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This work aims at evaluating the mechanical performance of joints in an AA6016 alloy, manufactured by means of two alternative processes: friction stir welding and adhesive bonding. Given its wide use in industrial fields such as automotive and aeronautics, an overlap configuration of joints was selected for the study, and a specific method based on the overlap length evaluation has been developed to allow comparisons among the cases. Two substrate thicknesses are considered in order to vary the overall joint stiffness, and the effect of such geometrical parameter on the mechanical behavior of the joined system is experimentally investigated. The experimental findings highlight that the stiffness of the joint may emphasize the effect of inherent defects due to process, severely affecting the mechanical and failure behavior of overlap-configured joints under static- or cyclic-load conditions.

In this study, the design of experiments (DoE) approach was used to identify the effect of low-pressure plasma surface treatment parameters on the lap-shear strength of adhesive bonded joints realized using different substrates. In particular, four different polymeric substrates were considered: 5- and 7-layer carbon-fiber reinforced polymers and polyamide 6 and 6.6. Two-level, full-factorial designs were used to investigate the effects of two varying principal parameters, namely, plasma power and treatment time, for each type of substrate. The analysis was carried out by considering different types of processing gases. The objective function was the tensile shear strength of the adhesively bonded joints. For each set of joints, the shear strength values​​ ​​were compared using the DoE approach to detect any systematic behavior among different substrates. Finally, it was possible to identify the set-up parameters that gave the best performance in terms of shear strength, considering any equivalent conditions from a statistical point of view. This aspect is particularly important in consideration of the process optimization of the manufacturing cycle; indeed, it allows the maximization of the joint efficiency by limiting the energy cost for treatment.

This paper deals with the interfacial adhesion properties of Ti-6Al-4V alloy substrates, prepared by using different treatment protocols selected on the basis of their different effectiveness, and bonded using a structural, high-strength epoxy adhesive. The alternative pre-bonding treatments were sodium hydroxide anodization and low-pressure-plasma treatment, the effects of which were compared to that of a base preparation via solvent degreasing of the substrates’ surface. The treated surfaces were joined according to standard protocols, and then tested for shear strength. The mechanical results were then correlated to surface characteristics of the substrates such as oxidation state and wettability. Parallel scanning Kelvin-probe measurements allowed us to focus our attention on the possible role of the electrical properties of the substrates. We observed that each treatment entails different behavior of the electrical potential of the surface, which correlates with the mechanical strength of the joints. The results suggest that an evaluation of the surface potential of titanium-alloy substrates might be a promising, indicative supplementary parameter for the evaluation of their pre-bonding surface conditions, allowing correlations with presence/absence of an oxide layer at the resin-substrate interface.

This paper focuses on the effects of the laser surface texturing process and joint configuration of stainless steel adherends on the adhesive tensile bond strength. Two different sources, a CO2 and a fiber laser, were used and compared. In particular, proper choice of laser parameters was explored with the aim of producing different roughness and peak-to-valley distance and different textures on the bonding area, which could increase the real contact surface. Furthermore, to more thoroughly understand the effect of the laser parameters on joint fracture load, the experimental campaign was conducted according to a Design of Experiment (DoE) framework and the results were analyzed with this methodology. The creation of particular textures and roughness levels were related to the resulting joint geometrical configuration and bond strengths. In particular, significant increases in joint bond strength were achieved using both laser sources. Furthermore, by optimizing the laser parameters, smaller laser spot scan path overlaps can be achieved as well as a more refined scale of surface texture and surface roughness. This thereby enables the joining of thinner sections of different materials.

This work reports a systematic and quantitative evaluation of the effects induced on the adhesive properties of carbon fiber reinforced polymer (CFRP) substrates by various vacuum cold-plasma treatments. In particular, surface activation of the CFRP substrates was performed using several combinations of exposure time, plasma power, and processing gas (air, O-2, Ar and N-2). By comparing these plasma treatments with conventional techniques of abrasion and peel ply, it was possible to substantially increase the performance of the adhesively bonded joints made by overlapping the CFRP substrates with a structural epoxy resin. On each differently treated surface, measurements of roughness and of wettability were performed, allowing the evaluation of the increase in surface energy after the plasma treatment. XPS analyses allowed the identification of the chemical state of the substrates and showed an in-depth functionalization of the outer layer of the CFRP material. The experimental results show that an engineered plasma treatment of the CFRP substrates allows one to modify the surface morphology and both wetting and chemical activation properties of the treated surfaces, resulting in an increased mechanical shear strength of the joints.

This experimental work focuses on the effect of various joint-design factors on the adhesive bonding of mixed short- (Onyx) and continuous-carbon-fiber reinforced Nylon-6 parts, 3D-printed via Fused Filament Fabrication technology. A low-pressure plasma treatment has been preliminarily performed to maximize the adhesion at the composite's surface-adhesive interface. This approach has allowed the definition of a performance benchmark beyond which the intrinsic non-homogeneity of the composite determines joint failures due to delamination of substrates. Starting from this condition, adhesive-bonding parameters such as adherend overlap length (12.5, 25, or 50 mm) and geometry, adhesive type (epoxy or polyurethane), and substrate configuration are considered, investigating their influence on both the mechanical behavior of single-lap joints and modality of crack propagation across the bonded laminates. Supported by failure analysis, the experimental findings highlight that higher load levels (up to +251.4%) can be achieved by increasing the joint stiffness using higher values of overlap length thanks to lower peel stresses at the adherend ends where material discontinuities are mainly concentrated. Alternatively, joining of continuous-fiber-only adherends is considered to provide a further high-performing approach that involves the exclusion of the Onyx shell from the areas designed to be bonded. Moreover, heterogeneous joining between additively-manufactured composites and conventional carbon-fiber-reinforced-polymer materials is also investigated, providing pivotal insights about the applicability of 3D-printed composites also in pre-existing composite structures.

This work reports a quantitative evaluation of the aging behavior of adhesively bonded joints manufactured using a toughened epoxy adhesive applied on CFRP substrates. Low-pressure plasma (LPP) treatment was employed, adopting air and pure oxygen as process gases and varying working parameters such as power and exposure time. A preliminary testing campaign was performed under standard laboratory conditions to evaluate shear strength of single-lap joints (SLJ), comparing effectiveness of LPP treatments to a traditional mechanical abrasion of the adherends. Even at this early stage, experimental findings highlighted how this physical method provides for competitive performance over abrasion, resulting in a remarkable enhancement of the shear resistance. Four sets of LPP-treatment conditions were selected and then subjected to accelerated aging (consisting of 56 cycles of 5 h at T = 70 °C, RH>90% and 5 h at T = −40 °C each). Their behavior was compared to that of abraded joints, employed as a reference. To assess the durability of the CFRP-epoxy adhesive system under accelerated aging conditions, tensile shear strength (TSS) testing and wedge cleavage test (WT) were performed in parallel. The experimental results showed that low-pressure plasma treatment of the CFRP substrates results in increased short-term quality of the adhesive joint as well as in enhancement of its durability even under severe aging conditions.

Polyolefins are considered among the most difficult polymeric materials to treat because they have poor adhesive properties and high chemical barrier responses. In this paper, an in-depth study is reported for the low pressure plasma (LPP) treatment of neutral polypropylene to improve adhesion properties. Changes in wettability, chemical species, surface morphology and roughness of the polypropylene surfaces were evaluated by water contact angle measurement, X-ray photoelectron spectroscopy and, furthermore, atomic force microscopy (AFM). Finally, the bonded joints were subjected to tensile tests, in order to evaluate the practical effect of changes in adhesion properties. The results indicate that plasma is an effective treatment for the surface preparation of polypropylene for the creation of bonded joints: contact angles decreased significantly depending on the plasma-parameter setup, surface morphology was also found to vary with plasma power, exposure time and working gas.

Friction stir welding (FSW) is a solid-state technique, which has assumed an increasingly important role in automotive, naval, and aeronautical industry over the years. Nowadays, thanks to its several benefits, FSW is used to weld any type of metallic, polymeric, or composite material. In recent decades, adhesive bonding has also enhanced relevance due to a request for much lighter structures to increase performance without increasing fuel consumption. From a mechanical perspective, welding has a high tensile strength despite a low fatigue resistance through the lack of joint elasticity. Therefore, the aim of this study is to investigate and compare static and dynamic behavior of welded, weld-bonded, and adhesive-bonded joints. After choosing the most suitable adhesive, surface preparation, consisting of sandblasting, was carried out. First of all, on the basis of previous experience in FSW, the process parameters of hybrid welding were determined. Both quasi-static and dynamic behavior of welded, adhesive-bonded, and weld-bonded joints, made in overlapped configuration, were then compared. Experimental tests showed that the adhesive limits the negative effect, due to the presence of the structural notch of FSW overlapped joints.

Adhesive bonding offers many advantages over mechanical fastening, but requires accurate surface preparation, which is widely recognized as the key step to producing reliable and durable adhesive bonds. The use of laser cleaning processes helps to increase the reactivity of the topmost layers of substrates, without affecting the bulk material properties. These methods are often applied to treat polymers, which are generally characterized by high chemical inertia, very low surface energy values and, consequently, poor adhesive properties and this is particularly true for polyolefins. Furthermore, laser treatment provides an alternative to the polluting and less accurate practices such as manual abrasion and primer. In this paper the effect of different laser pre-treatments on polypropylene (PP) and high density polyethylene (HDPE) substrates was studied, by microscopic investigation and single lap- shear tests. As a comparison, untreated and primer treated specimens were also included in the experimental campaign. The results show that laser treatment significantly affected the substrates’ surface and the mechanical properties of adhesive bonded joints. Finding the optimal combination of the working parameters, it is possible to enhance the strength of both untreated and primer-treated joints

The aim of this study was to evaluate the influence of surface pre-treatment on the durability of bonded joints between polyamide pieces. Various surface preparations of the substrates were carried out and a wedge test was performed to evaluate the joint behaviour over time. Joints composed of polyamide 6 and polyamide 66 were made using a two-component acrylic adhesive. Four different surface treatments were compared: only degreasing, degreasing + abrasion, degreasing + plasma and degreasing + abrasion + plasma. The tests carried out highlighted the extreme importance of choosing the right surface preparation if one wishes to guarantee high performance over time. For both substrates, superficial preparations with cold plasma yielded the best results.

The subject of this paper is the quantitative evaluation of the correlation which exists between the surface characteristics of parts in a titanium alloy, Ti-6Al-4V, and the mechanical strength of the joints, made linking these parts using a structural epoxy adhesive. In particular, the surfaces were prepared by using vacuum plasma as a prebonding treatment with various process gases (air, Ar and O2). For each of these treatments, tests were performed for the mechanical characterisation of the joints, and surveys on the wettability properties were carried out by using a sessile drop method in order to estimate the surface energy of plasma treated surfaces. The results were related to the morphological and electrical properties of the substrates, defined in parallel through SKPM assessment. Further XPS analyses of their chemical state allowed the evaluation of the effect of vacuum plasma on both contaminant removal and formation of a weak oxide layer. The results obtained highlighted interesting relationships between vacuum plasma surface preparation and mechanical resistance of the bonded joints, as well as synergy between the morphological, electrical and chemical properties present in adhesive phenomena.

The use of adhesives in place of traditional joining techniques as welding and riveting is becoming increasingly common in structural design. Compared to other conventional joining processes, adhesive bonding offers several advantages: including acoustic insulation, vibration attenuation, structure lightening, corrosion reduction and uniform stress distribution. These benefits can be offset if the surfaces to be bonded are not carefully cleaned, degreased and prepared. Many approaches to surface treatments based on physical or chemical modifications have been developed in the years in order to improve the surface activity. Although widely used and very efficient, these techniques present several disadvantages. Physical methods based on mechanical abrasion are supposed to extend bonding area as they increase the roughness, but cause an extensive degradation to the specimens and they are often not easy to reproduce. On the other hand, chemical treatments are typically used with the aim of modifying both morphology and chemical structure of the substrates layers, but they present serious environmental problems of waste disposal, which has moved investigations to an industrial alternative to these processes. According, the use of energetic methods of surface cleaning, such as laser and plasma cleaning processes have been recently consolidated since they are useful to modify the topmost layers of the substrates, increasing their reactivity, without affecting the bulk material properties. These treatments offer an effective and environment-friendly processing of different materials, providing strong and durable bonds with adhesives through modification of the surface morphology and functionalities. In this study, the effects of laser and low pressure plasma treatments on mechanical properties of adhesive bonded joints has been investigated and compared performing lap-shear tests.

In this paper, the effects of low pressure plasma treatment on surface energy of polyethylene samples and on shear properties of adhesive bonded joints based on these substrates have been investigated. In particular, the optimization of two plasma process parameters, exposure time and power input, was studied performing contact angle evaluation and lap-shear tests. The plasma treatment was also compared with a conventional primer treatment, for which it is a clean and effective alternative. As a measure of the durability of both treatments, the bond shear strength immediately after bonding was compared with that after a storage period in the laboratory environment. The experimental results show that the optimized plasma process may remarkably increase wettability properties of polyethylene surfaces and shear strength of bonded joints, even higher than those treated with primer and that these good properties remain quite unchanged even after some days of storage in a laboratory.

Hybrid joining is gaining wide space in automotive industry thanks to the possibility of concentrating advantages of different joining methods in a unique technique. Producers’ main interest is indeed to obtain quality joints, which are able to maintain their properties under severe load conditions, increasing the life of their components and, thus, safety for the final users. In particular, the use of adhesive-bonding combined with resistance spot welding (RSW) offers several advantages both over adhesive joints, increasing stiffness and static strength of the assemblies, and over the welded-only joints extending fatigue life of the components. This work relates the properties of hybrid RSWeld-bonded joints in an Advanced High-Strength Steel (AHSS) to the performance of joints made by using the two joining techniques separately, i.e. spot welding only and adhesive-bonding using a semi-structural epoxy-polyurethane adhesive. The experimental findings showed that this hybrid system presents excellent static and dynamic qualities over the two more conventional techniques and also suggest its use in production.

This paper presents a systematic investigation of the influence of sandblasting pretreatment parameters on the surface roughness and mechanical characteristics of adhesive-bonded joints. The preliminary surface treatment in a bonding process has two important aims: first of all, it eliminates contaminants (dust, grease, humidity and corrosion products) which can modify the wettability of the substrate, then it increases surface roughness and, consequently, the contact area between substrate and adhesive, creating a mechanical interlocking that maximizes adhesion. To enhance the strength and avoid the de-adhesive failure of the joint, it is therefore advisable to increase the contact substrate-adhesive by a mechanical treatment of sanding, grinding or preferably sandblasting, usually considered one of the most effective methods to control the desired level of surface roughness and joint strength. This process, apparently easy to manage, is controlled by a great number of operating parameters, which all contribute to creating a good result. With the aim of evaluating the influence that some of these parameters have on the mechanical characteristics of adhesive-bonded joints, an experimental campaign was carried out. In particular, a steel substrate, an epoxy adhesive and various types of sand, different in nature and granulometry were used. The variable parameters for the execution of blasting are sand, impact angle and pressure. The assessments departed from an investigation into their effect on surface roughness and thereafter the mechanical properties of the bonded joints were analyzed.

In aeronautics and aerospace construction, whenever a seam is needed between aluminum alloy parts, riveting, nailing or bolting are the preferred methods of junction. Friction stir welding technology has made possible the realization of high strength aluminum alloy joints, which are normally considered non-weldable with conventional welding techniques.

In this paper, laser weldability of neutral polypropylene has been investigated using fibre and carbon dioxide lasers. A design of experiment (DoE) was conducted in order to establish the influence of the main working parameters on the welding strength of the two types of laser. The welded samples were characterized by carrying out visual and microscopic inspection for the welding morphology and cross-section, and by distinguishing the tensile strength.The resulting weld quality was investigated by means of optical microscopy at weld cross-sections. The tensile strength of butt-welded materials was measured and compared to that of a corresponding bulk material.

This study reports a systematic investigation of the effects of various cold plasma treatment parameters on adhesion characteristics of two polyamide substrates. Polyamide 6 and polyamide 6.6 specimens were treated with a low pressure radio-frequency discharge plasma using different treatment time, power inputs and working gas (air, argon and oxygen). Contact angle measurements with sessile drop technique were carried out for estimation of surface wettability, as well as surface roughness evaluation and X-ray Photoelectron Spectroscopy (XPS) analysis. Then, untreated samples and cold plasma treated samples were adhesively bonded together to form overlap joints. Single lap shear tensile testing of these adhesively bonded joints was performed to investigate the effect of different surface treatments on the joint strength. The experimental results show that the optimized plasma process may remarkably increase wettability properties of polyamide surface and shear strength of the bonded joints.

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.

Fiber laser techniques are increasing their use in many applications, including modification of material surface properties. In particular they are often used for materials’ marking as a non-contact processing. In spite of this, the impact of the laser beam on the surface causes metallurgical and morphological changes. The developments during the laser-material interaction can also affect other surface properties, especially corrosion properties which are crucial in the case of Iron-Nickel alloys. Effect of laser marking on a Fe-Cr-Ni alloy using a Tm-fibre laser (IPG Photonics TRL1904; maximum power: 50W, wavelength: 1904 nm), is described in this paper. In order to evaluate the effect of the laser on corrosion properties a specific ageing test in salt spray has been performed. Moreover, superficial morphology analyses have been performed on samples before and after corrosion tests. Possibilities and limitations of laser marking on these alloys have been discussed, in particular from the point of view of the marked surface corrosion resistance preservation.

Purpose: This paper describes a new ultrasonic instrument (tipholder DB1 with crown prep tip inserts) designed to optimize prosthodontic margin repositioning and finishing. Materials and Methods: The insert movement was assessed, and it was demonstrated that tipholder DB1 provides its inserts with an elliptical-like movement, making the entire insert surface able to cut. Then, 20 extracted teeth were prepared using tipholder DB1, sonic instruments, and traditional drills. Dental surface roughness produced using each of the three tools was measured using a roughness tester. Results were compared using univariate analysis of variance and Bonferroni post hoc test. Results: The roughness produced using tipholder DB1 with crown prep insert presented no statistically significant differences compared to the roughness produced using sonic instruments and traditional drills. Conclusion: Tipholder DB1 with crown prep inserts is a promising treatment for margin repositioning and finishing.

Increasing payload and fuel efficiency of aircrafts have become the major issues for the aerospace industry, which has increased the development of more advanced materials with highly specific properties. Low density aluminium lithium alloys are attractive to the aerospace industry since the structural weight reduction that they allow leads to an improvement of aircraft performance. Significant interest has been shown in the use of advanced welding techniques like Friction Stir Welding, particularly due to the design and manufacturing benefits that they afford over established mechanical joining methods. A significant benefit of FSW is the fact that it implies only a few process parameters to control: tool rotational speed, travel speed and applied normal load are easily controlled. This work presents an approach to optimise FSW process parameters which govern the tensile strength and the fatigue life of AA8090 Al-Li alloy through their effects on plastic flow in the nugget and in the thermo-mechanically affected zone (TMAZ), and through the thermal alteration in the heat affected zone (HAZ) of the weld. Therefore a close relationship exists between microstructure and fatigue performance, the typical plastic flow of a FSW joint and the occurrence of defect types, specific to FS process, can influence the fatigue behaviour of the joints.

The joining of polymeric materials is a technology used in many industrial applications, from transport to telecommunications and the medical sector. A new technology for the joining of polymers is the laser welding process. In particular, fibre laser welding is a flexible technology which allows high process speed and the realization of good quality joints. Despite its application becoming more widespread in the production of assemblies of high precision, the application of laser technology for the welding of polymers has not been the subject of many studies up to now. This study focused on the welding of neutral polypropylene. The window process parameter was identified, without the use of additives to increase radiation absorption, and a mechanical characterization was conducted in order to evaluate the quality of the joints realized.

In this paper the behaviour of the plastic flow during Friction Stir Welding of AA7075 T651 plates, realized with different shaped tools, has been investigated. In particular, the influence of the shape of three tools was studied using copper strips placed along the welds. After welding, radiography and metallurgical analysis were used in order to investigate the marker movement and its fragmentation.

Fiber Reinforced Polymer (FRP) sandwich panels are increasing their application as structural and non-structural components in all kinds of construction. By varying the material and thickness of core and face sheets, it is possible to obtain sandwich structures with different properties and performance. In particular, their advantages as lightweight and high mechanical properties make them extremely suitable for the transport industry. One of the most critical aspects regarding composite materials for engineering application is their performance after hygrothermal aging. The panels used in this study are composed of low density core, made by thermosetting resin foam with microspheres and glass fibers rolled until obtaining the required thickness, and two face sheets of the same material but realized in high density. In this study, the authors focused on the bending behaviour of this kind of sandwich panel, as received and after severe aging cycles.

Current market demands drive companies to innovate their production techniques to improve products and simultaneously hold down costs. In the search for solutions aimed at an optimization of production processes, semi-finished products in aluminium alloys very definitely play a significant role. Moreover, the assembly of extruded parts with laminates may result in considerable difficulties, especially in relation to distortions or generated residual stress and process productivity. A solution to these issues may be found in the application of friction stir welding (FSW), characterized by low thermal input and high productivity. The asymmetry of the FSW process determines a different thermal input to the edges of the pieces to be joined. This aspect can be exploited when it is necessary to join pieces characterized by a very different thermal lag, as occurs in the case of joining extruded products and sheeting. This study aims to identify optimal FW parameters, placing particular attention on heat transfer...

For an effective application of polymers, it is essential to have good adhesion behaviour to ensure good mechanical properties and durable components. Unfortunately, in general terms, polymers are characterized by high chemical inertness, which leads to very low surface energy values and, consequently, poor adhesive properties; this is particularly true for polyolefins. In this study, the effects of low pressure plasma treatment on surface roughness of polyethylene and polypropylene samples and on shear properties of adhesive bonded joints based on these substrates have been investigated. In particular, the optimization of three plasma process parameters, exposure time, voltage and working gas, were studied performing roughness measurement, contact angle evaluation and lap-shear tests. The experimental results show that the optimized plasma process may remarkably change the surface morphology, increasing wettability properties of the surfaces and shear strength of the bonded joints. These good properties remain almost unchanged even after some days of storage in the laboratory.

Nickel alloys are very important in many aerospace applications, especially to manufacture gas turbines and aero engine components, where high strength and temperature resistance are necessary. These kinds of alloys have to be welded with high energy density processes, in order to preserve their high mechanical properties. In this work, CO2laser overlap joints between Inconel 718 sheets of limited thickness in the absence of postweld heat treatment were made. The main application of this kind of joint is the manufacturing of a helicopter engine component. In particular the aim was to obtain a specific cross section geometry, necessary to overcome the mechanical stresses found in these working conditions without failure. Static and dynamic tests were performed to assess the welds and the parent material fatigue life behaviour. Furthermore, the life trend was identified. This research pointed out that a full joint shape control is possible by choosing proper welding parameters and that the laser beam process allows the maintenance of high tensile strength and ductility of Inconel 718 but caused many liquation microcracks in the heat affected zone (HAZ). In spite of these microcracks, the fatigue behaviour of the overlap welds complies with the technical specifications required by the application.

In recent years, bonding techniques have been widely used to assist or even replace traditional joining techniques, such as welding and riveting. This study investigates how environmental conditions affect the mechanical behaviour of bonded joints. Analyses were carried out on single lap shear joints bonded by a two-component methacrylate-based flexible structural adhesive, extensively used in marine applications. The degradation of bonded joints in marine environment is mainly due to humidity and seawater-induced corrosion. Thus, the bonded samples were prepared for test by ageing in a climatic chamber with no applied stress (at controlled temperature and humidity) and immersion-drying cycles in a saline solution (seawater-like effect). Afterwards, tensile stress tests were performed. In order to evaluate the environment-induced degradation of the tested joints, the tensile strength of aged samples was compared with that of similar un-aged joints.

The purpose of the paper is to investigate the effects of low pressure plasma treatment on wettability of carbon fibre reinforced polymer samples and on shear properties of adhesive bonded joints based on these substrates. In particular, two plasma process parameters, exposure time and power input, were optimized, performing contact angle evaluation on lap-shear tests. The plasma treatment was also compared with a conventional mechanical abrasion and untreated and only degreased specimens. The experimental results show that choosing the optimal parameters is possible to improve the wettability of composite substrates and reduce the contact angle.

Combining great performances with high standards of safety and efficiency is the challenge for most aircraft manufacturers. Aircraft are subjected to extraordinary loads, repeated for millions of cycles also in extreme environmental conditions, hence every component must be designed and manufactured to maximize its fatigue resistance. Friction Stir Welding of aluminium alloys and surface finishing process as might be a good solution to reach this goal. The high performances reached in this field with the use of this technique could be further increased reducing the welded surface irregularity, which might promote the development of a dangerous fatigue failure mechanism. In this work, the effect of the welded surface finishing treatment on the fatigue behavior of AA8090 FSW butt joints was assessed. The results obtained from the dynamic tests highlighted the higher fatigue resistance of finished specimens, with respect to the as-welded joints.

The aim of this work is to investigate the Friction Stir Welding (FSW) process applicability to a typical aeronautical joint: the union of the stiffeners to sheets constituting the fuselage. To do this analysis FSW parameters of flat overlapped sheets were first identified by monitoring the temperature reached using thermography. 1.3 mm thick AA 2024 T3 overlapped sheets were welded with success and the thermal effect due to the use of different welding parameters was compared with the results of mechanical tests and microstructural investigations. FSW joint mechanical properties were compared with those detected by performing tensile tests of joints made by riveting, showing absolutely comparable values. FSW identified parameters were used to make overlap joints between flat panels and stiffeners, so as to realize a typical structure of the aeronautical sector and to compare, by pressurization tests, the resistance of such panels with similar riveted. The innovative character of this research is represented by cyclic pressurization tests, conducted as indicated by the FAR (Federal Aviation Regulation) rules on a scale model. Thus it was possible to verify that the fatigue strength of welded panels is such as to overcome the acceptability limits fixed in the aeronautical field.