CFD modelling of friction stir welding of thick plate 7449 aluminium alloy

Abstract

CFD modelling of friction stir welding has been conducted to understand and optimise the welding of thick, 7449 aluminium alloy for aerospace applications. The aim is to produce high strength, defect free welds that do not break the tool. The models compared different pin profiles and rotation speeds and were undertaken in two stages. The first stage involved creating a thermal model to better understand the generation and flow of heat. The second stage involved analysing the flow near the tool with a two-dimensional model. The traversing force results from the two-dimensional planar models compared favourably with experimental findings. The pressure distribution and deformation region size were compared for the different models. Novel maps of the deformation conditions experienced in each weld were produced. The analysis suggested reasons why some pin profiles and rotation speeds are preferable to others and explained the difference in the traversing force measurements.     

Laser welding of copper–nickel alloys: a numerical and experimental analysis

Abstract

Melt run trials were carried out on Cu–Ni bars using a CO₂ laser source in order to analyse the effects of welding parameters (i.e. laser power, welding speed) on geometrical characteristics and on the microstructure of the bead. Experimental results were then used to determine the source parameters to be employed in a finite element model (FEM) of the welding process, with particular attention paid to the thermal field induced by the laser beam. A specific procedure, named 'automatic remeshing technique', was used in order to minimise the computation time. The aim was to create a reliable numerical model, suitable for the optimisation, in practical cases, of welding processes of these kinds of materials. A good correlation, in terms of predicted cooling rates, with the values calculated from SDAS measurements, was observed.     

Microstructure and mechanical properties of laser welded austenitic high manganese steels

Abstract

The microstructure of laser welded austenitic twinning induced plasticity steel sheets joints was investigated by means of optical microscopy, SEM and electron backscattering diffraction in order to differentiate the fusion zone, heat affected zone and base material, as well as to establish present phases, grain size distribution and grain misorientation distribution caused by the welding process. Measurements of EDX were taken into account to evaluate the effect of Mn segregations. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joints. In addition, the twinning phenomena progress was assessed by investigating the texture evolution in the base material and fusion zone of samples plastically deformed by 5, 10 and 15%. Grain refinement was found in the fusion zone affecting substantially the mechanical properties of the welding, being the most resistant and harder region.     

Observations of the phenomenon of abnormal arc voltage occurring in pulsed metal inert gas welding of aluminium alloy

Abstract

At the time of arc reignition after short circuiting during electrode positive polarity, cathode spots are newly formed in the centre of the weld pool surface, where oxides scarcely exist. The work function of the cathode surface increases and the cathode spots concentrate because of the lack of sufficient oxides, leading to an increase in the potential gradient across the cathode fall space and the adjoining contraction space. Consequently, the arc voltage becomes abnormally high in spite of the short arc length. Moreover, when electrode polarity is switched from positive to negative immediately after a droplet has detached from the wire tip, cathode spots are newly formed on the surface of the molten metal remaining at the wire tip, where little oxide exists, leading to an abnormal increase in arc voltage as well. Therefore, the change in arc voltage does not necessarily indicate a fluctuation in the arc length.     

Experimental study of joint performance in spot friction welding of 6111-T4 aluminium alloy

Abstract

The effect of process parameters (cycle time, tool speed and axial force) on the specimen temperature measured 2 mm away from the weld in spot friction welding (SFW) of Al 6111-T4 is investigated. The temperatures were correlated to the lap shear load. Results revealed that, to achieve a good joint strength with the maximum lap shear load >2˙5 kN, temperatures should be greater than a threshold value, which is 350°C at a location close to the SFW joint in this study. By studying the specimen macrographs, two internal weld geometric features based on the cross-section area were identified and correlated to the shear and mixed failure modes of the lap shear tested specimens. A model was developed and validated using experimental data of the cross section area of SFW joint with either shear or mixed mode fracture. The model predicts that the SFW joint strength is maximised at the transition region between the shear and mixed mode fracture.     

Investigation of microstructure,surface and subsurface characteristics in titanium alloy friction stir welds of varied thicknesses

Abstract

Friction stir welding of titanium alloy (Ti–6Al–4V) was demonstrated on 3, 6, 9 and 12 mm thickness square groove butt joints. Complete microstructural and microhardness evaluations were conducted in addition to surface and subsurface examinations for each case. The 3 mm welds exhibited an extremely fine grained microstructure with evidence of processing temperatures below the beta transus temperature of the alloy. The 6, 9 and 12 mm samples possessed larger grains formed by a slower cooling rate from above the beta transus temperatures. The thick section weld exhibited a nearly uniform microhardness, while the thinner welds showed a slight, 6%, increase in hardness compared with the parent material.     

Inhibitive Effect of an Onium Compound on the Dissolution of Steel in Hydrochloric Acid

Abstract

An onium compound has been found to be an effective inhibitor for the dissolution of mild steel in hydrochloric acid in all practical conditions. Adsorption studies based on weight loss measurements show that adsorption of onium obeys the Freundlich adsorption isotherm. Rectilinear Arrhenius plots were obtained both in the presence and absence of inhibitor, but the activation energy is(paradoxically) much lower in the presence of the inhibitor. Onium polarises both the anode and cathode, the extent of the polarisation increasing with inhibitor concentration. The exchange current is considerably reduced in the presence of the inhibitor, inhibitor efficiencies calculated from this reduction agreeing well with values calculated from weight loss measurements. The combined use of KI and onium shows a synergistic effect.     

Effect of boundary conditions and heat source distribution on temperature distribution in friction stir welding

Abstract

Welding experiments on Al-6005A have been carried out using a fully instrumented milling machine. The power input was calculated from the measured torque and forces. The thermal cycles were measured at various locations close to the weld centreline. A finite element pseudo-steady-state uncoupled thermal model was developed, taking into account the influence of the welding parameters on the power input. The distribution of the total power input between surface and volume heat sources was also studied. The measured and predicted thermal cycles are in good agreement when proper contact conditions between the workpiece and the backing plate are introduced.     

Improving vibration weld joint strength through process and equipment modifications

Abstract

Vibration welding is a process used to join thermoplastic components. Currently, under optimised low pressure welding, the weld strength of butt joints of unreinforced polymer can be equivalent to the strength of unwelded material. However, in short glass fibre reinforced polymer, the optimised weld strength is significantly lower than that of unwelded material and is closer to the strength of the resin matrix. This lower strength is attributable to the unfavourable orientation of the short glass fibres in the weld zone. The fibres tend to align parallel and in the plane of the weld zone and thus provide no reinforcement in the direction perpendicular to the weld zone. In the present work the impact of various modifications to the existing vibration welding technology was examined, with the objective of increasing the current achievable weld strength of glass reinforced nylon. The introduction of a secondary vibratory motion perpendicular to the weld plane during welding resulted in strengths 20% higher than those of samples welded using the standard vibration welding process.     

Error analysis of forward and reverse heat conduction and convection calculations considering uncertainties in welding

Abstract

Numerical models of fusion welding traditionally compute temperature field for a given set of welding conditions in a forward manner. The reliability of computed temperature profile depends on the accuracy of a number of model input parameters, values of which are uncertain in nature. Here, the authors show that a genetic algorithm (GA) assisted integrated numerical model, following either convection or conduction based calculations, can identify the suitable values of the uncertain model input parameters and in turn provide reliable computed results. Powered with GA, the integrated model is used further in a reverse manner to predict multiple sets of welding conditions for a target weld geometry. The convection based calculations have been able to provide more reliable multiple welding variables in reverse calculations.