A thin-walled aluminum pipe is used for an arrow or a sports tent pole. For the sports tent makers, the strength of tent poles is the most competitive issue since the world market of sports tent demands the stronger tent poles persistently. The enhancement of the strength of the tube often requires additional tube-drawings and various heat treatments. No matter how precise the manufacturing system of a pipe is, the pipe after various process histories has residual strains and stresses with the curvature in itself. The curvature of a tube is eliminated by a roller leveling process, which is one of the most important factors in determining the quality of the tube. A roller leveler consists of several pairs of rotating staggered rollers where a pipe to be straightened is inserted and experiences serial bending and unbending during the leveling process.
As a substitution for A17001T6 tubes, which has the yield stress of 550 MPa and the elongation of 12%, an A17001 T9 tube is a good candidate for the enhanced strength, which has the yield stress of 663MPa and the elongation of 2%. The mechanical properties of the high yield stress and the very small elongation of Al 7001 T9 prohibit the pipe from being straightened with existing roller levelers designed for the A17001T6 pipe. The difficulties has to be overcome with identification of the leveling mechanism as well as the critical process parameters such as the intermesh of each roller and the slanted angle of rollers. Improper combination of process parameters results in the low quality of straightening, roll marks on the pipe's surface or crushing of the section.
The process parameters of a roller leveling process have to be identified based on the accurate quantitative evaluation of the stress, strain, and straightness of a pipe during and after the leveling process. The parameter values have been determined by experiences of skilled workers, simple mathematical models, or experiments with the trial-and-error, which are insufficient to provide the accurate quantitative information on the leveling mechanism and the process. In order to obtain the accurate quantitative, a systematic solution method is required.
In this paper, optimization of process parameters has been carried out for a multi-staggered-type 14-roller leveler. A finite element model of a multi-staggered 14-roller leveller was constructed and verified by experiments. Process parameters that have the significant effect on the leveling process are identified through screening finite element analysis with a fractional factorial experiment design. The analysis result with the full model shows that the most important parameters are the intermesh of third and fourth `loading rollers' on the right row of leveller and the slanted angle of the whole rollers as well as the allowable range of the significant parameters. Since finite element analysis with the full 14-roller leveler model requires excessive computation time, a fractional finite element model that consists of 7 rollers is constructed for the computational efficiency based on the result of the screening analysis. The main analysis is carried out with the fractional model and the Taguchi method for evaluation of the effects of the process parameters. The response variable is set to the plastic strain along the pipe length. Two noise factors are taken into account: the first is the initial residual curvature of a pipe; and the second is the variance of the plastic strain in the pipe. The optimum combination of process parameters is determined from the numerical analysis result and its validity is verified by experiments that show excellent coincidence with the analysis result. To guarantee the robustness of the leveller and enhance the straightness of the pipe after levelling process, the roller geometry is optimized. Three major geometric factors of the roller are considered: the first is the roller radius, the second is roller length, and the third is radius of roller curvature. The optimum combination of geometric parameters is determined from numerical analysis. The levelling process with optimum geometry of the roller shows excellent straightness of the tube after levelling.
