Papers by Author: Yücel Birol

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Authors: Feriha S. Birol, Yücel Birol, Margarita Slámová
Authors: M. Dündar, Yücel Birol, A.S. Akkurt
Authors: Nurşen Saklakoğlu, Yücel Birol, Şefika Kasman
Abstract: Owing to its superior flow and mould-filling capability, a fully globular structure is essential for semisolid processing technologies. The present work was undertaken to identify the cooling slope casting process parameters that, upon heating to the semisolid state, gives the required globular structure for the ETIAL 160 alloy. Of the two pouring temperatures investigated, 605 °C and 615 °C, the lower pouring temperature was found to provide more globular grains surrounded by liquid phases.
Authors: Margarita Slámová, Yücel Birol, M. Dündar, A.S. Akkurt, Miloš Janeček
Authors: Yücel Birol
Abstract: Hypereutectic Al-Si based alloys are expected to be more wear resistant than the AlSi8Cu3Fe alloy, currently used in compressor connecting rods in die cast grades. Die casting of such alloys, however, is not straightforward. Semi-solid processing was thus considered to overcome the problems encountered in casting hypereutectic Al-Si alloys. Two experimental hypereutectic AlSiCuFe alloys were obtained by adding 8 to 12 wt% Si to the AlSi8Cu3Fe alloy. The ingots of these alloys were melted and cooled to very near their liquidus points, 625°C and 675°C respectively, before they were poured into a permanent mould in order to produce nondendritic feedstock for thixoforming. This low superheat casting process largely replaced α-Al dendrites with relatively smaller α-Al rosettes. The slugs machined from the ingots thus obtained were thixoformed after they were heated in situ in the semi-solid range for 5 minutes in a laboratory press. Several thixoformed parts were heat treated to the T6 temper with an accompanying increase in hardness from 91-96 HB to 130-131 HB.
Authors: Yücel Birol
Abstract: Thixoforming offers the possibility of forming complex aluminum parts with a superior quality and a reduction of processing steps. The production of a fine, equiaxed, globular microstructure is essential for thixoforming. Strain Induced Melt Activation (SIMA) and the Cooling Slope (CS) casting processes were employed to produce AA6082 thixotropic feedstock in the present work.. SIMA process produces such a microstructure through recrystallization of heavily deformed billets and a subsequent heat treatment in the mushy zone. Molten metal with a suitable superheat is cast over a water-cooled, inclined metal plate into a permanent mould to produce the thixotropic billet in the CS casting route. The effect of cold work and heating temperature and time were investigated for the SIMA process where as the effect of CS length, casting temperature on the final microstructures were investigated for a fixed set of heating conditions in the CS casting route.
Authors: Yücel Birol
Abstract: Heat-treatable Al-Mg-Si aluminum alloys for automotive body panel applications are shipped and formed in T4 temper while still formable and are subsequently given a bake cycle to increase strength by age hardening while the paint is cured. However, the hardening response during the rather short industrial paint-bake is impaired upon natural ageing after the solution treatment. It is thus essential to counteract the adverse effect of natural ageing on bake hardening. While this is often accomplished by an additional ageing cycle before the paint-bake treatment, reversion treatments ought to be used once substantial natural ageing has first taken place to improve the bake hardening response of a twin-roll cast 6016 sheet. The present work was undertaken to improve the paint bake response of a twin-roll cast 6016 sheet by employing reversion treatments before the paint-bake cycle. Reversion treatments between 200°C and 250°C was effective in improving the bake hardening response of the twin-roll cast 6016 sheet which, when processed without reversion, failed to meet in-service strength requirements with a rather poor bake hardening response of 35 MPa.
Authors: Miroslav Karlík, J. Siegl, Margarita Slámová, Yücel Birol
Authors: Yücel Birol
Abstract: Semi-solid processing of aluminium and magnesium alloys has matured to become a well established manufacturing route for the production of intricate, thin-walled parts with mechanical properties as good as forged grades. This innovative forming technology, however, still remains as a major challenge in the case of steels. One major limitation in the thixoforming of steels has been the lack of suitable tool materials which could withstand the complex load profile and significantly increased forming temperatures which promote chemical interaction between forming dies and liquid steel. For industrial application to be attractive, the lifetime of the thixoforming tools has to be sufficient to ensure the cost effectiveness of this technology. Forming experiments using conventional hot working tool steel as die material have shown massive wear and plastic deformation. Several alternatives including copper alloys, titanium-zirconium-molybdenum based high temperature application alloy TZM, graphite and ceramics with and without suitable ceramic coatings have been tested with limited success. An attempt was made in the present work to investigate the applicability of CrNiCo-based superalloy dies in small scale forming tests in an effort to contribute to the development of semi-solid processing of steels.
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