Effect of low-speed waterjet pressure on the rock-breaking performance of unsubmerged cavitating abrasive waterjet

Unsubmerged cavitating abrasive waterjet(UCAWJ)has been shown to artificially create a submerged environment that produces shear cavitation,which effectively enhances rock-breaking performance.The shear cavitation generation and collapse intensity depend on the pressure difference between the intermediate high-speed abrasive waterjet and the coaxial low-speed waterjet.However,the effect of the pressure of the coaxial low-speed waterjet is pending.For this purpose,the effect of low-speed waterjet pressure on rock-breaking performance at different standoff distances was experimentally investigated,and the effects of erosion time and ruby nozzle diameter on erosion performance were discussed.Finally,the micromorphology of the sandstone was observed at different locations.The results show that increased erosion time and ruby nozzle diameter can significantly improve the rock-breaking performance.At different standoff distances,the mass loss increases first and then decreases with the increase of low-speed waterjet pressure,the maximum mass loss is 10.4 g at a low-speed waterjet pressure of0.09 MPa.The surface morphology of cavitation erosion was measured using a 3D profiler,the increase in both erosion depth and surface roughness indicated a significant increase in the intensity of the shear cavitation collapse.At a low-speed waterjet pressure of 0.18 MPa,the cavitation erosion surface depth can reach 600μm with a roughness of 127μm.

Contact Mechanism of Rail Grinding with Open-Structured Abrasive Belt Based on Pressure Grinding Plate

The current research of abrasive belt grinding rail mainly focuses on the contact mechanism and structural design.Compared with the closed structure abrasive belt grinding,open-structured abrasive belt grinding has excellent performance in dynamic stability,consistency of grinding quality,extension of grinding mileage and improvement of working efficiency.However,in the contact structure design,the open-structured abrasive belt grinding rail using a profiling pressure grinding plate and the closed structure abrasive belt using the contact wheel are different,and the contact mechanisms of the two are different.In this paper,based on the conformal contact and Hertz theory,the contact mechanism of the pressure grinding plate,abrasive belt and rail is analyzed.Through finite element simulation and static pressure experiment,the contact behavior of pressure grinding plate,abrasive belt and rail under single concentrated force,uniform force and multiple concentrated force was studied,and the distribution characteristics of contact stress on rail surface were observed.The results show that under the same external load,there are three contact areas under the three loading modes.The outer contour of the middle contact area is rectangular,and the inner contour is elliptical.In the contact area at both ends,the stress is extremely small under a single concentrated force,the internal stress is drop-shaped under a uniform force,and the internal stress under multiple concentration forces is elliptical.Compared with the three,the maximum stress is the smallest and the stress distribution is more uniform under multiple concentrated forces.Therefore,the multiple concentrated forces is the best grinding pressure loading mode.The research provides support for the application of rail grinding with open-structured abrasive belt based on pressure grinding plate,such as contact mechanism and grinding pressure mode selection.

CeO_(2)/MoSi_(2)改性ZrB_(2)-SiC陶瓷的烧结性能及耐烧蚀性能

无压烧结ZrB_(2)-SiC陶瓷可制备复杂形状构件,但致密化难且耐烧蚀性能有待提高,因此亟需研究能够改善陶瓷致密化的有效烧结助剂。本文采用CeO_(2)和MoSi_(2)为烧结助剂,在1850℃无压烧结1 h条件下制备了ZrB_(2)-SiC陶瓷,研究了CeO_(2)和MoSi_(2)对ZrB_(2)-SiC陶瓷烧结性能和耐烧蚀性能的影响。结果表明,当5%(体积分数)的烧结助剂中CeO_(2)和MoSi_(2)体积比为1∶1时,ZrB_(2)-SiC陶瓷的烧结性能、耐烧蚀性能最佳。烧结助剂在烧结过程中形成Ce-Mo液相,陶瓷颗粒在液相的表面张力下重排和传质,Ce-Mo液相促进陶瓷颗粒在毛细力下形成烧结颈,填充于陶瓷颗粒的间隙中,并在降低陶瓷晶粒的晶界能时促进致密化进程,最终获得无压烧结密度为5.02 g/cm^(3)、相对密度为89.71%、维氏硬度为14.04 GPa的ZrB_(2)-SiC陶瓷。相对于未添加烧结助剂时,实际密度提高了53%,维氏硬度提高了43%。在烧蚀过程中,烧结助剂适量补充SiO_(2)后可促进黏度适宜的玻璃相自愈合,并提高ZrO_(2)的稳定性,使ZrB_(2)-SiC陶瓷具有最好的耐烧蚀性能,此时ZrB_(2)-SiC陶瓷质量烧蚀率为-1.62 mg/s,线烧蚀率为0.33μm/s。

Microstructure, mechanical properties and two-body abrasive wear behaviour of hypereutectic Al-Si-SiC composite

The microstructure,mechanical properties,and the effects of sliding distance and material removal mechanism on two-body abrasive wear behaviour of hypereutectic Al-Si-Si C composite and its matrix alloy were investigated.The hypereutectic Al-Si-Si C composite was prepared by stir casting route.The hardness,ultimate tensile strength and yield strength of the composite are increased by 17%,38%,and 30%respectively compared with those of the matrix alloy,while the elongation of the composite is decreased by 48%compared with that of the matrix alloy.The wear rate of the materials is increased with increasing the abrasive size and the applied load and does not vary with the sliding distance.The wear surfaces and wear debris of the materials were characterized by high-resolution field emission scanning electron microscopy(HR FESEM)and wear mechanism was analyzed for low and high load regimes.

Rock breaking performance of the newly proposed unsubmerged cavitating abrasive waterjet

To improve the rock breaking ability, cavitating waterjet and abrasive waterjet are combined by using a coaxial low-speed waterjet generated around the periphery of a high-speed abrasive waterjet, and a new type of waterjet called unsubmerged cavitating abrasive waterjet(UCAWJ) is thus produced. The rock breaking performance of UCAWJ was compared with submerged cavitating abrasive waterjet(SCAWJ)and unsubmerged abrasive waterjet(UAWJ) by impinging sandstone specimens. Moreover, the effects of jet pressure, standoff distance, abrasive flow rate and concentration were studied by evaluating the specific energy consumption, and the area, depth, and mass loss of the eroded specimen. The results show that the artificially generated submerged environment in UCAWJ is able to enhance the rock breaking performance under the same operating parameters. Furthermore, the rock breaking performance of UCAWJ is much better at higher jet pressures and smaller standoff distances when compared with UAWJ. The greatest rock breaking ability of UCAWJ appears at jet pressure of 50 MPa and standoff distance of 32 mm, with the mass loss of sandstone increased by 370.6% and the energy dissipation decreased by 75.8%. In addition, under the experimental conditions the optimal abrasive flow rate and concentration are 76.5 m L/min and 3%, respectively.

Multi-physical Modeling and Adjusting for Ultrasonic Assisted Soft Abrasive Flow Processing

The polishing efficiency of the soft abrasive flow(SAF)method is low,which is not in line with the concept of carbon emission reduction in industrial production.To address the above issue,a two-phase fluid multi-physics modeling method for ultrasonic-assisted SAF processing is proposed.The acoustics-fluid coupling mechanic model based on the realizable k-ε model and Helmholtz equation is built to analyze the cavitation effect.The results show that the pro-posed modeling and solution method oriented to ultrasonic-assisted SAF processing have better revealed the flow field evolution mechanism.The turbulence kinetic energy at different ultrasonic frequencies and amplitudes is stud-ied.Simulation results show that the ultrasonic vibration can induce a cavitation effect in the constrained flow chan-nel and promote the turbulence intensity and uniformity of the abrasive flow.A set of comparative polishing experiments with or without ultrasonic vibration are conducted to explore the performance of the proposed method.It can be found that the ultrasonic-assisted SAF method can improve the machining efficiency and uniformity,to achieve the purpose of carbon emission reduction.The relevant result can offer a helpful reference for the SAF method.

Efect of Micro Abrasive Slurry Jet Polishing on Properties of Coated Cemented Carbide Tools

Owing to the popularization of coating technology, physical Vapor Deposition (PVD) coated tools have become indispensable in the cutting process. Additionally, the post-treatment of coated tools applied to industrial production can efectively enhance the surface quality of coating. To improve the processing performance of coated tools, micro abrasive slurry jet (MASJ) polishing technology is frst applied to the post-treatment of coated tools. Subsequently, the efects of process parameters on the surface quality and cutting thickness of coating are investigated via single-factor experiments. In the experiment, the best surface roughness is obtained by setting the working pressure to 0.4 MPa, particle size to 3 μm, incidence angle to 30°, and abrasive mass concentration to 100 g/L. Based on the results of the single-factor experiments, combination experiments are designed, and three types of coated tools with diferent surface qualities and coating thicknesses are obtained. The MASJ process for the post-treatment of coated tools is investigated based on a tool wear experiment and the efects of cutting parameters on the cutting force and workpiece surface quality of three types of cutting tools. The result indicates that MASJ machining can efectively improve the machining performance of coated tools.

Microstructure and Oxidation Behavior of ZrB_(2)-SiC Ceramics Fabricated by Tape Casting and Reactive Melt Infiltration

ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2) and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2) phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2) oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2) is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2) particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2) increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2) in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.

Experimental Abrasion Study of the Evolution of Gold Morphological Criteria in Different Sediments as a Function of Time

The use of gold as a tracer element for understanding the mode of emplacement of these formations requires a good knowledge of its dynamics. To this end, we have carried out experimental studies to investigate the evolution of contour states (regular, irregular), surface states (blunt, rough), shape (elongation) and dimension (size). The study of these criteria is carried out on SEM photos of particles, taken before and after the experiment, using Aphelion image processing software. Five experiments were carried out to study the influence of sediment granulometry and water dilution on particle morphological changes. The results of these experiments show that coarse sediments (>1 mm) are largely responsible for morphological changes in gold. They cause fragmentation and/or folding of the particle edges, depending on the magnitude of the impact forces. Strong impacts due to high water dilution cause staking on the particle surface. Fine sediments (<1 mm) slow down particle evolution as they cushion impacts and particles tend to flatten rather than fragment.

表面电性对TiO_(2)纳米管-SiC复合填料去除养殖废水中氨氮效率的影响研究

试验旨在探究二氧化钛(TiO_(2))纳米管-碳化硅(SiC)复合填料在净化养殖废水方面的性能。试验采用不同前驱体通过碱性水热合成方法合成3种不同粒径和晶相的TiO_(2)纳米管,测试了3种TiO_(2)纳米管的表面电性、X射线衍射图谱和场发射扫描电镜。研究TiO_(2)纳米管负载于SiC片上并覆盖微生物膜后,在紫外光照射和黑暗条件下去除养殖废水中氨氮污染物的性能。结果表明,晶相为锐钛矿的前驱体合成的TiO_(2)纳米管表面带负电,晶相为金红石和锐钛矿混合的前驱体合成的TiO_(2)纳米管表面带正电,而表面带正电的TiO_(2)纳米管能促进带生物负电的微生物的挂膜,进而促进对养殖废水中氨氮的去除作用;其中,负载P25 TiO_(2)纳米管的SiC片对氨氮的去除率在紫外光照射和黑暗条件下的去除率分别为86.1%和58.9%。研究表明,与黑暗条件相比,紫外光照射会影响TiO_(2)的光催化活性并提高TiO_(2)纳米管-SiC复合填料去除养殖废水中氨氮的效率。

Al_(2)O_(3)膜对高温环境下NiCrAlY-SiC复合镀层内部反应的阻隔及涂层耐磨性研究

为了解决以SiC颗粒为磨料的叶尖耐磨复合镀层在高温工况下SiC与涂层中Ni元素反应导致涂层耐磨性下降等问题,采用化学气相沉积(chemicalvapor deposition,CVD)工艺方法在SiC颗粒表面制备了Al_(2)O_(3)膜以阻隔SiC与涂层中Ni元素的反应。研究了0.5~1.0μm和5.0~10.0μm这2种厚度范围的Al_(2)O_(3)膜对NiCrAlY-SiC@Al_(2)O_(3)叶尖耐磨镀层中SiC颗粒与NiCrAlY镀层在1100℃高温环境下反应的阻隔效果,并测试了2种Al_(2)O_(3)膜厚度的NiCrAlY-SiC@Al_(2)O_(3)叶尖耐磨镀层在1100℃高温环境下保温500 h后的耐磨性。结果显示,5.0~10.0μm厚度的Al_(2)O_(3)膜可有效阻隔SiC颗粒与NiCrAlY镀层的高温反应,使NiCrAlY-SiC@Al_(2)O_(3)涂层在1100℃高温环境下保持良好的耐磨性,并与ZrO2陶瓷封严涂层形成良好的对磨匹配效果。

浆料涂刷-热压法制备2D C_(f)-ZrB_(2)-SiC复合材料的力学与烧蚀性能

连续碳纤维增强ZrB_(2)-SiC复合材料以其优异的抗氧化、抗烧蚀性能在航天飞行器热防护结构材料领域备受关注。本文采用浆料涂刷−热压法制备2D C_(f)-ZrB_(2)-SiC复合材料,探索运用微米级粉末浆料制备2D C_(f)-ZrB_(2)-SiC复合材料的可行性,研究烧结温度对材料微观结构与力学性能的影响,并测试材料的抗烧蚀性能。结果表明:采用微米粉末浆料涂刷−热压法制备的2D C_(f)-ZrB_(2)-SiC复合材料于2000℃烧结后具有最高的致密度与抗弯强度,开孔率达到8.01%,抗弯强度达到191.3 MPa;复合材料表现出较好的抗烧蚀性能,材料表面响应温度达到2600℃,经300 s等离子火焰烧蚀后线烧蚀率为3.51μm/s。

STUDIES ON MAGNETIC ABRASIVE MACHINING

This paper deals with the machining process using magnetic abrasives. Using an apparatus designed and made, a series of experiments are carried out, where the workpiece is cylindrical and the magnetic abrasives used are mainly Fe and Al 2O 3, for investigating the effects of machining time, working gap, rotating speed of workpieces, magnetic flux density on machining efficiency and surface roughness. At the end of this paper, the machining mechanism is also discussed.

吊顶窑用α-SiC结合SiC拱顶砖的服役行为研究

为解决燃气吊顶窑拱顶用耐火材料的污染和膨胀问题,以碳化硅颗粒和细粉为主要原料,水溶性树脂为结合剂,依次按三者质量比为65∶35∶4.5配料,机压成型为240 mm×170 mm×140 mm的坯体,在2400℃真空条件下保温3 h制备了α-SiC结合SiC砖。将该砖用于国内某氧化铝陶瓷棍棒公司的燃气吊顶窑拱顶,累计使用46窑次(合计276 h)后,该砖断裂严重。为此,对用后砖进行取样,借助热力学分析,检测了其距热面不同距离处的性能,分析物相组成和显微结构,并与原砖进行对比。结果表明:1)用后α-SiC结合SiC砖比原砖的显气孔率降低,体积密度增大,因为用后砖中生成了12%~14.2%(w)的方石英,由α-SiC结合转变为方石英结合,气孔孔径变小。2)用后砖与原砖比,常温抗折强度和荷重软化开始温度变化不大,推测材料断裂不是自身力学性能失效所致,而是因温度升降而频繁受到剪切力引起。

Fe_(3)O_(4)-SiC沥青混合料微波愈合性能研究

对不同掺量的Fe_(3)O_(4)-SiC沥青混合料微波热诱导愈合性能展开研究。首先,通过微波加热研究Fe3O4-SiC掺量对沥青混合料升温速率的影响;然后,通过DSR的频率扫描、温度扫描试验表征沥青的流动性,通过加载-间歇-加载模式的LAS试验评价沥青的愈合效果;最后,采用破坏-愈合-破坏模式的SCB试验研究沥青混合料愈合效果。结果表明:随着Fe_(3)O_(4)-SiC掺量增加,沥青混合料的微波升温效率得到改善,Fe3O4-SiC掺量为25%、50%、75%时升温速率较空白组分别提高了27.41%、43.44%、77.26%;频率扫描、温度扫描以及LAS试验表明,高温促进沥青流动,有利于愈合。SCB试验表明,Fe_(3)O_(4)-SiC掺量为50%时更适合用于沥青混合料微波热诱导愈合,既缩短加热时间,又保持了一定的愈合效果。

Abrasive Waterjet Machining Simulation by Coupling Smoothed Particle Hydrodynamics /Finite Element Method

In dealing with abrasive waterjet machining（AWJM） simulation,most literatures apply finite element method（FEM） to build pure waterjet models or single abrasive particle erosion models.To overcome the mesh distortion caused by large deformation using FEM and to consider the effects of both water and abrasive,the smoothed particle hydrodynamics（SPH） coupled FEM modeling for AWJM simulation is presented,in which the abrasive waterjet is modeled by SPH particles and the target material is modeled by FEM.The two parts interact through contact algorithm.Utilizing this model,abrasive waterjet with high velocity penetrating the target materials is simulated and the mechanism of erosion is depicted.The relationships between the depth of penetration and jet parameters,including water pressure and traverse speed,etc,are analyzed based on the simulation.The simulation results agree well with the existed experimental data.The mixing multi-materials SPH particles,which contain abrasive and water,are adopted by means of the randomized algorithm and material model for the abrasive is presented.The study will not only provide a new powerful tool for the simulation of abrasive waterjet machining,but also be beneficial to understand its cutting mechanism and optimize the operating parameters.

New Iron-based SiC Spherical Composite Magnetic Abrasive for Magnetic Abrasive Finishing

SiC magnetic abrasive is used to polish surfaces of precise, complex parts which are hard, brittle and highly corrosion-resistant in magnetic abrasive finishing（MAF）. Various techniques are employed to produce this magnetic abrasive, but few can meet production demands because they are usually time-consuming, complex with high cost, and the magnetic abrasives made by these techniques have irregular shape and low bonding strength that result in low processing efficiency and shorter service life. Therefore, an attempt is made by combining gas atomization and rapid solidification to fabricate a new iron-based SiC spherical composite magnetic abrasive. The experimental system to prepare this new magnetic abrasive is constructed according to the characteristics of gas atomization and rapid solidification process and the performance requirements of magnetic abrasive. The new iron-based SiC spherical composite magnetic abrasive is prepared successfully when the machining parameters and the composition proportion of the raw materials are controlled properly. Its morphology, microstructure, phase composition are characterized by scanning electron microscope（SEM） and X-ray diffraction（XRD） analysis. The MAF tests on plate of mold steel S136 are carried out without grinding lubricant to assess the finishing performance and service life of this new SiC magnetic abrasive. The surface roughness（Ra） of the plate worked is rapidly reduced to 0.051 μm from an initial value of 0.372 μm within 5 min. The MAF test is carried on to find that the service life of this new SiC magnetic abrasive reaches to 155 min. The results indicate that this process presented is feasible to prepare the new SiC magnetic abrasive; and compared with previous magnetic abrasives, the new SiC spherical composite magnetic abrasive has excellent finishing performance, high processing efficiency and longer service life. The presented method to fabricate magnetic abrasive through gas atomization and rapid solidification presented can significantly improve the finishing performance and service life of magnetic abrasive, and provide a more practical approach for large-scale industrial production of magnetic abrasive.

Two-Body Abrasive Wear of the Surfaces of Pangolin Scales

The Pangolin, a soil-burrowing animal, is covered with scales. These scales are often abraded by soil and rock and their surface is corrugated. The abrasive wear of the surface of the scales was examined. The scales were taken from a pangolin that had died of natural causes. The tests were run on a rotary disc abrasive wear tester. The abrasive material was quartz sand （96.5 wt.%） and bentonite （3.5 wt.%）. The morphology of the abraded surfaces and the abrasion were examined by stereoscopic microscopy and scanning electron microscopy. The concepts are proposed of ＂Guiding-Effect＂ and ＂Rolling-Effect＂ on the textured surfaces under free abrasive wear conditions and the critical dimensions of the ＂Rolling-Effect＂ are discussed.

Multiphase Flow and Wear in the Cutting Head of Ultra-high Pressure Abrasive Water Jet

Abrasive water jet cutting technology is widely applied in the materials processing today and attracts great attention from scholars, but many phenomena concerned are not well understood, especially in the internal jet flow of the cutting head at the condition of ultra-high pressure. The multiphase flow in the cutting head is numerically simulated to study the abrasive motion mechanism and wear inside the cutting head at the pressure beyond 300 MPa. Visible predictions of the particles trajectories and wear rate in the cutting head are presented. The influences of the abrasive physical properties, size of the jewel orifice and the operating pressure on the trajectories are discussed. Based on the simulation, a wear experiment is carried out under the corresponding pressures. The simulation and experimental results show that the flow in the mixing chamber is composed of the jet core zone and the disturbance zone, both affect the particles trajectories. The mixing efficiency drops with the increase of the abrasive granularity. The abrasive density determines the response of particles to the effects of different flow zones, the abrasive with medium density gives the best general performance. Increasing the operating pressure or using the jewel with a smaller orifice improves the coherency of p articles trajectories but increases the wear rate of the jewel holder at the same time. Walls of the jewel holder, the entrance of the mixing chamber and the convergence part of the mixing tube are subject to wear out. The computational and experimental results give a qualitative consistency which proves that this numerical method can provide a reliable and visible cognition of the flow characteristics of ultra-high pressure abrasive water jet. The investigation is benefit for improving the machining properties of water jet cutting systems and the optimization design of the cutting head.

Effects of nano-Al_2O_3 and nano-ZrO_2 on the microstructure, behavior and abrasive wear resistance of WC-8Co cemented carbide

WC-8wt.% Co cemented carbides with varied nano-Al 2 O 3 and nano-ZrO 2 contents were prepared respectively following conventional powder metallurgical procedures. Effects of nano-Al 2 O 3 and nano-ZrO 2 on the microstructure, behavior, and abrasive wear resistance were investigated. The result shows that a finer and more homogenous microstructure can be achieved by increasing nano-Al 2 O 3 , and increasing nano-ZrO 2 makes the microstructure more refined. Nano-Al 2 O 3 and nano-ZrO 2 could both help to give increased hardness. Transverse rupture strength is higher if the above nano-oxides are doped appropriately, whereas excess addition is deleterious. Abrasive wear resistance presents different variations with respect to increasing nano-Al 2 O 3 and nano-ZrO 2 . By contrast, increasing nano-ZrO 2 enhances the abrasive wear resistance more effectively than increasing nano-Al 2 O 3 . The influence of the two nano-oxides contents on the abrasive wear resistance does not almost vary with wear time, and the optimum addition level of nano-Al 2 O 3 in WC-8% Co cemented carbide is 0.3 wt.% from the stand of abrasive wear resistance. In addition, both of the nano-oxides can retard the increase of wear rate in long-term abrasive wear.