Effects of Asymmetric Thickness Mandrel on NC RDB Forming Quality of Ultra-Thin-Walled Tube

Abstract

To satisfy lightweight design requirements, aerospace ducts frequently employ ultra-thin-walled tubes with a diameter-to-thickness ratio (D/t) exceeding 100.However, ultra-thin-walled tubes present significant forming challenges, and the mandrel plays a critical role in their bending. Therefore, investigating the effect of mandrel structure on the quality of ultra-thin-walled tubes formed through NC bending is of considerable importance. In this study, utilizing the Abaqus nonlinear finite element platform, an asymmetric thickness ball design method is proposed. Based on the positioning of the asymmetric balls within the mandrel, seven distinct designs for asymmetric thickness mandrels are developed. This study conducts a finite element analysis of the NC rotary draw bending (RDB) process for ultra-thin-walled 304 stainless steel tubes and validates the corresponding experiments. The results indicate that as the asymmetric thickness mandrel is positioned further from the mandrel, the stress on the outer side of the tube near the bend initiation first increases and then decreases, while the stress on the inner side of the tube, after the midpoint of the bend, initially decreases and then increases. The use of asymmetric thickness mandrels significantly reduces both the thinning and thickening rates of the tubes, though their impact on improving the ellipticity is less pronounced. The core ball nearest to the mandrel is designated as Ball 1, with subsequent balls further from the mandrel labeled as Ball 2 and Ball 3, respectively. The placement of the asymmetric thickness balls improves the thinning rate in the order: Ball 2 > Ball 1 > Ball 3; enhances the thickening rate in the order: Ball 1 > Ball 2 > Ball 3; and optimizes the ellipticity in the order: Ball 3 > Ball 1 > Ball 2.