A numerical and experimental methodology for overcoming challenges arising from friction stir welding of dissimilar metals

Abstract

Friction stir welding is a rather recent solid state welding technique which presents several advantages over conventional fusion welding techniques. Different aspects of this process are thoroughly investigated in this work; experimentally the effect of the several process parameters on the resultant joint quality and strength is analyzed and it was found that placing the softer material in the advancing side could result in better mixing between the two sides. Additionally, intermetallic compounds were detected at the abutting interface between dissimilar materials through SEM/EDX analysis. The increase in the quantity of these intermetallic compounds is found to decrease the strength of the joints drastically.

Moreover, this process was reproduced using finite elements modelling software Deform in efforts to save long and costly experiments and measure state variables in the bulk of the welded nugget. Temperature profiles at specific points in the system are compared between the experiments and the ones resulting from the FEM. After testing several friction coefficients, a best fit was chosen in order to minimize the error between the maximum temperature measured at these specific points. Additionally, the volume fraction of the welded interface also compared favorably to the experimental volume fraction as obtained from EDX/SEM. Improvements to the results of the proposed model can be achieved by changing the flow stress models of the materials or by finding answers to allow the solution of the simulation to converge when the workpiece is modelled as two separate entities (each representing one material) or eventually by creating a user-sub routine that includes the temperature-time transformation plots to better reproduce the resultant structure between the dissimilar materials.

In an effort to understand the mechanisms that guides the deformation mechanism at an incoherent interface, molecular dynamics simulations are conducted using LAMMPS. In the first simulations, a pristine Al/Fe interface was created and compressed at constant strain rate of 5.107s-1. A remarkable mixing was found at the advanced stages of the simulations, at temperatures starting as low at 150K and going up to 900K. The RDF suggests that the created intermetallic compound has the structure of AlFe also known as the CsCl crystalline structure. In other simulations, different strain rates were tested and the nucleation of dislocations was observed across the incoherent interface. It was noticed that the first dislocations appeared in the bulk of the aluminum region. After that the dislocations pile up at the interface from the aluminum region and thus constitute a favorable nucleation site for the dislocations in the iron region. Two yield points were discerned on the stress strain plots at different temperatures corresponding to the nucleation of dislocations in the two regions Al and Fe respectively. The flow and yield stresses were then fitted into thermally activated equations as a function of temperature and strain rates. Finally, the valuable information obtained from these developed simulations were used for comparison in a different loading stress type; this type the mixing and resultant composition of the interface were analyzed following mixing from a nanometric tool rotating and feeding at the Al/Fe interface. Such a process did not yield a variation in the RDF before and after the passage of the tool which could be due to the extremely fast speeds of the tool out of other reasons.