Research On Surface Modification Of TC4 Titanium Alloy Via Friction Stir Welding Technology

The emerging solid-state friction stir welding technology, and its many derivative new processing methods, can not only realize the bonding of the titanium alloys, but also can be used on the surface modification of titanium alloy. The research on the surface modification mechanisms of TC4 alloy via friction-stir welding technology was performed in the present dissertation. The α+β dual-phase evolution rule in friction-stir zone and its influences were verified. The interaction mechanisms between Ti base microstructures and nitrigon atmosphere, dispersed solid ceramic and metal particles, and dissimilar metals were investigated during friction-stir behaviors. Several modified surface layers with new-typed structures or different chemical compositions were prepared on TC4 alloy, through different process routes, aiming to improve its properties.The microstructure characteristics of friction-stir zone in TC4 alloy was firstly investigated, to study forming mechanisms and driving α+β dual-phase evolutions by thermal-mechanical effects. It was established of the relationship among processing parameters, temperature changes, and structure evolution rules. The α/β evolution modes were proposed. On the basis of analyses on micro hardness features and anti-wear performances, the microstructure hardening mechanisms were discussed. The friction-stir zone experienced the initial plastic deformation, high temperature plastic flow, ‘the local forging’ and cooling. The thermal-mechanical conditions in ‘Stir extruding zone’ determined whether that the α/β phase transition was perfomed. The all-β-phase microstructure evolutions into ‘Forging zone’ determined final morphology. After cooling, a new α+β, or α+α’+β structure was obtained.Friction stir nitriding process was proposed. The surface area by nitriding with stirring behaviors and heat-activated misconstrues by plastic deformation was prepared on TC4 alloy substrate. The solid-gas adsorption, reactions and rapid diffusion mechanism were researched. The nitriding layer, with a large thickness and gradient structure characteristics, produced due to the high-temperature plastic-deformation accelerated Ti/N reaction-diffusion and the solutionprecipitation mechanisms, which benefited to generate nitride phases and nitriding layer. The nitriding layer was presented as gradient structure characteristics, with ‘dense nitride-phase layer region’, ‘half-dense nitride layer region’, ‘nitride particles dispersion layer region’, ‘poor nitriding stir zone’, which were driven by friction-stir mechanical behaviors of the tool.The pre-implantation of Ti C phase particles into TC4 alloy surface layer was conduced before the friction-stir processing, aiming to prepare a surface layer ceramic particle reinforced Ti alloy matrix composite structure. It was discussed of the Ti C particle actions of ‘refinement’, ‘stir and dispersed’, ‘dissolution and precipitation’, during the processing. It was found that the structure characteristic and particle actions could produce a hardening effect on the surface microstructure of TC4.According to the powder implantation friction stir processing and dissimilar material friction stir welding process methods, the plastic metals could be introduce to the friction-stir zone of TC4 alloy, to prepare the modificated surface layers. The research was give as follows:① Flame-resistant surface layer was prepared on TC4 alloy by pre-implanting Cu powders in friction stir surface modification process. The β-Ti phase stability of Cu were dispersed, inter diffused, and reacted with Ti in the friction-stir zone. Thus, the Ti-Cu alloy surface alloying layer with β-Ti phase rich region was obtainable. Accorting to the flame retardancy evaluation, the analyses of its flame retardant mechanisms were completed, expecting to suppress the spread of ‘titanium fire’.② The α-Ti phase stability of Al powders were introduced in to the friction-stir zone of TC4 alloy, aiming to prepare the Ti-Al structure layer on the TC4 substrate. The formed Ti-Al intermetellic phase particles could refine the Ti-based microstructure and promoted the micro-hardness.③ The pure Al bulky metal was friction stir welded with TC4, to be introduced into the surface layer of TC4 alloy, contributed by the friction-stir thermal-mechanical effect, aiming to realize the surface alloying on TC4 substrate. Basing on the Ti-Al metallurgical bonding, the dissimilar metal continuity and compactness of the Ti-Al alloying layer were further improved. The prepared alloying Ti-Al inter-layer on the surface of TC4 substrate played an important role to protect substrate against high-temperature oxidation.