The Effects of Geometry Size and Initial Microstructure on Deformation Behavior of Electrically-Assisted Micro-Compression in Ti-6Al-4V Alloy

In this study, electrically-assisted micro-compression (EAMC) tests were conducted for cylindrical specimens of Ti-6Al-4V alloy with four geometric sizes and three initial microstructures. The result showed that the specimen temperature nonlinearly increased with the square of current density. The quasi-static heat equilibrium equation was established to quantify the effects of the scale factor on the Joule heat temperature. Moreover, it was demonstrated that the Joule temperature scale effect had a greater effect on the flow stress than the sample size effect for specimens of different dimensions. It was noted that the 0.5 mm diameter sample displayed abnormal deformation behavior, which was related to surface oxidation leading a brittle surface layer. By comparison of the microstructures, it was found that the α→β phase transformation occured below the β transus temperature, which was attributed to the local Joule heat effect and the scattering of drift electrons during EAMC. Furthermore, the flow curves showed a strong dependence of the strength and ductility on the initial microstructure. The widmannstatten microstructure exhibited higher strength, smaller hardening rate and more easy flow localization compared with basket-weave microstructures, which was attributed to the low β phase content and narrow interlamellar spacing of α lamellae grains in the widmannstatten microstructure.

The Effect of Direct Electric Current on the Plastic Behavior of AA7075 Aluminum Alloy in Different States of Hardening

Electrically-Assisted Forming (EAF) techniques are interesting and promising for the automotive industry. Electrically-assisted tensile tests were carried out on specimens of AA7075 aluminum alloy in different states of hardening, namely T6 (the as-received state) and the supersaturated solid solution state. All the tests were carried out in quasi-static conditions under the application of direct electric current (DC) in the range of 90 to 540 A. The experimental results showed that with a DC density of 10 A/mm² the uniform strain and strain at fracture increased when the AA7075 was in the supersaturated solid solution state. A correlation between the mechanical results and microstructural features analysed through transmission electron microscopy was assessed. An explanation of the investigated phenomena based on the electron wind theory, heterogeneous microscale Joule heating and the Portevin–Le Chatelier (PLC) effect was finally proposed.