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Inconel 625 nickel alloy with its attractive high-temperature strength, excellent corrosion and oxidation resistance is mainly used for critical applications in demanding environments, in both as-cast and wrought conditions. Hot processing of this alloy is crucial for achieving its tailored mechanical properties due to the significant variation in microstructural changes with varying process parameters like temperature, strain, and strain rate. In this study, isothermal hot compression tests were carried out at temperatures ranging from 900 to 1100 °C, and under strain rates ranging from 0.01 to 1 s−1. The flow curves revealed three stages of deformation, including a substantial work-hardening stage followed by dynamic recovery and flow softening. Microstructural observations showed the occurrence of discontinuous dynamic recrystallisation (DDRX) as the dominant recrystallisation mechanism during the flow softening. Microstructural analysis suggested that the DRX was more sensitive to the test temperature as compared to the strain rate. An innovative material's constitutive model was developed, by combining Johnson–Cook (JC) and Avrami approaches, to predict work-hardening, dynamic recovery, and flow softening stages of deformation. The predicted flow behavior was in a good agreement with the experimentally measured data. The developed material model was integrated into DEFORM® 3D finite element (FE) simulation software as a user subroutine for the prediction of deformation behaviour in a double truncated cone (DTC) sample. Comparison between the experimentally measured data and the results of FE simulation on the DTC sample showed a very good convergence, indicating the suitability of the proposed material’s constitutive model for large scale simulations. Graphical Abstract

The effect of deformation heating on microstructure evolution during hot forging of Ti-6Al-4V was established. For this purpose, right-circular cylinders of Ti-6Al-4V with an equiaxed-α preform microstructure were preheated to a temperature between 1148 K (875 °C) and 1223 K (950 °C), and compressed to a 60-pct. height reduction in a screw press, yielding average true strain rates of ~ 5 to 20 s−1. Thermocouple measurements and corroborating finite-element-method (FEM) simulations quantified substantial deformation-heating-induced temperature increases. For all preheat temperatures, the heating transient led to an exposure above the equilibrium β transus temperature. Despite such temperature excursions, the volume fraction of equiaxed primary α in each forged billet was only slightly lower than that in the corresponding preheated condition. The source of such observations was rationalized on the basis of the (hypothesized) solute-concentration fields that develop during the heating and cooling transients experienced in high-rate deformation processing.

Background Determination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties. Objective This study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI). Methods These measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties. Results The XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation. Conclusion The ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

Defect-free welds between A-516 ferritic and 316L austenitic stainless steel were achieved using inertia friction welding (IFW) under varying friction and forge pressures. The microstructural evolution and the associated mechanical properties were characterised across different weld regimes. The welds were free from microcracks and demonstrated higher strength and hardness than the parent metal (PM), indicating enhanced properties. Carbide precipitates were not observed at the weld interface, implying no occurrence of sensitisation in the stainless steel. The presence of refined grains coupled with low grain orientation spread (GOS) values was the indication of continuous dynamic recrystallisation (CDRX) occurring during IFW.

Plastic effects during sheet metal forming can lead to undesirable distortions in formed components. Here, the three-stage work hardening and plastic strain recovery (“springback”) in a cold-rolled, α-phase commercially pure titanium is examined. Interrupted standard tensile tests with in situ x-ray diffraction and quasi-in situ electron backscatter diffraction show that twinning plays a minor role in both of these phenomena. The experiments give evidence that the observed work hardening plateau is the result of an abrupt activation and multiplication of 〈 c + a 〉 slip and a subsequent redistribution of load between grain families. The springback can be attributed to inelastic backwards motion and annihilation of dislocations, driven by backstresses from dislocation-based hardening during loading. The peak broadening behavior, observed by x-ray diffraction, suggests that the internal stress state is highest in the rolling direction, resulting in consistently higher springback magnitude along this direction.

Continuous drive friction welding (CDW) is a state-of-the-art solid-state welding technology for joining metallic components used in aerospace, oil and gas, and power generation industries. This study summarizes the results of mechanical and microstructural investigations on a modified AISI-8630 steel subjected to CDW. The effects of welding process parameters, including rotational speed, friction, and forge forces, during CDW were explored to determine an optimum welding condition. The mechanical properties of the weld, and microstructural characteristics across different regions of the weld were measured and examined. The microstructure characterization results suggest that the weld zone (WZ) experiences temperatures above the Ac3 and the thermo-mechanically affected zone (TMAZ) experiences temperatures between Ac1 and Ac3 of the material. Investigations with electron backscatter diffraction (EBSD) demonstrated the occurrence of strain-induced dynamic recrystallization in the weld. The weld demonstrated higher yield and ultimate tensile strengths at the expense of ductility and hardening capacity compared to the base metal (BM). The strain-hardening profiles of the welds exhibited a dual-slope characteristic, an indication of different levels of plastic deformation experienced by the constituent phases (i.e., martensite, bainite and ferrite) present in the microstructure. The maximum strength-to-ductility combination and static toughness values were obtained for the weld produced under the highest rotational speed, maximum friction force and an intermediate forge force of 1200–1400 rpm, 375–425 kN, and 600–650 kN, respectively.

Secondary hot-working on dual phase titanium alloys are essential for microstructural modification to tailor mechanical properties, which is typically challenging due to a narrow available processing window, especially during industrial scale manufacturing. Poor workability, strain induced porosity and adiabatic temperature rise in α/β phase region (i.e., sub-transus) are some of the main challenges faced. Cyclic thermal treatment (CTT) is an emerging technology showing potentials for microstructure modification (i.e., globularisation) in Ti-6Al-4V with significantly reduced mechanical work in the α+β region. This study summarises the results of CTT investigations conducted on a wrought Ti-6Al-4V alloy subjected to various thermo-mechanical conditions to develop different initial microstructures. Samples with uniform strain distributions were extracted from pre-forged samples and subsequently subjected to CTT using both conventional electric furnace (i.e., for slow heating and cooling rates), and induction heating (i.e., for faster heating and cooling rates). CTT of the samples forged at sub-transus temperature in conventional furnace led to maximum (i.e. ~ 100%) globularisation and significant coarsening of α grains, resulting in an equiaxed bimodal microstructure. On the other hand, CTT with induction heating method has resulted in a maximum of 80% globularisation fraction in samples forged to 60% reduction, and ~ 35% globularisation fraction in those forged to 20% reduction. The globularisation mechanisms during CTT of the sub-transus forged samples was dominated by the boundary splitting and thermal grooving. A Johnson-Mehl-Avarmi-Kolmogorov (JMAK) based model has been developed to predict the evolution of globularisation and grain growth during CTT. The developed JMAK model was then successfully incorporated into DEFORM® software as post-processing user subroutines. The predicted microstructure evolution by Finite Element (FE) simulations shown a good convergence towards the experimentally measured data following CTT.

In this work, high-cycle fatigue behavior of friction-stir welded AA6061-T6 was studied. An emphasis has been made on the inter-relationship between microstructure, residual stress and fatigue resistance. The welds were produced under optimized conditions, including a combination of relatively high welding temperature and rapid cooling rate, and subsequently undergone a standard post-weld aging heat treatment. The optimized welds exhibited excellent fatigue performance that was comparable (or even superior) to that of the base material. This result was attributed to a considerable grain refinement in the stir zone, subtle material softening in the heat-affected zone as well as to significant residual stress generated during the optimized FSW.

Microstructures observed by analytical scanning and transmission electron microscopy in the overlap region of a double-sided friction-stir-welded microalloyed steel (EH46) were recorded in detail. They are compared with microstructures in the thermomechanically affected region of the weld and with the base material. The differences suggest that the overlap region has been stirred in the single-phase ferrite, and consists mainly of small equiaxed ferrite grains with strain-induced precipitates, while the thermomechanically affected zone was processed in the austenite-ferrite-phase field, resulting in a mixture of bainite lath packets and ferrite grains. The almost complete absence of pearlite or cementite in the overlap region has led to the suggestion that it dissolved during friction stir welding, providing carbon for strain-induced precipitation. Also, in the complex microstructures of the overlap region, ferrite grains containing a high density of cell-like structures were observed, some of them having precipitates nucleated on their intersections. This implies that strain-induced continuous dynamic recrystallization has occurred.

Manufacturing high value components involves complex and non-linear thermo-mechanical processes to obtain optimum combination of microstructure and mechanical properties required for the final part. Among these, the ingot-to-billet conversion process, involving forging operations of upsetting and cogging, are critical to refine the as-cast coarse, elongated, and dendritic microstructure. In this study, the first stage of the ingot-to-billet conversion process has been investigated in type 316 austenitic stainless steel, aiming to propose a novel methodology for the characterisation of the as-cast material behaviour. Hot upsetting tests were carried out on cylindrical samples taken out from an industrial-scale ingot. The resulted microstructures were analysed, using advanced image analysis method, for the fraction and distribution of the recrystallised grains, highlighting the strong dependency of recrystallisation behaviour on the initial microstructure of the as-cast material. Using a finite element (FE) model considering the anisotropic behaviour of the material, originated from the preferential grain growth during casting, the deformation of the samples were predicted with a good accuracy. The results demonstrate the importance of considering the anisotropic plastic properties in the FE models to effectively predict the as-cast material deformation, shape and thus the thermo-mechanical characteristics applied during forging.

Microstructures of type 304 austenitic stainless steel, produced through thermo-mechanical processing, were analysed with large area EBSD and optical image analysis assessments of the attacked grain boundary cluster after DL-EPR testing. The thermo-mechanically processed microstructures were exposed to acidified potassium tetrathionate (K2S4O6) solution under tensile stress and the lengths and distributions of the initiated intergranular crack nuclei were assessed. The crack populations were quantified by fitting a Gumbel extreme value statistics distribution to evaluate their characteristic crack length. A factor (susceptibility parameter) is introduced to rank the degree of susceptibility to intergranular stress corrosion cracking of thermo-mechanically processed microstructures. This accounts for the network connectivity of the sensitised grain boundaries, the grain size and the degree of sensitisation. Similar rankings are obtained for this susceptibility parameter and characteristic crack lengths of the assessed microstructures, in which the thermo-mechanical treatments increased the population of grain boundaries with resistance to stress corrosion cracking. Keywords: Intergranular stress corrosion cracking (IGSCC), Grain boundary character distribution, Thermo-mechanical processing, Grain boundary engineering (GBE), Sensitisation, Corrosion

A constitutive model was developed to predict the high temperature sub-transus flow behavior of a metastable β Ti-10V-2Fe-3Al alloy, and its applicability for industrial scale forging process has been evaluated using a finite element simulation. Cylindrical samples extracted from a cogged billet of the alloy after β recrystallisation treatment were subjected to hot compression tests at sub-transus temperatures ranging from 720 °C to 780 °C with an increment of 15 °C under varying strain rates from 0.025 to 0.2 s−1. The sub-transus flow curves of the alloy exhibited work hardening followed by dynamic recovery and flow softening beyond the strain equivalent to peak stress. Formation of low angle grain boundaries within the prior β grains and dynamic recrystallisation of β phase were observed to contribute to the flow softening. A constitutive model based on Estrin Mecking and Avrami methods was developed with minimised number of material constants, using the data derived from the hot compression tests. The model was successfully verified using the deformation behaviour measured for the alloy in the region of interest. The constitutive model was implemented into a finite element package as a user subroutine to predict the flow behaviour of industrial scale billets during open die hot forging processes and validated using experimental trials. A fairly good predictive capability with more than 95% convergence for open die forging was achieved, confirming the suitability of the material model developed for predicting sub-transus forgeability of the Ti-10V-2Fe-3Al alloy during industrial scale forging.

Electron backscattered diffraction (EBSD) was used to analyze the evolution of microstructure and crystallographic texture during friction stir welding of dissimilar type 2205 duplex stainless steel (DSS) to type S275 low carbon-manganese structural steel. The results of microstructural analyses show that the temperature in the center of stirred zone reached temperatures between Ac1 and Ac3 during welding, resulting in a minor ferrite-to-austenite phase transformation in the S275 steel, and no changes in the fractions of ferrite and austenite in the DSS. Temperatures in the thermomechanically affected and shoulder-affected zones of both materials, in particular toward the root of the weld, did not exceed the Ac1 of S275 steel. The shear generated by the friction between the material and the rotating probe occurred in austenitic/ferritic phase field of the S275 and DSS. In the former, the transformed austenite regions of the microstructure were transformed to acicular ferrite, on cooling, while the dual-phase austenitic/ferritic structure of the latter was retained. Studying the development of crystallographic textures with regard to shear flow lines generated by the probe tool showed the dominance of simple shear components across the whole weld in both materials. The ferrite texture in S275 steel was dominated by D 1, D 2, E, $$ \bar{E} $$ E ¯ , and F, where the fraction of acicular ferrite formed on cooling showed a negligible deviation from the texture for the ideal shear texture components of bcc metals. The ferrite texture in DSS was dominated by D 1, D 2, I, $$ \bar{I} $$ I ¯ , and F, and that of austenite was dominated by the A, $$ \bar{A} $$ A ¯ , B, and $$ \bar{B} $$ B ¯ of the ideal shear texture components for bcc and fcc metals, respectively. While D 1, D 2, and F components of the ideal shear texture are common between the ferrite in S275 steel and that of dual-phase DSS, the preferential partitioning of strain into the ferrite phase of DSS led to the development of I and $$ \bar{I} $$ I ¯ components in DSS, as opposed to E and $$ \bar{E} $$ E ¯ in the S275 steel. The formations of fine and ultrafine equiaxed grains were observed in different regions of both materials that are believed to be due to strain-induced continuous dynamic recrystallization (CDRX) in ferrite of both DSS and S275 steel, and discontinuous dynamic recrystallization (DDRX) in austenite phase of DSS.

Design and control of microstructure of engineering parts made from nickel based superalloys with superior mechanical properties for high temperature applications, require the parts to be subjected to certain thermo-mechanical processing during forging. This often includes sequential straining and annealing at elevated temperatures followed by subsequent aging heat treatments at lower temperatures. In this study, the effect of strain magnitude on the evolution of microstructure during hot forging of a recently developed AD730 nickel based superalloy has been investigated. Microstructural heterogeneity was observed in a forged material manifested in a form of large non-recrystallized grains within the recrystallized matrix that is observed to be dependent on the level of deformation (i.e. strain magnitude). Analyses of microstructure indicated significant reduction in the fraction of low-angle grain boundaries and sub-structures with an increase in the applied strain, suggesting higher fraction of recrystallization with higher levels of strains. It was concluded that the lower strain levels were insufficient to provide enough driving force for complete recrystallization throughout the entire microstructure of the forged material.

Effective prediction of springback during sheet metal forming is critically important for automotive and aerospace industries, especially when forming metals with high strength-to-weight ratio such as titanium. This requires materials mechanical data during plastic deformation and their dependencies on parameters like strain, strain rate and sample orientation. In this study, springback is quantified experimentally as elastic strain recovery, degradation in Young’s modulus and inelastic strain recovery on unloading in a commercially pure titanium type 50A (CP-Ti-50A). The results show strain rate-dependent anisotropic mechanical behaviours and a degradation in Young’s modulus with increased level of plastic deformation. The level of degradation in Young’s modules increases gradually from 13% for samples parallel to the rolling direction (RD) to 20% for those perpendicular to the RD. A measurable nonlinear strain recovery was also observed on unloading that is orientation dependent. The level of springback is characterised as the sum of elastic recovery and the contributions from both the degradation in Young’s modulus and anelastic strain recovery. It is shown that the Chord modulus can estimate springback with a reasonable accuracy taking into consideration the elastic strain recovery, degradation in Young’s modulus and anelastic strain recovery.

Designing microstructure of components made from Inconel 718 nickel based superalloy (IN718) with tailored mechanical properties for high temperature applications, require sequential thermo-mechanical processing. This often includes straining and annealing at solution annealing temperature (i.e. ≈ 980 °C) followed by water quenching and subsequent aging heat treatments at lower temperatures. In addition to the microstructure development (i.e. precipitation) the aging heat treatment partially relieve the residual stresses generated at previous stages of forging and water quenching, however the stress field will not be completely relaxed. In this study, a series of experiments were conducted on round tensile specimens made from IN718 bar to investigate tensile stress relaxation behaviours at elevated temperatures used for aging heat treatments. The stress relaxation curves obtained can be described by a hyperbolic function with a non-zero asymptotic stress (σ ∞ ), which seems to be proportional to the initially applied stress (σ 0 ) for a fixed temperature. This behaviour is investigated at temperatures between 620 °C and 770 °C that is a temperature range used in industry to perform the aging heat treatments to obtain microstructures with tailored mechanical properties. It has been shown that the σ ∞ / σ 0 ratio has decreased rapidly with increasing temperature at this range. The relaxation behaviour has been assessed numerically and an empirical relationship has been defined for each temperature that can be used for modelling purposes.

The effects of varying rotational speed and inertia on the mechanical properties and microstructural evolution in 8630M low-alloy steel during inertia friction welding (IFW) were investigated. The results show that increasing rotational speed and decreasing inertia magnitude detrimentally affect the weld’s mechanical properties. Microstructural observations suggest that the temperature in the weld zone (WZ) and thermo-mechanically affected zone (TMAZ) reached Ac3 and in-between Ac1-Ac3, whereas that of the HAZ stayed below Ac1. Microstructure of the weld was dominated by simple shear texture rotated under the influence of the compression at early stages of IFW, implying that the deformation was a combination of shear and compression.

The dependency of anisotropic flow behaviour on crystallographic texture is investigated in Ti–6Al–4V alloy at 750 °C and 900 °C under constant strain rates of 10−2, 10−3 and 2 × 10−4 s−1. The evolution of microstructure and crystallographic texture during these test conditions has been studied using electron backscatter diffraction (EBSD). Anisotropic flow stress behaviour was observed at 750 °C irrespective of the applied strain rate. The maximum flow stress at this temperature was recorded for samples with their lengths perpendicular to the rolling direction (RD), which had //Transverse Direction (TD) ± 20°, Basal TD texture. The presence of a banded microstructure appeared to be the prime reason for the anisotropic tensile behaviours at lower temperatures. However, at the higher temperature of 900 °C isotropic deformation was achieved disregarding sample orientations, i.e., parallel or perpendicular to the RD. Rachinger grain boundary sliding along α-β boundaries, accommodated by intragranular slip, was seen to contribute towards the total strain in samples perpendicular to the RD deformed under 2 × 10−4 s−1 strain rate. As such, Rachinger grain boundary sliding is the dominant deformation mechanism in the direction perpendicular to the RD at 900 °C. On the other hand, although exhibiting isotropic flow behaviour, the same texture is not observed for the samples parallel to the RD at 900 °C under the same strain rate (2 × 10−4 s−1). Thus Rachinger grain boundary sliding is not thought to be the dominating deformation mechanism for this sample orientation and potentially Lifshitz sliding is active. It is concluded that despite not having a strong effect on flow behaviour, microstructural texture determines the mechanism (i.e., Rachinger, Lifshitz) by which the superplastic deformation is driven.

The evolution of residual stress and microstructure has been investigated in electron-beam welded Ti-6Al-4V alloy rings in order to develop an understanding of how the distribution of through-thickness residual stress correlates with microstructural evolution. A multiple technique approach to residual stress measurement was employed using a combination of different measurement techniques including X-ray diffraction (XRD), hole drilling method based on electronic speckle pattern interferometry (ESPI), and the contour method. It was found that there is a strong correlation between the change in residual stress and alpha phase morphology across the weld. The fusion zone exhibited highly tensile residual stress which was typified by an entirely acicular α′ microstructure formed by a displacive transformation within prior β grains on cooling. The tensile residual stress in the centre of the weld reduced towards the heat affected zone, transitioning to a compressive residual stress upon increasing distance from the weld centre. The transition from tensile to compressive residual stress correlates with a significant decrease in the volume fraction of α′ and an increase in the bimodal morphology of equiaxed primary alpha in a diffusional transformed beta matrix leading to elongated alpha in the base material.

Earing and thinning are often the major manufacturing problems occur during deep drawing processes. Thinning occurs when a section of a part undergoes localised deformation, and earing is the formation of wavy edges at the open end of a drawn part that must be trimmed at final stage leading to higher manufacturing costs. The anisotropic mechanical behavior of the initial sheet metal is the predominant source of thinning and earing problems. This work aims to establish a relationship between the properties of a sheet blank and thinning and earing issues during deep drawing by studying the evolution of crystallographic texture throughout the sheet forming process using crystal plasticity simulation modelling and experimental measurements. Firstly, to understand the impact of individual texture components on the mechanical properties of the material, Lankford coefficients for FCC crystal structure during uni-axial tensile loading were analysed using Visco-Plastic Self Consistent (VPSC) model. Subsequently, Finite Element (FE) analyses were carried out to study the effect of initial state of the material on earing and thinning issues occurred during deep drawing. It was observed that the existing Cube and Goss texture components evolved during annealing heat treatments were responsible for the generation of troughs along 45° to the rolling direction (RD) and peaks along the transverse direction (TD), respectively. Optical 3D scanning of a manufactured part confirmed that earing is less prominent in the case of as-rolled and shear-formed condition due to weakening of Cube and Goss texture components. Furthermore, a combination of FE simulation and the VPSC model has been used to simulate texture evolution during a standard earing cupping test at various points of interest. The results of texture evolution simulations were compared to those measured experimentally by electron backscatter diffraction (EBSD), and a good qualitative agreement is achieved.

The springback associated with cold deep drawing of sheet metals leads to undesired dimensional changes in the final products. This is often due to the heterogeneous plastic deformation in different areas of the intended geometry that creates various stress states throughout the part. The major objective of this study is to understand the interconnection between springback, level of plastic deformation, degradation of elastic modulus and strain recovery in a CP-Ti material. The mechanical properties of the sheet material and the dependency of mechanical properties on directionality are investigated by examining samples from three orientations of parallel to the rolling direction (RD), at 45° to RD and perpendicular to RD. The degradation of elastic modulus as a function of level of plastic deformation was explored for 0° and 45° samples by conducting multi-step uniaxial loading-unloading in tension.The experimental results showed that the mechanical properties vary for each direction, with the lowest elastic modulus along RD. A significant degradation was observed in elastic modulus (up to 50% reduction) with increased plastic deformation. This resulted in more strain relaxation compared to that associated with the initial elastic modulus. For stresses below 100MPa, a nonlinear (plastic) recovery was observed, resulting in additional relaxation in the total strain upon load removal in each step of the interrupted tests. This plastic recovery behaviour is observed to be dependent on sample orientation. It is concluded that accurate prediction of springback during sheet metal forming, requires a material model which takes into accounts the directional degradation of elastic modulus and the plastic recovery as a function of plastic deformation.

The effect of surface finish, applied stress and test duration on the developed populations of short crack nuclei has been studied for a thermally sensitised type 304 austenitic stainless steel, exposed to acidi fied potassium tetrathionate (K 2 S 4 O 6 ) solution. The crack populations can be quantified using extreme value statistics (Gumbel distribution) to obtain a characteristic crack length. The surface finish has a significant impact on crack development; a roughly machined surface, obtained with a greater depth of cut, is most susceptible to intergranular failure. The characteristic crack length of the crack population increases with tensile stress and test duration. Residual stress can cause cracking in the absence of an applied stress.

Aerospace gear components are required to demonstrate excellent load carrying and endurance characteristics. Case hardened steels are often utilised for these parts. During the manufacturing process of case hardened steel gears residual stresses are developed. The nature and distribution of these stresses are known to have a significant effect on distortion during heat treatment and machining processes. The fatigue performance of a gear subjected to cyclic loading is also strongly dependent on the nature and magnitude of manufacturing induced residual stresses. In this work the development of residual stress during the heat treatment and machining process has been assessed in case hardened steel alloy spur gears. Gears have been manufactured from two different initial conditions; as-received bar material, and hot forged billet. A spur gear geometry was rough machined prior to full heat treatment, and subsequent surface finishing. The evolution of bulk residual stress distribution within the gears throughout the manufacturing process was measured using the Contour Method. Surface residual stresses, complimentary to contour method, have been measured by X-ray diffraction. Gears from both manufacturing routes (i.e. machined from forged billet and as received bar) were found to develop tensile residual stress in the core of the gear following full heat treatment. High compressive stressed regions develop at the carburised case region at the exterior of the gear. The final residual stress magnitude and distribution within the gear was found to be independent of the initial forming process.

Hot rolled Ti-6Al-4V plate samples were taken from three different stages of an industrial heat treatment process; one as-rolled and two heat treated. This was followed by microstructure characterization using optical microscopy. Surface and through-thickness residual stress was determined using a combination of X-ray diffraction (XRD) and the contour method. Measured residual stress distributions showed similarities in distribution with that obtained for rolled Al-7050 alloy; including compressive troughs near the outer thickness on both sides, leading towards a tensile zone around the center with a local minima at the plate center thickness. Microstructure and residual stress data was then used to draw comparisons between the investigated conditions.

Fatigue fracture is one of the major degradation mechanisms in the low-alloy 4330 carbon steel pumps that are utilized in the hydraulic fracturing process operating under cyclic loading conditions. A weld cladding technology has been developed to improve the ability of these components to resist fatigue crack initiation by cladding them with a secondary material. This process introduces a residual stress profile into the component that can be potentially detrimental for fatigue performance. The cladding technology under examination is a low-alloy 4330 carbon steel substrate weld that is clad with the nickel-chromium–based superalloy IN625 and is investigated herein using several experimental residual stress measurement techniques. Understanding the magnitude and distribution of residual stress in weld clad components is of the utmost importance to accurately assess the performance of the component in service. This study summarizes the results of residual stress measurements that were determined using X-ray diffraction, i.e., hole drilling based on electronic speckle pattern interferometry, deep-hole drilling, and the contour method, to obtain the residual stress distributions from the surface of the weld clad, through the clad layer, and into the substrate material. The results of deep-hole drilling and the contour method show large-scale tensile residual stress in the clad layer and compressive residual stress in the majority of the substrate. However, the X-ray diffraction and hole drilling methods indicate the presence of short-scale compressive residual stress on the surface and near the surface of the clad layer. It was shown that these measurement techniques are complementary in assessing the residual stress profile throughout the entire component.

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Flow forming is a near net shape process for manufacturing of dimensionally accurate hollow components such as shaft in gas turbines, that is currently at its development stage for aerospace industry. The process has several advantages such as reducing material wastage, extremely fast manufacturing time, and eliminating extra manufacturing processes such as machining. Due to the nature of this complicated cold deformation process, significant magnitude of residual stress is introduced into the component. Understanding the magnitude and distribution of residual stress is essential to tailor the flow forming process to achieve parts within dimensional tolerances and desired mechanical properties. The present research is aiming to explore the generation and evolution of residual stress at various stages of flow forming process in a tubular component made from martensitic 15Cr-5Ni stainless steel, using different techniques of neutron scattering, x-ray diffraction (XRD) and hole-drilling based on electronic speckle pattern interferometry (ESPI). Residual stress measurements were carried out in preformed and flow formed components at surface, near-surface and in the bulk of components using XRD, ESPI based hole-drilling and neutron diffraction techniques. These measurements were conducted at different levels of reduction in the thickness of the original part (i.e. after 20% and 40%), by applying identical forming parameters for all samples. The XRD results show significant change in hoop and axial residual stress levels with a reduction in the wall thickness. This is more pronounced for the axial component where the average stress switches from relatively high tensile (∼ 450MPa) in the original part to significant compressive stress (∼ −600MPa) in the formed part, after 20% of reduction. The bulk residual stress components measured in the middle of thickness of the parts, using neutron scattering, show a general increase in the magnitude of residual stress by higher level of deformation (i.e. reduction in the wall thickness). The measured bulk stress components through the thickness were tuned to tensile after reducing the wall thickness by 40%. The results of XRD and neutron diffraction stress measurements suggest that the residual stress along the length of the samples (i.e. axial direction) is consistent with ±800 MPa and ±400 MPa after 20% and 40% reduction by forming process, respectively. The results of ESPI based hole-drilling show tensile hoop residual stress (≈600 MPa) and an abrupt fluctuation (i.e. tension-compressive-tension) in the axial residual stress near the surface of the part following flow forming. The stresses measured by ESPI based hole-drilling are complementary to the results of the XRD on surface and neutron diffraction in the bulk to reconstruct the residual stress profile form the surface through to the bulk.

The evolution of microstructure and crystallographic texture has been investigated in double-sided friction stir welded microalloyed steel, using electron backscatter diffraction (EBSD). The microstructure analyses show that the center of stirred zone reached a temperature between Ac1 and Ac3 during FSW, resulting in a dual-phase austenitic/ ferritic microstructure. The temperatures in the thermo-mechanically affected zone and the overlapped area between the first and second weld pass did not exceed the Ac1. The shear generated by the rotation probe occurs in austenitic/ferritic phase field where the austenite portion of the microstructure is transformed to a bainitic ferrite, on cooling. Analysis of crystallographic textures with regard to shear flow lines generated by the probe tool shows the dominance of simple shear components across the whole weld. The austenite texture at Ac1 - Ac3 is dominated by the B $$ \left\{ {1\bar{1}2} \right\}\langle 110\rangle $$ and $$ \bar{B} $$ $$ \left\{ {\bar{1}1\bar{2}} \right\}\langle \bar{1}\bar{1}0\rangle $$ simple shear texture components, where the bainite phase textures formed on cooling were inherited from the shear textures of the austenite phase with relatively strong variant selection. The ferrite portion of the stirred zone and the ferrites in the thermo-mechanically affected zones and the overlapped area underwent shear deformation with textures dominated by the D1 $$ \left\{ {\bar{1}\bar{1}2} \right\}\langle 111\rangle $$ and D2 $$ \left\{ {11\bar{2}} \right\}\langle 111\rangle $$ simple shear texture components. The formation of ultrafine equiaxed ferrite with submicron grain size has been observed in the overlapped area between the first and second weld pass. This is due to continuous dynamic strain-induced recrystallization as a result of simultaneous severe shear deformation and drastic undercooling.

Weld cladding of low alloy carbon steel generates compressive residual stress in the clad layer, in turn potentially improving resistance to fatigue failure, depending on the material used for cladding. The current paper summarises the results of investigations on the magnitude and distribution of residual stress in these weld clad components, undertaken using different techniques including X-ray diffraction, and incremental centre hole drilling based on both strain gauge rosettes and electronic speckle pattern interferometry. Results confirm the presence of tensile residual stress when cladding with Inconel 625 beyond the initial clad profile and compressive residual stress when cladding with 17-4 PH steel. The complementary nature of XRD and hole drilling techniques is highlighted with considerations regarding the weld clad profile and stress distribution with depth. Modelling of residual stress induced by weld cladding using a thermal transient analysis is presented. Simplification of the weld cladding process is shown to provide good correlation with experimentally measured residual stress. Complexities in modelling material behaviour and hence accurate prediction of residual stress are discussed. Chemical composition of the weld into the heat-affected zone and substrate is presented for both weld clad materials, highlighting the effects of alloying and diffusion on chemical composition. Given the complexities in obtaining accurate thermo-mechanical material properties required for modelling, and that residual stress profiles are measured to a limited depth into the clad layer, recommendations are made for the continuation of both experimental and simulation studies.

Understanding the residual stress state in brazed joints is crucial for the operational design and lifetime performance of the part in service. High-magnitude residual stresses are expected in the joined materials following cooling from brazing temperatures ( $\approx 950~^{\circ }\text{C}$ ) due to large mismatches in the thermal and mechanical properties. This paper aims at further understanding of the residual stresses caused when brazing tungsten to copper and tungsten to 316L austenitic steel using a eutectic gold-copper brazing alloy. These configurations are potentially useful for future diverter designs. Finite element analysis has been used to predict the brazing-induced stresses and residual stress measurements were carried out on the brazed joint by X-ray diffraction to validate the prediction model. Large residual stresses are predicted and measured in the tungsten; however, there is disagreement in the nature of the stress in the tungsten–copper configuration. Predicted stresses are highly tensile in nature close to the brazing interface, whereas the measured stresses are highly compressive. The disagreement is believed to be caused by the model not accurately simulating the complex brazing process. Residual stress measurements on the copper were not possible due to texturing during brazing, grain growth, and significant inelastic strains. There is excellent correlation between the measured and predicted stresses in the tungsten-316L configuration. High-tensile stresses were predicted in the tungsten (magnitude approximately 1000 MPa close to the braze interface) and high tensile stresses were measured (magnitude approximately 800 MPa in the same region). Joint misalignment of parent materials was also observed to significantly affect the residual stresses.

Standard test methods such as the electrochemical potentiokinetic reactivation test (EPR) and double-loop EPR test (DL-EPR) are commonly used to characterise sensitisation behaviour in austenitic stainless steels and nickel-based alloys. In this study, the DL-EPR test is augmented by large-area image analysis (IA) to characterise and quantify the networks of attacked grain boundaries. A new analysis approach that is based on a grain boundary cluster parameter is proposed to describe the network of corrosion susceptible grain boundaries, which may be estimated from electron backscatter diffraction (EBSD) data. This method may provide a better assessment of the relative DOS of different heats of austenitic stainless steels. Copyright 2011 Elsevier Ltd.

Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels occurs at susceptible grain boundaries after sensitisation. In this study, the effects of test duration, static stress (applied and residual) and microstructure orientation on the developed populations of short crack nuclei are reported for a sensitised type 304 austenitic stainless steel in an acidified potassium tetrathionate (K2S4O6 ) solution. The crack populations were analysed using the Gumbel distribution method, showing an increase in the characteristic crack lengths with increasing time and grain size. There is a weak, but measurable effect of stress on crack length. Tensile stress increases crack growth and compressive residual stresses introduced by surface machining are shown to be beneficial. A significant dependence on sample orientation is observed and this cannot be explained in terms of the bulk microstructure properties or characteristics, which showed no significant variations.

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>∑29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of ∑3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing. © 2010 Institute of Materials, Minerals and Mining.

The development of global microstructure characteristics has been compared to the local distribution and extent of Σ3 and Σ3n (1≤ n ≤3) grain boundary clusters as a function of thermomechanical processing in Type 304 stainless steel. A cold reduction of 5% produced GBE modified microstructures on annealing at 1050°C, containing almost one order of magnitude longer maximum cluster lengths than the corresponding annealing treatments for a reduction of 15%. Differences in the development of the distributions of cluster length scales were observed between the thermo-mechanical treatments. A re-conversion of the longest cluster obtained after GBE processing was observed with long annealing times. The local distribution of Σ3n boundary clusters was assessed, and regions with a low density of clusters are indicative of the onset of GBE conversion of microstructure. © (2010) Trans Tech Publications.

Understanding the residual stress state in brazed joints is crucial for operational design and life time performance of the part in service. High magnitudes residual stresses are expected in the joined materials following cooling from brazing temperatures (≈950°C) due to large mismatches in material properties such as coefficient of thermal expansion and Young’s modulus. This study aims at further understanding of the generation and distribution of residual stresses when brazing tungsten to copper using a eutectic gold-copper brazing alloy. This configuration is potentially useful for future divertor designs. Finite Element Analysis (FEM) has been used to predict the brazing induced stresses and residual stress measurements were carried out on the brazed joint by X-ray diffraction (XRD) to validate the prediction model. Large residual stresses are predicted and measured in the tungsten; however there is disagreement in the sign of the stress. Predicted stresses are highly tensile in nature close to the brazing interface, whereas the measured stresses are highly compressive. The disagreement is believed to be caused by the model not accurately simulating the complex brazing process. Residual stress measurements on the copper were not possible due to texturing during brazing, grain growth and significant inelastic strains and deformations. Misalignment of parent materials was also observed to significantly affect residual stresses.

Intergranular stress corrosion cracking (IGSCC) is a fracture mechanism in sensitised austenitic stainless steels exposed to critical environments where the intergranular cracks extends along the network of connected susceptible grain boundaries. A constitutive model is presented to estimate the maximum intergranular crack growth by taking into consideration the materials mechanical properties and microstructure characters distribution. This constitutive model is constructed based on the assumption that each grain is a two phase material comprising of grain interior and grain boundary zone. The inherent micro-mechanisms active in the grain interior during IGSCC is based on crystal plasticity theory, while the grain boundary zone has been modelled by proposing a phenomenological constitutive model motivated from cohesive zone modelling approach. Overall, response of the representative volume is calculated by volume averaging of individual grain behaviour. Model is assessed by performing rigorous parametric studies, followed by validation and verification of the proposed constitutive model using representative volume element based FE simulations reported in the literature. In the last section, model application is demonstrated using intergranular stress corrosion cracking experiments which shows a good agreement.

It is expensive, time consuming and can involve parts being transported elsewhere for weeks at a time, however, heat treatment for stress relief is a necessary evil for the manufacturing industry. Seen as essential for tackling residual stress in parts, its drawbacks are generally accepted as unavoidable. Despite its widespread use, investigations have found that heat treatment is not always appropriate for larger components and that using it may actually lead to distortion out of required tolerances and potential scrapping of parts. At the University of Strathclyde’s Advanced Forming Research Centre (AFRC), UK, part of the National Manufacturing Institute Scotland (NMIS), we are further investigating an alternative to heat treatment for relieving residual stress by collaborating with experts on the use of Vibratory Stress Relief (VSR).

The distortion behaviour of carburised and fully heat treated Ni-Cr-Mo martensitic steel (S156) has been experimentally evaluated. Dimensional measurements of Navy c-ring distortion coupons during interrupted heat treatment process for parts manufactured from two forming routes, hot forging and machined from as received bar, was performed. Metallurgical analysis was carried out to attempt to relate the observed microstructural characteristics with measured process induced distortion. The carburisation process was found to be the most severe in terms of inducing distortion. It was found that additional heat treatments during the process results in a larger final distortion. Machining parts from forgings results in higher distortions than that of those machined directly from as received bar due to the added thermal processing history. An FE simulation of the carburisation process for a c-ring coupon is presented.

It’s 1953 and Roy Woodward is a 25-year-old engineer trying to find an explanation for the causes of fatigue cracking and how these are linked to residual stress. Components are cracking prematurely and unexpectedly with no real understanding of why. Residual stress describes the stress locked within a component or material when all the externally applied loads and forces acting on it are removed. It can add to, or subtract from, the applied stresses and lead to unexpected consequences, such as the early failure of a part or distortion out of required tolerances – it’s a common, but even today, often unidentified, side effect of many manufacturing processes.

Titanium alloys, such as Ti-6Al-4V, are extensively used in critical aerospace applications. Heat treatments are often conducted during forging processes to produce final microstructures which exhibit advantageous mechanical properties. However, high thermal gradients present during processing may lead to the generation of undesirable levels of residual stress. Stress relief can be achieved by conducting aging or annealing treatments at elevated temperatures, yet there is limited quantitative understanding of how holding temperature affects the evolution of residual stress. Stress relaxation testing was conducted between 500-750°C and the resulting response was modified to describe creep strain. This was implemented in DEFORM™ finite element analysis software to model the evolution of residual stress during solution treatment followed by aging between 500-750°C, with comparison against solutions which did not consider creep strain. Stress relaxation phenomena was found to have an important impact on the reduction of residual stress and needs to be considered when carrying out thermo-mechanical processing at elevated temperatures.