03-08-2012, 12:43 PM
FICTION STIR WELDING
Friction_Stir_Welding(3).ppt (Size: 1.12 MB / Downloads: 40)
Introduction
Friction-stir welding (FSW) is a solid-state joining process and is used for applications where the original metal characteristics must remain unchanged as far as possible.
This process is primarily used on aluminum, and most often on large pieces which cannot be easily heat treated.
It was invented and experimentally proven by Wayne Thomas and a team of his colleagues at The Welding Institute UK in December 1991.
Principal of operation
In FSW, a cylindrical-shouldered tool, with a profiled threaded/unthreaded pin is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces of sheet or plate material, which are butted together.
Frictional heat is generated between the wear-resistant welding tool shoulder and nib, and the material of the work pieces.
Microstuctural Features
The microstructure of a friction-stir weld depends in detail on the tool design, the rotation and translation speeds, the applied pressure and the characteristics of the material being joined. There are a number of zones
stir zone is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equal and often an order of magnitude smaller than the grains in the parent material.
flow arm is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.
The thermo-mechanically affected zone occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller.
Heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding. The temprature Are power than those in the TMAZ.
Welding forces
During welding a number of forces will act on the tool:
A downwards force is necessary to maintain the position of the tool at or below the material surface.
The traverse force acts parallel to the tool motion and is positive in the traverse direction. Since this force arises as a result of the resistance of the material to the motion of the tool it might be expected that this force will decrease as the temperature of the material around the tool is increased.
The lateral force may act perpendicular to the tool traverse direction and is defined here as positive towards the advancing side of the weld.
Torque is required to rotate the tool, the amount of which will depend on the down force and friction coefficient (sliding friction) and the flow strength of the material in the surrounding region (sticking friction).
Generation and flow of heat
For any welding process it is, in general, desirable to increase the travel speed and minimize the heat input as this will increase productivity and possibly reduce the impact of welding on the mechanical properties of the weld
The welding cycle can be split into several stages during which the heat flow and thermal profile will be different :
Dwell. The material is preheated by a stationary, rotating tool in order to achieve a sufficient temperature ahead of the tool to allow the traverse. This period may also include the plunge of the tool into the work piece.
Transient heating. When the tool begins to move there will be a transient period where the heat production and temperature around the tool will alter in a complex manner until an essentially steady-state is reached.
Pseudo steady-state. Although fluctuations in heat generation will occur the thermal field around the tool remains effectively constant, at least on the macroscopic scale.
Post steady-state. Near the end of the weld heat may ‘reflect’ from the end of the plate leading to additional heating around the tool.