Journal of Mechanical Science and Technology 33 (8) (2019) 3981~3997 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online) DOI 10.1007/s12206-019-0136-1 Effect of pinless tool shoulder diameter on dieless friction stir extrusion joining of AA 5052-H32 and AA 6061-T6 aluminum alloy sheets † Tinu P. Saju 1 , R. Ganesh Narayanan 1,* and Barnik Saha Roy 2 1 Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Assam, 781039, India 2 Department of Mechanical Engineering, National Institute of Technology Agartala, Tripura, 799046, India (Manuscript Received April 17, 2018; Revised September 7, 2018; Accepted October 30, 2018) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract A new spot joining process called dieless friction stir extrusion is proposed, in which simultaneous mechanical interlocking (collar formation) and metallurgical bonding is the key aspect of joint formation. The pinless flat stir tool eliminates pinhole and hook formation, the common defects of friction stir spot welding. Aluminum alloy sheets such as AA 5052-H32 and AA 6061-T6 are spot joined and the effect of change in tool shoulder diameter (10-18 mm) on joint strength and joint formation is evaluated. Lap shear fracture load of 6.22 kN obtained at optimum tool shoulder diameter of 14 mm is higher than that of conventional spot joining techniques. Macrostructure analysis revealed that increase in tool shoulder diameter results in poor mechanical interlocking. Consequently, pin shear failure is com- mon at highest tool shoulder diameter. Critical weak zones of failure are identified. The change in tool shoulder diameter has significant impact on the external joint morphology. Keywords: Aluminum alloy; Dieless friction stir extrusion; Failure modes; Hardness; Lap shear test; Microstructure ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction Lightweight metals such as aluminum alloys have been re- placing conventional steel sheets for body panels, especially in automobiles [1]. Traditional fusion welding processes are getting obsolete by the emergence of efficient solid-state tech- nologies for joining these sheet metals. Solid-state joining techniques such as friction stir welding (FSW) and friction stir spot welding (FSSW) produce defect free joints with sound mechanical properties and metallurgical characteristics in aluminum alloys under optimized conditions [2]. However, studies show that the pinhole formation and hook defect have a detrimental effect on the formation and strength of these joints. Recent developments such as keyhole refilled FSSW [3] and probeless FSSW [4] can successfully eliminate these defects. However, the principle of frictional heat generation and further plastic deformation remains same for all these processes. Several friction stir based processes have been developed for joining dissimilar sheet metals such as aluminum alloys and automotive steel. Friction stir extrusion (FSE) was devel- oped to join dissimilar sheets, in which FSW was used to ex- trude and interlock aluminum into a pre-made groove in the steel sheet [5]. Joint strength upto 6 kN was obtained between AA 6061 sheet and low carbon steel sheet with slit saw groove. The quality of the joint depended on tool profile and the concerns of intermetallic bonding was eliminated. Friction stir forming (FSF) is a solid-state spot joining process in which the joint strength is contributed by simultaneous me- chanical interlocking and metallurgical bonding. The joint formation is enabled by the extrusion and further rivet head formation of the upper sheet metal through a pre-drilled hole in the lower sheet. The frictional heat flux generated by a pinless flat stir tool promotes plastic deformation of the upper sheet. Lazarevic et al. [6] studied the effect of process parame- ters such as rotational speed, plunge rate, plunge depth and sheet thickness on the joint strength of FSF joints between aluminum alloys such as AISI 5182, AA 6014, and GMW2 steel (mild steel) sheet. Quality joints were obtained with rivet head neck diameter of 3 mm and anvil cavity depth of 0.55 mm. The lap shear strength of the joints was better than that of self-pierce riveted joints. A detailed understanding about the stages of FSF, the effect of anvil cavity design and the micro- structure evolution was reported by Lazarevic et al. [7]. It was revealed that the material flows orthogonal to the tool axis and circumferential deformation of the material occurs with rota- tion of the tool. The effect of tool rotational speed on the for- mation of FSF joints between aluminum alloys such as AA 5052-H32 and AA 6061-T6 of 2 mm thickness was conducted * Corresponding author. Tel.: +91 3612582669, Fax.: +91 3612690762 E-mail address: ganu@iitg.ac.in † Recommended by Associate Editor Young Whan Park © KSME & Springer 2019