Investigation of Bobbin Tool Friction Stir Welding

Abstract

Friction stir welding is a solid-state welding technique for joining similar and dissimilar materials. Bobbin tool friction stir welding (BTFSW) is a variant of the conventional friction stir welding process (CFSW). Compared with conventional FSW, BTFSW eliminates limitations of CFSW using a two shoulders configuration. Many characteristics of BTFSW such as no need for the backing plate, joining of hollow extrusions, simplicity in fixture requirement, and zero downforces make it more attractive over conventional friction stir welding. To explore the potentials of BTFSW, a detailed investigation of the technique is essential in terms of the effect of process parameters, tool designs, and techniques to improve the mechanical properties of the BTFSW joints. Friction stir processing (FSP) is a technique of the processing of materials in the solidstate phase and is based on the principle of friction stir welding (FSW) to fabricate surface composites, to achieve superplasticity, and fine grains. Bobbin tool technique which works on a similar principle of CFSW has potential in processing the materials and fabricating surface composites similar to FSP. Due to its double shoulder arrangement, the bobbin tool technique can be explored to simultaneously fabricate double-sided surface composites. In the present investigation, the preliminary investigation focused to understand the effect of the process parameters, tool design, joint mechanism, and properties in BTFSW. The process parameters such as rotation speed and welding speed are investigated individually for their influence on BTFSW. Important tool design parameters such as shoulder diameters (20mm, 22mm, and 24mm), pin profiles (tapered with 3 flats, cylindrical with 3 flats), and shoulder profiles (4 spiral shoulder and concave shoulder) are investigated for their influence on joining during BTFSW. Further, the BTFSW has investigated for improving joint strength of Al 6061 alloy using the cooling assisted approach. Two novel approaches as different temperatures of the water (1℃ and 30℃) and methods of use (spraying and enveloping top surface) were investigated. The application of the bobbin tool technique for double-side composite fabrication is investigated in this study. The feasibility of simultaneous fabrication of surface composites on the top and bottom sides of the workpiece using B4C reinforcement particles into 6061 Al alloy was studied. The superplasticity study is further investigated using the bobbin tool technique. Further, the study of the feasibility of BTFSW for dissimilar Al-Cu joining using slot joint configuration is investigated. The present study revealed that the process parameters (rotation speed and welding speed) significantly affect the quality of joints. The study of process parameters revealed that the process parameters window in BTFSW is narrow. The study of shoulder diameter revealed an increase in temperature with increasing shoulder diameter. The tapered pin profile produced a defective joint while the cylindrical pin profile is most suitable for the defect-free joint. The optimum process parameters are found as the rotation speed of 380 rpm and welding speed 31.5 mm/min while optimum tool design consists of 24 mm shoulder diameter, 4 spirals on the shoulder, and cylindrical pin with 3 flats. The cooling with room temperature water jet (C-RJ) approach has been proposed to improve joint strength among natural air cooling (NAC), spraying with room temperature water mist (S-RM), and spraying with ice water mist (S-IM). The lowest hardness location was heat affected zone in NAC and it was the boundary of the thermo-mechanically affected zone and weld nugget zone in all cooling-assisted joints. The C-RJ cooling assisted joint resulted in the highest tensile strength of 189.16 MPa which is 8.17 % higher than the conventional bobbin friction stir welded joint. The bobbin tool friction stir processing (BTFSP) of B4C/Al 6061 revealed successful simultaneous fabrication of composite on top and bottom surfaces. Excellent dispersion of the B4C particles at the bottom surface of the workpiece was observed. The results indicated a more uniform distribution of B4C particles with 3 passes. The grain refinement and homogeneous distribution of the B4C significantly improved the microhardness and wear properties of the fabricated composites compared to asreceived Al. 3 pass BTFSPed samples reached maximum microhardness of 85 HV0.3 and 96 HV0.3 at the top and a bottom surface, which is 13% and 28% higher than BM, indicating substantial hardness improvement. The feasibility of the bobbin tool technique for superplasticity revealed the maximum elongation of 120% at 400℃ and 5.5 x 10−4 𝑠 −1 indicating the requirement of the ultra-fine microstructure for the material to exhibit superplasticity. Another investigation on the feasibility of the bobbin tool technique for dissimilar Al-Cu joining revealed a super hard stir zone for both joints. The UTS of the welds produced under BTFSW and CFSW was found as 58.70 ± 9.41 MPa and 29.29 ± 2.19 MPa respectively. Keywords: Bobbin tool friction stir welding, Composites, Cooling, Dissimilar, microhardness, Parameters, Processing, Superplasticity, Tool design