High-voltage electro pulse boring (EPB) has the advantages of high rock-breaking efficiency and good wall quality, and is a new and efficient potential method of rock breaking. The EPB process is defined as random because it is affected by many factors. At present, there is no suitable physical and mathematical model to describe the process and results of rock breakage in EPB, and the conclusions reached regarding rock-breakage mechanisms are not uniform. In this study, a complete damage model of high voltage EPB in granite is established, which includes a shock wave model and a damage model of high voltage EPB in granite. The damage model is based on the Particle Flow Code two-dimensional program. Use of a damage model of EPB accommodates the complete process of high voltage EPB, from discharge to production of a shock wave, and so rock-breaking via electro pulse can be simulated and calculated. The time-varying waveforms of shock waves with different electrical parameters are simulated and calculated on the basis of the model. Different shock wave forms are loaded into the surface and internal rock in the damage geometric model of EPB granite. Then, the breakage process of the rock surface and internally, and the mechanism of rock breakage using EPB are analyzed. This study provides a scientific basis for the quantitative expression and prediction of rock fragmentation in EPB in order to improve the drilling efficiency and reduction of energy loss in the process of EPB.

As the exploration and drilling of oil, natural gas and geothermal wells are expanding continuously, research into high-efficiency rock drilling technology is imperative. High-voltage electro pulse boring (EPB) has the advantages of high rock breaking efficiency and good wall quality, and is a new and efficient potential method of rock breaking. The design of electrode drill bits and the selection of drilling process parameters are the main obstacles restricting the commercialization of EPB. Accordingly, it is necessary to determine the influences on high-voltage EPB. In this study, based on the equivalent circuit of high-voltage electro pulse breakdown, a mathematical model of high-voltage electro pulse discharge in rock is established. Meanwhile, a numerical simulation model of high-voltage EPB of hard granite is established based on a coaxial cylindrical electrode structure, which is often used for electrode drill bits. The simulation analysis software Comsol Multiphysics (Comsol Multiphysics^®5.3a, COMSOL Co., Ltd., Stockholm, Sweden) is used to study the influences of granite composition, electrode spacing and electrode shape on the high-voltage EPB process. In addition, the influences of electrical parameters on high-voltage EPB are calculated according to a model of high-voltage electro pulse discharge in rock. Finally, it is demonstrated that high-voltage EPB is influenced by granite composition, electrical parameters, electrode spacing, and electrode shape, and the relationships between these factors are obtained. This study is of guiding significance for improving rock breaking efficiency, reducing energy loss, designing electrode drill bits and selecting drilling process parameters.

Metal matrix diamond composite samples were fabricated by selective laser melting (SLM) at different forming parameters to investigate the feasibility and new challenges when SLM is applied to diamond tools manufacturing. The surface topographies, Rockwell hardness, compactness, microstructure, and diamond thermal damage of the samples were investigated in this study. The fabricated samples had high porosity and relatively low Rockwell hardness and compactness, and some ridge-shaped bulges and textures were observed at the edges and surfaces. Microstructure analyses showed that diamond particles were homogeneously distributed and metallurgically bonded within the metal matrix. The thermal damage pits on the diamond crystals along the scanning direction were the dominant damage type for SLM, which was completely different from conventional vacuum brazing and hot-pressing sintering. Although some challenges need to be further studied, our results demonstrate that SLM has great potential to propel the development of metal matrix diamond tools.

In view of the stick-slip phenomenon in deep and hard rock drilling, a new type of torsional impactor that can provide torsional impact vibration was designed. According to the working principle and structural characteristics of the designed torsional impactor, this paper theoretically analyzes the influences of different structural parameters and motion parameters on the impact frequency, impact force, and impact torque of the torsional impactor. The results show that the impact frequency f is directly proportional to the rotational speed V_Z of the transmission shaft and the installed number n of torsional impact generating devices. Additionally, the impact force F is directly proportional to sine value of the impact angle α (i.e., sinα), impact hammer mass m, impact hammer rotation speed V_Z (i.e., transmission shaft rotation speed), and impact hammer rotation radius r and is inversely proportional to action time Δt of the impact hammer and impact anvil. Furthermore, the impact torque M is directly proportional to the impact force F and rotary radius r of the impact hammer. This article lays a foundation for further theoretical and experimental research of torsional impactors and provides a reference for the design and testing of torsional impactors.