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Friction Stir Welding (FSW) is one of the most used solid-state welding processes in the aeronautical, aerospace, ground transportation and naval fields. Stationary Shoulder Friction Stir Welding (SSFSW) is a recently introduced variant of the process allowing lower heat input into the joints, with beneficial effects in terms of joint mechanical properties, microstructure and top surface finish. In the paper, lap joints produced by SSFSW and made out of AA6082-T6 aluminum alloy sheets have been analyzed with the aim to investigate the effect of the stationary shoulder on the lap joints surface, metallurgical, and mechanical properties. The lap joints produced by SSFSW have been compared to the ones produced by “conventional” FSW in order to highlight the differences between the two processes. The top surface of the joints obtained with the two processes has been quantitatively evaluated. Finally, a dedicated numerical model has been utilized to highlight the causes of the observed differences through the distributions of the main field variables, namely temperature, strain and strain rate. It was found that SSFSW results in narrower weld nugget and smaller areas involved by micro hardness reduction caused by smaller peak temperature and high temperature zone. Additionally, better surface quality was found for the joint welded by SSFSW with lower heights and flash, allowing to avoid subsequent finishing operations needed for most industrial applications.

The manufacturing of aluminium alloy structural aerospace parts involves several steps: forming (rolling, forging …etc), heat treatments and machining. Before machining, the manufacturing processes have embedded residual stresses into the workpiece. The final geometry is obtained during this last step, when up to 90% of the raw material volume is removed by machining. During this operation, the mechanical equilibrium of the part is in constant evolution due to the redistribution of the initial stresses. This redistribution is the main cause for workpiece deflections during machining and for distortions - after unclamping. Both may lead to non-conformity of the part regarding the geometrical and dimensional specifications and therefore to rejection of the part or additional conforming steps. In order to improve the machining accuracy and the robustness of the process, the effect of the residual stresses has to be considered for the definition of the machining process plan and even in the geometrical definition of the part. In this paper, the authors present two new numerical approaches concerning the modelling of machining of aeronautical structural parts. The first deals with the use of an immersed volume framework to model the cutting step, improving the robustness and the quality of the resulting mesh compared to the previous version. The second is about the mechanical modelling of the machining problem. The authors thus show that in the framework of rolled aluminium parts the use of a linear elasticity model is functional in the finite element formulation and promising regarding the reduction of computation times.The manufacturing of aluminium alloy structural aerospace parts involves several steps: forming (rolling, forging …etc), heat treatments and machining. Before machining, the manufacturing processes have embedded residual stresses into the workpiece. The final geometry is obtained during this last step, when up to 90% of the raw material volume is removed by machining. During this operation, the mechanical equilibrium of the part is in constant evolution due to the redistribution of the initial stresses. This redistribution is the main cause for workpiece deflections during machining and for distortions - after unclamping. Both may lead to non-conformity of the part regarding the geometrical and dimensional specifications and therefore to rejection of the part or additional conforming steps. In order to improve the machining accuracy and the robustness of the process, the effect of the residual stresses has to be considered for the definition of the machining process plan and even in the geometrical defini...

A widespread usage of aluminum alloys for the fabrication of car-body parts is conditional on the employment of appropriate welding methods, especially if dissimilar welding must be performed with automotive steel grades. Dissimilar welding of aluminum alloys and steel grades poses some issues concerning the formation of brittle intermetallic compounds, difference in physical and chemical properties of the parent metals, and poor wetting behavior of aluminum. Friction stir welding is considered to be a reasonable solution to obtain sound aluminum/steel joints. A study on the join quality of dissimilar lap joints of steel and aluminum alloy sheets after friction stir welding is proposed here. A low carbon steel is joined with AA6016 aluminum alloy to study preliminarily the feasibility to assembly car-body parts. The joints, welded with tool rotation and feed rate varying in a wide range, have been studied from a visual examination and microstructural point of view. Optical microscopy has been used to characterize the microstructure of the examined sheets in as-received and welded conditions. Micro-hardness measurements have been carried out to quantitatively analyze the local hardness of the welded joints. Set welding process parameters are identified to assemble without the presence of macroscopic defects the examined steel and aluminum welded parts.

Ultrafine grained (UFG) materials are of great potential in industry due to their enhanced mechanical strength and other promising features, such as ability to superplastic deformation or excellent corrosion resistance. Nevertheless, one of the main limitations lies in their low thermal stability, which leads to excessive grain growth at elevated temperature. It influences mainly further processes performed at high temperature, such as joining. It causes detrimental problems during conventional fusion welding, as significant grain growth is observed and therefore the advantages as a result of small average grain size disappear. Therefore, the idea of applying solid state joining process seems to be suitable for UFG materials. Among the group of solid state welding processes there are methods based on friction. One of these methods, which is suitable for samples in the form of rectangular bars, is linear friction welding (LFW). Present study is the first comprehensive work describing the results of UFG aluminium welded using LFW method with additional emphasis put on a position of shear planes of welded samples. The results have shown, that UFG regime has not been preserved in a weld zone. Welding of UFG sample with an average grain size of 1 μm resulted in a grain growth in weld zone to 1.6−2 μm. Chosen parameters allowed to weld samples only locally, nevertheless, results from tensile tests performed on mini samples showed a great potential of the LFW method for joining UFG samples. The tensile strength of welds was in the range of 83–90 % of initial material.

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.

Explosion welding (EXW) is a solid-state joining process used to produce lap joints out of metal plates of dissimilar materials. During the process, a controlled explosive detonation results in a pressure wave pushing one of the plates to be welded, called flyer, against the other with high velocity. The high pressure and temperature generated, because of the impact energy decaying into heat, create the conditions for solid bonding phenomenon to take place. Due to the complexity of experimental tests, numerical simulation is considered a fundamental design tool for the process. Different approaches are found in literature to simulate the process. In this paper, a dual step Lagrangian–Eulerian approach is proposed to evaluate the effect of different explosives on the final quality of the weld. The numerical model was developed using the commercial software ABAQUS. A three dissimilar materials joint is considered, made of two outer plates of AA5083 aluminum alloy and A516 steel, respectively, and an intermediate layer made of AA1050 aluminum alloy. The model was first validated by comparing the wavy profile of the intermediate layer after the weld, and then used to evaluate the effect of different explosive by analyzing the distribution of the main process parameters as well as the presence of voids.

During the last few years, additive manufacturing has been more and more extensively used in several industries, especially in the aerospace and medical device fields, to produce Ti6Al4V titanium alloy parts. During the Selective Laser Melting (SLM) process, the heterogeneity of finished product is strictly connected to the scan strategies and the building direction. An optimal managing of the latter parameters allows to better control and defines the final mechanical and metallurgical properties of parts. Acting on the building direction it is also possible to optimize the critical support structure. In particular, more support structures are needed for the sample at 0°, while very low support are required for the sample at 90°. To study the effects of build direction on microstructure heterogeneity evolution and mechanical performances of selective laser melted Ti6Al4V parts, two build direction samples (0°, 90°) were manufactured and analyzed using optical metallographic microscope (OM) and scanning electron microscopy (SEM). Isometric microstructure reconstruction and microhardness tests were carried out in order to analyze the specimens. The obtained results indicate that the build direction has to be considered a key geometrical parameter affecting the overall quality of the obtained products.

Widespread use of aluminum alloys for the fabrication of car body parts is conditional to the use of appropriate welding methods, especially if dissimilar welding must be performed with automotive steel grades. Friction stir welding (FSW) is considered to be a reasonable solution to obtain sound aluminum-steel joints. In this context, this work studies the effects of tool position and force control in dissimilar friction stir welding of AA6061 aluminum alloy on DC05 low carbon steel in lap joint configuration, also assessing proper welding parameter settings. Naked eye and scanning electron microscopy (SEM) have been used to detect macroscopic and microscopic defects in joints, as well as to determine the type of intermixture between aluminum and steel. The joint strength of sound joints has been assessed by shear tension test. Results point out that tool force control allows for obtaining joints with better quality and strength in a wider range of process parameters. A process window has been determined for tool force conditions to have joints with adequate strength for automotive purposes.

Friction Stir Processing (FSP) has been demonstrated feasible to create local Metal Matrix Composites (MMCs) in light alloys matrix. In this research, local MMCs were produced contextually to the weld using Friction Stir Welding (FSW). SiC particles were added to AA7075 lap joints by creating a proper groove on the top surface of the bottom sheet. Different welds were produced with increasing number of tool passes. The effect of the multiple passes was investigated through shear tests, macro and micro observations, average grain size and microhardness measurements. The welded joints were compared to a reference weld produced with no reinforcements. It was found that poor mixing between matrix and reinforcement phase is obtained with one tool pass. On the contrary, proper mixing is obtained with three tool passes, resulting in an increase in the maximum load in shear tests of 50% with respect to the reference conditions.

Linear friction welding (LFW) is a solid-state joining process used to weld non-axisymmetric components. Material joining is obtained through the reciprocating motion of two specimens undergoing an axial force. During this process, the heat source is determined by the frictional work transformed into heat. This results in a local softening of the material and plays a key role in the onset of the bonding conditions. In this paper, a critical analysis of the different approaches used to simulate the LFW processes is provided. The focus of the paper is the comparison of different modeling strategies and the most relevant outputs available, i.e. temperature, strain and stress distribution, material flow, axial shortening and residual stress. Major issues arising due to the complexity of the process are discussed, highlighting strengths and weaknesses of each approach.

A numerical approach to model the selective laser sintering (SLS) of polypropylene is proposed. A 3D thermal model was developed and thus enables the prediction of the temperature fields and the extension of the sintered area in the powder bed taking into account the phase change during multiple laser passes. Powder–liquid, liquid–solid and solid–liquid phase changes were modelled during the SLS and the subsequent cooling processes. Then, a 3D thermomechanically coupled model was set up based on the temperature results of the thermal model in order to predict the distortion of the produced parts after cooling down. Different pre-heating temperatures were considered, highlighting their influence on the final part properties.

This paper presents the results of an experimental and numerical campaign on Linear Friction Welding of dissimilar AA2011-T8 and AA6082-T6 aluminum alloys. Experimental tests were carried out with constant oscillation amplitude and process time. Varying oscillation frequency, interface pressure, specimen geometry and mutual position were used. Grain size measurements, HV tests and EDX analysis were considered to characterize the microstructure of the joints as a function of the input process parameters. A thermal numerical model was utilized to predict the temperature profiles in the joints during the process. The obtained results allowed the identification of four weld categories: sound joints, “bonding limit” condition and two different unwelded joints. The investigation of the causes of the different joint behavior permitted to obtain a few design guidelines on the LFW of dissimilar alloys with different geometry.

Two different in-process control strategies were developed and compared with the aim to produce AZ31 magnesium alloy joints by friction stir welding on sheet blanks with a non-uniform thickness. To this purpose, sheets with dip or hump zones were welded by either changing the rotational speed or the tool plunging in order to keep constant the value of the vertical force occurring during the welding stage of the process. The influence of the main process parameters on the tool force, the micro- and macromechanical properties, and the joints microstructures in the dip and hump zones were analyzed. The results showed that using the rotational speed change-based approach, the hump zones are subjected to increased heat input with consequent increase of the heat-affected zone extension and average grain size (up to 11 μm). On the contrary, using the tool plunge change-based approach, the weld seam thickness in the dip zones is reduced, resulting in a decreased joint strength, with respect to tool rotation approach, equal to about 18%.

Linear Friction Welding (LFW) is a solid state welding process used to joint bulk components. In the paper, an experimental and numerical study on LFW of Ti6Al4V titanium alloy is presented. A laboratory designed LFW machine has been used to weld the specimens with different contact pressure and oscillation frequency. The joint microstructure has been experimentally observed with SEM and EDS. A dedicated numerical model, able to predict temperature, strain and strain rate distribution as well as the phase volume fraction evolution, has been utilized to predict the final microstructure in the welded parts. It was found that complete transformation of the alpha phase into beta phase occurs during the process. After cool down, martensitic structure is obtained at the core of the weld due to high temperatures reached during the process and high cooling rates occurring at the end of the reciprocating motion. Microhardness measurements confirmed increased hardness in the Weld Zone with respect to the parent material

Difficult to weld aluminium alloys can be effectively joined by different alternative processes. Friction Stir Welding (FSW), among the solid-state processes and adhesive bonding represent two very attractive techniques. They allow the production of highly resistant joints avoiding the formation of the typical fusion weld defects. The aim of this work is to identify, analyse and compare the mechanical properties of AA6016 aluminium alloy joints made out of 1 mm thick sheets. FSW lap joints were and epoxy bonded joints were produced. Using the FSW results as benchmark, the overlap required in the bonded joint was identified to ensure the same static strength. Once the geometric configurations of the joints are known, the static and dynamic resistance of welds and bonding have been compared.

In the present work, friction stir welding experiments on AZ31 magnesium alloy sheets, characterized by a variable thickness along the welding line, were carried out. The approach adapted during welding consisted in maintaining constant the heat input to the joint. To this purpose, the rotational speed of the pin tool was increased with decreasing thickness and decreased with increasing thickness in order to obtain the same temperatures during welding. The amount by which the rotational speed was changed as a function of the sheet thickness was defined on the basis of the results given by FEM simulations of the FSW process. Finally, the effect of the in-process variation of the tool rotational speed on the mechanical and microstructural properties of FSWed joints was analysed by comparing both the nominal stress vs. nominal strain curves and microstructure of FSWed joints obtained in different process conditions. It was observed that FSW performed by keeping constant the heat input to the joint leads to almost coincident results both in terms of the curve shape, ultimate tensile strength and ultimate elongation values, and microstructure.

In this study, two bonding criteria, previously used for porthole die extrusion, are applied to FSW starting from the local value of the main field variables calculated through a specifically developed 3D numerical model of the process. Their applicability and effectiveness have been assessed through an experimental and numerical campaign carried out with the main process parameters varying in a wide range. The pressure–time–flow criterion was demonstrated to be better suited for FSW processes when large welding speed is used.

Solid-state bonding occurs in several manufacturing processes, as Friction Stir Welding, Porthole Die Extrusion and Roll Bonding. Proper conditions of pressure, temperature, strain and strain rate are needed in order to get effective bonding in the final component. In the paper, a neural network is set up, trained and used to predict the bonding occurrence starting from the results of specific numerical models developed for each process. The Plata-Piwnik criterion was used in order to define a quantitative parameter taking into account the effectiveness of the bonding. Excellent predictive capability of the network is obtained for each process.

Lap joints of Ti–6Al–4V were produced and the effect of the main process parameters was studied through macro and micro investigations highlighting mechanical resistance, microhardness profiles, grain size and phase distributions. A dedicated numerical model was used to link the input process parameters to temperature and strain distributions and to the final microstructure in the welded joint. It is found that the strain produced in the stir zone by proper combination of process parameters plays a fundamental role in the final microstructure and mechanical properties of the joints.

A microstructural and mechanical investigation on lap joints welded by friction stir welding and made out of a 7075-T6 stringer and a 2024-T4 skin is presented. In particular, the metallurgical and mechanical properties of joints have been studied at different tool feed rates (V = 35, 50, 100 mm/min) and constant rotation speed (R = 500 r/min). Temperature distributions have been monitored during the process. It is found that in the welded area, the recrystallized zone (nugget) has an average grain size of about 3 µm and exhibits coarsened MgZn2 particles on grain boundaries. The maximum values of microhardness in the welded skin increase with the process temperature, while they just slightly vary with process temperature in the stringer. From the mechanical point of view, the joint welded at R = 500 r/min and V = 50 mm/min exhibits both the highest strength and average microhardness value in the welded area.

Friction Stir Welding (FSW) is a solid state welding process patented in 1991 by TWI; initially adopted to weld aluminum alloys, it is now being successfully used also for high resistant materials. Welding of titanium alloys by traditional fusion welding techniques presents several difficulties due to high material reactivity with oxygen, hydrogen, and nitrogen with consequent embrittlement of the joint. In this way FSW represents a cost effective and high quality solution. The final mechanical properties of the joints are strictly connected to the microstructural evolutions, in terms of phase change, occurring during the process. In the paper a 3D FEM model of the FSW welding process, based on a thermo-mechanical fully coupled analysis, is presented. The model, tuned both for the thermo-mechanical analysis and the phase transformation through experimental data, is able to predict the phase volume fraction in the typical zones of the joints at the varying of the main process parameters. The obtained results permit to assess that the tuned FEM model of the FSW process can be utilized as an effective design tool.

In the paper solid state bonding conditions obtained in friction stir welding (FSW) of AA5754-H111 butt joints are analysed, considering the so called zigzag line in the transverse section of the joints. A wide experimental campaign was carried out varying both tool advancing speed and tool rotational one. The effects of the process on the mechanical properties of the joint were highlighted and micro- and macro-observations were used in order to explain the reasons of the enhanced mechanical properties found for the welded material. Numerical results derived from a FEM model previously developed by the authors were utilised to point out the different mechanical and metallurgical behavior of the obtained joints.

A numerical and an experimental campaign were carried out with varying oscillation frequency and interface pressure. The local values of the main field variables at the contact interface between the specimens were predicted by a Lagrangian, implicit, thermo-mechanical FEM model and used as input of a dedicated Neural Network (NN). The NN, integrated in the FEM environment, was designed in order to calculate both a Boolean output, indicating the occurrence of welding, and a continuous output, indicating the quality of the obtained solid state weld. The analysis of the obtained results allowed three different levels of bonding quality, i.e., no weld, sound weld and excess of heat, to be correctly identified and predicted.

In the paper, a numerical and experimental study was carried out to highlight the effect of the distance d between the weld seam and the reinforcement on the residual stress distribution in Friction Stir Welded AA6082-T6 structures. An L-shaped profile was welded to a sheet metal with varying tool rotation and distance d from the weld seam. The Cut Compliance method was used to determine the resulting longitudinal residual stress. A dedicated FE model for FSW was set up, validated and utilized to predict the longitudinal residual stress in the assembled part. The analysis allowed the identification of a few design guidelines in order to reduce the detrimental effects of the residual stresses.

Dissimilar lap joints were produced by friction stir welding (FSW) out of Ti6Al4V titanium alloy and AA2024 aluminum alloy sheets. The joints, welded with varying tool rotation and feed rate, were studied by analyzing the maximum shear strength, Vickers microhardness and optical observations. A dedicated numerical model, able to take into account the presence of the two different alloys, was used to highlight the effects of the process parameters on temperature distribution, strain distribution, and material flow. The combined analysis of experimental measurements and numerical predictions allowed explaining the effects of tool rotation and feed rate on the material flow. It was found that tool rotation had a larger impact on the joint effectiveness with respect to feed rate. A competition between material mixing and heat input occurs with increasing tool rotation, resulting in higher joint strength when lower values of tool rotation are used.

Two different in-process control strategies, developed in order to produce sound joints in AZ31 magnesium alloy by Friction Stir Welding on sheet blanks with a non-uniform thickness, are presented and compared. To this purpose, sheets with dip or hump were machined and welded by either changing the rotational speed or the tool plunging in order to keep constant the vertical force occurring during welding. The mechanical strength of the joints was measured in the zones where the sheets before welding were characterised by different thicknesses. The sheets welded by the two different strategies are characterized by very similar ultimate tensile strength values. Finally, the results showed that the two approaches permit to successfully weld sheets with non-uniform thickness with a reduced loss in the mechanical strength.

The results of an experimental and numerical analysis on Friction Stir Welding of dissimilar lap joints made out of AISI304 and Ti-6Al-4 V thin sheets are presented. The mixed joints, welded with varying process parameters, have been characterized from a mechanical and metallurgical point of view. A numerical model able to take into account the behaviour of the two different materials has been used. The numerical results have been utilized to explain the joint properties and the occurring material flow. The effect of the heat input, tilt angle and sheets mutual position has been investigated. It is found that both hooking defects and tunnels may occur with incorrect choice of process parameters. Good nugget integrity is obtained when the titanium sheet is used as upper sheet and high heat input is conferred to the joint. Finally, welding speed influences the nugget morphology as low values contribute to minimize the formation of the hooking defect.

Linear Friction Welding (LFW) is a solid-state joining process applied to non-axisymmetric components. LFW involves joining of materials through the relative motion of two components undergoing an axial force. In such process the heat source is given by the frictional forces work decaying into heat determining a local softening of the material and eventually bonding conditions. In the paper the authors present a designed and assembled laboratory fixture for LFW operations and the results of an experimental and numerical campaign aimed to weld steel parts. The dedicated fixture permitted to highlight the effect of the most important process parameters. Process conditions allowing effective bonding conditions were highlighted and local conditions of pressure and temperature determining effective bonding of the specimens were determined.

One of the most relevant aspects of friction stir welding is the possibility to weld different materials. In the present paper, the authors present an improved continuum finite element model for the simulation of friction stir welding processes aimed to obtain T joints, made of a stringer in AA7175-T73511 and of a skin in AA2024-T4. The model, taking into account the thermomechanical behaviours of the two different materials, is utilised to study the occurring material flow and residual stress state. Numerical results are compared with experimental observations: the model is able to predict the material flow, obtaining important information on the joint failure mode.

FSW of titanium alloys is nowadays one of the most challenging welding operations, even with a solid state process, due to the thermo-mechanical and thermo-chemical characteristics of such materials. Due to the relevant application of titanium alloys in the aeronautic and aerospace industries, in the recent years few attempts were carried out to develop FSW processes aimed to maximize the mechanical performances of the welded parts. In the paper a new fixture is presented allowing obtaining effective FSW joints of titanium blanks, which were investigated through mechanical and metallurgical tests highlighting the peculiarities of FSW of titanium alloys.

Spot welding can be considered a very common joining technique in automotive and transportation industries as it permits to obtain effective lap-joints with short process times and what is more it is easily developed through robots and automated systems. Recently the Friction Stir Spot Welding (FSSW) process has been proposed as a natural evolution of the already known Friction Stir Welding (FSW) process, allowing to obtain sound spot joints that do not suffer from the insurgence of typical welding defects due to the fusion of the base material. In the paper, a modified Friction Stir Spot Welding (FSSW) process, with a spiral circular movement given to the tool after the sinking stage, is proposed. A continuum based numerical model for Friction Stir Spot Welding process is developed, that is 2D Lagrangian implicit, coupled, rigid-viscoplastic. This model is used to investigate the distribution of the main field variables, namely temperature, strain and strain rate, as well as the Zener-Hollomon parameter which, in turn, strongly affects the Continuous Dynamic Recrystallization (CDRX) process that takes place in the weld nugget. Numerical and experimental results are presented showing the effects of the process parameters on the joint performances and the mechanical effectiveness of the modified process.

Aluminum alloys are widely used in the aircraft industries even if such materials present lower ductility with respect to steels, anisotropy phenomena and, more important, they are often “difficult” to be welded or even “non-weldable”. In the last years the friction stir welding process (FSW) was proposed and applied in order to get good mechanical and technological performances of the joints. In this paper, an experimental and numerical investigation on the lap joining of AA2198-T4 aluminum alloy blanks by FSW is presented. In particular the joints strength and metallurgical properties are investigated by varying the joint configuration and the tool geometry and rotational speed. It is found that the use of cylindrical–conical pin tools and the correct choice of the relative sheet positioning increase the welded nugget extension and integrity improving the mechanical performances of the obtained joints.

Friction stir welding (FSW) is an energy efficient and environmentally "friendly" (no fumes, noise, or sparks) welding process, during which the workpieces are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. Since its invention in 1991 by TWI, such process has been reaching a continuously increasing popularity among aerospace, automotive and shipbuilding industries due its capability to weld unweldable or difficult-to-weld light alloys in different joint morphologies. In this paper a wide experimental campaign is carried out in order to obtain T and lap joints characterized by dimensions of industrial interest from two sheets, the stringer and the skin, of different materials. In particular a peculiar clamping fixture has been designed in order to assure a perfect contact between the stringer and the skin and to develop the “transparency” welding; a proper welding tool has been utilized allowing to obtain at the same time a sound joint and the designed final component fillet radii. Finally T-pull tests together with macro and micro observations have been developed at the varying of the main process parameters.

Friction stir spot welding (FSSW) has been proposed as an effective technology to spot weld the so-called “difficult to be welded” metal alloys. In the paper, a variation of the FSSW process has been considered. A tool path is given after the sinking phase nearby the initial penetration site; in this way a larger welding spot is obtained and more material is involved in the bonding process. The process mechanics of such modified FSSW process is highlighted and the joint strength undergoing tensile tests is considered at the varying both of the assigned tool path and of a few process parameters. Macro- and micro-analyses are made in order to analyze the local material microstructure evolution. It is found that improved performances, with respect to the “traditional” FSSW process, are obtained for all the considered case studies.

Friction-stir welding (FSW) is a relatively new but already well known solid-state welding process whose main advantage with respect to fusion welding processes is the possibility to successfully weld light alloys, traditionally considered difficult to weld or unweldable. Despite the good mechanical performances that can be obtained, there exists the possibility to further improve the joints' effectiveness through post-welding heat treatments that are however time and cost-expensive and, therefore, not best suited for industrial applications. In the present paper, the authors report the results of an experimental campaign, developed on FSW of AA7075-T6 aluminum alloy, aimed to investigate the possibility to enhance the joint performances through in process heat treatments. Welded joints were developed under three different conditions, namely, free air, forced air, and with water flowing on the surface of the joint itself. The influence of the external refrigerants was investigated at the varying of the specific thermal contribution conferred to the joint. Both mechanical and metallurgical investigations were developed on the welded joints highlighting both improvements of mechanical performances of the joints and reductions in the softening of the material when external refrigerants are used. © 2008 Springer-Verlag London Limited.

Friction stir welding (FSW) is a solid state welding process used to weld difficult to be welded or unweldable materials as aluminium alloys. In the last years, other materials have been successfully tested as magnesium, titanium and nickel based alloys. Dissimilar joints can be obtained by FSW, but issues arise concerning the correct choice of the process parameters. In the paper, the results of an experimental and numerical campaign aimed to produce dissimilar AZ31-AA6016-T6 butt joints are presented. The effect of sheet mutual position and main process parameters was investigated. It was found that intermetallics are the main cause of the poor quality of the joints. Sound joints can be produced only if the magnesium alloy is in the advancing side.

The results of experimental activity on friction stir welding (FSW) of aluminum alloys are reported. Butt joints of two different materials, namely AA2024-T4 and AA7075-T6, were investigated from a metallurgical point of view. Grain dimensions and insoluble particle densities were investigated both in the parent materials and in the joints. Furthermore, the effect of post-welding heat treatments on the joint strength was studied.

In this paper the results of an experimental investigation into the local mechanical and morphological characterization of AA6082-T6 friction stir-welded (FSW) butt joints are illustrated. The softening effects of the welding process are highlighted through the use of micro-indentation tests on the top surface of the specimens. The effect of post-welding heat treatments was investigated, also.

Although friction stir welding (FSW) has been successfully used to join materials that are difficult-to-weld or unweldeable by fusion welding methods, it is still in its early development stage and, therefore, a scientific knowledge based predictive model is of significant help for thorough understanding of FSW process. In this paper, a continuum based FEM model for friction stir welding process is proposed, that is 3D Lagrangian implicit, coupled, rigid-viscoplastic. This model is calibrated by comparing with experimental results of force and temperature distribution, then is used to investigate the distribution of temperature and strain in heat affect zone and the weld nugget. The model correctly predicts the non-symmetric nature of FSW process, and the relationships between the tool forces and the variation in the process parameters. It is found that the effective strain distribution is non-symmetric about the weld line while the temperature profile is almost symmetric in the weld zone.

Linear friction welding is a solid-state joining process for non-axisymmetric components in which joining of materials is obtained through the relative motion of two components under pressure. In the process the heat source is given by the frictional forces work decaying into heat determining a local softening of the material and eventually bonding conditions. A dedicated fixture was equipped with sensors for the in-process acquisition of variables regarding kinematics, dynamics and temperature levels. The results of an experimental campaign aimed to weld AA6082-T6 aluminum alloy parts are presented and a process window is identified for the used alloy.

In the paper a continuum based FEM model for Friction Stir Welding of different material lap joint made out of thin stainless steel and titanium sheets is proposed. The simulation campaign was made out using the 3D Lagrangian implicit code DEFORM{trade mark, serif} by means of a rigid-visco-plastic approach. The model, already set up and tuned for FSW process of similar materials and geometrical configurations takes into account the different mechanical and thermal behavior of the two materials and the microstructural evolution of the considered titanium alloy in the same joint. Additionally, it is able to predict temperature, phase, strain and strain rate distributions and evolution at the varying of the main process variables. The phase evolution models take into account only the main phases called Alpha, Beta and Alpha+Baeta so that, at the end of the simulation, the complete phase distribution in the welded zone.

Friction Stir Welding (FSW), as a solid state welding process, seems to be one of the most promising techniques for joining titanium alloys avoiding a large number of difficulties arising from the use of traditional fusion welding processes. In order to pursue cost savings and a time efficient design, the development of numerical simulations of the process can represent a valid choice for engineers. In the paper an artificial neural network was properly trained and linked to an existing 3D FEM model for the FSW of Ti–6Al–4V titanium alloy, with the aim to predict both the microhardness values and the microstructure of the welded butt joints at the varying of the main process parameters. A good agreement was found between experimental values and calculated results.

Friction stir welding (FSW) is an energy efficient and environmentally "friendly" (no fumes, noise, or sparks) welding process, during which the sheets are welded together in a solid-state joining process. FSW is mature for simple configurations but a significant lack of knowledge is found when dealing with different designs such as T-sections, corner welds and box sections. The present work explores the feasibility of producing corner fillet geometries using FSW. Although such a kind of geometry has traditionally been considered unfeasible for the process, it seems to have a great potential to be used for T-joint configurations, a recurrent design pattern in transport applications. A specific tool has been developed and a set of welds has been produced with it. The process has been analysed by both micro and macro observations and a dedicated numerical model tuned by comparison of the thermal histories experimentally measured by embedded thermocouples. According to the obtained results, the proposed model is able to represent a useful design tool for the process. Main problem pending to be solved is how to avoid the formation of a tunnel defect in the weld centre line due to a suck effect of the tool on the stirred material.

This paper presents the results of a combined experimental and numerical investigation focused on the effects of an in process water cooling treatment aimed at improving the final quality of friction stir welded butt joints in terms of mechanical resistance and metallurgy of the processed material. Micro and macro observations, together with the evolution of an already developed finite element tool, have been used to analyze specimens obtained under different process conditions. Water cooling was found to enhance joint strength, reducing the material softening usually observed in the thermo-mechanically affected zone area, with no detrimental effect on nugget integrity.

Friction stir welding (FSW) now definitively reached a large interest in the scientific community and what is more in the industrial environment, due to the advantages of such solid state welding process with respect to the classic ones. The latter aspects are relevant also with reference to joints characterized by a complex geometry. What is more, advanced FEM tools permit to develop effective engineering of the processes; quantitative results can be acquired from numerical simulations once basic information, as the process mechanics and the material flow, are certain. Material flow plays a fundamental role in FSW since it determines the effectiveness of the joints or, in turn, the insurgence of defects. In the paper, the material flow in the FSW of aluminum alloys T-joints is investigated at the varying of the most relevant technological and geometrical parameters with numerical simulations and experiments. In particular, to investigate the metal flow, a wide campaign of experimental tests and observations was developed utilizing a thin foil of brass as marker, placed at the interface of the two blanks to be welded. Some relevant conclusions on the process mechanics and on the actual material flow determining the material bonding are outlined, permitting an insight of the FSW of T-joints.

Friction stir welding (FSW) has been reaching a continuously increasing popularity among joining processes since its invention in 1991. Although mainly used for aluminium alloys, it has been successfully applied also to steels. In the present paper, a continuum based FEM model for FSW of steels is proposed, which is three-dimensional Lagrangian implicit, coupled, rigid viscoplastic. The model, whose potential has been analysed through temperature distribution comparisons, is able to predict temperature, strain and strain rate distributions, together with thermal and mechanical loads on the welding tool, at varying main process variables. In this way, the FEM model can be used for process and tool design.

The potential of friction stir welding (FSW) has thoroughly been investigated by several authors, however their focus has primarily been on butt joints. T-joints are of paramount interest for transportation industries especially due to their capacity to straighten panels. Welding of T-joints is very challenging due to thin walls, poor location of the rib–web interface and the requirements for corner-fillets. This paper investigates FSW of T-joints of two popular aluminum alloys, i.e. 2024-T4 and 6082-T6, and the role played by the material characteristics on joining. First, an experimental study is carried out with specially designed fixture to determine the effect of process conditions. Then, the joints are metallurgically and mechanically evaluated. Finally using a numerical model of the process previously developed by the authors, the thermal and plastic flow fields for the two alloys are calculated and compared. It is found that the material dependent thermal and plastic fields affect the state of TMAZ, HAZ and nugget-region in the joint and that the low-strength high-work hardening alloy 6082 provides a much better joint integrity than the higher-strength low-hardening 2024 primarily due to the greater penetration of the plastic zone in the former.

Friction stir welding (FSW) is an energy efficient and environmentally “friendly” (no fumes, noise, or sparks) welding process, during which the workpiece are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. Significant microstructural evolution takes place during FSW: in particular continuous dynamic recrystallization (CDRX) phenomena result in a highly refined grain structure in the weld nugget and strongly affect the final joint resistance. In the paper two different analytical models aimed to the determination of the average grain size due to continuous dynamic recrystallization phenomena in FSW processes of AA7075-T6 aluminum alloys have been implemented in a 3D FEM model and numerical analyses of the welding processes have been performed to verify their effectiveness.

Tailor welded blanks (TWBs) are conventionally produced by laser or traditional welding processes. In either case, the joints are created by solid-liquid-solid phase transformations that result in undesirable microstructures and tensile residual stresses detrimental to joint performance. This study investigates feasibility of an alternate joining process, friction stir welding (FSW). The joining of AA7075-T6 blanks of different thickness is investigated through FE analyses and controlled experiments. It is found that for a successful joint, the welding parameters have to be carefully designed so that the resulting metal flow and the temperature history during FSW are consistent for the two thicknesses.

The increasing use of structural light alloys in the aeronautical, automotive and transportation industry is pushing researchers to find new solutions for the production of innovative components. Mixed joints made out dissimilar alloys represent a challenge for engineers to the difficulties arising in welding materials characterized by significantly different mechanical, thermal and chemical properties. In the paper, an overview of the most used process to produce dissimilar joints of aluminum, magnesium and titanium is given. Both fusion based and solid state welding processes can be used. Although the joining of these materials is possible, particular attention must be taken to the choice of process parameters in order to avoid the formation of intermetallics, often resulting in brittle behavior and poor mechanical properties of the joints.