Solid State Phenomena Vols. 116-117

Abstract: The study on new rheoforming technologies of semisolid alloys has recently been one main subject interesting many professors, experts and industrials, and so many new methods about preparing semisolid alloy slurry and rheoforming process have been put forward. The semisolid slurry of AlSi7Mg alloy was prepared by being poured at a low superheat and stirred by an electromagnetic field at a low power for a short time, and then the slurry was further soaked for proper time and finally rheoformed into a key-shaped die cavity in this paper. The influence of slurry temperature, injection specific pressure and injection speed on the rheoforming process was also studied. The experimental results show that if AlSi7Mg alloy melt is poured at 630°C or 650°C and meanwhile stirred by an electromagnetic field at a low power for a short time, the most primary α-Al grains solidified in the slurry are spherical and only a few are rosette-like. If the slurry after made is then soaked in the liquidus-solidus range for some time, the ripening of primary α-Al grains is developed further and they become more spherical and larger. The experimental results also show that the slurry temperature, injection pressure and speed have a great influence on filling ability. The higher the slurry temperature, injection pressure and speed, the higher the slurry filling ability, and the shorter the core distance from the ingate, the more easily the die cavity is filled. With the given key-shaped casting, if only the slurry temperature should be equal to or more than 585,, or the injection pressure is equal to or more than 20 MPa, or the injection speed should be equal to or more than 1.73m/s, the key-shaped die cavity can be filled completely. The microstructural distribution in the casting is very homogeneous and it shows that the slurry prepared by the new method is very suitable for rheoforming and helpful to obtaining high quality castings.

Abstract: Asymmetrical rolling was performed by rolling AA 1050 sheets with different velocities of upper and lower rolls. In order to study the effect of roll gap geometry on the evolution of strain states and textures during asymmetrical rolling, the reduction per rolling pass was varied. After asymmetrical rolling, the outer thickness layers depicted shear textures and the center thickness layers displayed a random texture. With decreasing reduction per an asymmetrical rolling pass, the thickness layers depicting shear textures increases. The strain states associated with asymmetrical rolling were investigated by simulations with the finite element method (FEM).

Abstract: Metal matrix composites (MMCs) have been used for aviation, automobile and nuclear application due to their highlight properties such as superior strength, high specific stiffness, wear, and creep resistance at elevated temperature. For developing MMCs by liquid infiltration, preform will be required. In this research paper, developed a hybrid fiber preform and investigated their microstructure properties. Graphite nano fiber (GNF) and Alumina fiber were used for fabrication of the preform. The main objective of developing a preform is i) attainment of uniform distribution of reinforcement ii) minimization of mechanical and chemical damage. Since, it is extremely difficult to disperse nano-size fibers uniformly into the preform. An attempt has been made for hybrid preform with alumina micro-fiber and graphite nano fiber. Microscopic investigations revealed good disperse of the nanofibers in the preform.

Abstract: An alternative method has been proposed for the continuous and sample production of SSM slurry for the rheo-forming process. The process named “Low Superheat Pouring with a Shear Field (LSPSF)” dose not use the conventional stirring process, instead, it uses solidification conditions to control nucleation, nuclei survival and grain growth by means of low superheat pouring, vigorous mixing and rapid cooling during the initial stage of solidification combined with thereafter a much slower cooling. The method has been applied to A356, 201 and A380 Al-alloys. The primary phases present in average equivalent diameter of 40-70μm, 35-50μm and 50-70μm for A356, 201 and A380, respectively. The morphology of primary phases is nearly spherical with shape factor of 0.78-0.86, 0.71-0.83 and 0.85-0.96 for A356, 201 and A380, respectively. For each of those alloys, there is no eutectic entrapped within the primary phase. The advantages of the LSPSF include process simplicity with high efficiency, easy incorporation into existing metal forming installation without infrastructure changes and a wide process window for pouring temperature.

Abstract: A rotating vane-cup rheometer is used to determine the rheological properties of semi-solid slurries, and a procedure is established for characterizing the rheology with emphasis given to the proper and self-consistent evaluation of the material constants.

Abstract: The aim of this work is to study the solidification conditions necessary to produce good quality/low defect metal alloy strip when thixorolling directly from the semi-solid state. To facilitate the study lead/tin alloys were chosen for their relatively low operating temperature. The objective is to extrapolate these findings to the higher temperature aluminium alloys. Three alloys (70%Pb- 30%Sn, 60%Pb-40%Sn, 50%Pb-50%wtSn) were used particularly to study the influence of the solidification interval. The equipment consists of a two roll mill arranged as an upper and lower roller, where both rollers are driven at a controlled speed. The lower roller is fed with semi solid alloy through a ceramic nozzle attached to the lower end of a cooling slope. Several types of nozzle and their position at the roller were tested. This produced different solidifications and consequently different finished strip. The alloys were first cast and then poured onto the cooling slope through a tundish in order to create a continuous laminar flow of slurry and uniformity of metal strip quality. The pouring was tested at different positions along the slope. The cooling slope was coated with colloidal graphite to promote a smooth slurry flow and avoid the problem of adherence and premature solidification. The metallic slurry not only cools along the slope but is also initially super-cooled to a mush by the lower roller whilst at room temperatures, thus enabling thixorolling. It was also found that the nozzle position could be adjusted to enable the upper roller to also contribute to the solidification of the metallic slurry. However the rollers and the cooling slope naturally heat up. Temperature distribution in these zones was analysed by means of three thermocouples positioned along the cooling slope and a fourth in the base of the semi solid pool within the nozzle. The objective being to design an optimum pouring and cooling system. The formed strip was cooled down to room temperature with a shower of water. Microstructures of the thixorolling process were analysed. The differences in solidification conditions resulted in differing qualities of finished strip and corresponding defect types, all of which are a serious quality issue for the rolled product.

Abstract: Microstructure evolution was investigated during the solidification of succinonitrile-5at.% water transparent alloy and Sn–15 wt.%Pb alloy under mechanical stirring through in-situ observation and quenching, separately. The results showed that the evolution of primary microstructures under stirring experienced the growth of single grain particle and the successive agglomerating and coarsening of multi-particles when the particle size reached a certain value. The increase of stirring rate promoted the globular growth of solidification microstructure after it nucleated in the melt and increase the grain size. Thus, the microstructure during semi-solid processing could be refined by a controlled stirring and cooling process, which depended on the optimization among the stirring rate, cooling rate and temperature at which the stirring rate is changed.

Abstract: We have developed new type semi-solid injection process, that is, runner-less injection process. In order to investigate the effects of solid fraction on microstructure and casting defects of AZ91D in new type semi solid injection process, semi-solid forming testing machine which has the same system as a runner-less injection machine has been made on an experimental basis. Its temperature controlling system has been established to obtain the homogeneous solid-liquid coexisted state in its injection cylinder. AZ91D billets are injected into a permanent mold by this machine in the semi-solid state. A shearing in the part of nozzle of injection cylinder is the most important to reveal thixotropic property of alloy slurry in semi solid forming process by injection machine. So it needs controlling of solid fraction to affect thixotropic property. In order to decrease casting defects and hold homogeneous structure, solid fraction more over 50% is needed. But when the solid fraction increases more than 50%, primary solid particles grow coarser, and then controlling method is required to suppress coarsening. In the case of less than 50% of solid fraction, liquid part preferentially fills inside the permanent mold and alloy slurry continue to fill the mold behind alloy liquid. Then large casting defects form at the boundary of both flows.

Abstract: This paper focuses on a rheo-forming of arm parts fabricated by an electromagnetic stirring system (EMS). This forming process takes place under high pressure of high pressure die casting and thin-walled casting is possible. Also, the productivity is higher than for low pressure die casting because of the shorter cycle time. Rheo-forming is advantageous because forming is performed in the semi-solid state with laminar flow and the gas content is low, which makes welding possible. Therefore, this paper examines arm parts with EMS and has investigated the mechanical properties after T6 and T5 heat-treatment.

Abstract: The quality of semisolid casting largely depends on the formability of the semisolid feedstock. Despite of the semisolid casting process conditions, the initial microstructure of the feedstock plays a significant role in determining the metal formability under the semisolid state. In this study, the effect of initial microstructure of A356 alloy on the mechanical forming response in its semisolid state was investigated. A wide range of the initial microstructures varied from a very coarse dendritic structure to a fine globular structure were produced in A356 alloy using the Controlled Nucleation Method, particularly by controlling pouring temperature during solidification. Cylindrical specimens with 12 mm in diameter and 10 mm in height were compressed to a height reduction of 8 mm at constant strain rates from 1.39
10-1 /s to 1.39
10-3 /s. Strain rate jump tests were also carried out in order to evaluate the strain rate sensitivity at high fraction solid of 0.9. The materials produced with a low superheat exhibit a fine globular structure. They showed a very low compression stress in the semisolid state compared with the materials poured at high temperatures, which have coarse and dendritic structure.