Additive manufactured (AM) offers significant design freedom for aerospace components, but many AM surfaces require machining to meet functional and dimensional requirements. Alloy 718 remains challenging to machine due to its high hot-hardness and strong work-hardening behaviour, and recent studies suggest that AM-induced microstructural variations may further influence machinability. This work compares the machinability with two solid end mill concepts, a cemented carbide tool and a binderless CBN (BL-CBN) tool, when machining Alloy 718 produced by Laser Powder Bed Fusion (PBF-LB) in three material conditions: As-built, Hot Isostatic Pressed (HIP), heat-treated (AMS 5662) using a wrought Alloy 718 as reference. The comparison focuses on tool wear and surface integrity, including surface topography, residual stresses, and near-surface deformation. Machining tests were performed on small blade-shaped samples representative of a thin-walled aerospace component geometry. The results show clear differences in machinability between the two tool concepts. The cemented carbide tool is highly sensitive to the material condition, leading to higher surface roughness and lower compressive residual stresses compared to the BL-CBN tool. In contrast, the BL-CBN tool provides stable performance across all AM conditions. For the relatively short cutting lengths studied, machining-induced deformation remained low for both tools in the AM materials. However, wrought Alloy 718 material remains the most challenging material to machine, as it caused highest tool wear, deepest residual stresses and largest plastic deformation

Ni-based superalloys components are utilised in some of the most hostile engineering environments. The exceptional high-temperature performance of these alloys presents significant challenges during manufacturing. Machining is a critical process, enabling the precise shaping of forgings into functional components. During material removal, plastic deformation of the workpiece material and friction generate substantial heat and high stresses, which can affect the integrity of both tool and workpiece. CBN (cubic boron nitride) tools are commonly employed for the finish turning of high-value Ni-based superalloy components. High-pressure cooling (HPC) is implemented to prevent premature tool failure by cooling the cutting tool and aiding chip breakage. In the study presented, the influence of HPC - and cutting speed on the resulting workpiece residual stresses were investigated during axial turning of an advanced wrought disk alloy (AD730®). It was found that both HPC application and cutting speed influence residual stress depth profiles, which are known to enhance fatigue life and can therefore be optimised to improve in-service performance. Consequently, this research has significant implications for industries such as aerospace and power generation. Copyright

Barkhausen noise (BN) is industrially relevant quality control method which is used for process control of i.e. grinding. Similarly to any non-destructive testing measurement also BN measurement requires reference samples. Previously, laser irradiation method with optical pyrometer control was found to be an effective method to produce controlled and reproducible heating effect to metal surfaces. In this study, the long-term stability and changes in the laser processed reference samples were inspected. During the long-term stability survey, certain guidelines which are helpful for preparing the reference samples and evaluating the long-term usability of them were noticed. It was found that the sample initial structure should be as stable as possible prior laser processing. One key finding was that carburized, case-hardened samples are unstable for reference samples since the retained austenite decomposition over time may affect the BN signal levels making these samples unsuitable for reliable long-term use. In addition, the laser irradiation temperature needs to be optimized to produce significant change to the BN signal level. However, too high temperature would produce too altered area compared to production which might observe more easily mechanical effects from the dynamical use with BN sensors.

Hard turning offers a cost-effective alternative to traditional grinding, yet the tool wear progression limits the broader industrial adoption. During hard turning, the surface microstructure of AISI 52100 steel transforms into a nanocrystalline structure known as white layer, accompanied by significant surface residual stresses. With optimal cutting conditions, surfaces develop nanocrystalline microstructures with high compressive stresses, known as mechanically induced white layers (M-WLs). In contrast, improper cutting conditions generate thermally induced white layers (T-WLs), associated with tensile stresses. This study investigates the effect of feed rate, cutting speed, and tool wear on the different white layers formed and their influence on the surface integrity. Microstructural analysis reveals that the M-WL formed by dynamic recovery mechanism exhibited fragmented nanocrystalline grains with ∼26 % higher hardness than the bulk material. The presence of elongated lamellar grains with ∼7 % higher hardness in the material drag zone beneath the M-WL suggests the occurrence of a grain subdivision process that initiates M-WL formation. This grain subdivision mechanism generated lamellar grains composed of geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs), reflecting progressive strain accommodation during severe plastic deformation. In contrast, T-WL is generated by continuous dynamic recrystallization mechanism and features nanograins with ∼27 % higher hardness and an underlying over-tempered dark layer with ∼16 % lower hardness than the bulk material. The M-WL exhibits surface roughness of ∼5 times lower and better surface compressive stress than the T-WL. This research demonstrates a promising hard turning strategy for producing advantageous M-WL with nanocrystalline grains and improved surface integrity

When hot forging components, wear can occur in the tool after a period of use, leading to incorrect geometry in the final component. This necessitates replacing the worn tool with a new one, which is costly. The current approach is to repair the tool using machining that removes the worn surface which is less efficient from a circularity standpoint. A more sustainable approach is to maximize the tool life by carefully adjusting the material and process parameters to slow the wearing process and repair without removing material as much as the cost is justified. Factors such as sliding distance, normal forces between the billet and forging tool, and the hardness of the tool all influence wear during forging. This study focuses on analytics of the process using measurements of the tool conditions and wear simulation based on Archard's law. The tool was analysed using stress, geometry, and hardness measurements. Several strategies to maintain or increase hardness, thereby extending tool life, are proposed. These include adjusting heat treatment before forging, modifying machining parameters, extending cooling time during hot forging, and replacing the current coolant with a more effective one.

Metal additive manufacturing surface topographies are complex and challenging to characterise due to e.g. steep local slopes, re-entrant features, varying reflectivity and features of interest in vastly different scale ranges. Nevertheless, average height parameters such as Ra or Sa are commonly used as sole parameters for characterisation. In this paper, a novel method for selecting relevant parameters for evaluation is proposed and demonstrated using a case study where the smoothing effects after three processing steps of the electro chemical post-process Hirtisation of a metal AM surface are quantified. The method uses a combination of conventional areal texture parameters, multiscale analysis and statistics and can be used to efficiently achieve a detailed and more relevant surface topography characterisation. It was found that the three process steps have different effects on the surface topography regarding the types and sizes of features that were affected. In total, Sdq was reduced by 97 %, S5v was reduced by 81 % and Sa was reduced by 78 %. A surface texture with much lower average roughness, less deep pits and less steep slopes was produced, which is expected to be beneficial for improved fatigue properties.

Layer removal with electropolishing is a well-established method when measuring residual stress profiles with lab-XRD. This is done to measure the depth impact from processes such as shot peening, heat treatment, or machining. Electropolishing is used to minimize the influence on the inherent residual stresses of the material during layer removal, performed successively in incremental steps to specific depths followed by measurement. Great control of the material removal is critical for the measured stresses at each depth. Therefore, the selection of size of the measurement spot and electropolishing parameters is essential. The main objective in this work is to investigate how different electrolytes and electropolishing equipment affect the resulting surface roughness, geometry, microstructure, and consequently the measured residual stress. A second objective has been to establish a methodology of assessing the acquired electropolished depth. The aim has been to get a better understanding of the influence of the layer removal method on the accuracy of the acquired depth. Evaluation has been done by electropolishing one ground and one shot peened sample of a low-alloy carbon steel, grade 1.1730, with different methods. The results showed a difference in stresses depending on the electrolyte used where the perchloric acid had better ability to retain the stresses compared to the saturated salt. Electropolishing with saturated salt is fast and results in evenly distributed material removal but has high surface roughness, which is due to a difference in electropolishing of the two phases, ferrite, and pearlite. Perchloric acid electropolishing is slower but generates a smooth surface as both ferrite and pearlite have the same material removal rates but may cause an increased material removal for the center of the electropolished area. In this work, it is suggested to use perchloric acid electropolishing for the final layer removal step. © 2023, The Author(s).

Milling is traditionally the most used machining method when manufacturing complex gas turbine components. In particular those made from nickel-based superalloys. However, for larger free form surfaces, grinding may be an efficient alternative that could be used throughout the complete manufacturing route, from roughing to finishing. Hence, in this work the two processing methods has been compared in regard to surface integrity and productivity. Machining tests have been performed on case plates of heat-treated Alloy 718 using best practise setting for roughing and finishing with grinding and milling. The surface integrity of the work pieces was evaluated regarding surface topography, residual stresses, and deformation. This comparison showed that the main advantage with grinding is the ability to switch between roughing and finishing by just altering the depth of cut. Further, grinding offers lower surface roughness, compressive residual stresses, and significantly lower degree of deformation. From a productivity perspective, deep grinding may offer high material removal rates and ability to machine several work pieces in the same setup. However, grinding is limited to simpler free form geometries and may result in minor surface damages and abrasive surface residue. For selection of machining strategy, advantages and drawbacks shown in this work need to be considered for the application at hand in respect to productivity, surface integrity and requirements on fatigue life. 

Progressive induction hardening is an in-line steel heat treatment method commonly used to surface harden powertrain components. It produces a martensitic case layer with a sharp transition zone to the base material. This rapid process will induce large residual stresses, where a compressive state in the case layer will shift to a tensile state in the transition zone. For fatigue performance, it is important to quantify the magnitude and distribution of these stresses, and moreover how they depend on material and processing parameters. In this work, x-ray diffraction in combination with a layer removal method is used for efficient and robust quantification of the subsurface stress state, which combines electropolishing with either turning or milling. Characterization is done on C45E steel samples that were progressively induction hardened using either a fast or slow (27.5 or 5 mm/s, respectively) scanning speed. The results show that although the hardening procedures will meet arbitrary requirements on surface hardness, case depth and microstructure, the subsurface tensile stress peak magnitude is doubled when using a fast scanning speed. However, the near-surface compressive residual stresses are comparable. In addition, the subsurface tensile residual stress peak is compared with the on-surface tensile stresses in the fade-out zone.

Interest in hard-turning is steadily increasing due to its obvious benefits in terms of desirable surface integrity and improved operational efficiency. Surface microstructural variations can occur during machining due to cutting speed, tool geometry, and process conditions. Th ese variations create nanostructured white layers (WL), categorized as mechanically induced white layers (M-WL) or thermally induced white layers (T-WL). This study explored the role of retained austenite (RA) content (<2%, 12%, and 25%) on WL generation in AISI 52100 bearing steel, offering insights for optimizing hard-turning. The findings showed that, regardless of RA content, samples exhibited M-WL with no dark layer beneath the white layer when utilizing a cutting speed (VC) of 60m/min using a fresh insert. Increasing tool flank wear to 0.2mm led to the formation of T-WL and surface tensile residual stresses in specimens with higher RA content (12% and 25%). This effect was also observed at 260m/min with a fresh cutting insert. Machining at 260m/min with a worn tool (VB) of 0.2mm resulted in T-WL and surface tensile residual stresses, independent of RA content. Additionally, a 0.2mm tool wear caused a significant shift in the maximum subsurface compressive residual stre sses to greater depths, irrespective of RA content.