Effect of Plasma Arc Remelting on Microstructure and Tribological Properties of TiZr-based Alloy br

Owing to their outstanding resistance to corrosion in oil and gas environments, titanium alloys have been used to produce high-pressure gas wellhead sealing parts, including large-area valve plates and seats, which are always washed by high-speed airflow containing acidic gases. It is well known that the valves must be opened and closed frequently, and severe wear phenomena may occur on the contact surfaces. As vital structural components, the sealing parts must possess high hardness on the surface to enhance their anti-wear property. However, severe adhesive wear can always be observed when a titanium surface slides against any other surface, and the poor tribology behavior has restricted its large-scale applications. At the same time, the application of titanium alloys in a corrosive environment can be extended by improving their surface properties. Given the disadvantages of high cost, long period, and a thin modified layer for the common surface modification techniques, systematic research by plasma transferred arc remelting with a remelting current ranging from 80 A to 140 A has been conducted to improve the wear resistance of Ti-20Zr-6.5Al-4V alloy (hereinafter referred to as T20Z alloy). The cross-section microstructure, phase composition, and microhardness were analyzed using a scanning electron microscope(SEM), X-ray diffractometer(XRD), and Vickers hardness tester, respectively. The UMT-2 multifunctional tribometer and three-dimension profilometer were used to evaluate the tribological properties of T20Z alloys, and the wear mechanism was also analyzed comprehensively. The results showed that the remelted samples consisted of a surface remelted zone, heat-affected zone, and interior substrate on the cross-section, and the corresponding microstructures were fine lamellar, martensitic and coarse lamellar structures, respectively. It is important to note that no elements such as N and O were introduced into the remelted zone during the remelting process in the argon atmosphere. For a remelting current of 80 A, the thickness of the remelted zone was approximately 1.2 mm, and its microstructure had been refined significantly. The average & alpha; lamellae thickness in the remelted zone was only 0.31 & mu;m, which was not more than one-tenth of that in the substrate. Considering the well-known Hall-Petch relationship, the hardness of the remelted zone, which could be as high as 600 HV0.2, was increased by 225 HV0.2 compared to the hardness of the substrate owing to the grain refinement. The heat-affected zone with a hardness of 450 HV0.2 was located between the surface remelted layer and the substrate. By increasing the remelting current to 140 A, the thickness of the remelted zone increased gradually from 1.2 mm to 1.98 mm. Due to the large heat input under the condition of a high remelting current, the decreased supercooling degree and the driving force for the grain nucleation occurred, resulting in a large lamellae thickness of 0.56 & mu;m. Although the remelting current had some influence on the lamellae thickness and the lamellae thickness increased slightly, the hardness was improved to a varying extent compared to that of the substrate. The friction-wear tests indicated that the most apparent wear resistance enhancement was obtained when the remelting current was 80 A, and the wear rate of the remelted specimen decreased by 25.89% compared to that of the untreated specimen. Meanwhile, the dry sliding friction coefficient of the remelted specimen was similar to that of the untreated specimen and exhibited good stability. Based on the analysis of the wear trace morphology of the surface remelted specimen, the underlying mechanism of dry sliding friction at room temperature was abrasive wear. Thus, the plasma arc remelting method is beneficial in obtaining a thick and hard remelted layer with a significantly refined microstructure and improves the wear resistance of the surface remelted T20Z alloy, especially when the remelting current is low.