Linear Friction Welding Research Articles

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.

Characterisation of joint properties through spatial mapping of cracks in fatigue specimens, extracted from the linearly friction welded steel coupon

Linear friction welding (LFW) is widely used for the fabrication of high-value components in the aero-industry. The versatility of use and high integrity of joints produced by LFW, suggest that this technology could contribute greatly to sustainable engineering of the future. For example, in near-net-shape manufacturing, LFW is predicted to offer a substantial reduction in material waste and processing time, in addition to offering the capability of bonding material pairs currently difficult to fusion weld. LFW of steel components is seldom researched in published literature, with little information available on fatigue strength and fracture behaviour of such joints. This may hamper the confidence needed to popularise LFW in wider industry. Thusly, authors conducted a discovery-oriented fatigue study, followed by metallography and micro-hardness tests, conducted on a 100Cr steel LFW coupon and corresponding batch of parent material. The weld coupon was dissected into a set of fatigue specimens, to allow mapping of the joint's internal properties through individual fatigue estimates and fracture morphology of each specimen in the set. A prototype fatigue machine was constructed, designed to deliver independent displacement and force controlled bending or torsional loadings, representing complex real-world conditions more accurately than common, uniaxial tests. The hour-glass shape of fatigue specimens allowed for targeting strictly the weld region, inducing fracture at the weakest material section, which was found to lay far outside the immediate contact interface. Referencing distances from fracture points to the theoretical weld interface plane in each fatigue trial, resulted in discovery of a macroscopic pattern. Interpolated to a crack initiation plane, this pattern is proposed to reflect the heat-affected zone, across the greater LFW coupon. Authors also studied crack paths and suggest a link between fracture direction, residual stresses in joint and angular orientation of the crack initiation plane in respect to the theoretical weld interface.

Intergrowth Bonding Mechanism and Mechanical Property of Linear Friction Welded Dissimilar Near‐Alpha to Near‐Beta Titanium Alloy Joint

Linear friction welding (LFW) of dissimilar near‐alpha TA19 (Ti–6Al–2Mo–4Zr–2Sn) to near‐beta TB2 (Ti–5Mo–5V–8Cr–3Al) titanium alloys is conducted with two preset axial shortenings to further explore the bonding mechanism of LFW. The microstructure evolution of the joints is analyzed by using an optical microscope, scanning electron microscope, and transmission electron microscope. The tensile properties and Vickers hardness of the joints are tested. The results show that there are obvious differences in the macro‐ and microstructure characteristics on both TA19 and TB2 sides of the joint resulting from the different thermomechanical coupling effects, and different dynamic recovery/recrystallization behaviors due to their differences in high‐temperature strength, thermal conductivity, and alloying elements. It is found that for a sound joint (larger axial shortening), a large number of high‐angle grain boundaries are formed in the weld zone (WZ) on the TA19 side through continuous dynamic recrystallization, and a large number of low‐angle grain boundaries are obtained in the WZ on the TB2 side via dynamic recovery. The mutual migration of grain boundaries on both sides of the weldline leads to the formation of intergrowth grains, making the dissimilar TA19 to TB2 joint a reliable bonding.

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.

LFW (Linear Friction Welding)

LFW (Linear Friction Welding)

Thermo-mechanical finite element simulation for linear friction welding of superalloys

Thermo-mechanical finite element simulation for linear friction welding of superalloys

Microstructure, Tensile Properties, and Fatigue Behavior of Linear Friction-Welded Ti-6Al-2Sn-4Zr-2Mo-0.1Si.

This paper presents the microstructural characteristics and mechanical properties of linear friction-welded (LFWed) Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) in as-welded (AWed) and stress relief-annealed (SRAed) conditions. The weld center (WC) of the AWed Ti-6242 consisted of recrystallized prior-β grains with α’ martensite that were tempered during SRA at 800 °C for 2 h and transformed into an acicular α + β microstructure. The peak hardness values, obtained in the AWed joints at the WC, sharply decreased through the thermomechanically affected zones (TMAZs) to the heat-affected zone (HAZ) of the Ti-6242 parent metal (PM). The SRA lowered the peak hardness values at the WC slightly and fully recovered the observed softening in the HAZ. The tensile mechanical properties of the welds in the AWed and SRAed conditions surpassed the minimum requirements in the AMS specifications for the Ti-6242 alloy. Fatigue tests, performed on the SRAed welds, indicated a fatigue limit of 468 MPa at 107 cycles, just slightly higher than that of the Ti-6242 PM (434 MPa). During tensile and fatigue testing, the welds failed in the PM region, which confirms the high mechanical integrity of the joints. Both the tensile and fatigue fracture surfaces exhibited characteristic features of ductile Ti-6242 PM.

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.

Analysis of the Microstructure and Mechanical Properties during Inertia Friction Welding of the Near-\u03b1 TA19 Titanium Alloy

The current research of titanium alloy on friction welding process in the field of aero-engines mainly focuses on the linear friction welding. Compared to the linear friction welding, inertial friction welding of titanium alloy still has important application position in the welding of aero-engine rotating assembly. However, up to now, few reports on inertial friction welding of titanium alloy are found. In this paper, the near-alpha TA19 titanium alloy welded joint was successfully obtained by inertial friction welding (IFW) process. The microstructures and mechanical properties were investigated systematically. Results showed that the refined grains within 15‒20 μm and weak texture were found in the weld zone due to dynamic recrystallization caused by high temperature and plastic deformation. The weld zone consisted of acicular α′ martensite phase, αp phase and metastable β phase. Most lath-shaped αs and β phase in base metal were transformed into acicular martensite α′ phase and metastable β phase in thermo-mechanically affected zone and heat affected zone. As a result, the microhardness of welded joint gradually decreased from the weld zone to the base metal. Tensile specimens in room temperature and high temperature of 480 °C were all fractured in base metal illustrating that the inertia friction welded TA19 titanium alloy joint owned higher tensile strength compared to the base metal.

Microstructure and Mechanical Properties of Linear Friction Welding Joint of GH4169 Alloy/S31042 Steel

Microstructure and Mechanical Properties of Linear Friction Welding Joint of GH4169 Alloy/S31042 Steel

Grain size and misorientation evolution in linear friction welding of additively manufactured IN718 to forged superalloy AD730™

Selective Laser Melted (SLM) Inconel718 (IN718) superalloy was linear friction welded (LFWed) to forged AD730™ Nickel-based superalloy. Successful joints free of micro-porosity, micro-cracking, and oxides were obtained. Microstructure variations across the weld line developed during LFW were examined using different techniques, including laser confocal microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The microstructure was also evaluated, particularly in terms of Grain size and misorientation changes were determined and correlated with microhardness evolution in different regions of the weld joint. The characteristics of the microstructure on both sides of the weld joint was analyzed and related to the deformation and temperature paths imposed during the LFW process. Dynamic recrystallization (DRX) occurred on both sides of the dissimilar weld line and it was found that Discontinuous DRX (DDRX) and Continuous DRX (CDRX) took place in the WZ and in the TMAZ, respectively. In order to study the influence of the starting microstructure in the LFW experiments, LFW of a homogenized SLM IN718 sample was analyzed and compared with the non-homogenized sample. A clear change in the size and grain misorientation levels of the heat and thermomechanical affected zones were observed between the two conditions. The differences were related to a greater degree of strain induced in homogenized sample and the increasing effect of the solid solution strengthening mechanism caused by a partial dissolution of the second-phase strengthening particles in the matrix.

Effect of heat treatment on the microstructure, microhardness and impact toughness of TC11 and TC17 linear friction welded joint

The service life of whole blisks for the aero-engine compressor is sharply decreased due to the dissimilar linear friction welded (LFWed) joint of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) and Ti–4Mo–4Cr–5Al–2Sn–2Zr (TC17) alloys, which have the different microstructure and mechanical properties. To optimize the microstructure and performance of the joint, and improve service life of the whole blisks, the post-weld heat treatment (PWHT) with annealing, solution plus aging were adopted for the welded joint. The results show that the nucleation rate of primary αP phase increases obviously, which are spheres, rods and strips, and orthorhombic martensite αs phases transform into hexagonal martensite αs phases interlaced evenly on the β phase matrix on the WZ of TC17 side; meanwhile, acicular hexagonal martensite αs phases transform into lamellar αs phases on the WZ of TC11 side after solid solution and dual aging. The difference of the microhardness on both sides of the joint is narrowed significantly. The impact toughness of the joint is improved sharply from 9.3 J/cm2 after welding to 27.6 J/cm2 with an increase of 200%. In addition, the dissolution and growth model of primary αp phases were established, which can reasonably predict the nucleation and grow process of α phase. Furthermore, the narrowed discrepancy of the microhardness and increased impact toughness can be attributed to the decreased slice thickness of martensite αs phase, based on the dislocation slip mechanism and distorted discordant model.

Friction welding of conventional Ti-6Al-4V alloy with a Ti-6Al-4V based metal matrix composite reinforced by TiC

Titanium alloys are supreme structural materials primarily due to their high specific strength. However, their wide use is largely restrained by the high cost of raw titanium compared to other metals commonly used in structural alloys. Layered structures of titanium alloys allow substantial increase of the material utilisation ratio and therefore draw significant attention. The rational ways of layered parts fabrication are bonding or joining of individually optimised layers into a final complex structure. The use of friction welding to join the parts is one of the most attractive ways of achieving a desirable result, since it is a solid state and near-net-shape process that modifies the structure of connected parts only locally. The study goal was to validate feasibility of the layered structures of Ti-6Al-4V (Ti-64) alloy and metal matrix composite (MMC) on its base with 10% of TiC fabricated by rotary friction welding (RFW) and linear friction welding (LFW). Both initial structures, Ti-64 and MMC, were made using low-cost blended elemental powder metallurgy. RFW and LFW were successfully used to bond the sections of the alloy and its composite. TiC particles stabilise the structure and are not fragmented by friction welding under used processing parameters.

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.

Formation of interfacial reaction layer for stainless steel/aluminum alloy dissimilar joint in linear friction welding

One of the mechanisms for the dissimilar materials joining is the formation of interfacial reaction layers. However, this reaction is disrupted owing to the presence of oxide layers and voids, leading to lower strength, and the elimination of these factors is essential. Thus far, studies on linear fraction welding (LFW) with regard to the relationship between the joint strength and the microstructure of the base alloy and interface are lacking. Therefore, herein, the influence of microstructural changes in base alloys and interfacial structures on the joint tensile strength was investigated for the dissimilar joint between the stainless steel and an A5083 alloy after high-frequency linear friction welding at 245 Hz. After the initial joining process up to 1 s, the joint strength decreased as the friction increased. The crystal orientation analyses using electron backscatter diffraction demonstrated a minimal change in the microstructure of the A5083 alloy, particularly in the grain size in the geometric dynamic recrystallization region. This variation in the microstructure was minimal, regardless of the friction time, which was consistent with the result of the hardness distributions. The observation of microstructure using transmission electron microscopy revealed the formation of metastable Al-Fe intermetallic compounds and Al-Mg-Cr compounds derived from the reaction between the oxide layer and alloying elements in the A5083 after experiencing friction for a short time. Conversely, we confirmed the formation of a fragile boundary within the interfacial reaction layer after the exposure to friction for an extended period due to differences in the formation process of the layer: mechanical mixing or interdiffusion. Hence, it was proposed that mechanical removal as well as the interfacial reaction between the oxide layer and the Mg contained in the aluminum alloy function in removing the surface oxide layer, which plays a significant role in designing the interfacial structure and joint properties.

Linear friction welding of a beta titanium alloy: experimental investigations on microstructure evolution and mechanical properties

ABSTRACT Linear friction welding was used to join Ti–3Al–8V–6Cr–4Mo–4Zr alloy under different pressures, and the microstructure, texture evolution and mechanical properties of the joint were investigated. Results show that the incomplete continuous dynamic recrystallisation (CDRX) occurs during welding, which refines the grains and produces a large number of subgrains in the thermo-mechanically affected zone (TMAZ) and weld centre zone (WCZ). At a higher friction pressure, the width of the WCZ becomes narrower, with the decrease of grain size and the increase of recrystallisation degree. In addition, the texture in the WCZ has changed from {112}<111> to {110}<111>, while the texture in the TMAZ remains at {110}<112>. The tensile properties of the two joints are similar to the base metal (BM), and the microhardness in WCZ and TMAZ are also close to that in BM due to the incomplete CDRX and the decrease of alpha phase.

Linear friction welding of Ti-6Al-4V alloy fabricated below β-phase transformation temperature

The interfacial temperature, microstructure and hardness distribution of the linear friction welded Ti-6Al-4V joints obtained at different applied pressures were investigated. The sound Ti-6Al-4V alloy joints were successfully fabricated below the β-phase transformation temperature. The increased pressure considerably decreased the welding temperature. The dynamically recrystallized ultra-fined α equiaxed grains of 0.2 μm were formed in the weld center of the joint by applying pressure of 550 MPa. Moreover, the hardness of the weld center was higher than that of the base material without the softened region in thermo-mechanical affected zone, which suggests the improved mechanical properties of the joints.

On the effect of a thermal treatment on the tensile and fatigue properties of weak zones of similar Ti17 linear friction welded joints and parent material

The tensile and fatigue properties and the damage mechanisms of as welded (LFW–AW) and post-weld heat treated (PWHT) similar Ti17 linear friction welded (LFW) joints as well as Ti17 parent material (PM) have been studied together with their underlying microstructures. The LFW–AW joints exhibited a depletion of the α phases in the weld centre zone (WCZ) and in the thermo-mechanically affected zone (TMAZ), which was previously qualified as a weak zone. Stereo Digital Image Correlation (SDIC) analysis of the strain field for monotonic loading of the PM showed strain heterogeneity that was linked by EBSD and Schmid's factor analysis to the large β grains containing α precipitates. Compared to the PM tensile specimens, the LFW–AW joint highlighted lower ultimate tensile strength (UTS) and a strongly reduced and scattered macroscopic ductility. Maps and spatio-temporal graphs of normalized strain rate fields obtained by SDIC clearly highlighted localization at the weak zone. For the target fatigue life of 105 cycles, the fatigue strength of the AW joint was about half that of the PM with fatigue crack initiation at the β grain boundary in the weak zones. Remarkably, the PWHT made the AW joints comparable to the PM in terms of microstructure and mechanical properties. The former weak zones recrystallized into β grains with α, the UTS and fatigue strength were almost recovered, macroscopic and local ductility were enhanced, failure during monotonic and cyclic loadings occurred far from the former weak zones.

Joining of Dissimilar Alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si Using Linear Friction Welding.

Dissimilar joints between Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) were manufactured using linear friction welding. The weld quality, in terms of the microstructure and mechanical properties, was investigated after stress relief annealing (SRA) at 750 °C for 2 h and compared with the as-welded (AWed) results. The central weld zone (CWZ) microstructure in the AWed condition consisted of recrystallized prior-β grains with α’ martensite, which transformed into an acicular α+β structure after SRA. The hardness in the AWed condition was highest in the CWZ and decreased sharply through the thermomechanically affected zones (TMAZ) to the parent materials (PMs). After SRA, the hardness of the CWZ decreased, mainly due to tempering of the α’ martensite microstructure. Static tensile testing of the dissimilar welds in both the AWed and stress relief annealed (SRAed) conditions resulted in ductile fracture occurring exclusively in the Ti-6Al-4V side of the joint. The promising results on joining of Ti-64 to Ti-6242 provide valuable insight for tailoring performance of next-generation aero-engine products.

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.