An investigation on metallurgical characteristics of tungsten based tool materials used in friction stir welding of naval grade high strength low alloy steels S. Ragu Nathan a, ⁎, V. Balasubramanian b , S. Malarvizhi b , A.G. Rao c a Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India b Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India c Marine Materials Division, Naval Materials Research Laboratory (NMRL), Ambernath, Mumbai 421 506, Maharastra, India abstract article info Article history: Received 26 August 2015 Received in revised form 20 November 2015 Accepted 11 December 2015 Available online 15 December 2015 A non-consumable tool is a vital requirement for friction stir welding (FSW) of high melting point alloys such as steel and titanium. In this investigation, an attempt was made to understand the pre-weld and post-weld micro- structural characteristics of three tungsten based alloy FSW tools viz. 90%W, 95%W and 99%W. A naval grade high strength low alloy (HSLA) steel plates of 5 mm thickness were welded using the above tools with a tool rotational speed of 600 rpm and welding speed of 30 mm/min. Microstructural characteristics of the FSW tools, before and after welding, were analyzed using optical microscopy (OM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). From this investigation, it is found that the tool made of 99% W doped with 1% La 2 O 3 exhibited microstructural stability at elevated temperatures during FSW process. © 2015 Elsevier Ltd. All rights reserved. Keywords: Friction stir welding HSLA steel Tool material Tool life Microstructure 1. Introduction Friction stir welding (FSW) is a promising solid state joining tech- nique for welding of high melting point alloys such as steel, titanium and nickel. FSW is known for green, high productive welding technolo- gy with lower emission of harmful gases as compared to fusion welding process [1]. Mechanical action in the form of frictional stirring on the base material modify the coarse grain microstructures into fine grains due to plastic deformation and fast cooling rate [2]. Welding of steels will be affected by both the temperature and composition which extensively affects the microstructure evolution. Tool wear and plastic deformation are the two major problems encountered during FSW of high melting point alloys (L80 steel) [3]. The tool wear is either due to mechanical damage or chemical affinity of the tool and work piece; however, the plastic deformation is associated with the variation in stress, strain rate and temperature during FSW. Therefore, the tool must withstand high frictional and resultant forces experienced by the pin during initial plunge stage [4]. Most of the tool failures are reported during plunge stage, thus resulting in poor stirring and non-uniform grain refinement of the parent material in stir zone and thereby, violating the primary advantage of the FSW process itself. However, the tool should withstand and counter various forces generated at the initial plunge stage along with other factors and the tool also should traverse in the weld direc- tion. The tool pin is responsible for plasticizing the stir zone, excavating the softened material from advancing side to retreating side and consol- idating beneath it so as to begin the next cycle [5]. Almost care is required to avoid interaction gap between consecutive cycles, which eliminates defects like wormholes, pin hole, tunnel defect, etc. at the advancing side [6]. Jiye et al. (2014) explained the tool wear mechanism of W–La tool and advocated the use of conical pin with large pin length as compared to smaller one for better stability [7]. Hence, the taper conical larger pin was mostly recommended. During the tool transverse motion, the tool shoulder should support the pin by generating optimum heat to plasti- cize the parent material which in-turn reduce the flow stresses. This supportive heat generation should be uniform throughout the weld length. The variation in flow stresses during welding will lead to pin damage resulting in poor weld quality. Consolidation of the material extruded is secondary function of the shoulder. The tool distortion such as expansion or contraction, rubbing wear and if any one of the situation prevails will lead to poor weld quality, loss of tool pin, and severe plastic deformation [8]. Much of the tool degradation may be attributed to the high heat (temperature around 1200 °C) and stresses generated during FSW of steel. The brittle tendency of the polycubic boron nitride (PCBN) tool used for FSW of steel and titanium alloys can cause tool breakage due Int. Journal of Refractory Metals and Hard Materials 56 (2016) 18–26 ⁎ Corresponding author. E-mail addresses: ragucemajor@gmail.com (S. Ragu Nathan), visvabalu@yahoo.co.in (V. Balasubramanian), jeejoo@rediffmail.com (S. Malarvizhi), gouravdrdo@gmail.com (A.G. Rao). http://dx.doi.org/10.1016/j.ijrmhm.2015.12.005 0263-4368/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM