Solid State Phenomena Vols. 116-117

Abstract: An important step in the processing of semi-solid metals is the inductive re-heating of the feedstock material. The heating should lead to an uniform billet temperature in order to obtain good forming results. The billet is supposed to be heated to the target temperature as fast as possible and at the same time it must be guaranteed, that the outer area of the billet does not melt prematurely. Conventionally the open-loop trajectories consist of simple power over time diagrams and are generated by extensive experiments. By using an open-loop control scheme it is possible to chose a desired trajectory for the middle axis temperature of the billet which respects the given constraint on the heating process. By taking advantage of the flatness property of the system, an open loop trajectory for the coil current can be calculated which ensures the desired behavior of the axis temperature. The shape of the trajectory is determined by the shape of the desired trajectory and the temperature dependent material properties, which have to be known with the needed accuracy. The losses of the converter and induction coil are estimated online so that the induced power is known. The trajectory ensures that the billet is heated to a temperature just below the solidus temperature without overheating of the billet’s surface and with a very homogeneous temperature distribution. The Experiments have been conducted using A356 aluminum alloy.

Abstract: Thixoextrusion for 7003 and 7075 Al wrought alloys with different content of Cu was carried out with respect to isothermal holding temperature in the partially remelted semisolid state. Especially, despite higher extrudability index of 7075 Al wrought alloy compared with the extrudability index of 7003 Al wrought alloy, the maximum extrusion pressures of 7075 Al wrought alloy were about the same as the maximum extrusion pressures of 7003 Al wrought alloy at temperature with the liquid fraction 10% (7003-624
, 7075-609
). The hardness values of thixoextruded specimens were higher than that of as-quenched specimens only after partial remelting. The microstructures of thixoextruded specimens did not show any dendrites or rosette-like structures. The average grain sizes of thixoextruded specimens were smaller than average grain sizes of homogenized specimens.

Abstract: The microstructural evolution and mechanical response in compression in the semi-solid state of previously extruded AZ91 alloys containing two levels of Ca additions (1mass%Ca (AZC911) and 2mass%Ca (AZC912)) have been studied. Ca additions have a significant effect on microstructural evolution and compression behavior of the AZ91 alloy. At 515°C, the liquid fraction in AZC911 is larger than that in AZC912, so that the compression stress of the AZC912 alloy was found to be much larger than that of the AZC911 alloy. This behavior is explained through DSC analysis which suggests that some solid Al2Ca phase remains in AZC912 alloy at this temperature. Increasing the remelting temperature for this alloy leads to more liquid and coarsening of the solid particles occurs with increasing holding time.

Abstract: Recently, a rheocasting process has been interested to produce a structural part for an automobile industry and so the slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important to produce a high quality and cost effective part in the rheocasting process. An electromagnetic (EM) stirrer employing for a slurry making process was designed and prepared to induce simultaneously both a circumferential and vertical fluid flow of a melt in order to control a rotation angle of EM stirring of a poured melt. In the present study, the semi-solid slurry of Al-15%Cu alloy was produced in the EM stirrer and its microstructure and the orientation distribution function calculated from the pole figure data was examined in accordance with a various rotation angle of EM stirring between 0˚ (circumferential flow) and 90˚ (vertical flow). The size and morphology of primary α phase was affected with a rotation angle of EM stirring and the finest and the most globular primary α phase could be obtained at a rotation angle of EM stirring of 45˚ and 60˚. Also, due to the EM stirring the tendency to random orientation was appeared. Also, the tendency of random orientation was the most at a rotation angle of 45˚ and 60˚. Therefore, it was considered that the rotation angle of EM stirring of 45˚ and 60˚ was the most effective to induce the non-dendritic growth of primary solid phase of EM stirred Al-Cu alloy.

Abstract: This paper focuses on fabricated slurry by electromagnetic stirring (EMS) with A6061 and A7075 wrought aluminum alloys. For this EMS process, it is important to find the optimal electromagnetic stirring conditions such as pouring temperature, stirring current and stirring time. After electromagnetic stirring according to each condition, the billets fabricated by EMS were investigated, as was the microstructure and cooling temperature of molten alloy which was directly cooled from liquid state to the semi-solid state during EMS. In use of cooling time and temperature data, it is possible to determine the forming time and stirring time required, respectively.

Abstract: Thixoextrusion process has been developed for improving productivity, reducing extrusion pressure, extending die life and even cost saving due to low energy consumption compared with conventional extrusion process. The present study focuses on AZ31 Mg wrought alloy to investigate the potential industrial applications of thixoextrusion process. The maximum extrusion pressures of thixoextrusion process were very lower than that of hot extrusion process. The average grain sizes of thixoextruded specimens were bigger than average grain sizes of hot extruded specimens. However, the grains of the thixoextruded specimens did not show any asymmetry. Also, the microstructures of thixoextruded specimens did not show any dendrites or rosette-like structures. The values of tensile strength and hardness of thixoextruded specimens were lower than that of hot extruded specimens. The specimens manufactured by the thixoextrusion were usually rather soft and their tensile strength was lower than those of hot extruded specimens. If necessary, their low tensile strength can be improved through appropriate heat.

Abstract: The name “high strength brass” is given to the wrought and cast alloys indicating their particular virtue of high strength, which can be achieved by additions of Al, Fe, Mn and Sn. Forgings made from copper base alloys offer a number of advantages over products made by other processes. However, because for forging more heat must be applied to the ingot which was solidified once, there are some disadvantages in the economy of energy and time. In this study, we investigated the microstructures and mechanical properties of high strength brass made by semi solid forging and compared them with those of conventionally forged product and gravity die casting. No shrinkage or gas hole was found in semi solid forgings. Fine equiaxed crystals developed at the center of semi solid forgings, while grains in the corner of semi solid forgings were elongated perpendicular to the pressure direction. The grains of semi solid forgings were smaller than those of conventional forgings and gravity die castings. It is suggested that a rapid heat transfer condition due to applied pressure is responsible for grain refinement. Tensile and yield strengths of semi solid forgings were as high as those of hot forgings but elongation was positioned between that of conventional forgings and gravity die castings.

Abstract: This study examined the application of color measurement to determining the chemical composition of Cu alloys such as Cu-Zn, Cu-Sn, and Cu-Al using a color recognition system. Measured data showed that the hue intensity changed exponentially when Zn, Sn, or Al in the Cu binary systems and fitted well with an exponential equation within the experimental error. This indicates that the obtained equations can be used for predicting the compositions of Cu-Zn, Cu-Sn, and Cu-Al systems by measuring the hue intensity.