Effects of tool pin profile in friction stir welding: a review on microstructural evolution and mechanical performance of alloy joints

PurposeThe main purpose of the present work is to study the effect of tool pin profiles on mechanical properties of welded plates made with two different aluminium alloy plates.Design/methodology/approachThe welded plates were fabricated with the three different kinds of pin profiled tools such as taper cylindrical, taper threaded cylindrical and stepped cylindrical pin profiles. Tensile properties of welded plates were evaluated using tensile testing machine at room temperature. Microstructures studies were carried out using scanning electron microscope.FindingsTensile properties were improved with the use of taper threaded cylindrical pin tool in friction stir welding process when compared with taper cylindrical and stepped cylindrical pin tools. This is due to refinement of grains and mixing of plasticized material occurred with generation of sufficient heat with the taper threaded pin tool. Through these studies, it was confirmed that friction stir welding can be used to weld Al6061 and Al2014 aluminium alloy plates.Research limitations/implicationsIn the present study, the friction stir welding is performed with constant process parameters such as tool rotational speed of 900 rpm, transverse speed of 24 mm/min and tilt angle of 1°.Practical implicationsAluminium alloys are widely using in automotive and aerospace industries due to holding a high strength to weight property. These aluminium alloy blanks can be developed with friction stir welding method with better properties.Originality/valueVery limited work had been carried out on friction stir welding of aluminium alloys of Al 6061 and Al2014 with different tool pin profiles. Furthermore, this work analyzed with tensile properties of welded plates correlated with weld zone microstructures.

The main objective for the existing research needs to identify the appropriate friction stir weld tool pin profile for welding distinct aluminum alloys AA6061 and AA5052. To investigate their effect on tensile strength, micro-hardness and wear characteristics of dissimilar joints, four separate tool pin forms were utilized: straight cylindrical (SC), straight threaded cylindrical (STC), tapered cylindrical (TC), and straight square (SS). Joints were produced with different tool pin profiles even at a constant rotational speed of 1000 rpm and two different table traverse speed of 60 and 80 mm/min. A SC tool pin design has been found to generate surface galling defects, while a TC tool pin profile generates inadequate heat diffusion as well as defective morphology of the surface. The results showed that tool pin profiles had an important influence on the mechanical characteristics of dissimilar joints. Threaded cylindrical tool pin profile yielded a higher tensile strength of 231 MPa besides a joint efficiency around 74.5 %, while the square tool pin profile obtained a maximum microhardness of 94 HV in the stir region. Grain size in the stir zone was enhanced through adjustments in the design of the tool pin profile and the average grain size for the square and threaded cylindrical tool pin profile was 35 and 39 µm. The highest hardness value was observed in the weld NZ near to advancing side of AA6061, and the minimum hardness value was observed throughout the HAZ near retreating side of AA5052 Al Alloys. Among all pin profiles, STC and SS pin profiles had been identified to be appropriate tool pin profiles of friction stir welding of AA6061–AA5052 Al alloy.

AA6061 aluminium alloy (Al-Mg-Si alloy) has gathered wide acceptance in the fabrication of light weight structures requiring a high strength-to-weight ratio and good corrosion resistance. Compared to many of the fusion welding processes that are routinely used for joining structural aluminium alloys, the Friction Stir Welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and recast. This process uses a nonconsumable tool to generate frictional heat in the abutting surfaces. The welding parameters such as tool rotational speed, welding speed, axial force, etc., and tool pin profile play a major role in deciding the joint properties. In this investigation, an attempt has been made to study the effect of rotational speed and tool pin profile on mechanical properties of AA6061 aluminium alloy. Five different tool pin profiles (straight cylindrical, tapered cylindrical, threaded cylindrical, triangular, and square) have been used to fabricate the joints at five different tool rotational speeds (800–1600 RPM). Tensile properties, microhardness, and microstructure of the joints have been evaluated. From this investigation it is found that the joints fabricated using square pin profiled tool with a tool rotational speed of 1200 RPM exhibited superior mechanical properties compared to other joints.

Friction stir welding (FSW) is categorized as solid state welding process which has attracted many researchers and industrialist due to its environmental friendly, clean and hazard free process. This study is undertaken to investigate the effect of tool pin profile and process parameters of FSW on AA5083 aluminum alloy. The marine grade aluminum alloy had a wide acceptance in the fabrication of light weight structure requiring high strength and good corrosion resistance properties. The current investigation provides an insight on the influence of various FSW parameters under the effect of different tool pin designs. Three tool pin profiles which include cylindrical, cylindrical thread and cylindrical tapered with combination of various level of rotation and transverse speeds were experimented and evaluated during the FSW process. Optimization on the mechanical properties such as tensile strength and hardness result were analyzed using ANOVA. Result showed that the optimum FSW parameters obtained were at 1400 rpm, transverse speed of 75 mm/min when using cylindrical pin profile.

Submerged friction stir welding (FSW) is used to improve the weld zones mechanical properties in the present study. This research aims to obtain the optimized process parameters used to fabricate the AA6063 Submerged FSW joint. In the Submerged FSW process, the most important influential factors are tool rotational speed, traverse speed, and pin profile in a seawater environment. The different workpieces are friction stir welded while submerged in seawater at different tool rotational speeds, traverse speeds, and tool pin profiles such as square pin, cylindrical taper pin, and threaded pin. The produced weldments were tested for the mechanical properties of higher tensile strength, microhardness, corrosion rate, and the microstructure of weldments was characterized by using a scanning electron microscope, transmission electron microscope, and X-ray diffractometer. The corrosion rate is investigated by using an electrochemical analyzer by potential dynamic polarization open-circuit technique. For this investigation, The Taguchi method with the L9 orthogonal array design of experimentation is adopted. The maximum UTS was acquired in the weld joint fabricated with 1250 r/min of tool rotational speed, 45 mm/min traverse speed, and a square tool pin. The stirred zone is tested for microhardness. High hardness is achieved with high tool rotational speed and low traverse speed with a square tool pin profile. The corrosion rate is also decreased with high tool rotational speed, low traverse speed, and a square tool pin profile.

Present paper explores the effect of the various tool pin profiles viz. cylindrical, threaded cylindrical, threaded cam and threaded triflute on the joint quality of Friction stir welded aerospace grade aluminium alloy (AA7475). 2.5 mm thick plates of AA7475-T761 are welded using Friction stir welding (FSW) and subsequently, the welds are characterized using optical microscopy (OM) and mechanical testing. Microstructural examinations reveal that the tools with threaded cylindrical, threaded cam, and threaded triflute pin profile lead to the formation of tunneling defect and joint line remnant (JLR) in the welds probably due to imbalance in material movement caused by insufficient plunge and sticking of base material around and on the bottom of the pin. Further, it is also found that cylindrical pin profile results in the maximum tensile strength (291.8 MPa) and threaded cam pin profile gives minimum tensile strength (247.5 MPa). Low tensile strength is observed due to sticking of 0.1 mm thick layer of aluminium on the bottom of the pin leading to reduction in the active plunge causing defects formation and deteriorating the joint quality.

The influence of rapid plastic deformation in the generation of welding heat during friction stir welding (FSW), supplementing the frictional heat generation by the tool shoulder, forms the thrust of the present investigation. Several researchers have highlighted the role of tool shoulder in the generation of frictional heat and suggested that the tool-material interface friction as the sole mechanism for heating. The configuration of tool pin profile is seldom studied for its contribution to welding heat through rapid plastic deformation at high strain rates (103/s), especially while welding thick plates. An attempt has been made to understand the dependence of deformation heat generation with different tool pin profiles in welding 5 mm thick AA2014-T6 aluminum alloy, maintaining the same swept volume during the tool rotation. An attempt has also been made to correlate the influence of process response variables such as force and torque acting on the tool pin. To quantify the physical influence of tool pin profile, temperature measurements were made in the region adjacent to the rotating pin, close to nugget in the thermo-mechanically affected zone (TMAZ). It has been observed that the temperature rises at a relatively rapid rate in the case of hexagonal tool pin compared to the welds produced employing other tool pin profiles. It is observed that during FSW, extensive deformation experienced at the nugget zone and the evolved microstructure strongly influences the mechanical properties of the joint. The present study is also aimed at understanding the influence of tool profile on the microstructural changes and the associated mechanical properties. Transverse tensile samples failed at the nugget/TMAZ boundary due to localized softening. Hexagonal tool pin profile welds have shown higher tensile strength, low TMAZ width, and high nugget hardness compared to other tool pin profile welds.

A three dimensional computational fluid dynamics (CFD) model has been developed to study the effect of tool pin profile on the material flow and temperature development in friction stir welding (FSW) of high specific strength AA 7068 alloy. Numerical simulations were carried out using a RNG k-e turbulence model. Three tool pin profiles, viz. cylindrical, conical and straight cylindrical threaded were considered for the simulation. The temperature distribution and material flow pattern obtained from the simulation were compared for different pin profiles. Simulation results predicted Temperature distribution and material maxing was better in straight cylindrical tapered thread pin welds. Weld joints were fabricated using the straight cylindrical threaded pin with the same parametric combinations as in the simulation. Peak temperature measured in the experiment was less than that obtained by simulation. Hardness measurements taken at different weld regions has showed that about 71% of that of the base metal hardness is obtained with the threaded tool pin. The microstructure study revealed a defect free weld joint. Precipitates distributed in the microstructure indicate sufficient heat input to join the material without dissolving precipitates. The developed numerical model is helpful in optimising FSW process parameters.

In this work, effects of different tool pin profiles of flat and tapered shoulder geometries on the peak temperature in friction stir welding are investigated analytically. The developed models used for the analytical investigations considered the welding process as a combination or mixture of the pure sliding and the pure sticking. From the results, the amount of heat generation and the peak temperature are directly proportional to the number of edges in the pin profiles in such a way that the heat generated and peak temperature in the profiles increases from the triangular pin profile to hexagonal pin profile. Also, the rate of heat generation and the peak temperature in flat shoulder are greater than in tapered/conical shoulder. The results in this work are validated with experimental and the past theoretical results and good agreements are achieved.

Influences of tool pin profile and tool shoulder diameter on the formation of friction stir processing zone in AA6061 aluminium alloy

In the current study, the effect of different tool pin profiles on fatigue crack growth rate, tensile strength, microstructural behaviour, and microhardness of friction stir welded joints of AA7075-T651 was studied. The friction stir welding joints known as J1 and J2 were formed using two different tool pin profiles: square pin tool profiles and left-hand threaded cylindrical tool pin tool profiles. All pin profile has scrolled shoulder. The results showed that the fatigue crack growth rate for the J2 joint was lesser than that of J1 joint, whereas the average grain size of J2 joint was lower than the J1 joint which led to the higher tensile strength of the J2 joint when compared to the J1 joint. Both joints were found to have residual stresses that were compressive in the longitudinal direction. The grain size of the weld nugget zone, internal residual stresses and strengthening precipitates impede the fatigue crack growth rate. The crack closure and non-homogenous microstructure play an important role in the variation of the fatigue crack growth rate.

Dissimilar material joining is an important concept with friction stir welding which gives an effective and substitute solution to fusion welding of aluminium alloys. The effect of different tool pin profiles on mechanical properties of cast aluminium alloy A319 and wrought aluminium alloy AA6063 were analyzed. The friction stir welding tool pin profiles comprises of round, hexagonal and square shape. The joints were tested for their strength using mechanical tests like tensile and micro hardness tests. Vickers micro hardness profiles for all the welded specimens were obtained. Optical macrographs revealed that the distinction between the two materials at the stir zone.

The present work aims to model the three-dimensional heat transfer coupled with the material flow model of 7075-T6 aluminium alloy material using the three tool pin profiles, square, pentagon, and hexagon, in the friction stir welding process. The temperature rise and fluid flow behaviour from the top to the bottom of the tool were evaluated. Also, the strain rate and dynamic viscosity variation were found near the tool pin and shoulder region. The results showed that the frictional contact heat between the shoulder surface and workpiece was responsible for the maximum heat generation. The probe area was minimum in the square tool pin geometry, which results in high heat generation due to the maximum shoulder surface contact area with the workpiece model. Furthermore, the analytical formula for calculating the heat generation on the tool shoulder/workpiece interface and the tool pin/workpiece contact region were also evaluated. The numerical modelling of heat generation was evaluated by COMSOL Multiphysics V5.3a software.

Influence of tool pin profile on the metallurgical and mechanical properties of friction stir welded Al–10 wt.% TiB 2 metal matrix composite

In the present research work, industrial grade pure copper and brass have been joined using friction stir welding. Effect of tool pin profile along with stirrer rotation speed and welding speed on the friction stir welding performance in terms of breaking load and percentage elongation has been investigated. Three tool pin profiles were designed viz., cylindrical, tapered threaded and square. These were used to fabricate the joints at three different rotation speeds (1,000, 1,100, 1,200 rpm) and welding speeds (25, 35, 40 mm/min.). The experiments were designed according to Taguchi-based L-9 orthogonal array. Variance analysis has been carried out to obtain the influence and percentage contribution of process parameters on welding performance. Optimisation results reveal that most significant factor is the pin profile. Square pin profile at 1,200 rpm along with 35 mm/min welding speed produces mechanically sound and metallurgically defect-free weld joints.