Effects of Heat Input on Characteristics of Micro-arc Oxidation Coatings on TC4 Titanium Alloy

Micro-arc oxidation technology is an environmentally friendly surface modification technology, which improves the surface properties such as wear resistance, corrosion resistance, thermal shock resistance, hardness and other properties by in-situ synthesis of functional ceramic coatings on the valve metal surface. Some scholars have found that during the micro-arc oxidation process, a large amount of heat is released, the temperature of the electrolyte increases rapidly, which seriously affects the corrosion resistance and wear resistance of the coating. The work aims to study the effects of heat input on the characteristics of the micro-arc oxidation coating of TC4 titanium alloy by adjusting the heater power so as to provide a theoretical basis for improving the refrigeration system in micro-arc oxidation. The TC4 titanium alloy was cut into a rectangle of 15 mm×15 mm×2 mm as the base material and polished and with 400#, 800#, 1200#, 2000# sandpaper in turn. The test adopted pulse power supplies (DWL20-6), the output mode adopted constant current, the current density was 5.2 A/dm2, the frequency was 100 Hz, the duty cycle was 40%, and the time was 15 min. The electrolyte composition was Na2SiO3 (12 g/L), (NaPO3)6 6 g/L, Na2WO4 1.5 g/L, NaOH 0.5 g/L, C3H8O3 3 mL/L, EDTA 0.3 g/L, C6H15NO3 1 mL/L. Before the test, the initial temperature was controlled to 20 ℃ by the cool system, the constant heat input during the micro-arc oxidation process was controlled to 0, 70, 140, 210, 280 W respectively. The surface morphology, cross-sectional morphology and elements of the coatings were analyzed by scanning electron microscopy (ZEISS EVO MA15) and X-ray energy dispersive spectrometer (OXFORD X-max). The phase composition of the coatings was analyzed by X-ray diffractometer (DX-2700B). The microhardness of the coating was measured by a digital microhardness tester (HXD-2000TM/LCD, China). The polarization curves of the solution samples against 3.5wt.% NaCl solution were measured using an electrochemical workstation (Gamry Reference 3000, USA). Reciprocating friction using a multifunctional material surface property testing machine (MFT-4000). With the increase of heat input, the coating thickness increased first and then decreased. The main phases of the coating were Rutile TiO2, Anatase TiO2 and a small amount of SiO2. The heat input improved the corrosion resistance of the coating, and the corrosion rate gradually decreased from 4.516×10–3 mm/a to 3.109×10–4 mm/a. The wear resistance of the coating surface first increased and then decreased with the increase of heat input. When the constant heat input was 140 W, the TC4 titaniumalloy micro-arc oxidation coating had the best comprehensive performance, its thickness was 21.56 μm, the surface roughness RA was 1.47 μm, and it had good corrosion resistance and wear resistance. The heat input affected the ion transport during the formation of the coating, and increased the ratio of Ti atoms to O atoms in the coating, which was beneficial to the formation of the TiO2 coating. Get rid of the bondage, the Ti and O atoms participating in the reaction were reduced, and the growth rate of the coating was reduced. The coating thickness increased from 18.53 μm to 21.56 μm and then decreased to 17.67 μm with the heat input. The main phases of the coating were Rutile, Anatase and a small amount of SiO2. By controlling the external heat input, the structure of the TC4 titanium alloy micro-arc oxidation coating can be improved, and the performance of the coating can be improved. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.