Claims:We claim:
1. A multi-mode friction stir welding (FSW) tool comprising a main shaft (1) connected to the spindle of the FSW machine, the main shaft (1) has a detachable pin (2) connected to it, the detachable shoulder (3) is connected to floating shoulder (4) supported by a resilient device (11), the former being adapted to be in use or in stationary mode and there is provided a shoulder body (5) enclosing the floating shoulder (4) and detachable shoulder (3), the shoulder body (5) being connected to a pulley unit (6) and the upper portion of the tool is fitted with a removable mode selector collar (7), which can be selectively removed or installed for the tool to perform different variations of friction stir welding, the pulley unit (6), shoulder body (5), floating shoulder (4) and detachable shoulder (3) comprises shoulder assembly and it is connected to the main shaft (1) by bearing means (8), whereby the forces acting on the shoulder in axial and tangential directions can be transmitted to the main shaft (1),the tool being provided with a coaxial type drive which makes pin (2) and shoulder (3) suitable for independent rotation which can be independently controlled for their speed and direction of rotation so that all the variants of FSW such as Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW, Micro FSW (All variants) and High Speed FSW (All variants) can be performed by the same tool.
2. The friction stir welding (FSW) tool as claimed in claim 1 wherein said bearing means comprises of two tapered roller bearings (8) and the pin (2), which is connected to the main shaft (1) of the tool through threads, is detachable and replaceable just like the detachable shoulder (3) which is also replaceable and both are made of material having low thermal conductivity.
3. The friction stir welding (FSW) tool as claimed in claims 1 and2 wherein the collar (7) is removed and the shoulder assembly is independently rotated by using an external motor through the V-belt drive connected to the pulley unit (6) whereby the pin (2) and shoulder (3) are made to rotate independently of one another in same or opposite direction with same or different speed, thereby achieving Dual Rotation FSW (DRFSW) or Counter Rotation FSW (CRFSW).
4. The friction stir welding (FSW) tool as claimed in claims 1 and 2 wherein the collar (7) is removed and Stationary Shoulder FSW (SSFSW) is performed by keeping the shoulder (3) stationary.
5. The friction stir welding (FSW) tool as claimed in claim 1 and 2 wherein mode selector collar (7) is installed and it arrests the shoulder assembly with main shaft(1), thereby achieving Regular FSW.
6. The friction stir welding (FSW) tool as claimed in claim 1 and 2 wherein the floating shoulder (4) in the shoulder assembly is floating type, which can move in the axial direction through the splines machined inside the floating shoulder (4) and over the shoulder body (5).
7. The friction stir welding tool as claimed in claim 1 and 2, wherein said floating shoulder (4), supported by resilient device(11) is adapted to move up and down relative to the pin (2) to maintain shoulder contact with the workpiece constantly, results in larger quantity of heat being generated thereby significantly reducing the axial force required during welding.
8. The friction stir welding tool as claimed in claim 1 and 2, wherein the height of the pin (2) is equal to the displacement of the floating shoulder (4) in the upward direction, the lifting of floating shoulder (4) stops when pin (2) reaches the desired depth and over-penetration of the tool into the workpiece is avoided by presetting the sheet thickness in the tool using a sheet thickness adjustor (12) and locknut (13), making the tool suitable for welding sheets having a wide range of thickness and sheets with varying thickness.
9. The friction stir welding tool as claimed in claims 1 and 2, wherein the tool is adapted to gradually retracting the pin (2) from the workpiece while maintaining contact between shoulder (3) and the workpiece, said resilient device (11) attached to said floating shoulder (4) always ensuring a predetermined contact force which reduces the chance of plasticized material to flow outside, thereby eliminating formation of keyhole at the end of welding.
10. The friction stir welding tool as claimed in claims 1and 2 wherein the pin (2) can be replaced with a modified micro pin (16) at the tip portion having an integral shoulder (17) whereby the tool is capable of performing Regular µFSW, Counter Rotation µFSW, Dual Rotation µFSW, Stationary Shoulder µFSW in the same manner as RFSW, CRFSW, DRFSW and SSFSW and said tool is capable of performing friction stir welding process, in respect of materials with thicknesses of 1000 µm or less.

Dated this 11th day of October, 2017.
(S. GHOSH)
for seenergi IPR
Applicant’s Agent
, Description:MULTI–MODE FRICTION STIR WELDING TOOL

FIELD OF THE INVENTION
The present invention relates in general to tools and their parts used for friction stir welding (FSW) and in particular to a multi–mode friction stir welding tool having the capability to perform all the variants of friction stir welding such as Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW, Micro FSW (µFSW) and High Speed FSW.

BACKGROUND AND PRIOR ART
Friction Stir Welding (hereinafter referred to as FSW) is a solid state welding process invented in 1991 by The Welding Institute (TWI), UK. In FSW, the combined rotation and translation of a specially designed tool having a shoulder and pin creates heat by friction and causes plastic deformation of the weld material. Since the maximum temperature developed during the process is within 90% of melting temperature of the base metal, the joint developed would have superior properties like fine grain, lesser distortion, lesser residual stress, better ductility, etc. Dissimilar metals can be joined using this technology. FSW can be effectively used for welding aluminium, copper, magnesium, titanium, steel and so on.
In the prior art, there are several types of tools for performing FSW. However, for each type of FSW, i.e. Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW and Micro FSW, a dedicated tool has to be used. Out of these variants, the Stationary Shoulder FSW was invented for welding thermo plastics, which is a major research area. The rest of the variants are used for welding metals.
A single FSW tool which can perform all the variants of FSW has not been reported anywhere in the prior art. In addition, huge axial force needs to be exerted during the welding in the prior art. Also, a keyhole is created at the end of welding in the prior art during withdrawal of the tool. A single tool for both metal and thermo plastics welding is also not disclosed in the prior art.
Tool wear and tool failure are very common while welding hard metals like steel and titanium. In the prior art tools, replaceable type shoulder and pin are not disclosed. In the existing uni-body design of pin and shoulder eliminates the possibility of using material with low thermal conductivity for making pin and shoulder. This is primarily because these materials are very difficult to machine. Thus possibility of changing the tool materials to give replaceable tools is limited in current state of art.
The present invention seeks to overcome these drawbacks of the prior art.
The present invention is a multipurpose friction stir welding tool having the capability to perform all the variants of friction stir welding such as Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW, Micro FSW (µFSW) and High Speed FSW.
In addition, advantages include substantial reduction in welding force in the axial direction, facility to adjust pin height for different sheet thickness, welding of workpiece with varying thickness, and elimination of keyhole at the end of welding. The tool is very compact and light in weight, weighing less than 3 kg.
Moreover, the present invention has detachable and replaceable type pin and shoulder parts to substantially reduce tooling costs.
Such a tool is not known in the prior art.

OBJECTS OF THE INVENTION
The primary object of the invention is to provide a multi–mode friction stir welding tool which is capable of performing all the variants of FSW such as Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW, Micro FSW (All variants) and High Speed FSW (All variants).
Another object of the invention is to provide a friction stir welding tool which can be used for both metal and thermo plastics welding.
Another object of the invention is to provide a friction stir welding tool having multi–mode friction stir welding spindle which results in reduction of force developed during welding.
A further object of the invention is to provide a friction stir welding tool having multi–mode friction stir welding spindle which is suitable for welding of work pieces of wide range of thickness and varying thickness by introducing a thickness selector and a locknut in the tool.
Yet another object of the invention is to provide a friction stir welding tool having multi–mode friction stir welding spindle which eliminates key hole at the end and has detachable and replaceable pin and shoulder so that the tooling cost can be reduced significantly.
Another object of the invention is to provide a frictionstir welding tool having multi–mode friction stir welding spindle which reduces the axial force developed during friction stir welding by introducing a floating shoulder supported by a resilient device and by reducing the heat loss from the work piece through the tool, by using low thermal conductive material for fabricating pin and shoulder.
Yet another object of the present invention to provide a multi–mode friction stir welding tool which is capable of performing Micro friction stir welding (µFSW) which is the adaptation of the friction stir welding process to materials with thicknesses of 1000 µm or less.
It is another object of the present invention to provide a multi–mode friction stir welding tool which is capable of performing High Speed FSW which includes High Rotational Speed of the spindle (6000 to 24000 rpm) as well as high traverse speed of the tool (more than 100 mm/min).
How the foregoing objects are achieved will be clear from the following description. In this context it is clarified that the description provided is non-limiting and is only by way of explanation.

SUMMARY OF THE INVENTION
Accordingly, the present invention provides multi-mode friction stir welding (FSW) tool comprising a main shaft connected to the spindle of the FSW machine. The main shaft has a detachable pin connected to it. A shoulder is in connectivity with the pin and it is either detachable from or integral with the pin. This shoulder is connected to floating shoulder supported by a resilient device, the former being adapted to be in use or in stationary mode and there is provided a shoulder body enclosing the detachable shoulder and floating shoulder. The shoulder body is connected to a pulley unit and the upper portion of the tool is fitted with a removable mode selector collar, which can be selectively removed or installed for the tool to perform different variants of friction stir welding.
Preferably, the pulley unit, shoulder body, floating shoulder and detachable shoulder comprises shoulder assembly and it is connected to the main shaft by means of two tapered roller bearings, whereby the forces acting on the shoulder in axial and tangential directions can be transmitted to the main shaft.
More preferably, the pin, which is connected to the main shaft of the tool through threads, is detachable and replaceable just like the detachable shoulder which is also replaceable and both are made of material having low thermal conductivity.
Most preferably the pin can be replaced with a modified micro pin at the tip portion having an integral shoulder whereby the tool is capable of performing Regular µFSW, Counter Rotation µFSW, Dual Rotation µFSW, Stationary Shoulder µFSW in the same manner as in Regular FSW, Counter Rotation FSW, Dual Rotation FSW and Stationary Shoulder FSW and said tool is capable of performing friction stir welding process in respect of materials with thicknesses of 1000 µm or less.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The nature and scope of the present invention will be better understood from the accompanying drawings, which are by way of illustration of a preferred embodiment and not by way of any sort of limitation. In the accompanying drawings:-
Figure 1a is a line diagram of the multi–mode friction stir welding tool having spindle with detachable pin and floating shoulder according to the present invention.
Figure 1b is a sectional view along line A-A of the tool shown in figure 1a, showing the mode selector collar inserted.
Figure 2a is a line diagram of the tool showing the mode selector collar inserted when the tool perform Regular FSW.
Figure 2b is a sectional view along line A-A of the tool shown in figure 2a, showing the mode selector collar inserted when the tool perform Regular FSW.
Figure 3a is a line diagram of the tool when mode selector collar is removed when the pin and shoulder can rotate independently when the tool performs Dual Rotation FSW.
Figure 3b is a sectional view along line A-A of the tool shown in figure 3a, showing mode selector collar removed and pulley unit being rotated in the direction of the pinto perform Dual Rotation FSW.
Figure 4a is a line diagram of the tool when mode selector collar is removed and the pin and shoulder are being rotated in opposite directions as the tool performsCounter Rotation FSW.
Figure 4b is a sectional view along line A-A of the tool shown in figure 4a, showing mode selector collar removed and pulley unit being rotated in the opposite direction of the pin for Counter Rotation FSW operation.
Figure 5a is a line diagram of the tool when mode selector collar is removed and only the pin is being rotated, the pulley unit drive being switched off as the tool performs Stationary Shoulder FSW operation.
Figure 5b is a sectional view along line A-A of the tool shown in figure 5a, showing mode selector collar removed and pulley unit drive being switched off for Stationary Shoulder FSW operation.
Figure 6a is a line drawing of the tool showing the modified pin inserted in place with the integral shoulder, so that the tool can perform µFSW.
Figure 6b is the sectional view along line A-A of the tool shown in figure 6a.
Figure 6c is the magnified view of the bottom region of figure 6b showing the modified pin along with floating shoulder for performing µFSW.
Figure 7 shows an exploded view of the multi–mode friction stir welding tool having spindle with replaceable pin and floating shoulder according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION
Having described the main features of the invention above, a more detailed and non-limiting description of a preferred embodiment will be given in the following paragraphs with reference to the accompanying drawings.
In all the figures, like reference numerals represent like features. Further, the shape, size and number of the components shown are by way of example only and it is within the scope of the present invention to change their shape, size and number without departing from the basic principle of the invention.
All through the specification including the claims, the technical terms and abbreviations are to be interpreted in the broadest sense of the respective terms, and include all similar items in the field known by other terms, as may be clear to persons skilled in art. Restriction or limitation if any referred to in the specification, is solely by way of example and understanding the present invention.
Referring to figures 1a and 1b, the multi–mode friction stir welding tool having spindle with replaceable pin and floating shoulder according to the present invention comprises of a main shaft (1) which is connected to the spindle of the FSW machine. A detachable pin (2) is connected to the main shaft (1) via threaded joint. There is also a detachable shoulder (3), which is connected to a floating shoulder (4), the latter being supported by a resilient device (11), which can be a spring. Both the detachable pin (2) and the detachable shoulder (3) can be replaced when needed. Further, as explained a little later with reference to figures 6a, 6b and 6c, the pin can be replaced with a modified micro pin where it takes a modified shape (16) and this is within the scope of the invention. Further, the floating shoulder (4) can be selectively put to use or can be dormant, depending upon the requirement. For example, during Regular Micro Friction Stir Welding operation, floating shoulder (4) is not required.
The detachable shoulder (3), the floating shoulder (4) are enclosed in the shoulder body (5). A plurality of splines is provided over the shoulder body and inside the floating shoulders to facilitate the upward and downward movement of the floating shoulder (4). The shoulder body (5) is connected to a pulley unit (6) through a threaded joint. A grub screw (14) is used to avoid over-tightening and loosening during reversing the direction of rotation.
The force acting on pin (2), which is developed during friction stir welding, is transferred to the main shaft (1) directly, while the force acting on the shoulder (3) is transferred through the floating shoulder (4), shoulder body (5) and pulley unit (6) to the main shaft via two tapered roller bearings (8). Two lock nuts (9) and (10) are used to keep the main shaft in position. Lock nuts (9) and (10) can also be used to adjust the tightness of the tapered roller bearings (8).
The tool is provided with a mode selector collar (7), which is removably fitted in the upper portion of the tool and is used to make the tool perform different variants of friction stir welding (FSW). By inserting the collar (7), the relative motion between main shaft (1) and shoulder (3) can be arrested and conventional friction stir welding can be performed as shown in figures 2a and 2b. Here, the tool performs Regular Friction Stir Welding (RFSW).
However, when the mode selector collar (7) is removed, the pin and shoulder can rotate independently, enabling the other variants of FSW to be performed. This would be clear from figures 3a, 3b, 4a, 4b, 5a and 5b. All these figures depict the state of the tool when collar (7) has been removed. The shoulder assembly can be rotated independently by using a separate motor and a V belt drive (not shown) connected to the pulley unit (6). The pin (2) rotates in a direction similar to the shoulder (3) or opposite to it, but what is important is that pin (2) and shoulder (3) rotate separately. As explained a little later with reference to figures 5a and 5b, in this stationary shoulder friction stir welding process, it is the pin (2) which rotates, but the shoulder (3) remains stationary.
Referring to figures 1a and 1b again, the floating shoulder (4) is supported by resilient device (11) so that initially before plunging into the workpiece, the tip of the pin (2) and tip of the shoulder (3) are at the same level as shown in fig 1. This is accomplished by the help of two set screws (15) which prevent the floating shoulder from coming out in the direction of spring force. When welding begins and the tool plunges into the workpiece, an upward force pushes the shoulder (3) against the spring force as shown in figures 2a and 2b. Since the floating shoulder (4) is assisted by the spring force, the contact between shoulder (3) and the workpiece is ensured by the spring force while the pin goes into the material. The upward movement of the shoulder (3) and floating shoulder (4) is controlled by a thickness adjustor (12) along with a lock nut (13). In actual practice the travel distance of floating shoulder (3) is equal to the thickness of workpiece.
Figure 2a is a line diagram of the tool showing the mode selector collar inserted when the tool perform Regular FSW. Figure 2b is a sectional view along line A-A of the tool shown in figure 2a, showing the mode selector collar inserted when the tool perform Regular FSW.
These two figures 2a and 2b indicate that the tool is performing Regular Friction Stir Welding (RFSW). This would be particularly clear from figure 2a, which indicates a position different from that indicated in figure 1a. Further, from figure 2a it would be also clear that relative motion between the pin (2) and the shoulder (3) is arrested. The structural and functional aspects of the friction stir welding tool, would be clear from the description above, with reference to figures 1a and 1b. Accordingly, hereinafter, only the special functional aspects are discussed without repeating the structural aspects of the friction stir welding tool and the routine function performed by each part.
As shown in figures 3a and 3b, the pin (2) and shoulder (3) are made to rotate independently in the same direction. In this mode, as particularly seen in figure 3a, the tool performs Dual Rotation FSW, i.e. the pin and shoulder are made to rotate in same direction but with different speed. It would be also clear from figure 3b, that the collar (7) is removed.
Figures 4a and 4b show that the pin (2) and the shoulder (3) are rotated in opposite directions. This mode, as it would be particularly clear from figure 4a, is Counter Rotation FSW. Here, also, it can be seen from figure 4b that the collar (7) is removed.
Figures 5a and 5b depict that only the pin (2) rotates and the external motor driving the shoulder is turned off and only the pin (2) rotates. This would be clear from figure 5a. The tool performs Stationary Shoulder FSW in this mode. Here, also, it can be seen from figure 5b that the collar (7) is removed.
Figure 6a, 6b and 6c show the details of spindle having modified pin (16) for performing Micro FSW. Micro friction stir welding (µFSW) is the adaptation of the friction stir welding process to materials with thicknesses of 1000 µm or less. It can be seen from figure 6b and also from the enlarged version in figure 6c, a modified detachable micro pin (16) is used for performing µFSW. From figure 6c, it would be clear that the modified Pin consists of a micro pin (16) along with an integrated shoulder (17). The detachable shoulder (3) and floating shoulder (4) assistance can be turned on or off according to the requirement. The spindle is capable of performing all the variants of µFSW such as Dual Rotation µFSW and Counter Rotation µFSW when the assistance from detachable shoulder (3) and floating shoulder (4) is on and also regular µFSW when the assistance from detachable shoulder (3) and floating shoulder (4) is off. It can also perform Stationary Shoulder µFSW by keeping the floating shoulder (3) stationary. Although not shown in figure 6b, it should be understood that the collar (7) is removed during Dual Rotation µFSW, Counter Rotation µFSW and Stationary Shoulder µFSW. The collar (7) remains in place, only during regular µFSW.
Figure 7 shows the exploded view of the proposed tool, in which the placement of the different parts of the tool mentioned above is clearly shown.
Hence, from the description hereinbefore, it would be clear that the multipurpose tool for friction stir welding according to the present invention is capable of performing all the variants of FSW such as Dual Rotation FSW, Counter Rotation FSW, Stationary Shoulder FSW, Regular FSW, Micro FSW (all variants) and High Speed FSW (all variants). This is the principal advantage achieved by the friction stir welding tool according to the present invention.
The additional advantages include reduction of force developed during welding, welding of work pieces of wide range of thicknesses and varying thickness, elimination of key hole at the end of the welding job, detachable and replaceable pin and shoulder and so on. In the present embodiment, the pin and shoulder are independently designed so that the pin and shoulder can rotate separately.
The rotating pin (2) and shoulder (3) can be independently controlled for their speed and direction of rotation. A floating type shoulder (4) supported by a resilient device (11) is designed in such a way that it can perform axial movement relative to the pin (2). Thus, during plunging of the tool into the workpiece, tips of both the pin and the shoulder make contact with the workpiece. This results in larger quantity of heat being generated due to the increased contact area between the tool and the workpiece, leading to softening of the workpiece quickly. Hence, the force during plunging reduces significantly. As plunging advances, the shoulder is lifted upwards by compressing the resilient device.
The floating shoulder (4) supported by the resilient device (11) always ensures detachable shoulder (3) contact with the workpiece, irrespective of the thickness of workpiece. Thus thickness variation of the workpiece does not affect the quality of the weld obtained. Travel of the floating shoulder (4) relative to the pin is adjusted with sheet thickness adjustor (12) and lock nut (13) so that the thickness of the sheet to be welded can be varied within a given range. During the retracting stage of the tool after welding, the pin retracts from the workpiece by maintaining the contact between shoulder (3) and workpiece with the help of resilient device (11). Thus, the keyhole at the end of welding can be eliminated, which is an added advantage of the present invention. This tool design facilitates the possibility to make the pin and shoulder with materials having lower thermal conductivity, which reduce heat loss through the pin and shoulder. This helps in softening the material quickly and maintaining the axial force at lower level.
In the present embodiment, the pin and the shoulder are designed for independent rotation as in the case of a coaxial spindle. The pin (2), which is connected to the main shaft (1) of the tool through threads, is detachable and replaceable. The shoulder assembly consists of the pulley unit (6), shoulder body (5), floating shoulder (4) and a detachable shoulder (3). The shoulder assembly is connected to the main shaft (1) via two tapered roller bearings (8). Thus the forces acting on the shoulder in axial and tangential directions are transmitted to the main shaft. The shoulder (3) is floating type, which can move in the axial direction through a plurality of splines machined inside the floating shoulder (4) and over the shoulder body (5).
The shoulder assembly can be independently rotated by using an external motor through V-belt drive, not shown here, which is connected to the pulley unit (6). Thus Dual Rotation FSW and Counter Rotation FSW can be achieved. Stationary Shoulder FSW is also possible in the present embodiment by keeping the shoulder unit stationary. For carrying out regular welding, the specially designed mode selector collar (7) is made to firmly hold the shoulder (3) with the main shaft (1).
In accordance with the friction stir welding tool of the present invention, detachable shoulder (3) and floating shoulder (4) can perform axial movement relative to the pin (2). Pin (2) and shoulder (3) can independently rotate. Their speed and direction of rotation can be independently controlled.
For obtaining a sound weld, the height of pin should nearly be equal to the thickness of the sheets to be welded. The movement (range) of the floating shoulder (4) relative to the pin (2) can be preset by a sheet thickness adjustor (12) and locknut (13) provided on the shoulder body (5) of the shoulder assembly. Thus, the present invention allows welding sheets of different thicknesses with same tool.
More advantageously, the height of the pin (2) is equal to the displacement of the floating shoulder (4) in the upward direction. The lifting of floating shoulder (4) stops when pin (2) reaches the desired depth and over-penetration of the tool into the workpiece is avoided by presetting the sheet thickness in the tool using a sheet thickness adjustor (12) and locknut(13), making the tool suitable for welding sheets having a wide range of thickness.
The resilient device (11) always ensures that the contact of shoulder end with the sheet to be welded is maintained and hence the thickness variation due to the surface irregularities does not affect the quality of the weld produced.
In the retraction stage, the resilient device allows gradual retraction of the pin from the workpiece while maintaining the contact between the shoulder and sheet surface. The resilient device (11) attached to the floating shoulder (4) always ensure a predetermined contact force. This reduces the chance of plasticized material to flow outside, thereby eliminating formation of keyhole at the end of welding. In addition, the pin and shoulder can be fabricated with materials having lesser thermal conductivity which significantly reduces axial force by reducing the heat loss through the tool from the workpiece.
During FSW, while the tool plunges into the workpiece, plasticized material coming out of the workpiece is suppressed by the shoulder. In the present invention, the shoulder is assisted by a spring, the stiffness of which affects the flow of plasticized material coming out. When the stiffness of the spring is less, more material tends to come out and vice versa.
The material used for making pin, shoulder, floating shoulder and main shaft is H13 Tool Steel (Hot Die Steel) and for all other parts it is Oil Hardened Non Shrinking Steel (OHNS).The machining of the parts of the tool involves turning, boring, drilling, CNC milling, thread cutting and tapping. All the machining operations are performed with an accuracy of ±0.010 mm. The pin, shoulder and floating shoulder are subjected to heat treatment procedure to achieve a hardness of 50 HRC.
In conventional friction stir welding tools, the pin initially makes contact with workpiece during the plunging stage. Since the area of pin tip is very small compared to shoulder area, the contribution of heat generation from the pin tip is very low. Thus large axial force is generated during the plunging phase.
This problem is eliminated in the present invention by ensuring that both pin and shoulder make contact with the workpiece from the beginning of the plunging itself. Thus frictional heat development in the plunging phase is maximised by maintaining the contact between shoulder and workpiece by the resilient device (11). There are two advantages for using this technique:
1. Reduction of axial force developed during plunging phase,
2. Avoiding the possibility of plasticized materials coming out during retraction phase.
As stated before, the shoulder and pin are independently operated in the present invention. Thus the amount of heat generation can be controlled by adjusting the rotational speed of the shoulder. For example, both pin and shoulder rotate with same speed in conventional friction stir welding, but the present invention allows the shoulder and the pin to rotate with same or different speeds in same direction, and same or different speeds in opposite directions. By controlling the rotational speed of shoulder, the heat generation can be controlled precisely, which eliminates the chance of melting of the material.
The floating shoulder supported by the resilient device always maintains the contact between the shoulder and the workpiece. Thus, the surface irregularities do not affect the weld quality. In addition to that, the withdrawal of the tool from the workpiece is gradual as the shoulder maintains contact with the workpiece during the withdrawal stage also. This eliminates the chance of plasticized materials coming out during the retraction phase. Since the retraction is gradual due to maintaining the shoulder contact, keyhole formation is also eliminated.
The present invention uses material having low thermal conductivity for fabricating the detachable and replaceable pin (2) and shoulder (3) in order to reduce the axial force by reducing the loss of heat conducted through the tool to the machine’s spindle. This was not present in the prior art. The existing single body design of prior art pin and shoulder eliminates the possibility of using low thermal conductive material for making pin and shoulder. This is primarily because these materials are very difficult to machine. Thus, possibility of changing the tool materials in current state of art is limited.
The multi–mode friction stir welding tool, according to the present invention is capable of performing High Speed FSW which includes High Rotational Speed of the spindle (6000 to 24000 rpm) as well as high traverse speed of the tool (more than 100 mm/min).
The present invention has been described with reference to some drawings and a preferred embodiment purely for the sake of understanding and not by way of any limitation and the present invention includes all legitimate developments within the scope of what has been described herein before and claimed in the appended claims.