L. Quintino

Performance of bi-adhesive bonded aluminium lap joints

The stress concentration towards the ends of a bonded lap joint depends to some extent on the relative stiffnesses of the adherend and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress concentration, giving rise to potentially higher joint strength. The paper presents the results of a study of the application of two adhesives with different stiffnesses along the overlap length in single lap joints. A stiff adhesive was applied in the middle portion of the overlap, while a low modulus adhesive was applied towards the edges prone to stress concentrations. The results show measurable increase in strength of the bi-adhesive bonded joints compared with those in which single adhesives were used over the full length of the bondline.

Fume emissions during gas metal arc welding

The control of exposure to welding fumes is of increasing importance in promoting a healthy, safe and productive work environment. This article describes the effects of shielding gas composition on the amount and composition of welding fumes produced during gas metal arc welding (GMAW). The amount of fumes generated during welding was measured for steady current over a range of wire-feed speeds and arc voltages using the standard procedures contained in ANSI/AWS F1.2 [American Welding Society. ANSI/AWS F1.2. Laboratory method for measuring fume generation rates and total fume emission of welding and allied processes. Miami, Florida; 1992]. Results of these measurements show that the fume formation rates (FFRs) increase with CO2 and O2 in the shielding gas mixture. The lowest FFRs were obtained with the mixtures of Ar + 2%CO2 and Ar + 3%CO2 + 1%O2. The highest FFRs were obtained with the mixtures of Ar + 18%CO2 and Ar + 5%CO2 + 4%O2. The welding fumes contains mainly iron, manganese, silicon, titanium and sodium under oxide forms. The fume cluster particles have dimensions between 0.5 and 2 µm. The FFR was found to be governed by the transfer modes of molten metal, i.e. the current intensity and arc voltage, as well as by the shielding gas mixtures composition. Thus these parameters have to be taken into consideration before designing a welding process. Whenever possible, users of GMAW should use the lowest current intensity. However, when this is not possible, due to the constraints of process productivity, welders should use higher currents, but with Ar + 2%CO2 and Ar + 3%CO2 + 1%O2 shielding mixtures, which will lead to smaller fume emissions.

Analysis of Beam Material Interaction in Welding of Titanium with Fiber Lasers

Ti-6Al-4V is one of the most widely used titanium alloy in industrial applications because of its lightweight and corrosion resistance. The new generation of high power fiber lasers presents several benefits, namely, high power, low beam divergence, and compact size. These lasers can be used in a diversity of materials as the low wavelength that characterizes them allows absorption by almost all metals and alloys. This article presents a research about the weldability of the Ti-6Al-4V alloy using a fiber laser. Weld beads produced with different processing parameters were morphologically characterized under optical microscopy and the microstructures obtained were investigated.

Influence of Pre-treatments on the Surface Condition of 2024-T3 Aluminium Alloy

SUMMARY The influences of various pre-treatments of interest in adhesive bonding on the surface condition of 2024-T3 alloy have been examined. In particular, the morphologies of films formed during alkaline etching (NaOH), electropolishing (perchloric acid/ethanol), acid pickling (Na2Cr2O7/H2SO44 and anodzing (H3PO4), and the consequences of film growth to the composition of the alloy, have been determined using transmission and scanning electron microscopies and Rutherford backscattering spectroscopy. The pretreatments result generally in scalloped surfaces, with deeper cavities associated originally with second phase particles. The topography of the surface is generated primarily in the pre-treatment stages, with subsequent anodizing having a lesser influence. Enhanced attack of the alloy in the vicinity of grain boundaries occurs during acid pickling, which appears to be accentuated during subsequent anodizing. Alkaline etching and electropolishing produce compact films, respectively about 5–8 and 2–3 nm thick, on the main alloy surface. In contrast, acid pickling produces an open-textured, porous film, about 30 nm thick and of fibrous appearance in section, with a thin barrier layer at the alloy/film interface. The porous films produced by anodizing in phosphoric acid are of typical thickness in the range 200–350 nm, with a 10–18 nm thick, barrier layer. The formation of the various films results in significant enrichment of copper in the alloy to the range 4.2 × 1015 to 6.4 × 15 Cu atoms cm−2.

Numerical simulation of mono- and bi-adhesive aluminium lap joints

In single-lap bonded joints the stresses are maximum at the edges, where failure usually begins, while in the centre stresses are the lowest. Stress concentration towards the ends of a bonded lap joint depends, to some extent, on the relative stiffness of the adherend and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress concentration, giving rise to potentially higher joint strength. This paper discusses the failure mechanism of bi-adhesive joints, where a flexible adhesive is used at the joint edges, while a less ductile adhesive is used in the centre of the overlap. The results show an increase in shear strength of the bi-adhesive-bonded joints compared with those in which single adhesives were used over the full length of the bondline. The increase of the apparent lap-shear strength was qualitatively predicted, through finite element modelling.

Laser Welding of Structural Aluminium

This chapter starts with an overview of the fusion welding processes used in aluminium welding and further progresses by analysing in detail the characteristics of laser welding of aluminium. Laser sources for welding are available for a few decades but new concepts are coming to the market. The chapter addresses the most commonly used lasers for materials processing, CO2 and Nd-YAG (neodymium–yttrim aluminium garnet) and their interaction with aluminium alloys in welding applications. More recent laser types are also included, namely fibre lasers and disc lasers as, though only more recently available in the market, their potential is foreseen as being interesting for welding of aluminium. Hybrid laser MAG (Metal Active Gas) welding has proven to lead to good results in welding aluminium plates namely for long seam welding.

Materials Behavior in Laser Welding of Hardmetals to Steel

Tool manufacturing industry faces the problem of permanently joining hardmetals to steel holders with high shear strength. The mostly used welding process still is brazing. However, brazed joints have poor lifetimes, mostly when high temperatures are achieved and often break in operation. In a previous study about the ability of CO2 and Nd:YAG lasers to weld hardmetals to steels, it was found that Nd:YAG lasers, working in continuous wave mode, could be used especially for welding hardmetals with Co content around 12%. This article discusses the materials behavior under laser radiation and analyzes the microstructural features observed.

Developments in NDT for Detecting Imperfections in Friction Stir Welds in Aluminium Alloys

Friction stir welding (FSW) has dramatically changed how aluminium alloys can be welded. The quality of FS welds is usually excellent, but some imperfections periodically occur. The geometry, location, and microstructural nature of these imperfections bear no resemblance to the imperfections typically found in aluminium fusion welds. Consequently, it has been difficult to identify FS weld imperfections with common non-destructive testing (NDT) techniques. Therefore, further development of NDT techniques must be done to enable the detection of FS weld imperfections. This paper presents an integrated, on-line, NDT inspection system for FS welds, which employs a data fusion algorithm with fuzzy logic and fuzzy inference functions. It works by analyzing complementary and redundant data acquired from several NDT techniques (ultrasonic, Time of Flight Diffraction (ToFD), and eddy currents) to generate a synergistic effect that is used by the software to improve the confidence of detecting imperfections. The system was tested on friction stir welded AA5083-H111 specimens. The results indicate that by combining the output from various NDT processes, an improvement in finding imperfections can be obtained compared to using each NDT process individually. The methodology implemented in the QNDT_FSW system has given good results and improved reliability in the NDT of friction stir welds.

Fatigue Behaviour of Friction Stir Welded Steel Joints

The development and application of friction stir welding (FSW) technology in steel structures in the shipbuilding industry provide an effective tool of achieving superior joint integrity especially where reliability and damage tolerance are of major concerns. Since the shipbuilding components are inevitably subjected to dynamic or cyclic stresses in services, the fatigue properties of the friction stir welded joints must be properly evaluated to ensure the safety and longevity. This research intends to fulfill a clear knowledge gap that exists nowadays and, as such, it is dedicated to the study of welded steel shipbuilding joints in GL-A36 steel, with 4 mm thick. The fatigue resistance of base material and four plates in as-welded condition (using several different parameters, tools and pre-welding conditions) were investigated. The joints culminate globally with defect-free welds, from which tensile, microhardness, and fatigue analyses were performed. The fatigue tests were carried out with a constant amplitude loading, a stress ratio of R=0.1 and frequency between 100 and 120 Hz. The experimental results show the quality of the welding process applied to steel GL-A36 which is reflected in the mechanical properties of joints tested.

Laser Welding of Dissimilar Aluminium Alloys with Filler Materials

Aluminium alloys have great potential in industrial applications due to their high strength and low weight, specially the AA5xxx and AA6xxx series that have particular applications in the aerospace, military and information technology industries. Laser welding is quite used for joining these materials despite their high reflectivity against laser beam radiation. Nd:YAG lasers are used in pulsed wave mode to overcome a set of difficulties, such as the material absorptivity, its thermal expansion coefficient and thermal conductivity. Autogenous welding of dissimilar aluminium alloys brings additional difficulties due to chemical, thermal and mechanical incompatibility, so the use of a correctly selected filler material improves the compatibility of both alloys being joined. This paper describes the work performed aiming to study laser welding of dissimilar alloys of AA6xxx series and of AA5xxx to AA6xxx series with filler wire. A Nd:YAG laser was used in a pulsed wave mode and a set of filler wires was tested aiming to select the filler presenting better performance. The quality of the welds was evaluated by macro and microstructural analysis. Scanning electron microscopy and hardness measurements were also performed. The results indicate that filler wires with low magnesium and high silicon contents allow for a better weld quality, free of porosity (individual or in clusters). However, the mechanical strength has to be considered in order to assure adequate service performance of the components or structures.