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    Effect of two-pass friction stir processing on the microstructure and mechanical properties of as-cast binary Al-12Si alloy
    (Elsevier Science Sa, 2015) Aktarer, S. M.; Sekban, D. M.; Saray, Onur; Kucukomeroglu, T.; Ma, Z. Y.; Purcek, G.
    The effect of two-pass friction stir processing (FSP) on the microstructural evolution, mechanical properties and impact toughness of as-cast Al-12Si alloy was investigated systematically. Severe plastic deformation imposed by FSP resulted in a considerable fragmentation of the needle-shaped eutectic silicon particles into the smaller ones. The length of eutectic Si particles decreased from 27 +/- 23 mu m to about 2.6 +/- 2.4 mu m. The average aspect ratio of 6.1 +/- 5.1 for eutectic Si particles in the as-cast state decreased to about 2.6 +/- 1.0 after FSP with a corresponding increase in their roundness. The hardness, strength, ductility and impact toughness of the alloy increased simultaneously after two-pass FSP. The increase in the yield and tensile sirength values after FSP was about 20% and 29%, respectively. The FSPed alloy exhibited 25% elongation to failure and 15% uniform elongation which were almost seven times and five times higher, respectively, than those of the as-cast alloy. The hardness of the alloy increased from 58 Hv0.5 for the as-cast state to about 67 Hv0.5 after FSP. The absorbed energy during impact test increased to about 8.3 J/cm(2) after FSP, which is about seven times higher than that of the as-cast alloy. Improvements in all mechanical properties were mainly attributed to the radical changes of the shape, size arid distribution of the eutectic silicon particles along with the breakage and refined of the large alpha-Al grains during two-pass FSP. (C) 2015 Elsevier B.V. All rights reserved.
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    Microstructure, mechanical properties and formability of friction stir processed interstitial-free steel
    (Elsevier Science Sa, 2015) Sekban, D. M.; Saray, Onur; Aktarer, S. M.; Purcek, G.; Ma, Z. Y.
    The microstructure, mechanical properties and stretch formability of fine-grained (FG) interstitial-free steel (IF-steel) formed by friction stir processing (FSP) was investigated systematically. One-pass FSP drastically refined the microstructure with aid of dynamic recrystallization (DRX) mechanism during processing and formed volumetric defect free basin-like processed region (PR) with a mean grain size of 5 mu m (initial grain size was 40 mu m). This microstructural evolution brought about a considerable increase in both hardness and strength values of IF-steel without considerable decrease in ductility values. Also, strain hardening dominated deformation behavior was obtained with the FSPed samples as an essential property for the engineering application. Coarse-grained (CG) IF-steel demonstrated high formability with an Erichsen index (El) of 2.88 mm. Grain refinement by FSP yielded very close El value of 2.80 mm with increasing punch load (F-El). Force-displacement curves obtained in each process conditions reflected a similar membrane straining regimes where samples uniformly thinned under biaxial tension loads with aid of strain hardening capability. The formation of FG microstructure by FSP reduced the roughness (orange peel effect) of the free surface of biaxial stretched sample by decreasing the nonuniform grain flow leading to the so-called orange peel effect. It is concluded that a good balance of strength, ductility and strain hardenability along with equivalence formability to CG condition can be achieved by FSP as a single step practical procedure. (C) 2015 Elsevier B.V. All rights reserved.