White Layer Formation Research Articles

CHARACTERIZATION OF SURFACE, SUBSURFACE DAMAGE AND THEIR EFFECT ON FATIGUE LIFE AFTER MACHINING OF AEROSPACE-GRADE ALUMINUM ALLOY (Al 6082-T6)

Aluminum alloys are widely used in the automotive and aerospace industries due to their lower mass-to-strength ratio than other metallic alloys. Apart from their inherent properties, aluminum alloys like other metallic alloys show a significant change in their mechanical properties according to the machining parameters. The research literature on obtaining optimum mechanical properties of aluminum alloys that undergo machining is very limited. Moreover, the combined effect of several parameters on the machinability of aluminum alloys has not yet been explored. In this paper, the effect of three machining parameters (Depth of Cut (DoC)), feed rate (FR), and cutting speed (CS) on the subsurface damage and fatigue life of aerospace-grade aluminum alloy (Al-6082-T6) is observed. Samples are prepared using a full fractional approach to effectively capture the effect of all input parameters. Thereafter, samples were subjected to surface roughness, micro-hardness, and fatigue life tests. Results of surface roughness and micro-hardness tests are compared with fatigue life. The general linear model was employed to capture the percentage effect of each input parameter on the output parameters. The results showed that DoC was the main contributing factor that caused subsurface damage, while surface roughness and fatigue life were mainly affected by FR and CS. Optical microscope images showed a white layer formation that had higher hardness than the base metal. Overall, this research work proposes the input parameters that can be used to achieve minimum surface damage and fatigue life.

Rolling Element Bearing Damage in the Presence of Applied Electric Current

Modern wind turbines and battery electric vehicles have bearings in close proximity to electrical generators, motors, and inverters. These systems have motivated new research focused on damage mechanisms associated with low- and high-level electric currents in rolling element bearings. This work presents a study on the influence of applied electric current and lubricant selection on bearing surface damage. Direct current was applied to a rotating tapered roller bearing with axial load and oil lubrication. Tests were conducted with various levels of applied current and two different oils (additized and non-additized). Micropitting, white etching cracking (WEC), and the formation of a surface white layer were investigated using both optical microscopy and scanning electron microscopy. Some of the tested bearings had black oxide surface treatment on their rollers and raceways. The black oxide was evaluated with regard to the prevention of the WEC damage mode. The occurrence of the described damage modes depended on the type of oil used and the application of electric current. Higher electric currents increased bearing surface damage in these tests.

Subsurface conditioning in BTA deep hole drilling for improved component performance

When bores with high length-to-diameter ratios (l/D > 10) and large diameters (D > 40 mm) are required, usually, the Boring and Trepanning Association (BTA) deep hole drilling process is used. Common industrial applications of this process are aerospace engineering and petrol exploration, where drilled components range from landing gears and engine shafts to drill collars. Since such parts tend to be particularly costly and highly safety–critical, ensuring favorable surface integrity during drilling is crucial to guarantee their reliability and performance. This study aims to identify correlations between the BTA deep hole drilling process and the resulting surface integrity using experimental and simulative approaches. The impact of feed and cutting speed on the thermomechanical loads and the resulting surface integrity are analyzed, also taking into account the occurrence of dynamic process disturbances. Particularly, the formation of white etching layers (WEL) is investigated using well-established, conventional techniques such as optical microscopy and microhardness testing. Additionally, innovative micromagnetic methods are employed. Magnetic Barkhausen noise (MBN) analysis is qualified as a well-applicable approach for rapid, non-destructive detection of WEL. To enhance understanding of MBN analysis and increase its robustness, the underlying mechanisms, governing the magnetic behavior of the subsurface are elucidated in detail by X-ray diffraction (XRD), electron backscatter diffraction (EBSD), magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE) microscopy. The methodology will serve as a basis for controlled subsurface conditioning in BTA deep hole drilling.

Artificial neural network-based modelling and prediction of white layer formation during hard turning of steels

During hard machining, steels subjected to very high thermal and mechanical loads can result in microstructural/phase changes such as the formation of a white layer. This layer, which is often harder than the raw material, is considered detrimental to the fatigue performance and in-service life of machined parts. This paper proposes a comprehensive study of white layer formation during hard machining of steels using statistical analysis and artificial neural networks (ANN) modeling. To this end, two steals, named AISI 52100 and AISI 4340, commonly used in the manufacturing of structural machines’ components and extensively studied in the last decade, have been considered in this study. First, Taguchi method combined with response surface methodology (RSM) was applied to analyze and to optimize the machining parameters regarding the white layer thickness. Second, an ANN model is developed to predict the white layer thickness during hard machining of the studied steels using a large amount of machining data. Three training algorithms were tested to find the most robust configuration. The equivalent carbon parameter was introduced for the first time in machining modeling which make the proposed ANN-based model capable of predicting the white layer thickness for different hardened steels. The results show a significant agreement between predictions and experimental results, avoiding costly experimental machining tests.

Characterization of the performance of microbial-based metalworking fluids in grinding using cell analogues

In grinding, the applied metal working fluid plays a decisive role in terms of minimizing (i) process energy consumption and (ii) thermal impact on surface and subsurface of the workpiece owing to lubrication and cooling effect of the fluid. For microbial-based metalworking fluids (MWFs), the presence of bacterial cells has been shown to be beneficial for MWF performance in grinding. The interaction of the cell bodies with other components of MWFs and their influence on the performance in grinding operations has not been investigated so far due to the difficulty of ensuring defined boundary conditions. As a novel approach, the present work investigates the individual influence of the presence of cell analogues and other MWF properties on the performance of water-based MWF in surface grinding of ball bearing steel. The evaluation is based on the specific material removal rate, which, when exceeded in the process, results in the onset of white layer formation. It has been shown that the physical presence of bacterial cells and extracellular polymeric substances have positive effects, while droplet size has only a minor influence on MWF performance.

Formation of white etching layer on rails due to coupled thermal and mechanical actions

The formation mechanism of the white etching layer (WEL) on the rail steel material has been confirmed via two different routes; thermal-induced WEL by phase transformation and mechanical-induced WEL by severe plastic deformation. In this study, a combination of thermal and mechanical processes was performed to examine the synergistic interaction of two formation mechanisms of the WEL on rails. An experiment was conducted on the rail steel material using a Gleeble thermo-mechanical simulation system. The results confirmed that the martensitic WEL is formed by phase transformation from rapid quenching at a temperature below the critical point of material phase transformation, under 550 °C, when high hydrostatic pressure is applied. The application of combined thermal and mechanical action induced martensite with the presence of retained austenite. The microstructure of the transition zone between the WEL and the base material contains partial and full dissolution of cementite, broken pearlite lamella, and dislocation density, confirming that both thermal and mechanical processes are involved in the phase transformation of the material.

Details of pearlite to austenite transformation in steel: Experiments and phase-field modeling

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

Thermochemical oxidation of commercially pure titanium; controlled formation of robust white titanium oxide layers for biomedical applications.

This study addresses controlled formation of white rutile surface layers on commercially pure (CP) titanium by gaseous thermochemical oxidation. The formed oxide layers were investigated with light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), transmission electron microscopy - energy-dispersive X-ray spectroscopy (TEM-EDXs), spectrophotometry, thermogravimetric analysis (TGA), and Vickers micro-indentation. The oxidation response of CP titanium in different single gas systems, O2, N2O, or CO2, at temperatures ranging from 750 °C to 1000 °C, showed that the formed oxide scales exhibit oxide stratification, irrespective of the applied gas. Additionally, a two-step oxidation process was found to result in controlled growth of robust, dense, adherent white titanium oxide layers. The two-step process entails a first oxidation step in an atmosphere of CO/CO2 at 750 °C; a second oxidation step is performed in N2/N2O at 650 °C. The oxidation in a CO/CO2 atmosphere results in the incorporation of carbon in the forming oxide layer. TEM-EDXs analysis after oxidation in CO/CO2 revealed that carbon resides in thin interlayers between slightly stratified 200-300 nm layers of rutile. This unique “composite” oxide layer containing carbon was found to be robust and densely adhering to the substrate. After the second oxidation step in N2O carbon was “retracted” (oxidized) from the oxide‑carbon composite layer, resulting in an aesthetically pleasing, adherent white oxide layer. Finally, this two-step process, leading to robust white oxide layer formation, is show-cased on a biomedical demonstrator part for dental applications.

Nonlocal thermomechanical coupled analysis of wheel-rail contact using a peridynamic model

Nonlocal peridynamic (PD) theory is widely used in damage and thermomechanical coupling simulations. This study applied ordinary state-based PD thermomechanics to model the wheel-rail contact bringing temperature-dependent materials. A two-dimensional coupled thermomechanical model was developed to predict the evolution of the thermomechanical response, thermal damage, and wear in railway rail. The model was verified based on the displacement and temperature fields of the rail. The model results correlated well with the finite element model and analytical method results. The influence of the vehicle speed, rail surface cracks, and wheel idling on the thermomechanical response of the rail was also evaluated in detail. The results show that the influence of the vehicle speed on the temperature field is much greater than that of the contact stress. The contact temperature under the critical creepage could lead to the formation of a brittle white etching layer (WEL). Cracks on the rail surface increased the contact stress and temperature peaks. Thermomechanical loading also promoted rail surface damage. Continuous thermomechanical loading due to wheel idling softened and removed rail surface materials, forming wear pits and a WEL, known as wheel burns.

High-stress abrasive wear performance of medium-carbon direct-quenched and partitioned, carbide-free bainitic, and martensitic steels

Experimental steels, a direct-quenched and partitioned (DQP) steel and a carbide-free bainitic steel (CFB), were tested along with a commercial martensitic 500 HB grade wear resistant steel in high-stress abrasive conditions. The three steels had different microstructures consisting of varying fractions and morphologies of martensite, retained austenite, and bainitic ferrite. The results showed that the CFB steel had a lower mass loss compared to the martensitic 500 HB steel with a similar hardness level. The DQP steel had a higher initial hardness and outperformed the other two steels. Wear surface characterization revealed that the investigated steels had significant work hardening of the wear surface, except with different mechanisms. Transformation induced plasticity (TRIP) increased the hardness of the DQP and CFB steels, while the fully martensitic 500 HB had more white layer formation on the wear surface resulting in increased hardness.

Heterogenous structure and formation mechanism of white and brown etching layers in bainitic rail steel

The failures of conventional pearlitic rail steels are strongly affected by the formation of hard and brittle white and brown etching layers (WEL and BEL, respectively) on the rail raceway during service. Bainitic rail steels with an excellent strength–toughness balance are promising candidates for next generation rail steels. It is generally difficult to generate hard and brittle WEL/BEL in bainitic rails because of the low carbon content. This study reports, for the first time, the formation of a unique multilayer heterostructured WEL/BEL in a field-tested bainitic rail. The heterogenous structures of WEL/BEL were characterized using transmission electron microcopy, focused ion beam, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, electron probe microanalysis, atom probe tomography, and nano-indentation techniques. Results demonstrate that WEL is mainly composed of fine-grained martensite or ferrite and retained austenite, whereas BEL is a composite structure containing nanocrystalline martensite or ferrite, retained austenite, cementite, and oxide or O-rich particles. The oxide or O-rich particles are primarily formed within BEL through local oxidation, resulting in secondary partitioning of the alloying elements between the oxidation product and its surrounding matrix. BEL is hard but brittle due to the formation of brittle twinned martensite, cementite, and oxide particles, whereas the adjacent WEL is relatively soft and ductile. Based on these findings, the heterostructured WEL/BEL formation mechanism is preliminarily discussed in terms of inhomogeneous grain refinement, localized oxidation, chemical alterations (including both chemical segregation in the initial microstructure and secondary partitioning of the alloying elements during local oxidation), and phase transformation.

Investigation on White Layer Formation in Dry High-Speed Milling of Nickel-Based Superalloy GH4169

To investigate the formation mechanism of the white layer on the machined surface during high-speed milling of nickel-based superalloy GH4169, several cutting parameters were selected for milling experiments. Energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD) were employed to characterize element distribution, phase transformation, and microstructure changes in the machined surface of the superalloy and then reveal the formation mechanism of the white layer on the machined surface. The results show that the white layer appears on the machined surface of GH4169, which is dense and has no obvious structural features. The total amount of elements in the white layer remains unchanged, but the distribution of elements such as C, N, O, Fe, and Ni changes due to phase change. The formation mechanism of the white layer is due to the dynamic recovery and dynamic recrystallization caused by the heat–force coupling effect, which leads to the grain refinement of the material and thus forms the white layer. This investigation can provide theoretical support to improve the service life of the parts in actual machining.

In-situ micro-cantilever bending studies of a white etching layer thermally induced on rail wheels

A complex and untraceable mechanical and thermal loading situation in rolling-sliding contacts can lead to the formation of white etching layers (WELs), depicting critical crack initiation sites. For our detailed studies, to obtain a holistic view of the microstructural characteristics and micro-mechanical properties, we prepared a WEL on a decommissioned (after 200,000 km service life) rail wheel by laser surface treatments with a defined energy input. This WEL is predominantly martensitic down to a depth of 30–40 μm, after which a transition to the deformed ferritic-pearlitic microstructure of the hypoeutectoid rail wheel steel is present. The martensitic region is with 6.98 ± 0.68 GPa significantly harder than the transition zone (5.17 ± 0.39 GPa) and the deformed ferritic-pearlitic base material (3.30 ± 0.33 GPa). In-situ V-notched micro-cantilever bending experiments of the martensitic (and thus most brittle) region show crack initiation and propagation – mostly along the boundaries of the martensitic grains – besides a plastic behavior. Applying the elastoplastic fracture mechanics allows to derive the local fracture toughness KIQ, which is 16.4 ± 1.2 MPam1/2 for this martensitic region of the WEL. The results outpoint the application of micro-cantilever bending tests in addition to hardness testing as a promising tool to discuss the relationship of microstructural characteristics with its micro-mechanical properties.

Study on white layer formation during machined surface evolution in high-speed machining of rail steel

Study on white layer formation during machined surface evolution in high-speed machining of rail steel

Modeling of microstructural workpiece rim zone modifications during hard machining

Surface integrity determines, among other factors such as tool material and wear, the chemical composition and microstructure of the workpiece material as well as selected cutting parameters, the performance of machined parts. The aggressive nature of the cutting process influences the surface integrity and causes altered microstructure on the workpiece surface. Most popular example is the White Layer formation, which reduces the fatigue resistance of components. To study the influence of cutting parameters on White Layer thickness for a steel of grade AISI 4140, in this paper, a physics-based dislocation density-based model is implemented to predict the White Layer formation under the assumption of dynamic recrystallization. As a material model, a modified coupled damage mechanics model based on Johnson–Cook is implemented, which considers in comparison to the original one, damage induced softening using damage initiation and fracture indicators as well as strengthening mechanisms through grain refinement and dislocation damping at high strain rates. Dry orthogonal cutting is modeled via Coupled-Eulerian–Lagrangian method in Abaqus/Explicit. The developed model is validated based on experimental data regarding grain size and White Layer thickness. The model overestimated the grain size on the machined workpiece rim zone by a factor of 2–4, while the White Layer thickness was predicted with an error of max. 0.36 μm.

Modeling of Microstructural Evolution in Hard Turning of AISI-52100

White layers formed during machining of AISI-52100 decrease the fatigue life of dynamically loaded components. It is critical to predict the temperature and microstructure evolution of machined surfaces. This research examined 3D simulation of hard turning using a round insert. Voce-Johnson Cook constitutive law and hardness-based microstructural evolution model were incorporated. To reduce the excessive computational time, the 3D simulation was simplified to a 2D simulation. The predicted cutting forces, temperature, and microstructure changes were validated against experimental data. The results showed the accuracy of the process simulation for process optimization in order to avoid white layer formation.

Surface integrity of laser-assisted turning of the GH4169 alloy.

The unique properties of GH4169, such as low thermal conductivity, high strength at high temperatures, and work hardening characteristics, make it difficult to machine. As an effective way to improve the machinability of materials, laser-assisted machining (LAM) has the advantages of reducing the cutting force, restraining chatter, improving productivity, and prolonging tool life. This study investigates the surface integrity of GH4169 in LAM, including the surface roughness, surface defects, microhardness, residual stress, and white-layer formation. First, the depth of the heat-affected layer from 380 to 760r/min was determined to ascertain the depth of cutting. Secondly, at different cutting parameters, the surface roughness was found to be significantly reduced. LAM leads to a more severe plastic deformation, producing a thicker hardened layer and higher surface hardness on the fabricated substrate, but its internal hardness is less than that of samples made by conventional machining (CM), owing to thermal softening. Both machining methods produce residual tensile stresses, which are larger in LAM. Thirdly, the results of a surface integrity experiment show that white-layer generation is a consequence of the combination of phase change, heat, and plastic deformation. Finally, a comprehensive fuzzy evaluation considering the surface roughness, surface defects, microhardness, residual stress, and thickness of the white layer is established, which verifies that LAM significantly improves the surface integrity of GH4169, setting up a theoretical foundation and technical reference for the laser-assisted turning of nickel-based superalloys.

Mechanical white etching layer formation kinetics in pearlitic steels: A phenomenological model based on microstructural characterization

In order to predict the lifetime of railtrack components, numerical models are required to better describe the apparition of microstructural defects that lead to material failure. In pearlitic steels, the severe mechanical loading or/and the local temperature increase result in the formation of a hard and brittle phase that is prone to cracking, which is called White Etching Layer (WEL).The current study proposes a phenomenological model, based on the microstructural evolution, to describe the mechanically-induced-WEL kinetics of formation. First, a detailed multiscale microstructural characterization was performed in order to identify suitable microstructural indicators for the WEL formation. The grain size, orientation and morphology, and internal disorientation were characterized for different steps of mechanical WEL formation. This could be achieved thanks to the use of a dedicated test bench that allows to monitor the wheel–rail contact condition (sliding, number of cycles, and contact pressure) independently. The proposed model was validated using tests performed in extreme load and sliding conditions.The numerical and experimental results indicate that the most severe conditions (contact pressure and sliding ratio) catalyze WEL formation, and that the same deformation state can be achieved through different loading paths.

Formation mechanism of brown etching layers in pearlitic rail steel

The formation of White Etching Layers (WELs) in rails can lead to the initiation of microcracks, thus reducing the service time of rails. However, the WEL formation mechanism is not yet understood. Brown Etching Layers (BELs) are the intermediate microstructure located between parent pearlite and WEL. In this work, we use BELs as a microstructural tracer to understand the early stages of WEL formation. We study the formation mechanisms of BELs in an industrial R350HT rail steel retrieved from a service application. Atom probe tomography, complementary transmission electron microscopy, and transmission Kikuchi diffraction provide us access to high resolution microstructure characterization. Carbon-depleted cementite lamellae are sheared and cut by dislocations. Several BEL areas with cementite particles exist in the direct vicinity of parent pearlite. Energy dispersive X-ray spectroscopy reveals redistribution of carbon and inhomogeneous distribution of manganese in BEL areas. These results suggest a correlation between cementite decomposition and the formation of BEL. Deformation-induced cementite decomposition causes local alterations in the microstructure regarding defect density and carbon composition, leading to austenite formation in the parent pearlite aided by the temperature ingress during rail/wheel contacts.

Prediction and analysis of material removal rate and white layer thickness during wire electrical discharge turning (WEDT) process

This paper seeks to propose an analytical model for material removal rate (MRR) and white layer thickness (WLT) during wire electrical discharge turning (WEDT) process. This approach considers the predicted crater radius and depth (based on overlapping of craters) for the determination of MRR. The discharge energy (ranging from 4.217 J to 12.228 J) was utilized for the evaluation of the process responses. Besides, a detailed analysis of material removal is visually demonstrated with the aid of a high-speed camera captured at 15,000 fps. The images/videos from the camera help to visualize the material removal process at various discharge energies and input settings. A non-uniform increase in MRR at discharge energy ranging from 4.217 J to 8.475 J was observed, presumably due to the unpredictable wire vibration as confirmed from the high-speed camera. The white layer formation during the process was analyzed using SEM images. Beyond the discharge energy value of 10.276 J (approx.), substantial increase in molten pool formation causes the WLT to shift to a higher incremental slope. The predicted MRR and WLT values are in good agreement with the measured results with an absolute average error of 7.09% and 5.12%, respectively. A detailed EDS analysis confirms the variation in surface elemental composition before and after the WEDT process.