R.M. Miranda

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.

Determination of airborne nanoparticles from welding operations

The aim of this study is to assess the levels of airborne ultrafine particles emitted in welding processes (tungsten inert gas [TIG], metal active gas [MAG] of carbon steel, and friction stir welding [FSW] of aluminum) in terms of deposited area in pulmonary alveolar tract using a nanoparticle surface area monitor (NSAM) analyzer. The obtained results showed the dependence of process parameters on emitted ultrafine particles and demonstrated the presence of ultrafine particles compared to background levels. Data indicated that the process that resulted in the lowest levels of alveolar deposited surface area (ADSA) was FSW, followed by TIG and MAG. However, all tested processes resulted in significant concentrations of ultrafine particles being deposited in humans lungs of exposed workers.

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.

Comparison of deposited surface area of airborne ultrafine particles generated from two welding processes

This article describes work performed on the assessment of the levels of airborne ultrafine particles emitted in two welding processes metal-active gas (MAG) of carbon steel and friction-stir welding (FSW) of aluminium in terms of deposited area in alveolar tract of the lung using a nanoparticle surface area monitor analyser. The obtained results showed the dependence from process parameters on emitted ultrafine particles and clearly demonstrated the presence of ultrafine particles, when compared with background levels. The obtained results showed that the process that results on the lower levels of alveolar-deposited surface area is FSW, unlike MAG. Nevertheless, all the tested processes resulted in important doses of ultrafine particles that are to be deposited in the human lung of exposed workers.

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.

Austenite grain growth, microstructure and hardness in the heat-affected zone of a 2.25 Cr-1Mo steel

Abstract Submerged arc weldings were made on thick plates of a 2.25 Cr-1Mo steel (where the composition is in approximate weight per cent), using different heat inputs in the range 1.1–5.8 MJ m−1. The microstructures in the heat-affected zone (HAZ) were identified, and the austenite grain size and hardness were measured. The microstructure is predominantly bainitic, but some martensite was observed at low heat inputs. The observations in real weldings were compared with those made in welding simulations, using dilatometric tests. In these, the relevant transformation temperatures were determined. The austenite grain size was correlated to the thermal cycle, which was experimentally obtained at one point of the HAZ in each welding experiment. The grain size D, obtained by the linear intercept method, is well described by the kinetic equation Dn−D0n = kI where I is the “kinetic strength” of the thermal cycle which depends on the activation energy Q for grain growth. A method based on the dilatometry results was developed which allows the determination of the grain growth exponent n, independently of Q. With this method, we obtained n = 3.17. This is consistent with the welding data. The best value of Q was found to be 180 kJ mol−1. The grain size in welds was found to be comparable with that measured in the dilatometric specimens, for the same “kinetic strength” of the cycles.

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.

On the Mechanisms for Martensite Formation in YAG Laser Welded Austenitic NiTi

Extensive work has been reported on the microstructure of laser-welded NiTi alloys either superelastic or with shape memory effect, motivated by the fact that the microstructure affects the functional properties. However, some effects of laser beam/material interaction with these alloys have not yet been discussed. This paper aims to discuss the mechanisms for the occurrence of martensite in the heat-affected zone and in the fusion zone at room temperature, while the base material is fully austenitic. For this purpose, synchrotron radiation was used together with a simple thermal analytic mathematical model. Two distinct mechanisms are proposed for the presence of martensite in different zones of a weld, which affects the mechanical and functional behavior of a welded component.

Shape memory effect of laser welded NiTi plates

Laser welding is a suitable joining technique for shape memory alloys (SMAs). This paper reports the existence of shape memory effect (SME) on laser welded NiTi joints, subjected to bending tests, and correlates this effect with the microstructural analysis performed with X-ray diffraction (XRD). All welded samples were able to recover their initial shape after bending to 180°, which is a remarkable result for industrial applications of NiTi involving laser welding.

Characterization of airborne particles generated from metal active gas welding process.

Abstract This study is focused on the characterization of particles emitted in the metal active gas welding of carbon steel using mixture of Ar + CO2, and intends to analyze which are the main process parameters that influence the emission itself. It was found that the amount of emitted particles (measured by particle number and alveolar deposited surface area) are clearly dependent on the distance to the welding front and also on the main welding parameters, namely the current intensity and heat input in the welding process. The emission of airborne fine particles seems to increase with the current intensity as fume-formation rate does. When comparing the tested gas mixtures, higher emissions are observed for more oxidant mixtures, that is, mixtures with higher CO2 content, which result in higher arc stability. These mixtures originate higher concentrations of fine particles (as measured by number of particles by cm3 of air) and higher values of alveolar deposited surface area of particles, thus resulting in a more severe workers exposure.