New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract

A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.     

Residual stresses simulation for friction stir welded joint

Abstract

As a solid state joining technique, friction stir welding (FSW) can produce high strength, low distortion joints efficiently. Compared to fusion welding, residual stresses in FSW joints are expected to be low due to a relatively low heat input. However, apart from the heat input, the force from the tool also plays an important role in the development of welding stresses. In the present paper, a semicoupled thermomechanical finite element model containing both thermal load and mechanical load was established to simulate the development of welding stresses during FSW process; an autoadapting heat source model was employed in the thermal analysis; the fixture was also included in the mechanical analysis model. The simulation results showed that due to the effect of the tool force, the longitudinal residual tensile stresses became smaller and were asymmetrically distributed at different sides of the weld centre; the peak of the tensile residual stresses at the retreating side was lower than that at the advancing side. Calculated and experimental results were compared.     

Evaluation of 2D, 3D and applied plastic strain methods for predicting buckling welding distortion and residual stress

Abstract

Welding induces residual stresses which in thin section structures may cause buckling distortion. The magnitude of longitudinal residual stress is critical in the prediction of buckling distortion, which affects numerous welding applications in the ship building, railroad and other industries. The objectives of this paper are to overview and evaluate modelling procedures for bucking distortion. Moving source two-dimensional (2D), three-dimensional (3D) small deformation, 3D large deformation, and 2D–3D applied plastic strain analyses are evaluated by comparing computed residual stress and distortion against experimental measurements. Guidelines for modelling welding distortion are developed along with an assessment of the efficiency and limitations of the various analysis methods.     

Experimental study on distortion of Al-6013 plate after friction stir welding

Abstract

Friction stir welding (FSW) experiments with different panel dimensions and welding parameters have been designed to study the distortion of FSW. The FSW experiments were carried out with a load control facility to make the welding parameters reliable. The distortion of FSW is much smaller than that of arc welding, but it is still very significant. Three-dimensional distortion measuring system was applied to further study distortion trends. The results show that the distortion after FSW is in saddle shape, with convex bending in longitudinal direction and concave bending in transverse direction. This distortion pattern is in contrary with that of traditional arc welding. It is also found that increasing the panel length increases the longitudinal distortion but almost do not influence the transverse distortion. Increasing the rotation speed increases both longitudinal distortion and transverse distortion. The influence of welding speed on distortion is not very clear.     

Simplified method for establishing constitutive equations and flow stress data for welding stress modelling

Abstract

A new method, called the representative σ–T curve approach, has been proposed by which the temperature and microstructural dependency of the flow stress in welding stress simulation tools can be relatively easily determined. The method is based on the ideal plasticity approach and on considering the evolution of the material properties during the temperature cycle in Satoh testing as representative for that taking place in all material points in the heat affected zone in a real welding operation. The method requires prior knowledge to the relevant Continuous Cooling Transformation (CCT) diagram.     

Residual stress engineering in friction stir welds by roller tensioning

Abstract

The authors investigate the efficacy of applying rolling pressure along the weld line in thin butt welds produced using friction stir welding (FSW) as a means of controlling the welding residual stresses. Two cases are examined and in each case, comparison is made against the as welded condition. First, for FSW of AA 2024 aluminium alloy, roller tensioning was applied during welding using two rollers placed behind and either side of the FSW tool. Very little effect was seen for the down forces applied (0, 50, 75 kN). Second, for FSW AA 2199 aluminium alloy, post-weld roller tensioning was applied using a single roller placed directly on the FS weld line. In this case, significant effects were observed with increased loading, causing a marked reduction in the longitudinal tensile residual stress. Indeed, a load of just 20 kN was sufficient to reverse the sign of the weld line residual stress. Only slight differences in Vickers hardness were observed between the different applied loads. Furthermore, unlike some methods, this method is cheap, versatile and easy to apply.     

Stress corrosion cracking of AZ61 magnesium alloy friction stir weldments in ASTM D1384 solution

Abstract

Stress corrosion cracking (SCC) behaviour of a wrought magnesium alloy (AZ61) friction stir weldment was assessed in ASTM D1384 test solution at two strain rates. The analyses have shown that both the parent and the weldment are susceptible to SCC at a nominal strain rate of 10^–6 s^–1. Fractographic evidence clearly reveals the susceptibility, especially of the fine grained region of the friction stir weld nugget to SCC. The susceptibility to cracking has been observed to increase with decreasing strain rate and under this condition (10^–7 s^–1), the behaviour of the parent and the friction stir weldment were nearly the same.     

Themo-elasto-viscoplastic modelling of friction stir welding

Abstract

A coupled two-dimensional Eulerian thermo-elasto-viscoplastic model has been developed for modelling the friction stir welding process. First, a coupled thermo-viscoplastic analysis is performed to determine the temperature distribution in the full domain and the incompressible material flow around the spinning tool. Next, an elasto-viscoplastic analysis is performed outside the viscoplastic region to compute the residual stress. Both frictional heat and plastic deformation heat generation are considered in the model. Furthermore, this is the only known model computing residual stress accounting for plasticity caused by both thermal expansion and mechanical deformation due to material spinning. The computed residual stress is verified by comparing to experimentally measured data.     

Modelling of 7050 aluminium alloy friction stir welding

Abstract

A thermal model combined with a microstructural and yield strength model has been developed to give a prediction of precipitate evolution and strength in the as welded and post-weld heat treated condition for friction stir welding of 7xxx aerospace aluminium alloys. This fully coupled model is applied to an overaged high strength 7050 aluminium alloy friction stir welded using a range of welding rotation and translation speeds. The evolution of the microstructure has been predicted as a function of the process parameters. The resulting microstructural evolution is shown to be a complex function of both peak temperature observed during the weld cycle and heating/cooling rates. Yield strength has been calculated from the microstructural predictions and a comparison between predicted yield strength and measured hardness has been used to test the modelling approach. Reasonably good agreement between model and experiment is found over the wide range of process parameters investigated.     

Developing predictive tools for friction stir weld quality assessment

Abstract

This research programme explores predictive tools that assess friction stir weld quality in aluminium alloys through dynamic characterisation. The study focuses on the correlations between dynamic interrogations measures of friction stir welded panels with the weld energy, as welded mechanical properties and the microstructure. 7136-T76 aluminium extrusions were joined at unique weld energies, and to characterise and identify the friction stir welds through non-destructive techniques, theoretical modelling and lab scale dynamic testing were conducted to establish the correlation between the weld energy and the associated spectral characteristics of the beam (natural frequencies/mode shapes). In this non-destructive evaluation study, the modal parameters were measured and were correlated with the friction stir weld microstructure and the physical parameters of the welded components, such as axial and flexural rigidities. The viability of weld parameter identification and weld quality assessment of friction stir welding beams using dynamic interrogation techniques is demonstrated.     

Peak temperatures and microstructures in aluminium and magnesium alloy friction stir spot welds

Abstract

The peak temperatures during friction stir spot welding of similar and dissimilar aluminium and magnesium alloys are investigated. The peak temperatures attained during friction stir spot welding of Al 6111, Al 2024, and AZ91 are within 6% of their solidus temperatures. In dissimilar AZ91/Al 6111 spot welds the peak temperature corresponds with the α-Mg solid solution and Mg₁₇Al₁₂ eutectic temperature of 437°C. An a-Mg plus Mg₁₇Al₁₂ eutectic microstructure is produced in dissimilar friction stir spot welds when material displaced during pin penetration into the lower sheet material contacts the upper sheet material at the eutectic temperature.     

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.     

Distortion control in welding by mechanical tensioning

Abstract

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.     

Modelling microstructural evolution during multiple pass friction stir welding

Abstract

The effect of multiple weld passes on the microstructure and properties of friction stir welds in aerospace aluminium alloy AA 7050 has been investigated with the aid of a microstructure model. Each weld pass leads to a reduction in minimum strength in the heat affected zone, the decrease being greatest after the first pass. The model predicts that this is due to partial dissolution and coarsening of the strengthening precipitatates, which up to weld pass 3 is dominated by dissolution, and beyond weld pass 3 by coarsening. Microstructural evolution in the nugget is predicted to involve complete precipitate dissolution during each pass, followed by reprecipitation on the refined grain boundaries. A post-weld aging treatment after multiple passes is predicted not to be effective in improving minimum strength since precipitation occurs via growth and coarsening of existing particles, not nucleation of new precipitates.     

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.     

Tool design and stir zone grain size in AZ31 friction stir spot welds

Abstract

The influence of tool design and tool rotational speed variations on the torque, energy output, stir zone temperature and average grain size in the stir zones of AZ31 friction stir spot welds was investigated. The average stir zone grain size decreased by ～1 μm in AZ31 friction stir spot welds made using a three-flat/threaded tool design and tool rotational speeds of 2250 and 3000 rev min^?1. However, there was no statistically significant influence of tool design on the average grain sizes in friction stir spot welds made using tool rotational speeds of 1500 and 1000 rev min^?1. There was no evidence of grain growth in the stir zones of AZ31 friction stir spot welds. Similar torque, calculated energy output and stir zone temperature values were found in AZ31 friction stir spot welds made using threaded and three-flat threaded tool designs and tool rotational speeds from 1000 to 3000 rev min^?1.     

Elasto-visco-plastic analysis of welding residual stress

Abstract

In the past three decades, extensive research has focussed on the application of numerical methods for the computation of residual stress. Most commonly, the simulations involved performing weakly coupled thermal mechanical finite element analyses in Lagrangian reference frames assuming rate independent elasto-plastic material response. Nearly all approaches assumed rate independent elasto-plastic material response, which is most appropriate at low to moderately elevated temperatures. At, the high temperatures near the fusion zone, the material response becomes rate dependent and an elasto-visco-plastic model would be more suitable. In 1989, Brown et al. (Int. J. Plast., 1989, 5 , 95–130) proposed a rate dependent constitutive equation (commonly known as Anand's model) to describe the plastic evolution of metals at high temperatures. The objective of this work is twofold: evaluate the suitability of Anand's elasto-visco-plastic model in computing welding residual stress and investigate the feasibility of an Eulerian implementation of Anand's model in modelling welding residual stress. Such an implementation has the potential to reduce computational cost in modelling welding processes, since it is a steady state analysis as compared to the common time incremental Lagrangian analyses. An Eulerian reference frame is also more advantageous in modelling processes with large deformation such as friction stir welding, rolling and extrusion since excessive mesh distortion and re-meshing are no issues as the case of Lagrangian models (Int. J. Mater. Form., 2008, 1 , 1287–1290).     

Relationships between weld parameters,hardness distribution and temperature history in alloy 7050 friction stir welds

Abstract

Aluminium alloy 7050 was friction stir welded using three different ratios of tool rotation rate to weld travel speed. Welds were made using travel speeds of between 0·85 and 5·1 mm s^?1. Weld power and torque were recorded for each weld. An FEM simulation was used to calculate the time–temperature history for a subset of the welds. For each weld the hardness distribution with and without post-weld heat treatment was determined. The hardness distributions within the welds are rationalised based on the friction stir welding parameters and the resulting temperature histories. The analysis provides a basis for manipulation of weld parameters to achieve desired properties.