Residual Stress in Friction Stir Welding and Laser-Assisted Friction Stir Welding by Numerical Simulation and Experiments

Abstract

The friction stir welding (FSW) has become an important welding technique to join materials that are difficult to weld by traditional fusion welding technology. In this technique, the material is not led to fusion, and the joint is the result of the rotation and movement along the welding line of the tool that causes softening of material due to frictional heat and the stirring of the same. In FSW, the temperature does not reach the fusion value of the materials, and this helps to decrease the residual stress values. However, due to the higher force involved in the weld and, thus, the rigid clamping used, the residual stresses are not low in general in this technique. As the presence of high residual stress values influences the post-weld mechanical properties, e.g. fatigue properties, it is important to investigate the residual stress distribution in the FSW welds. In this chapter, two numerical models that predict temperatures and residual stresses in friction stir welding and laser-assisted friction stir welding will be described. Experimental measurements of temperatures and residual stress will be carried out to validate the prediction of the models.

Numerical optimisation of laser assisted friction stir welding of structural steel

“…The additional heat source softens the material before the welding process, and this has the beneficial effect of reducing the stresses experienced by the tool. Various researchers [1,[11][12][13][14] have used different types of assisted heat sources to analyse the behaviour of the entire process. Shi et al [11] investigated the FSW process on Aluminium by inputting ultrasonic vibrations to the workpiece.…”

“…A relatively simpler, yet advanced, version of the conventional FSW is the laser-assisted FSW (LAFSW), which is the most extensive modification applied to the FSW process to date [12,[16][17][18][19][20]. The experimental setup of LAFSW incorporates the FSW machine accompanied by a laser heat source.…”

“…Various experimental and modelling studies have been undertaken for LAFSW of Aluminium and other low melting temperature alloys. Casavola et al [12] made an experimental setup for LAFSW of Aluminium 5754 and performed various tests by altering the laser power and distance between the laser and the FSW tool. It was concluded that a closer laser distance with the FSW tool was more suitable to produce a strong preheating effect on the workpiece than a far distance irrespective of the laser power [12].…”

Stationary shoulder friction stir welding of military aircraft sheet alloys

“…Residual stresses are formed in FSW due to the non-uniform heat distribution and plastic deformation which occur, leading to the formation of different zones, and the choice of process parameters strongly influences the extent of residual stress within the weld. RS generation in FSW is also partially generated due to the large forces and resultant rigid clamping required which prohibits material contraction during cooling and thus introduces RS (Casavola, 2018).…”

“…appear to be spurious results based on the degree of fluctuation throughout the hole depth and in comparison with the EoW retreating side; if additional time and resources had been available both the missing and spurious data sets would be repeated. As described previously, RS generation in FSW is partially generated due to the large forces involved, significant thermal gradients, and resultant rigid clamping required which prohibits material contraction during cooling and thus introduces RS (Casavola, 2018). Additionally, the undeformed and un-heated material surrounding the weld will also impart constraints, regardless of the clamping (Mishra et al, 2014).…”