Surface Hardening of Commercially Pure Titanium by Non-vacuum Electron Beam Cladding of Powder Mixtures

Abstract

Introduction. Modern engineering copes with different tasks associated with the modification of the structure of the surface layers of metallic materials using high-temperature heating sources. Structural transformations that occur during this treatment make it possible to increase the strength, corrosion and tribological behavior of metals. Titanium and its alloys are widely used in modern industry, but its distribution is limited by a high coefficient of friction and low resistance to wear. An insufficient attention is payed to the problem of titanium and its alloys hardening with the use of high-temperature sources of heating. Analysis of the works related to high-speed heating of titanium-base alloys showed that the laser beam is most often used as a source of surface heating. The Ti-6Al-4V titanium alloy predominantly performs the function of the base material. The samples obtained by surfacing powder mixtures containing titanium diboride (TiB2) and boron carbide (B4C) possess high hardness and wear resistance. However, the thickness of the coatings formed this way does not exceed 1 mm. To produce modified layers of increased thickness it is rational to use the method of electron beam treatment of materials in air. The aim of the work is to study the possibility of cladding of a powder mixture containing boron carbide to modify surface layers of cp-titanium by the method of non-vacuum electron beam treatment. Materials and Methods. Cp-titanium is used as the base material. Plates of base material were treated with a highly concentrated electron beam discharged into air. Powder mixtures with different content of boron carbide powder (10, 20 and 30 wt. %) were used to form particles of the high-strength phase in the surface layers. Modified materials were analyzed by optical and scanning electron microscopy. Studies of abrasion resistance were carried out under friction conditions on fixed and loosely fixed abrasive particles. Results and discussion. The mechanical and tribotechnical characteristics of modified titanium layers are largely determined by structural transformations occurring in the surface layers of the material. The treatment of a titanium alloy with a high-concentration electron beam in air allows obtaining modified layers with a thickness of more than 1 mm. Cladding of a powder mixture containing boron carbide leads to the formation of high-strength particles in the surface-alloyed layers, which have a significant effect on the properties of the base material. Addition to the cladding mixture 10 wt. % of a boron carbide powder allows obtaining qualitative layers containing finely dispersed particles of titanium monoboride and titanium carbide. The volume fraction of the high-strength phase in these layers is similar to 20%. Increasing the concentration of boron carbide in the original powder mixture to 30 wt. % leads to the formation in the structure of modified layers of large primary crystals of titanium boride and titanium carbide of dendritic morphology. An increase in B4C concentration also leads to an increase in the volume fraction of the strengthening phase to 40...44 %. A characteristic feature of these samples is the presence of conglomerates of fine particles in the lower coverage zone. The average microhardness of the hardened layers reaches 4 250-6400 MPa. In the conditions of friction on fixed of abrasive particles, the maximum wear resistance exceeds 2. 4 times the same index of the reference sample was recorded during the testing of the alloy obtained by cladding the mixture with 30 wt. % B4C. The same samples showed an eightfold increase in the wear resistance values when the abrasive particles were loosely attached to the material.