Residual stress distribution in a copper-aluminum multifilament composite fabricated by rotary swaging

Rotary swaging is a promising technique for the fabrication of clad Cu/Al composites. Residual stresses appearing during the processing of a special arrangement of Al filaments within the Cu matrix and the influence of the bar reversal between the passes were studied by (i) neutron diffraction using a novel evaluation procedure for pseudo-strain correction and (ii) a finite element method simulation. The initial study of the stress differences in the Cu phase allowed us to infer that the stresses around the central Al filament are hydrostatic when the sample is reversed during the passes. This fact enabled the calculation of the stress-free reference and, consequently, the analysis of the hydrostatic and deviatoric components. Finally, the stresses with the von Mises relation were calculated. Hydrostatic stresses (far from the filaments) and axial deviatoric stresses are zero or compressive for both reversed and non-reversed samples. The reversal of the bar direction slightly changes the overall state within the region of high density of Al filaments, where hydrostatic stresses tend to be tensile, but it seems to be advantageous for avoiding plastification in the regions without Al wires. The finite element analysis revealed the presence of shear stresses; nevertheless, stresses calculated with the von Mises relation show similar trends in the simulation and in the neutron measurements. Microstresses are suggested as a possible reason for the large width of the neutron diffraction peak in the measurement of the radial direction.Web of Science165art. no. 210

Microstructure and Mechanical Properties of Laser Additive Manufactured H13 Tool Steel

Hot working tool steel (AISI H13) is one of the most common die materials used in casting industries. A die suffers from damage due to friction and wear during its lifetime. Therefore, various methods have been developed for its repair to save costs to manufacture a new one. A great benefit of laser additive manufacturing (cladding) is the 3D high production rate with minimal influence of thermal stresses in comparison with conventional arc methods. Residual stresses are important factors that influence the performance of the product, especially fatigue life. Therefore, the aim of this contribution is to correlate the wide range of results for multilayer cladding of H13 tool steel. X‐ray and neutron diffraction experiments were performed to fully describe the residual stresses generated during cladding. Additionally, in‐situ tensile testing experiments inside a scanning electron microscope were performed to observe microstructural changes during deformation. The results were compared with local hardness and wear measurements. Because laser cladding does not achieve adequate accuracy, the effect of necessary post‐grinding was investigated. According to the findings, the overlapping of beads and their mutual tempering significantly affect the mechanical properties. Further, the outer surface layer, which showed tensile surface residual stresses and cracks, was removed by grinding and surface compressive residual stresses were described on the ground surface

Load partition and microstructural evolution during in situ hot deformation of Ti-6Al-6V-2Sn alloys

Two Ti–6Al–6V–2Sn alloys, with globular and lamellar microstructures, are deformed at 750 °C during tensile and compression tests. The lamellar microstructure shows softening and higher peak stress values than the globular microstructure as a consequence of the Hall–Petch effect. In-situ high energy Synchrotron diffraction experiments allow characterization of the load partition between α- and β-phases, plastic deformation mechanisms and texture evolution. The α-phase deforms mainly by rotation while the β-phase deforms by misorientation formation, acting merely as load transfer agent. The Taylor factor evolution of the α-phase and the annihilation of dislocations are analyzed qualitatively and quantitatively. The Taylor factor is connected to both the softening observed in the alloy with the lamellar microstructure and the texture development

Load Partition and Microstructural Evolution at High Temperature in Multiphase Lightweight Metallic Materials

Aluminium and titanium alloys surround our lives and are essential when weight minimization is required together with relatively high strengths. At room temperature, strength can be improved by reinforcing these alloys with ceramics phases, resulting in the formation of metal matrix composites. The introduction of rigid intermetallic phases is an alternative option when the production of the composites raises the costs.The study of the load partition between phases under the effect of external (thermo-) mechanical loads is crucial to understand the behaviour of multiphase alloys during real service conditions. The lack of scientific works analysing the load partition in Al and Ti multiphase alloys at high temperature is the main motivation of the present work.Bulk diffraction techniques capable to penetrate up to a few cm within materials are necessary to follow in situ the evolution of the load partition under the effect of external loads. Neutron sources with high flux and the third-generation synchrotron diffraction sources are ideal for this purpose compared with traditional x-ray laboratory sources. Furthermore, acquisition times up to 3 min for the neutron diffraction and below 1 s for the synchrotron diffraction make them suitable tools for in situ analysis at high temperature, where relaxation effects can play a relevant role.The aim of this work is the study of the load partition and the microstructural evolution at high temperature in selected multiphase lightweight metallic materials by means of in situ neutron and synchrotron diffraction techniques.Cast AlSi12 and AlSi10Cu6Ni2 alloys, undergoing solution treatments during 1h and 4h at 500°C, are compared with their respective as cast conditions during high temperature compression tests. The effect of the load partition between different phases is analyzed by in situ synchrotron diffraction. The eutectic Si in the aluminium piston alloy is able to bear a higher load than in the AlSi12 alloy in corresponding heat treatment conditions and for identical externally applied strains. Cu, Ni and Fe aluminides, forming highly contiguous 3D networks with the eutectic Si are responsible for this behaviour. The decrease of the load carrying capability in the AlSi12 with solution treatments is due to the disintegration of 3D Si networks, which on the other hand, is partially preserved by the aluminides after solution treatments in the AlSi10Cu6Ni2 alloy. The role played by aluminides on damage onset in the AlSi10Cu6Ni2 alloy is revealed by the sudden decrease of the load born by Al2Cu at the point of maximum strength of this alloy.In a different section, three SiC-particle reinforced composites produced by powder metallurgy, two of them by wet blending using 2124 and 6061 Al-alloys, and one by ball milling using a 2124 Al-alloy, are subjected to thermal cycling in order to explain the differences observed in their creep resistance. The effect of the blending route on the stress relaxation during thermal cycling is studied by in situ neutron diffraction. The thermal stresses partially relax at T above 90°C. The higher creep resistance of the composite produced by ball milling shows less relaxation of thermal stresses in comparison with the wet blended composites. The average axial compressive thermal stresses are approximately 50 MPa, 15 MPa and 10 MPa after heating to 230 - 300°C in the matrices of the ball milling 2124, wet blending 6061 and wet blending 2124 composites, respectively, which relax at rates smaller than 5 x 10^(-9) - 3 x 10^(-8) s^(-1).Finally, a Ti662 alloy reinforced with 12% and 20% volume fractions of TiC particles and produced by powder metallurgy, is subjected to high temperature tensile tests to compare the load bearing capability of the different phases as well as the different deformation mechanisms. Two Ti662 matrices, produced by powder metallurgy and ingot metallurgy are used as reference alloys. Synchrotron diffraction is the selected tool for this study. The smaller alpha-lamella size of the PRMs in comparison with the unreinforced powder metallurgy matrix is responsible for their higher ductility. Furthermore, the presence of the TiC particles does not play an effective reinforcing role because of their inhomogeneous spatial distribution although local strain evolution shows that they are able to bear part of the load. The beta-phase plastifies practically at the beginning of the tensile tests. The alpha-phase deforms by a mechanism of grain rotation in the Ti662 alloy produced by powder metallurgy as a consequence of the geometrical reorientation of the lamellae and crystallographic rotation within the lamellae. On the other hand, the beta-phase shows an increase of crystallographic misorientation within individual grains. The Ti662 ingot metallurgy alloy, because of its smaller grain size, allows following in situ the texture evolution

Neutron diffraction study of residual stresses in a W-Ni-Co heavy alloy processed by rotary swaging at room and high temperatures

Residual stresses were studied in tungsten heavy alloy bars produced by powder metallurgy and deformed by rotary swaging at room temperature (RT) and at 900 degrees C. Neutron diffraction technique was used to scan the residual stresses across the bars. Both tungsten particles and NiCo2W solid solution matrix were analysed. Maximum axial stresses of similar to 300 MPa and similar to 200 MPa were observed for the tungsten phase at the centre in the RT and in the high-temperature deformed samples, respectively. Compressive residual axial stresses were found close to the sample surface, showing that rotary swaging is a suitable deformation method for tungsten heavy alloys to provide an appropriate surface modification for its use in metallic parts undergoing, e.g., fatigue. Residual stresses developed in the NiCo2W-phase are larger than those found in the tungsten particles although with a secondary role in the overall equilibrium conditions due to its lower strength and smaller volume fraction. Total stresses for each phase were separated into macro- and microstresses. Macrostresses can be mainly influenced by the incompatibility of the elliptical cross-section of the sintered sample with the head of the rotary machine while microstresses are mainly developed by the elastic mismatch between the constituent phases.Web of Scienc

In-Situ Synchrotron X-Ray Diffraction of Ti-6Al-4V During Thermomechanical Treatment in the Beta Field

This work aims to identify the mechanisms of restoration occurring in Ti-6Al-4V during hot plastic deformation and subsequent heat treatment. The allotropic phase transformation that occurs during cooling distorts the interpretation of the restoration mechanisms taking place at high temperatures. Therefore, analysis of deformed samples by conventional microscopy have led to controversies in the interpretation of the main dynamic restoration mechanism. Additionally, static restoration of the microstructure can occur during slow cooling, modifying the microstructure. These facts were mainly the reasons why discontinuous dynamic recrystallization and/or dynamic recovery has been reported as the main dynamic restoration mechanism for Ti-6Al-4V. In this work, we use in-situ synchrotron X-ray diffraction combined with conventional microscopy to determine the dynamic and static mechanisms of restoration during and after deformation at different strain rates. The results show dynamic recovery as main mechanism of restoration during deformation in the β field, denoted by sub-grain formation and a misorientation dependency of the strain rate. After deformation, static recrystallization, grain growth, and coarsening of the β grains can be observed, especially at strain rates higher than 0.1 s−1. It is also demonstrated that the nucleation of new grains can occur within the very first seconds of the isothermal heat treatment

Texture and differential stress development in W/Ni-Co composite after rotary swaging

Knowledge of texture and residual stresses in tungsten heavy pseudoalloys is substantial for the microstructure optimization. These characteristics were determined in cold and warm rotary swaged W/NiCo composite with help of neutron diffraction. The results were discussed in view of the observed microstructure and mechanical properties. The investigated bars consisted of tungsten agglomerates (bcc lattice) surrounded by NiCo-based matrix (fcc lattice). No preferential crystallographic orientation was found in the as-sintered bar. A strong texture was formed in both the tungsten agglomerates ( fiber texture parallel to the swaging axis) and in the NiCo-based matrix ( fiber texture) after rotary swaging. Although usually of double-fiber texture, the fiber of the fcc structures was nearly missing in the matrix. Further, the cold-swaged bar exhibited substantially stronger texture for both phases which corresponds to the higher measured ultimate tensile strength. The residual stress differences were employed for characterization of the stress state of the bars. The largest residual stress difference (approximate to 400 MPa) was found at the center of the bar deformed at room temperature. The hoop stresses were non-symmetrical with respect to the swaging axis, which was likely caused by the elliptical cross section of the as-sintered bar.Web of Science1312art. no. 286

Refinement of the Ti-17 microstructure after hot deformation: Coupled mesoscale model

The thermo-mechanical processing of Ti-alloys comprises several steps where complex deformation and temperature cycles are achieved. In this work, the static recrystallization behaviour of a Ti-17 alloy is investigated using ex-situ characterization and in-situ synchrotron radiation experiments aiming to understand the operating mechanisms and to establish the recrystallization kinetics. Hot compression in the β- field for different strain rates is applied to provide different initial microstructures before isothermal heat treatments and continuous cooling. Strain induced boundary migration is the main operating nucleation mechanism during static recrystallization. A simple mesoscale model is proposed to couple the evolution of the microstructure during hot deformation followed by annealing considering the heterogeneity of deformation within the β-grains, for the nucleation and growth of grains and the formation of the substructure by static recovery. Electron backscattered diffraction measurements are used after isothermal annealing and continuous cooling treatments to validate the model. A strong influence of the localization of deformation in the vicinity of the prior β-high angle grain boundaries is observed and empirically implemented in the mesoscale model. The strong influence of the temperature is attributed to the difference in high angle grain boundary mobility during static recrystallization. Grain refinement is not successfully achieved up to the investigated strain due to the insufficient nucleation rate with respect to the growth rate. However, a homogenous recrystallized microstructure is observed. The model can predict the microstructure for any starting microstructure, even beyond the experimental validatio

Load partition during hot deformation of AlSi12 and AlSi10Cu6Ni2 alloys: a quantitative evaluation of the stiffness of Si networks

An eutectic AlSi12 alloy contains a rigid 3D network formed by the eutectic Si in the as-cast condition, which disintegrates during solution treatment. Synchrotron tomography proved that a near eutectic AlSi10Cu6Ni2 alloy also exhibits a 3D network with higher and more stable stiffness due to the presence of aluminides that retain the initial as-cast microstructure during the solubilization treatment and increase the volume fraction of rigid phases. In order to evaluate the load borne by different phases during hot deformation, in situ synchrotron experiments were carried out revealing an underestimation of the load transfer from the soft α-Al matrix to the hard silicon 3D network in the AlSi12 alloy. By taking into account the additional stiffness introduced by the local interconnectivity, the stresses in different phases in the near eutectic AlSi10Cu6Ni2 alloy were calculated. Additionally, the analysis of the aluminide Al$_2$Cu allowed to identify its influence in the global damage of the hybrid 3D network formed by the Si and aluminides in the near eutectic AlSi10Cu6Ni2 alloy

Load partition during hot deformation of AlSi12 and AlSi10Cu6Ni2 alloys : a quantitative evaluation of the stiffness of Si networks

An eutectic AlSi12 alloy contains a rigid 3D network formed by the eutectic Si in the as-cast condition, which disintegrates during solution treatment. Synchrotron tomography proved that a near eutectic AlSi10Cu6Ni2 alloy also exhibits a 3D network with higher and more stable stiffness due to the presence of aluminides that retain the initial as-cast microstructure during the solubilization treatment and increase the volume fraction of rigid phases. In order to evaluate the load borne by different phases during hot deformation, in situ synchrotron experiments were carried out revealing an underestimation of the load transfer from the soft α-Al matrix to the hard silicon 3D network in the AlSi12 alloy. By taking into account the additional stiffness introduced by the local interconnectivity, the stresses in different phases in the near eutectic AlSi10Cu6Ni2 alloy were calculated. Additionally, the analysis of the aluminide Al$_2$Cu allowed to identify its influence in the global damage of the hybrid 3D network formed by the Si and aluminides in the near eutectic AlSi10Cu6Ni2 alloy