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The microstructure and mechanical properties of FSPed HSLA steel

Aktarer S. M., Küçükömeroğlu T.

Journal of Achievements in Materials and Manufacturing Engineering

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2016

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Vol. 75, nr 2

55--60

EN

Purpose: High-strength low-alloy (HSLA) steels have been used in a wide range of applications from automotive to ship building industry due to their low weight, formability and good weld ability. However, especially in the automotive industry, it is still attempted to improve this steel for the better formability and strength properties. Grain refinement is a well-known method to improve mechanical properties of metallic materials without changing their chemical compositions. Friction stir processing (FSP) is a new method of enhancing the properties of metals as a result of grain refinement by severe plastic deformation, which is based on the basic principles of Friction Stir Welding (FSW) technique. The purpose of this study is to investigate microstructural alteration and the main mechanical properties of HSLA after friction stir processing. Design/methodology/approach: HSLA steel sheet with a thickness of 1.5 mm was processed using a tungsten carbide (WC) tool consisting of a cylindrical shoulder and a cylindrical conical pin. The parameters of FSP are kept to a fixed tool rotation speed of 1600 rpm, traverse speed of 170 mm.min-1 and down force of 5 kN. The evaluation after and before FSPed of HSLA steel was performed by optical microscope, scanning electron microscope, tensile test and hardness measurement. Findings: After FSP, refined microstructure brought about a considerable increase in both hardness and strength values. The increase in the yield and tensile strength after FSP was about30% and 34%, respectively. Research limitations/implications: Electron backscatter diffraction (EBSD) mapping could not be done in this study. The EBSD mapping should be performed for detailed microstructural characterization of processed zone such as grain size distribution and misorientation angle distribution. Practical implications: FSP can be applied to other steel to obtain high strength steel without any decrease in their ductility properties by means of grain boundary strengthening mechanism. Originality/value: FSP, as a severe plastically deformation technique, is applied to many aluminium alloys and steels. However, only few studies were reported on FSPed HSLA steels. Moreover, further investigations are needed to identify the microstructural and mechanical properties of the FSPed HSLA steels.

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Friction stir processing of the AZ91 magnesium alloy with SiC particles

Iwaszko J., Kudła K., Fila K.

Archives of Materials Science and Engineering

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2016

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Vol. 77, nr 2

85--92

EN

Purpose: Purpose The main purpose of the research was friction modification by means of material stirring (FSP – Friction Stir Processing) of the surface layer of the AZ91 magnesium alloy with SiC particles. Design/methodology/approach: For the introduction of SiC particles and the formation of the composite structure in the surface layer of the magnesium alloy, the original multi chamber technology (MChS), developed as part of this study, was used. The scope of research verifying the effectiveness of the friction modification included both macro- and microscopic evaluation of the structural changes triggered by the treatment. Findings: The research results showed that friction modification of the AZ91 magnesium alloy leads to a strongly refined structure and intensively dispersed SiC ceramic particles in the surface layer of the magnesium alloy, resulting in the formation of the composite structure of the metal-ceramic type. In the stirred zone (SZ), a prevalence of equiaxed grains sized 2–15μm was observed, whereas the degree of refinement of structure depended on the treatment parameters, especially on the rotation speed of the stir tool. In the thermomechanically affected zone (TMAZ), deformed grains dominated, the location of which corresponded to the direction of the displacement of the plastified material during the FSP treatment. SiC particles have been found both in the SZ and in the TMAZ. Practical implications: The obtained results prove that using the FSP technology to modify the surface layer of magnesium alloys with SiC particles is an effective and promising solution with a high application potential, which allows for forming the material structure to a great extent. Originality/value: The structural research has shown that the Multi Chamber System technology enables a controlled and virtually lossless introduction of an additional phase in the course of the single-stage treatment, and minimizes the dislocation of the powder beyond the working area of the working tool.

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Mikrostruktura i właściwości stopu magnezu az91 po modyfikacji tarciowej z przemieszaniem materiału

Fila K., Iwaszko J.

Hutnik, Wiadomości Hutnicze

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2015

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Vol. 82, nr 5

354--358

PL

W pracy dokonano oceny wpływu modyfikacji tarciowej z przemieszaniem materiału (FSP − Friction Stir Processing) na strukturę i właściwości stopu magnezu AZ91. Zakres badań obejmował ocenę makro- i mikrostrukturalną warstwy wierzchniej stopu, rentgenowską analizę fazową oraz pomiar mikrotwardości. W materiale poddanym modyfikacji tarciowej stwierdzono silne rozdrobnieniu struktury oraz homogenizację materiału, a także obecność obszarów o zróżnicowanej strukturze, tj. strefy wymieszania, strefy odkształcenia termoplastycznego oraz strefy wpływu ciepła. W strefie wymieszania dominowały równoosiowe ziarna o długości od 1 μm do 5 μm, przy czym stopień rozdrobnienia struktury zależny był od zastosowanych parametrów obróbki, a zwłaszcza prędkości obrotowej narzędzia roboczego. W strefie odkształcenia termoplastycznego przeważały ziarna odkształcone, a ich lokalizacja korespondowała z kierunkami przemieszczania się uplastycznionego materiału podczas obróbki. Stwierdzonym zmianom strukturalnym towarzyszył wyraźny wzrost twardości materiału. Badania rentgenostrukturalne wykazały stabilność składu fazowego w warunkach towarzyszących modyfikacji tarciowej.

EN

The study assessed the impact of the friction stir processing (FSP) on the structure and the properties of AZ91 magnesium alloy. The research comprised both macro- and microstructural assessment of the surface layer of the alloy, an X-ray phase analysis, and the measurement of microhardness. The friction stir processed material showed considerable structure refinement and homogenization, as well as the presence of areas with differentiated structure, i.e. a stirred zone, a thermoplastic deformation zone and a heat-affected zone. In the stirred zone, equiaxed grains sized from 1 μm to 5 μm were dominant, whereas the degree of the structure refinement depended on the applied processing parameters, particularly on the rotational speed of the working tool. In the thermoplastic deformation zone, deformed grains were dominant and their location corresponded to the direction of the stir motion of the material plasticized during the processing. The structural changes were accompanied by an evident increase in the material hardness. X-ray diffraction revealed the stability of the phase composition under the circumstances accompanying the friction stir processing.