| Modern machining industry requires cemented carbide inserts to perform stronger in areas of processing efficiency, service life and maintenance cost, it shows the significant value to research and develop the effective surface modification techniques for cemented carbide. As a new developed technique, high current pulsed electron beam (HCPEB) shows substantial potentials to improve the surface performance of materials. Within a short pulse of several μs, the electron beam energy deposits into the surface layer of material where the superfast melting and solidification processes occur along with a high amplitude dynamic stress impact, resulting in a modified surface layer with dense micostructure, homogeneous composition distribution, deep thickness～μms. In the present research, the surface modification of cemented carbides by HCPEB was conducted with two types of widely used cemented carbides known as YG8, YG6 with the aims to reveal the evolution regularity of surface micro structure and service properties and further explore an new effective surface modification technique for cemented carbide materials. The main research results are listed as following.For the HCPEB surface modification of YG6 cemented carbide, the treating parameters were chosen including accelerating voltage 27 kV, pulse numbers 1,5,10,20 and 35. A remelted layer formed on the top surface with a thickness over 0.8um. With the increasing number of pulses, the WC grains were refined drastically of grain size～0.1 μm after 20 pulses of HCPEB irradiation. Craters and micro-cracks were noticed on all specimens. When the pulse number reached 20, micro-cracks began to heal and only a few craters remained with samll size ～1.1 μm.HCPEB surface modification experiment was carried out on YG8 cemented carbide with parameters as follows:number of pulses 1,3,5,10,15,25 and 50, accelerating voltage 23.4 kV,27 kV. Under the same accelerating voltage, the surface remelting became more severe with the increasing number of pulses where WC particles showed a rounder shape and a remelted layer of thickness～1 μm. Meanwhile, the average size of crater decreased from 3.16 μm to 0.63 μm. Also, the micro-cracks emerged on the modified surface, and developed into a micro-crack network after 50 pulses of irradiation. The planar density of craters formed under high accelerating voltage is lower than that of low accelerating voltage. The best surface modification effects were obtained when using 25 pulses and 27 kV.According to the micostructur characterization, the fcc phase WCi-x, Co3W9C4 and graphite were from on the modified surface of cemented carbide. In further analysis by TEM and EBSD, the grain size of WC1-x phase on the surface layer of YG6 was with 20-100 nm, and the average grain size of Co3W9C4 and graphite was～10 nm and less than 200 nm, respectively, indicating the formation of nano composite micro structure. After 20 pulses of HCPEB irradiation, the nano-phases covered 39% of the surface area with a relatively uniform distribution. It is established that under the HCPEB irradiation, the YG cemented carbides transformed into a metastable micro structure composed of fcc+M12C+graphite+ liquid.The surface properties of YG6 cemented carbide were measured. The tendency of decrease firstly and then rising up after a certain number of treating pulse was observed in microhardness and wear tests. The microhardness value was increased by 36% as compared to the initial sample. The wear depth and wear rate showed the same regularity. The specimen irradiated with 20 pulses of 27kV perform best with wears trace depth decreasing about 82% when wear rate decreased to 12.3 mm3min-1,35% of the initial sample. The corrosion resistance of HCPEB treated sample was improved greatly with a lowest corrosion current density of 1.495 ×10-5 A/cm2 as compared to the initial sample of 6.415×10-5 A/cm2.Likewise, microhardness of YG8, increased with the more HCPEB pulses applied to a certain level and then decreased. The sample irradiated with 25 pulses of 27kV showed the best microhardness of 2262 HK, improved by 38% as compared with the initial sample.Results above show that the surface layer of cemented carbide can be transformed to a nano composite microstructure by HCPEB treatment. For the first time, the in-situ segregation of graphic nano-particle from WC grains was discovered. The microhardness and wear resistance of cemented carbides were improved effectively. Although, there are still problems remaining for further study, such as the inflence of Co content in initial cemented carbide on the formation of graphic nano-particles and their effect on abrasive resistance, and how to reduce the generation of surface microcracks. |
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