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

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

Towards understanding the thermal history of microstructural surface deformation when cutting a next generation powder metallurgy nickel-base superalloy

Despite the ongoing progress in metallurgical characterisation of machined surfaces, knowledge of the thermal conditions under which they originate during the workpiece-flank interaction is still lacking. When cutting advanced superalloys, little is known about temperature evolution in the machined part volume, where workpiece material interacts with the tool flank. In this work, the characteristics of the thermal field and the resulting surface metallurgy induced in a next generation nickel-base superalloy have been studied for cutting scenarios involving different combinations of thermo-mechanical boundary conditions. Analysis of the thermal field evolution in the workpiece subsurface has allowed the heating and cooling rates induced by cutting to be revealed, allowing description of two distinct types of thermal cycle, with a Heating-Peaking-Cooling (H–P–C) and a Heating-Quasi-isothermal Deformation-Cooling (HQC) structure depending on the process aggressiveness. Subsurface thermal history has been found to relate with the severity of the cutting-induced deformation, as it combines information on thermal field magnitude and on the process rates. Furthermore, thermal balance equations have been applied to study the rate of the heat generation in the machined subsurface due to its own plastic deformation while interacting with the tool flank. This has revealed that the highest rate of heat generation induced by plastic deformation occurred in thin surface layers at the beginning of the workpiece-flank contact, which has been associated to the conditions under which white layers (WLs) are generated. Energy balance analysis has furthermore indicated the development of a less severe and less impulsive deformation process at higher subsurface depths, which has been linked to the formation mechanism of material drag (MD) layers. In this way, the thermal history of machined surfaces has been related to their resulting metallurgical integrity, allowing in-depth understanding of the physical conditions developing when cutting next-generation superalloys

Can higher cutting speeds and temperatures improve the microstructural surface integrity of advanced Ni-base superalloys?

Future Ni-base superalloys are designed to deliver outstanding mechanical behaviour at high temperatures, which may translate in significant machining challenges. In this work, a paradigm is presented by which is proven how machining of these materials could benefit from increased cutting speeds and temperatures provided that they are able to promote shear localisation and thermal softening in the chip deformation zones, whilst retaining high-temperature strength within the machined surface. In this way, thermal control of chip formation leads to both lower cutting forces and energies, as well as enhanced surface integrity with lower levels of microstructural reconfiguration

Temperature-dependent shear localisation and microstructural evolution in machining of nickel-base superalloys

Understanding the microstructural evolution mechanisms in machining of advanced materials is essential to achieve excellent surface integrity levels within the manufacture of safety–critical components. However, as thermal and mechanical effects are coupled in conventional cutting operations, it is difficult to attribute their individual role on microstructural evolution and integrity. To investigate the temperature-dependency of microstructural evolution in cutting, a new experimental set-up has been developed to perform machining experiments under controlled temperatures. Results show that an onset in chip shear localisation with nanocrystalline grain refinement can be induced uniquely by an increase in cutting temperature under fixed cutting parameters, which microstructurally controls the transition from continuous to serrated chip formation. Increase in mechanical effects at HT leads to the formation of a continuous chip grain refinement layer, associated to a change in energy partition at the tool-workpiece interface. These small-scale behaviours are found to control the reduction in cutting forces and energy at higher temperatures, with a decrease of ∼ 25–30%. Nevertheless, despite the lower deformation energy, HT cutting induced larger amounts of microstructural deformation because of thermal softening effects, further disclosing the role of thermal effects on the interplay between shear localisation, microstructural evolution and surface integrity

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

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

IXPE instrument integration, testing and verification

The Imaging X-ray Polarimetry Explorer (IXPE) is a scientific observatory with the purpose of expand observation space adding polarization property to the X-ray source's currently measured characteristics. The mission selected in the context of NASA Small Explorer (SMEX) is a collaboration between NASA and ASI that will provide to observatory the instrumentation of focal plane. IXPE instrument is composed by three photoelectric polarimeters based on the Gas Pixel Detector (GPD) design, integrated by INFN inside the detector unit (DU) that comprises of the electrical interfaces required to control and communicate with the GPD. The three DUs are interfaced with spacecraft through a detector service unit (DSU) that collect scientific and ancillary data and provides a basically data handling and interfaces to manage the three DUs. AIV has been planned to combine calibration of DUs and Instrument integration and verification activities. Due the tight schedule and the scientific and functional requirements to be verified, in IAPS/INAF have been assembled two equipment's that work in parallel. The flight model of each DU after the environmental tests campaign was calibrated on-ground using the Instrument Calibration Equipment (ICE) and subsequently integrated in the instrument in the AIV-T process on a AIV and Calibration Equipment (ACE), both the facilities managed by Electrical Ground Support Equipment (EGSE) that emulate the spacecraft interfaces of power supply, functional and thermal control and scientific data collection. AIV activities test functionalities and nominal/off-nominal orbits activities of IXPE instrument each time a calibrated DU is connected to DSU flight model completing step by step the full instrument. Here we describe the details of instrumentation and procedures adopted to make possible the full integration and test activities compatibly with calibration of IXPE Instrument

First detection of X-ray polarization from the accreting neutron star 4U 1820-303

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz

X-Ray Polarimetry of the Dipping Accreting Neutron Star 4U 1624-49

We present the first X-ray polarimetric study of the dipping accreting neutron star 4U 1624$-$49 with the Imaging X-ray Polarimetry Explorer (IXPE). We report a detection of polarization in the non-dip time intervals with a confidence level of 99.99%. We find an average polarization degree (PD) of $3.1\pm0.7$% and a polarization angle of $81\pm6$ degrees east of north in the 2-8 keV band. We report an upper limit on the PD of 22% during the X-ray dips with 95% confidence. The PD increases with energy, reaching from $3.0\pm0.9$% in the 4-6 keV band to $6\pm2$% in the 6-8 keV band. This indicates the polarization likely arises from Comptonization. The high PD observed is unlikely to be produced by Comptonization in the boundary layer or spreading layer alone. It can be produced by the addition of an extended geometrically thin slab corona covering part of the accretion disk, as assumed in previous models of dippers, and/or a reflection component from the accretion disk

A polarimetrically oriented X-ray stare at the accreting pulsar EXO 2030+375

Accreting X-ray pulsars (XRPs) are presumably ideal targets for polarization measurements, as their high magnetic field strength is expected to polarize the emission up to a polarization degree of ~80%. However, such expectations are being challenged by recent observations of XRPs with the Imaging X-ray Polarimeter Explorer (IXPE). Here we report on the results of yet another XRP, EXO 2030+375, observed with IXPE and contemporarily monitored with Insight-HXMT and SRG/ART-XC. In line with recent results obtained with IXPE for similar sources, analysis of the EXO 2030+375 data returns a low polarization degree of 0%-3% in the phase-averaged study and variation in the range 2%-7% in the phase-resolved study. Using the rotating vector model we constrain the geometry of the system and obtain a value for the magnetic obliquity of ~$60^{\circ}$. Considering also the estimated pulsar inclination of ~$130^{\circ}$, this indicates that the magnetic axis swings close to the observer line of sight. Our joint polarimetric, spectral and timing analysis hint to a complex accreting geometry where magnetic multipoles with asymmetric topology and gravitational light bending significantly affect the observed source behavior.Comment: A&A accepted. Proofs versio