The Mechanics of Bi-metal Drawing of Rod and Tube

Abstract

Due to the increasing use of bi-metal products in electrical, mechanical, nuclear and aerospace engineering, the mechanics of bi-metal drawing of tube and rod was studied. The research was done for three different processes, namely the bi-metal drawing of tube on rod, the bi-metal plug drawing of tube on tube and the bi-metal tube sinking process. Using the equilibrium approach for the analysis of the bi-metal drawing of tube on rod process, theoretical expressions were derived for the prediction of the die pressure, the draw load and the interfacial shear stress between the different material combinations. It was found that the magnitude and direction of the shear stress at the bi-metal interface and the bi-metal thickness ratio had a marked effect on draw load. The theoretical expressions gave good agreement with experiments on conventional rod drawing and bi-metal drawing of tube on rod. Nomographs were produced so that lengthy equations are simplified using graphical anamorphosis. A combination of nomographs was used to obtain useful data from different combinations of variables. The plug drawing of bi-metal tube was simulated by a mathematical model using the equilibrium approach. The theory enables the draw load, plug load and mean die pressure for different material combinations and thickness ratios to be predicted. A computer programme was presented to predict these parameters with different reductions in area and under different frictional conditions at the die/tube, tube/plug interfaces. These were compared with the experiments using different combinations of stainless steel, mild steel, brass and copper tubes. The agreement between theoretical prediction and experiments was good. The bi-metal tube sinking process was analysed by the upper bound approach. A computer programme was written to give the optimum number of the trapezoidal elements in the deformation zone required on the outer and inner tubes for the least work. The theory was compared with experiments. A reasonable upper bound was obtained. The dimensional variations in bi-metal sinking experiments were also noted. Following these theoretical and experimental investigations, the force necessary to draw axisymmetric bi-metal combinations can be predicted with satisfactory reliability and both nomographs and computer programmes have been produced to assist in making the necessary calculations. The coefficients of friction at the tool/tube interfaces were measured by the rotating die method which gave a reasonably good estimation of the frictional conditions.