**Deformation Behavior Causing Excessive Thinning of Outer Diameter of Micro Metal Tubes in Hollow Sinking**

#### **Takuma Kishimoto 1,2, \*, Hayate Sakaguchi 1 , Saki Suematsu 1 , Kenichi Tashima 3 , Satoshi Kajino 4 , Shiori Gondo <sup>4</sup> and Shinsuke Suzuki 2,5,6**


Received: 25 August 2020; Accepted: 27 September 2020; Published: 1 October 2020

**Abstract:** The deformation behavior of microtubes during hollow sinking was investigated to clarify the mechanism of the excessive thinning of their outer diameters. Stainless-steel, copper, and aluminum alloy tubes were drawn without an inner tool to evaluate the effect of Lankford values on outer diameter reduction. Drawing stress and stress-strain curves were obtained to evaluate the yielding behavior during hollow sinking. The observed yielding behavior indicated that the final outer diameter of the drawn tube was always smaller than the die diameter due to the uniaxial tensile deformation starting from the die approach end even though the drawing stress was in the elastic range. The results of a loading-unloading tensile test demonstrated that the strain remained even after unloading. Therefore, the outer diameter is considered to become smaller than the die diameter during hollow sinking due to microscopic yielding at any Lankford value. Furthermore, the outer diameter becomes smaller than the die diameter as the Lankford value increases, as theorized. As the drawing stress decreases or the apparent elastic modulus of the stress-strain curve increases, the outer diameter seems to approach the die diameter during unloading, which is caused by the elastic recovery outside the microscopic yielding region.

**Keywords:** micro tube; hollow sinking; plastic anisotropy; surface quality; size effect
