Microstructural Characteristics of Surface Composite on Titanium Alloy Fabricated via a Novel Method of Friction-Stir Nitriding

Abstract

Abstract. The microstructural evolutions and properties of the surface composite produced via the friction stir nitriding (FSN) method on Ti-6Al-4V alloy were investigated. Due to the optimized process parameters and suitable mixed gas proportion for a limited combustion reaction between Ti and N 2 , a composite layer with the thickness of ~200 m beneath an outermost thin TiN layer was fabricated. The composite layer was composed of three microstructure types. Formation mechanisms for microstructure evolutions during FSN on titanium alloy were detailed in terms of three steps. IntroductionTitanium alloys would have much wider usage in various industrial sectors if their wear-resistance could be significantly improved [1]. To solve this problem, the surface micro-structure of titanium can be modified via nitriding techniques, such as plasma, laser, and gas nitriding [2]. The nitrogen has a high solubility in -Ti to significantly strengthen the surface layer due to the formation of the compound layer containing of TiN and/or Ti 2 N [2]. In the present paper, a novel nitriding method called friction-stir nitriding (FSN) is proposed. Multiple research results announced that friction stir processing (FSP) is emerging as a very attractive method used primarily to provide the localized modification of microstructures in the surface layers of metal components or to in-situ form a wear-resistant composite surface [3][4][5]. Actually, FSN is employed for titanium and its alloys depending on the basic principles of FSP technique and a limited combustion reaction of Ti in a mixed atmosphere of Ar 2 and N 2 gases during FSP procedure. It is well-known that the wearable TiN x compounds can be synthesized via a rapid combustion reaction between Ti and N 2 gas [6]. Interests in this alternative nitriding method of FSN for preparing composite surface layer embedded in the Ti matrix are motivated by low cost production, the considerable surface-layer thickness and no debonding issues due to the inherent material continuity between the treated layer and substrate.