Influence of Bobbin tool design on Quality of welds made by FSW of Aluminium Alloys

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 18 (2019) 5177–5184

www.materialstoday.com/proceedings

ICMPC-2019

Influence of Bobbin tool design on Quality of welds made by FSW of Aluminium Alloys L. V. Kamblea* , S. N. Somanb a

b

Associate Professor, Mechanical Engineering Department, The M. S. University of Baroda, Vadodara, 390001, India. Professor & Head, Metallurgical and Materials Engineering Department, The M. S. University of Baroda, 390001, Vadodara, India.

Abstract Friction Stir Welding (FSW) an Eco-friendly process developed by The Welding Institute (TWI), UK, in 1991. Since its inception, sufficient developments have taken place in this area which we call as conventional or standard friction stir welding (CFSW). But the area of double shoulder or self-reacting tool is very lean in the literature and has a potential to explore it further. There is a need to understand the causality to study the effect of tool design parameters on weld quality. The heat developed in case of Bobbin Friction Stir Welding (BFSW) is different than CFSW due to two shoulders; and hence care must be taken in designing the tool. In general, FSW considered to be defect free solid state joining process, but it is not always true. Root canal, surface tunnel, joint line remnant, voids are some of the defects observed. The present paper will throw some light in the direction of development of bobbin tools, so that an idea will be conceived regarding design requirements and limitations in the fabrication of fixed gap bobbin tools. The tools developed were validated by actual experimental investigations and characterization for different aluminium alloys. A square pin fixed gap bobbin tool was found to be suitable for defect free welding of Al alloys. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Bobbin tool, rooster tail, hour glass, surface tunnel, microstructure

1. Introduction This Conventional friction stir welding (CFSW) is a matured process currently adopted successfully in marine, aerospace, railway and automotive industries. The conventional FSW mostly applied to aluminium alloys, is found

* Corresponding author. Tel.: 91-9427453209; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019

5178

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

to be advantageous in context of joint design, mechanical properties with low distortion, process repeatability with lesser defects, safety and environment. It is a process independent of position and has a potential to reduce cost and weight of assembled structures. The only limiting factor of CFSW as compared to fusion process is the welding speed. Another drawback of the process is the requirement of rigid clamping to prevent part movement due to process loads and strong backing plate or anvil to resist the axial thrust [1]. To overcome these limitations, a self-reacting tool having an additional shoulder at bottom in place of an anvil can be used. These improvements result into reduction of process loads and an increase in welding speed. Bobbin tool FSW, though was developed along with CFSW [2], but not become much popular because of limitations observed in design and fabrication of tool. Bobbin tool has number of potential advantages which will make it attractive in easy adoption. It was a perception of the industries that expensive and complex equipment are required for bobbin tool FSW. Recent updates at TWI in fixed bobbin FSW and a new novel variant called floating bobbin tool FSW have demonstrated excellent welding capability using simple equipment which could be readily implemented on existing machines (used for CFSW), low cost machines [3]. The weld regions in FSW can be divided into three zones [4, 5, 6] viz, heat affected zone (HAZ), thermomechanically affected zone (TMAZ) and the weld nugget or stir zone. Each weld zone is shown in Fig.1 for BFSW. It is observed that CFSW and BFSW have different region sizes and forms. e.g. by referring to the weld nugget zone, CFSW inherited the basin shaped nugget while BFSW has the form of an hour-glass [4, 7]. In both the processes, the size of the nugget area decreases toward the center, and in BFSW process this area widens again towards lower surface. This is because the pin dimensions are smaller than the shoulder. Retreating side (RS)

Top surface

Advancing side (AS)

Bottom surface A = Parent Metal, B = Heat affected zone (HAZ), C = Thermo-mechanically affected zone (TMAZ), D = weld nugget. Fig. 1: BFSW macro structural cross section Al alloy in butt joint [6]

Till the date, enough attention is given to tool design features, process parameters (spindle and travel speed) and its effect on weld quality by previous researchers on CFSW as compare to BFSW. Although these factors play similar roles in both the processes, the additional shoulder in BFSW and the full pin penetration, create additional interactions. This is because for single shoulder tool pin, material is always available around the tool-substrate interface, but this is not the case in bobbin tools. The situation is even more acute when dealing with difficult-toweld and thin materials. Besides that, although the literature describes a variety of specifically designed tools, the details of the design including dimensions of the features, and the process responses have not yet been revealed, especially for BFSW and partly for CFSW. 2. Development of bobbin tool, result and discussion Tool is a key component in FSW process. But very less number of published literature have been found for BFSW till the date, compare to CFSW. The primary features of Bobbin tool as shown in Fig. 2, are upper shoulder, pin and the lower shoulder. It is an extension of CFSW tooling with one additional shoulder attached to the pin at its

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

5179

bottom. With this perception, the bobbin tool was designed in the beginning. It is very difficult to add shoulder features in fixed gap bobbin tool for thin sheet welding.

Fig. 2: General features of Bobbin tool

The bobbin tool was manufactured keeping the same dimensions used in CFSW. They are shoulder diameter D=20mm, pin diameter d=6mm for joining 6mm thick Aa6101 Al alloy [8]. The experiment was carried out on HASS make CNC machine, with 1000 rpm tool rotational speed and 200 mm/min traversing speed. In the beginning bonding was good, but all of a sudden tool failure occurred (Fig.3) after 25- 30mm of tool travel; at junction of top shoulder and pin.

Fig. 3: Failure of cylindrical pin bobbin tool during welding of AA6101 Al alloy

The factors for tool failure were properly studied and observed that there are several covert factors playing the role in heating and bonding of material. One of the probable reason may be generation of excessive heat during the process. The second cause may be the weaker section of pin and stress concentration; as we have kept the same design as CFSW with additional shoulder connected at bottom of the pin. The third reason may be higher traversing speed resulted into cold welding and failure of tool at top side of plate. Hence the optimization of welding parameters is required. At the same time sufficient dwell period should be provided to plasticize and spin the material. M. K. Sued et al. [9] worked extensively on bobbin tool. Weld-on-plate experiment was carried out and found in his initial study that, the pin with four flat feature gives better weld quality for thin plate aluminium. Author adopted a systematic approach in designing the bobbin tool as shown in Fig. 4. Different shoulder features and pin features were laid down and dimensions were finalized based on literature and experience.

5180

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

Fig. 4: Tool features adopted in design of Bobbin Tool

With the experimentation and intuition we developed a new tool with a little modification in pin size and shape and shoulder features. The tool material selected was H-13 tool steel having compositions shown in Table 1 and Fig.5. Shoulder diameter was kept same as D=20 mm, while pin design was changed to square cross section from cylindrical for proper spinning and mixing of material. The pin square is 6 x 6 mm2. Two different shoulder features were selected instead of flat surface viz. (i) concave – convex (Fig. 6) and (ii) concave – concave (Fig. 7). Both the tools were used for welding of AA 6082 Al alloy in butt configuration on Vertical Machining Center. Table 1: Composition of H-13 tool steel

Element

Measured Weight%

Standard Weight%

C

--

0.40

Si

0.99

1.1

V

1.21

1.0

Cr

5.44

5.0

Mn

0.16

0.5

Mo

1.24

1.50

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

Fig. 5: the EDS (Energy Dispersive spectroscopy) analysis of H-13 tool steel.

Fig. 6: Fixed gap bobbin tool with square pin, concave-convex shoulder BT1 Table 2: Process parameters for BT1 tool

Code no. Spindle speed rpm Linear speed mm/min

BS1 800 48

BS2 600 48

BS3 600 24

Fig. 7: Fixed gap bobbin tool with square pin, convex-convex shoulder BT2

5181

5182

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184 Table 3: Process parameters for BT2 tool

Code no. Spindle speed rpm Linear speed mm/min

B1 1000 24

B3 800 24

The experiment was carried out using both the tools with the process parameters given in Table 2 and 3. It is general assumption that BFSW will result into sound weld, but it is not always true. The defects like poor penetration or root tunnel were overcome, but surprisingly a new defect called surface tunnel type defect was observed in samples welded by BT1 tool (Fig. 8).

Fig. 8: Top and bottom surface of sample welded with concave-convex shoulder bobbin tool BT1.

A concave shoulder in CFSW serves as a reservoir of material during tool traversing, but in bobbin tool this reserve resulted into excessive material flow in the form of ribbon and flash. Due to this surface material was also pulled with tool travel and damaged the surface and proper surface bonding was not achieved. While in case of BT2 tool (Fig. 9) little flash was found but ribbon was not observed. The surface was clean and no additional surface irregularities. The welds were tested by X-Ray radiography to see the defect along the joint line. Characterization was also carried out.

Fig. 9: AA6082 Al alloy plates welded with convex-convex shoulder, square pin bobbin tool BT2.

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

5183

2.1 Microstructural Analysis of BFSW samples

Fig.10: Microstructure of AA 6082 T6 Al alloy welded with fixed gap bobbin tool BT2 at 1000 rpm spindle speed and 24 mm/min traversing speed (At magnification of 100X at different zones)

Fig. 10 shows microstructure analysis of BFSW sample on either side of plate thickness measured at 100X magnification at different zones like HAZ, TMAZ, nugget zone, and base metal. Fig. 11 shows microstructure at 100X and 200X magnification of the sample shown at top and bottom side respectively. The AA6082 Al alloy plates were welded by square pin, convex-convex shoulder at constant welding speed of 24mm/min and rotational speed 1000rpm and 800rpm.

Fig. 11: Microstructure of AA 6082 T6 Al alloy welded with fixed gap bobbin tool BT2 at 800rpm spindle speed and 24 mm/min traversing speed (At magnification of 100X (upper) and 200X (lower))

Optical microstructure shows the base metal contain solid solution of aluminium matrix Mg2Si and silicon particles. HAZ contain solid solution of aluminium matrix Mg2Si and silicon particles. TMAZ contain solid solution of aluminium matrix Mg2Si (slightly large) and silicon particles. Nugget contain fine grain solid solution of aluminium matrix Mg2Si and fine silicon particles (Fig. 10-11). SEM microstructures show the soundness in all the zones of BFSW sample. In X-Ray radiography, there was no observation of any type of defect. It is observed from the Fig. 10-11 that curvature of the hour glass increase with the rotational speed and protruding away from the advancing side towards stir zone, while it is in diffused state on retreating side.

5184

L. V. Kamble et al./ Materials Today: Proceedings 18 (2019) 5177–5184

3. Conclusions Tool design is a key to successful welding in Bobbin friction stir welding process. Bobbin tool should not be designed directly keeping the same dimensions as CFSW, but different parameters like stress concentration, pin weakness, heat generation, etc. need to be considered while designing the tool.One should not be under the impression that bobbin tool always will give sound weld. In depth study is required before its design and fabrication.BFSW can overcome root tunnel or incomplete penetration type defects. But surface tunnel defect was observed in many cases in BFSW preferably in concave-convex shoulder bobbin tool.There was also tool failure observed during the welding of Al alloys, in which cylindrical pin flat shoulder tool was used. Pin was a weaker section in this tool. Convex-convex shoulder with square pin bobbin tool resulted into sound weld and was very successful in X-Ray radiographic test.Very fine equiaxed grain structure was observed in weld nugget in case of weld carried out by BT2 convex-convex shoulder tool.Thickness variations are generally observed in sheet manufacturing. An indigenously designed tool is capable of taking care of this thickness variations.A clear hour glass microstructure was observed at advancing side and little diffusing structure at retreating side of weld. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

T. J. Minton, “Friction stir welding of commercially available superplastic aluminium,” PhD. Thesis, Dep. Eng. Des. Brunel Univ. London., no. September, p. 252, 2008. W. M. Thomas, C. S. Wiesner, D. J. Marks, and D. G. Staines, “Conventional and bobbin friction stir welding of 12% chromium alloy steel using composite refractory tool materials,” Sci. Technol. Weld. Join., vol. 14, no. 3, pp. 247–253, 2009. TWI Ltd, “Assessment of Bobbin Friction Stir Welding for the Joining of Aluminium Alloys,” 2008. R. S. Mishra and Z. Y. Ma, “Friction stir welding and processing,” Mater. Sci. Eng. R Reports, vol. 50, no. 1–2, pp. 1–78, 2005. R. S. Mishra and M. W. Mahoney, “Friction Stir Welding and Processing,” ASM Int., p. 368, 2007. W. M. Thomas and C. S. Wiesner, “Recent Developments of FSW Technologies : Evaluation of Root Defects , Composite Refractory Tools for Steel Joining and ...,” Trends Weld. Res. Proc. 8th Int. Conf., pp. 25–34, 2009. J. Schneider, A. Nunes, and M. Brendel, “The Influence of Friction Stir Weld Tool Form and Welding Parameters on Weld Structure and Properties : Nugget Bulge in Self-Reacting Friction Stir Welds,” 8th Int. Symp. Frict. Stir Weld., 2010. L. Dubourg and P. Dacheux, “Design and properties of FSW tools : a literature review,” 6th Int. Symp. Frict. Stir Weld., 2006. M. K. Sued, D. Pons, J. Lavroff, and E. H. Wong, “Design features for bobbin friction stir welding tools : Development of a conceptual model linking the underlying physics to the production process,” Mater. Des., vol. 54, pp. 632–643, 2014.