Kadir Kocatepe

Tensile fracture behavior of AA7075 alloy produced by thixocasting

Abstract Tensile fracture behaviors of thixocast, artificially aged thixocast (thixocast+T6) and as-extruded AA7075 alloys were investigated. The microstructural and fractographic observations were carried out using optical microscope (OM) and scanning electron microscope (SEM). Experimental studies showed that as-extruded and thixocast+T6 specimens exhibited considerably more excellent mechanical properties than as-thixocast AA7075 specimen. T6 artificial heat treatment with prolonged solution treatment significantly improved the tensile properties of the thixocast AA7075 alloy. The tensile properties of as-extruded and thixocast+T6 specimens were close to each other. In as-thixocast specimen having remarkable micro-cracks, fracture was intergranular brittle type. The ductile fracture surfaces were observed in as-extruded and thixocast+T6 specimens. In as-thixocast specimens, decohesion started between the eutectic–matrix interfaces and propagated through grains. Micro-void coalescence was the dominant form of fracture in thixocast+T6 heat treated specimens. The micro-voids nucleation was initiated at the interface between the matrix and multinary eutectic structure.

EXPERIMENTAL STUDY ON THE EFFECT OF LIQUID METAL SUPERHEAT AND CASTING HEIGHT ON INTERFACIAL HEAT TRANSFER COEFFICIENT

In this article, the interfacial heat transfer coefficient (IHTC) is investigated as a function of superheat temperatures (100°C, 150°C, and 200°C) and casting heights (100, 150, and 200 mm). The experiments were conducted for a liquid alloy (Al-Si 12.9%) on water-cooled copper chill during vertically upward solidification of a eutectic Al-Si alloy casting. A finite difference method (FDM) is applied for the numerical used for solution of inverse heat conduction problem (IHCP), the so-called Becks method. Computer-guided thermocouples were connected with the chill and casting, at six positions and the time-temperature data were recorded automatically. As the lateral surfaces are very well heat isolated, the unidirectional solidification process started vertically upward at the interface surface. The measured time-temperature data files were used by FDM using explicit technique. The experimental and calculated temperatures have shown excellent agreement. The IHTC increases as superheat temperatures increases. However, the casting height (100, 150, and 200 mm) has no significant effect on the IHTC. It changed only maximum peak values of the IHTC and increased air gap formation time with increasing casting height.

Effects of Pouring Distance and Holding Time on the Thixotropic Microstructure of A357 Alloy

Abstract Cooling slope casting process is one of the methods used to produce feedstock material having non-dendritic structure for thixocasting of A357 aluminum alloy. In the present work, the effect of various pouring distances of cooling slope on the microstructure of A357 aluminum alloy was investigated. For this purpose, the molten metal at 630 °C superheat temperature was poured on the surface of water-cooled slope. Reheating of feedstock ingots at a temperature of 572 °C with various holding times was also performed to achieve a spheroidal microstructure. Experimental results showed that optimum spheroidal microstructure of thixocasted A357 ingots having a cell size of 45 μm and a shape factor of 0.60 of primary Al were obtained when a pouring distance of 350 mm on the cooling slope is used. A holding time of 5 min at 572 °C reheating temperature of feedstock ingots with 350 mm pouring distance provided optimum thixotropic properties.

Effects of the thixocasting injection velocity on tensile properties of an A357 Al alloy

Abstract In this study, the effects of injection velocity on tensile properties and hardness of A357 aluminum alloy produced by thixocasting process were investigated. For this purpose, feedstock produced by cooling slope process were first held at reheating temperature of 572 °C and then formed at injection velocities of 1, 1.5, 2 and 2.37 m × s−1, respectively. In order to make comparisons, other specimens were produced by high pressure die casting method via melted metal at the casting temperature of 630 °C using same injection velocities. Tensile test and hardness measurements were conducted for all specimens. The best tensile test properties were obtained for the specimens produced by thixocasting process at injection velocity of 2.37 m × s−1. For this specimens, the values of 0.2 % proof strength, tensile strength and total elongation were obtained as 89.9 MPa, 237.3 MPa and 20.7 %, respectively. Furthermore, its hardness were measured as 74 HV.

ÖTEKTİK Al-Si ALAŞIMINDA SOĞUTUCU YÜZEY PÜRÜZLÜLÜĞÜNÜN ARAYÜZEY ISI TRANSFER KATSAYISINA ETKİSİ

Ara yuzey isi transfer katsayisinin metalden kaliba olan isi transfer orani uzerinde onemli rolu vardir. Dokumlerin makro ve mikro yapilari esas olarak isi transfer oranina baglidir. Bu calismanin amaci su sogutmali celik sogutucu (H13) uzerinde tek yonlu dikey olarak katilasan otektik Al-Si alasiminda sogutucu yuzey puruzlulugunun ara yuzey isi transfer katsayisina etkisini incelemektir. Ara-yuzey isi transfer katsayisi sogutucu yuzey puruzlulugune bagli olarak, Sonlu Farklar Metoduna dayali FORTRAN programi ile belirlenmistir. Sonuc olarak; isi transfer katsayisi sogutucu yuzey puruzlulugu azaldikca artmistir, maksimum isi transfer katsayisi degerleri; 0.15 µm sogutucu yuzey puruzlulugunde 3320 W/m 2 K, 0.22 µm sogutucu yuzey puruzlulugunde 3302 W/m 2 K ve 2.47 µm sogutucu yuzey puruzlulugunde 2954 W/m 2 K olarak elde edilmistir, ara yuzey isi transfer katsayilari zamanin guclu bir fonksiyonu olarak belirlenmistir, numerik cozumun hassasligi Fourier sayisi azaldikca artmistir, optimum cozum hesaplama zamani goz onunde tutularak Fo=0.0625 ve Dx=0.25 mm ile elde edilmistir