钨合金的磨削加工去除机理

目的 解决钨合金磨削加工去除机理不明晰的问题。方法 基于单磨粒刻划有限元仿真、单磨粒刻划和磨削加工实验，探究钨合金的磨削加工去除机理。结果 在刻划过程中，划痕的不同位置材料的去除特性存在显著差异。在单颗粒刻划切入端，材料依次发生了塑性变形、隆起、微裂纹，再到钨相与黏结相的混杂交融。在划痕中段以材料去除为主，出现了材料微卷起和材料卷起现象，沿着刻划方向卷起现象越来越严重。在划痕切出端，划痕边缘和尾部均出现了“飞边”现象，且相较于切入端，切出端的形貌较差，实验与仿真吻合。此外，在不同相位处，材料的去除特性也存在一定不同。在钨相区域，同时存在脆性特征和塑性特征。在黏结相区域，刻划深度较浅时主要呈现塑性变形、塑性流动等特征，刻划中端深度较大时主要呈现与钨相的混杂和交融。在钨相与黏结相的相界处，相邻钨颗粒呈现不同的损伤或去除特征，且相界会阻断特征形貌的传递。最后，磨削后的钨合金表面存在单颗粒刻划痕上出现的所有去除特征，与单颗粒划痕的去除特征吻合。不同的是，磨削后划痕底部出现了区域性和放射状的裂纹。结论 钨合金的两相特性使得磨削表面的去除特征较复杂，存在塑性变形、微裂纹、微卷起、卷起、裂纹和两相交...     

Fundamental investigation on nano-precision surface finishing of electroless Ni–P-plated STAVAX steel using magnetic compound fluid slurry

Electroless nickel–phosphorus (Ni–P) plating used in a range of hot embossing metal molds/dies and injection metal molds/dies must be manufactured to nano-precision roughness for proper operation of the molds/dies. We therefore developed a novel polishing technique for mirror surface finishing of this kind of magnetic material using a magnetic compound fluid (MCF) slurry. The effects of the magnetic and gravitational forces acting on the carbonyl iron particles (CIPs) and abrasive particles (APs) within the MCF slurry were studied first, and the behaviors of the CIPs and APs in the presence of an external magnetic field were predicted. Then, experiments were performed to confirm the predictions by investigating the distribution of the CIPs and APs on the working surface of the MCF slurry. Finally, four MCF slurries containing CIPs and APs with different diameters were employed to finish the Ni–P-plated STAVAX steel specimen at different working gaps. The results revealed that for the magnetic workpiece, the resultant vertical force attracted CIPs towards the work surface, whereas APs were pushed away from the work surface. However, the CIPs and APs showed opposite behaviors with the non-magnetic workpiece. The percentage of APs distributed on the working surface increased and the distribution became more even as either the diameter of the CIPs or the working gap increased, whereas that of CIPs had the opposite tendency. The MCF slurry containing bigger CIPs and smaller APs should be employed and the working gap should be set at a smaller value in order to perform mirror surface finishing of a magnetic Ni–P-plated surface. Under the experimental conditions in this work, the Ni–P-plated surface quality improved significantly, and a mirror surface roughness (Ra) of 4 nm was successfully achieved without leaving scratches or particle adhesion when using an MCF slurry containing CIPs 7 μm in diameter and APs 1 μm in diameter, showing that MCF slurries containing commercial CIPs are applicable to the nano-precision finishing of magnetic materials.     

Pose and trajectory control of shield tunneling machine in complicated stratum

The present study is focused on developing a method for controlling the pose and trajectory of a shield tunneling machine (STM) applied in complicated stratum. Lacking method to determine target motions of thrust cylinders and suitable electro-hydraulic control system are major restrictions for the STM to realize automatic pose control. To overcome these bottlenecks, a mathematical method for determining the target motions of thrust cylinders is proposed based on kinematic analysis of the thrust mechanism. With this method, target motions of thrust cylinders when the STM excavates along any specific curves can be obtained and used as the input signal of the pose control system. A multi-cylinder control system is proposed based on master/slave control strategy to control the length of each kinematic chain in order to adjust the pose of the thrust mechanism. Experiments are carried out to evaluate the performances of this control system. The experimental results verify that the proposed pose control system is effective in controlling the pose and trajectory of the shield machine no matter it advances along a straight or a curved tunnel axis. Considering the complex loads during the experiments, the proposed system has great potential for applying in practical tunnel construction.     

碳纤维增强树脂复合材料齿槽加工中分层缺陷的形成机制

针对碳纤维增强树脂复合材料(Carbon fiber-reinforced plastic,CFRP)齿槽加工中易产生分层缺陷的问题,尤其以切出侧最严重。根据齿槽加工中分层缺陷的形成过程,重点构建CFRP齿槽加工切出侧最表层材料分层形成的临界切削力模型,并以平纹编织CFRP为研究对象,采用T型铣刀进行试验,从力学角度揭示齿槽加工中分层缺陷的形成机制。结果表明:齿槽上下两侧的经向纤维分层缺陷较小;A情形(有支撑)、B情形(无支撑)下纬向纤维分层缺陷产生的临界切削力均较小,因此,齿槽加工中分层缺陷均易产生,尤其是B情形;A、B两种情形下单束纤维临界力均在边缘附近呈较大值,中间部位临界力变化较小及纤维的变形均以中间部分较大,边缘部位变形小,由此导致A、B两种情形下的分层缺陷均以单束纤维为边界形成半月弧形“波峰”;随着每齿进给量f_z和切削速度V_c的增大,两种情形下的分层缺陷均呈增大趋势。试验结果与临界力理论推导结果基本一致。      

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture,chip formation is a severe plastic deformation process including large strain,high strain rate and high temperature.And the strain rate in high speed cutting process can be achieved to 105 s~(-1).30CrMnSiNi2Asteel is a kind of important high-strength low-alloy structural steel with wide application range.Obtaining the dynamic mechanical properties of30CrMnSiNi2Aunder the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining.The dynamic compressive mechanical properties of30CrMnSiNi2Asteel were studied using split Hopkinson pressure bar(SHPB)tests at 30-700°C and3000-10000s~(-1).The stress-strain curves of 30CrMnSiNi2Asteel at different temperatures and strain rates were investigated,and the strain hardening effect and temperature effect were discussed.Experimental results show that 30CrMnSiNi2Ahas obvious temperature sensitivity at 300°C.Moreover,the flow stress decreased significantly with the increase of temperature.The strain hardening effect of the material at high strain rate is not significant with the increase of strain.The strain rate hardening effect is obvious with increasing the temperature.According to the experimental results,the established Johnson-Cook(J-C)constitutive model of 30CrMnSiNi2Asteel could be used at high strain rate and high temperature.     

Manufacture of a new kind diamond grinding wheel with Al-based bonding agent

A new kind diamond grinding wheel with Al-based bonding agent was prepared in this paper. The influence of sintering temperature to the relative density (R.D.), hardness and service life of diamond grinding wheels with AlSnTi, AlSnTiNiCo, AlSnTiNi and AlSnNiCo bonding agent was studied. The microstructure of different bonding agent sintered at different temperature was observed. The service life of the Al-based grinding wheels was compared with Cu-based or resin-based ones. The results showed that the AlSnTiNiCo is the best composition system in this research. The best sintering temperature is 300 °C. The sample has a high relative density after sintered at 300 °C. The retention of Al-based bonding agent to diamond grit is strong. The service life of this Al-based diamond grinding wheel is about three times as long as that of resin-bonded grinding wheel.     

Precision internal grinding of bearing steel based on the state control of oxide layer with electrolytic in-process dressing

The oxide layer state directly relates to the grinding efficiency and machining quality in electrolytic in-process (ELID) grinding. However, no proper and detailed information has been found about internal grinding by controlling the state of the oxide layer. Therefore, in this investigation, some experiments have been conducted based on the active control of the oxide layer state to study internal grinding performance and ground surface qualities. The experimental comparison with the traditional ELID grinding has been carried out to prove the advantages of the proposed method in dynamic equilibrium. The influences of the grinding and electrical parameters on the workpiece surface quality, force characteristics and material removal rate of internal machining of bearing steel have been investigated in detail. The experimental results show that the new method can realize a stable internal grinding process even with finer abrasive wheels of W10 and W2.5.     

Kevlar纤维增强复合材料激光-铣削组合加工试验及可行性分析

Kevlar增强复合材料(Kevlar fiber-reinforced plastic,KFRP)在武器装甲防护领域受到了强烈的关注和广泛的应用。在机械加工中极易产生拉毛、难以排屑、刀具磨损严重等问题,在激光加工中依然存在诸多难点。为同时规避KFRP机械加工缺陷和激光加工的短板问题,本文提出KFRP激光-铣削组合加工,分析KFRP激光-铣削组合加工工艺的可行性。试验表明,KFRP激光加工中存在明显的纤维炭化区、树脂熔融区,其中,在树脂熔融区,纤维基本未受损。激光加工的最大切缝宽度在0.5 mm以下,深度不超过2 mm,激光加工工艺参数对切缝宽度a的影响相对较小,但对切缝深度h、纤维炭化区宽度A、树脂熔融区宽度A_m影响显著。激光-铣削组合加工中,切屑呈大块状和絮状,当纤维烧蚀不完全时,容易出现抽丝现象。KFRP激光-铣削组合加工,不仅能有效的避免纤维拉毛现象,还能大幅度降低切削力和切削温度,从而提高加工质量,降低加工难度。      

Cutting composites: A discussion on mechanics modelling

Using the cutting of long fibre reinforced polymer and ceramic particle reinforced aluminium as the examples, this paper tends to understand some common features in the mechanics modelling of machining composites. It demonstrates that an accurate characterisation of matrix deformation, matrix–reinforcement interaction, and reinforcement deformation are key factors for the establishment of a model to reflect the principal material removal mechanisms. A precise understanding of these factors can be achieved through a logic process of mechanism exploration, model derivation and verification.     

Magnetorheological finishing process for hard materials using sintered iron-CNT compound abrasives

The use of magnetorheological fluids for finishing is one of the most promising smart processes for the fabrication of ultra-fine surfaces, particularly three-dimensional millimeter or micrometer structures. This process is not readily applicable to hard-surface materials, like an Al₂O₃–TiC hard disk slider, if a conventional rotating tool is used. This is due to the rotational speed and the resulting actual impressed abrasion energy limits, and the consequent low efficiency of the material removal rate. In this study, the main mechanism responsible for the decrease of the material removal rate on hard materials for a wheel-type magnetorheological finishing process is examined, both theoretically and experimentally, and a solution to this problem is devised via two approaches. The first uses a rectilinear alternating motion to improve processing conditions, and the second focuses on the use of more effective abrasives, namely magnetizable abrasives made of iron powders sintered with carbon nanotubes, which are new abrasives that have not yet been introduced in the field of surface finishing. Furthermore, it is shown that these abrasives increase the lifetime of consumables (magnetorheological fluid and abrasives) and the material removal rate.