Micromachines 2021, 12, 1299. https://doi.org/10.3390/mi12111299 www.mdpi.com/journal/micromachines Article Effect of Punch Surface Grooves on Microformability of AA6063 Backward Microextrusion Tatsuya Funazuka 1, *, Kuniaki Dohda 2 , Tomomi Shiratori 1 , Ryo Hiramiya 3 and Ikumu Watanabe 4 1 Academic Assembly Faculty of Engineering, University of Toyama, Toyama 930-8555, Japan; shira@eng.u-toyama.ac.jp 2 Department of Mechanical Engineering, Northwestern University, Evanston, IL 60201, USA; dohda.kuni@northwestern.edu 3 Graduate School of Science and Engineering for Education, University of Toyama, Toyama 930-8555, Japan; m2071232@ems.u-toyama.ac.jp 4 Research Center for Structural Materials, National Institute for Materials Science, Ibaraki 205-0047, Japan; WATANABE.Ikumu@nims.go.jp * Correspondence: funazuka@eng.u-toyama.ac.jp; Tel.: +81-76-445-6792 Abstract: In order to apply conventional forming processes at the micro scale, the size effects caused by material properties and frictional effects must be taken into account. In this research, the effects of tool surface properties such as punch surface grooves on microextrudability, assessed using ex- trusion force, shape of the extrusion, and Vickers hardness, were investigated using an AA6063 billet. Microscale grooves of 5 to 10 µm were fabricated on the punch surface. The extrusion force increased rapidly as the stroke progressed for all the grooves. Comparing the product geometries showed that, the smaller the groove size, the lower the adhesion and the longer the backward ex- trusion length. The results of material analysis using EBSD showed that a 5 µm groove depth punch improved the material flowability and uniformly introduced more strain. On the other hand, mate- rial flowability was reduced and strain was applied nonuniformly when a mirror-finish tool was used. Therefore, the tribology between the tool and the material was controlled by changing the surface properties of the punch to improve formability. Keywords: microextrusion; size effect; microtexture; grain size; aluminum alloy 1. Introduction In recent years, the improvement of production of microscale parts using plasticity processing technology has attracted attention in various fields, including medicine, elec- tronics, and chemistry [1]. Among these technologies, microextrusion processing has at- tracted significant attention from industries as a microcomponent-forming processing technology. When conventional macroscale processing technologies such as extrusion are applied to the micro scale, problems arise in terms of repeatability and accuracy. Engel et al. clarified the effect of decreasing the product size on tribology using double-cup extru- sion tests at the micro scale, and found that as the product size decreased, there were fewer pockets in the tool–billet contact area to hold lubricant. It was reported that as the product size decreased, the pockets that held lubricant in the tool–billet contact area de- creased and the direct contact area increased, resulting in higher friction [2]. We also in- vestigated the processing temperature suitable for microforming, and showed that stable forming is possible when processing at high temperatures where dislocation migration becomes active, and that the variation in product accuracy due to size effects is reduced [3]. In a series of studies on microextrusion [4–7], Cao et al. investigated the effects of microstructure and interface conditions such as grain size, shape, and orientation of the Citation: Funazuka, T.; Dohda, K.; Shiratori, T.; Hiramiya, R.; Watanabe, I. Effect of Punch Surface Grooves on Microformability of AA6063 Backward Microextrusion. Micromachines 2021, 12, 1299. https://doi.org/10.3390/mi12111299 Academic Editor: Josko Valentinčič Received: 14 October 2021 Accepted: 20 October 2021 Published: 22 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and institu- tional affiliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (http://crea- tivecommons.org/licenses/by/4.0/).