Preparation And Oxidation Behavior Of Aluminized Coating On TC4Titanium Alloy Via Friction Stir Processing

Although titanium and its alloys possess an excellent comprehensive performance, the poor oxida-tion resistance, especially for the most widely used of TC4titanium alloy, limits their use at elevatedtemperature. To solve this problem, the solid-state processing method of friction stir lap welding（FSLW）, which is different from the traditional coating interface structure, was used to fabricatealuminized cladding or coating on the surface of TC4alloy. By the annealing treatment after welding,the metallurgical bonding performance was improved. In addition, the ball milling process was alsoused to control the coating surface composition. The composite structure of “powder coating+alumi-num cladding+substrate” was fabricated to improve the oxidation resistance of coating.The results showed that by changing the geometry of the stir tool, process parameters, dissimilarmetal of Titanium and aluminum was welded. The mechanical milling was used as a post-treatment fora suitable coating thickness. The as-processed Ti/Al interlayer was Ti-rich in chemical composition andhad an interval banding structure, due to the FSLW thermal-mechanical effects. On the one hand, theprocess of friction stir welding heat input reduces the plastic flow occurs at the interface resistance oftitanium alloy, and promote the diffusion of titanium and aluminum elements; On the other hand, the“milling” behavior of pin leaded to the mechanical mixing and mutual migration of weld material at theinterface, and reduced the atomic diffusion distance of inner Ti/Al, increased the number of diffusionpath of atoms.After annealed at500℃, the Ti/Al interlayer was replaced by diffusion reaction. The diffusionlayer exhibited a typical island structure with Al and Al3Ti biphasic coexistence, which has a densestructure with thickness of60m. The formation reason of diffusion layer was before the annealingprocess, the mechanical miscible and ingredient miscible has been formed, which increased the contactarea of elements, and reduced the diffusion distance. Since the effect of friction stir lap of pre-mixed,the diffusion reaction during the heat treatment eventually led to the formation of considerable thick-ness of diffusion layer with an island Al-Al3Ti binary phase structure.Cyclic oxidation weight gain experiments showed that the coating played a good antioxidant effect.After the oxidation, the sample surface formed a dense structure of α-Al2O3and θ-Al2O3mixed mor-phology, and was accompanied by a small amount of Al2Ti and Al3Ti. In the oxidation process, the suf-ficient amount of Al in the surface hinder the diffusion of O, meanwhile, ensured the diffusion of Ti andAl at the interface occurs in a dilute O environment. The failure mechanism of coating was divided intothree stages: the first stage was the surface oxide layer generated and flaking stage; the second stage was the aluminum compound formation and oxidation stage; the third stage is the oxidation and failureof the intermediate layer. These three stages were not independence, but along with the oxidation of thewhole story.To further improve the coating properties, the milling process was used to prepare Cr and Ni-CrAlCoY composite coatings on aluminum cladding surfaces. The results show that the preparation ofthe coating process mainly included particles attached, particles implantation, particles cold weldingand particles flaking. Which NiCrAlCoY coating exhibited a better oxidation resistance.