# Effect of Surface Micro-Hardness Change in Multistep
###### Abstract  
The machined surface quality, especially the micro-hardness of machined surface layers, is strongly correlated to the friction and wear characteristics of titanium alloy engineering parts. Therefore, to explore relationship of the local surface micro-hardness change in multistep machining and the surface wear resistance of the machined parts is urgently necessary. The machined surfaces were acquired through two-step (roughing and finishing) and three step (roughing, semi-finishing, and finishing) cylindrical turning experiments. The dry friction and wear tests were carried out by UMT-2 friction and wear tester on the multistep final machined surface along the feed direction. The surface wear microtopography and subsurface microstructure were observed and analyzed by scanning electron microscope. The micro-hardness variation in the local area of the finishing surface will cause the extension of unstable friction time stage while withstanding the cyclic and alternating contact stresses, and the soft-hard alternating area should be the sources of friction and wear defects, for instance cracks, peeling pits, fracture striations and even the wear fracture zone to crack propagation and peeling off. This will be of great significance to accurately control the machined surface quality and adaptively improve the surface wear resistance of titanium alloy components. multistep machining; micro-hardness; standard deviation; friction and wear +
Footnote †: journal: Applied  
14 July 2021  
13 August 2021  
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**Copyright:** 2021 by the authors. License MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/40/](https://creativecommons.org/licenses/by/40/)]([https://creativecommons.org/licenses/by/40/](https://creativecommons.org/licenses/by/40/))). **Key Laboratory of High Efficiency and Clean Mechanical Manufacture of MOE, School of Mechanical Engineering, Shandong University, Jinan 250061, China; liahsea@163.  
wear at room temperature, and the main mechanisms of damage at high temperature were oxidative wear and fatigue delamination. [PERSON] et al. [5] found that plating Cu on the surface of titanium alloy could significantly improve the surface wear resistance, and the wear resistance of the Cu coating layer was obviously superior to that of the titanium alloy matrix. The wear scar of the titanium alloy matrix was furrow morphology, and the wear mechanism was stripping wear and adhesive wear; the wear mechanism of the Cu coating layer was stripping wear and fatigue abrasion. By changing the experimental environment temperature, [PERSON] et al. [6] studied the effect of the temperature on the friction and wear of the titanium alloys, and it was found that the surface was not oxidized at a low temperature, but at a higher temperature, there was significant oxidation on the wear scar surface, and oxidative wear occurred on the surface. [PERSON] et al. [7] performed the friction and wear of titanium alloy Ti-6 Al-4V under dry sliding conditions by pin disc wear tester. It was found that the appearance and disappearance of surface friction oxides would change the friction and wear properties and even the wear mechanism of titanium alloys due to the change of sliding speed. Under the experimental conditions, stripping wear and oxidative wear would occur at low speeds, while at high speeds, there was only oxidative wear. In order to improve the wear resistance, not only could the material be pretreated, but the cutting parameters and cutting process could also be optimized. [PERSON] et al. [8] used a ball-end milling cutter to explore the influence of the tool milling path on the friction and wear of the die steel AISI D2. It was found that the ability of the material to resist friction and wear was closely related to the tool milling path.