The microstructural evolutions and variations in mechanical performance of electron beam welded (EBW) Ti-6Al-4V (Ti64) alloy have been investigated. The effects of heat treatment on the microstructure of welded samples have been studied after post-welding solution treatment and ageing. The martensitic phase α' has been confirmed using transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) has been used to investigate the phase and grain morphology. Results showed that the martensitic α′ phase coarsened, the size of heat-affected zone (HAZ) changed and grains in the base materials (BMs) had grown after the post-weld heat treatments (PWHT). The tensile behaviour of electron beam welded Ti64 has been investigated using in situ tensile testing monitored by optical microscopy. The deformation and failure were directly revealed during the in situ tensile process. Results showed that the EBW Ti64 samples have different failure locations after receiving different post-weld heat treatments. The relationship between the post-weld heat treatments, microstructural evolution and mechanical properties of EBW Ti64 were investigated. Thermodynamic databases were used to predict mechanical properties—including the yield strengths—of the titanium alloy for different grain sizes, representing different post-weld heat treatment operations, and these were embedded into a finite element modelling framework to simulate the tensile testing specimens to understand the mechanical fields experienced such as stresses and strains, just prior to failure.

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A detailed microstructural characterisation of the emerging weld-line grain structure, for bead-upon-plate welds in Ti-6Al-4V (Ti64) of differing plate thickness, was performed. The microstructure studied was formed during both steady state and non-steady state sections within the weld path, with the non-steady state portion being taken from the end of the plate as the weld bead and heat source overhang the edge of the plate. This allows for the effects of welding process conditions on the microstructural evolution to be determined. The weld pool geometry and 3D tomography of the weld-induced defects have been investigated. Detailed characterisation of microstructure and texture for different welding parameters and for steady and non-steady states have been used to identify physical parameters for the microstructure predictions that are difficult to obtain otherwise. The different states significantly affect the weld crown shape and formation, weld toe, weld bead depth and width. However, the heat affected zone (HAZ) remains unchanged. Regarding the microstructural evolution, both the steady and non-steady states have similar microstructure and texture. No defects were observed in the steady state section of welds, but sub-surface spherical pores have been observed in the non-steady state section of a weld. Finite element modelling to simulate the thermal-metallurgical-mechanical fields within the steady and non-steady state sections of the welds was considered, and the cooling rates predicted within steady state and non-steady sections were interrogated to improve the theoretical understanding of the microstructure and defect formation differences in these Ti64 EB weld regions.

The velocity–versus-time rundown curves from two experimental Ti-6Al-4V inertia friction welds were analysed and differentiated several times, to produce rotational acceleration, jerk, jounce (or snap), crackle and pop versus-times curves for each weld. Titanium alloys and their mechanical properties are known to be highly sensitive to strain rate as the material is deformed, though nothing has ever been considered in terms of the higher-order time-derivatives of position. These curves have been studied and analysed further, for a more complete understanding of the derivative trends. Rotational acceleration and jerk traces both display behavior patterns across the two welds as the part rotates under action from the flywheel. The rotational snap also displays a pattern in this derivative during the final approximately 0.5 s of welding, as the energy dissipates. Evidence of a distinct oscillatory pattern in the rotational crackle and pop terms was noted for one weld when differentiating over a larger time-base, though could not be replicated in the 2nd weld. The higher derivative curves allow distinction of different process regimes, indicating that inertial energy mostly influences the time-base of dynamically steady-state phase. Qualitative differences between initial energies are evident in higher derivatives.

Manufacturing processes such as welding subject the α/β titanium alloy Ti-6Al-4V to a wide range of temperatures and temperature rates, generating microstructure variations in the phases and in the precipitate dimensions. In this study, the metallurgical and numerical modelling of Ti-6Al-4V when subjected to a high energy density welding process was affected by a series of analytical equations coded in Sysweld commercial specialist FE welding software. Numerical predictions were compared with experimental results from laser welding tests on plates with different thicknesses, initial microstructural morphologies, and operating conditions. The evolution of the microstructure was described by using a diffusion-based approach when the material was operating in the α + β field, whilst empirical equations were used for temperatures above the β-transus temperature. Predictions made by the subroutines within the FE model were shown to match with reasonable trends when validated using experimental characterisation methods for various metallurgical features, including the α particle size, β grain size, martensitic needle thickness, and relative phase volume fractions.

The microstructural kinetics of β grain growth in the β field of a Ti-6Al-4V alloy was studied by a series of controlled heat treatments at constant temperature rates. Heating rates of 5°C/s, 50°C/s and 500°C/s were considered, stopping at different peak temperatures. The thickness evolution of martensitic needles and lamellar α laths, formed on cooling, was also investigated, by soaking the material above its β-transus temperature and cooling down at 5°C/s, 50°C/s, 100°C/s and 300°C/s till ambient temperature. Quantitative microstructural analyses were used to measure the particle dimensions. The β grain growth kinetics was reasonably well described by a modified Avrami equation. The thickness of α lamellae was a function of the cooling rate and the β grain dimension in which they nucleated. The martensite needle thickness was shown to be a function of the cooling rate to which the material was subjected.

A microstructural characterisation and computational modelling framework for simulating the coupled thermal mechanical cycles with two different initial parent microstructures, and their impact upon the emerging microstructure features including beta grain size and phase volume fractions during electron beam welding (EBW) of Ti-6Al-4 V (Ti64) alloy is presented. Ti64 plates (measuring 5.5 mm in thickness) with different thermal processing have been electron beam welded using differing process parameter sets. These experimental welds have been fully characterised to provide information on the effect that a prior heat treatment has upon the micro structural evolution caused by the EB welding. Microstructure characterisation was investigated using both light microscopy and scanning electron microscopy. The results from this microstructure characterisation have been used to identity physically-based parameters required for microstructure predictions.

Complex heat treatment operations and advanced manufacturing processes such as laser or electron-beam welding will see the metallic workpiece experience a considerable range of temperatures and heating/cooling rates. These intrinsic conditions will have a significant bearing upon the microstructure of the material, and in turn upon the thermo-mechanical properties. In this work, a diffusion-based approach to model the growth and shrinkage of precipitates in the alpha + beta field of the common titanium alloy Ti-6Al-4V is established. Further, the numerical model is extended using a JMA-type approach to explore the dependency of the beta-transus temperature on extremely high heating rates, whereby dissolution alone is too slow to accurately describe the alpha to beta-phase transformation. Experimental heat treatments at rates of 5, 50, and 500 °C/s were performed, and metallographic analysis of the samples was used to validate the numerical modeling framework predictions for lamellar shrinkage, while data from the literature has been used to evaluate the numerical modeling framework for the growth of equiaxed microstructures. The agreement between measurements and numerical predictions was found to be good.

The prediction of solidification microstructures associated with additive manufacture of metallic components is fundamental in the identification scanning strategies, process parameters and subsequent heat treatments for optimised component properties. Interactions between the powder particles and the laser heat source result in complex thermal fields in and around the metal melt pool, which will influence the spatial distribution of chemical species as well as solid-state precipitation reactions. This paper demonstrates that a multi-component, multi-phase precipitation model can successfully predict the observed precipitation kinetics in Inconel 625, capturing the anomalous precipitation behaviour exhibited in additively manufactured components. A computer coupling of phase diagrams and thermochemistry (CALPHAD)-based approach captures the impact of dendritic segregation of alloying elements upon precipitation behaviour. The model was successful in capturing the precipitation kinetics during annealing considering the Nb-rich and Nb-depleted regions that are formed during additive manufacturing.

Selective-laser melting (SLM) is one of the most rapidly developing and promising of all the so-called "Additive Manufacture" routes due to its capability to produce component geometries that would prove impossible using traditional manufacture. A selective-laser melting fabricated cuboid component was built using powder CM247LC, using standard methods, and this was subsequently analysed using neutron tomography methodology to allow for three-dimensional visualisation of the exterior and the interior of the component. The resulting neutron radiographs were processed and analysed for evidence of both porosity and grain boundary segregation within the component. (C) 2018 Elsevier B.V. All rights reserved.

Inertia friction welding has been used across the aerospace, automotive, and power-generation industries for the fabrication of complex axisymmetric components for over forty years. The process involves one axisymmetric piece being held stationary and another piece being brought into contact set to rotate about its axis of symmetry by a flywheel with the system under an applied load across the joint. Plasticization at the joint interface through the frictional heating sees the two pieces bond together. The titanium alloy Ti-6Al-4V has been widely studied for inertia welding applications. A successful selection of processing parameters (flywheel energy and mass, applied load) allows an inertia welding process which produces a very high-integrity weld, with a minimal heat-affected zone (HAZ) and thermomechanically affected zone (TMAZ), formed as a narrow band at the interface and extending further into the material. The width of this narrow band of heated material is dependent upon the process parameters used. A series of experimental inertia friction welds were performed using Ti-6Al-4V, and a finite element (FE) modeling framework was developed using the FE code Deform in order to predict the widths of the HAZ and TMAZ at the weld interface. The experimentally observed HAZ boundaries were correlated with the thermal fields from the FE model, while TMAZ boundaries were correlated with the Von Mises plastic strain fields.

A multi-phase, multi-component mean-field model has been developed for simulating the intermetallic precipitation kinetics in Inconel 718. The aim of this work is to develop predictive capability to aid in process optimisation and explore precipitation kinetics during additive manufacturing (AM). The model has been calibrated to available experimental data, and then applied to predict precipitation kinetics during typical solid solution treatment and aging operations, and during AM. It is shown that a Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) based modelling approach provides a unified particle growth rate which can capture the growth, coarsening and dissolution of γ′, γ* and δ precipitates under relevant heat treatment conditions. To apply the model to AM, finite element simulations of a simple rectangular build have been carried out, using a property switching method to simulate the material deposition. The component level simulation provides the thermal fields to calculate precipitation kinetics during deposition, also allowing for the examination of the heat affected zone in the substrate. The modelling approach can capture the repeated nucleation and dissolution of precipitates that occurs during AM. The model shows good agreement with experimental data when applied to predicting precipitation kinetics during heat treatment.

This paper presents a computational framework to study the differences in process-induced microvoid and precipitate distributions during selective laser melting (SLM) of two nickel-based superalloys representative of low (IN718) and high (CM247LC) volume fraction precipitate-strengthened alloys. Simulations indicate that CM247LC has a higher propensity to form process-induced microvoids than IN718. Particle sintering is predicted to be strongly influenced by the powder size distribution. For deposition thickness of approximately 40 μm, thermal gradients during cooling are predicted to be larger for CM247LC than IN718 and consequently expect the development of larger residual stresses for a high volume fractionγ′ alloy. A coupled mean field/finite-element approach has been used to predict the precipitate distributions across a simple rectangular build and during a subsequent hot isostatic pressing (HIP) cycle. Unimodal and multi-modal particle distributions are predicted for IN718 and CM247LC at the end of the SLM, respectively. A higher volume fraction ofγ′ is predicted for CM247LC at the end of the SLM process. During HIP, simulations indicate a dramatic increase in theγ′ volume fraction in CM247LC, which can result in a reduction in stress relaxation and lead to a ductility drop.

Ceramic cores are used in casting processes to create complex internal shapes within the final component. The work presented here uses Computational Fluid Dynamics (CFD) analysis to predict the filling and solidification behaviour of ceramic core material during the Ceramic Injection Moulding (CIM) process in the large, complex geometries now typical of modern cores. The aim of the study is to develop a predictive capability to identify key defects in the core that might otherwise only be observed after a number of expensive manufacturing processes. Manufacturing trials using short shots have been carried out in order to validate the transient flow patterns of the paste; this has highlighted the occurrence of jetting, weld lines and flow defects that are highly dependent on the injection parameters and runner designs employed. Analysis of the modelled solidification, shear rates, stagnation points and phase migration has driven die and process optimisation in a production environment.

High energy-density beam welding, such as electron beam or laser welding, has found a number of industrial applications for clean, high-integrity welds. The deeply penetrating nature of the joints is enabled by the formation of metal vapour which creates a narrow fusion zone known as a “keyhole”. However the formation of the keyhole and the associated keyhole dynamics, when using a moving laser heat source, requires further research as they are not fully understood. Porosity, which is one of a number of process induced phenomena related to the thermal fluid dynamics, can form during beam welding processes. The presence of porosity within a welded structure, inherited from the fusion welding operation, degrades the mechanical properties of components during service such as fatigue life. In this study, a physics-based model for keyhole welding including heat transfer, fluid flow and interfacial interactions has been used to simulate keyhole and porosity formation during laser welding of Ti-6Al-4V titanium alloy. The modelling suggests that keyhole formation and the time taken to achieve keyhole penetration can be predicted, and it is important to consider the thermal fluid flow at the melting front as this dictates the evolution of the fusion zone. Processing 2 induced porosity is significant when the fusion zone is only partially penetrating through the thickness of the material. The modelling results are compared with high speed camera imaging and measurements of porosity from welded samples using X-ray computed tomography, radiography and optical micrographs. These are used to provide a better understanding of the relationship between process parameters, component microstructure and weld integrity.

A fluid dynamics approach to modelling of fusion welding in titanium alloys is proposed. The model considers the temporal and spatial evolution of liquid metal/gas interface to capture the transient physical effects during the heat source–material interaction of a fusion welding process. Melting and vaporisation have been considered through simulation of all interfacial phenomena such as surface tension, Marangoni force and recoil pressure. The evolution of the metallic (solid and liquid) and gaseous phases which are induced by the process enables the formation of the keyhole, keyhole dynamics, and the fully developed weld pool geometry. This enables the likelihood of fluid flow-induced porosity to be predicted. These features are all a function of process parameters and formulated as time-dependent phenomena. The proposed modelling framework can be utilised as a simulation tool to further develop understanding of defect formation such as weld-induced porosity for a particular fusion welding application. The modelling results are qualitatively compared with available experimental information.

High energy-density beam welding, such as electron beam or laser welding, has found a number of industrial applications for clean, high-integrity welds. The deeply penetrating nature of the joints is enabled by the formation of metal vapour which creates a narrow fusion zone known as a “keyhole”. However the formation of the keyhole and the associated keyhole dynamics, when using a moving laser heat source, requires further research as they are not fully understood. Porosity, which is one of a number of process induced phenomena related to the thermal fluid dynamics, can form during beam welding processes. The presence of porosity within a welded structure, inherited from the fusion welding operation, degrades the mechanical properties of components during service such as fatigue life. In this study, a physics-based model for keyhole welding including heat transfer, fluid flow and interfacial interactions has been used to simulate keyhole and porosity formation during laser welding of Ti-6Al-4V titanium alloy. The modelling suggests that keyhole formation and the time taken to achieve keyhole penetration can be predicted, and it is important to consider the thermal fluid flow at the melting front as this dictates the evolution of the fusion zone. Processing induced porosity is significant when the fusion zone is only partially penetrating through the thickness of the material. The modelling results are compared with high speed camera imaging and measurements of porosity from welded samples using X-ray computed tomography, radiography and optical micrographs. These are used to provide a better understanding of the relationship between process parameters, component microstructure and weld integrity.

A supply of medium carbon boron steel rod has been used industrially to produce the "rib-like" rod structures for mechanical conveyor systems, used across a number of non-safety critical industries, such as agricultural harvesting. The steel rod is resistive-heated and subsequently mechanically deformed such to produce a small region of flattened proportions, to allow for easier mechanical attachment to a belt system to attach all rods to the conveyor system. It has been noted industrially that after the flattening operations have taken place, a region at the shoulder of the flattened section is susceptible to cracking problems. The root cause of this cracking was desired to be understood, hence three likely causations for the cracking were explored, namely (i) mechanical stresses at the region, (ii) micro-segregation of the alloying elements at the location, and (iii) overheating. A 2D axi-symmetric finite element framework was developed to predict the stresses generated in the flattened section. This model showed that there were some areas of concern regarding the predicted effective stress and strain distributions, compared to the material flow stresses, thus potentially a mechanical reason for the cracking to occur. Microscopy methods were considered to understand the microstructure of the surrounding material and the nature of the cracks. However, these suggested that there was no likely element segregation to cause a significant variation in material property. Finally, temperatures generated by the resistive heating procedure were measured, and this does suggest that the material may have been overheated, thus producing coarser austenite grains whilst the material is held at elevated temperatures for a short time, and so producing inferior mechanical properties in this small region of heated material. The effects of overheating are impossible to eliminate without a complete re-melt of the steel. Thus, the research has demonstrated that a combination of overheating, and in-situ stress and strain distributions, could be the root cause of the cracking. (C) 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.