Strength, failure and microstructure development for friction stir welded AA6061-T6 joints with different tool pin profiles

Abstract

The influence of seven different tool pin profiles was investigated to appraise the tensile, flexural strength, fracture behavior, and microstructure evolution in friction stir welded similar AA6061-T6 joints. Results indicate that each tool pin profile affects the material flow/intermixing in different ways; consequently, the weld nugget feature and grain size in the stir zone (SZ) were also observed to be different. Relatively finer grains with homogenous microstructure in SZ were observed for joints prepared using cylindrical grooved with straight flutes (CGF) and cylindrical grooved (CG) tool pins. The highest tensile strength and flexural load were observed for joints obtained with CGF tool pin followed by CG tool pin. A relationship between average yield strength of the joint and inverse square root of average SZ grain size was established for these joints. EBSD analysis revealed that the SZ is the mixture of low angle grain boundaries and high angle grain boundaries (HAGBs) and the fraction is controlled by different pin profiles. The HAGBs and high shear texture components $B/\overline{B}$ and $C$ were found predominant in welds prepared with CGF and CG tool pins. Fractographic analyses showed the presence of numerous dimples in sound bonded regions indicating the ductile fracture; however, tearing/rupture and shear failure were witnessed near the vicinity of the tunnel region.

Introduction

Different materials including lightweight metals like aluminum and other alloys are successfully joined by friction stir welding (FSW), a process which was developed and patented by The Welding Institute, UK [1]. The FSW gained significant importance due to its ability to produce sound quality similar and dissimilar joints without any need of welding consumables–shielding gases, electrodes, fluxes, fillers, etc. [2]. In FSW, the joints are produced below the solidus temperature of the base materials [2]. Thus, being a solid-state welding process eliminates the welding defects formed during solidification of molten metal observed in other fusion welding processes. In FSW, a rotating tool first plunges gradually into the plates to be welded followed by traveling along the length of the weld. The friction and plastic deformation (due to an interaction between the tool and workpiece in the localized region) during plunging of the rotating tool generates significant amount of heat and introduces flow of plasticized material around the tool pin. Traveling of the rotating tool completes the weld along the plate length. However, defects formation such as tunnels/voids, cracks, etc. can sometimes be detected in the weld nugget. Such problems can be resolved by selecting the optimal process parameters like tool plunge depth, rotational speed of tool, welding speed, tool tilt angle, etc. Apart from welding parameters, studies showed that the tool pin profile significantly affects the flow and intermixing in FSW, and therefore, selecting correct pin profile is important to avoid such defects.

Esmaeili et al. [3] studied different kinds of welding defects during dissimilar FSW between brass and aluminum 1050. They concluded that the improper process parameter settings lead to improper material flow, which forms defects like tunneling, crack, surface groove, fragment defect, and spongy structure. The location and distribution of surface defects during FSW process were analyzed by Ranjan et al. [4] by image processing approach. The effect of rotational speed of tool, welding speed and post-weld aging treatment were investigated on microstructural and mechanical properties during FSW of AA6063-T4 [5]. They observed that mechanical strength slightly improved and percentage elongation decreased after post-weld aging treatment. Li et al. [6] also examined the influence of tool rotation and welding speed on microstructure and mechanical strength of stationary shoulder friction stir welded AA6061-T6. They observed that the welding speed has more influence on tensile strength than rotational speed of tool and joint strength increases with increase in welding speed. Raturi et al. [7] investigated different rotational and traverse speed of tool, different tool pin profiles and preheating to study their effect on weld quality, joint strength and failure of dissimilar FSW between AA6061-T6 and AA7075-T651. They observed that tool pin profiles, tool rotation, and weld feed rate play an important role on weld quality and joints strength. Wang et al. [8] studied the effect of additional water cooling on mechanical performance of friction stir welded AA5083-H19 and reported that the welded joints obtained at low rotation speed with additional water cooling showed strength almost equal to base metal. Fernández et al. [9] analyzed different tool pin profiles like cylindrical pin with two horizontal flutes and cylindrical pin with grooves on vibro-acoustic signal and weld quality in FSW of AA1050-H24. Ramanjaneyulu et al. [10] studied different tool pin profiles on mechanical and metallurgical characterization of FSW joints and reported that the joints prepared by hexagonal tool pin exhibited higher strength with higher nugget hardness and lower thermo-mechanically affected zone width. The effect of tool pin profiles (threaded cylindrical and square) on microstructural and mechanical properties was investigated by Khodaverdizadeh et al. [11] during FSW of pure copper, and the study reported that the joints prepared with square pin exhibit higher mechanical strength and fine grain structure. The influence of different pin profiles and rotational speed of tool on tensile strength and zone formation were investigated by Elangovan and Balasubramanian [12] during FSW of AA2219 and their study reports that the joints obtained using square tool pin were mechanically sound and defect-free. Imam et al. [13] analyzed three different pin profiles (square, tapered square, and tapered cylindrical) for strength and joint quality of friction stir welded AA6063-T4 and their study reported that joints obtained using square pin were defect-free. Padmanaban and Balasubramanian [14] investigated different tool material, tool shoulder diameter, and tool pin profiles to study their effect on microstructure and joint strength of friction stir welded Mg alloy. From their study, they reported that the joints obtained using tool with shoulder diameter 18 mm and threaded pin profile yielded higher strength. Ilangovan et al. [15] also stated that tool with threaded pin profile results in good flow, intermixing, and produce defect-free sound welds during dissimilar FSW between AA6061-AA5086. Chowdhury et al. [16] analyzed different rotational speed, welding speed, and tool pin thread orientation (left and right-hand thread) on tensile strength, microstructure, and strain hardening behavior during FSW of Mg alloy. They reported that tool pin with a left-hand thread and rotating clockwise generated good quality weld due to downward material flow. Sahu and Pal [17] studied the joining of AA1050 alloy plates of different thickness by single pass and double pass FSW process, and their study showed that the tensile, flexural, impact strength, and hardness of welded nugget improved in double pass FSW process. Kadian and Biswas [18] analyzed different tool pin profiles for material flow characteristics in friction stir welded AA6061 using finite element method and observed that the tool with circular pin profile results in better material mixing compared to flat tool pins.

Aforesaid literature indicates that different tool pin profiles and process parameters do considerably affect the material flow, intermixing, and thereby microstructure evolution and mechanical strength of FSW joints. The variation in failure modes and joint strength might be attributed to the material flow and intermixing between the materials of plates being welded. Therefore, the material flow, subsequent formation of zones, intermixing and bonding between the plates to achieve maximum joint strength can considerably be dependent on different tool pin profiles The aim of the present study was thus to appraise the effect of seven different tool pin profiles i.e., cylindrical grooved, cylindrical grooved with straight flutes, cylindrical full threaded, cylindrical full threaded with flutes, cylindrical half threaded, square and cylindrical tapered on the joint tensile strength, yield strength and flexural load (three-point bend test) during FSW of AA6061-T6. Detailed microstructural studies of welded joints and fractographic analyses using optical and field emission scanning electron microscope (FE-SEM) were performed. Profound investigation on microstructural development and strength-microstructure correlation, grain morphology, grain boundary misorientations distribution, etc. were performed using electron backscatter diffraction. Energy dispersive spectroscopy was also carried out to reveal phase formation and their distribution in the weld zone.