Volkan Kilicli

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.

The influence of the austenite dispersion on phase transformation during the austempering of ductile cast iron having a dual matrix structure

Abstract In the present work, an unalloyed ductile cast iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.% and 0.047 Mg wt.% was austempered for various times at both the conventional austenitizing temperature (single phase austenitic region) and the different intercritical austenitizing (partially austenitizing) temperature ranges to investigate the effect of parent austenite dispersion at the austenitizing temperature on the nature of austenite decomposition during austempering. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular austenitizing temperature and austempering time. It was concluded that in addition to proeutectoid and bainitic ferrite, the new ferrite (also called epitaxial ferrite) introduced into the partially austenitized structure during austempering and new ferrite formation and its content was dependent on the scale of the parent austenite dispersion at the austenitizing temperature, variation in austenitizing temperature and austempering time. The new ferrite formed only in the structure with coarse parent austenite dispersion.

Östemperlenmiş Küresel Grafitli Dökme Demirlerde İndüksiyonla Yüzey Sertleştirmenin Yorulma Sınırı Üzerine Etkisi

Bu calismada, ostemperlenmis kuresel grafitli dokme demirlerde induksiyonla yuzey sertlestirmenin yorulma siniri uzerine etkisi arastirilmistir. Bu amacla, GGG70 sinifi kuresel grafitli dokme demir numuneler; 900°C’de 90 dk ostenitlenmis ve ardindan 375°C sicakliktaki tuz banyosunda 120 dk ostemperlenmistir. Daha sonra dokulmus ve ostemperlenmis kosullardaki yorulma numunelerine yuksek frekansli induksiyonla yuzey sertlestirme uygulanmistir. Dokulmus halde, ostemperlenmis, dokulmus halde + induksiyonla sertlestirilmis ve ostemperlenmis+induksiyon ile sertlestirilmis numunelerin yorulma sinirlari, donel egmeli yorulma testi cihazi belirlenmistir. Deneysel sonuclar, test edilen numuneler arasinda en iyi yorulma siniri sonuclarini ostemperlenmis+yuzeyi induksiyonla sertlestirilmis numunelerin sergiledigini gostermistir.

Reo-Dökümle Üretilmiş AA7075 Alüminyum Alaşımının Metalografik İncelenmesi

Bu calismada reo-dokum yontemiyle sekillendirilen AA7075 aluminyum alasiminin mikroyapisi ve dokum hatalari metalografik olarak arastirilmistir. Al-Zn alasimlarindan endustride en cok kullanilan ve dovme aluminyum alasimi olan AA7075 alasimi yari-kati metal sekillendirme tekniklerinden reo-dokum yontemiyle sekillendirilmistir. Reo-dokumde dondurmeli entalpi dengeleme (swirled enthalpy equilibrium device-SEED) islemi ile uretilen numuneler kullanilmistir. Deneysel sonuclar, AA7075 alasiminin reo-dokumle sekillendirilebilecegini gostermistir. Reo-dokumle uretilmis AA7075 aluminyum alasiminda dentritik mikroyapi yerine kuremsi sekilde mikroyapi olusmustur. Reo-dokumle uretilmis AA7075 aluminyum alasiminda, sivi fazca zengin olan bolgelerde mikro segragasyonun sicak yirtilmaya neden oldugu anlasilmistir. Radyografik muayene sonucunda merkezde cekme boslugu ve sicak yirtilmanin varligi tespit edilmistir. Numunelerin kenar bolgelerinde sivi segregasyonundan dolayi sertlik degerinin daha dusuk oldugu tespit edilmistir.