High Thermo-mechanical Loads Research Articles

Advanced calculation of the stress distribution in milling tools during cutting under consideration of residual stresses and tool wear

Milling is an irreplaceable technology for finishing turbomachinery components made of nickel-based alloys. Cutting tools in milling are exposed to high thermomechanical loads causing tool wear or ultimately tool failure. The initial condition of the tool’s surface layer determines the propagation of tool wear. “Surface layer” denotes the zone near the tool surface, which is affected mechanically, thermally, and chemically during the manufacturing process of the tool (e.g. due to grinding, polishing). So far, only limited knowledge is available on the specific effects of wear-induced changes in tool geometry as well as of residual stresses on the distribution of stresses in the cutting wedge of milling tools. This publication presents a method to consider residual stresses of cutting tools and tool wear in a 2D FEM chip formation model for detailed analysis of certain tool failure phenomena in the milling process. In order to represent a depth distribution of residual stresses in the surface layer of the uncoated tool in the simulation, residual stress measurements at grinded milling tools were performed. Tool wear was taken into account by adapting the geometry of the cutting tool. The geometry changes were based on measurements of the width of the flank wear land and of the cutting edge radius. Principal stresses and von Mises equivalent stress distribution were analyzed. The described method may improve the understanding of surface layer effects on tool life of milling tools. With decreasing chip thickness, a second, more localized zone of tensile stresses was formed at the rake face subsurface, which can be critical in terms of fracture. With compressive residual stresses and with increased flank wear, these critical tensile stresses were reduced or even eliminated.

Study on dedicated drill bit design for carbon fiber reinforced polymer drilling with improved cutting mechanism

Carbon fiber reinforced polymer (CFRP) has significant applications in the aerospace and automobile industries. However, the relatively high thermomechanical loads generated when drilling CFRP is considerably detrimental to the cutting tool. To address this issue, in this work, a step-change in the performance of CFRP drilling was conducted by switching the cutting model from crushing/compression fracture to the shear cutting of the composite workpiece to suppress the thermomechanical loads using a novel web thinning design. To evaluate the performance of the proposed drill bit, a comprehensive comparison was carried out between the dedicated and conventional drill bit designs from thermal, mechanical, and chip formation points of view. The results show that the dedicated drill bit outperformed the conventional drill bit in terms of thrust force, temperature elevation, burr, and exit-delamination. An in-depth observation revealed that the shear cutting became dominant in the drilling with the proposed drill bit while the crushing/compression was dominant with the conventional drill bit. The transition of the phenomena was induced by the properly designed rake angle. The mechanism of material removal in CFRP drilling using the dedicated drill bit design provides a new design direction to modifying the rake angle for improving the efficiency and quality of composite material drilling.

In-process measurement of Barkhausen noise and resulting productivity increase potential in grinding of case hardened steel

The micromagnetic Barkhausen Noise (BN) measurements bear the capability of providing non-destructively information about the condition of the surface integrity of ground workpieces. No unambiguous statements about the surface and subsurface area state can be made by evaluating single measured micromagnetic values if a superposition of different effects in the microstructure as a consequence of high thermomechanical loads occur. Especially for highly stressed components in automotive industry or in wind energy systems, case hardenend steels are deployed. These steels are often finished at the end of their process chain by a grinding process. At this point, the workpieces to be ground have already achieved enormous created added value. For productivity reasons, grinding should be carried out close to the process limit. To avoid undesirable phase transformations in the microstructure, process models such as the process model from Malkin can be used. If a grinding process consists of several process steps it must be ensured that the final process step leads to the intended surface and subsurface properties. In order to make the process design as productive as possible, an in-process measurement signal which correlates to the current state of the surface and subsurface area can help to achieve this result. Ideally this signal is applicable to generate an adaptive grinding process.Such a signal is investigated in this paper by application of BN as an in-process measurement technology. The paper shows the reliability of BN measurements as well as that there are different maximum BN amplitudes for different material removal rates. It is expected that these BN values correlate particularly to the residual stresses in the surface and subsurface area since Malkin's grinding burning limit was used to exclude (strong) phase transformations. Considering a micromagnetic parameter measured in-process a reduction of process time of 37% and thus a significant process improvement was achieved.

Cutting force based surface integrity soft-sensor when hard machining AISI 4140

AbstractWorkpiece rim zone modifications during hard machining can be explained with the high thermo-mechanical loads induced by the cutting process. The formation of White Layers with a fine-grained microstructure by dynamic recrystallization (DRX) is one of those surface modifications that can negatively affect the functionality of a machined part by changing the residual stress state and facilitating crack initiation. As a consequence, the fatigue life of the machined parts is reduced. It is therefore of great interest to understand the thermo-mechanical conditions which induce White Layers formation in order to be able to control them by in-situ measurements if necessary. For this purpose, a cutting force based Soft-Sensor is developed in this study which enables the in-process estimation of White Layer thickness. Therefore, a cutting force based analytical model is used to estimate the resulting temperature fields and correlated with validated numerical chip formation simulations. In addition, the predictions of the White Layer thickness of the analytical model are then compared using light microscopy and the results of the numerical finite element model, in which a DRX model is additionally implemented.

A three-dimensional axial fuel relocation framework with discrete element method to support burnup extension

Under normal operation, oxide nuclear fuel pellets subjected to high thermomechanical loading and irradiation damage can develop complex crack networks, separating the fuel pellet into a pile of fragments. During a Loss of Coolant Accident (LOCA), these pellet fragments, or fine powder especially when originating from the high-burnup fuel pellet periphery, may relocate axially into the ballooned region of the zircaloy cladding. In this work, the axial relocation process of fuel fragments and powder is modeled based on the three-dimensional Discrete Element Method (DEM) in the Abaqus software. A Voronoi tessellation technique is adopted to generate random convex polyhedrons in order to mimic a coarse fragmentation pattern. A parametric study in terms of the mean fragment size from 1.3 mm to 3.0 mm was conducted to investigate its effect on the axial profile of fuel mass fraction and filling ratio after fuel relocation. Then a sensitivity study of the randomness in fragmentation on axial fuel relocation was performed. For high-burnup fuels, in order to characterize the porous High Burnup Structure (HBS), the pellet rim was discretized into separate spherical shape particles. The effect of powder fraction on fuel relocation was demonstrated, and the results indicated that higher powder fraction can significantly increase the fuel mass fraction and filling ratio level in the ballooned region, which showed qualitative agreement with experimental observations. Finally, the verification against a one-dimensional theoretical relocation model was conducted to verify the robustness and high fidelity features of the DEM model in simulating axial fuel relocation under LOCA. The modeling approach serves as a mechanistic framework to support future burnup extensions for the oxide nuclear fuel.

Nondestructive characterization of elastic modulus of APS Ni–5Al/10hBN coating on stainless steel 304 under high temperature

The failure mechanisms of coatings, under high thermo-mechanical loads, can be examined through finite element simulations. However, mechanical properties such as Young’s modulus and Poisson’s ratio are necessary to achieve an accurate model. In this study, nickel–aluminum with hexagonal boron nitride (NiAl/hBN) is deposited on stainless steel 304 substrate using atmospheric plasma spray technique. The effect of the hydrogen content in the plasma-formed gas on the microstructure, hardness, and porosity of the deposited layers is examined. Also, the mechanical properties of the coating are measured at ambient temperatures using non-contact laser ultrasound technique. Results indicated that the flow rate of hydrogen has a major impact on the coating structure and elastic properties. The inhomogeneity in the coating can be minimized by decreasing the hydrogen flow rate. The coating Young’s modulus was increased at room temperature, which slowly decreased while increase in atmospheric temperature. The behavior of Young’s modulus at high temperature causes decrease in flexural strength. In addition, the back calculations are well agreed with experimental results.

Development of a Methodology for Strain Field Analysis during Orthogonal Cutting

Functionality of all machined parts depends on their dimensional tolerance, surface condition and subsurface characteristics, which are often summarized as surface integrity. Key aspects of surface integrity are a change of the residual stress state and white layer formation. Reasons for any workpiece modification of the surface rim zone can be found in the occurring high thermo-mechanical loads during the cutting process. Over the last years, many researchers published simulation based approaches to determine both thermo-mechanical loads and changes in workpiece microstructure induced by machining. Nevertheless, the direct measurement of relevant state variables such as plastic strain and temperature fields at relevant cutting conditions is still a key challenge. Directly obtaining those state variable fields from the process will enable the development of fast computing analytical models to in-process estimate surface integrity features. As a basis for later modelling of dynamic recrystallization during cutting, a methodology to calculate the accumulated equivalent plastic strains alongside one particle movement path throughout the shear zone is developed in this paper. Therefore, an orthogonal cutting test bench has been used to obtain high resolution images when cutting AISI 4140 which were then analyzed using Digital Image Correlation (DIC).

Selection of machining condition on surface integrity of additive and conventional Inconel 718

Abstract Inconel 718 is a nickel-based superalloy used in steam turbines, jet engines, etc., where it is subjected to high thermomechanical loads. Inconel 718 is the most preferred alloy in the above applications due to its excellent resistance to fatigue and creep. Inconel 718 is mainly strengthened by the precipitates which were formed during the heat treatment process by the combination of elements like Nb, Ti, and Al along with the base element Ni. Selective Laser Melting (SLM) is one of the most used additive manufacturing processes to manufacture components with high dimensional accuracy in a flexible manner with less material usage. The samples fabricated by SLM were subjected to two heat treatments: Hot Isostatic Pressing (HIP) to improve the density followed by the Aeronautic Heat Treatment (AHT) in order to improve the material properties. The heat-treated SLM samples have the same mechanical properties of the conventional Inconel 718 fabricated by the Cast and Wrought (C&W). The Central Composite face-centered Design (CCD) was used for the Design of Experiments (DOE) in which three different cutting speeds (vc) and feed rates (fz) were considered whereas the depth of cut (ap) is kept constant. Machining experiments were conducted under dry, Minimum Quantity Lubrication (MQL) and emulsion (Wet) conditions on both the cast and wrought (C&W) Inconel 718 and additive fabricated SLM Inconel 718. The optimum cutting condition for C&W and SLM Inconel 718 samples was determined based on three criteria: minimum specific cutting energy (Wc), minimum tool wear (Vb) and minimum surface roughness (Ra). From the overall results, it is being inferred that the SLM Inconel 718 produced better results compared to C&W Inconel 718.

New strategies for improvement of numerical model accuracy in machining of nickel-based alloy

Accuracy and reliability of the numerical results is one of the most important challenges in the scientific community for finite element modeling (FEM) of cutting processes. It becomes more essential in machining of difficult-to-cut materials such as nickel-based alloys where high thermo-mechanical loads are induced into the workpiece. In this paper, new strategies were adopted to improve accuracy of FE modeling of cutting process of Inconel718 superalloy. Firstly, a novel hybrid strategy was established to simultaneously calibrate controllable simulation parameters. It was implemented based on the design of experiment, intelligent systems and FEM of cutting process. Using validation with experimental results, a great improvement was obtained for prediction of cutting forces, maximum temperature, and chip geometry compared with numerical results obtained previously by conventional calibration procedure. At the second stage of the paper, a grain size-based flow stress was developed by implementation of advanced FORTRAN user-subroutine in FE code to take into account the effect of changes in material properties during the chip formation. According to the results, implementation of grain size based flow stress has a significant effect on enhancement of simulation accuracy. Finally, it can be concluded that, the innovated strategies presented in this paper provide fundamental and useful approaches to improve precision in modeling of other manufacturing processes.

Machining influence on the fatigue resistance of Inconel 718 fabricated by Selective Laser Melting (SLM)

Selective laser melting (SLM) is one of the most used additive manufacturing processes, using which complex parts can be built with high dimensional accuracy in a flexible manner with less material usage. The aerospace industry relies on SLM technique to manufacture components like turbine and compressor blades which has a high demand in its dimensional tolerance.Inconel 718 which is a nickel based super alloy used in steam turbine, jet engines where it is subjected to high thermomechanical loads. Inconel 718 is the most preferred alloy in the above applications due to its highest strength-to-weight ratio, its excellent resistance to fatigue, creep and corrosion. Inconel 718 is mainly strengthened by the precipitates which were formed during the heat treatment process by the combination of elements like Nb, Ti and Al along with the base element Ni.In this paper microstructure analysis of as-built and heat treated samples were done along with the evaluation of its mechanical characterization. Followed by the machining test under dry and emulsion conditions using the same cutting parameters. The surface integrity evaluation were done before performing four point bending fatigue test. The fatigue fracture of the sample corresponding to number cycles to failure along with the fractographic analysis of the dry and emulsion machined samples was investigated. The fatigue crack failure is compared to each process of machining to determine the best machining condition for Inconel 718. The best machining condition is dry compared to emulsion as it gives better surface roughness, more compressive residual stress due to which dry machined samples sustained more cycles to failure.

On understanding the microstructure of SiC/SiC Ceramic Matrix Composites (CMCs) after a material removal process

The unique material nature (e.g. hard, brittle, heterogeneous and orthotropic) of SiC-based Ceramic Matrix Composites (CMCs) highly affects the outcomes of machining process by inducing high thermo-mechanical loads during material removal. This can result in severe material damage which in turn causes a reduction of the in-service life of critical structural ceramic components (such as in aero-engines or nuclear reactors). In this study, the phenomenon by which the material removal mechanism during drilling influences the CMC surface integrity are discussed by characterising the fracture and deformation phenomena on the CMC's constituents - i.e. SiC and Si materials. Moreover, the strain induced to the surface, together with the changes in chemical composition are characterised via micro Raman spectroscopy and related to the principles of residual stresses upon cutting. This results in a novel understanding of the material removal process that governs cutting of SiC-based CMCs while emphasising how the different microstructure, morphology and nature of ceramics behave under the same cutting conditions. This study has therefore led to a comprehension of how the microstructure of complex hierarchical ceramic materials such as SiC/SiC CMCs is affected by a mechanical cutting process and opens avenues to understand the structure damage under other machining operations (e.g. milling, grinding).

Analysis of active cooling panels in a scramjet combustor considering the thermal cracking of hydrocarbon fuel

In this paper, a procedure is demonstrated for the numerical analysis of a cooling system using active panels for high-speed combustion chambers subjected to high thermo-mechanical loads. A promising alloy for high heat loads, namely, niobium Cb-752, is considered in this study. In addition to sensible heat transfer, endothermic heat absorption through the cracked hydrocarbon fuels is a viable option for cooling the chamber panels. The focus of the panel design is to minimize the weight with the safe thermo-mechanical characteristics of the panel. The present analysis is carried out in three steps. First, a one-dimensional (1D) analytical model is developed. The second and third steps are three-dimensional (3D) analyses without and with consideration of fuel endothermicity, respectively. The optimal channel dimensions obtained from the 1D analysis provide inputs for the 3D analysis. The channel dimensions obtained from the 1D analysis do not satisfy the targeted parameters in the 3D analysis. Thus, the channel is redesigned in 3D and tested with and without cracking. The channel dimensions are optimized for the ranges of heat fluxes (160–220 W/cm2) in the combustor and mass flow rates (0.011–0.015 kg/s) of the fuel in the channel. Reducing the weight of the panel is sought by considering the benefit of heat absorption through the endothermic cracking process in the fuel channel. The combination of an increased width and cracking performs better than the other non-cracking cases. The study presents the most efficient configuration suitable for applications in high-speed combustion chambers under the given heat flux conditions. By increasing the channel width and cracking (for 3D), the weight of the panel is reduced by 10.58%. The fuel exit temperature, fuel cracking behavior in the channel, structural stress distribution and weight of the panel are analyzed in terms of the operating conditions.

Temperature dependent mechanical properties of metallic HVOF coatings

The complex failure mechanisms of metallic coatings under high thermomechanical loads can be analyzed in finite element simulations. For this purpose, mechanical properties are needed to ensure a realistic modelling approach. The development of new coatings and the usage of coating processes different from other investigations in the literature require new measurements to obtain realistic material parameters.In the present study, tensile- and compression tests as well as vibrating reed experiments were carried out to determine Young's modulus as well as strength and yield stress of HVOF sprayed metallic coatings at different temperatures. Free standing specimens of NiCrAlY, NiCuCrAl and Rene80 coatings were produced to avoid an unknown influence of the substrate to the experimental results.The values of Young's modulus from the three measurement methods show a good agreement at moderate temperatures. At elevated temperatures, a discrepancy between the quasi-static tests (tensile/compression) and the vibrating reed experiments has been observed, caused by creep and plasticity due to the lower strain rates and larger strains in the quasi-static tests.After a pre-exposure of the coatings to high temperatures, an increase of Young's modulus was observed. The mechanism of this effect has not been understood yet, but may be caused for example by recovery or a closure of defects and micro-cracks in the coatings.All coatings have a relatively large compressive strength and yield stress greater than 1 GPa at moderate temperatures, but exhibit a brittle/ductile transition at elevated temperatures where a sharp reduction of the yield stress and increase of ultimate strain could be observed.

Analysis of multipoint cutting tool temperature using FEM and CFD

Nimonic C-263 is a British nickel–chromium–cobalt–molybdenum alloy specially meant to be used in high-temperature and high-strength applications. Consequently, it produces high thermo-mechanical loads on the machining tool when machined. As a result, excessive heat is generated during dry drilling, between Nimonic C-263 and tungsten carbide tool coated with Alcorna, which in turn reduces the durability of the tool. In order to determine the temperature distribution of the tool, the coupling of finite element machining simulations along with computational fluid dynamics simulations is performed. The temperature distribution of the tool under dry condition is simulated via the DEFORM 3D software and the Altair AcuSolve software. Further investigation under cooling condition is performed using Altair AcuSolve software. The mass flow rate of the coolants is kept constant when the temperature distribution is obtained during the CFD analysis under cooling condition. Silver nano coolant has high heat reduction compared to other coolants and is found to generate 34% less heat than dry condition.

Modeling the rubbing contact in honeycomb seals

Metallic honeycomb labyrinth seals are commonly used as sealing systems in gas turbine engines. Because of their capability to withstand high thermo-mechanical loads and oxidation, polycrystalline nickel-based superalloys, such as Hastelloy X and Haynes 214, are used as sealing material. In addition, these materials must exhibit a tolerance against rubbing between the rotating part and the stationary seal component. The tolerance of the sealing material against rubbing preserves the integrity of the rotating part. In this article, the rubbing behavior at the rotor–stator interface is considered numerically. A simulation model is incorporated into the commercial finite element code ABAQUS/explicit and is utilized to simulate a simplified rubbing process. A user-defined interaction routine between the contact surfaces accounts for the thermal and mechanical interfacial behavior. Furthermore, an elasto-plastic constitutive material law captures the extreme temperature conditions and the damage behavior of the alloys. To validate the model, representative quantities of the rubbing process are determined and compared with experimental data from the literature. The simulation results correctly reproduce the observations made on a test rig with a reference stainless steel material (AISI 304). A parametric study using the nickel-based superalloys reveals a clear dependency of the rubbing behavior on the sliding and incursion velocity. Compared to each other, the two superalloys studied exhibit a different rubbing behavior.

Damage mechanisms of metallic HVOF-coatings for high heat flux application

The copper wall of regeneratively cooled liquid fuel rocket-combustion chambers is exposed to high thermomechanical loads. Although it is cooled by liquid hydrogen in internal cooling channels, surface temperatures of more than 800°C on the hot-gas side are reached. To lower this temperature and to protect the copper against oxidation, a metallic coating system is developed. It is applied with high velocity oxygen fuel spray (HVOF) and consists of a bond coat and a top coat. As top-coat materials, two candidates were tested in this study: A nickel-based and a cobalt-rhenium-based alloy. For the bond-coat, a new NiCuCrAl alloy has been developed previously.The purpose of this article is to investigate the failure mechanisms of the new metallic coating systems. For this purpose, two tests are applied. On the one hand, laser-cycling experiments are conducted to test the coatings with a thermal gradient between top-surface and substrate and to investigate the influence of high heating rates. On the other hand, the coatings were tested isothermally in a furnace to investigate the coatings without any thermal gradient. Interface delamination, buckling, formation of vertical cracks and kirkendall porosity are identified as relevant failure mechanisms and the conditions under which they will occur are discussed.

Improvement in the brake disc design for heavy vehicles by parametric evaluation

Design of the brake disc geometry for a given brake disc material provides an opportunity for improvement in the fatigue life of the brake disc. High thermomechanical loads at braking lead to substantial local plastification and also induce tensile residual stresses in certain areas of the brake disc. This contributes to shortening of the fatigue life of the brake disc by possible initiation and growth of cracks. In the present paper, a simulation approach for evaluation of brake disc designs with respect to thermomechanical performance is developed and applied. Brake disc performance is analysed using commercial finite element software by employing a constitutive model for grey cast iron implemented in a Fortran subroutine. The thermal loading consists of consecutive severe braking cycles at a constant brake power and a constant speed, with cooling between the brake cycles. Based on a previous experimental study, three different assumptions are made regarding the spatial distribution of the thermal load at braking. A standard commercial brake disc made from grey cast iron having straight vanes is used as the reference case. Geometrical modifications are introduced in the ventilation arrangement using a design-of-experiments approach, studying both straight cooling vanes and different pillar layouts. A preliminary assessment of the fatigue life of the brake discs is carried out. The results indicate that the introduction of different pillar arrangements instead of straight vanes make it possible to decrease the mass of the brake disc by up to 13% or to increase the fatigue life of the brake disc by about 50%.

Super-strong materials for temperatures exceeding 2000\u2009\xb0C

Ceramics based on group IV-V transition metal borides and carbides possess melting points above 3000 °C, are ablation resistant and are, therefore, candidates for the design of components of next generation space vehicles, rocket nozzle inserts, and nose cones or leading edges for hypersonic aerospace vehicles. As such, they will have to bear high thermo-mechanical loads, which makes strength at high temperature of great importance. While testing of these materials above 2000 °C is necessary to prove their capabilities at anticipated operating temperatures, literature reports are quite limited. Reported strength values for zirconium diboride (ZrB2) ceramics can exceed 1 GPa at room temperature, but these values rapidly decrease, with all previously reported strengths being less than 340 MPa at 1500 °C or above. Here, we show how the strength of ZrB2 ceramics can be increased to more than 800 MPa at temperatures in the range of 1500–2100 °C. These exceptional strengths are due to a core-shell microstructure, which leads to in-situ toughening and sub-grain refinement at elevated temperatures. Our findings promise to open a new avenue to designing materials that are super-strong at ultra-high temperatures.

Unified Constitutive Modeling towards Enhanced Structural Integrity

Design and analysis of critical components in energy (nuclear, solar and fossil power), aerospace, automobile and chemical industries based on detailed inelastic analysis can enhance structural integrity and thereby economy. Especially for the components exposed to very high temperature thermomechanical fatigue loading, unified inelastic analysis based life prediction may enhance accuracy. A unified constitutive model (UCM) with features of strain rate-dependence, static recovery, mean-stress evolution, strain range-dependence, and finally creep damage is developed. The modified UCM is validated against simulating a broad set of strain-controlled isothermal and anisothermal fatigue and fatigue-creep responses, and stress-controlled creep responses of Haynes 230. Some of these results are presented to demonstrate improved simulations by the modified UCM. Importance of damage parameters in improving simulations in the tertiary creep regime is observed.

Modelling of grey cast iron for application to brake discs for heavy vehicles

Cast iron brake discs are commonly used in the automotive industry, and efforts are being made to gain a better understanding of the thermal and mechanical phenomena occurring at braking. The high thermomechanical loading at braking arises from interaction between the brake disc and the brake pads. Frictional heating generates elevated temperatures with a non-uniform spatial distribution often in the form of banding or hot spotting. These phenomena contribute to material fatigue and wear and possibly also to cracking. The use of advanced calibrated material models is one important step towards a reliable analysis of the mechanical behaviour and the life of brake discs. In the present study, a material model of the Gurson–Tvergaard–Needleman type is adopted, which accounts for asymmetric yielding in tension and compression, kinematic hardening effects, viscoplastic response and temperature dependence. The material model is calibrated using specimens tested in uniaxial cyclic loading for six different temperatures ranging from room temperature to 650 °C. A special testing protocol is followed which is intended to activate the different features of the material model. Validation of the model is performed by using tensile tests and thermomechanical experiments. An application example is given where a 10° sector of a brake disc is analysed using the commercial finitie element code Abaqus under a uniformly applied heat flux on the two friction surfaces. The results indicate that the friction surface of the hat side and the neck can be critical areas with respect to fatigue for the uniform heating studied.