In order to better understand the interface bonding behavior of linear friction welding (LFW) of a near-β titanium alloy, the thermo-physical simulation for deformation behavior and microstructure evolution of a near-β TB2 titanium alloy was carried out by using hot compression tests with specially designed hat-shaped specimens under different compression displacements, temperatures and strain rates which were decoupled in the simulation process. The results show that the peak shear stress in bonding zone (BZ) during hot compression increases with decreasing deformation temperature and increasing strain rate, and the width of BZ decreases with increasing deformation temperature and strain rate. The margin of BZ has largely deformed grains, and the center of BZ has fine equaxied recrystallized grains. The mechanism of the joint is continuous dynamic recrystallization (CDRX) and the degree of CDRX in BZ increases with increasing compression displacement, deformation temperature and strain rate. In addition, {112}[111] texture forms in the margin of BZ, and {110}[001] texture forms in the center of BZ.

The parameters of a constitutive model of a fused deposition modeling (FDM) additively manufactured part were calculated with a novel computational tool of inverse optimization. The finite element (FE) model was used to calibrate the printed part's properties, incorporates a composite shell section of linear elastic orthotropic material under plane stress, various design configurations following a Taguchi L18 orthogonal array, were modelled in Abaqus CAE 2021 following the classical laminate theory (CLT). The material studied was PLA. The plastic mechanical behavior was taken into account with various stress-based failure criteria normally used in fiber- reinforced composite materials. Initially an investigation based on experiments of 3D printed FDM specimens determined stress-strain curves for various types of geometry and loading. Then, inverse optimization was performed with the interior point (IP) method, which calculates material parameters of the constitutive model of the parts. It was shown that the experimental response of 3D printed FDM processed parts can be estimated accurately numericallywith material parameters calculated with the new inverse optimization tool proposed.

Linear friction welding (LFW) is a solid-state welding process used primarily in manufacturing of aeroengine blisks. Ti60 near-alpha titanium alloy was developed for blisks operating at 600 degrees C, and shows excellent thermal stability and high-temperature oxidation resistance. In order to promote the wider use of such blisks, the mechanisms of microstructure evolution including phase transformation and dynamic recrystallization of welded Ti60 joint were studied. The effects of phase transformation and dynamic recrystallization on microhardness and tensile strength were investigated as well. Results show that the microstructure of the thermo-mechanically affected zone (TMAZ) deforms severely along the friction direction, forming {0001}[11 (2) over bar0](alpha) texture. beta -> metastable beta then -> alpha + beta transformations occur, leading to lamellar alpha precipitating from the deformed beta. Sufficient continuous dynamic recrystallization makes the WZ almost composed of fine equiaxed grains of 10 mu m size. In addition, alpha -> beta with the subsequent alpha' transformation occurs in the weld zone (WZ), producing extensive distribution of acicular alpha' martensite inside the recrystallized grains. Under the effects of strain hardening, precipitation strengthening and fine grain strengthening, the hardness in WZ increases significantly (i.e., 381 HV) compared to base metal (BM) (335 HV), with the tensile strength of the joint (943 MPa) also increasing over that of the BM (914 MPa). However, because of inhomogeneous deformation, the elongation of the joint (13.3%) is slightly lower than that of BM (14.1%).

Bobbin tool friction stir welding (BT-FSW) is a promising solid-state welding process for fabricating closed or hollow profiles due to its self-supporting nature, which is achieved using a combined tool consisting of two shoulders and a penetrating pin, releasing the backing plate. The joint reliability is mainly affected by its macro-/ micro-features including geometric defects and non-uniform grains. However, the underlying thermo-physical process has not been fully understood to clarify how the defects form and grains evolve during welding and how to control them. In this paper, a 3D thermo-mechanically coupled Eulerian-Lagrangian model was developed to analyze the material flow behavior and help understand the defect forming mechanism and recrystallization behavior during BT-FSW of aluminum alloy, wherein tracing particles were specially embedded. The calculated results revealed a complex material migration in the domain driven by the rotating tool. The flow behavior in horizontal/vertical directions at local regions was asynchronous, and eventually converged on the advancing side (AS) to normally form a sound joint, where the combined effects of shearing and squeezing of the material flow throughout thickness affected the morphology of S-line defect. In addition, the non-uniform thermo-mechanical cycling caused an abrupt change in grain structure near the TMAZ/SZ-AS transition region due to the combined continuous and discontinuous dynamic recrystallization, and geometrical effect of strain. The final joint failure was the result of competition between the two softening regions, S-line defect and TMAZ/ SZ-AS. These can be applied for further fundamental investigation of parameter optimization or welding structure design, and promote the exploration of post-processing methods to improve the joint performance.

The precipitation behavior and development of grain structure have been investigated to reveal their association with the microhardness and corrosion resistance of probeless friction stir spot welded (P-FSSW) joint of an Al–Li alloy. Results identified the primary strengthening phase of T1 (Al2CuLi) to be completely dissolved in the stir zone (SZ), while equiaxed grains were formed. Grain refinement was directly associated with the combined effect of continuous dynamic recrystallization and geometrized effect of imposed strain. However, as localized grain boundary bulging indicated, limited discontinuous recrystallization also participated in the microstructure evolution. The hardness dropped noticeably in the heat affected zone (HAZ) due to the weakening of precipitation and dislocation strengthening, which resulted from partial dissolution or coarsening of T1 phase and recovery effect. Grain refinement and solution strengthening were responsible for the slight increase in hardness of the SZ. In addition, the SZ had minute increased pitting resistance due to the homogenization of intermetallic particles, precipitate dissolution and refined grains.

An improved tool was designed to enhance weld formation for adjustable-gap bobbin-tool friction stir welding. No macro defects are observed as the welding speed increases from 120 to 240 mm/min. Microstructure analysis shows the maximum density of geometrically-necessary dislocations reaches 2.52 presenting at the stir zone (SZ). The Guinier-Preston-Bagaryatsky zones in heat affected zone (HAZ) and SZ are partially dissolved, and the relative volume fraction of S and theta phases in SZ is larger than that in HAZ. The hardness of the joints exhibits a W-shaped distribution. The minimum hardness is obtained at the HAZ. As the welding speed increases, the tensile strength initially increases up to 400 MPa at 180 mm/min, and then decreases. Poor mechanical properties at 60 mm/min and 300 mm/min are related to void defect.

This paper presents experimental results of standard three-point bending tests, for fused deposition modeling (FDM) specimens made of acrylonitrile–butadiene–styrene (ABS), as well as ABS plus. Both types of specimens were built with different building strategies. Furthermore, specimens of the aforementioned materials were subjected to standard Charpy’s impact test. The Charpy’s tests have been conducted using notched and unnotched specimens. Hereby, the flexural and impact strength of the specimens were quantitatively determined. Moreover, the findings of the implemented parametric analysis were discussed and correlated with the ones of the corresponding literature for bulk and FDM built materials. In addition, the fracture areas of the specimens were examined on a scanning electron microscope (SEM) to determine the failure patterns of the filaments’ strands. The research findings proved that ABS and ABS-plus FDM-printed specimens exhibit inferior flexural strength and bending capacity when compared to the bulk material. Finally, the derived impact strength results proved that ABS and ABS-plus FDM-printed specimens display a decreased impact strength when compared with the ones of the corresponding bulk materials.

Sheets of AA2198 alloy with various surface conditions were welded with probeless friction stir spot welding (P-FSSW). Results show that the oxide layer on the original lap-weld surface produces continuously distributed oxide impurities at the interface of the P-FSSWed joint with a large amount of voids. The visual flow at the interface provides a persuasive explanation of local preferential abrasion. Following surface grinding, local abrasion increases with surface roughness and results in the dispersion of voids and oxides, which contributes to the improvement of metallurgical connection. The corresponding mechanical strength of the P-FSSWed joints shows a relatively significant increase, while the fracture mode remains affected by the hook defect regardless of the surface state.

A third generation Al-Li alloy has been successfully welded by probeless friction stir spot welding (P-FSSW). The joints presented symmetrical ‘basin’ shapes and two distinct regions were observed: the stir zone (SZ) and the thermo-mechanically affected zone (TMAZ), which were characterized by recrystallized grains and deformed grains, respectively. In order to study the relationship between the P-FSSWed joint morphology and mechanical strength, a phenomenological model of various geometric features of these joints, including the stir zone width, the stir zone edge angle (SEA) and the hook angle (HA), was established. According to the model, the actual stir zone was described by an ellipse which has its limitation, no matter how to change the welding parameters. The tensile/shear strength was related significantly to the morphology of hook defect, and so was the fracture mode. Additionally, a one-to-one correlation existed between shoulder diameter and sheet thickness to achieve a sound weld, which was useful in manufacture to yield high-quality joints.

In this work, a third generation Al-Li alloy has been successfully spot welded with probeless friction stir spot welding (P-FSSW), which is a variant of conventional friction stir welding. The Box-Behnken experimental design in response surface methodology (RSM) was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83kN was obtained under a dwell time of 7.2s, rotation speed of 950rpm and plunge rate of 30mm/min. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction (EBSD) and differential scanning calorimetry (DSC) were used for microstructure evolution and property analysis. The precipitation of GP zone and Al3Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.

A solid-solution strengthened Ni-based superalloy was welded with linear friction welding (LFW) under different process parameters. Microstructural examination of the sound joints showed a refined microstructure in the weld zone (WZ) and the thermo-mechanically affected zone (TMAZ), with limited oxide presence at the edge of the weld for certain welding conditions. The flow line feature was present in the WZ and TMAZ due to carbide rearrangement. However, the mechanical properties of the joints, i.e. microhardness and tensile strength, were higher than those of the base metal. The weld microstructure underwent a discontinuous dynamic recrystallization during LFW, which was accompanied by limited continuous dynamic recrystallization and static recrystallization, which lead to a remarkable increase in the fraction of low-angle grain boundaries (LAGBs) and a dramatic decrease of the Σ3 twin boundaries in WZ and TMAZ. In comparison to WZ, the TMAZ had a higher fraction of LAGBs because of the presence of stronger microtexture and less recrystallization.

AA2198 joints were produced using the probeless friction stir spot welding (P-FSSW). Numerical modeling of the process based on the Coupled Eulerian-Lagrangian technique was developed to investigate the material flow. The modeling and experimental observations provide a consistent interpretation of the circumferential material flow which has some inward radial flow at the top surface. As the welding process advances, the flow is directed downwards, producing a spiral contracting pattern. The simulation results show that the hook defect is formed as the material in the bottom sheet is squeezed upwards in a symmetrical fashion with respect to the centerline, matching well with the experimental results.

The precipitate evolution and material flow visualization during probeless friction stir spot welding (P-FSSW) of an Al-Li alloy has been investigated experimentally and numerically in this work. Due to severe heat input during welding, the main strengthening phases of T1 fully dissolved in the stir zone followed by the re-precipitation of GP zone and δ′ phases, assisted with the refinement of the intermetallic particles and grains. The history of material flow was three-dimensionally visualized with a tracer, studied with X-ray tomography. The experiments and the simulation results provided a consistent interpretation of a circumferential material flow which moved inwards radially near the top surface. Upward flow in the bottom sheet improved with dwell time, causing the interface to bend. Local abrasion between the two sheets due to flow difference led to the disruption of oxide layer, and formed a connection at the interface.

The effect of offset of the centre of a tensile specimen to the weldline on global tensile properties of friction stir welded AA2024 joints was investigated using experiments and numerical analysis. The size and geometry of these discrete zones, such as the central low hardness zone, the low hardness zone-I near the thermo-mechanically affected zone, the low hardness zone-II near the base metal and the base metal, were determined from a cross-sectional hardness map. Results show that tensile specimens with different area of the joint placed in the centre of the specimen do not affect the tensile strength and fracture path of a joint, strain is affected. Predictions based on local tensile properties follow well the measured global tensile behaviour.

Using cold spraying (CS), a surface layer with a modified microstructure and enhanced mechanical properties was formed on a 3.2 mm thick friction stir welded (FSWed) AA2024-T3 joint. The combined effect of ''shot peening effect (SPE)" and ''heat flow effect (HFE)" during CS were used to enhance joint mechanical properties. The microstructure evolution of the FSWed AA2024-T3 joints in the surface layer following CS coatings and their effect on mechanical properties were systematically characterized with electron back-scattered diffraction, transmission electron microscopy, differential scanning calorimetry and mechanical tests. Based on these experiments, a grain refinement, finer and more S phases, and improved amount of Guinier-Preston-Bagaryatsky (GPB) zones produced by CS treatments are proposed. The deposition of aluminum coating on the joint, lead to hardness recovery in the stir zone and the development of two low hardness zones as the density of GPB increased. The tensile properties of FSWed AA2024-T3 joints improved with the application of the aluminum coatings. Experiments and analysis of the enhanced mechanical properties mechanism indicate that SPE with a high plastic deformation and HFE with an intensive heat flow are necessary for the production of refined grains and increased numbers of GPB zones. (C) 2018 Elsevier B.V. All rights reserved.

The microstructural evolution across the friction-stir-welded dissimilar AA5083-H112 to AA2024-T351 aluminum alloy joints was characterized via electron backscatter diffraction (EBSD), aiming to identify the effect of inhomogeneous microstructures on the tensile properties and cyclic deformation behavior along with the influence of loading history. Results show that the top region of the stir zone (SZ) mainly consisted of AA2024 which was initially positioned on the retreating side during welding. On the AA5083 side, the lowest density of the geometrically-necessary dislocations (GNDs) assessed from the local misorientations of 0°–2° appeared in the SZ, while it occurred in the heat-affected zone (HAZ) of the AA2024 side. The fractions of recrystallized grains in the AA5083 SZ and AA2024 SZ were ~73% and ~34%, respectively. Strain localization and the resultant failure occurred in the lowest hardness zone of the AA5083 side. The extent of cyclic hardening increased as the stress amplitude increased. Both plastic strain amplitude and plastic strain energy density decreased with increasing number of cycles at higher total strain amplitudes. In the stepwise cyclic deformation tests in the form of ascending-descending loading, the prior cyclic deformation process tended to generate more stabilized structure and performance.

Linear friction welding (LFW) is a solid-state joining process that is an established technology for the fabrication of titanium alloy bladed disks (blisks) in aero-engines. Owing to the economic benefits, LFW has been identified as a technology capable of manufacturing Ti-6Al-4V aircraft structural components. However, LFW of Ti-6Al-4V has seen limited industrial implementation outside of blisk manufacture, which is partly due to the knowledge and benefits of the process being widely unknown. This article provides a review of the published works up-to-date on the subject to identify the “state-of-the-art”. First, the background, fundamentals, advantages and industrial applications of the process are described. This is followed by a description of the microstructure, mechanical properties, flash morphology, interface contaminant removal, residual stresses and energy usage of Ti-6Al-4V linear friction welds. A brief discussion on the machine tooling effects is also included. Next, the work on analytical and numerical modelling is discussed. Finally, the conclusions of the review are presented, which include practical implications for the manufacturing sector and recommendations for further research and development. The purpose of this article is to inform industry and academia of the benefits of LFW so that the process may be better exploited.

Double-side probeless friction stir spot welding（DP-FSSW） of AA2198 alloy was conducted to investigate the microstructure and mechanical properties. Compared with common single-side probeless friction stir spot welding（P-FSSW）, the plastic strain during DP-FSSW is nearly symmetrical with respect to the bondline to suppress the extension of hook defect, which is detrimental to the joint mechanical strength.With DP-FSSW, a fully metallurgically bonded region has formed due to severe plastic deformation at high temperatures. Tensile/shear tests show that the joint strength could exceed 8 kN, which is comparable to P-FSSW and refill FSSW, and all fractures happen in a shear failure mode as cracks extend along the interface of two sheets. The microhardness profile exhibits a uniform distribution along the thickness direction, in which the hook defect shows the lowest value.

Linear friction welding (LFW) is a relatively new type of solid-state welding method emerged in the 1980s,and it is mainly used for the welding of similar and dissimilar non-circular cross-section metals.At present,LFW has been successfully used in the titanium alloy blisks of aircraft engines in the developed western countries.Although the blisk simulators have been welded successfully,the basic theory research is still inadequate.According to the open literatures,most works have been carried out on LFW of similar and dissimilar metals in terms of the microstructure,mechanical properties,process parameters and numerical simulation.Besides,the correlated characters between the microstructure and mechanical properties have been found.This article reviewed the important research results on LFW titanium alloys,and the deficiencies at present have been put forward.Finally,the application statuses and development prospects of LFW titanium alloys have been considered.