1. Friction Stir Welding Submitted by, VISHNU PRABHAKAR Roll no:-11218 M-tech TKMCE 1

2. 2 Introduction Friction Stir Welding (FSW) is a solid-state joining process FSW was developed by “The Welding Institute” in 1991 FSW is still in the process of research and development It is a dry welding process The original metal characteristics remain unchanged. FSW is different from other conventional welding techniques

3. Principle of operation 3 A constantly rotating cylindrical-shouldered tool with a profiled nib is traversely fed at a constant rate into the joint between two clamped pieces of material. The nib is slightly shorter than the weld depth required, Friction heat, heat generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting.

4. Heat Generation A fraction of plastic deformation energy is stored in the form of increased defect densities Deformation increases the dislocation density, the amount of grain surface and grain edge per unit volume and by cutting precipitates may force them to dissolve local interfacial heat generation due to friction is the product of frictional force and the sliding velocity Interfacial deformation heat is the product of shear stress and the velocity of the workpiece material which sticks to the tool as it moves 4

5. Heat Transfer During tool–pin, insertion and extraction heat generation occurs at a constant rate the weld profile and properties remain roughly constant during the welding phase. The temperature and velocity fields in pseudo-steady state are commonly obtained by solving the continuity, momentum and energy equations for incompressible single–phase flow assuming steady state. Of the heat generated at the shoulder-workpiece interface, some of it is transported into the tool material while the rest enters the work–piece. 5

6. 6 Material flow on di ff erent horizontal planes (a) 0.35 mm, (b) 1.59 mm and (c) 2.28 mm below the top surface for a 304 stainless steel plate of thickness 3.18 mm.

7. Tool Design  Tool design influences heat generation, plastic flow, the power required, and the uniformity of the welded joint.  The shoulder generates most of the heat and prevents the plasticized material from escaping from the work– piece,  Both shoulder and the tool–pin affects the material flow  The tapered threads in the whorl design induce a vertical component of velocity that facilitates plastic flow. 7

8. Various tool designs 8

9. Process Description Butt Joints 9

10. The material is transferred from the leading edge of the tool to the trailing edge of the pin Forged by the intimate contact of the shoulder-pin profile. stirring motion tends to break up oxides on the faying surfaces, allowing bonding between clean surfaces. To achieve full closure pin has to pass very close to the backplate, since only limited amount of deformation occurs below the pin, and then only close to the pin surface. To avoid open root (lack of penetration) the tool axis and the workpiece are tilted by a small angle, θ, typically in the 2-4 O range drawback of limiting the ability to execute nonlinear welds and can also limit the welding speed. 10

11. The start and end of the joint will not be fully welded In steel and other high melting alloys, a small- diameter hole is predrilled in the butt line. The weld start and end regions be machined off. 11

12. Lap Joints In a lap joint there is no butt line the pin must penetrate through the top member. lap welds need out of plane stirring, across the interface of the two members being welded. A second shoulder is introduced in tool for lap welding 12

13. the pin must penetrate completely through the top member, and extend some distance into the bottom member. The notches on either side of the joint are potential sites for crack initiation and, as such, they have a profound effect on mechanical properties. Lap joints are not as strong as butt joints, they have adequate static and fatigue properties to replace fastened joints. 13

14. Processing Variables Welding Parameters rotational speed (rpm), travel speed, normal force, lateral force, tool attitude (tilt angle), shoulder plunge, penetration ligament (butt joints), penetration into the bottom member (lap joints). 14

15. Slower travel speeds and lower rotational speeds for harder alloys or thicker sections. Increasing the rotational speed or decreasing travel speeds increase heat input and welding temperatures. Plunge depth is defined as the depth of the lowest point of the shoulder below the surface of the welded plate Plunging the shoulder below the plate surface increases the pressure below the tool The plunge depth needs to be correctly set, ▫ To ensure the necessary downward pressure ▫ To ensure that the tool fully penetrates the weld. Excessive plunge depth may result in pin rubbing on the backing plate surface 15

16. Tool Design & Materials In early days simple cylinder one-piece steel tools were used Later threaded pins were used This enhanced mixing and the use of higher speeds and better quality, void free welds. scrolled shoulders enable welding around corners. The scroll shoulder eliminate weld surface undercutting and the flash that extrudes under the tool shoulder, Flat ended pins are used for better stirring action and weld penetration in butt joints, cooling of the tool, to increase its life The two-piece tool allows the use of pin materials suited for specific applications 16

17. Tool design (materials and configuration) influence joint profile, microstructure and properties. Tool materials affect the welding process, friction coefficients, hence heat generation. Tool material must be sufficiently strong, tough and hard wearing, at the welding temperature. It should have a good oxidation resistance and a low thermal conductivity to minimize heat loss and thermal damage to the machinery further up the drive train. Hot-worked tool steel such as AISI H13 has proven perfectly acceptable for welding aluminium alloys within thickness ranges of 0.5 – 50 mm 17

18. Advantages Good mechanical properties in the welded condition Improved safety - absence of toxic fumes or the spatter of molten material. No consumables — A threaded pin made of conventional tool steel, e.g., hardened H13 Easily automated on simple milling machines — lower setup costs and less training. Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool. Generally good weld appearance and minimal thickness under/over-matching reducing the need for expensive machining after welding. Low environmental impact. 18

19. Defects Exit hole left when tool is withdrawn. Large down forces required with heavy-duty clamping necessary to hold the plates together Less flexible than manual and arc processes (difficulties with thickness variations and non-linear welds). Often slower traverse rate than some fusion welding techniques, although this may be offset if fewer welding passes are required. 19

20. Applications Of Friction Stir Welding Aerospace The main rationale for employing FSW in the manufacture of aerospace components is weight savings Friction Stir Welding eliminates rivets, fasteners, and the need for an overlap sheet configuration. The butt-joint configuration facilitates joint evaluation and quality assurance. FSW offers the means to join Al-Li alloys High strength and low weight is always a desirable combination. 20

21. Ship Building Imagine a large catamaran that can be constructed from building blocks, just like a toy boat. All the pieces would fit perfectly together, ensuring mastery of dimensional accuracy and simplifying any necessary modifications. FSW represents a first step towards this type of construction approach in shipbuilding. The low heat input during joining assures less residual stress, resulting in precisely welded components that require minimal fit-up work friction stir welded products are ready-to-use. With proper design, the elements are ready-to-use directly after welding. One limiting factor, often mentioned when discussing FSW, is the relatively high downforce cneeded when performing the weld. The surface finish should be of high quality, as the aesthetic properties of the root side of the joint will follow the backing bar. 21

22. Automobile Application In principle, all aluminium components in a car can be friction stir welded Minor modifications to the structure may be needed in order to make it more suitable for FSW, but these should not be insurmountable. In larger road transport vehicles, the scope for applications is even wider and easier to adapt With FSW, plates of different thicknesses can be joined securely with a high quality weld Overlap joints are also possible with FSW, providing an alternative solution to resistance-spot-welded or seam-welded pieces. 22

23. SUMMARY Friction stir welding has been a major revolution to industry. Hard materials and engineering alloys can now be welded efficiently using this process. FSW is an emergent technology that can be used to overcome significant limitations of other joining processes. Its inherent mechanical property advantages and operating cost advantages make it ideal for automotive application. The process has demonstrated its capabilities and been approved as a novel method for joining aluminium and other metals. The welding process improves existing structural properties and leaves the weld “cold”. In some cases, if proper care is taken, weld properties equal those of the base material. Significant advances have been achieved and are ongoing in developing and evaluating FSW technologies, especially in the areas of materials, structures, weld tools, and process innovations. The selected overaging treatments also improve corrosion resistance of these alloys. In structures development, FSSW holds promise for competing favorably with conventional joining technologies 23