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 / t 0 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 / t 0 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 / t 0 and to improve the forming result as well as the process stability.
Highlights
Micro-Hydroforming is a down-scaled forming process, based on the expansion of metal tubes with outer diameters below about 1000 μm by internal pressurization within a die cavity which corresponds to the final workpiece shape
This paper presents experimental and theoretical results for the microhydroforming 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
An important cause of size-effects is resulting from the increasing ratio of grain size to tube wall thickness dk/t0 in micro-hydroforming when reducing the scale compared to conventional tube hydroforming process size
Summary
Micro-Hydroforming is a down-scaled forming process, based on the expansion of metal tubes with outer diameters below about 1000 μm by internal pressurization within a die cavity which corresponds to the final workpiece shape. Experimental bursting tests of micro-tubes and the comparison of the results with tests of macro-tubes in [4] have shown that such influences have to be considered in micro-hydroforming. For conventional hydroforming it has already been identified that a superimposed axial compressive force improves the formability of the tube metal [6]. Special focus is on the correlations between the microstructure of the workpiece material, the stress state during forming with and without the application of superimposed axial forces and the resulting stability of the forming process
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