Vol.:(0123456789) 1 3 Metals and Materials International https://doi.org/10.1007/s12540-019-00472-3 Friction Stir Welding of Thick AA2519 Alloy: Defect Elimination, Mechanical and Micro‑Structural Characterization Mohammed Ubaid 1  · Dhruv Bajaj 1  · A. K. Mukhopadhyay 2  · Arshad Noor Siddiquee 1 Received: 1 August 2019 / Accepted: 16 September 2019 © The Korean Institute of Metals and Materials 2019 Abstract Friction stir welding (FSW) of thick aluminium sections will pave way for remarkable new defence applications, provided the accompanying challenges are successfully addressed. These challenges include elimination of tunnel defects, loss of strength in heat-treatable aluminium alloys, tool design and excessive process forces. Accounts of defect elimination, detailed micro-structural and mechanical characterization for thick section FSW are scarce in the published literature. Further, special strategies such as bobbin tool, pre-drilled hole for plunging and inverted double pass are generally deployed to overcome these challenges. This makes the process less productive and complex. In the present work, armor grade aluminum alloy AA2519-T87 plates having a thickness of 15.4 mm have been successfully joined with a joint efciency of greater than 75% without supplementary strategies. The disparity in the efects of tool rotational speed and welding speed in the formation of a sound weld has been addressed in detail to enhance the understanding of FSW of thick sections. Micro-hardness measure- ments have been carried out throughout the transverse cross-section of the welds and correlated with diferent zones and their corresponding microstructures. Scanning electron microscopy and X-ray energy dispersive spectroscopy have been used to characterize the coarse phase particles present in the weld zones. Keywords Friction stir welding (FSW) · Aluminium alloy AA2519 · Thick plates · Defect elimination · Joint efciency · Mechanical properties 1 Introduction Wrought aluminum alloys with copper as major alloying element are of prime importance to several strategic and technology intensive sectors. Classifed under the 2xxx series, these alloys fnd usage in cryogenic, aerospace and a variety of critical defence applications [1]. AA2519 is a relatively new armor grade aluminum alloy currently being utilized to develop lightweight combat structures. Charac- teristics like high-strength to weight ratio, high toughness as well as high resistance to stress corrosion cracking make AA2519 alloy suitable for ballistic applications [2, 3]. Being an age-hardenable alloy, this alloy derives its strength from strengthening precipitates and hence fusion welding of such alloys is unfavorable [1, 4]. A solid-state joining process such as friction stir welding (FSW) has proved to be the key enabler towards utilizing such an aluminum alloy for the targeted applications [5–12]. FSW studies conducted on AA2519 plates having a thick- ness of less than 8 mm have demonstrated the role of tool design in the material fow and heat balance in the welded plates. Tapered as well as straight cylindrical pin profles have exhibited defect free welds [13]. Moreover, underwater FSW (UFSW) has yielded a better joint efciency owing to suppression of precipitate dissolution and coarsening [14]. Studies aimed at optimizing traverse speed have also been conducted. In case of UFSW, lower traverse speeds resulted in sound welds [15]. Characterization of weldments from a microstructural viewpoint, establishing a relationship of micro-hardness values corresponding to various zones has also been performed [13–15]. Rao et al. [16] performed FSW on 10 mm and 16 mm thick plates of AA7075 alloy in order to study the mechanical prop- erties of the welds. Sound welds with a joint efciency of 70% and 53% were observed for 10 mm and 16 mm weldments, * Dhruv Bajaj maildhruv08@gmail.com 1 Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi 110025, India 2 Aeronautical Materials Division, Defence Metallurgical and Research Laboratory, Hyderabad 500066, India