Scale-bridging micromagnetic evaluation of surface integrity and conditioning mechanisms in deep hole drilled AISI 4140 steel
Creators: name: Strodick, Simon
Publisher: name: TU Dortmund
Year: 2024
Subjects: subject: Surface integrity
subject: Surface condition
subject: White etching layer
subject: Micromagnetic evaluation
subject: Deep hole drilling
subject: Boring Trepanning Association (BTA)
subject: 660
subject: Oberflächeneigenschaft
subject: Werkstückrandzone
subject: Barkhausenrauschen
subject: Zerstörungsfreie Werkstoffprüfung
subject: BTA-Tiefbohren
subject: Magnetische Werkstoffprüfung
subject: Mikrostruktur
subject: Plastische Deformation

description: In the face of global warming, modern production engineering is confronted with the pressing challenge of developing methods for a more sustainable production. A key strategy in manufacturing with vast potential for climate change mitigation is to improve material and resource efficiency by enhancing the surface integrity of components through adapted manufacturing processes. As numerous surface and subsurface properties are closely linked to performance, tailoring surface integrity enables the production of components with significantly improved capability, reliability and lifetime. This general connection between the design of machining processes, the resulting surface integrity of components and their performance also applies to the field of deep hole drilling. A widely employed process for machining large diameter bores with high length-to-diameter ratios is Boring Trepanning Association (BTA) deep hole drilling. This process is commonly used in fields like aerospace engineering or petroleum industry to machine components, such as landing gears, drill collars, or hydraulic cylinders. A major obstacle in assessing the surface integrity at the critical inner surface of deep drilled components is the hindered accessibility of the borehole wall, as it complicates or fully prevents the use of many conventional approaches, such as X-ray diffraction. In addition to this, some of the most common methods for assessing aspects of surface integrity like microhardness testing or optical microscopy of cross-sections are either fully-destructive or semi-destructive and thus their application is restricted to a limited amount of spot checks, while damaging the workpiece. In contrast, magnetic Barkhausen noise (MBN) analysis is an entirely nondestructive technique, based on relatively compact sensors that can be positioned inside of deep drilled boreholes. Consequently, this method is a particularly promising approach for a holistic evaluation of surface integrity in ferromagnetic deep drilled components. On this account, this thesis aims at qualifying MBN for the nondestructive, efficient, and reliable assessment of the surface integrity in deep drilled components. Since MBN is affected by multiple aspects of surface integrity which overlap and interfere, the mechanisms governing the MBN analysis need to be elucidated and separated. Therefore, in a first step, a thorough evaluation of surface integrity in BTA deep hole drilling is performed. This includes interconnected analyses of the microstructure, the microhardness gradients and the residual stress states in subsurfaces of deep drilled specimens. To analyze the impact of cutting parameters on surface integrity, the cutting speed and the feed in drilling are varied. It is observed that white etching layers (WEL) form at the surface of bores, when using relatively high cutting speeds and feed rates. These WEL bring about a significant increase in hardness. For instance, it is found that the microhardness inside the WEL can exceed the microhardness of the bulk material by more than three times. To investigate further into the microstructural evolution in the subsurface during deep hole drilling, electron backscatter diffraction analyses are performed. The results indicate a strong refinement of the grains resulting from severe plastic deformation as well as swept grains in the material adjacent to the WEL. As the nanocrystalline structures observed are close to the limit of the lateral resolution in electron backscatter diffraction, transmission Kikuchi diffraction is carried out additionally to validate the findings obtained by electron backscatter diffraction. In addition to this, electron channeling contrast imaging and scanning transmission electron microscopy are used to evaluate grain size gradients, grain morphologies, voids, and lattice defects such as dislocation structures and stacking faults. Building on these fundamental findings on surface integrity, scale-bridging micromagnetic approaches are combined in the next step, to identify interrelations between the different aspects of surface integrity and the micromagnetic properties. The evolution of the magnetic microstructures in external magnetic fields is evaluated by magneto-optic Kerr effect microscopy. In addition to this, magnetic domain structures are evaluated by the Bitter technique. Magnetic force microscopy is employed for an analysis of the magnetic microstructure with particularly high spatial resolution. Inside of white etching layers, no distinct magnetic domains are observed by any of the approaches. In specimens free of WEL, domain patterns are found, which expand to the very surface. These multi-instrumental investigations provide the scientific basis for the application of MBN. In MBN analysis, it is observed that WEL lead to significantly lower maximum MBN amplitudes, due to the inherent structure and properties of WEL. The high number of dislocations, stacking faults and grain boundaries inside the WEL, evidenced by the microstructural analyses, all act as pinning sites for domain wall motion. Based on these observations, it is assumed that the WEL barely contributes to the generation of Barkhausen noise in MBN analysis and that this results in the particularly low MBN amplitudes for specimens with WEL. Based on these findings, a multi-sensory MBN-based test stand is developed, which allows for the rapid characterization of surface integrity in BTA deep drilled components. It is used to compare the results of different systems for MBN analysis, employing both multipurpose sensors and a sensor designed to fit a drill head for BTA deep hole drilling. To analyze the impact of WEL on the mechanical properties of deep drilled components a procedure is developed, inspired by the tube flattening test according to ISO 8492. Results are provided for the compression of components when applying quasi-static and cyclic loads.