Incremental Sheet Forming (ISF) has gained significant attention as a flexible and cost-effective manufacturing method for producing low to medium volume and customised sheet metal components. However, its industrial uptake remains limited by gaps in knowledge related to surface quality and geometric accuracy, which are strongly governed by tribological conditions at the tool-sheet interface. This study experimentally compares five distinct lubrication strategies – Hydro slide (HG32) oil, flood emulsion coolant, minimum quantity lubrication (MQL) using soybean oil, supercritical CO₂ (scCO₂), and hybrid scCO₂ + MQL during 25 ISF trials of 1 mm thick stainless-steel grade 304 sheets formed into an 84 mm × 84 mm square pyramid geometry. Process performance was evaluated using vertical forming forces, a friction indicator derived from tangential (in-plane) and vertical force measurements, surface roughness parameters (Ra and Rz) measured using the Alicona optical topography functionality, and geometric accuracy using full field 3D scanning. The results showed that HG32 consistently produced the lowest vertical forming forces but resulted in the poorest surface quality, characterised by pronounced surface smearing, higher friction indicator values, and increased Ra and Rz roughness parameters. The higher friction indicator values suggest a greater contribution of tangential forces relative to vertical forces, suggesting increased tool-sheet friction and reduced lubrication effectiveness. In contrast, MQL achieved the best overall surface quality, a lower friction indicator value suggesting more favourable tool-sheet sliding conditions and improved lubrication effectiveness and the most stable geometric performance, despite generating higher forming forces than HG32. Full field deviation mapping showed that geometric deviations were concentrated along diagonal regions for all lubrication strategies. Overall, the findings provide a detailed process quality dataset and clear guidance on selecting lubrication strategies to balance force demand, surface quality, and dimensional accuracy in ISF of hard to form stainless-steel parts.

As manufacturers seek to improve cutting process sustainability, alternative metalworking fluid (MWF) strategies are increasingly being explored. For the first time, this study characterises tool wear when shoulder milling both Grade 5 titanium (Ti-6Al-4V) and Grade 2 commercially pure titanium (CP-Ti) with a novel through-spindle Aurion Machining Technologies ionised air (IA) MWF setup. Preliminary results show that during CP-Ti milling trials, the IA strategy led to a tool life of between 85% and 91% of that which was achieved with soluble oil emulsion MWF, whilst during Ti-6Al-4V milling the observed tool life with IA was between 158% and 278% greater than with emulsion coolant at analogous cutting conditions (2.58 and 3.78 times respectively). In addition, IA generated a 26.2% reduction in surface roughness after Ti-6Al-4V milling, potentially indicating a change in tool–surface interaction behaviour. These benefits are compounded as IA returns to its original condition rapidly after utilisation, meaning low environmental and health impact with no waste MWF liquids/gases/mist, clean metal cuttings and low delivery power. Beyond these promising results, cooled and dried but non-ionised air was also shown to perform strongly (regarding tool wear in Ti-6Al-4V milling), relative to emulsion MWF, such that at 190 m/min cutting speed it generated 90% of the tool life which was achieved by the IA strategy. Whilst these preliminary findings require confirmation through repeat testing, IA remains of clear interest for further experimental investigation across a range of subtractive processes. Moreover, this work highlights the potential benefits, niches and configurations for air-based MWFs in general, with further mechanistic and tribological exploration warranted.

Due to recent developments across the aerospace, power generation and defense sectors, the demand for flat-surfaced components with extremely high surface quality is rapidly increasing. In this regard, although abrasive machining processes often produce fine, contaminated swarf that is frequently relegated to landfill, these processes remain critical for the engineering sector. Motivated by increasing sustainability and circularity pressures, this narrative review examines the current state of the art in recycling and repurposing the chips, tooling and cutting fluids that are typically generated or consumed within grinding processes. In doing so, a number of methodologies for extracting useful materials from swarf slurries are identified, including pyrometallurgical routes (applied successfully to Ni–Co alloys, for example), hydrometallurgical strategies (e.g., iron leaching from ferrous swarf) and, in the case of non-metallic materials such as CMCs and CFRPs, chemical processing methods. Various means of separating abrasive constituents and removing contaminants from grinding swarf are also highlighted, within which centrifugation and heat treatment are found to be particularly useful for non-ferrous materials such as titanium alloys or composites, whilst ferrous materials are largely magnetically separated. Prospective applications for spent abrasive tooling are also explored, including reuse as shot, waterjet machining feedstock, road surface additives, or mortar in the context of cement production. Likewise, heat- and radiation-based strategies for prolonging cutting-fluid life are highlighted, and their associated sustainability benefits and limitations discussed, despite ultimate disposal still being relegated to fuel usage or landfill. Ultimately, this review identifies the scarcity of grinding-specific recycling process data and highlights the need for robust, publicly accessible recycling strategies for novel material systems.

The formation of the built-up edge (BUE) is critical in machining ductile materials such as C45, particularly under dry conditions. It affects the surface integrity, tool conditions, and accuracy of the produced parts. The method to identify the stages of BUE formation without interrupting the process remains unclear. This work investigates the correlation of BUE formation during dry orthogonal cutting of C45 steel with uncoated carbide inserts to the chip characteristics. Firstly, by analysing the measured cutting force, the change in cutting-edge radius during cutting is explained by the BUE accumulation. Then, BUE formation stages are correlated with chip condition, including the presence of deformation bands, underside chip roughness and shear angle, and thickness, curling, chemical composition and colour of the chips. Two cutting tests were conducted at two feed rates and a constant speed. In each test, the chips were collected in three transient regions: Beginning, middle, and end of the cut. Then, each test is repeated under the same cutting conditions, and the produced chips were collected and analysed. In each transient region, five chip segments were collected and analysed. The cutting force reflects a change in process mechanics evidenced by radial force surpassing the tangential component during the cut, which is due to the increased cutting-edge radius with the continuous progression of BUE formation. The findings for the chip under these conditions indicate reduction in chip underside roughness along the cutting process, an increase in chip thickness, and straightening of the chip shape. The chemical composition analysis of the chip underside shows a decrease in carbon content and an increase in oxygen content as the cutting progresses. The chip colour indicates a transition from dark blue at the beginning of the cut to grey at the end. As a result, this study advances knowledge of the stages of built-up edge formation by investigating chip conditions.Image, application 1Application 1

Modern aircraft assembly involves drilling holes in multimaterial stacks, followed by fastening to create a structural joint. Stringent quality requirements to ensure structural integrity necessitate an accurate assessment of the hole quality. Carbon fiber-reinforced polymer (CFRP) is widely used in aircraft structures, and delamination and uncut fibers are prominent defects associated with CFRP drilling. Furthermore, metal-CFRP composite structures, owing to their higher cost and hence the need to reduce waste, present even more stringent requirements on hole quality. Defects such as delamination and uncut fibers in this composite also differ in how they manifest. An automated evaluation framework and robust hole quality metrics are therefore required to address this challenge. In this article, an automated image processing-based evaluation framework for rapid and objective assessment of delamination and uncut-fiber defects is developed. This robust framework introduces combined metrics, leveraging a weighted $p$ -norm approach to aggregate five factors for delamination and five for uncut fibers with $p = 1$ , 2, and $\infty $ . A method to choose weights for the different $p$ -norms is also identified to create robust measures. Grayscale images captured via optical microscopy serve as inputs, and the framework outputs both the combined metrics and ten individual metrics for detailed analysis. The multiobjective space with the associated Pareto front and population-based ranking derived from the weighted $p$ -norm-based metrics is introduced as a means to identify relative hole quality and to associate these with the machining parameters. A dataset comprising 24 holes from two metal-CFRP composite workpieces produced in a robotic drilling operation is used for the demonstration of the framework. The results demonstrate the effectiveness of the proposed framework and its utility for real-world applications.

The machining of near net shapes (NNS) produced by laser powder bed fusion (L-PBF) presents challenges regarding the stiffness of components. Notably, complex geometries featuring thin-walled, slender and hollow regions are of particular interest to additive manufacturing technologies, yet to meet the dimensional and surface quality requirements of functional parts, machining is often deemed necessary. Compliant regions of a workpiece are prone to chatter, and workholding becomes difficult with complex surfaces. Previous works have explored the pure stiffening of flexible workpieces with solid elements such as buttresses and lateral stiffeners. Meanwhile, advances have been made in mesostructural design for a range of metamaterial functions due to the maturation of L-PBF.Building upon these two concepts, the present contribution investigates what damping and stiffening effects a lattice support structure would have on the chatter stability of a flexible workpiece produced by L-PBF. A dynamic model of a cantilever beam supported by a spring and a viscous damper is proposed to predict the vibrational behaviour of a workpiece supported by a lattice support structure. A preliminary modal test is carried out to acquire damping behaviour to inform the model, and provide a deeper understanding of the relationships between lattice parameters and damping.This study is part of an ongoing discussion into the post-processing of NNS parts produced by L-PBF. It presents the concept of additive design for machining, and prompts investigation into how a mesostructural support could be designed to enhance machining operations. As the proposed structure is an addition to the functional part, it should be sacrificial, and to be sacrificial it should be removed effectively and efficiently. The present contribution seeks to provoke a discussion around these emerging concepts.

Whilst subtractive manufacturing has been de-risked significantly over recent decades, the emergence of new unfamiliar materials is proving to be a significant challenge for social sustainability. Given this rapidly evolving landscape, this review serves to outline the current available data on the occupational health implications of various existing and emerging material species, ranging from radioactive metals to composite materials. A structured search of sources up to January 2025 was conducted using databases such as Google Scholar, PubMed and Web of Science in addition to various authoritative occupational health reports, prioritising the literature directly pertaining or analogous to machining-related hazards. Evidence highlights the complexity of the machining environment, with occupational hazards ranging from toxicological factors to fire risks (i.e., due to swarf pyrophoricity). Case studies outline both relatively benign pathologies (e.g., dermatitis and sensitisation) and much more severe health complications (e.g., carcinogenicity, systemic organ damage and death), underscoring the need for continuous assessment and updating of exposure controls, even for materials traditionally regarded as safe.

Airliner assembly processes involve components being pre-assembled into a 'stack', which is then drilled through. Manufacturers have strict hole quality requirements and need confidence in hole quality, since defects such as burrs and delamination can affect structural integrity. Human experts can be empowered to perform hole quality inspection through the provision of useful information. Visual representations of signal features and their association with the spatial and temporal features in the hole quality is a powerful mechanism which to facilitate quality inspection. This paper proposes a novel sensor signal integration framework to map sensor signals from the time domain to the relative spatial domain as indicated by the drill bit position. Kalman filter based rotational position estimation from fibre-optic signal and relative drilling depth estimation from laser signal provided the relevant spatio-temporal information for the mapping. The resulting spatial domain mapping enables visualisation of signals for the detection of any defect related anomalous patterns for a human expert to inspect hole quality. Its potential is demonstrated on a real-world drilling trial of different quality holes.

Rotational Vibration-assisted Incremental Sheet Forming (RV-ISF) has emerged as a flexible process for manufacturing complex geometries with significant potential for wider applications. An experimental study was performed on a square pyramid geometry to investigate the effect of tool geometry and process parameters such as rotational speed and step size on the forming process of AA5251 H22 alloy sheets. The process was monitored using a suite of sensors to measure vibrations, force, and temperature, as well as machine tool data. Data from the eddy current sensor show that increasing rotational speed generally reduces vibration amplitude for RV-ISF tools by 27%, indicating more frequent but shorter-duration tool-sheet contact events at higher speeds, while finer step sizes increase vibration activity by 8%. Force and thermal measurements were also obtained, highlighting the influence of rotational speed, step size, and tool design on the thermo-mechanical response, with RV-ISF tools reducing frictional heating more effectively than conventional ISF tools by 22%. The results explain how tool design and process optimization can enhance vibrational and thermal softening in the incremental sheet forming process. This study contributes to the understanding of RV-ISF tool geometries by comprehensively monitoring the process with sensors and machine data and provides insights for improving part quality in incremental sheet metal forming.

Industry standard assessment of machined surface integrity typically requires a form of destructive testing to capture microstructural features such as white layers or distorted layers. The X-ray diffraction surface integrity inspection method (XRD-SIIM) offers a non-destructive alternative for the detection and sizing of fatigue-limiting microstructural features, without the sacrifice or alteration of a component which is required for cross-sectional microscopy. In this study XRD-SIIM was used to measure the surface integrity across a nickel-based superalloy gas turbine disc segment. XRD-SIIM microstructural feature detection and sizing models were trained and validated from a set of surfaces generated in a turning trial with a newly developed calibration procedure allowing these models to be adjusted for different instrument setups to facilitate measurement of the turbine disc segment. The non-destructive test was then able to show comparable inspection results to industry standard microscopy of microstructural features when inspecting the representative component, with over 90% of the outside surface of the component accessible by the diffractometer, and the internal surfaces accessible with appropriate hardware. Surface curvature only became a limiting factor for inspection of geometrical features with a small radius of curvature such as seal fins and the corner radii on fir tree sections. (c) 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)

Complete inspection of workpiece surface integrity invariably involves a form of destructive testing to enable the assessment of microstructural defects such as machining-induced white layers and near-surface plastic deformation. The incumbent offline and destructive microscopy inspection process is incompatible with both a digital and sustainable manufacturing vision of zero waste, as such, a non-destructive technique which utilises a novel X-ray diffraction surface integrity inspection method (XRD-SIIM) has been developed. This approach has been designed to complement traditional machinability-type assessments of tool life and machined surface topography, establishing a new process flow for validation. In this paper, for the first time, non-destructive on-machine validation of workpiece microstructural surface integrity is demonstrated, via a comparative investigation into the effect of insert grade, cutting speed and coolant delivery method on the depth of the imparted plastic deformation depth. It is shown that XRD-SIIM allows repeatable, non-destructive determination of deformed layers within a typical machining centre enclosure, with comparable findings to the incumbent cross-sectional microscopy approach. The generation of surface integrity digital fingerprints of a machining operation facilitates rapid comparison between testing variables, with a transition to an objective quantifiable assessment rather than one which open to subjectivity. In turn, XRD-SIIM expedites the development and benchmarking of new operations, tooling, materials, or coolant.

High performance powder-based Ni-based superalloys exhibit exceptional in-service properties at elevated temperature, however this leads to reduced machinability and the potential for significant machining induced damage. Field assisted sintering technology (FAST) is capable of consolidating powder rapidly and efficiently, allowing for precise control of the microstructure via the dissolution of strengthening phases. In this study, subsolvus and supersolvus dwell temperatures were utilised to produce fine and coarse grain forms of an advanced Ni-based disk alloy. Surface integrity and machining forces were then evaluated after single point turning for a range of surface speeds. Higher cutting forces and lower depths of subsurface damage were generated when machining the fine grain (subsolvus) material when compared to the coarser grained (supersolvus) material. For both material conditions tested, higher surface speeds led to a reduced depth of subsurface deformation due to increased local temperatures, promoting workpiece softening. In addition, at higher cutting speeds the deformation of near surface gamma' precipitates were observed to be greater. These results demonstrate that the FAST process can be utilised to control microstructure, and as a result, tailor the machinability of Ni-based superalloy material. (c) 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)

Aircraft assembly entails drilling numerous holes, often in multi-material stacks, then joining parts with fasteners fitted through the holes. There are stringent quality requirements on the holes, and assessment of hole quality is crucial to ensuring the integrity of the joints. Carbon fibre reinforced polymer (CFRP) is a commonly used material in aircraft structures due to its desirable properties. However, it is susceptible to defects not associated with metals, such as delamination and uncut fibres. While there have been multiple metrics for assessment of these defects proposed in literature, relatively few attempts to consolidate them have been seen. Furthermore, common measurement methods used for assessing these defects (e.g. 3D-microscopy) are well established, but can be time-consuming; manual interrogation of raw inspection data can also be highly subjective. To address these challenges, this paper proposes a set of combined metrics along with an automatic image processing framework for fast assessment of delamination and uncut fibre defects. The combined metric for the delamination region is aggregated from across six factors and uncut fibre uniform metrics from across five factors. The image processing framework receives grayscale images as inputs, taken from an optical coordinate measuring machine, and outputs the combined metrics along with eleven separate metrics. Experimental results, using a preexisting dataset from twenty-four holes on two workpieces from a real robotic drilling operation, are given to demonstrate the effectiveness of the proposed combined metrics and the corresponding image processing framework.

Titanium alloys display anisotropic deformation properties due to the hexagonal close-packed (hcp) crystal structure of the α-phase. When subjected to localised deformation during machining, this behaviour influences fluctuations in the cutting force response of the material as the tool encounters grains of different orientations. In this research, cutting force signals acquired during face turning of Ti–6Al–4V possessing a lamellar α colony structure have been spatially mapped demonstrating the ability to identify microstructural features such as prior-β grain boundaries, grain boundary α, and α colonies. Measured cutting forces have been correlated to texture using orientation information acquired from large area EBSD analysis. A relationship between the misalignment of the crystallographic a slip vector with respect to the cutting direction and the passive cutting force response has been established, demonstrating a rise in cutting forces as this misalignment is increased. This novel approach to in-process materials evaluation offers manufacturers the potential of a powerful digital quality assurance tool, with the results presented here demonstrating the possibility for rapid characterisation of entire component surfaces, revealing microstructural features, and inferring the crystallographic orientation of macrotextured regions in Ti–6Al–4V.

Aircraft assembly involves drilling vast numbers of holes, with stringent quality requirements, in multi-material stacks. To achieve high quality functional holes, one of the process parameters that is controlled carefully is the spindle speed. Variations in spindle speed from the programmed value can be indicative of the quality of the manufactured part and thus can be useful for process monitoring. Estimation of the tool angle has potential benefits in monitoring the spatial localisation of hole defects. Rotational speed estimation from encoded type detectors is an established methodology but the literature on parameter-free angle estimation is sparse. To address these challenges, this paper proposes a set of novel strategies for speed and angle estimation. The strategy includes the use of a fibre optic sensor with a novel encoding structure to facilitate angle estimation, and the analysis of the sensor signal using a staged signal processing framework. Experimental results from a real robotic drilling operation of a multi-material stack are given to demonstrate the effectiveness of the estimation strategy.

Polycrystalline diamond (PCD) is currently under development as a new generation of cutting tool material for titanium alloy machining applications. The unrivaled high temperature hardness possessed by PCD offers the potential for higher levels of productivity compared to tungsten carbide, the current industry standard tool material, through facilitating higher cutting speeds. This study investigates the performance of various PCD tool grades during square shoulder milling of Ti-54M. The influence of PCD grain size on dominant wear mechanism has been established, revealing that a smaller, sub 1 μm, grain size offers improvements in tool life due to superior fracture toughness compared to larger grained material. For fine grained PCD, loss of tool material through a cyclic process of workpiece adhesion followed by grain pull-out was identified to be the predominant wear mechanism, contrasting the mechanical fracture dominated wear observed for the larger grained PCD grades. The influence of insert microgeometry was also investigated through honing of the cutting edge radii. An increased tendency for edge fracture was demonstrated when machining with larger radii tooling which was attributed to increased cutting forces. Finally, the study has compared the surface integrity response of the workpiece following PCD and carbide machining, revealing considerably lower levels of microstructural damage and cutting forces when machining with PCD. This highlights the potential benefits of PCD in finishing applications, whereby high speed machining can be employed to reduce the impact on component surface integrity.

The degradation behaviour of textured CVD α-alumina has received significant attention within academic literature with regard to steel turning. In contrast, the research performed on Ni-based superalloys is yet to be brought up to the same level of understanding. Hence, the objective of this report will be to offer insights with regard to the degradation mechanisms of the textured alumina layer of CVD Ti(C,N)/α- Al2O3 coated carbides when turning a Ni-based superalloy, and to explore the role of different wear mechanisms that contribute to cutting tool failure. Adhesive wear was observed to be the primary wear mechanism responsible for coating failure, which can result in sudden mechanically driven grain pull-out and delamination events. Grain-pull out was observed across the entire contact zone; delamination was observed to occur most frequently around the trailing edge and depth-of-cut notches, and towards the end of the contact zone as the chip separates from the tool. Evidence of characteristic plastic deformation induced ridges in the alumina coating surface were also revealed within the sliding zone, leading to gradual wear of the alumina layer. These findings demonstrate that there are both similarities and differences in alumina degradation behaviour when compared to previous studies conducted on steel turning.

Machining-induced white layers and severely deformed layers are undesirable surface integrity features which can be formed when machining high-strength aerospace alloys. An orthogonal milling process has been designed and performed to assess the impact of cutting speeds, tool wear, cutting edge radius and climb vs conventional milling on white layer formation and plastic strain distribution. The plastic deformation in the machined surface associated with the formation of white layers in Ti-6Al-4V has been quantified using micro-grids of different length scales printed using the electron beam lithography technique. It was found that white layers formed via the severe plastic deformation mechanism, at equivalent plastic strain values in excess of 1.2 and in regions of the cutting arc with the instantaneous chip thickness of less than the cutting-edge radius and ploughing and rubbing being the dominant mechanisms. The results indicated that the magnitude of the measured strains and the depth of plastically deformed material was greater at lower cutting speeds, during climb milling and when machining with a larger cutting edge radius and tool flank wear land.

In this study, a recently developed x-ray diffraction method is shown to be capable of detecting thermally-induced white layers formed during hard turning, as well the identification of grinding-induced rehardening and tempering. The x-ray method is evaluated against established industry methods for detecting thermally-induced machining defects, Barkhausen noise testing and nital etching. Detection using x-ray is facilitated by a sensitivity to fundamental properties of these surface integrity features, lattice strain and grain size, rather than a secondary property such as residual stress. X-ray inspection offers quantitative results unlike, nital etching, but does not achieve full part coverage in one step.

Accurate fatigue life predictions of titanium alloy components requires an understanding of how the machining affected metallurgical and micro-mechanical subsurface condition influences fatigue crack nucleation and growth. This study investigates the influence of surface integrity features generated during carbide and high-speed polycrystalline diamond machining on the fatigue behaviour of coarse and fine-grained Ti-6Al-4V. Mechanically induced compressive residual stresses, promoted by higher feed rates and the larger cutting edge radii of carbide tools, have been demonstrated to provide an overriding enhancing effect on fatigue life due to crack initiation suppression and reducing the deleterious effects of microstructural deformation and surface imperfections.