2023 年 64 巻 2 号 p. 485-491
The tool pin profile damage during successive passes in friction stir welding (FSW) of dissimilar aluminium alloys AA6061-T6 and AA7075-T651 is recorded during the present investigation. A right-hand threaded with three intermittent flat faces tool pin at 900 rpm tool rotation and 98 mm/min of weld speed without and with the application of secondary heating (using butane gas torch) is used for joint fabrication. Two separate tools are used to obtain weld length of total 1 metre each without and with secondary heating. Tool profile damage is progressively monitored using scanning electron microscopy along with measuring tensile strength of the joints obtained. The leading edge of the threaded profile (at the end of flat face) distorts with mechanical damage and chipping while the incident material is being plasticized and pressurized by flat region. Secondary heating helps in eased material flow by material softening leading to load reduction and thereby tool wear. The tools experienced slight bending, tearing near shoulder. For both the tools certain amount of wear is being noticed however joint strength is retained without significant adverse effect even after completion of welding for a meter of total weld length.
Friction stir welding (FSW) is a well demonstrated joining technique especially for different grades of similar and dissimilar aluminum alloys. FSW is a solid-state joining process that uses non-consumable tool harder than the material to be joined, to stir the materials along the faying or abutting line. The process institutes a high rotating tool pin first plunged into the material and then traversed along the adjoining surfaces in order to obtain the joint predominantly through solid-state diffusion. For joint efficacy the tool geometry integrated of shoulder, pin and contour over the pin becomes the decisive elements moreover the process parameters. The tool geometry determines the amount of frictional heat produced and degree of plastic deformation for better material mixing.1) The contours over the pin or tool profile more or less determines the microstructure of the final joint that further controls the joint mechanical properties.2) Threads are one of the desirable geometrical features while designing the FSW tool pin due to its imperative advantages and facile fabrication. Inclusion of a thread improves the material mixing by inhabiting the vertical flow of material and excess heat generation due to increased surface area for friction.3) The continuous thread can expedite vertical flow creating a void at the bottom. The flat faces introduced over the cylindrical profile reduce the vortex movement, increases torque and strain in flowing material.4)
Prado et al.5) performed FSW of commercial 6061-T6 aluminum alloy and metal matrix composite (6061-T6 and Al2O3 (20%)) using right-handed screw threaded tool pin of standard grade tool steel (HRC 62). The tool wear was measured by taking photographs, then printing, taking cutouts along the tool outline and weighing. They mentioned material filling within the threads as one of the main reasons for wear. Shindo et al.6) carried out FSW of metal matrix composite (Al alloy and SiC (20% by vol.)) at 1000 rpm and varying welding speeds of 1, 3, 6 and 9 mm/s and observed that tool wear decreases with increase in welding speed. Fernandez and Murr7) used threaded pin profile and reported that with less than 10% of tool wear the threaded profile erodes itself to achieve the self-optimized shape. Hasan et al.8) used tungsten–rhenium–hafnium carbide dome shaped tool rotating at 2000 rpm to weld 6 mm thick austenitic stainless-steel plates. Their study concluded that tool pin shape wears out due to sliding wear and tool wear out decrease the stirring action. Chen et al.9) used PCBN tool with cylindrical tapered probe having two different profiles on the probe namely, spiral (identical to threads) and chamfered (flat faces) to FSW stainless steel plate. Their study showed the least wear resistance for spiral probe profile compared to that with flat faces. Okada et al.10) focused on tool composition to reduce tool wear while friction stir welding Al-Steel. They used galvannealed steel with Zn–Fe coating and conserved its toughness by achieving fine and uniform carbide distribution during casting. Sahlot et al.12) presented the quantitative wear of H13 steel tool pin profile for FSW of CuCrZr alloy and observed increase in wear rate with tool rotational speed near the tool pin although relatively less wear near the pin root was observed. They also reported that change in tool pin profile resulted in poor material flow, degraded weld quality and can also introduce defects.
The increased applicability of aluminium alloys such as AA6061-T6 and AA7075-T651 necessitates the detailed study of tool pin profile damage during their FSW. The present experimental investigation thus aims to interpret the extent of tool wear during successive passes after FSW of dissimilar aluminum alloys with and without employing secondary or additional heating (by butane gas flame), moreover the primary heat generated due to the process mechanics. FSW is carried out by a threaded pin with intermediate flat interruptions keeping other process parameters (tool rotation, weld speed) at optimum level to result defect free joint of better mechanical properties. Tool profile wear and damage are appraised observing under field emission scanning electron microscope (FESEM) and joint strengths are also assessed extracting transverse tensile samples in order to observe any change/degradation in joint quality that may have induced due to the successive damage in tool profile.
To determine the tool wear and damage, dissimilar FSW between 6.1 mm thick plates of AA6061-T6 and AA7075-T651 are performed. The H13 die steel tool of shoulder diameter 20 mm and pin length 5.9 mm (Fig. 1(a)) is used for all welding. FSW is performed at tool rotation of 900 rpm and welding speed of 98 mm/min by placing AA6061 on advancing side (AS) and AA7075 on retreating side (RS) as shown in Fig. 1(b). The 6 mm diametric cylindrical pin is engrooved with right hand threads of 1 mm pitch and 0.8 mm depth. The threading is discontinued (machined) by creating flat surfaces at three equally distant (120° apart) locations and hereafter referred as TIF (threaded with intermittent faces). The two separate TIF tools used for obtaining joints at two different weld conditions (see Fig. 1(c)) i.e., without and with the application of secondary heating, referring as welding condition-1 and welding condition-2 respectively, while the other process parameters are maintained constant. In present context secondary or additional heating refers to the external heating provided with the help of butane gas torch other than the heat produced due to the process itself. The portable cannister of butane gas with mounted torch is used for secondary heating. The plate surfaces are heated for about 1 minute prior to the start of the welding and measured using infrared thermometer (Extech 42570). The heating is continued during tool plunging, tool traversing and extended till the completion of welding. While during preheating the flame is travelled over the entire surface in order to reach the temperature of the plates in the range of 150 ± 5°C; during plunging and tool traverse, the flame is directed along the joint abutting line and travelled ahead of the moving tool.
Schematic representation of (a) TIF tool, (b) welded plate and (c) arrangement of each experimental run.
The type and extent of damage experienced by the tool profile during FSW is recorded after each welding pass of approximately 172 mm length. Two separate TIF tools are used for two different sets of six successive welds of total 1032 mm (6 × 172 mm) length, one with and the other without the use of secondary heating. After each weld (172 mm) the tools are dipped in NaOH solution to get rid of the adhered Al alloy and examined using FESEM and EDS for profile damage, wear and metallurgical changes. The profile damage is recorded at two different locations on each tool designated as Position-1 and Position-2. These two different locations (Position-1 and Position-2) has been marked on each fresh tool in order to record the damage at same site after every run/weld pass. To observe the effect of profile damage on the change in strength of successive joints, the uniaxial tensile test is performed preparing standard tensile samples as per ASTM E8 (Fig. 1(b)).
In order to perceive the manner work material interacts and moves around the pin creating different velocity and pressure zones, a schematic illustration is presented in Fig. 2. In the direction of tool travel the work material encountering from the front is called leading side (LS) while the back side where material is leaving is called the trailing side (TS) as shown in Fig. 2(a). Similarly, the AS is the side having tangential velocity vector of tool rotation and welding velocity are in the same direction while for RS both velocity vectors are in opposite direction. The plastic deformation around the pin happens categorically and can be sub divided into the rotational and transitional zones.13) In rotational zone the material tries to adhere to the pin surface and rotates several times before leaving. The width of the rotational zone completely depends on process parameters and its concentricity will be decided by the stick and slip conditions.
Schematic illustration of (a) material flow direction around the rotating and traversing tool pin, (b) forces exerted by the work material on tool pin and (c) work material moving around the pin cross section.
The other important zone is transitional zone as shown in Fig. 2(a) by the streamlines around the pin starting just outside the rotational zone and ends with the static parent material. In this zone, material enters at LS and leaves at TS, and exists due to the translatory motion of the tool forming shearing layer at LS outside the rotational zone. The asymmetric material flow in transition zone exists as flow velocity decreases when material moves from TS towards LS. This prompts the accumulation of the material at the back of the pin specially on AS. On AS a stagnation point is created in translation zone due to the encounter of two opposing material flows where one flow is in rotational and the other is in welding direction. As the work material incidents on the pin, it bifurcates and undergoes two different types of deformation and flow. Some of the deformed/plasticized material makes its way through the threaded profile and get deformed accordingly while rest of the material is pressurized to extrude by the flat face. Figure 2(b) schematically displays the forces being exerted by the work material on the tool pin surface during plunging and welding. During plunging the tip of the tool pin experiences the maximum axial thrust and the force will tend to minimize during welding.14) The leading edge of each threaded section experiences the initial material blow or impact during every rotation of the tool. At the leading edge the work material experiences abrupt change in the flow and ascends over the threaded profile tend to cause the mechanical deformation, micro-breakage, chipping wear of the edge (see tool status at Position-1 after fourth, sixth run, Fig. 3).
FESEM micrographs of fresh tool pin and tool pin after second, fourth and sixth run of dissimilar AA6061-AA7075 FSW joints without the application of secondary heating (welding condition-1) with their corresponding high magnification images at two different locations (Position-1 and Position-2) around the pin.
The work material further squeezes inside the groove of the thread and can be looked upon as an extrusion process. During this extrusion analogous material flow, the work material exerts pressure on threads in both the upward and downward direction (normal to the surface profile). These forces to an extent are countered by the other forces generated in the consecutive thread and thus will lead to minimal profile damage. These forces can however adversely affect the tool tip as thread at the pin tip will receive the low counter force from the tool tip surface. Figure 2(c) represents the cross-sectional view of tool pin illustrating the material being transported by flat faces around the periphery. In the direction of tool rotation, the material flowing ahead of the leading edge of the thread profile material is pushed creating a high-pressure material zone. The trailing edge of the consecutive thread is dragging the work material with itself due to friction creating a low-pressure material zone. The leading edge thus will be stressed more as compared to the trailing edge and will make vulnerable to faster damage. Although the existence of pressure gradient along the flats can lead to increased material flow velocity and creates a larger deformation zone, the same is numerically simulated by Su et al.2)
Figure 3 represents the SEM images at two different locations (Position-1 and Position-2) of the tool pin employed for welding condition-1 during AA6061-AA7075 dissimilar FSW without the application of secondary heat. The schema in Fig. 3 displays tool pin FESEM images taken at two different locations (Position-1 and Position-2) where each column started with a picture of fresh tool followed by the image of tool pin after second (344 mm weld length), fourth (688 mm weld length) and sixth (1032 mm) run. For Position-1 the tip of the tool pin (welding condition-1) seems to be deformed after the second run, but substantial chipping and failure can be observed after the fourth run. The tool tip experiences maximum deformation during plunging and leads to the predominantly bending of the initial thread. The deformed thread might create hinderance to the uniform material flow and will undergo further mechanical failure due to thrust forces, subsequent damage and chipping. It can be observed in the Position-1 as significant portion of the tool used at welding condition-1 got wear out starting from the leading edge and is extended to some distance. In Postion-2 the tool tip (welding condition-1) initially got bent and folded closer to the successive thread and apparently acting as one. This helps both the crests to bear the loads collectively without undergoing any further wear even after the sixth run. The threads other than at the tip does not show any major wear or profile damage. Although the sharp edges formed during threading got blunt in the initial welds and are majorly intact even after sixth run. The initial tool wear is generally anticipated specially for the intricate profiles like threads. The tool will initially undergo wear and will slowly achieve a self-optimized shape restricting wear to great extent.11) The tool pin acts as the cantilever beam experiencing the maximum deflection at the tip and can be observed for the tool pin in Fig. 3 (Position-1) after the sixth run of welding condition-1. Figure 4 represents the FESEM images at two different locations (Position-1 and Position-2) of the tool pin used for welding condition-2 i.e., for fabricating FSW joints with the application of secondary heat. The tip of the tool, used for welding condition-2, at Position-1 is observed slightly bent after the second run and seems to establish the optimized shape without further degrading even after the sixth run. With the use of secondary heating, the flow of the soften materials become easier, forces over the tool tip reduces and thus lower bending is observed compared to the case of without secondary heating. The tip of the tool observed from the Position-2 seems to be almost undamaged till the completion of sixth run and thus secondary heating can be influencing parameter to control tool damage especially the tool pin tip.
FESEM micrographs of fresh tool pin and tool pin after second, fourth and sixth run of FSW joints with the application of secondary heating (welding condition-2) with their corresponding high magnification images at two different locations (Position-1 and Position-2) around the pin.
Figure 5 presents the FESEM micrograph at the root of the tool pin (used for welding condition-2) near shoulder along with the high magnification images exaggerating the fine crack formed over the surface. During FSW, the tool traversing along the abutting line experiences bending load and a twisting. However, the magnitude of the load and the twist experienced by the designed tool will depend on the process parameters like rotational and welding speed in addition to the size and length of the pin. A comparatively higher welding speed for same rotational speed can lead to reduction in heat accumulation decreasing material softening and thus increasing the load on tool pin. The stress and the bending moment concentrate at the tool pin root near shoulder and it can be observed that crack initiated within the trough of the thread near the shoulder-pin base region. At location B the single continuous crack can be observed at the thread groove while at location A, multiple cracks of different lengths can be observed.
Low and high magnification FESEM image of the tool pin (used for welding condition-2) depicting crack near the root of the tool pin.
The FESEM micrographs over the region where threaded section ends and flat region starts for the two tools used to fabricate joints in without and with secondary heating conditions are shown in Fig. 6 as a representative case. Alongside the FESEM images, the element overlay map with aluminium marked on it and the weight percentage for the two different tool surfaces (irrespective of the weld length) are also presented. The elemental mapping shows maximum aluminium adherence for both the tools are chosen to be showcased. The elemental mapping is conducted over the region and the traces of aluminum are witnessed with the percentage composition of 0.29% and 0.26%. The aluminium migrated from base plates got cling to the tool surface due to hot adhesion and bonded strongly enough that does not get removed with NaOH cleaning. The ridged topology of the surface generated during the tool fabrication process by machining allows bonding of soft material like aluminium especially at high temperatures during FSW.
FESEM image, element overlay (aluminum), % composition over the region of interest for tool used for fabricating FSW joints (a) without and (b) with secondary heating condition.
The tensile stress-strain plots, yield strength (YS) and ultimate tensile strength (UTS) of dissimilar joints obtained without and with secondary heating during FSW after different weld lengths are shown in Fig. 7. In particular to inspect the effect of tool degradation on joint quality, tensile tests are performed after second, fourth and sixth run. Although a successive damage has been recorded for the tool pin in both the cases but the tensile test results do not represent any consequent adverse effect on the joint strength and the YS, UTS of the joints are retained ever after welding of six runs with more than a meter total weld length.
Tensile stress v/s strain, UTS and YS plot for FSW joints obtained (a) without secondary heating (welding condition-1) and (b) with secondary heating (welding condition-2).
From the afore-presented experimental results, the following conclusions are drawn.
