This work presents an alternative processing route to the conventional powder HIP—forge route for Nickel-based superalloys. Demonstrating how the field-assisted sintering technology (FAST) process can be exploited to successfully diffusion bond or functionally grade two or more Nickel-based superalloys from powder feedstock. The robustness of the process has been further demonstrated by the successful bonding of one alloy in powder form to another in the solid form. Chemical and microstructural analysis of the diffusion bond between the alloys is characterised, in both cases, with a short diffusion zone—in agreement with thermodynamic model predictions. A gradual transition in microhardness across the bond region was measured in all samples. A machinability assessment was also carried out through a simple face turning operation. Analysis of the cutting forces and machined surface shows signs of a directionality when machining across the bond region between two alloys, indicating that care must be taken when machining multi-alloy FAST-DB components.

The widespread use and development of inertia friction welding is currently restricted by an incomplete understanding of the deformation mechanisms and microstructure evolution during the process. Understanding phase transformations and lattice strains during inertia friction welding is essential for the development of robust numerical models capable of determining optimized process parameters and reducing the requirement for costly experimental trials. A unique compact rig has been designed and used in-situ with a high-speed synchrotron X-ray diffraction instrument to investigate the microstructure evolution during inertia friction welding of a high-carbon steel (BS1407). At the contact interface, the transformation from ferrite to austenite was captured in great detail, allowing for analysis of the phase fractions during the process. Measurement of the thermal response of the weld reveals that the transformation to austenite occurs 230 °C below the equilibrium start temperature of 725 °C. It is concluded that the localization of large strains around the contact interface produced as the specimens deform assists this non-equilibrium phase transformation.

Ni-based superalloys IN718 and IN713LC have been joined through linear friction welding (LFW) in this study. The variation of microstructure across the weld line developed during linear friction welding and after post weld heat-treatment (PWHT) has been investigated. Their effects on microhardness have also been studied. A clean weld region which is free of micro-porosity, micro-cracking and oxides, was achieved. Dynamic recrystallisation (full and partial) occurred on both sides of the weld, which produced much finer grains in the recrystallised zone. Dissolution of 'parent' gamma'/gamma '', towards the weld line was observed on each side of the weld. However, reprecipitation of gamma' was only found in the as-welded IN713LC. All these were found to have a huge impact on the hardness profile. A softer heat affected zone (HAZ) was found in the IN718 side with the lowest hardness value achieved at an axial position 0.6 mm from the weld line. The increased dissolution of gamma'/gamma '' towards the weld line resulted in decreasing hardness towards the weld line. However, the formation of refined grains closer to the weld line increased the hardness towards the weld line from axial position 0.6 mm. In contrast, a harder HAZ was found in the IN713LC side, which resulted from the formation of finer reprecipitated gamma' and recrystallised grains. PWHT brought about reprecipitation and/or further reprecipitation of gamma'/gamma '' in the IN718 and IN713LC HAZs, resulting in stronger HAZs.

AbstractThis study details the development and validation of a finite element methodology to robustly simulate the inertia friction welding (IFW) process. There are many difficulties involved in modelling IFW. These include the short and violent process to complete a weld, as well as the challenges in obtaining experimental data throughout the process to complement, validate and inform the modelling effort. The objectives here are to model the macroscale multiphysical process leading to an accurate prediction of key process output variables, ultimately leading to a reliable method for predicting the post weld microstructure.

The variation of microstructure and microtexture across a Ti-6Al-4V (Ti64) linear friction weld was investigated using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Pole figures and misorientations distribution obtained from  phase show four distinct regions within the weld, with different textural characteristics. Nevertheless, the main texture components remain the same in centre weld zone and thermal mechanical affected zone with the basal poles of the  phases located at around 0, 60 and 90 to the sample normal (Z0) direction. The results indicate that the texture components are strongly related to the amount of transformed  phase. The deformation of the primary  grains has a limited effect on the texture development. 1 Materials and Surface Science Institute, University of Limerick, Ireland (This is the current address of the first author, the work presented here was done in the University of Birmingham ) ACCEPTED MANUSCRIPT

The variation of microstructure and microtexture across a Ti-6Al-4V (Ti64) linear friction weld was investigated using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Pole figures and misorientations distribution obtained from α phase show four distinct regions within the weld, with different textural characteristics. Nevertheless, the main texture components remain the same in centre weld zone and thermal mechanical affected zone with the basal poles of the α phases located at around 0°, 60° and 90° to the sample normal (Z0) direction. The results indicate that the texture components are strongly related to the amount of transformed β phase. The deformation of the primary α grains has a limited effect on the texture development.

Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130°C and 1250°C spanning strain rates from 0.01s−1 to 4000s−1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall–Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener–Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000s−1 where mechanical testing’s results were absent.

The microhardness and microstructure of linear friction welded Ti–6Al–2Sn–4Zr–6Mo (Ti6246) alloys were studied, in both as-welded and post-weld heat-treated conditions. It has been found that the as-welded Ti6246 has a lower microhardness value of about 360HV in the central weld zone than that of the base material of about 420HV. Post-weld heat-treatment of the Ti6246 weld at 600°C for 1h has led to the hardness increase of about 180HV at the central weld zone. Transmission electron microscopy studies show that the microstructure at the central weld zone of the as-welded Ti6246 consists of fine grains with dense acicular orthorhombic α″ martensite. The soft α″ martensite is believed to account for the low hardness measured in the as-welded conditions. Phase transformation from orthorhombic α″ to hexagonal α occurred during the PWHT, resulting in the observed hardness increase.

Finite element (FE) process modeling of inertia friction welding between dissimilar high-strength steels, AerMet® 100 and SCMV, has been carried out using the DEFORM™-2D (v10.0) software. This model was validated against experimental data collected for a test weld performed between the materials; this included process data such as upset and rotational velocities as well as thermal data collected during the process using embedded thermocouples. The as-welded hoop residual stress from the FE model was also compared with experimental measurements taken on the welded component using synchrotron X-ray and neutron diffraction techniques. The modeling work considered the solid-state phase transformations which occur in the steels, and the trends in the residual stress data were well replicated by the model.

The inertia welding process has been recognized to yield very high magnitudes and rates of deformation with a steep temperature gradient across the weld leading to near melting point at the weld interface. The elevated temperature properties of SCMV and Aermet 100 steels were studied by means of compression testing on a single shot servo-hydraulic test frame. A wide combination of ram velocities, strain rates, temperatures and grain sizes were used to mimic the conditions in inertia welding and determine true stress true strain curve. The implications of these test variables on flow stress were described by the Zener - Hollomon parameter. The second derivative of the flow curves were used to find the critical strain for the onset of dynamic re-crystallization (DRX). The kinetics of dynamic re-crystallization was found by applying the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and was used to find a relationship between the fractions of re-crystallized austenite with respect to strain and time. Prior austenite grain (PAG) boundaries were revealed and studied by optical microscopy. Strain, strain rate and temperature were found to be the main determining factors in controlling the flow stress. The austenite grain size was found to influence the flow stress (Hall-Fetch relationship), final PAG size and the kinetics of DRX. Constant strain rate tests were prepared in a manner as to de-couple the PAG size with respect to the flow stress. Flow curves showed flow softening by DRX for constant strain rate whereas constant velocity curves showed persistent work hardening or DRV as the strain rate increased even though DRX was seen in the microstructure. The results from compression testing were applied to laboratory made inertia welds to establish any similarities or trends.

Characterization of dissimilar linear friction weld (LFW) of Ti-alloys has been carried out. The microstructure of a Ti-64 with Ti-6246 weld was analyzed using scanning and transmission electron microscopy. The microtexture of the weld was examined using electron back-scattered diffraction (EBSD) and the microhardness was measured. The dissimilar LFW weld shows an interface at the central weld zone (CWZ) with strong contrast. The element distribution measured using energy-dispersive X-ray spectrometer suggests that only limited atomic diffusion occurred during welding. In the weld region, the hardness of Ti-64 shows an increase while that of Ti-6246 shows a decrease, compared with the respective parent materials in as-welded condition. After post-weld heat treatment, a hardness increase was observed in Ti-6246, while Ti-64 shows no significant hardness increase. Although the EBSD pattern quality is poor in the CWZ at the as-welded condition, the pattern quality of the post-weld heat-treated sample shows that both the Ti-64 and Ti-6246 have similar texture.

The addition of lithium to aluminium alloys is known to afford the dual advantages of increasing mechanical performance while lowering density. These characteristics make Al-Li alloys particularly desirable for aerospace applications. However, the complex precipitation pathways and extensive nanometer-sized decomposition products, termed “nanoprecipitates,” make characterization difficult and thus limits optimization of the property sets of commercial alloys. This investigation uses thermal analysis and electrical resistivity methods to further understanding of the evolution of the various nanoprecipitates during isochronal aging of an Al-8.7 at. pct Li alloy. The results indicate decomposition via the following pathway: Spinodal-Ordering → Congruent Ordering + Spinodal Decomposition + Dissolution of Small Spinodally ordered regions → Growth of δ′ → Dissolution of δ′ → Nucleation and Growth followed by Dissolution of the δ phase.

The Inertia Friction Welding (IFW) process is a high-temperature and high pressure process, with heavy plastic deformation, high power density, fast heating and fast cooling of the weld material. The microstructure produced in the weld line （WL）zones is therefore very different from parent material. A detailed microstructural investigation of the WL zones has been conducted using transmission electron microscopy and scanning electron microscopy. It has been shown that the morphology, energy status and microchemistry of grain boundaries in the WL zones are quite different from those in the parent material. It is also observed that, compared to a bi-modal distribution of intragranular ¢ particles in the parent material, a unimodal distribution of very fine spherical ¢ particles is produced in high density in the WL zones. This work provides a detailed understanding of the physical and chemical changes occurring across the weld line.

Microstructure in the weld line (WL) zone produced by inertial friction welding has been studied for a series of RR1000–RR1000 welds using transmission electron and scanning electron microscopy. A fully recrystallised fine grain structure was found to form throughout the WL zone, characterized by straight and smooth grain boundaries, high energy status and modified grain boundary chemistry due to very fast cooling after welding. Very fine γ′ particles with unimodal size distribution were reprecipitated in the WL zone. The fine γ′ particles are spherical in shape, high in number density and characterized by an imbalanced chemistry, containing less Al, Ti, Ni, Ta γ′-forming elements but more Cr, Co, Mo γ-forming elements than all types of parent γ′ particles. As a result, a hard and strong WL zone was produced by the inertial friction welding.

In this paper, the levels of residual stress in the vicinity of linear friction welds in Ti–6Al–4V (Ti-64), a conventional α– β titanium alloy, and Ti–6Al–2Sn–4Zr–2Mo (Ti-6242), a near α titanium alloy with higher temperature capability, are mapped and contrasted. The alloys have significantly different high temperature properties and the aim of this work was to investigate how this might affect their propensity to accumulate weld residual stresses and their response to post-weld heat treatment. Measurements are reported using high energy synchrotron X-ray diffraction and the results are compared to those made destructively using the contour method. The strain free lattice plane d 0 variation across the weld has been evaluated using the biaxial sin 2 Ψ technique with laboratory X-rays. It was found that failure to account for the d 0 variation across the weld line would have led to large errors in the peak tensile stresses. Contour method measurements show fairly good correlation with the diffraction results, although the stresses are underestimated. Possible reasons for the discrepancy are discussed. The peak tensile residual stresses introduced by the welding process were found to be greater for Ti-6242 (∼750 MPa) than for Ti-64 (∼650 MPa). Consistent with the higher temperature capability of the alloy, higher temperature post-weld heat treatments have been found to be necessary to relieve the stresses in the near α titanium alloy compared to the α+ β titanium alloy.

The application of Linear Friction Welding (LFW) to aerospace Ti-alloys components offers significant weight reduction and cost saving advantages. The severe thermomechanical deformation associated with LFW results in a localised, yet complex microstructural development, which is the subject of the present investigation. In this work, the microstructural and microhardness developments due to LFW of beta-forged Ti-6246 were investigated. Metallurgical characterisation of the welds was performed using scanning electron microscopy (including electron backscatter diffraction) and microhardness. It was found that the thermomechanical deformation in the weld resulted in the formation of dynamically recrystallised metastable beta-phase in the weld, which indicates that welding at the weld line occurred above the beta-transus regardless of welding parameters. The width of the dynamically-recrystallised beta-region, and the beta-grain size was found to decrease with the increase in the consolidation (forging) pressure applied during LFW. Following post-weld heat treatment, fine alpha-precipitates were found to form in the weld line, resulting in a sharp increase in hardness.

Linear Friction Welding (LFW) is a novel welding technology, which utilises a combination of frictional heating and plastic deformation to join difficult-to-weld materials. However, when joining high temperature materials large residual stresses are generated, which can be detrimental to the joint performance. In this work, the residual stress development due to LFW in beta-forged Ti-6246 aerospace alloy was investigated using energy dispersive X-ray diffraction, at the European Synchrotron Radiation Facility in Grenoble, France, focusing on the influence of the consolidation pressure on the stress development. As Ti-6246 is a two-phase alloy, calculating the stresses in the alloy required the characterisation of strain in both phases separately. For accurate strain measurements, additional measurements of the stress-free lattice spacing as a function of position across the weld line were carried out using a laboratory based x-ray source in combination with the biaxial sin(2)Psi approach. The advantages and limitations of this technique, with respect to resolution and accuracy, are highlighted and discussed.

Microstructural variation across inertia friction welded Aermet 100 (ultra-high strength secondary hardening steel) and SCMV (high strength low alloy Cr–Mo steel) has been studied. Retained austenite was mapped in two dimensions across the inertia friction welds using synchrotron-X ray diffraction. Microhardness testing and SEM were used to characterize the general microstructure variation across the weld line in the as-welded and post-weld-heat-treated (PWHT) conditions. Four distinct microstructural zones were observed for the as-welded condition in the heat-affected zone. Subsequent PWHT resulted in a reduction of the peak hardness on adjacent sides of the weld with all other areas increasing in hardness. These changes in hardness are explained based on tempering of the martensitic structure and precipitation reactions during PWHT.

An abstract is not available for this content so a preview has been provided. Please use the Get access link above for information on how to access this content.