The new challenges of machining Ceramic Matrix Composites (CMCs): review of surface integrity

Ceramic Matrix Composites (CMCs) are currently an increasing material choice for several high value and safety-critical components, fact that has recently originated the need of understanding the effect of several machining processes. Due to the complex nature of CMCs - i.e. heterogeneous structure, anisotropic thermal and mechanical behaviour and generally the hard nature of at least one of the constituents (e.g. fibre or matrix) - machining become extremely challenging as the process can yield high mechanical and thermal loads. Furthermore, the orthotropic, brittle and heterogeneous nature of CMCs result in different material removal mechanisms which lead to unique surface defects. Hence, this review paper attempts to provide an informative literature survey of the research done in the field of conventional and non-conventional machining of CMCs with a main focus on critically evaluate how different machining techniques affect the machined surfaces. This is achieved by exploring and recollecting the different material characterisation techniques currently used to observe and quantify the mechanical and thermal surface and subsurface damages and highlight their governing removal mechanisms

A quick method for evaluating the thresholds of workpiece surface damage in machining

© 2019 This paper proposes a Pendulum-Based Cutting Test (PBCT) methodology which allows quick cutting tests for surface integrity evaluation along with providing cutting energies associated with particular level of workpiece surface damage, this is backed by an unified cutting energy model that links damage level of machined surface with energy partition in the cutting area. PBCT method could rapidly define the energy transferred to the workpiece that incurs particular magnitude of surface damage without using conventional machine tools and monitor the cutting process while only limited amount of materials is required. A demonstration of the proposed method is presented for Inconel718

On modelling of cutting force and temperature in bone milling

© 2018 Elsevier B.V. Cutting force and temperature are the key factors to be controlled during the orthopaedic surgery which could result in mechanical damage and necrosis of the bone tissue. Mechanistic modelling of the bone cutting process is expected to be an efficient method to understand and control these process challenges. However, due to the special structure and properties of the bone tissue (consist of osteon fibres and interstitial lamellae matrix), the conventional metal cutting models are not applicable in bone cutting process. This paper presents a novel cutting force and temperature mechanistic models for milling of bone. A cutting stress model of bone material was developed which takes into account its anisotropic characteristics based on the orthogonal cutting data. The cutting force coefficients are predicted incorporating the osteon orientation, tool geometry and edge effect with unified mechanics of cutting approach. Furthermore, a model of the induced cutting temperature based on heat flux developed during the process was proposed to predict the temperature distribution on bone cut surface. The experimental results showed a better consistency with the proposed model compared with the conventional Johnson-Cook model under different cutting conditions. A necrosis (potential cell injury from thermal effect) penetration depth was also proposed to evaluate the extent of thermal damage of bone tissue by the developed models. The proposed model can be used to assist the robotic surgery, to optimize the cutting parameters as well as to guide the orthopaedic tool design

Predictive model of the surface topography for compliant grinding of brittle materials

During uneven and time-dependent compliant grinding of brittle materials, the surface topography is difficult to predict as ductile and brittle regions are coupled due to compliance occurring in macro/micro tool-workpiece contact. This paper proposed a predictive model for surface topography prediction by considering its ductile-brittle transition in compliant grinding. Shape adaptive grinding and monocrystalline silicon were chosen as an example to validate the proposed model based on progressive grinding tests (spot, line, area). Feed-Spindle Projection Angles are further investigated, revealing that 0° angle can obtain a ground surface with lower area roughness and smaller brittle fragments than 45° and 90

Incident laser modulation by tool marks on micro-milled KDP crystal surface: Numerical simulation and experimental verification

© 2019 Elsevier Ltd Micro-milling has been accepted as the most promising method to repair the micro-defects on the surface of KH2PO4 (KDP) optics. However, surface tool marks are inevitably introduced during the micro-milling repairing process, and could possess great potential risks in lowering the laser-induced damage threshold of KDP optics. The primary cause of laser damage growth of nonlinear crystals has been considered as its internal light intensification. In this work, how the tool marks impact the incident laser modulation as well as the laser-induced damage resistance of micro-milled KDP optics was theoretically and experimentally investigated. The results indicate that periodic tool marks can cause diffraction effect and result in significant relative light intensity modulation (IRmax), up to 5.6 times higher than that inside smooth crystal surfaces. Although the change trends of IRmax with respect to tool marks on both surfaces of KDP optics are similar, the IRmax induced by the rear-surface tool marks is nearly twice higher than that induced by the front-surface tool marks, which means the rear surface with tool marks are more vulnerable to be damaged. The period of tool marks determines the modulation degree and distribution patterns of light intensity inside KDP crystal while the residual height of tool marks can only slightly regulate the modulation degree of light intensity. The tool marks with a period of 1 μm normally give rise to serious light intensification and should be strictly excluded, while the period of tool marks from 10 μm to 20 μm is conducive to the laser damage resistance of micro-milled KDP optics, which were verified by the tests of transmittance capacity and laser damage resistance, and is supposed to be preferred in the actual repairing process of full-aperture KDP optics

A digital approach to automatically assess the machining-induced microstructural surface integrity

© 2020 Elsevier B.V. When it comes to advanced materials for safety-critical applications, the evaluation of the machining-induced microstructural surface integrity represents a primary aspect within the assessment of part quality. Nowadays, presence and extent of machining-induced microstructural anomalies in the workpiece subsurface is manually measured by human inspection of digital micrographs. In the present work, computer-based performance of this task is achieved through a set of algorithms designed to automatically identify microstructural anomalies resulting from material removal operations. Digital surface integrity assessment has been demonstrated with application to scanning electron micrographs exhibiting different levels of microstructural deformation and obtained under different imaging conditions. Furthermore, the digitally detected material condition has been investigated with the support of in-depth field emission gun scanning electron microscopy (FEG-SEM) and electron backscatter diffraction (EBSD) analysis. This has allowed the relationship between the material evidence observed through different strategies to be established. Finally, the set of algorithms has been applied to study the microstructural condition of a large material region, by performing sequential processing of a series of micrographs. In this way, the measurement procedure has been calibrated and its capability to perform surface-integrity evaluation on large areas in an automated and standardised way has been demonstrated

What micro-mechanical testing can reveal about machining processes

For many years, the machining community has dedicated significant efforts to investigate the microscopic scale level phenomena during the material removal process. On one hand much research has been carried out in relation to workpiece surface integrity after machining and the methods for its study. On the other hand, many studies have been dedicated to replicate machining conditions at microscopic scales using high resolution setups. Although these two topics seem to be little related, there is an opportunity of the machining community to take the advantage of the advanced testing/investigation setups that enable these two strands of research to be performed at very high resolution and repeatability, thus giving new pathways for research in this field. Here we are flagging up to the community the research opportunities offered by micro-mechanical testing that can be performed using in-situ scanning electron microscopes (SEM) or other high-resolution imaging instruments. As such, this review paper discusses the recent research advances in using in-situ micro-mechanical testing for: (i) understanding the phenomena occurring in the workpiece (sub) surfaces after machining operation by performing very high resolution micro-mechanical testing (e.g. compression/bending of micro-pillars/beams) within particular zones of machined superficial layers; (ii) studying the material removal mechanisms at micrometric level using common indenters or dedicated edges to understand how the workpiece materials (e.g. groups/single grains) react to cutting conditions. Finally, we comment on possible future research topics using micro-mechanical testing in-situ in high resolution imaging instruments and how this could help to advance the understanding of machining processes

The Effect of Laser Ablation Pulse Width and Feed Speed on Necrosis and Surface Damage of Cortical Bone

Bone cutting is of importance in orthopaedic surgery but is also challenging due to its nature of brittleness—where severe mechanical and thermal damages can be introduced easily in conventional machining. Laser machining is a new technology that can allow for complex cut geometries whilst minimising surface defects i.e., smearing, which occur in mechanical methods. However, comparative studies on the influence of lasers with different pulse characteristics on necrotic damage and surface integrity have not been reported yet. This paper for the first time investigates the effects of laser type on the necrotic damage and surface integrity in fresh bovine cortical bone after ex-situ laser machining. Three lasers of different pulse widths, i.e., picosecond, nanosecond and continuous wave lasers have been investigated with different feed speeds tested to study the machining efficiency. The cutting temperature, and geometrical outputs have been measured to investigate the thermal influence on the cooling behaviour of the bone samples while high-speed imaging was used to compare the material removal mechanisms between a pulsed and continuous wave laser. Furthermore, an in-depth histological analysis of the subsurface has revealed that the nanosecond laser caused the largest necrotic depth, owing to the high pulse frequency limiting the dissipation of heat. It has also been observed that surface cracks positioned perpendicular to the trench direction were produced after machining by the picosecond laser, indicative of the photomechanical effect induced by plasma explosions. Therefore, the choice of laser type (i.e., in terms of its pulse width and frequency) needs to be critically considered for appropriate application during laser osteotomy with minimum damage and improved healing

On the importance of interface stability in cellular automata models: Planar and dendritic solidification in laser melted YSZ

Laser-processing technologies are often applied to enhance the properties of ceramics, such as laser glazing of Yttria Stabilised Zirconia (YSZ). However, very limited attention was paid to the solidification phenomena and mechanisms of YSZ. In this paper, two coexisting solidification behaviours of laser-melted YSZ have been identified, namely grain bending (crystals with curved grain boundary geometry under the surface) and grain surface sealing (in-plane surficial crystals cover vertical columnar grains). Although these phenomena have been reported, this is the first time the two phenomena coexist in the same material. A new cellular automata (CA) approach has been proposed to explain the formation mechanisms of these two phenomena. This new CA method consists of the separation of growth modes into dendritic and planar growth, whose critical transition value is calculated based on the supercooling theory. Besides, the proposed model for planar growth is far less computationally expensive than the widely used decentred octahedron algorithm. A good agreement with the EBSD data of longitudinal cross-sections and the top surface has been observed which proves that the proposed method can become a more realistic and efficient way to predict the grain microstructure in laser processing, allowing to capture dendritic and planar growth simultaneously