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Purpose The purpose of this paper is to investigate the effect of remelting each layer on the homogeneity of nickel-titanium (NiTi) parts fabricated from elemental nickel and titanium powders using laser powder bed fusion (LPBF). In addition, the influence of manufacturing parameters and different melting strategies, including multiple cycles of remelting, on printability and macro defects, such as pore and crack formation, have been investigated. Design/methodology/approach An LPBF process was used to manufacture NiTi alloy from elementally blended powders and was evaluated with the use of a remelting scanning strategy to improve the homogeneity of fabricated specimens. Furthermore, both single melt and up to two remeltings were used. Findings The results indicate that remelting can be beneficial for density improvement as well as chemical and phase composition homogenization. Backscattered electron mode in scanning electron microscope showed a reduction in the presence of unmixed Ni and Ti elemental powders in response to increasing the number of remelts. The microhardness values of NiTi parts for the different numbers of melts studied were similar and ranged from 487 to 495 HV. Nevertheless, it was observed that measurement error decreases as the number of remelts increases, suggesting an increase in chemical and phase composition homogeneity. However, X-ray diffraction analysis revealed the presence of multiple phases regardless of the number of melt runs. Originality/value For the first time, to the best of the authors’ knowledge, elementally blended NiTi powders were fabricated via LPBF using remelting scanning strategies.

Refill Friction Stir Spot Welding (RFSSW) is a technology used for joining solid materials that was developed in Germany in 2002 by GKSS-GmbH as a variant of the conventional friction stir spot welding (FSSW) [1]. In the RFSSW technology, the welding tool consists of a fixed outer part and rotating inner parts, which are called a pin and a sleeve. The tool for RFSSW is designed to plasticize the material of the parts to be joined by means of a rotary movement. The design of the tool allows independent vertical movement of both elements of the welding tool. This allows obtaining spot welds without creating holes that could weaken the structure. The main advantage of RFSSW is the potential for replacing the technologies that add weight to the structure or create discontinuities, such as joining with screws or rivets. Thus, RFSSW has great potential in the automotive, shipbuilding and aviation industries. Furthermore, the technology can be used to join different materials that could not be connected using other joining methods. The main objective of this work is to understand the physical and mechanical aspects of the RFSSW method – including the residual stress state inside the weld and around the joint. The results of the investigations can help to determine optimal parameters that could increase the strength and fatigue performance of the joint and to prove the significant advantage of RFSSW connections over other types of joints. The work assumes the correlation of two mutually complementary investigation methods: numerical analyses and experimental studies carried out with diffraction methods. The comparison between numerical and experimental results makes potentially possible the determination of degree of fatigue degradation of the material by observing the macroscopic stress state and the broadening of the diffraction peak width (FWHM), which is an indicator of the existence of micro-stress related to the dislocation density and grain size.

Methods of incremental manufacturing, i.e. 3D printing, have been experiencing significant growth in recent years, both in terms of the development of modern technologies dedicated to various applications, and in terms of optimizing the parameters of the process itself so as to ensure the desired mechanical and strength properties of the parts produced in this way. High hopes are currently being pinned on the use of highly penetrating types of radiation, i.e. synchrotron and/or neutron radiation, for quantitative identification of parameters characterizing objects produced by means of 3D printing. Thanks to diffraction methodologies, it is feasible to obtain input information to optimize 3D printing procedures not only for finished prints but also to monitor in situ printing processes. Thanks to these methodologies, it is possible to obtain information on parameters that are critical from the perspective of application of such obtained elements as stresses generated during the printing procedure itself as well as residual stresses after printing. This parameter, from the point of view of tensile strength, compression strength as well as fatigue strength, is crucial and determines the possibility of introducing elements produced by incremental methods into widespread industrial use.

In this work, the evolutions of stresses in both phases of the Al/SiCp composite subjected to thermal cycling during in situ compression test were measured using Time of Flight neutron diffraction. It was confirmed that inter-phase stresses in the studied composite can be caused by differences in the coefficient of thermal expansion for the reinforcement and matrix, leading to a different variation of phase volumes during sample heating or cooling. The results of the diffraction experiment during thermal cycling were well predicted by the Thermo-Mechanical Self-Consistent model. The experimental study of elastic-plastic deformation was carried out in situ on a unique diffractometer EPSILON-MDS (JINR in Dubna, Russia) with nine detector banks measuring interplanar spacings simultaneously in 9 orientations of scattering vector. For the first time, the performed analysis of experimental data allowed to study the evolution of full stress tensor in both phases of the composite and to consider the decomposition of this tensor into deviatoric and hydrostatic components. It was found that the novel Developed Thermo-Mechanical Self-Consistent model correctly predicted stress evolution during compressive loading, taking into account the relaxation of thermal origin hydrostatic stresses. The comparison of this model with experimental data at the macroscopic level and the level of phases showed that strengthening of the Al/SiCp composite is caused by stress transfer from the plastically deformed Al2124 matrix to the elastic SiCp reinforcement, while thermal stresses relaxation does not significantly affect the overall composite properties.

This work deals with the problem of X-ray stress determination on the samples dedicated to fatigue tests. A number of research studies point out the fact that the processing of hard, difficult to machine materials like nickel superalloys, reveals more than one trend of residual stress versus working parameters of behaviour (Lavella and Berruti, 2010). Many papers have shown that the residual stresses are dependent on a combination of a number of factors. When the above is taken into account simultaneously with the requirements of the internal General Electric specification for the fatigue tests samples preparation (Metallic test specimen preparation, low stress, 2017) the problem of turning and grinding parameters gathers significance. It is well known that the quality of the surface layer, produced during machining, is of vital importance for the fatigue life specially for the components of aircraft produced form nickel superalloys e.g. Inconel 718 (Kortabarri et al., 2011). That is why the surface layer’s properties are described in detail by the standards. The aim of the work is to determine one of the most influential features from the point of view of fatigue life, i.e. the stress state on the surface layer with one non-destructive method – the diffraction analysis.

The aim of the investigations was to determine the effect of parameters of refill friction stir spot welding (RFSSW) on the fracture load and failure mechanisms of the resulting joint. RFSSW joints were made in 7075-T6 Alclad aluminium alloy sheets using different welding parameters. The load capacity of joints was determined under tensile/shear loadings. Finite element-based numerical simulations of the joint-loading process were carried out, taking into account the variability of elasto-plastic properties of weld material through the joint cross-section. The influence of welding parameters on selected phenomena occurring during the destruction of the joint is presented. The considerations were supported by a fractographic analysis based on SEM images of fractures. It was found that there is a certain optimal amount of heat generated, which is necessary to produce the correct joint in terms of its load capacity. This value should not be exceeded, because it leads to weakening of the base material and thus to a reduction in the strength of the joint. Samples subjected to uniaxial tensile shear load showed three types of failure mode (tensile fracture, shear fracture, plug type fracture) depending on the tool rotational speed and duration of welding. Prediction of the fracture mode using FE-based numerical modelling was consistent with the experimental results. The samples that were damaged due to the tensile fracture of the lower sheet revealed a load capacity (LC) of 5.76 KN. The average value of LC for the shear fracture failure mechanism was 5.24 kN. The average value of the LC for plug-type fracture mode was 5.02 kN. It was found that there is an optimal amount of heat generated, which is necessary to produce the correct joint in terms of its LC. Excessive overheating of the joint leads to a weakening of the base metal and thus a reduction in the strength of the joint. Measurements of residual stresses along the axis specimens showed the presence of stresses with a certain constant value for the welded area on the side of the 1.6 mm thick plate.

The present work deals with the investigation of the micromechanical behaviour of a duplex steel, which consists of two phases (austenite and ferrite) exhibiting significantly different mechanical properties. The stresses in both phases were experimentally determined in the elastic and plastic ranges of deformation using synchrotron radiation diffraction experiments. The used methodology enabled to determine the values of initial stresses and to study the evolutions of the principal phase stresses and the second order stresses during the elastic, as well as the plastic range of deformation. With the help of a self-consistent model, the critical resolved shear stresses and the work hardening parameters for slip systems, active in each phase, were also estimated. Comparison of the measured phase stresses and lattice strains evolutions with the model results showed a good agreement between prediction and experiment when the initial stress state in the sample and shape of grains approximated by ellipsoidal inclusions were taken into account. The overall outcome of the work is the determination of stress partitioning between the two phases of a polycrystalline material for all deformation stages, determined directly from diffraction experiment. The results enabled analyses of von Mises, hydrostatic stresses as well as second order stresses evolutions in both phases during tensile deformation. Finally, the experimental data were successfully compared with predictions of the self-consistent deformation model.

The stress partitioning between phases, phase stress relaxation as well as origins of Al/SiCp composite strengthening are studied in the present work. In this aim, the measurements of lattice strains by neutron diffraction were performed in situ during tensile test up to sample fracture. The experimental results were compared with results of elastic–plastic self-consistent model. It was found that thermal origin phase stresses relax at the beginning of plastic deformation of Al/SiCp composite. The evolution of lattice strains in both phases can be correctly simulated by the elastic–plastic self-consistent model only if the relaxation of initial stresses is taken into account. A major role in the strengthening of the studied composite plays a transfer of stresses to the SiCp reinforcement, however the hardness of Al metal matrix is also important.

The publication describes how diffraction methods and mathematical bases can be used for measurement of various types of stresses in single-phase and multiphase materials. Firstly, the paper defines the stresses and classifies them from the scale of their interactions point of view. Subsequently, the phenomenon of radiation diffraction on the crystalline lattice is presented including formulas describing this phenomenon and the dependencies enabling stress measurements. The key part of the paper is the description of one of the second order stress estimation methods based on diffraction data and a self-consistent model.

The TOF neutron diffraction measurements were done for Al/SiCp metal–matrix composite (17% of SiC) subjected to T6 thermal treatment. Using three separated diffraction peaks of SiC phase and four peaks of Al phase, the lattice strains were measured for both phases independently during in situ tensile test. The experimental results were presented in comparison with elastoplastic model, which allows to find the values of parameters determining plastic deformation of Al matrix (critical resolved shear stress and hardening parameter). Additionally, the results of TOF method were compared with those which were obtained with monochromatic neutron radiation (LLB, Saclay). In the latter experiment Al/SiCp composite containing 25% of SiC was measured. It was shown that after elastoplastic deformation the mismatch stresses determined for both phases relax during tensile deformation.

The fatigue phenomena, as being one of the crucial importance for aircraft exploitation life, was investigated with different methods. The most important topic is to determine the state of the material and subsequently the moment of fatigue crack initiation. Many authors investigated the fatigue induced material damage and evolution of structure with different methods i.e.: scanning electron microscopy (Zhang et al. 2008), electron transmission microscopy (Hirsch and Whelan 1960), optical microscopy and two-beam interferometry (Kim and Laird 1978) or /and with modelling (Dingli et al. 2000). Nowadays, as the possibilities of diffraction methods were developed, so the conjoining of the diffraction image with the changes in the material structure can be applied to investigate the fatigue process in materials (Tahara et al. 2009; Huang et al. 2010).

In the previous work [1] it was shown that strain hardening had considerable impact on the hardness of Inconel 718. In order to verify the weakening of the material associated with the damage mechanism in the tested material, the hardness tests were performed on the stretched specimen which was subsequently heat treated. The tests revealed that, after heat treatment, the measured hardness was reduced with an increasing degree of the plastic deformation present prior to heat treatment.

Measuring the in-depth stress state is of vital importance for materials scientists. Strain gauges methods are capable of yielding information only about the surface stress state. Diffraction methods using synchrotron or neutron radiation, which allow totally non-destructive stress measurements inside the material, are not widely available. In this context, the best widely available method combines the X-ray diffraction stress measurements and gradual removal of the outer layer by means of electropolishing. Here, this method was applied to the specimen made of 1H13 stainless steel cut with under water on a corundum cut-off wheel. The idea was to investigate how deeply an additional stress state resulting from cutting was introduced and whether the technique of combining of X-ray diffractometry and electropolishing can be used widely for determining the stress state inside the specimen.

Electropolishing is the sole reliable method of removing the outer layer of the specimen without changing its stress state. This feature of the electropolishing procedure allows researchers to investigate the in-depth stress distribution. Developing of the method in a diffraction laboratory is crucial because there is no universal theory for the electropolishing procedure allowing the removal of the layers of different thickness. This is due to the multiplicity of different factors affecting the electropolishing results. A factor of vital importance from the point of view of indepth stress measurements is the thickness of the electropolishing layer. Hence the importance of the procedures for the electropolishing of a layer of a precisely defined thickness. This work deals with the problem of the selection of the parameters in the electropolishing process for two types of materials: stainless steel and aluminium alloy. The tests of mutual correlation of current intensity, voltage applied and time of the procedure and its results are presented in the paper.

Synchrotron X-ray diffraction was applied to study the evolution of lattice strain and stresses in both phases of pearlitic steel during a tensile test. The advantage of the methodology used in this work is the possibility of experimental study of stress localisation, which is directly determined from measurements and can be used to study the process of strain strengthening of lamellar pearlite. It was found that in the elastic range of deformation, both cementite and ferrite are loaded similarly due to the nearly equal elastic properties of both phases, while plastic deformation leads to significant load transfer from ferrite to cementite. Due to the complexity of the lamellar microstructure of the material, the classical elastic-plastic self-consistent model does not correctly predict the partitioning of the stresses between phases during plastic deformation. Therefore, the grain-matrix interaction given by the self-consistent model was modified and successfully applied to simulate the interaction between phases. The synchrotron experiment allowed us to determine the critical resolved shear stresses of ferrite phase in the pearlitic steel subjected to different thermal treatments. The role of cementite in material strengthening was evaluated on the basis of the evolution of von Mises stress, experimentally determined in both phases. It was found that during plastic deformations, the von Mises stress does not change significantly in ferrite compared to an important increase in elastically deformed cementite. Therefore, the partitioning of stresses between phases is mainly responsible for the strain strengthening of the tested pearlitic steel exhibiting fully lamellar microstructure.

The grain scale of materials is an area still open for investigations within the field of materials science. The most helpful tools to perform this type of research are diffraction methods. Within the research project presented in this paper two experiments were carried out employing two different types of radiation: neutron (ISIS) and synchrotron (ESRF). The aim of the work was to describe the stress state in the necking zone during the occurrence of a damage phenomenon (Fig. 1.) in separate phase and to check the level of the homogeneity. The supplemental tools were the finite elements method and self-consistent modeling – it testified, confirmed and completed our experimental results and allowed us to formulate the justifiable conclusions.

The paper presents the results of diffraction stress measurement in Al/SiC composite and in 2124T6 aluminum alloy during the in situ tensile test. The main aim of the work is to observe the stress values for different stages of tensile test for the composite after applying two types of thermal treatment and for the alloy used as a matrix in this composite, to identify the type of hardening process. The experimental results were compared against the calculations results obtained from the self-consistent model developed by Baczmański [1] - [3] to gain the information about the micromechanical properties (critical resolved shear stress τcr and hardening parameter H) of the examined materials. This comparison allowed researchers to determine the role of reinforcement in the composite as well as the impact of the heat treatment on the hardening of the material.

International audience; In the present work, the mechanical behavior of phases in duplex steel during tensile test was studied. Special interest was taken in the analysis of damage process just before failure. In this aim two diffraction methods: in-situ time of flight neutron diffraction and X-ray synchrotron diffraction were applied. Using diffraction data, the slip mechanism on crystallographic planes during plastic deformation was investigated. In the case of aged UR45N steel, it was found that significant softening caused by damage process was initiated in the ferritic phase. The lattice strains measured in situ by two above mentioned diffraction methods were compared with prediction of the self-consistent model.

The methods most commonly used for the determination of the elastic lattice deformation and distortion are diffraction methods, which enable to perform measurements of stresses and elastic properties of polycrystalline materials. The main advantages of diffraction methods are associated with their non-destructive character and the possibility to be used for macrostress and microstress analysis of multiphase and anisotropic materials. Diffraction methods enable taking measurements selectively only for a chosen alloy phase. This is very convenient when several phases are present in the sample since measurements of separate diffraction peaks allow the behaviour of each phase to be investigated independently. In this work, a method for analysis of diffraction with synchrotron radiation is described. The methodology is based on the measurements of lattice strains during “in situ” tensile testing for several hkl reflections and for different orientations of the sample with respect to the scattering vector. Some initial results are presented.

International audience; Diffraction methods for lattice strain measurement provide useful information concerning the nature of grains behaviour during elastoplastic deformation. The main advantage of the diffraction methods is the possibility of studying mechanical properties of polycrystalline materials separately in each phase and in groups of grains with a specific orientation. In this work we present application of the neutron and X-ray diffraction to study “in situ” deformation of two phase stainless steels during tensile loading. The experimental results are compared with self-consistent model.

International audience; In this work a new method for analysis of neutron diffraction results obtained during “in situ” tensile load is proposed and tested. The methodology is based on the measurements of lattice strains during “in situ” tensile test for several hkl reflections and for different orientations of the sample with respect to the scattering vector. As the result the full stress tensor for preferred texture orientations in function of applied stress can be determined with help of crystallite group method. The experimental data are presented and compared with self-consistent model calculations performed for groups of grains corresponding to the measured hkl reflections.

The goal of this work was the determination of lattice strains distribution in two phases of duplex steel during plastic necking. Subsequently, the stress heterogeneity in the neck was studied in order to determine the reason for the damage initiation and to verify the hypothesis that the damage begins in the ferritic phase. To do this, X-ray synchrotron radiation was used to scan the ‘in situ’ variation of the interplanar spacings along the necking zone for samples subjected to tensile loading. A self-consistent model and FEM simulation were applied for the experimental data interpretation.It was found that for advanced necking the phase lattice strains, especially those measured at some distance from the neck centre, show a large inversion of the loads localised in both phases compared to the undamaged state (the lattice strains in the ferrite become smaller than in the austenite). This effect indicates stress relaxation in the ferrite which is connected with the damage phenomenon. Correlation of the experimental results with the modelling shows that the value of von Mises stress is responsible for the initiation of the ferritic phase softening. Keywords: Elastoplastic deformation, Necking, Damage, Polycrystalline material, Self-consistent model, Synchrotron diffraction

In the present work, the evolution of the phase stresses in a 0.68 wt%C pearlitic steel is analyzed by synchrotron diffraction during uniaxial tensile loading, at room temperature. The diffraction measurements were done at ESRF beamline ID15B (Grenoble, France). The microstructure of the studied material, obtained after an austenitizing at 1050°C for 7 minutes followed by cooling under blowing air, corresponds to fully pearlitic steel with a cementite volume fraction of about 12.5%. As expected, the results show a clear effect of elastic and plastic anisotropy in the both phases. For the interpretation of the diffraction data, different models are compared. In elastic range and for small plastic deformation, the self-consistent model presents the best agreement with the experimental data. For large plastic deformation, this model does not predict correctly the stress partitioning between the phases as well as the macro behavior of the studied steel. Therefore a mixture model “(1-x)*self-consistent model + x*Taylor” was used to take into account the interaction between the phases.

The paper deals with damage and fracture behavior of ductile metals caused by stress-state-dependent growth and coalescence of defects on the micro-level. The continuum damage model has been generalized to take into account the effect of stress state on damage and failure criteria as well as on evolution equations of damage strains. Different branches are considered corresponding to various micro-mechanisms depending on stress state: growth of voids, formation of micro-shear-cracks and their combination. To be able to get more insight in this complex damage and failure behavior series of three-dimensional micro-mechanical numerical analyses of void containing unit cells have been performed. These calculations cover a wide range of stress states in tension, shear and compression domains. The numerical results are used to show general trends caused by growth and coalescence of micro-defects. They are also used to develop equations for stress-state-dependent damage criteria, to propose evolution equations for damage strain rate tensors, and to introduce a coalescence condition and a fracture criterion formulated in terms of damage strain tensors.

International audience; Owing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale, i.e. phase and grain scale. In situ synchrotron diffraction (transmission mode) during tensile tests and modified self-consistent elastoplastic model were used to study elastic and plastic phenomena occurring in polycrystalline specimens during deformation. The evolution of stress for grains which belong to different phases of duplex stainless steel and pearlitic steel was analyzed.

The paper deals with determination of stress and strain field induced during riveting and FEM modelling of this process. Stress and strain measurements during riveting allow for better understanding the phenomena that occur during this process and to determining stress and strain field which exists in the joint after riveting. The results will be also used for validation of FEM models.

X-ray diffractometry is one of the basic methods of stress measurement. This method was used to measure stress distributions around rivets as described further in this paper. There were two types of riveted samples, six types of samples made of rivet wire (after different types of treatment) and a aluminium sheet sample with three measurement areas: plate with both cladding and anodized layer, plate after removing the anodized layer and plate after removing both cladding and anodized layer. Riveted samples were prepared to measure the stress distribution around the rivets and the samples of wire and the plate with three areas were prepared to check the effect of different types of treatment on stress state.