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Microstructure, microhardness and tensile properties of FSWed DP 800 steel

100%

Kucukomeroglu T. , Aktarer S. M.

Journal of Achievements in Materials and Manufacturing Engineering

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2017

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tom Vol. 81, nr 2

56--60

EN

Purpose: Dual phase (DP) steels are widely used in the automotive industry due to their properties of a high balance of strength and formability. However, it is known that conventional welding of high strength steel leads to some undesirable results such as hardness decrease in the heat affected zone. Friction stir welding (FSW) is a new solid state joining method, which is used to join these steels due to its advantage of low heat input. The aim of this study is to evaluate the microstructural change and mechanical properties of friction stir welded DP800 steel. Design/methodology/approach: DP 800 steels with 1.5 mm thickness were subjected to friction stir welding, by using a tungsten carbide (WC) tool. The tool was tilted 2°, and downforce of the tool was kept constant at 6 kN. During processing, the tool rotation and traverse speed were fixed at 1600 rpm and 170 mm∙min-1, respectively. Findings: The friction stir welded region comprises martensite, bainite, refined ferrite. The average microhardness of stir zone has increased from 260 HV0.2 to about 450 HV0.2. The tensile sample shows a decrease in the ultimate tensile strength (σUTS) about 3%, from 827 MPa to 806 MPa for the joint. The yield strength (YS) of the joint is about 566 MPa and the value is near that of DP800. Research limitations/implications: The tungsten carbide tool used for the friction stir welding has suffered deterioration in the pin profile after 1 meter welding operation. It may be advisable to drill a pre-hole in the specimens for a longer tool life. Practical implications: Tool wear for industrial applications will be a major problem. Therefore, the use of tools with high wear resistance such as polycrystalline cubic boron nitride may be recommended. Originality/value: Works on friction stir welding of dual phase steels are limited and they mostly focus on spot welding. Also, this study systematically investigates the microstructure and mechanical properties of dual-phase 800 steels after the friction stir welding.

2

The microstructure and mechanical properties of FSPed HSLA steel

100%

Aktarer S. M. , Küçükömeroğlu T.

Journal of Achievements in Materials and Manufacturing Engineering

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2016

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tom 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.

3

Applicability of Friction Stir Welding to steels

80%

Çam G. , İpekoğlu G. , Küçükömeroğlu T. , Aktarer S. M.

Journal of Achievements in Materials and Manufacturing Engineering

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2017

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tom Vol. 80, nr 2

65--85

EN

Purpose: The friction stir welding (FSW) method is widely considered to be one of the most significant developments in joining technology to emerge in the last 30 years. The technique has originally been developed for joining difficult-to-fusion-weld Al-alloys, particularly for high strength grades and now widely used in various industrial applications, such as transport industries. On the other hand, the application of FSW to high temperature materials such as steels is hindered due to the problems associated with the stirring tools although there is a wide interest for the application of this technique to these materials. Design/methodology/approach: The aim of this review is to address the current state-of-the-art of FSW of steels, focusing particularly on microstructural aspects and the resulting properties of these joints and discuss the future prospects of this technique for steels. For instance, the use of FSW can be advantageous for joining steels in some special applications where conventional fusion welding processes fail to produce sound cost effective joints, and the high tooling costs of FSW can be justified (i.e. underwater joining of steel pipes or hot plate welding in steel mills). In this study, only structural steels (mainly plain C steels), ferritic stainless steels, austenitic stainless steels and duplex stainless steels will be considered and the other types of steels are out of the scope of this work although some examples are included in the discussion. Research limitations/implications: The tools experience high temperatures in FSW of steels, i.e., above 1000°C. The number of tool materials which can withstand such temperatures is very limited. In addition, the welding of many common steels can be readily conducted by various conventional fusion welding methods. These joining methods are very flexible, easy-to-perform and well established in industrial applications, which further prevents the application of FSW to these materials. These limitations are to be overcome for commercial exploitation of this technique for joining steels.