Molecular Dynamics Simulation of Micro Mechanisms in Slip Deformation Theory of Crystals

Based on the discussion that there should be a micromechanism that causes a macroscopic slip of mono-crystal copper, molecular dynamics simulations with the analytical displacement feld around a crack tip have been carried out. The result of the simulation shows that macroscopic shear slip in an f.c.c. mono-crystal copper occurs as discrete time events. This is because cross-slips occur in many places in a material such that a macroscopic shear slip is blocked until some critical state of deformation. A macroscopic shear slip then occurs suddenly at the critical state in which the area of disordered atomic arrangement has stretched from one end of a crystal to the other end. The reason why macroscopic shear slips occur in the directions of the slip planes of a crystal is attributed to the fact that the areas of disordered atomic arrangement develop only along those directions.     

A study on the cutting mechanism of microcutting using molecular Dynamics

Ultraprecision metal cutting (UPMC) technology, which makes possible submicrometre form accuracy and nanometre roughness, is developed to reach the 1 nm nominal (undeformed) thickness of cut. At this thickness level, the finite element method (FEM) cannot be used to solve the problem. Molecular Dynamics can be applied to this small cutting depth.In this paper using molecular dynamics simulation, microcutting with a subnanometre chip thickness, the cutting mechanism for the microcutting condition, i.e. tool edge configuration, cut material and cutting speed, are evaluated.As the result, the simulation of the cutting mechanism at subnanometre depth of cut is evaluated.     

Mechanochemical effect on the microstructure and mechanical properties in ultraprecision machining of AA6061 alloy

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the microgroove surface. There is a reduction in cutting and thrust forces with the application of ink during microcutting. Moreover, the microhardness of the ink-affected microgroove is lower than that of the ink-free microgroove. Numerous substructured grains exist in the ink-affected microgroove zone whilst deformed grains dominate in the ink-free microgroove zone produced by microcutting. Furthermore, the mechanochemical effect can facilitate the nucleation of precipitates in the microgroove zone and induce the formation of subgrains with multiple orientations. According to the analysis and calculation, the main texture components of the ink-affected sample are Goss {110}001 and R {124}211, and that of the ink-free sample are Brass {110}112, Copper {112}111 and S {123}634. Besides, a clear difference of slip systems is found between the ink-free and ink-affected microgrooves, and the results show that R texture is easier to form on the ink-affected microgroove.     

Chip formation during microscale cutting of a medium carbon steel

Microscale orthogonal cutting tests were conducted on normalized AISI 1045 steel and the resulting chips examined using optical and Scanning Electron Microscopy (SEM). As the uncut chip thickness approaches the size of the smallest average grain type in the material, chip formation changes from continuous to a new type of chip called a quasi-shear-extrusion chip. Results indicate that the pearlite and softer ferrite grains play distinct roles in the plastic deformation process. A Finite Element (FE) model was developed to illustrate the behaviour of the chip formation process during microscale cutting of alternating hard and soft layers of material. The FE model is not an optimization tool, but simply an aid in understanding the mechanics of the microcutting process. The resulting chip morphology is compared to the FE model, and discussed. Stick–slip friction observed at the tool–chip interface is found to affect the transition between shearing and ploughing during the cutting process, chip curl, and the plastic deformation process throughout the chip. The ramifications of the results and model predictions are presented.     

Effect of composition and microstructure on slurry abrasion response of hardfaced martensitic stainless steel

Hardfaced martensitic stainless steel alloy was deposited on mild steel substrate by flux cored arc welding method. The slurry abrasion studies of weld-deposited hardfaced steel were performed using slurry abrasion test rig with 250–300 μm silica sand particles. The effect of weld compositional gradation on the abrasive wear resistance of hardfaced stainless steel at a distance of 0.6, 1.2, 2.4, 3.0 and 3.6 mm from the top surface was studied. The observed abrasion rates were rationalized in terms of mass loss, hardness and distance from the top surface i.e. diluted surfaces beneath the top surface. The abrasive wear mass loss increased with increasing distance beneath the top surface, which was attributed to the coarsening and morphology change in martensite phase. The results of the present work indicated change in morphology of martensite with increase in the distance beneath the top surface. The operating abrasive wear mechanisms involved ploughing, microcutting and indentation.     

Microcutting using a micro turn-milling machine

A turn-milling machine, which is a device having two spindles that rotate a tool and workpiece, can be used to fabricate various shapes by cutting. However, little progress has been made in microcutting using a turn-milling machine. Therefore, a micro turn-milling machine designed especially for microtools was developed and used to fabricate micropins with various cross sections. Cemented tungsten carbide microtools, which are useful both as a milling cutter and as a turning tool, were processed by electrical discharge machining and employed for microcutting. As a result, various micropins were successfully fabricated, including not only cylindrical pins but also those with cross sections having square, triangle, and cross shapes. These noncylindrical micropins have the smallest reported cross-section dimensions for pins fabricated by cutting to the best of our knowledge. Furthermore, cutting characteristics such as the machined shape and surface quality were investigated, which showed that better micropin shapes were obtained by up-cut turn-milling than by down-cut turn-milling or by turning. In contrast, turning was superior to turn-milling in terms of the surface quality.     

ATOMIC SCALE LEVEL CHIP FORMATION OF AMORPHOUS METAL INVESTIGATED BY USING AFM AND MD-RPFEM SIMULATION

A cutting device has been developed by utilizing the scanning system of atomic force microscope (AFM) in machining, which aims at clarifying the ultimately small size removal as well as observing them in high resolution. As an application of the proposed method, the micro-chip deformation is investigated by cutting an amorphous metal Fe₇₈B₁₃Si₉. It is shown that a lamellar structure, which is formed due to the localized shear occurred in a very narrow region and at very high strain rate, appears even at nanometer scale cutting. The simulation of this process using combined molecular dynamics (MD) and rigid-plastic finite element method (RPFEM) is also presented in this paper. The simulation shows that the localized shear would occur at the atomic scale by the cooperative group-movement of atoms. The high kinetic state of atoms would play a role of advancing this process for causing such localized shear.     

耐温耐磨有机硅基复合涂料的冲蚀磨损研究

研究了陶瓷颗粒增强体 ( Si C、Al2 O3)及不同冲蚀条件对有机硅基复合材料涂料 (以下简称有机硅复合涂料 )耐冲蚀性能的影响。发现基体组成、陶瓷颗粒种类、含量及冲蚀条件对有机硅复合涂料耐冲蚀性有很大的影响。合适的选择颗粒增强体 ,复合涂料的冲蚀率较纯有机硅涂料降低了 1～ 2个数量级。SEM冲蚀表面形貌分析表明 :低速冲蚀时涂层的磨损是因塑性变形和疲劳引起 ,而高速冲蚀时则由塑性变形、显微犁耕和显微切削造成。传统的耐温耐磨复合涂料多为纤维增强 ,因此研究开发新型颗粒增强复合涂料有较大的意义。     

Response of Ti–6Al–4V and Ti–24Al–11Nb alloys to dry sliding wear against hardened steel

Titanium alloys have been of great interest in recent years because of their very attractive combination of high strength, low density and corrosion resistance. Application of these alloys in areas where wear resistance is also of importance calls for thorough investigations of their tribological properties. In this work, Ti–6Al–4V and Ti–24Al–11Nb alloys were subjected to dry sliding wear against hardened-steel counter bodies and their tribological response was investigated. A pin-on-disc type apparatus was used with a normal load of 15–45N and sliding speed of 1.88 ms⁻¹. In the steady state, it was demonstrated that Ti–24Al–11Nb had a substantially higher wear resistance (about 48 times) than that of the Ti–6Al–4V alloy tested under a normal load of 45 N. Severe delamination is found to be responsible for the low wear resistance of Ti-6Al-4V. In the case of Ti–24Al–11Nb, two wear mechanisms have been suggested: delamination with a lower degree of severity and oxidative wear. It is thought that the ability of Ti–24Al–11Nb to form a protective oxide layer during wear results in a much lower wear rate in this alloy.