Showing papers in "International Journal of Machine Tools & Manufacture in 2020"

Review of ultrasonic vibration-assisted machining in advanced materials

Abstract: Compared to conventional machining (CM), ultrasonic vibration-assisted machining (UVAM) with high-frequency and small-amplitude has exhibited good cutting performances for advanced materials. In recent years, advances in ultrasonic generator, ultrasonic transducer, and horn structures have led to the rapid progress in the development of UVAM. Following this trend, numerous new design requirements and theoretical concepts have been proposed and studied successively, however, very few studies have been conducted from a comprehensive perspective. To address this gap in the literature and understanding the development trend of UVAM, a critical overview of UVAM is presented in this study, covering different vibration-assisted machining styles, device architectures, and theoretical analysis. This overview covers the evolution of typical hardware systems used to achieve vibratory motions from the one-dimensional UVAM to three-dimensional UVAM, the discussion of cutting characteristics with periodic separation between the tools and workpiece and the analysis of processing properties. Challenges for UVAM include ultrasonic vibration systems with high power, large amplitude, and high efficiency, as well as theoretical research on the dynamics and cutting characteristics of UVAM. Consequently, based on the current limitations and challenges, device improvement and theoretical breakthrough play a significant role in future research on UVAM.

High-performance integrated additive manufacturing with laser shock peening –induced microstructural evolution and improvement in mechanical properties of Ti6Al4V alloy components

Abstract: High-performance integrated additive manufacturing with laser shock peening (LSP), is an innovative selective laser melting (SLM) method to improve mechanical properties, and refine microstructure in the surface layer of metallic components. Phase, residual stress distribution, surface micro-hardness, tensile properties and microstructural evolution of SLMed and SLM-LSPed specimens in horizontal and vertical directions were examined. In particular, typical microstructural features in the surface layer were characterized by transmission electron microscopy (TEM) observations. Results indicated that surface micro-hardness subjected to massive LSP treatment had significantly improved, tensile residual stress was transformed into compressive residual stress by LSP-induced plastic deformation, and both SLMed specimens in two directions exhibited a good combination of the ultimate tensile strength (UTS) and ductility. Meanwhile, high-density dislocations and a large number of mechanical twins were generated in the coarse α′ martensites by laser shock wave (LSW), and gradually evolved into refined α′ martensites. Furthermore, according to the included angle between LSW and the deposited plane, two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented, and the influence mechanisms of the included angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens were revealed. The hybrid additive manufacturing technology combined SLM with LSP realizes the high-efficiency and high-quality integrated manufacturing of the formed metallic components for practical applications.

Direct Observation of Pore Formation Mechanisms during LPBF Additive Manufacturing Process and High Energy Density Laser Welding

Abstract: Laser powder bed fusion (LPBF) is a 3D printing technology that can print parts with complex geometries that are unachievable by conventional manufacturing technologies. However, pores formed during the printing process impair the mechanical performance of the printed parts, severely hindering their widespread application. Here, we report six pore formation mechanisms that were observed during the LPBF process. Our results reconfirm three pore formation mechanisms - keyhole induced pores, pore formation from feedstock powder and pore formation along the melting boundary during laser melting from vaporization of a volatile substance or an expansion of a tiny trapped gas. We also observe three new pore formation mechanisms: (1) pore trapped by surface fluctuation, (2) pore formation due to depression zone fluctuation when the depression zone is shallow and (3) pore formation from a crack. The results presented here provide direct evidence and insight into pore formation mechanisms during the LPBF process, which may guide the development of pore elimination/mitigation approaches. Since certain laser processing conditions studied here are similar to the situations in high energy density laser welding, the results presented here also have implications for laser welding.

Packing quality of powder layer during counter-rolling-type powder spreading process in additive manufacturing

Abstract: Powder spreading is an essential procedure in powder-bed-based additive manufacturing, and the resultant packing quality of the powder layer has important effects on the quality of the final products. In this work, the counter-rolling-type powder spreading is investigated by experiments and numerical simulations. Non-invasive in-situ measurements are performed to evaluate the packing qualities of the powder layer such as surface roughness and packing density, where the effect of the spreading speed is studied. It is found that both the surface quality and packing density of the powder layer decrease with the increase of spreading speed. Besides, the sensitivity of the surface roughness of the powder layer increases with the spreading speed, i.e., the higher the spreading speed is, the more remarkably the surface quality decreases. Numerical simulations using the discrete element method are performed to investigate the dynamics of the powder spreading in terms of the velocity, contact force and coordination number of powder particles, providing new insight to the physical mechanisms underlying the counter-rolling-type powder spreading at particulate scale.

Compliant grinding and polishing: A review

Abstract: Compliant grinding and polishing refers to a class of fine material processing methods relying on one or more system element being compliant with the workpiece surface in a controllable and reversible manner, thus being distinct from conventional rigid wheel grinding and polishing. The resulting surface adaptability greatly facilitates the ultra-precision machining of freeform surfaces, with a level of accuracy and productivity difficult to achieve with conventional rigid processing systems. Aiming for a comprehensive review of signal advances on this topic in the last decades, this paper introduces a multi-level compliance categorization system to sort between distinctive processes. For each category, various perspectives are explored that pertain to principles, methodologies, and physical properties such as material removal and surface integrity. Finally, relative merits are analyzed, together with a discussion of the outlook and future trends.

A feeding-directional cutting force model for end surface grinding of CFRP composites using rotary ultrasonic machining with elliptical ultrasonic vibration

Abstract: End surface grinding of carbon fiber reinforced plastic (CFRP) composites using RUM with elliptical ultrasonic vibration has been proven to be effective in improving surface quality and simultaneously decreasing cutting forces. The cutting force is considered as one of the key output variables to evaluate the machining performance of the cutting process. Investigating cutting force and its modeling development provides great help to understand the effects of input variables and material removal mechanisms of RUM end surface grinding of CFRPs with elliptical ultrasonic vibration. However, there is no investigation on modeling cutting force for this process. This investigation will, for the first time, present a mechanistic feeding-directional cutting force model for such a process. This model is developed based on the material removal mode of brittle fracture. The approaches of the modeling development start from the analysis of one single abrasive grain, including the material removal volume, the effective cutting time, the average indentation depth, and the impact grain force in one ultrasonic vibration cycle. The designed pilot experiments are performed to verify this mechanistic model. The trends of predicted cutting forces are consistent well with those of experimental results. In addition, it can be also applied for predicting the effects of input variables (including depth of cut, feedrate, tool rotation speed, ultrasonic amplitude, abrasive size, and abrasive concentration) on feeding-directional cutting forces.

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

Abstract: 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.

Active vibration suppression in robotic milling using optimal control

Abstract: Six degree-of-freedom (6-dof) industrial robots are attractive alternatives to Computer Numerical Control (CNC) machine tools for milling of large parts because of their low-cost, greater versatility, and larger work volume. However, 6-dof industrial robots are significantly more compliant than CNC machine tools, which makes them prone to vibrations during milling. An additional complexity of industrial robots is their pose-dependent vibration characteristics. This paper presents a pose-dependent optimal control methodology to actively suppress tool tip vibrations generated by the periodic milling forces in robotic milling. Discretely sampled robot structural modal parameters as a function of robot configuration (pose) are used to develop a data-driven Gaussian Process Regression (GPR) model. The model is then utilized to solve the Linear Quadratic Regulator (LQR) optimal control problem to obtain pose-dependent controller gains necessary for vibration suppression. The pose-dependent controller is implemented on a 6-dof industrial robot and its performance evaluated through process-independent offset mass experiments and through milling experiments. The methodology is shown to be effective in decreasing the tool tip vibrations and improving the machining accuracy in robotic milling.

Fundamental aspects and recent developments in metal surface polishing with energy beam irradiation

Abstract: Energy beam polishing technologies have revolutionised the metal surface precision finishing procedure of various materials owing to their specific capability to provide non-contact, selective, and automated polishing processes; further, these technologies help enhance the mechanical properties of the materials. However, the inadequate understanding of the fundamental mechanism of the energy beam finishing process may limit the practical application of such technologies. Furthermore, to cope with the rapid development in various technologies associated with energy beam surface finishing, there is a strong requirement for researchers to highlight the current obstacles and predict promising trends. In this review paper, the main benefits and characteristics of energy beam surface finishing are first summarised. The various parameters that influence the energy beam finishing results are then discussed. For different metallic materials, the experiment parameters are not very identical; therefore, the discussions will be conducted in a sequence. The surface roughness level that can be obtained for the assigned material is considered a key aspect for evaluating the finishing effect, and the variation in mechanical property is considered as a supplement. We also review the fundamental evolution of the energy beam finishing models, existing applications in various fields, and limitations that stand in the way of satisfying industry requirements. The paper will be concluded by introducing the future development trends in energy beam surface finishing technologies, for instance, the large-area and short-time finishing, which play a decisive role on whether the current research can be applied for factory production and provide economic benefits.

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

Abstract: 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.

A novel method to continuously map the surface integrity and cutting mechanism transition in various cutting conditions

Abstract: This research proposes an innovative method to study the machined surface integrity and the variation of cutting mechanisms during a continuous rapid transition between different cutting conditions with an example for a nickel-based superalloy as workpiece material. For the first time, a machined surface covering a wide range of cutting speeds in a single cutting test has been generated, able to capture a clear variation (from serrated to continuous) of chip morphologies and provide a way for in-depth understanding of cutting phenomena. Different material characterisation techniques, including Scanning Electron Microscope (SEM), Electron Back-Scattered Diffraction (EBSD), and X-ray diffraction (XRD), were used to quantitatively evaluate the plastic deformation of the machined surface captured at various cutting speed in a single test. The results show a high potential for the application of this method to continuously study the cutting mechanism transition between different cutting conditions and rapidly characterise the machining behaviour of advanced materials, from the point of views of surface integrity and portioning of cutting energy.

Surface texture formation mechanism based on the ultrasonic vibration-assisted grinding process

Abstract: It is widely accepted that surface textures can significantly improve the tribological properties of surfaces. The purpose of this research was to study surface texture formation mechanism through the ultrasonic vibration-assisted grinding process. First, a single grain cutting process model was established, and the cutting trajectory length and shape were calculated. Second, the single grain profile models were established that considered the dressing effect. Then, by considering continuous cutting process of large amounts of grains, the surface texture formation mechanism model was established. The calculation results were verified by experiments. Finally, the characteristic parameters that described the surface texture topography were proposed and the influence law of the process parameters was analyzed. The results of this research showed that the established theoretical model could accurately describe the ultrasonic vibration-assisted grinding process and the topographical characteristics of the surface texture. By controlling the process parameters, surface textures with controllable topography could be processed. This work can provide a substantially theoretical basis for processing surface textures efficiently.

Influence of grit geometry and fibre orientation on the abrasive material removal mechanisms of SiC/SiC Ceramic Matrix Composites (CMCs)

Abstract: SiC/SiC Ceramic Matrix Composites (CMCs) have been identified as a key material system for improving aero engine performance as they offer low density, high strength and stiffness, and superior environmental resistance at high temperatures. Nevertheless, due to their heterogeneous, hard and brittle nature, these materials are considered among the most difficult-to-machine, and grinding arises as one of the preferred choices for their processing. Therefore, understanding of the material removal mechanism and influence of the abrasive grit geometry when grinding CMCs is a critical enabler for achieving high component quality at highest efficiency and minimum cost. With the aim to reduce the uncertainties associated with the stochastic nature of the abrasive particles, grits of different shapes and sizes have been accurately created by Pulse Laser Ablation (PLA). In order to reproduce the grinding process kinematics, scratch tests in a circular trajectory have been carried out with single abrasive grain with different geometries and sizes, and arrays of overlapped grains to determine the influence of shape, size and spacing on the surface integrity of SiC/SiC CMCs after grinding. The morphology of the various constituents of the workpiece has been assessed regarding the direction of the scratch with respect to the orientation of the fibres. Results reflect a higher influence on the process forces by the grain shape rather than fibre orientation. Moreover, after the inspection of the abraded individual CMC constituents, a change in the mechanisms governing the process for the different abrasive grain geometries have been identified, despite the brittle material removal mode displayed by all of them. Explanation of the ground surface morphology in an analytical and comprehensive manner through a contact mechanics approach shows that the crack onset location is governed by the grains shape but its direction of propagation depends on the fibre orientation.

Experimental and theoretical study of internal finishing by a novel magnetically driven polishing tool

Abstract: Surface finish greatly affects the friction, wear, corrosion, heat transfer and lubrication properties of internal surfaces which find wide applications in medical, automobile, aerospace and mould and die industries. However, improving the internal surface finish is extremely challenging due to the restricted tool accessibility of conventional manufacturing processes. This paper develops a novel magnetic polishing tool to deterministically polish internal surfaces. Repeatability tests, single point polishing experiments and gap variation experiments are conducted to evaluate the performance of the proposed polishing tool. Experimental results substantiate the good repeatability and localized polishing capability with a material removal rate of 15 μm/min and achievable surface roughness of 0.258 μm Ra. Furthermore, a theoretical model is developed to reveal the material removal mechanism based on the contact mechanics model and sliding wear theory. The developed model can successfully predict the two-dimensional and three-dimensional polished profiles under different gaps which are defined by the distance between the externally driven magnet and the outer surface of the workpiece. The localized polishing capability is, for the first time, achieved in internal surface finishing and the theoretical study establishes a novel framework for modelling the polished profile evolution in pressure-copying polishing processes.

A novel modeling of volumetric errors of three-axis machine tools based on Abbe and Bryan principles

Abstract: The modelling of volumetric errors of machine tools has been widely used by the method of Homogeneous Transformation Matrix (HTM) based on rigid body kinematics analysis for a long time. Without spindle induced errors and thermal errors, to establish a closed-loop HTM for a three-axis machine tool, the total of 21 geometric errors are the primary elements to be known. It is well known that the measured points of translational errors are directly related to the volumetric error at the tool cutting point through rigid body kinematics. In the generalized HTM method, however, this relationship is missing. This report, therefore, proposes a new comprehensive approach to formulate the volumetric errors based on the famous Abbe principle in order to derive the error term in the motion direction, and Bryan principle to derive the error terms in orthogonal to the motion direction. The proposed methodology is simple in concept, rational in physical meaning and easy in implementation. Experimental results, including faster multi-degree-of-freedom error measurements, volumetric error analysis and compensation on a testbed of small machine tool, validate the correctness and effectiveness of this method.

Interactive effect of grain size and crystal structure on deformation behavior in progressive micro-scaled deformation of metallic materials

Abstract: Progressive micro-scaled deformation is an efficient approach to fabricating microparts directly using metal sheets. However, how the size effect and material crystal structure affect the deformation of metallic materials in the deformation process has not yet been well understood. In this research, a progressive micro-scaled deformation of metallic materials with different crystal structures and grain sizes was conducted to realize a multi-stage deformation and produce microparts directly using metal sheets via piercing, two step extrusion, and blanking. A unified mechanism-based constitutive model was proposed to describe the grain size dependent flow stress by incorporating dislocation glide and deformation twinning. The constitutive equations were implemented into the finite element model to simulate the progressive micro-scaled deformation. The deformation behavior of the microparts made of FCC pure copper, BCC pure iron, and HCP pure titanium with different size scales were thus extensively studied based on the material flow, strain pattern, microstructure evolution, and forming defect formation. The results show that the length of cylindrical micropin and the extrudates of the microformed parts by using FCC pure copper and BCC iron are decreased with the coarsening of grains. Deformation twins were prevalently formed in the produced micropin and micropart using HCP pure titanium with coarse grains and the dependency of their length on the initial grain size became unnoticeable. The irregular geometric defects include burr, incline, rollover, and bulge. The burr and rollover of extruded micropart are generally deteriorated with the increasing grain size for the three used materials, whereas the dimension of incline is not significantly affected by the initial grain size. All of these provide more insights into the progressive micro-scaled deformation of metallic materials for making bulk microparts.

Effect of joint interfacial contact stiffness on structural dynamics of ultra-precision machine tool

Abstract: The contact stiffness of the joint interface significantly affects the dynamic characteristics of a machine tool structure. Herein, an improved theoretical model is proposed to calculate the joint interfacial contact stiffness. This model is established based on fractal theory. To mitigate the plastic and elastic contact problems, the Stronge contact model is adopted to express the contact force in the plastic regime innovatively, whereas the elastic contact force is interpreted using Hertz theory; the domain extension factor is compensated to revise the microcontact size distribution. Furthermore, the rotational contact stiffness is first derived, which renders the contact model more comprehensive. A representative test structure is designed, and the contact model is validated by comparing the simulated contact stiffness with experimental results. The transfer matrix method for multibody systems is incorporated with fractal contact model to establish a dynamic model of an ultra-precision machine tool; this ensures that the dynamic characteristics of the machine tool can be predicted more precisely. Experiments involving a vibration response test and a modal test demonstrate the feasibility of the fractal contact and dynamic models. Based on these two models, the effect of the interfacial contact stiffness on the structural dynamics is investigated. As the preloads at two joints loosen by 15%, the maximum vibration displacements increase by 1.74% and 2.90%, respectively, indicating that the joint interface between the column and lathe bed is more sensitive. The procedure presented herein will facilitate the design and optimization of the complex structures.

Fabrication of surface microstructures by mask electrolyte jet machining

Abstract: This paper presents a novel electrochemical processing technique, mask electrolyte jet machining (MEJM), for the fabrication of surface microstructures. MEJM combines jet electrochemical micromachining (Jet-ECM) and through-mask electrochemical micromachining (TMEMM), combining the advantages of TMEMM, which is a high-throughput process, and of Jet-ECM, with its adjustable flow field. The effects of a mobile nozzle on electrolyte flow are investigated, and a new modeling approach for large translational movements is proposed. An analysis of the accuracy and reliability of the proposed method is presented. Microprotrusions and microdimples are produced to high precision and with excellent consistency of dimensional variation (maximum standard deviation 2.171 μ m). The results suggest the possibility of an affordable technique for batch fabrication of surface microstructures with high efficiency and precision.

Deterioration of form accuracy induced by servo dynamics errors and real-time compensation for slow tool servo diamond turning of complex-shaped optics

Abstract: Tool servo diamond turning is a promising method for generating complex-shaped optics. Considering the dynamic oscillation of the tool, servo dynamics are essential for the accuracy of the form generated with tool servo turning. Although static geometric errors and multiple compensation strategies for these errors have been studied extensively, the effects of dynamics errors of the servo axis on surface quality have received less attention. In this study, a dynamic surface generation model is established by considering the coupling effect between the servo axis dynamics and the cutting force. Using a micro-lens array (MLA) as an example, the components of the servo dynamics error of the servo motion, including dynamic deformation, resonant vibration, and trajectory tracking error, are characterised. The relationship between the servo dynamics error and the resulting form error is then theoretically identified with experimental verification. To compensate for the servo dynamics error in real-time, the concept of a cooperative tool servo (CTS) is proposed by incorporating a slow tool servo (STS) into a fast tool servo using a master-slave control strategy. Relative to using a STS for turning an MLA, the peak-to-valley (PV) servo motion error when using a CTS decreases from 4.8 μm to 0.3 μm experimentally, and the corresponding PV form error for one hexagonal lenslet decreases from 4 μm to 1.4 μm, demonstrating the effectiveness and superiority of a CTS for dynamics error compensation in the manufacturing of complex-shaped optics.

A novel self-centring drill bit design for low-trauma bone drilling

Abstract: Drilling is one of the most common procedures in orthopaedic surgery. However, drilling-induced trauma occurs frequently and affects the processing damage and position accuracy of the holes, which strongly influence the postoperative recovery. Therefore, there is an urgent need to design a dedicated drill bit that can satisfy low-trauma requirements such as low cutting force, low temperature, self-centring, and low surface damage during orthopaedic surgery. In this work, a novel three-step drill structure is proposed to modify the cutting conditions at the entrance and exit of drilling, to effectively reduce the mechanical and thermal damages and improve the position accuracy in bone drilling. As the first step drill, a unique tip with thinned web was adopted by considering the drill skidding mechanisms under a non-perpendicular drilling condition. The second step was achieved by using an optimal point angle for balancing the effects of the cutting force and temperature. Moreover, a transition arc design was proposed as the third step to adjust the point angle during the finishing stage for switching the cutting mechanism from ‘fracture & shear crack’ cutting to ‘shear’ cutting in association with a certain range of feeding rates. This could reduce the mechanical and thermal damages to the finished hole surface. Drilling experiments under various process conditions demonstrated that the proposed drill design significantly reduced the drilling force, temperature, and damage and also improved the position accuracy of the holes compared to the conventional drill design. The proposed design provides an effective tool to achieve low-trauma bone drilling in orthopaedic surgery.

Geometric errors identification considering rigid-body motion constraint for rotary axis of multi-axis machine tool using a tracking interferometer

Abstract: The measurement and identification of the four position-independent geometric errors (PIGEs) and six position-dependent geometric errors (PDGEs) in the rotary axis are necessary to reduce their contributions to the overall machining errors of a multi-axis machine tool. In this paper, a new geometric error identification method using a tracking interferometer is presented by considering the rigid-body motion constraint in multilateration. The rigid-body motion constraint is introduced to establish a new coordinate calculation model for the measurement points, and an identification process is presented to separately identify the PIGEs and PDGEs by deriving identification models based on established geometric error models. The main novelty of the proposed method lies in the consideration of the rigid-body motion constraint, which makes the identification more robust against random factors. Monte Carlo simulations and verifying experiments were performed for validation. The results show that the PIGEs and PDGEs in the rotary axis can be successfully separated and identified by the new method. Improved identification accuracy compared with the traditional method is achieved. The maximum angular positioning error was reduced by 84% after compensation. A lower uncertainty in the identified errors compared with those obtained by the traditional method and instrument software is achieved. The proposed method is validated by comparing the identification results with those obtained by the instrument software and laser interferometers.

In-situ synthesised interlayer enhances bonding strength in additively manufactured multi-material hybrid tooling

Abstract: Interfacial bonding reliability is a critical issue of metallic multi-material components due to the tendency to delaminate arising from the difference in physical and chemical properties between materials. Here we propose a novel approach to enhance the interfacial bonding through the in-situ synthesis of an interlayer in additively manufactured multi-material hybrid tooling via laser powder bed fusion (LPBF). The effects of laser parameters on tuning the interlayer formation and resulting bonding strength are investigated. The interfacial microstructure evolution, the in-situ formation mechanism of the interlayer, and the interface bond mechanisms are investigated. Intense Marangoni convection and inter-diffusion between two materials in interfacial melt pools, along with Cr redistribution segregation, facilitate the in-situ formation of a Cr-rich interlayer during LPBF process. The in-situ phase transformation behaviour in the interlayer is explained through the Schaeffler-Delong diagram. Mechanical tests, including flexural, tensile and nano-hardness tests, reveal that a strengthened/hardened interface (stronger than parent material) is obtained. The underlying interfacial bonding mechanism of the multi-materials is discussed in terms of the in-situ formed interlayer, Cr segregation and elemental diffusion, in-situ austenite formation together with intrinsic characteristics of the LPBF process. It is found that the in-situ formed interlayer serves to alleviate the interfacial mismatch with Cr-segregation leading to strengthening in the interface, while in-situ austenite formation counteracts residual tensile stress in the melt pool. Hybrid tooling developed in this way integrates complex geometry, improved productivity and high bonding strength.

A tool-based hybrid laser-electrochemical micromachining process: Experimental investigations and synergistic effects

Abstract: This paper proposes a novel tool-based hybrid laser-ECM process which exploits synergy of laser and electrochemical process energies along the same machining axis, thereby enhancing the potential of both processes while compensating and minimizing their limitations. This process combines features from jet-ECM and water jet guided laser processes into a new micromachining process. In this study, details of this tool-based hybrid laser-electrochemical micromachining process are presented and an experimental study on process-material interaction is performed using Inconel IN718 as workpiece material. According to the experimental results, material removal rates of the order of 0.6 mm3/min are obtained. It has been observed that while the process response is material-dependent as well as ECM parameter dependent, the effective laser pulse energy reaching the workpiece surface is the main factor influencing the surface characteristics. Additionally, the electrolyte flow rate affects material removal and also influences laser coupling into the tool-electrode. It has been observed that within a specific process window i.e. pulse-energy 30–45 μJ, flow rate 32–48 ml/min, IEG 20–30 μm, voltage 20–25 V; high quality surfaces are observed with less defects. At pulse energies higher than 60 μJ, the process speed becomes higher but the surface becomes rough due to combined material removal mechanisms taking place. Furthermore, metallographic investigations on the machined surface reveal presence of multiple removal mechanisms such as laser removal, laser assisted electrochemical removal and electrochemical removal depending on the applied laser pulse energy. Overall, this study has shown that hybrid laser-electrochemical micromachining has a high potential to machine advanced metallic alloys with conductivity variations even for high aspect ratio features and needs further research developments.

Analytical and experimental investigations on the mechanisms of surface generation in micro grinding

Abstract: Micro grinding is an emerging technology for producing structured surfaces on hard and brittle materials. A micro pencil grinding tool (MPGT) consists of a layer of superabrasive grits, bonded to a solid cylindrical surface. Randomly distributed and geometrically undefined grits interact with the workpiece surface at random positions. These random grit positions and protrusions lead to a difference in the size of undeformed chips. This study presents an analytical method to understand the undeformed chip geometry, that considers grit kinematics. Kinematic simulation of grit trajectory paths in longitudinal direction showed a reduced number of active grits in micro grinding with an increase in speed ratio and with reduced tool dimensions. MPGTs with different diameters, grit sizes, and planar grit densities have been used to perform the experiments on a 16MnCr5 hardened steel material. The influence of maximum radial height grits on surface generation in micro grinding has been verified experimentally for up and down grinding modes. Microscopic observations of ground surfaces have shown the distinct differences between up and down grinding modes, which are similar to the surface generation in milling processes. Moreover, the formation of linear grooves with uniform depth and width unlike conventional surface grinding at lower speed ratios indicated the influence of individual grits on surface generation. Trajectory path simulation results have also shown the same observation.

Coaxial waterjet-assisted laser drilling of film cooling holes in turbine blades

Abstract: Film cooling holes (FCHs) of nickel-based single crystal turbine blades were drilled by 532 nm Nd:YVO4 nanosecond laser in coaxial waterjet-assisted environment. Microstructure of the side wall of the FCHs was mainly investigated by means of transmission electron microscopy. The average thickness of heat affected zone (HAZ) around FCHs decreases with increasing of water flow rate. The main phase within HAZ evolves from β-NiAl to β-NiAl + γ-Ni with the increase in the water flow rate. Some γ-Ni particles in the HAZ twined along (111) plane. A small portion of the FCHs are free of HAZ when drilled by coaxial waterjet-assisted laser drilling at a laminar water flow rate ≥3.1 m/s. There are no processing-induced defects including HAZ, microcrack, and phase transformation around the FCHs when drilled at the water flow rate ≥5.1 m/s. The FCHs with high surface quality can be drilled by the coaxial waterjet-assisted laser drilling. Finally, effects of fluid water on drilling quality of the FCHs were discussed.

Laser processing of freeform surfaces: A new approach based on an efficient workpiece partitioning strategy

Abstract: A novel method for laser processing freeform surfaces is proposed and demonstrated in this article. The method employs empirical data on the 3D limitations of a given laser process, namely the negative effects of focal offset and angle of incidence on the process performance, to partition a freeform surface into triangular laser processing fields. In this way, processing efficiency can be maximized by minimizing part repositioning while fully utilizing the capabilities of high dynamics galvo scanners. In this proof of concept, the surface of 3D printed Ti–6Al–4V spherical shells was improved by more than 90% and subsequently textured, using the proposed method. Conclusions were made about the advantages of this new approach for processing freeform surfaces consistently and efficiently.

Wire electrochemical micromachining: An overview

Abstract: Wire Electrochemical Micromachining (Wire ECMM) process has become a topical field of research in today's context of micromachining. This is due to the agility which the process offers, that makes it as one of the most suitable option for machining of micro parts and features. Process capabilities of the Wire ECMM such as protracted tool life, surface integrity, material removal rate, surface finish, burr and heat affected zone-free surface gives it an edge over Wire Electric Discharge Machining (WEDM), its counterpart in thermal based machining process. However, the control of process parameters in Wire ECMM is not easy and requires knowledge of their influence. At present, Wire ECMM is still a lab-based technique. This article aims to provide a detailed and well-arranged literature survey of the advancements made in Wire ECMM to date along with an insightful discussion on the science of electrochemical dissolution. Key factors influencing the process performance are identified and reviewed. Different analytical and numerical models proposed in the process are also reviewed critically. This article is expected to assist the researchers/practicing engineers in identifying the existing research gaps and contributing towards making Wire ECMM an industrially viable option for micromachining.

Dynamic evolution of powder stream convergence with powder feeding durations in direct energy deposition

Abstract: During direct energy deposition (DED), changes in the powder feeding behavior significantly affect the deposition process. In this study, a high-speed photography method was developed to observe the dynamic evolution of the powder stream for various powder feeding durations, and the synchronous evolution of the inner channel characteristics of the nozzle were also measured. Combined with powder stream simulations, the contribution of the evolution of each channel characteristic on the powder divergence was discussed. The results show that as the powder feeding duration increased, the inner channel characteristics of the nozzle changed dynamically, which led to the powder stream divergence. For a powder feeding duration of 60 h, the nominal powder spot size increased by 41.4% at the plane 15 mm below the nozzle exit. As the powder feeding time increased, the inner wall roughness, inner channel diameter, and the exit size of the nozzle also increased owing to the scouring erosion from the high-speed powder particles. Subsequently, changes in the inner wall roughness and inner channel diameter of the nozzle tended to be smaller owing to the formation of a work hardening layer. It was also demonstrated that the changes in the inner wall roughness of the nozzle had a negligible effect on the powder stream divergence, an increase in the inner channel diameter of the nozzle extended the maximum divergence range of the particle trajectory at the nozzle exit, and the expansion of the nozzle exit size reduced the constraint causing the allowed trajectory range of the ejected powder particles to increase. The deposited height gradually decreased as the powder feeding duration increased, resulting in a 38.7% decrease after 60 h.

Region adaptive scheduling for time-dependent processes with optimal use of machine dynamics

Abstract: In time-dependent processes, such as bonnet and fluid jet polishing, surface quality and accurate processing critically depend on careful planning of the tool feed CNC feedrate commands are usually generated from a dwell time map calculated by deconvolution or numerical iteration These methods are time-consuming, numerically unstable, and fail to consider dynamic stressing of the machine tool In this research, Gaussian mixture model (GMM) is proposed to model experimental tool influence functions (TIF) This leads to a general analytical convolution model integrating processing depth, volumetric removal rate of TIF, path spacing and feedrate Based on this model, a novel direct feedrate scheduling method is proposed, which is suitable for any kind of smooth time-dependent processing beam Optimal feedrate scheduling within dynamic constraints of the machine tool is achieved by establishing acceptable path spacing and feed ranges, whilst dynamic stressing of the machine tool is optimized concurrently through adaptive path spacing Simulations and experiments demonstrate the enhanced stability and usefulness of the proposed feedrate model in deterministic material removal It also verifies that path adaptability allows for improved machine tool dynamics, without incurring a process accuracy penalty

Plasma-assisted electrochemical machining of microtools and microstructures

Abstract: A novel hybrid electrochemical machining (ECM) approach combining a pulsed cathodic plasma and an electrochemical machining process, namely, plasma-assisted electrochemical machining (PA-ECM), is proposed in this study to strengthen the capacity of ECM, which entails both high efficiency and precision. The plasma characteristics, material removal behavior, surface topography and machining precision of PA-ECM for microtool fabrication are experimentally investigated under various conditions. The results show that PA-ECM can be realized under optimized electrical potentials with a vapor gaseous skin and electrolytic plasma layer formed around the cathode tool, of which the kinetic and thermal energies can enhance both the kinetics of the electrochemical reaction and mass transport during the ECM process. Through the design of the pulse voltage waveform, the formation and transportation of gaseous bubbles and plasmas can be well controlled. It has been shown that PA-ECM is effective and efficient for improving both the material removal rate and form accuracy in machining microtools. In the presence of plasma, a microrod tool with a high aspect ratio of 55:1 is successfully machined by PA-ECM in 5 s from its original diameter of 200 μm to approximately 18 μm, which seems to be the highest machining rate achieved so far. Additionally, in comparison to traditional ECM under the same conditions, PA-ECM provides a noticeable improvement in the microrod tool straightness error from 66.8 μm to 14.6 μm owing to the side surface insulation effect of the gaseous skin. The resulting surface roughness Ra is drastically reduced from 1096 nm to 46 nm, demonstrating that PA-ECM provides an innovative way to considerably improve the ECM efficiency without compromising the surface finish. Furthermore, the PA-ECM of microholes and microstructures are exhibited with improved precision, demonstrating the capacity of PA-ECM for micromachining.