The influence of axial compressive stresses on the formability and scattering of process parameters in micro-hydroforming processes of tubes

Cologne University of Applied Sciences Cologne, Betzdorfer Strasse 2, 50679 Cologne, Germany

^a Corresponding author: e-mail: christoph.hartl@fh-koeln.de

Abstract

Feasible product geometries manufactured with micro-hydroforming, as well as process stability are crucially influenced by the microstructure of the used tube material. The higher ratio of grain size to tube wall thickness d_k/t₀ in micro-hydroforming, compared to conventional tube hydroforming, leads to an increase of scattering of process parameters and instabilities. This paper presents experimental and theoretical results for the micro-hydroforming of tubes made from stainless steel and from platinum with a focus on the correlations between the microstructure of the workpiece material, the stress state during forming and the resulting stability of the forming process. Investigated tube dimensions were 800 µm outer diameter and 40 µm wall thickness of the steel tubes, respectively 1650 µm and 76 µm of the platinum tubes. The average ratio d_k/t₀ was about 0.3 for the steel material and about 0.9 for the platinum alloy. It has been shown that superimposed axial stresses are suitable to reduce the restricting influence of an increased ratio d_k/t₀ and to improve the forming result as well as the process stability.