Linear Friction Welding Process Research Articles

Plastic flow and interfacial bonding behaviors of embedded linear friction welding process: Numerical simulation combined with thermo-physical experiment

In this study, a new linear friction welding (LFW) process, embedded LFW process, was put forward, which was mainly applied to combination manufacturing of long or overlong load-carrying titanium alloy structural components in aircraft. The interfacial plastic flow behavior and bonding mechanism of this process were investigated by a developed coupling Eulerian-Lagrangian numerical model using software ABAQUS and a novel thermo-physical simulation method with designed embedded hot compression specimen. In addition, the formation mechanism and control method of welding defects caused by uneven plastic flow were discussed. The results reveal that the plastic flow along oscillating direction of this process is even and sufficient. In the direction perpendicular to oscillation, thermo-plastic metals mainly flow downward along welding interface under coupling of shear stress and interfacial pressure, resulting in the interfacial plastic zone shown as an inverted “V” shape. The upward plastic flow in this direction is relatively weak, and only a small amount of flash is extruded from top of joint. Moreover, the wedge block and welding components at top of joint are always in un-steady friction stage, leading to non-uniform temperature field distribution and un-welded defects. According to the results of numerical simulation, high oscillating frequency combined with low pressure and small amplitude is considered as appropriate parameter selection scheme to improve the upward interfacial plastic flow at top of joint and suppress the un-welded defects. The results of thermo-physical simulation illustrate that continuous dynamic recrystallization (CDRX) induces the bonding of interface, accompanying by intense dislocation movement and creation of many low-angle grain boundaries. In the interfacial bonding area, grain orientation is random with relatively low texture density (5.0 mud) owing to CDRX.

Thermal Stress Characteristics of Dissimilar Joints Joining Ti-64 and CCM via Linear Friction Welding

Linear Friction Welding (LFW) is a unique welding process which enables dissimilar materials to be welded without filler material. In this study, bi-metallic Ti-64-CCM joints were manufactured utilizing LFW and subsequently analyzed. A coupled thermal-deformation model was created to accurately model both thermal and deformation profiles along the weld interface. Ti-64 and CCM stock were machined in accordance with ASTM standards for tensile testing. Temperature distribution of the two material surfaces were measured using an IR camera. Tensile testing was then performed to analyze weldability and bonding strength. A transient decoupled thermomechanical simulation model was generated to simulate the LFW process. The simulation model produced results within 5% error of both experimental results. The tensile testing results indicate that successful bonding of Ti-64 to CCM is possible for an oscillation amplitude of 2 mm under large forging pressure. The results also show that parameter optimization is essential for successful LFW, as the decrease in percent error from test set 1 to test set 4 was 62.5%. From the tensile testing and thermal data collected, it was concluded that bonding of Ti-64 to CCM via LFW is possible but further research into the necessary process parameters is needed.

Computational modelling of dynamic recrystallisation of Ni-based superalloy during linear friction welding

Linear friction welding (LFW) is an advanced joining technology used for manufacturing and repairing complex assemblies like blade integrated disks (blisks) of aeroengines. This paper presents an integrated multiphysics computational modelling for predicting the thermomechanical-microstructural processes of IN718 alloy (at the component-scale) during LFW. Johnson–Mehl–Avrami-Kolmogorov (JMAK) model was implemented for predicting the dynamic recrystallisation of γ grain, which was coupled with thermomechanical modelling of the LFW process. The computational modelling results of this paper agree well with experimental results from the literature in terms of γ grain size and weld temperature. Twenty different LFW process parameter configurations were systematically analysed in the computations by using the integrated model. It was found that friction pressure was the most influential process parameter, which significantly affected the dynamic recrystallisation of γ grains and weld temperature during LFW. The integrated multiphysics computational modelling was employed to find the appropriate process window of IN718 LFW.

Characterisation of Microstructure and Mechanical Properties of Linear Friction Welded α+β Titanium Alloy to Nitinol

A variable area nozzle integrated into the design of a high-bypass-ratio turbofan engine effectively saves up to 10% in aircraft fuel consumption. Additionally, noise emissions can be lowered at airports during take-off and landing by having better control of the nozzle diameter. Shape memory capabilities of Nitinol alloys could be availed in the form of actuators in the construction of such a nozzle. However, these Nitinol actuators must be joined to Ti-6Al-4V, a prominent alloy making up most of the rest of the nozzle. Because of the huge differences in the physical and metallurgical properties of these alloys, fusion welding is not as effective as solid-state welding. In the current study, a linear friction welding process was adopted to join Ti-6Al-4V to Nitinol successfully. The effect of friction welding on the evolution of weld macro and microstructures; hardness and tensile properties were studied and discussed. The macrostructure of Ti-6Al-4V and Nitinol’s dissimilar joint revealed flash formation mainly on the Ti-6Al-4V side due to its reduced flow strength at high temperatures. Optical microstructures revealed fine grains in Ti-6Al-4V immediately adjacent to the interface due to dynamic recrystallisation and strain hardening effects. In contrast, Nitinol remained mostly unaffected. An intermetallic compound (Ti2Ni) was seen to have formed at the interface due to the extreme rubbing action, and these adversely influenced the tensile strength and elongation values of the joints.

Corrigendum to “Hot deformation behavior and constitutive model of GH4169 superalloy for linear friction welding process” [J Manuf Process 32 (2018) 469–481

Corrigendum to “Hot deformation behavior and constitutive model of GH4169 superalloy for linear friction welding process” [J Manuf Process 32 (2018) 469–481

Dissolution of delta phase in Ni-based superalloy during linear friction welding: integrated multiphysics computational process modelling

Linear friction welding (LFW) is an increasingly popular solid-state joining method for challenging applications such as integrated blade disk of aero-engines. However, the influence of friction-generated heat on the material microstructural evolution, material deformation and resultant mechanical performance of the manufactured components is not well understood. A novel integrated multiphysics computational modelling is presented for predicting the component-scale microstructural evolution of IN718 alloy during LFW. A modified time-temperature equivalence formulation was implemented for predicting the evolution of the δ phase, which was coupled with thermomechanical modelling of the LFW process. There is reasonably good agreement between the computational modelling results of this paper and the experimental results from the literature in terms of δ phase volume fraction and weld temperature. The integrated multiphysics computational modelling predicts the influence of process parameters on thermomechanical and microstructural processes of IN718 LFW. By systematically analysing the influence of 10 different LFW process parameter configurations, the friction pressure was identified as the most influential process parameter determining the extent of δ phase dissolution and weld temperature during LFW.

Linear Friction Welding of an AZ91 Magnesium Alloy and the Effect of Ca Additions on the Weld Characteristics.

Solid-state welding offers distinct advantages for joining reactive materials, such as magnesium (Mg) and its alloys. This study investigates the effect of linear friction welding (LFW) on the microstructure and mechanical properties of cast AZ91 (Mg–9Al–1Zn) and AZ91–2Ca alloys, which (to the best knowledge of the authors) has not been reported in the literature. Using the same set of LFW process parameters, similar alloy joints—namely, AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca—were manufactured and found to exhibit integral bonding at the interface without defects, such as porosity, inclusions, and/or cracking. Microstructural examination of the AZ91/AZ91 joint revealed dissolution of the Al-rich second phase in the weld zone, while the Mn containing phases remained and were refined. In the AZ91–2Ca/AZ91–2Ca joint, the weld zone retained Ca- and Mn-rich phases, which were also refined due to the LFW process. In both joint types, extensive recrystallization occurred during LFW, as evidenced by the refinement of the grains from ~1000 µm in the base materials to roughly 2–6 µm in the weld zone. These microstructural changes in the AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca joints increased the hardness in the weld zone by 32%. The use of digital image correlation for strain mapping along the sample gage length during tensile testing revealed that the local strains were about 50% lower in the weld zone relative to the AZ91 and AZ91–2Ca base materials. This points to the higher strength of the weld zone in the AZ91/AZ91 and AZ91–2Ca/AZ91–2Ca joints due to the fine grain size, second phase refinement, and strong basal texture. Final fracture during tensile loading of both joints occurred in the base materials.

Flat hardness distribution in AA6061 joints by linear friction welding

It is known that one of the main concerns associated with the conventional welding of precipitation-strengthened Al alloys is the formation of softening regions, resulting in the deterioration of mechanical properties. In this study, we show that linear friction welding (LFW) can completely suppress softening regions in precipitation-strengthened AA6061-T6 alloy by introducing a large shear strain and by controlling the interfacial temperature. We found that the LFW process resulted in an extremely low interfacial temperature; it decreased as the applied pressure increased from 50 to 240 MPa. This approach can essentially suppress both softening and hardening regions, leading to uniform hardness distribution in Al joints. The high-pressure LFW process demonstrated here can thus provide an innovated guidance to obtain high-performance Al alloy joints and be extended to other precipitation-strengthened Al alloys, which undergo high-temperature softening.

The study of the welding relief shape influence on the linear friction welding process

The study of the welding process of asymmetric parts made of Ti-6A1-4V alloy was made to expand the field of application of the process of linear friction welding and to improve the use of technology of existing assemblies. During of the study, it was found that the process of welding T-joints has features in a longer duration of the unsteady stage and an increase in the amplitude value of the shear force during the steady state. Welded joints do not contain seam flaws over the area of the welded joint, which it made unnecessary to assign an allowance along the perimeter of the welded section. The change in conditions at the joint is expressed in an increase in the microhardness of the welded seam in comparison with the Ti-6A1-4V joints, which are produced by the traditional technology more than 50 HV.

MODELING OF TITANIUM ALLOYS PLASTIC FLOW IN LINEAR FRICTION WELDING

The article presents the results of the analysis of the plastic flow of titanium alloys in the process of the Linear Friction Welding (LFW). LFW is a high-tech process for joining critical structural elements of aerospace engineering from light and high-temperature alloys. Experimental studies of LFW modes of such alloys are expensive and technically difficult. Numerical simulation was carried out for understanding the physics of the LFW process and the formation laws of a strong welded joint of titanium alloys. Simulation by the SPH method was performed using the LS DYNA software package (ANSYS WB 15.2) and the developed module for the constitutive equation. The new coupled thermomechanical 3D model of LFW process for joining structural elements from alpha and alpha + beta titanium alloys was proposed. It was shown that the formation of a welded joint occurs in a complex and unsteady stress-strain state. In the near-surface layers of the bodies being welded, titanium alloys can be deformed in the mode of severe plastic deformation. A deviation of the symmetry plane of the plastic deformation zone from the initial position of the contact plane of the bodies being welded occurs during a process of LFW. Extrusion of material from the welded joint zone in the transverse direction with respect to the movement of bodies is caused by a pressure gradient and a decrease in the alloy flow stress due to heating. The hcp-bcc phase transition of titanium alloys upon heating in the LFW process necessitates an increase in the cyclic loading time to obtain a welded joint.

Linear friction welding of dissimilar Ni-based superalloys: microstructure evolution and thermo-mechanical interaction

Linear friction welding (LFW) technology, as an advanced manufacturing approach to fabricate the bimetallic turbine blisks, is receiving increasing attention from aerospace industries. Current work experimentally investigated the microstructural evolution and microhardness in LFW of FGH4096 and GH4169 superalloys. A complete dynamic recrystallization (DRX) was obtained in friction interface zone (FIZ) with no obvious secondary phases including δ and carbide phases on GH4169 side, and primary and secondary γ′ phases on FGH4096 side. The thermal-mechanically affected zone (TMAZ) was characterized by incomplete DRX where a significantly lower amount of low angle grain boundaries existed together with partially dissolved secondary phases depending upon the distance from weld line. The above-mentioned distinct microstructural behavior, manifesting different thermo-mechanical fields at different areas in the joint, necessitated the further analysis of numerical simulations of LFW of two dissimilar alloys. A thermo-mechanical coupling model was developed to predict the temperature, plastic strain and stress distributions. Satisfactory comparisons between numerical and experimental results were observed. The findings in the work are believed to be one of few currently available to focus on the study of metallurgical and thermo-mechanical coupling behavior of the dissimilar nickel-based superalloy joint made by the LFW process.

On the influence of tooling behaviour over axial shortening mechanisms in linear friction welding of titanium alloys and modelling plasticisation effects

Linear Friction Welding (LFW) has often been studied with the principal goal of describing complex physical interactions at the weld interface. Most published research has focused on the LFW process itself and under many assumptions: simplified welding geometries, over-constrained and rigid assemblies, and idealised clamping conditions. Equipment and tooling characteristics must be considered alongside the process itself for a more comprehensive understanding of LFW. The following study is aimed at addressing observed unaccounted variance in process outputs by investigating Ti6Al4V alloy welds processed under uniform conditions. A method for machine characterisation and a new experimental data analysis workflow is proposed, based on data processing techniques in spectral, temporal, and spatial domains. Research findings presented in this paper describe new plasticisation phenomena which accompany axial shortening in LFW welds. A new kinematic model is proposed, explaining macroscopic interactions at the contact interface. It allows for identification of shortening behaviour prevailing in the process, estimation of relative shape thickness of the thermo-mechanically affected zone, as well as potential for macroscopic misalignment between two joint substrates. Post-weld metrological investigations indicate a link between joint misalignment and effects observed, suggesting that tooling could influence weld shortening behaviour by disturbing planarity at the contact interface.

Microstructure Characterization and Mechanical Properties in Individual Zones of Linear Friction Welded Ti-6Al-4V Alloy

Linear friction welding (LFW) offers a new approach to manufacture aerospace components while improving the buy-to-fly ratio. However, the fundamental knowledge associated with the LFW process, including the attendant microstructural evolution and corresponding mechanical behavior is still rather limited. In this research effort, subscale tensile coupons were prepared and tested to determine the properties of each discrete zone of the linear friction welded specimen, namely the welded zone, thermomechanically affected zone, and parent material. The results show that the yield strength of the welded zone is 20 pct higher than the parent material and the thermomechanically affected zone is 13 pct higher than the parent material. Materials characterization, including optical microscopy, scanning electron microscopy, electron backscattered diffraction-based orientation microscopy and transmission electron microscopy, was conducted to develop an understanding of the microstructure–property relationships. The highly refined nature of the microstructure makes final interpretations challenging, but the evidence suggests that the mechanical behavior is dominated by phenomenon that operate at the 1 to 50 nm length scale, including strain hardening and highly refined features that hinder slip.

Molecular dynamics study of the effect of alloying elements and imperfections on linear friction welding of Cu and Ni metals

There are many physical phenomena and properties associated with diffusion in linear friction welding (LFW) process. Many of these phenomena are not observable by known and conventional methods. The molecular dynamics (MD) is an advantageous and practical method to study the diffusion process and atomic behavior of welded materials. In the current research, we used MD simulations to model the LFW process between copper and nickel metals. The effect of defects in the structures and also the addition of alloying elements to the copper structure was also studied. Our findings revealed that the surface and subsurface imperfections in the copper structure were gradually eliminated in the friction stage. In contrast, these imperfections in nickel structure disappeared in the forging stage. Also, we showed that adding alloying elements to copper structure leads to an increase in the amount of the mean square displacement (MSD) of the atoms and the diffusion coefficient in copper.

Thermomechanical modelling for the linear friction welding process of Ni-based superalloy and verification

This paper presents a fully coupled thermomechanical model for the linear friction welding process of Inconel-718 nickel-based superalloy by using the finite element method. Friction heat, plastic work, and contact formulation were taken into account for two deformable plastic bodies oscillating relative to each other under large compressive force. The modelling results of the thermal history at the weldline interface and thermal field at a distance away from the rubbing surfaces were compared to instrumented data sourced from related publications for model verification. Optimal linear friction welding process parameters were identified via comparison of weld temperature to the liquidus temperature of Inconel-718 alloy. Comparison of interface temperature showed a consistent range of values above the solidus melting temperature (1250 ℃) and below the liquidus melting temperature (1360 ℃) of Inconel-718 alloy. For the first time, a visible linear friction welding process window is identified using a thermomechanical computational modelling method. Through computational modelling, the influence of welding process parameters on the heat transfer and deformation of weld was systematically investigated.

Study on microstructure and mechanical property of linear friction welding on 9Cr reduced activation ferrite/martensite steel

The present work is concerned with the application of linear friction welding (LFW) process accessible for the weld-jointing of 9Cr reduced activation ferrite/martensite (RAFM) steels. Optical and electron microscopic characterization of microstructures were performed in various regions of the weld joint. Hardness, tensile and Charpy impact tests on the joined samples were processed to examine the mechanical property and reliability of the weld joints. It indicates that, hot plastic deformation induced by linear friction triggers continual dynamic recrystallization in the weld zone, along with high-density dislocation substructures formed by such severe deformation, which leads to a good combination of mechanical performances in the weld joint. Such linear friction welding comes beyond the rotational process restriction in conventional friction stir welding, and avoid significant oxide inclusions, porosities and coarsened grains brought by heat input as well. The work proves that the present LFW technique works well in the welding of 9Cr RAFM steels and inspires us of a future study on optimizing process parameters of the welding process for a better performance.

Mechanical and metallurgical characterization of AA6082-T6 sheet-bulk joints produced through a linear friction welding based approach

In the last decades, new flexible manufacturing processes have been developed to face the demands, by many industrial fields, for highly customized complex functional parts. The peculiar design of these components often overcomes conventional sheet metal and bulk metal forming processes capabilities. In order to face this issue, new hybrid techniques, capable of exploit key advantages of different processes, have to be developed. In this study, a method to obtain sheet-bulk joints, based on the Linear Friction Welding process, is proposed. The feasibility of the technique was investigated through an experimental campaign carried out with varying pressure and oscillation frequency using AA6082-T6 aluminum alloy. The main mechanical and metallurgical properties of the produced joints, including typical material flow defects, were highlighted. It was found that sound hybrid sheet-bulk joints can be produced by the proposed approach. Finally, it was highlighted how the height of the weld center zone plays a key role on the mechanical properties of the produced joints.

“Microstructure and mechanical characterisation of titanium alloy linear friction welds”

The increasing use of composite materials in new aircraft builds leads to a significant demand for titanium alloy structural parts. The increasing costs and popularity of this material, together with restrictions in supply and processing, are driving the aerospace industry to make increasingly efficient use of available material. Linear Friction Welding (LFW) is a rapid, high integrity, solid-state forging process that has the potential to decrease the buy-to-fly ratio, production time and time to market of aerostructure components. Its dependability has already been proven for the production of key components in some of the latest generation aero engines. The LFW process is yet to be used for aerostructures. This is primarily due to the LFW process not being widely known, but also due to a lack of performance data on aerostructures manufactured by LFW being available to the supply chain and design community. To address this issue, a large series of titanium alloy weldments was produced and assessed via metallographic examinations and mechanical testing in both as-welded and post-weld heat-treated condition. The matrix of experiments was able to capture the LFW process window of this titanium alloy, and to measure the impact of the parametric conditions. Metallographic examination revealed a high integrity weld free from contaminants and oxides at the weld interface; with a characteristic recrystallised Widmanstätten martensitic Beta weld centre zone microstructure, in as welded condition, and a finegrained equiaxed recrystallised to alpha-beta microstructure in post-weld heat-treated condition. As-welded joints were tested under tensile and alternating fatigue conditions to provide an extended set of joint performance data. Joints demonstrated tensile performance equivalent to that of the parent material in all cases, with near-parent fatigue properties and improved (reduced) fatigue scatter in the post-weld heat-treated condition.

LINEAR FRICTION WELDING – PROCESS DEVELOPMENT AND APPLICATIONS IN AEROSPACE INDUSTRY

Linear Friction Welding (LFW) is a solid-state joining process very well adapted to titanium alloys, producing high integrity joints with fine grain, hot-forged microstructure and narrow heat affected zone. The first industrial application of this process was found in aircraft engines, for the manufacturing of “blisks” (“bladed disks”): linear friction welding the blades onto a disk provides economic savings and reduces the manufacturing time, compared to machining the whole blisk from solid. While the diffusion of LFW process in the blisk manufacturing market is still at the early stages and have promising growth potential, the process is now being developed for aircraft structures such as clips, brackets, hinges, fittings, and larger parts like seat rails, wing ribs, lintels and fuselage frames. The LFW process allows not only to manufacture a given part at a lower cost, it also open new part design possibilities, that were not available with traditional manufacturing processes. The manufacturing process of Ti-6Al-4V structural and engine parts by LFW is explained, highlighting advantages, limitations and part design best practices. Several LFW candidate parts are introduced and evaluated through feasibility, mass savings, post weld operations and overall cost savings.

Finite element modelling for temperature, stresses and strains calculation in linear friction welding of TB9 titanium alloy

In the present paper, finite element method (FEM) simulation of linear friction welding (LFW) TB9 titanium alloy was carried out using ABAQUS software. The constitutive model of TB9 alloy was established using Johnson-Cook constitutive equations, and then a two-dimensional (2D) coupled thermo-mechanical model was established to simulate the temperature, the Mises stress and the equivalent plastic strain fields of TB9 during LFW process. The axial shortening and macroscopic morphology of the linear friction welded joint were obtained by simulation results and verified by experiments. Finally, by consideration of the temperature fields of the simulated welded joint, the authors experimentally investigated the microstructure of the actual welded joint.