Experimental and Numerical Investigations on Microcoining of Stainless Steel 304

Abstract

Increasing demands for miniature metallic parts have driven the application of microforming in various industries. Only a limited amount of research is, however, available on the forming of miniature features in high strength materials. This study investigated the forming of microfeatures in Type 304 stainless steel by using the coining process. Experimental work was performed to study the effects of workpiece thickness, preform shape, grain size, and feature size on the formation of features ranging from 320μmto800μm. It was found that certain preform shapes enhance feature formation by allowing a favorable flow of the bulk material. In addition, a flow stress model for Type 304 stainless steel that took into consideration the effects of the grain and feature sizes was developed to accurately model and better understand the coining process. Weakening of the material, as the grain size increased at the miniature scale, was explained by the Hall-Petch relationship and the feature size effect.

Microforming Processes

“…In general, unfilling could occur in creating protrusion, while a close contact between blank and tooling can usually be attained in forming cavity by indentation [16]. Kim et al [74,75] conducted the experiment of forming micro-sized protrusions on the blank surface through microcoining process using stainless steel 304 and pure copper, as shown in Fig. 4.35a.…”

Accuracy of Model Force Prediction in Closed Die Coining Process

“…By incorporating the expanded Hall-Petch expression from [20] and [30] into Eq. (4), the next formulation for forging force is obtained, in which the forging force depends on true strain and the crystalline grain size:…”

“…K hp is interpreted as the resistance of the grain boundary to deformation (parameter from a HallPetch model), and d is the size of the crystalline grain. Parameters α SE and β SE are determined on the basis of the findings of [20] and [30]. The size factor (λ) is obtained from the diagram on Fig.…”

“…Many papers are concerned with the study of the size effects in micro-forming, [7] and [11] to [13], and their influence on the various microforming processes, such as micro-indentation [14], micro-imprinting [15], micro-extrusion, [16] and [17], and the bulk forming processes of sheet metal, [18] and [19]. However, relatively few works deal with the coining process and the real problems of production, such as residual stress, elastic deformation and incomplete die filling, [1] to [3], [10] and [20] to [22], lubricant residues and surface damage [23], and tool development and manufacturing, [24] and [25]. That is why this process is still not well-researched and wellknown, especially from the micro-forming point of view.…”

Size effects on formability in microscale laser dynamic forming of copper foil

“…However, the flow curve increases when the ratio further decreases from 191 to 49 in hydraulic bulge test. Kim et al (2008) developed a flow stress model, which considers the grain and feature size effect to examine the forming behaviour in the coining process. The results revealed that the forming depth is affected by the specimen thickness, grain size and feature size.…”