Christopher B. Smith

Effect of Compliance and Travel Angle on Friction Stir Welding With Gaps

This paper presents an investigation of the effects of friction stir weld tool travel angle and machine compliance on joint efficiency of butt welded 5083-H111 aluminum alloy in the presence of joint gaps. Friction stir welds are produced with a CNC mill and an industrial robot at travel angles of 1 deg, 3 deg, and 5 deg with gaps from 0 mm to 2 mm, in 0.5 mm increments. Results indicate that the more rigid mill resulted in higher joint efficiencies than the relatively compliant robot when welding gaps greater than 1 mm with a 3 deg travel angle using our test setup. The results also show that when gaps exceed 1 mm welds made with a travel (tilt) angle of 5 deg are able to generate higher joint efficiencies than welds made with a travel angle of 1 deg and 3 deg. Based on tool geometry and workpiece dimensions, a simple model is presented that is able to estimate the joint efficiency of friction stir welds as a function of gap width, travel angle, and plunge depth. This model can be used as an assistive tool in optimizing weld process parameters and tool design when welding over gaps. Experimental results show that the model is able to estimate the joint efficiency for the test cases presented in this paper.

Toward an Efficient Parallel Eigensolver for Dense Symmetric Matrices

We describe a parallel algorithm for finding the eigenvalues and eigenvectors of a dense symmetric matrix, with an emphasis on the dense linear algebra operations. We follow the traditional three-step process: reduce to tridiagonal form, solve the tridiagonal problem, then backtransform the result. Since the different steps have different algorithmic characteristics, this problem serves as a perfect vehicle for exploring some issues associated with parallel linear algebra calculations. In particular, we examine the effects of matrix distribution and blocking on the computational performance of tridiagonalization and backtransformation. Through experiments on an Intel Paragon, we demonstrate that block storage of the matrix is not necessary for a highly efficient block algorithm. The performance of our approach compares very favorably with that of the corresponding ScaLAPACK routines.

Combined Temperature and Force Control for Robotic Friction Stir Welding

The objective of this research is to develop a closed-loop control system for robotic friction stir welding (FSW) that simultaneously controls force and temperature in order to maintain weld quality under various process disturbances. FSW is a solid-state joining process enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings compared to traditional fusion welding processes.During FSW, several process parameter and condition variations (thermal constraints, material properties, geometry, etc.) are present. The FSW process can be sensitive to these variations, which are commonly present in a production environment; hence, there is a significant need to control the process to assure high weld quality. Reliable FSW for a wide range of applications will require closed-loop control of certain process parameters.A linear multi-input-multi-output process model has been developed that captures the dynamic relations between two process inputs (commanded spindle speed and commanded vertical tool position) and two process outputs (interface temperature and axial force).A closed-loop controller was implemented that combines temperature and force control on an industrial robotic FSW system. The performance of the combined control system was demonstrated with successful command tracking and disturbance rejection. Within a certain range, desired axial forces and interface temperatures are achieved by automatically adjusting the spindle speed and the vertical tool position at the same time. The axial force and interface temperature is maintained during both thermal and geometric disturbances and thus weld quality can be maintained for a variety of conditions in which each control strategy applied independently could fail.Copyright