great deformation can achieve completely in view of low flow stress of semi-solid slurry.     

Filling ability of semi-solid A356 aluminum alloy slurry in rheo-diecasting

The effects of slurry temperature, injection pressure, and piston velocity on the rheo-filling ability of semisolid A356 alloys were studied by the reho-diecasting methods. The results show that the slurry temperature of the semi-solid A356 aluminum alloy has an important effect on the filling ability; the higher the slurry temperature, the better is the filling ability, and the appropriate slurry temperature for rheo-filling is in the range of 585-595℃. The injection pressure also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the injection pressure is in the range of 15-25 MPa. The piston velocity also has a great effect on the filling ability, and it is appropriate to the rheo-filling when the piston velocity is in the range of 0.072-0.12 m/s. The filling ability of the slurry prepared by low superheat pouring with weak electromagnetic stirring is very good and the microstructural distribution in the rheo-formed die castings is homogeneous, which is advantageous to the high quality die casting.     

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg_2Si in situ composites

The effect of high-speed direct-chill(DC) casting on the microstructure and mechanical properties of Al–Mg_2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg_2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg_2Si particles(from 28 to 12 μm), the spacing of eutectic Mg_2Si(from 3 to 0.5 μm), and the grains of AA6061 alloy(from 102 to 22 μm). The morphology of the eutectic Mg_2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg_2Si phase and the grains in the Al–Mg_2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg_2Si composites and AA6061 alloy billets.     

Isothermal grain growth of reactive spray formed 7075 alloys in semi-solid state

The grain growth behavior in reactive spray formed 7075+2.91vol%TiC Al alloy was studied and compared with that of spray formed 7075 Al alloy at semi-solid state. The effects of in-situ TiC particles on the microstructure of spray formed 7075 Al alloy were also investigated. The specimens were heat-treated isothermally at various temperatures between the solidus and liquidus of 7075 Al alloy for times in the range of 10-60 min, then quenched in water. The microstructure of reheated specimens was characterized using scanning electron microscopy and optical microscopy. The grain size was measured using a mean linear intercept method.Results show that the in-situ TiC particles can effectively retard grain growth and refine the grain at a limited size. The grain growth exponent in Arrhenius equation increases from 2 to 3, which indicates that the in-situ TiC particles have the significant pinning effect on grain coarsening in the semi-solid state.     

DSC analyses of static and dynamic precipitation of an Al–Mg–Si–Cu aluminum alloy

In the present investigation, both static and dynamic precipitations of an Al–Mg–Si–Cu aluminum alloy after solid-solution treatment(SST)were comparatively analyzed using differential scanning calorimetry(DSC). Dynamic aging was performed in the SST alloy through equal channel angular pressing(ECAP) at different temperatures of room temperature, 110, 170, 191 and 300 1C. For comparison, static artificial aging was conducted in the SST alloy at 191 1C with two aging times of 4 and 10 h. The DSC analyses reveal that the dynamic precipitation has occurred in the ECAPed samples, while the activation energies associated with the strengthening precipitates in the dynamic samples are considerably higher than the energies in the SST and static aged samples. The higher activation energies are probably attributed to the smaller grains and higher dislocation density developed after ECAP. The results in the present investigation allow the prediction of the type of the dynamic precipitates to influence the strength of the ultrafine grained alloy during ECAP at various temperatures.     

Effects of P addition on the microstructure and mechanical properties of Mg–5%Sn–1.25%Si alloy

The effects of Al-P addition on the microstructure and mechanical properties of as-cast Mg–5%Sn–1.25%Si magnesium alloy were investigated. The results show that the phases of the as-cast alloy are composed of α-Mg, Mg2 Sn, Mg2 Si, little P, and AlP. The Chinese character shape Mg2 Si phase changes into a granular morphology by P addition because AlP can act as a heterogeneous nucleation core for the Mg2 Si phase. When 0.225wt% of Al–3.5%P alloy is added, the mechanical properties of the Mg–5%Sn–1.25%Si alloy are greatly improved, and the tensile strength increases from 156 to 191 MPa, an increase of 22.4% compared to the alloy without P addition. When the amount of Al–3.5%P reaches 0.300wt%, a segregation phenomenon occurs in the granular Mg2 Si phase, and the tensile strength and hardness decrease though the elongation increases.     

Microstructural evolution during reheating of A356 machining chips at semisolid state

The microstructural evolution of A356 machining chips in the semisolid state was studied at different temperatures and holding times. The results showed that the elongated α-Al grains first recrystallized in the semisolid state and then became globular with a high shape factor (SF). Both the temperature and the holding time clearly affected the grain size and SF. When the heating temperature or holding time was increased, the grain size and SF gradually increased and finally became stable. Moreover, the Vickers hardness of primary α-Al grains gradually decreased with increasing heating temperature. The optimal slurry for semisolid processing, with a good combination of grain size and SF, was obtained when the chips were held at 600℃ for 15 min. The semisolid slurry of A356 chips exhibited a lower coarsening rate of α-Al grains than those produced by most of the conventional semisolid processes. The coarsening coefficient was determined to be 436 μm3·s-1 on the basis of the linear Lifshitz-Slyozov-Wagner (LSW) relationship.     

Numerical simulation on the rheo-diecasting of the semi-solid A356 aluminum alloy

The effect of injection pressure, piston velocity, and the forming temperature of semisolid slurry on the filling behavior of the semi-solid A356 aluminum alloy was investigated by simulation methods. The simulation results show that these processing parameters have an important effect on the filling behavior of the semi-solid A356 aluminum alloy. The slurry flows steadily in the cavity when the injection pressure, the piston velocity, and the forming temperature are low, but it is prone to turbulent flow when the injection pressure, the piston velocity, and the forming temperature are much higher. Therefore it is necessary to determine the appropriate processing parameters to get a steady flow of the slurry in the cavity.