Effect of mechanical activation of WC-based powder on the properties of sintered alloys

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 1 2021 Introduction Hard alloys based on tungsten carbide and cobalt (WC-Co) are widely used in the manufacture of cutting, drilling tools, wear-resistant parts due to their high hardness, strength, wear resistance, and good fracture toughness [1-3]. As a rule, by changing the structure of WC–Co hard alloys, for example, the composition of the alloy, the size of the carbide grains, the volume content of the binder component, it is possible to adjust the hardness, impact toughness, and strength [4]. The sintering process and the mechanical properties of such materials are signi fi cantly affected by the composition and microstructure, especially the grain size of the carbide phase [5], the content, and distribution of the cobalt binder [6-8]. At the same time, the presence of the η phase (W 3 Co 3 C) has a signi fi cant effect on the properties [3], and multi-stage sintering leads to a decrease in the grain size of the carbide phase, a decrease in the porosity of the sample. Paper [9] reports that using the method of rapid sintering with pulsed current activation can effectively suppress grain growth. The use of nanocrystalline materials is known to improve physical and mechanical properties. Thus, paper [10] showed that when the particle size of TiC powder decreases from 380 to 60 nm, the hardness of sintered samples increases from ~ 28 (HV) to 32 GPa. It is possible to reduce the size of powder particles by using high-intensity mechanical processing in a planetary ball mill [11]. This method is relatively inexpensive and easy to implement [12]. In the process of high-intensity mechanical processing of powders, a state with a very small size of the coherent scattering region can be formed [13, 14], and [15, 16] showed that mechanical processing activates sintering with achieving a higher density and a smaller grain size [11, 17]. During such processing, the shape of the particles may also change, which, as a rule, will affect the physical and mechanical properties. For example, [18, 19] report that the lamellar shape of the particles allows increasing both the hardness of the material and viscosity. It was shown in [20] that hard alloys with lamellar WC grains have a higher fracture toughness than hard WC-Co alloys with prismatic WC grains. However, it is known that mechanical processing does not always lead to a positive result, since such processing may cause contamination of powders, their oxidation, etc. [21, 22]. Thus, despite many research and practical publications devoted to the effect of high-intensity machining on the properties of alloys, the data on the effect of high-intensity machining of WC-Co powders on the morphology of particles, structure, phase composition, and physical and mechanical properties of sintered hard alloys are insuf fi cient. The purpose of this work is to study the effect of high-intensity mechanical activation of VK-8 powder on the structure and properties of powders and sintered samples. To achieve this purpose, the following tasks were set: 1) to study the morphology of the particles and their size by scanning electron microscopy before and after machining; 2) to study the changes of the phase composition and parameters of the fi ne crystal structure by X-ray diffraction and X-ray structural analysis after machining; 3) to study the microstructure of the sintered samples by optical and scanning electron microscopy; 4) to study the changes of the phase composition and parameters of the fi ne crystal structure by X-ray diffraction and X-ray structural analysis of the sintered samples; 5) to study the hardness of the sintered samples. Research Methodology The industrial powder of tungsten carbide of the VK-8 grade produced by Virial Ltd. was studied. The powder was processed in the planetary ball mill AGO-2 (Russia), the diameter of the steel balls was 0.7 cm, the ratio of powder to balls was 1:10, the rotation speed of the planetary disk was 1820 rpm, which provides an acceleration of 60g. The mechanical activation time was 10...300 seconds. Pressing of the samples was carried out on a hydraulic press by cold one-sided pressing at a pressure of 200 MPa with pressure exposure of 15 s, sintering of the samples was carried out in a vacuum furnace SNVE 1.3.1/16 (Russia) according to the following mode: a) heating from room temperature to 800 °C at a heating rate of 5 °C/min, followed by maintaining this temperature for 30 minutes;