A method for evaluating dynamic viscosity of alloys during friction stir welding

Abstract

The constitutive laws for dynamic viscosity, used today in CFD-models for the analysis of viscous flows during friction stir welding, aggregate data obtained from diverse standard mechanical trials (including, among others, hot compression, tension, torsion, etc.). However, the real stress–strain condition in the stir zone during FSW is that the metal dwells in the condition of overall, multiaxial deformation at high strain rates. Therefore, a method for determining the dynamic viscosity of alloys via spot-FSW trials has been proposed. The method is based on the measurements of the forge force and spindle torque of a pinless tool during the spot-FSW trials as well as the thickness of the stir zone. The results of the measurements are then processed by means of a computational procedure, which reproduces: the temperature field in the workpiece based on the torque and forge force data; strain rate in the stir zone based on its thickness; dynamic viscosity based on the strain rate and temperature. The method has allowed the calculation of a variety of discrete values of the dynamic viscosity for aluminum alloy AA5083 in dependence on the temperature and strain rate at various locations of the stir zone boundary. These discrete data have finally been approximated to receive a corresponding temperature-and-strain rate dependent polynomial.

Keywords

• Friction stir welding (FSW)
• Dynamic viscosity
• Aluminum alloy AA5083
• Experimental-and-calculation method
• Computational fluid dynamics (CFD)-model