Friction Stir Welding and Thermodynamic Processes

In FSW, many thermodynamic processes interact ocuur at the same time including the varied rates of heating and cooling, plastic deformation ,physical flow of the processed matetial around the tool. Defects like porosity and hot cracking are absent in Friction Stir Welding as it a solid state welding process. Joining occurs below the melting point of the weldable material. So the base metal doesn’t undergo bulk melting at the joint.

In various welding processes generally materials are welded by reducing the resistance to deformation by supplying the needed energy in the form of heat.However the supplied heat tendsw to create conditions that leads to microstructural changes such as recrystallization, grain orientation growth and coarsening or dissolution of the strength giving precipitates. Such microstructural changes depend upon the chemical composition of the various materials involved in welding.In FSW formation of defects is still observed because it lacks the potential for imbalances between the different processing zones. Faults such as non bonding , void formation can appear at very cold conditions,due to insufficient material flow .

Defects such as flash formation,nuggets collapse within the stir zone and deterioration of the strength properties of the joint can appear at very hot conditions due to extensive material flow.In addition to these flow related defects other defects related to geometry also exists such as lack of penetration and lack of joining which mainly occurs due to manual error or operating errors.There are various defects such as voids,flash,onion microstructure,zigzag etc.Some effects in detail.

Bands in weld nuggets can be seen as onion ring structures in friction stir welds for al alloys.These onion ring structures have bright and dark bands and the spacing between these bands is nothing but equal to the distance the tool travels in one complete rotation.Increase in rotations of the tools results in increase of spacing between alternate bands.Increase in material transport per weld length also increases the spacing between alternate bands.Amount of heat energy that is produced per unit time depends on stirring,mixing and rotatory speed of tool. 

Rotatory and transverse speed of the tool basically governs the peak temperature during the whole welding process and also the time of the whole welding process. The translation of the tool entrains the material from the advancing side,the material is rotated around the pin and deposition takes place on the rear of the retreating side. Material is carried from retreating side of the weld and is deposited to fill in the material cavity in the wake of the pin . Thus, the FSW nugget consists of a mixture of two mechanical streams and therefore formation of rings occur.

On Increasing temperature influence in the formation and the subsequent roles that the bands play in the formation of a crack path in a weld nugget placed under cyclic loading is observed. The differences in the intermetallic particles ,size and shape in the bands are result of hotter welds. For constant rotational speed, softening of weld nugget is reduced as the feed rate or translational tool velocity is increased and it can be observed in the change in microstructure.

If the tool pin rotates at very high speed material experiences very hot temp conditions.So due to this excessive heat genearation takes place which results in softening of material around the boundary of two shoulder surface flash occurs due to excess material flow.Large amount of material is ejected if large increase in pressure occurs.Lack of penetration occurs mainly due to improper tool pin length as compared to thickness of workpiece and change in weld thickning along the weld line.Extrusion of material around the pin takes place due to high pin depth resulting in weld flash.Weld flash formation takes place on retreading side if large tool tilt angles are used.

It is very common defect in Friction Stir Welding. Void formation has high correlation with with fluid dynamics which is involved inside the weld nugget.High productivity can be achieved by increasing the welding speed but it also results in void formation under the top surface of the weld.Increasing the speed further will result in bigger holes