Research On Micro-extrusion Forming Of H62 And Its Size Effects

Micro-forming as one of the key micro-fabrication technologies is becoming more and more important since 1990s, and the development is very rapid. In micro-forming, the specimens are so small size and high accuracy. Due to the size effects, the know-how of conventional forming cannot be simply transferred to micro-forming.Firstly, there are many unjust conceptions and ambiguous appellations in micro-forming research,accordingly, the size effects are newly defined from the view of micro-forming, which are classified as material attributive size effects and technical condition size effects. The quantifying is demonstrated, which acts on positive function in evaluating size effects by using known data.Secondly, a precision micro-forming die was designed and developed. Since the extrusion parts size is much small, high manufacture and centering precision on the die parts are required. This paper introduced single cup and double cup extrusion die and the manufacture method of forming-parts, as well as some key technologies. For increasing the guide precision guide post and ball bearing, float shank and punch self-guide are introduced.Then, the extrusion of single-cup and double-cup are performed with a series of H62 brass specimens from 4 to 0.5mm in diameter (height/diameter=1:1) at room temperature, and the aim is to study the influence of specimen size, deformation degree and deformation velocity on the material flow stress. The results show that the material flow stress decreases rapidly and size effects occurs with the decrease of the specimen diameter; the material flow stress differences increase with the increase of relative deformation degree and size effects occurs; the material flow stress differences increase with the increase of deformation velocity and size effects occur distinctly under the same relative deformation degree; the stress of double-cup extrusion is bigger than the single-cup's under the same diameter and relative deformation degree.Lastly, the friction effects in the micro-forming have been studied with the extrusion experiment of series of H62 brass. The results show that the friction increases distinctly with the decrease of the specimen diameter with lubrication and the friction coefficient increases, then an open-closed lubricant pockets are used to explain it.