Tube formability assessment for tube hydroforming

摘要

Determination of forming limit of tubes can be difficult when it involves many process-related variables and the use of costly hydroforming equipment. Therefore, the industry currently relies on sheet or tube longitudinal properties to estimate tube hydroformability. This paper introduces a simple method for experimentally assessing formability of tubes as material intrinsic characteristics for tube hydroforming, and presents results on the correlation between mechanical properties and failure location as well as forming limit at a small negative minor strain state. One commercial hydroforming tube, and one aluminum trial tube, both made by a continuous tube mill and seam welded using high-frequency (HF) induction welding, were evaluated for their forming characteristics. It is found that during tube-free expansion, the failure location of the tubes is affected by the nonuniformity of transverse tensile properties around the tube circumference, and failure initiates at lower strength/hardness location. Forming limit at -5.5% minor strain for the tubes studied here are also determined. It is found that the two tubes studied here have similar limit strain, which can be related to their similar level of n-value at their respective UTS, and similar uniform elongation, both obtained by uniaxial hoop tension test. The forming limit strain at free-expansion were obtained through the use of a newly developed test method using a ring specimen to simulate free expansion of tubes. The test method, referred here as the Zero-Gage Ring Hoop Tension Test (ZG-RHTT), is an extension of a recently developed method of uniaxial tube transverse tensile test.