| AISI H13steel is widely used in hot work die steel, but after quenching and tempering, its hardness is lack. Additionally, with poor working conditions, its surface often fails due to thermal wear and thermal fatigue that make the mould scrap, and seriously affect production efficiency. Laser cladding (LC) is a new kind of surface modification technologies, compared with conventional surface technology, it has many outstanding characteristics, such as fast cooling speed, low dilution rate, optional area cladding as well as easy automation, so LC has good prospects for development and potential advantage of surface repair, especially in parts of large and expensive equipment partial failures. Co-based self-fluxing alloy and TiC has good high temperature strength and wear resistance, but laser cladding pure TiC powder has the disadvantages of easy cracking, poor formability and so on, while Co-based alloy has good wettability on melting, this is helpful to obtain good dense and smooth cladding layer. Therefore, using TiC/Co-based mixed powder is available to obtained not only good quality, but also good high temperature oxidation resistance and wear resistance of cladding layer.In order to improve the wear resistance of AISI H13hot work tool steel, Co50alloy coating and TiC/Co-based composite coatings with different compositions of Co50-TiC precursor powders (10%TiC+Co50,20%TiC+Co50and30%TiC+Co50), were prepared on the H13steel surface by6kW transverse-flow CO2laser. The bonding characteristics, phase composition, microstructure morphology of the coatings were investigated by XRD, SEM and OM; using micro-hardness tester and high-temperature wear tester studied the micro-hardness distribution and the wear behaviors at different temperatures of the coatings. The research was focused on the morphological characteristics and growth mechanism of TiC whose influence on the microhardness and high-temperature wear properties of TiC composite coatings were also investigated. The results indicated that:(1) Co50alloy coating as well as TiC/Co composite coatings with the contents of TiC (wt.%) less than20%showed good metallurgical bonding characteristics with the H13steel substrate surface, but when the TiC content was more than20%, the composite coating failed either at the coating/substrate interface. Co50alloy coating was mainly composed of y-Co dendrite and eutectic between y-Co dendrites, while TiC/Co composite coating which contained of TiC particles, dendrite and fine eutectic.(2) Co50alloy coating mainly contained of y-Co solid solution and Cr1.12Ni2.88phase, but with the increasing of TiC content, the substrate phase composition of TiC/Co based coatings tended to be simple. Phase composition of Co+10%TiC coating was consisted of TiCo3, Cr2Ni3as well as Cr-Ni-Fe-C phases, and so on, while phase composition of Co+20%TiC coating was Cr2Ni3and y-Co, and phase composition of Co+30%TiC coating mainly composed of y-Co solid solution.(3) TiC/Co based coatings were presented in terms of TiC origins which can be divided into two kinds, i.e. undissolved TiC and in-situ TiC particles. In-situ TiC particles is highly dependent on the TiC content. When TiC contents were low, TiC particles in preset powder layers decomposed Ti and C, caused re-precipitation in-situ TiC at the same time. In addition, the contents of TiC in preset powder layers were found to have a significant effect on the morphology characteristics of TiC particles, as well as TiC morphologies in various regions of TiC/Co-based composite coatings were different. In10%TiC+Co50composite coating, in-situ TiC contained of fine TiC, eutectic mixture TiC, polygon TiC and flower-buster; from the coating bottom upwards, the number of flower-buster TiC increased, the diameter of TiC particles were growing. In-situ TiC of20%TiC+Co50coating mainly consisted of fine TiC and developed dendrite TiC; from the bottom to the coating surface, the diameter of undissolved TiC showed obvious gradient.30%TiC+Co50coating had no in-situ TiC and undissolved TiC did not appear significant gradient.(4) After laser cladding processing, cross-section microhardness of H13steel surfaces have been substantially improved, the average cross-section microhardness of Co-based coating and TiC/Co based coatings were significantly higher than that of H13steel substrate (208HV0.2), are499HV0.2,552HV0.2,590HV0.2and824HV0.2, respectively, about2.4-4.0times higher than the H13steel. In comparision, microhardness of TiC/Co based coatings were higher than that of Co-based coating, with the increasing of TiC content, microhardness of TiC/Co based coatings was increased, the highest hardness could be increased up to824HVo.2, about4times greater than the substrate. This result plays a beneficial role to improve the wear resistance of H13steel substrate surface. (5) With the increasing of the wear load, in all cases, friction coefficient of Co50coating and H13steel showed a decreasing tendency. As the sliding velocity increased, friction coefficient of Co50coating and H13steel showed a trend of first decreasing then increasing. With the increasing of the sliding velocity, the wear rate of H13steel was first increased and then decreased, when the sliding speed of200r·min-1, the wear rate reached the maximum value; while Co50coating was first decreased and then increased to the contrary. Friction coefficient of the Co50coating was found to be strongly influenced by temperatures, at room temperature,200℃and700℃the coating has a large friction coefficient. In contrast, not much difference in the friction coefficient of20%TiC+Co50coating was as observed as various temperature, further, the average friction coefficient was relatively stable.(6) Co+20%TiC coating showed better wear behavior than H13steel, Co50coating and10%TiC+Co50coating, as well as had a more stable fiction coefficient; The wear mechanism at room temperature was caused by brittle spalling wear, adhesive wear and plough wear; while high-temperature wear mechanism was mainly caused by oxidation wear and fatigue wear.Overall, comparing with the H13steel substrate, Co+20%TiC composite coating has good comprehensive properties, its hardness and high-temperature wear resistance can meet the use requirements. |
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