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Study On In Situ Synthesized Ceramic Phase Reinforced Fe-based Coating By Laser Cladding

Posted on:2010-08-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:B S DuFull Text:PDF
GTID:1101360278974487Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
As one of the three most commonly encountered failure modes (fatigue, wear, corrosion) of engineering components, wear causes great economic loss in commercial application. The fabrication of wear resistant coatings on metallic materials through modifying the physical and chemical properties of surface becomes an important way to improve the quality of products, develop the technology of maintenance and reproduction, and extend the service time of mechanical products. In the present study, laser (CO2, Nd: YAG) was employed with combination of in situ technology to fabricate TiB2, TiC, TiB2+TiC reinforced Fe-based wear resistant coatings on low carbon steel. Systematic analysis was carried out to study the microstructure and phase constituent of coating, formation mechanism of reinforcement and wear properties of coating. Besides, factors that have influence on microstructure and properties of coating were also investigated.It is found that constituent of preplaced powder and the related effect on processing are key factors of synthesizing in situ ceramic phase reinforced Fe-based coating by laser cladding. Using FeTi30+FeB16 as precursor, TiB2 reinforced Fe-based coatings were produced by CO2 laser cladding. TiB2 particles with blocky and strip shape are distributed uniformly in the clad layer. Phase constituent of TiB2+α-Fe is obtained when the atomic ratio of Ti to B in the preplaced powder is between 1:1.8 to 1:2. These coatings also show better cracking resistance. In situ TiC reinforced Fe-based coatings were fabricated by CO2 laser cladding using FeTi30+graphite as precursor. By considering the burning amount of graphite and Ti during laser cladding, the selection of atomic ratio of Ti:C=1:1.3 can facilitate the formation of TiC, improve its content and avoid the presence of brittle phases such as Fe2Ti and high-carbon martensite. TiC with flower-like and dendritic morphology is dispersed in the Fe-based matrix.In situ synthesized TiB2+TiC reinforced Fe-based coating was fabricated by Nd:YAG laser cladding using preplaced powder of Fe+Ti+B4C Brittle Fe3(B,C) phase presented in the coating and the content of reinforcements was relatively low when the amount of Ti and B4C in the preplaced powder was selected based on the reaction of 3Ti+B4C=2TiB2+TiC and relatively higher power density was used. By using Fe45-Ti41.12-B4C13.88(wt.%) as precursor and smaller power density, phases presented in the coating evolved into TiB2, TiC andα-Fe, and the content of reinforcements also increased. Dese and defect-free coating with metallurgical joint to the substrate was obtained. Reinforcements dispersed uniformly in the matrix, and TiB2 shows the morphology of block and strip while TiC takes on the shape of equiaxed and flower. This is because of the competing growth and deviation of concentration environment for TiB2 and TiC. Moreover, due to the influence of dilution ratio, the content and size of reinforcements increase with the increasing of scan speed while the increasing of laser power results in the decreasing of content and size of reinforcements.It is shown by thermodynamic analysis that in Fe-Ti-B, Fe-Ti-C and Fe-Ti-B-C system TiB2 and TiC possess lower Gibbs free energy from 300K to 2000K, and thus they are stable phases. The atomic ratio of elements for forming reinforcements is of importance to the phase constituent of coatings. During laser cladding, the laser-materials interaction induces the heating effect and formation of low-melting point eutectic. Fine reinforcements (TiB2 and TiC) can be formed by diffusion and reaction. Furthermore, the coupling effect of laser heating and exothermic reaction make the reinforcement remelt into the liquid alloy. Reinforcements are formed by nucleation and growth. In situ synthesized reinforcements of TiB2 and TiC exhibit faceted nature, which shows that the rapid solidification process during laser cladding does not transform the solid-liquid interface from smooth to rough.The (0001) and {10(1|-)0} planes of TiB2 have lower interface energy and thus tend to grow more slowly, leading to the final morphology of TiB2 with (0001) asbasal plane and {10(1|-)0} planes as prismatic faces. Due to the driving force induced bydiffusion and constitutional supercooling in front of the interface of nucleus, TiC tends to lose its interface stability. The dendrite arm of TiC grows along the [001] direction, which leads to the flower-like and/or radial dendrite shape of TiC. The faces shown at the tip of TiC dendrite are close-packed {111} planes. In addition, club-shaped and branch-like TiC was found owing to the eutectic reaction during solidification. In the (TiB2+TiC)/Fe coating, TiB2 and TiC nucleate separately, but their growing habitus is not changed. Phenomenon of TiB2 growing on TiC particles was observed. The interface between in situ synthesized reinforcement and matrix remains strong, clean and free from deleterious and amorphous phase.It was shown by dry sliding wear test at room temperature that in situ synthesized TiB2/Fe, TiC/Fe and (TiB2+TiC)/Fe possess superior wear resistance. Compared with the substrate, wear coefficient of the coating decreased by 0.1-0.15 under the same wear environment. Uniformly distributed reinforcements with high amount can effectively decrease the adhesion and abrasion during sliding with the counter-wheel, resulting in the substantial increase in wear resistance. Wear mechanism of in situ TiB2/Fe coating is micro-ploughing and peeling of reinforcement, and its wear volume is 1/17-1/19 of that of Q235 substrate; Wear mechanism of in situ TiC/Fe coating is micro-ploughing and selective adhesion, and its wear volume is 1/15-1/16 of that of Q235 substrate; Wear mechanism of in situ (TiB2+TiC)/Fe coating is micro-ploughing and scratching, and due to the simultaneous presence of TiB2 and TiC, reinforcements are distributed more uniformly with smaller free distance, leading to the best wear resistance (1/21 of Q235 metal).
Keywords/Search Tags:Laser cladding, Coating, In situ synthesis, Metal matrix composites, Wear properties
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