| Titanium alloys have been used in aerospace, marine, petro-chemical, etc. mainly because of their light weight, high strength and excellent corrosion resistance. However, the low hardness and poor wear resistance limit their wide application. The composite coating fabricated by laser cladding with high efficiency has many advanced features, for example, excellent bonding strength between coating and substrate, highly refined structure, the low dilution rate and limited heat affected zone.In this paper, laser cladding was used for surface strengthening of Ti-6A1-4V titanium alloy, a widely used alloy in industry. Ti, Al and Si powders were clad on the surface of the substrate. Argon gas was used as the shielding gas during the cladding process. As a result, ceramic hard phases Ti5Si3, Ti7Al5Si12, Ti3AlC2, TiAl, Ti3Al and Al3Ti were in situ synthesized, which are beneficial to increase the hardness and wear resistance of the laser cladding coatings.The type, content and distribution characteristics of the in situ formed ceramic phases in the coating were controlled by designing the pre-placed materials and adjusting the process parameters. By means of the investigations on the chemical composition, phase constitution, microstructure, hardness and wear resistance of the coatings, the in situ formation mechanism of ceramic phases was analyzed. Besides, the interface structure and the strengthening mechanism of the coatings were revealed. What’s more, the action mechanisms of TiC (or B4C) and Y2O3 on the solidification process of the molten pool were studied.Results indicated that composite coatings showing metallurgical bonding with the substrate were fabricated on Ti-6A1-4V titanium alloy by laser cladding with Ti, Al and Si mixed powders. During the solidification process of the molten pool, the multiple in situ formed phases nucleated and grew. Owing to the reciprocal inhibition of the diverse phases, the coatings showed fine and uniform microstructures. The surface quality, microstructure, and properties of the laser cladding coatings could be affected by both the cladding materials composition and process parameters. Under the experimental condition of 0.2-0.3 MPa (argon atmosphere),950-1100 W (laser power), and 5 mm·s-1 (scanning rate), composite coating with high hardness and excellent wear resistance were obtained by laser cladding with Ti-35Al-15Si (wt.%) materials system.Subsequently, TiC (or B4C) was added in the pre-placed materials, in order to further improve the properties of laser cladding coatings. With the addition of B4C, TiC, TiB and TiB2 ceramic phases were in situ formed in the molten pool, beneficial to further improvement of the microhardness and wear resistance. Nevertheless, the added content should be controlled to a proper scope. In this paper, it was found that 20wt.% TiC or 1 Owt.% B4C was the best choice to fabricate coatings with better properties.Rare earth oxide Y2O3 could refine the microstructure of the cladding coating significantly, and the action mechanism was summarized as follows. Firstly, partial undissolved Y2O3 acted as the heterogeneous nucleation cores in the molten pool. Secondly, partial Y2O3 decomposed into Y and O2, and Y tended to distributed on the grain or phase boundaries, restricting the movement of the boundaries. However, proper amount of Y2O3 should be added, avoiding the increasing brittleness of the composite coating. In this paper,2 wt.% Y2O3 was found appropriate. By adding appropriate amount of TiC (or B4C) and Y2O3 simultaneously, laser cladding coatings showing better properties were obtained. The laser cladding coatings fabricated with 20wt.% TiC and 2 wt.% Y2O3 exhibited a high hardness five times that of the substrate. High temperature wear resistance test was carried out at 800℃. Results showed that the surface of the substrate and the cladding coatings were covered by a layer of oxide film, which mainly consisted of Al2O3, TiO2 and SiO2. The wear volumes of the cladding layer with the different components were 50πmm3 (Substrate),485μmm3 (Ti-45A1-15Si),4507πmm3 (Ti-35Al-15Si),432πmm3 (Ti-25Al-15Si),42πmm3 ((Ti-35Al-15Si)-1Y2O3),408πmm3 ((Ti-35Al-15Si)-20TiC),396πmm3 ((Ti-35Al-15Si)-10B4C),385πmm3 ((Ti-35Al-15Si)-20TiC-lY2O3) and 381πmm3 ((Ti-35Al-15Si)-20B4C-1Y2O3) respectively. As the temperature increases, the friction coefficient and the wear volume increased. With the increase of load, the friction coefficient decreased and the wear rate increased. The wear mechanism of the cladding coatings was composed of oxidation wear, spalling wear and adhesive wear. While the wear mechanism of the substrate was mainly a combination of oxidation wear and adhesive wear.In this paper, multi-component ceramics reinforced coatings were fabricated by laser cladding on Ti-6A1-4V titanium alloy. The in situ formation mechanism of ceramic phases was analyzed. The interface structure and the strengthening mechanism of the coatings were revealed. The action mechanisms of TiC (or B4C) and Y2O3 on the solidification process of the molten pool were studied. This paper is expected to provide experimental and theoretical basis for the application of laser cladding in industry. |
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