Modelling alpha case formation and embrittlement for Ti-6Al-4V produced by additive manufacturing and subjected to thermomechanical post-processing

Comunicación presentada en: 5th Iberian Conference on Structural Integrity que corresponde con el 38 Congreso del Grupo Español de Fractura GEF2022 celebrado en Coimbra, Portugal del 30 de marzo al 1 de abril de 2022La Fabricación Aditiva permite la producción optimizada de geometrías complejas. Las propiedades de tracción entallada de la aleación Ti-6Al-4V se estudian numéricamente en este trabajo considerando el método de Selective Laser Melting (SLM) y diferentes condiciones de post-proceso. La formación de una capa frágil enriquecida con oxígeno o “alpha case” también se reproduce aquí para atmósferas no inertes aprovechando esquemas numéricos para el agrietamiento asistido por hidrógeno. La reducción local de la energía de fractura en función de la concentración de oxígeno se implementa en un modelo de Phase Field para la nucleación y propagación de grietas. Se modelizan diferentes probetas de tracción entallada en Comsol Multiphysics. El análisis es secuencial: la entrada y difusión de oxígeno se simulan para diferentes temperaturas y tiempos de permanencia, reproduciendo procesos típicos de SLM y HIP y diferentes presiones parciales de oxígeno. En un segundo paso, se simula el ensayo mecánico de tracción y se resuelve el problema de Phase Field considerando una reducción lineal de la tenacidad a fractura en función de la concentración de oxígeno. Se evalúan los efectos del valor de la longitud característica y del comportamiento plástico. Los resultados muestran que el marco numérico actual, después de una necesaria calibración de parámetros, es capaz de predecir la influencia del post-proceso termomecánico en la fractura de componentes entallados.Additive Manufacturing enables cost-effective production of complex geometries. Notched tensile properties of Ti-6Al4V alloy are here numerically studied considering the Selective Laser Melting (SLM) method and different postprocessing conditions. The formation of an oxygen-enriched brittle layer or an “alpha case” in Ti-6Al-4V is also reproduced for non-inert atmospheres. Exploiting other Finite Element frameworks for environmentally assisted cracking, e.g. hydrogen embrittlement models, the local reduction of fracture energy as a function of oxygen concentration is implemented in a Phase Field model for crack nucleation and propagation. Different notched tensile specimens are modelled in Comsol Multiphysics. The analysis is sequential: oxygen uptake and diffusion are simulated for different temperatures and dwell times, reproducing typical SLM and HIP processes and different partial pressures of oxygen. In a second step, mechanical tensile testing is simulated, and the damage Phase Field scheme is solved considering a linear reduction of fracture toughness as a function of oxygen concentration. The effects of the characteristic length value and of plastic behaviour are evaluated. Results show that the present framework, after parameter calibration, is able to predict the influence of thermomechanical post-processing on notch fracture.The authors gratefully acknowledge financial support from the Junta of Castile and Leon through grant BU002-P20, co-financed by FEDER funds

Stress-intensity factor solutions for embedded elliptical cracks in round bars subjected to tensile load

In this paper, stress-intensity factor solutions are presented for an embedded elliptical crack in a round bar subjected to tensile load. The stress-intensity factors (SIF) are presented in a tabulated form and were obtained from three-dimensional finite-element analyses of this crack configuration. The solutions provide the stress-intensity factor as a function of three dimensionless parameters representative of the crack size, the crack aspect ratio of the elliptical flaw and its relative position in the cross section. The dimensionless parameters cover ranges that allow most internal flaw shapes in practice to be considered. In order to validate the numerical model developed, some particular cases are compared with solutions of embedded elliptical flaws in different geometries available in the literature. Afterwards, a sequential methodology for fatigue crack growth is presented, including the conditions for the recategorization from the internal elliptical crack to a semi-elliptical surface crack. A comparison of the predicted crack paths with experimental results of fatigue crack propagation initiated from internal defects in round bars is also presented. This experimental validation shows the capability of the proposed SIF solutions for the study of the fatigue crack propagation initiated from internal defects in this geometry.JCyL project reference BU-002-P20, co-financed with FEDER funds

Stress-intensity factor solutions for the simulation of fish-eye fatigue crack growth in round bars subjected to tensile load

Trabajo presentado en: The 7th International Conference on Crack Paths (CP 2021), organised by TC3 Fatigue of Engineering Materials and Structures of the European Structural Integrity Society (ESIS). The CP 2021 edition will take place in a virtual format (September 21st to 24th, 2021)The fatigue crack growth in round bars initiated from internal defects leads to the formation of a circular crack pattern usually so-called fish-eye. This failure mechanism is found in the current additive manufacturing techniques in which internal defects, such as pores or lack of fusion, are the main cause of fatigue crack initiation. Moreover, this fatigue mechanism becomes the predominant failure mode in the Very High Cycle Fatigue (VHCF) regime. With the aim of adequately studying these fatigue crack situations, this paper presents a set of solutions for the stress-intensity factor calculation for embedded elliptical cracks in a round bar subjected to tensile load. The stress-intensity factors (SIF) are presented in a tabulated form and were obtained from three-dimensional finite-element analyses. The SIF solutions are provided as a function of three dimensionless parameters that include the crack size, the crack aspect ratio, and its relative position in the cross section. After that, a sequential methodology for fatigue crack growth simulation is presented, and a comparison with experimental results of fatigue crack propagation initiated from internal defects in round bars is also presented. Finally, by varying the initial crack position and the initial crack aspect ratio, several aspects related to the evolution of the fatigue crack shape in this geometry are analyzed.The authors gratefully acknowledge financial support from the Junta de Castilla y Leon (Spain) through grant BU-002-P20, co-financed by FEDER funds

Closed-form equations for the calculation of stress intensity factors for embedded cracks in round bars subjected to tensile load

Fatigue crack propagation initiated from internal defects is a typical mechanism observed in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) of cylindrical tensile specimens subjected to uniaxial cyclic loads. To study the fatigue crack propagation of these embedded cracks by means of a fracture mechanics approach, solutions for the Stress Intensity Factor (SIF) for different crack configurations are needed. In this paper, a set of closed-form equations for the calculation of the SIF of embedded cracks in round bars subjected to tensile load is presented. Two sets of solutions are provided, which allow for different levels of approach to be considered. The first solution provides the SIF for the vertex points of an internal elliptical crack as a function of three dimensionless parameters related to crack size, crack position and crack aspect ratio. The second solution is a simplification for eccentric circular cracks located at any position of the cross section. The methodology necessary for the application to the study of fatigue crack propagation is also presented, and a comparison with those obtained from experimental tests is included, which exhibits a very good capacity for prediction.The authors gratefully acknowledge financial support from the Junta de Castilla y Leon (Spain) through grant BU-002-P20, co-financed by FEDER funds

Impresión 3D como herramienta docente en asignaturas de Resistencia de Materiales

[EN] 3D printing is an increasingly popular technology that is being proposed from different approaches as a teaching tool. Besides its application in the production of visual and representative objects, 3D printing is proposed here for the manufacture of specimens that are then tested with the aim of characterising mechanical properties of materials within the teaching framework of Solid Mechanics courses. This methodology is able to engange sutdents in the desing, printing and test of the specimens. As a result, the process of material characterisation in the lab is faster and might be completely integrated as a supporting teaching tool of different testing procedures related to the Structural Integrity of materials and components.[ES] La Impresión 3D es una tecnología en auge que está siendo propuesta desde diversos enfoques como una herramienta docente. Además de su utilidad como técnica de creación de objetos visuales y representativos, aquí se propone la fabricación de probetas típicas para la caracterización de las propiedades mecánicas de distintos materiales dentro del contexto docente de la Resistencia de Materiales. La metodología desarrollada hace partícipe al alumno en el proceso de diseño, impresión y ensayo de las probetas. De este modo, el proceso de caracterización se agiliza en el laboratorio permitiendo su completa incorporación como apoyo a la docencia de diversos ensayos relacionados con la Integridad Estructural de materiales y componentes.Díaz, A.; Cuesta, I.; Alegre, JM. (2019). Impresión 3D como herramienta docente en asignaturas de Resistencia de Materiales. En IN-RED 2019. V Congreso de Innovación Educativa y Docencia en Red. Editorial Universitat Politècnica de València. 1076-1084. https://doi.org/10.4995/INRED2019.2019.10474OCS1076108

Hydrogen uptake and diffusion kinetics in a quenched and tempered low carbon steel: experimental and numerical study

To better understand hydrogen uptake kinetics, electrochemical permeation tests have been performed in a quenched and tempered low-alloy steel. Hydrogen uptake and transport has been studied with three different surface roughness, in four different solutions (1 M H2SO4, 1 M H2SO4+As2O3, 0.1 M NaOH and 3.5% NaCl) and two different hydrogen charging current densities (1 and 5 mA/cm2). A strong effect of the charging solution, current density and surface roughness has been demonstrated. In 1 M H2SO4 + As2O3 solution and 5 mA/cm2, hydrogen recombination on the surface of the samples is strongly reduced and interstitial diffusion prevails due to the trap saturation ( ). However, in 1 M H2SO4, 0.1 M NaOH and 3.5% NaCl, hydrogen transport is dominated by trapping and detrapping processes ( ). Permeation transients are numerically reproduced through Finite Element simulations and compared to the experimental results. The relationship between hydrogen diffusion kinetics at the microstructural level and surface effects is clearly established by a mapping strategy obtained from the wide range of experimental results, combined with a numerical approach.The authors would like to thank the Spanish Government for the financial support received to perform the research projects RTI2018-096070-B-C33 and PID2021-124768OB-C21. This work was supported by the Regional Government of Castilla y León (Junta de Castilla y León) and by the Ministry of Science and Innovation MICIN and the European Union Next Generation EU/PRTR (MR4W.P2 and MR5W.P3). L.B. Peral is also grateful for his Margarita Salas Postdoctoral contract (Ref.: MU-21-UP2021-030) funded by the University of Oviedo through the Next Generation European Union

Numerical study of hydrogen influence on void growth at low triaxialities considering transient effects

Assuming that hydrogen enhances localised plasticity, as one of the leading mechanisms proposed in the literature, the void growth and coalescence are modified by local softening and ductile failure features depend on hydrogen accumulation. It is anticipated that strain rate plays an important role in hydrogen-informed void mechanisms, however, coupling voids, transient hydrogen diffusion, rate-dependent hydrogen-material interactions and intrinsic hardening, remains a challenge. In this study, the simulation of a void unit cell in a hydrogen pre-charged material is reconsidered here for the first time to incorporate transient effects, i.e. the kinetic redistribution of hydrogen around a void subjected to a high strain rate and a constant stress triaxiality. A coupled diffusion-mechanics scheme is implemented in a set of ABAQUS subroutines in order to analyse the interaction of hydrogen with the material response. The influence of strain rate is also considered when defining the cell boundary conditions through the limiting cases of equilibrium and insulated unit cells. The competition between the two inherent mechanisms, namely, hydrogen softening and strain rate hardening, is studied with the implemented framework. Results show that transient effects determine hydrogen concentrations and strongly dictate failure mechanisms: shearing might occur due to the hydrogen induced softening for moderate strain rates even though the cell is insulated. However, for very fast loading it is demonstrated that the fast creation of traps due to plastic deformation results in hydrogen depletion and necking failure is observed.MINECO Refs: MAT2014-58738-C3-2-R and RTI2018-096070-B-C3

Analysis of hydrogen permeation tests considering two different modelling approaches for grain boundary trapping in iron

The electrochemical permeation test is one of the most used methods for characterising hydrogen diffusion in metals. The flux of hydrogen atoms registered in the oxidation cell might be fitted to obtain apparent diffusivities. The magnitude of this coefficient has a decisive influence on the kinetics of fracture or fatigue phenomena assisted by hydrogen and depends largely on hydrogen retention in microstructural traps. In order to improve the numerical fitting of diffusion coefficients, a permeation test has been reproduced using FEM simulations considering two approaches: a continuum 1D model in which the trap density, binding energy and the input lattice concentrations are critical variables and a polycrystalline model where trapping at grain boundaries is simulated explicitly including a segregation factor and a diffusion coefficient different from that of the interior of the grain. Results show that the continuum model captures trapping delay, but it should be modified to model the trapping influence on the steady state flux. Permeation behaviour might be classified according to different regimes depending on deviation from Fickian diffusion. Polycrystalline synthetic permeation shows a strong influence of segregation on output flux magnitude. This approach is able to simulate also the short-circuit diffusion phenomenon. The comparison between different grain sizes and grain boundary thicknesses by means of the fitted apparent diffusivity shows the relationships between the registered flux and the characteristic parameters of traps.The authors gratefully acknowledge financial support from the project MINECO Refs: MAT2014-58738-C3-2-R and RTI2018-096070-B-C33. E. Martínez-Pañeda acknowledges financial support from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant Agreement No. 609405 (COFUNDPostdocDTU)

Analysis of the Influence of the thickness and the hole radius on the calibration coefficients in the hole-drilling method for the determination of non-uniform residual stresses

The Hole-Drilling method is a semi-destructive technique useful for obtaining residual stress distributions by drilling and measuring relieved strains. The standard for this method, i.e., ASTM E837 – 13a, is based on the Integral Method and facilitates obtaining the coefficient matrices required to solve the inverse problem and to calculate the residual stress at depths of up to 1.00 mm. A possible deviation from the coefficients given by this standard is searched when the piece has a small thickness or the hole diameter is not 2.00 mm. FEM simulations are performed with the aim of analysing these effects and proposing new matrices, expressions and correlations for conditions outside the usual thickness and diameter limits. A parametric sweep over a wide range of thicknesses and hole diameters has been implemented in ANSYS to establish a consistent and automated numerical procedure for widening the applicability of the Hole-Drilling method.Junta of Castile and Leon through grant no. BU053U16 (Cofunded by European Regional Development Fund/European Social Fund)

Effect of HIP post-processing at 850 °C/200 MPa in the fatigue behavior of Ti-6Al-4V alloy fabricated by Selective Laser Melting

Hot Isostatic Pressing (HIP) is a thermomechanical post-processing technique widely used in Additive Manufacturing parts to reduce internal defects, such as entrapped-gas-pores or lack-of-fusion, which have a great influence on the mechanical and fatigue properties of the material. In this paper, the effect of a non-conventional HIP-cycle on the fatigue behavior of a Ti-6Al-4V alloy manufactured by Selective Laser melting (SLM) is studied. The HIP-cycle examined in this study is carried out at pressure of 200 MPa and a temperature of 850 ◦C for 2 h. Moreover, the cooling process is faster than that obtained from conventional furnace cooling rates, with the aim to limit the microstructural coarsening effects that affect the fatigue behavior. For the study, an extensive experimental fatigue program was carried out which included a first batch of SLM specimens tested under as-built conditions, a second batch of SLM specimens subjected to the present HIP process, and a third batch of specimens of a reference wrought processed material obtained by rolling and annealing processes. The microstructure of the material, before and after HIPping, is analyzed and a fractographic analysis is carried out to study the mechanism of crack initiation and its relation to the fatigue behavior. The results show that the present HIP-process allows for very good material densification, a microstructure that shows minimal coarsening effects, and good fatigue properties comparable to the conventional wrought processed material.The authors gratefully acknowledge financial support from the Junta de Castilla y León (Spain) through grant BU-002-P20, co-financed by FEDER funds