Effect of Die Surface Conditions on Microstructural Changes and Force Behavior in Micro-Extrusion of Pure Copper

8 pages•Published: August 6, 2026

Abstract

In recent years, copper-based conductive and heat dissipation components have required fine microstructures for miniaturization and enhanced functionality. Micro-forming is an excellent processing method characterized by high productivity and suitability for mass production. Due to size effects, microforming is strongly influenced by tribological conditions, making the proper setting of die surface conditions crucial. In recent years, extensive research has been conducted on surface treatments for dies, including diamond-like carbon (DLC) coatings. One of the objectives of these studies is to reduce forming force and thereby extend die life. Forming force is determined by the sum of the frictional force between the die and material surfaces and the deformation resistance of the material. Since the deformation resistance of the material is closely related to plastic flow and the formation of deformation zones within the material, it is important to understand how tribological conditions, such as die surface properties, influence these factors. However, in microforming, the effects of die surface conditions on internal deformation mechanisms and, consequently, on force behavior remain unclear.
In this study, we investigated the effect of die surface conditions on internal deformation and force behavior during micro-extrusion of pure copper. The tests were conducted using a mirror surface die with a ground finish and a DLC-coated die. The maximum force was lower for the DLC die than for the mirror surface die, demonstrating that the DLC coating provides a lubricating effect even in micro-extrusion of pure copper. Focusing on the force behavior, a convex peak behavior, considered to be influenced by changes in the microstructure during processing, was observed for both dies, and the maximum force was recorded during this behavior.
To evaluate the relationship between microstructural changes during deformation and force behavior, EBSD observations were performed on samples obtained from interrupted tests. Regardless of the die used, heterogeneous deformation occurred near the sample tip immediately after the start of the test, with deformation concentrated near the die contact surface. Nanoindentation tests revealed significant work hardening in regions where deformation was concentrated. The area exhibiting significant work hardening expanded radially inward within the billet until the maximum force was reached and then contracted thereafter. These series of phenomena were common to both dies.
To further assess the effect of die surface conditions on internal deformation, observations focusing on dislocation density were conducted. There are two types of dislocations: geometrically necessary (GN) dislocations and statistically stored (SS) dislocations. GN dislocations are required for material shape changes, and their density is known to correspond one-to-one with strain gradients. Under appropriate observation conditions, GN dislocation density near the maximum force was measured for each die to evaluate the influence of die surface conditions on internal deformation. The results showed a tendency for lower GN dislocation density in the DLC die compared to the mirror surface die. This suggesting that die surface conditions affect not only friction between the die and material surfaces but also deformation within the material.

Keyphrases: deformation behavior, dlc, micro forming, plastic deformation, pure copper, size effect

In: Numpon Mahayotsanun (editor). Proceedings of The 11th International Conference on Tribology in Manufacturing Processes & Advanced Surface Engineering, vol 4, pages 22-29.

Links:	https://easychair.org/publications/paper/1tjZ
	https://doi.org/10.29007/vhh6