09-02-2013, 04:54 PM
FRICTION WELDING
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INTRODUCTION
The term welding is used to cover a wide range of bonding techniques. Now-a-days many processes of welding have been developed and probably there is no industry which is not using welding process in fabrication in its products in some form or other. This is most rapid and easiest way of fabrication and assembly of metal parts. The research carried in this field has given various way and methods to weld practically all metals. Means have also been found out to weld dissimilar metals. In comparison with other processes, welding gives 100% strength of joint and it is relatively easy and quick process.
This process is being used in Automobile industry, aircraft machine frames, structural works etc. Many of cost products are now-a –days being fabricated by welding various parts together. Such construction has advantage that products are lighter and stronger. Building, bridges and structures can be built lighter causing weight reduction and do not require additional plates as in case of riveting welded joints have high corrosion resistance also.
Design for Heat Balance :
When similar metals or dissimilar metals having about the same forging temperature and thermal conductivity are friction welded, there are no restrictions on relationship of cross sectional area of bar or tube to the size of plate to which it is welded. Heating rates as high as 100,000 f/sec make weld cycle short.
In welding dissimilar metals with widely different forging temperature and thermal conductivity adjustment of size or area adjacent to weld interface may be necessary in one of workpieces.
For example, In welding nickel base alloy shaft 2.5mm in diameter to an alloy steel shaft, the alloy steel shaft should be larger than nickel base alloy.
Design for Flow of Weld Upset :
In applications where presence of weld upset on one or more work metal surfaces is undesirable and where the upset can not be removed after welding traps can be incorporated into the joint design to provide clearance for flow of weld metal from interface.
When a plate is welded to ma shaft extending through hole in plate enough clearance must be provided to prevent rubbing of shaft against adjacent surface as it uses energy of flywheel unpredictably and diminishes reproducibility of weld.
PRINCIPLE OF OPERATION OF FRICTION WELDING
The principle of operation of this process is changing of mechanical energy into heat energy. One component is gripped and rotated about its axis while other component to be welded to it, is gripped and does not rotate but can be moved axially. When contact is made between both rotating and nonrotating parts, heat is developed at contact faces due to friction and applied pressure ensures that temperature rises to that required for welding Rotation is stopped and forging pressure is applied. Causing more extrusion at joint area, forcing out surface oxides and impurities in form of flash. The heat is concentrated and localized at interference grain structure is refined by hot work and there is little diffusion across the interference that the welding of dissimilar materials is possible.
Inertia Welding Process :
In inertia drive welding, a flywheel and the rotating part are mounted in a spindle, which is driven to the desired speed. The drive source is then disengaged, and two parts make contact under a preset axial force. The free rotating flywheel which decelerates under the same applied force or later under a large force. Meanwhile kinetic energy stored in the flywheel and spindle is converted into frictional heat at the abutting surfaces. The weld is complete when the flywheel comes to a stop. A subsequent higher forging force may be used after the flywheel has stopped.
Linear Friction Welding :
Linear Friction Welding (LFW) is seen a key technology for the aerospace industry as it enables the joining of difficult to bond materials, can be used as a repair process, and to build the complex structures required for today's gas turbines. Essentially, it is a non-melting fusion process producing high integrity welds with little prior surface preparation required.
As the parts to be welded are forced into intimate contact, a fully reversed motion is imposed on part of the system. This generates frictional heat in the immediate region about the weld plane, thereby softening a finite volume of material. As the weld proceeds, a portion of this visco-plastic layer is extruded at the periphery of the weld interface, in rippled sheets of metal known as flash. This should ensure that any interfacial contaminant is expelled. The combination of fast joining times of the order of a few seconds, and the direct heat input at the weld interface, gives rise to relatively small heat affected zones. This, by judicious selection of components geometry, this also limits process induced distortions.