磨料水射流对脆性材料的冲蚀研究

磨料水射流技术作为一种特种加工技术，具有无刀具接触、无热影响区和加工范围广等优势，在众多领域得到应用。为了探究磨料水射流对脆性材料的冲蚀效果，构建和设计了磨料水射流外流场冲蚀仿真模型与磨料水射流冲蚀实验。以30 mm×50 mm的喷嘴外流场域为计算域，建立磨料水射流冲蚀仿真模型，并分析射流冲蚀过程中压力分布、水与磨料的速度分布及它们在射流中心线上的衰减规律。通过对氧化铝陶瓷材料的冲蚀实验，分析工艺参数对冲蚀孔径的影响，并结合仿真结果对比分析了射流束宽度与冲蚀孔径的关系。结果表明：水的速度随着喷嘴距离的增大而减小且分布范围变宽，射流宽度呈线性增大，磨料速度随喷嘴距离的增大而减小且分布范围基本不变；射流中心线上水的速度与磨料速度呈三段式衰减，水的第1段速度衰减段长度比磨料的长，但水的第2段速度衰减段长度比磨料的短；射流束能量的有效利用部分逐渐减小，但在15~25 mm的靶距范围内其有效利用部分较稳定，为40%；冲蚀孔径随喷嘴距离增大呈线性增大。研究结果为磨料水射流切割、铣削及抛光加工的参数选择提供实验依据，同时为磨料水射流加工过程仿真提供参考。     

An investigation on machined surface quality and tool wear during creep feed grinding of powder metallurgy nickel-based superalloy FGH96 with alumina abrasive wheels

In this study, the machined surface quality of powder metallurgy nickel-based superalloy FGH96 (similar to Rene88DT) and the grinding characteristics of brown alumina (BA) and microcrystalline alumina (MA) abrasive wheels were comparatively analyzed during creep feed grinding. The influences of the grinding parameters (abrasive wheel speed, workpiece infeed speed, and depth of cut) on the grinding force, grinding temperature, surface roughness, surface morphology, tool wear, and grinding ratio were analyzed comprehensively. The experimental results showed that there was no significant difference in terms of the machined surface quality and grinding characteristics of FGH96 during grinding with the two types of abrasive wheels. This was mainly because the grinding advantages of the MA wheel were weakened for the difficult-to-cut FGH96 material. Moreover, both the BA and MA abrasive wheels exhibited severe tool wear in the form of wheel clogging and workpiece material adhesion. Finally, an analytical model for prediction of the grinding ratio was established by combining the tool wear volume, grinding force, and grinding length. The acceptable errors between the predicted and experimental grinding ratios (ranging from 0.6 to 1.8) were 7.56% and 6.31% for the BA and MA abrasive wheels, respectively. This model can be used to evaluate quantitatively the grinding performance of an alumina abrasive wheel, and is therefore helpful for optimizing the grinding parameters in the creep feed grinding process.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00305-2     

CBN grain wear and its effects on material removal during grinding of FGH96 powder metallurgy superalloy

Grinding with cubic boron nitride (CBN) superabrasive is a widely used method of machining superalloy in aerospace industries. However, there are some issues, such as poor grinding quality and severe tool wear, in grinding of powder metallurgy superalloy FGH96. In addition, abrasive wheel wear is the significant factor that hinders the further application of CBN abrasive wheels. In this case, the experiment of grinding FGH96 with single CBN abrasive grain using different parameters was carried out. The wear characteristics of CBN abrasive grain were analyzed by experiment and simulation. The material removal behavior affected by CBN abrasive wear was also studied by discussing the pile-up ratio during grinding process. It shows that morphological characteristics of CBN abrasive grain and grinding infeed direction affect the CBN abrasive wear seriously by simulation analysis. Attrition wear, micro break, and macro fracture had an important impact on material removal characteristics. Besides, compared with the single cutting edge, higher pile-up ratio was obtained by multiple cutting edges, which reduced the removal efficiency of the material. Therefore, weakening multiple cutting edge grinding on abrasive grains in the industrial production, such as applying suitable dressing strategy, is an available method to improve the grinding quality and efficiency. The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00412-2     

微小音速喷嘴临界背压比测试装置

音速喷嘴工作时的背压比小于临界背压比是实现并保持临界流的关键。现行国际标准建议,对于喉部雷诺数Red小于2×10⁵的音速喷嘴,需保持0.25的临界背压比或进行临界背压比的实际测量。基于此,建立了一套微小音速喷嘴临界背压比测试装置,该装置能够实现喉径为0.3~3mm,流量范围为0.05~5m³/h音速喷嘴临界背压比的测量,测量结果的扩展不确定度为0.56%(k=2)。     

喷射器极限工况特性实验研究

喷射器作为热驱动喷射式制冷系统的核心部件,其性能会影响整个制冷系统的运行效率。极限工况是指喷射器从可以工作状态到不能工作状态的极端工况,对该工况下喷射器的特性研究具有重要意义。本文自行设计并搭建了以R134a为制冷剂的喷射式制冷系统极限工况的实验装置,分别对引射流体质量流量为零的极限工况下不同喷射器工作流体压力及喷射器出口背压对缩放喷嘴出口背压的影响规律进行了实验研究。结果表明:极限工况下,喷嘴出口背压同时受工作流体压力和喷射器出口背压的影响,随工作流体压力升高而降低,随喷射器出口背压升高而升高。同时,得到该喷射器在工作流体压力为1.5~3.2MPa,且喷射器出口背压在0.66~0.96 MPa范围内的最低引射流体压力,为工程应用提供参考。     

Turning of glass with abrasive waterjet

This article reports research results on abrasive waterjet (AWJ) turning of glass. Glass rods, 25 mm in diameter, were turned by using AWJ to investigate the effects of several process parameters on the surface quality of the machined glass surfaces. The parameters studied are rotational speed, stand-off distance, water pressure, nozzle traverse speed, and abrasive flow rate. The results were also compared with those obtained from conventional machining of glass. The results showed that higher traverse rates were associated with an increase in material removal rate and thus an increase in surface roughness and waviness values. The sensitivity of surface quality to rotational speed was more than that to the traverse speed. Good surface finish was achieved at lower traverse speeds and higher turning speeds. Higher stand-off produced rougher surface finish. The best finish was generated when the nozzle consumed 300 g min^-1 of abrasives. Higher pressures did not produce smoother surface finish.     

Statistical modeling of high-carbon cast-steel particle flow through a blast-cleaning metering system

The selection and adjustment of an effective abrasive mass flow rate is one of the most important requirements for efficient blast-cleaning processes. Steel grit is one of the most widely used abrasive materials in the industry, and the adjustment of effective steel grit mass flow rates can improve efficiency and decrease costs. Systematic investigations into the metering behavior of steel grit have not been performed yet. The paper deals with a systematic investigation into the flow of high-carbon cast-steel grit particles through a metering valve. The investigation involves abrasive mass flow rate measurements, and the results are statistically interpreted based on design of experiments and analysis of variance. Four process parameters are varied, namely static air pressure, nozzle diameter, valve opening, and particle size range. Abrasive mass flow rate increases if air pressure, nozzle diameter, or valve opening increases, and it decreases if particle size becomes smaller. Only valve opening and nozzle diameter provide statistically significant effects. Interaction effects are statistically insignificant for all parameter combinations. It is concluded that a three-parameter linear regression model is suitable to statistically describe the relationships in the scope of the evaluation effort.     

Study and evaluation of abrasive water jet turning process performance on AA5083

In this study, the effect of processing parameters on surface roughness and macro surface characteristics was analyzed during the machining of Ø30 mm and 300 mm aluminum alloy AA5083 abrasive water jets. As the processing parameters (up to 10 mm min⁻¹, 15 mm min⁻¹, 20 mm min⁻¹ and 25 mm min⁻¹), abrasive flow rate (50 g min⁻¹, 150 g min⁻¹, 250 g min⁻¹ and 350 g min⁻¹), the lathe chuck rotational speed (25 min⁻¹, 50 min⁻¹, 75 min⁻¹ and 100 min⁻¹) and the nozzle approach distance (2 mm, 5 mm, 8 mm and 11 mm) were used in experiments. In experimental studies, the pump pressure (360 MPa) was used as a constant, in the form of an abrasive Garnet (100 mesh), and the nozzle diameter as 0.76 mm. According to the findings, the best results in terms of surface roughness were obtained as a result of turning speed and abrasive flow rate. When the macro surface characteristics were examined, it was found that the lathe chuck rotational speed increased, the rate of nozzle progression was low, the rate of abrasive flow was high and the nozzle approach distance was lower and the smoother surfaces were obtained.     

Experimentelle und theoretische Untersuchungen zu Hochdruck-Sicherheitsventilen – Auslegung und Design unterstützt durch numerische Rechenmethoden (CFD)

High-pressure safety valves with set pressures of more than 200 bar are required in industry, e.g. for polyethylene and synthesis gas applications. They are presently sized according to ISO 4126-1. Only equations for ideal gases are presented there, and there are no indications as to how the real gas factor and the adiabatic exponent for real gases are to be calculated. For this reason, an equation for the critical mass flow rate of a real gas through a nozzle was derived and compared with the model according to EN-ISO 4126-1 and with experimental data. It is recommended that the current ISO-standard be supplemented by the nozzle flow model for real gases. The first numerical calculations (ANSYS-CFX) show that the discharge coefficient for a high pressure safety valve measured at moderate pressures can be extrapolated to very high pressures if it is used in conjunction with the nozzle flow model for real gases. However, this numerical result must yet be validated for further valve types by experiment. For this purpose, BASF has set up a high pressure valve test facility in Ludwigshafen.     

Influence of nozzle design and process parameters on surface roughness of CFRP machined by abrasive jet

Carbon fiber reinforced polymer composites find extensive applications in the areas of automotive, aircraft and aerospace, medical sciences, and electronics. Their usage in automated engineering industry needs good quality in components and excellent surface finish. The aim of this research work is to make holes on carbon fiber reinforced polymer composite material using a newly designed nozzle with and without internal thread. Surface roughness (Ra) was calculated across the depth of the machined hole. The effect of air and abrasive mixture pressure (P), stand-off distance (L), nozzle diameter (D), and abrasive size (S) on surface roughness was carried out for two different nozzles. The experimental result showed offer of good surface finish on CFRP materials from the newly designed nozzle with internal thread.     

Surface Waviness Controlled Grinding of Thin Mold Inserts Using Chilled Air as Coolant

The ongoing trend of miniaturization in electronic related parts, which is an average of two times every 5 to 7 years, introduces machining challenges. Surface grinding of thin miniature parts is an ardent task due to its warpage, induced by high specific grinding energy (10-100 J/mm³). Therefore, coolant is used to avoid thermal damage, improve surface integrity, and prolong wheel life. However, coolant, the incompressibility media, introduces high forces at the grinding zone, that create dimensional and shape instability. This article presents the results of a new grinding method when using chilled air as coolant media for thin mold insert, two-stage vapor compression refrigeration cycle was adopted for production of chilled air of temperature, -35°C; pressure, 0.2-0.3 MPa; and flow rate, 0.4 m³/min. Along with chilled air, traces of vegetable oil mist was also impinged to the grinding zone. Both chilled air and vegetable oil mist were applied through two independent paths of a specially designed twin compartment nozzle for maximizing the penetration.     

悬浮式烘箱风嘴结构的优化设计及实验验证

目的 创新设计风嘴泄压孔结构,优化风嘴和基材之间由于气流撞击而形成的涡流,导致基材表面风速温度堆积不均匀的问题。方法 采用Fluent软件,建立单一的风嘴仿真模型,并利用RNG k-ε湍流模型进行数值仿真模拟,分析风嘴有无泄压孔结构对基材干燥特性的影响,并在7种泄压孔设计结构中确定了相对最优方案,通过搭建实验台验证最优风嘴泄压孔结构设计的合理性。结果 实验研究结果表明,通过增设相对最优风嘴泄压孔结构,基材表面涡流产生区域平均风速提高了3.32 m/s,均方差降低了0.86;平均温度提高了24.76 K,均方差降低了5.96,表明设计有泄压孔结构的风嘴能明显改善由于涡流堆积造成的风速和温度较低的现象。结论 设计泄压孔结构的风嘴在两狭缝之间区域的风速和温度均有明显提升,涡流区域的压力产生明显下降,改善了风嘴两狭缝之间由漩涡引起的风速、温度较低现象。     

Thermodynamic modelling and parameter determination of ejector for ejection refrigeration systems

This paper presents a detailed thermodynamic modelling method of an ejector for ejection refrigeration system. In this model, the primary flow in the ejector was assumed to fan out from the nozzle without mixing with the secondary flow in a certain downstream distance, so that a hypothetical throat was formed where the secondary flow reached the sound speed. However, the area of this hypothetical throat remained unknown. Therefore, based on several sets of experimental results, the present study developed empirical correlations of the hypothetical throat area to aid further modelling. The ratio of the hypothetical throat area to the mixing area was correlated with two dimensionless variables: one was the ratio of nozzle throat area to the mixing area, and the other one was the primary and secondary flow pressure ratio. The model has been validated by the measured primary mass flow rates and the critical back pressures.     

Machining and surface finishing of brittle solids

Ceramic materials are finished primarily by abrasive machining processes such as grinding, lapping, and polishing. In grinding, the abrasives typically are bonded in a grinding wheel and brought into contact with the ceramic surface at relatively high sliding speeds. In lapping and polishing, the ceramic is pressed against a polishing block with the abrasives suspended in between them in the form of a slurry. The material removal process here resembles three-body wear. In all these processes, the mechanical action of the abrasive can be thought of as the repeated application of relatively sharp sliding indenters to the ceramic surface. Under these conditions, a small number of mechanisms dominate the material removal process. These are brittle fracture due to crack systems oriented both parallel (lateral) and perpendicular (radial/median) to the free surface, ductile cutting with the formation of thin ribbon-like chips, and chemically assisted wear in the presence of a reactant that is enhanced by the mechanical action (tribochemical reaction). The relative role of each of these mechanisms in a particular finishing process can be related to the load applied to an abrasive particle, the sliding speed of the particle, and the presence of a chemical reactant. These wear mechanisms also cause damage to the near ceramic surface in the form of microcracking, residual stress, plastic deformation, and surface roughness which together determine the strength and performance of the finished component. A complete understanding of the wear mechanisms leading to material removal would allow for the design of efficient machining processes for producing ceramic surfaces of high quality. The research was supported in part by the National Science Foundation through grants MSS 9057082, Jorn Larsen-Basse, Program Director and DDM 9057916, Bruce Kramer, Program Director.     

纤维增强复合材料磨削制孔加工技术研究进展

纤维增强复合材料具有优良的物理、化学和力学性能,在航空航天、汽车、新能源等高新技术领域应用广泛。相比传统钻铣刀具,磨料工具在纤维增强复合材料制孔时,加工后的分层、毛刺、撕裂及热损伤等缺陷更小,且磨料工具可以稳定加工硬度更高的纤维增强陶瓷基复合材料。首先,综述了纤维增强复合材料在磨削制孔过程中的切屑形成、磨削轴向力、磨削温度等磨削加工机制;其次,探讨了近年来国内外在纤维增强复合材料磨削制孔技术中的制孔加工缺陷及其评价方法;然后,分析了纤维增强复合材料磨削制孔质量及其影响因素;此外,综述了纤维增强复合材料磨削制孔刀具及其磨损机制等方面的研究现状;最后,对纤维增强复合材料磨削制孔加工技术研究进行了总结和展望。      

音速喷嘴一元流动模型详解及其激波计算

针对文丘里音速喷嘴,阐述了如临界背压比等概念,指出了ISO 9300中背压比定义存在容易造成歧义的缺陷。然后基于一元等熵流动理论,从数学上证明了：当文丘里喷嘴喉部压力与上游滞止压力之比达到临界压比时,喉部产生音速,通过喷嘴的质量流量达到最大值;推导了实际条件下喷嘴的流量公式,导出的流量公式相较于ISO 9300给出的相应公式,增加了喉部状态参数下的压缩性系数修正项■。最后从气体动力学基本方程出发,讨论了在较大背压比范围内,喷嘴扩散段中产生激波的机理,给出了激波产生的位置、激波前、后的压力和马赫数的一元流动计算模型,并运用数值模拟方法对计算结果进行了验证,同时还与Craig A的实验数据作了对比。对最小出口压比对比的结果显示,一元流动模型与实验数据的最大误差≤17%。     

Influence of minimum quantity lubrication parameters on grind-hardening process

Currently, dry grind-hardening has been studied to achieve a deep grinding harden layer. For dry grind-hardening process, the ground surface may have a poor surface quality. In our previous research, minimum quantity lubrication (MQL) has been proved as an effective way to improve the surface quality in grind-hardening process. Unfortunately, the relationship between the surface quality and parameters of MQL is still not clear. In this paper, different spraying parameters (fluid flow, air pressure, nozzle location, spraying distance) are investigated in surface grinding experiments of annealed steel AISI 5140. Grind-hardening performances are analyzed for grinding force and surface quality, such as surface roughness, micro-hardness, and grinding harden layer depth. Consequently, the results show that MQL can improve the grinding surface roughness and the micro-hardness of the grinding harden layer. The surface roughness can be improved with a better lubrication, which can be achieved with the appropriate spray direction. The surface roughness and the harden layer depth are reduced with the increasing fluid flow rate and air pressure, while they increase with the spraying distance.     

Research on the Mechanism of Abrasive Particles Accelerated and a Cutting System of a Premixed Abrasive Jet

Forces acting on abrasive in the process of speeding up have been analyzed. Motion differential equation of abrasive in a pipeline and nozzle has been given,respectively. Mechanisms of abrasive particles accelerated in a premixed abrasive jet has been analyzed. The study shows that driven by high-pressure water,velocity of an abrasive is near to velocity of water in pipeline through the acceleration distance. In the taper section of a nozzle,water and abrasive particles are greatly accelerated at the same time. But velocity of an abrasive always lags behind velocity of water. A premixed abrasive jet cutting system has been introduced. The structure and working principles of the system have been given. The system is an assembly of abrasive screening and filling. By use of the premixed abrasive jet cutting system established,cutting experiments have been made to test the main parameters which influence the cutting performances such as working pressure,standoff and traverse velocity,and the nozzle diameter affecting cutting chink width.     

Effect of water supply pressure on atomization characteristics and dust-reduction efficiency of internal mixing air atomizing nozzle

As a new type of nozzle, the internal mixing air atomizing nozzle has been widely used in the field of dust reduction via spraying. In this study, the effect of water supply pressure on the atomization characteristics and dust-reduction efficiency of the internal mixing air atomizing nozzle was investigated. Firstly, the FLUENT software was used to simulate the flow field inside and outside the nozzle under different water supply pressures. The numerical simulation results showed that as the water supply pressure increased, the internal pressure and water flow velocity in the mixing chamber of the nozzle increased while the air flow rate decreased sharply, resulting the continuous decrease in the relative velocity between gas and liquid. Meanwhile, as the water supply pressure increased, the fragmentation scale of the liquid jet at the outlet of the nozzle was prolonged and the atomization of the liquid was limited. Secondly, based on the custom-developed dust reduction experimental system via spraying, the atomization characteristics of the nozzle were investigated. According to the experimental results, when the water supply pressure increased, the water flow rate and air flow rate of the nozzle had exponential increase and decrease, respectively. As the water supply pressure increased, the range, droplet volume fraction, droplet size, and velocity all increased, while the atomization angle first increased and then decreased. Finally, the dust reduction experiment via spraying was performed under different water supply pressures. The results showed that with the increase of water supply pressure, the dust-reduction efficiency for both the total dust and the respirable dust first increased and then decreased.     

微磨料浆体射流形成过程的能量传输

为了研究微磨料浆体射流形成的能量传输与转换,用碳化硅磨料对40CrMnMo7钢进行冲击,建立数学模型描述冲击力与射流压力、喷嘴效率、射流流量、水力功率等参数的相互影响,通过测量射冲击力、射流流量来预测射流系统的单位水力功率的出力状况。结果表明,该数学模型可以预测射流冲击力与水力功率之比,由系统压力模型计算的冲击力比实验结果小20%;在射流的滞止区内,压力与冲击力不呈线性关系,在相同压力下,磨料粒子具有更大的动能,微磨料浆体射流冲击力比纯水射流的大20%,单位水力功率提供的射流冲击力随着射流压力的增大而减小;射流冲击力随着喷嘴直径、射流压力的增大而增大,在靶距较小的情况下,冲击力随着靶距的增大而增大,当靶距达到一定值时,冲击力随着靶距的增大而减小。