The surface integrity of machined metal components is critical to their in-service functionality, longevity and overall performance. Surface defects induced by machining operations vary from the nano to macro scale, which cause microstructural, mechanical and chemical effects. Hence, they require advanced evaluation and post processing techniques. While surface integrity varies significantly across the range of machining processes, this paper explores the state-of-the-art of surface integrity research with an emphasis on their governing mechanisms and emerging evaluation approaches. In this review, removal mechanisms are grouped by their primary energy transfer mechanisms; mechanical, thermal and chemical based. Accordingly, the resultant multi-scale phenomena associated with metal machining are analyzed. The contribution of these material removal mechanisms to the workpiece surfaces/subsurface characteristics is reviewed. Post-processing options for the mitigation of induced surface defects are also discussed.

Surface integrity in metal machining - Part I: Fundamentals of surface characteristics and formation mechanisms

Machine vision significantly improves the efficiency, quality, and reliability of defect detection. In visual inspection, excellent optical illumination platforms and suitable image acquisition hardware are the prerequisites for obtaining high-quality images. Image processing and analysis are key technologies in obtaining defect information, while deep learning is significantly impacting the field of image analysis. In this study, a brief history and the state of the art in optical illumination, image acquisition, image processing, and image analysis in the field of visual inspection are systematically discussed. The latest developments in industrial defect detection based on machine vision are introduced. In the further development of the field of visual inspection, the application of deep learning will play an increasingly important role. Thus, a detailed description of the application of deep learning in defect classification, localization and segmentation follows the discussion of traditional defect detection algorithms. Finally, future prospects for the development of visual inspection technology are explored.

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State of the Art in Defect Detection Based on Machine Vision

Atomic force microscopy (AFM) tip-based nanofabrication is as a powerful method to machine nanostructures. However, the traditional AFM scratching process suffers from low processing efficiency and a low rate of material removal. Thus, an AFM tip-based nanomilling method was previously proposed to improve the machining efficiency and material removal rate; however, the machining mechanism of the nanomilling process, especially on hard-brittle silicon crystals, is not well understood. In this study, we used an AFM tip-based nanomilling approach to machine nanochannels on single-crystal silicon to investigate the machining mechanism (i.e., the material removal state, undeformed chip thickness, the brittle-to-ductile transition, and subsurface damage). For the first time, we established a theoretical model for tip-based nanomilling with a constant normal load to predict the machined depth. Nanochannels fabricated with a wide range of feed directions, crystal orientations, and tip trajectories were obtained, which provided a reference for machining high-quality nanochannels and an in-depth understanding of the nanomilling of single-crystal silicon. The experimental results demonstrated that we could machine a nanochannel without pile-ups by selecting a specific trajectory with a feed smaller than 4 nm in the 0° direction, a normal load no greater than 120 μN, and a crystal orientation at rotation angles of 135°, 157.5°, or 180°. Moreover, transmission electron microscope analysis of the subsurface of the nanochannel revealed a large number of dislocations, stacking faults, and a layer of amorphous silicon. Our findings are of great significance for obtaining high-quality nanochannels with a predicable machined depth and understanding the nanomilling mechanism of hard-brittle materials.

Towards understanding the machining mechanism of the atomic force microscopy tip-based nanomilling process

Soft-brittle crystals, e.g. KH2PO4 (KDP), are difficult-to-cut due to their high brittleness which can easily generate crack during the machining process. The conventional method to machine this kind of material is by inducing ductile cutting mechanism at room temperature with ultra-precision machining, which can only remove materials at nanoscale level and hence yields very low material removal rate. While some thermal-assisted processes have been recently attempted to improve the machinability of some difficult-to-cut materials, e.g. ceramics, there is no systematic understanding of the temperature effect on material removal mechanism of soft-brittle KDP crystals yet. In this work, the temperature effect on the material removal mechanism has been investigated for the first time using nano-scratch technique. While a decreased hardness and elastic modulus have been observed with the increase of temperature, an increase of fracture toughness has been revealed with a contradictory tendency, indicating a higher capacity of plastic deformation at elevated temperature. In contrast to the almost totally brittle scratch at room temperature (RT) caused by crack propagation and edge chipping, the scratch at 170 °C can achieve more ductile-regime surfaces with a larger critical undeformed cutting depth (3.61 μm), e.g. a significant increase of 8.60 times compared with that at RT (0.42 μm). Moreover, the TEM analysis on the subsurface microstructures shows that a great number of nano grits was generated in the subsurface at RT as the result of crack propagation and interaction, while at elevated temperature some crystallographic lattice misaligned structures (LMS) and nano crystals have been brought about due to the nucleation and evolution of thermal-activated dislocations, which explains the higher plasticity of KDP at elevated temperature. The results present in this paper are of great significance for understanding the specific temperature effect on the brittle-to-ductile transition of the cutting mechanism for future designing thermal-involved processes to machine soft-brittle materials.

Temperature effect on the material removal mechanism of soft-brittle crystals at nano/micron scale

Thin structures are often required for several engineering applications. Although thick sections are relatively easy to produce, the cutting of thin sections poses greater challenges, particularly in the case of thermal machining processes. The level of difficulty is increased if the thin sections are of larger lengths and heights. In this study, high-aspect-ratio thin structures of micrometer thickness (117-500 µm) were fabricated from D2 steel through wire electrical discharge machining. Machining conditions were kept constant, whereas the structure (fins) sizes were varied in terms of fin thickness (FT), fin height (FH), and fin length (FL). The effects of variation in FT, FH, and FL were assessed over the machining errors (FT and FL errors) and structure formation and its quality. Experiments were conducted in a phased manner (four phases) to determine the minimum possible FT and maximum possible FL that could be achieved without compromising the shape of the structure (straight and uniform cross-section). Thin structures of smaller lengths (1-2 mm long) can be fabricated easily, but, as the length exceeds 2 mm, the structure formation loses its shape integrity and the structure becomes broken, deflected, or deflected and merged at the apex point of the fins.

https://www.mdpi.com/2072-666X/12/1/1/pdf

High Aspect Ratio Thin-Walled Structures in D2 Steel through Wire Electric Discharge Machining (EDM).

Reaction-bonded silicon carbide (RB-SiC) ceramic, one of the best candidates for large optical mirrors, is difficult to machine because of its high hardness and brittleness. A hybrid process called electrical discharge diamond grinding (EDDG) exhibits potential for improving the machinability of RB-SiC by combining electrical discharge machining (EDM) and diamond grinding. However, this hybrid process leads to damages that differ from those in conventional processes owing to the simultaneous actions of EDM and diamond grinding. In the present study, surface and subsurface damages induced by the interactions between EDM and diamond grinding during the EDDG of RB-SiC were examined. The effect of the discharge energy was considered. The surface and subsurface topographies and microstructures were characterized via scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy. The EDM and grinding zones exhibited distinctive surface topographies and different dominant material removal mechanisms. An increase in the discharge energy facilitated ductile removal of the material and decomposition of SiC. Thus, a thinner subsurface damage layer was obtained compared with that in the less-thermally affected zone. The decomposed C and material migration tended to increase with the discharge energy. Owing to the interactions between EDM and diamond grinding, the subsurface was a mixture of amorphous/crystalline C, polycrystalline/nanocrystalline SiC, and a crystalline SiC matrix.

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Surface and subsurface damage of reaction-bonded silicon carbide induced by electrical discharge diamond grinding

In this paper, the hydrophobic polymer surface with regular and controllable micro-textured structures was effectively and rapidly fabricated by micro-injection molding using a novel micro-textured...

Precise WEDM of micro-textured mould for micro-injection molding of hydrophobic polymer surface

Precise micro-grinding machining was proposed to fabricate regular and controllable micro-grooved array structures on the surface of mold cores to realize the mass production and manufacturing of micro-structured polymer components by micro injection molding in this paper. First, the 3D topographies and section profiles of micro-ground mold cores and micro-formed polymers with different micro-structure parameters were presented. Then, the surface roughness of mold cores and polymers were compared. Next, the relationships between machining accuracy of mold core ground by micro-grinding and filling rates of micro-structured polymer formed by micro injection molding were investigated. Finally, the influences of micro injection molding parameters on the filling rate of micro-structures polymer were investigated. It is shown that the micro-structured polymer can be effectively and rapidly fabricated using the proposed method. The experimental results indicate the highest form accuracy of the micro-grooved mold core and the filling rate of micro-structured polymer can reach to 4.05 µm and 99.30%, respectively. It is found that the filling rate of the micro-structured polymer roughly increased with increasing machining accuracy of the mold core. The injection pressure had the greatest influence on the filling rate of the injection formed polymer, while the melt temperature had the least influence.

https://www.mdpi.com/2072-666X/10/4/253/pdf

Fabrication of Micro-Structured Polymer by Micro Injection Molding Based on Precise Micro-Ground Mold Core.

In this paper, micro-structured polymer parts were efficiently and accurately fabricated by micro-injection molding using a micro-structured mold core machined by wire electrical discharge machining (WEDM). The objective was to realize low-cost mass production and manufacturing of micro-structured polymer products. The regular micro-structured mold core was manufactured by precise WEDM. The micro-structured polymer workpieces were rapidly fabricated by micro-injection molding and the effects of the micro-injection molding process parameters on replication rate and surface roughness of micro-structured polymers were systematically investigated and analyzed. It is shown that the micro-structured polymer can be rapidly and precisely fabricated by the proposed method. The experimental results show the minimum size machining error of the micro-structured mold core and the maximum replication rate of micro-formed polymer were 0.394% and 99.12%, respectively. Meanwhile, the optimal micro-injection molding parameters, namely, jet temperature, melt temperature, injection velocity, holding pressure and holding time were 195 °C, 210 °C, 40 mm/min, 7 Mpa and 5 s, respectively. The surface roughness Ra at the groove bottom and top of the micro-structured polymer workpieces achieved minimum values of 0.805 µm and 0.972 µm, respectively.

https://www.mdpi.com/2073-4360/11/10/1591/pdf

Efficient and Precise Micro-Injection Molding of Micro-Structured Polymer Parts Using Micro-Machined Mold Core by WEDM.

The invention provides an in-site electro-discharge repairing device and a method for a milling head. The in-situ electro-discharge repairing device comprises a first driving assembly, the milling head and a working table, wherein the milling head is driven by the first driving assembly to rotate, the working table is used for bearing a workpiece to be machined, the working table is provided withan electro-discharge repairing structure, and the electro-discharge repairing structure comprises a repairing wheel, a second driving assembly used for driving the repairing wheel, and a pulse power supply used for discharging and repairing, and a first pulse power supply is electrically connected with the repairing wheel and the milling head correspondingly. According to the in-site electro-discharge repairing device and the method for the milling head, the repairing wheel and the first pulse power supply are arranged, so that the repairing wheel and the milling head are close to discharge, when the milling head is abraded, the discharging and the repairing can be directly carried out on the position where the milling head is located, and dismounting and transposition repairing are not needed, so that a plurality of installation errors are avoided, and the repairing precision and the repairing efficiency of the milling head are improved.

Chen Fumin

Papers

Surface and subsurface damage of reaction-bonded silicon carbide induced by electrical discharge diamond grinding

Precise WEDM of micro-textured mould for micro-injection molding of hydrophobic polymer surface

Fabrication of Micro-Structured Polymer by Micro Injection Molding Based on Precise Micro-Ground Mold Core.

Efficient and Precise Micro-Injection Molding of Micro-Structured Polymer Parts Using Micro-Machined Mold Core by WEDM.

In-site electro-discharge finishing device and method for milling head