Friction stir welding of dissimilar A319 and A356 aluminium cast alloys

Abstract

In the present investigation, the microstructure and mechanical characteristics of dissimilar A319 and A356 cast Al alloys plates joined by friction stir welding (FSW) were evaluated. The effect of tool rotational and welding speeds as well as the post-weld heat treatment (PWHT) on such properties was investigated. Post-weld heat treatment was carried out at a solutionising temperature of 540°C for 12 h followed by aging at 155°C for 6 h. For the as welded specimens, the welded zone (WZ) exhibited higher hardness values when compared with the A319 and A356 parent alloys. The peak hardness at the WZ was found to increase by increasing the tool rotational speed and/or reducing the welding speed. In contrast, the post-weld heat treated (PWHTed) specimens exhibited lower hardness values at the WZ than the parent alloys. For PWHTed specimens, the peak hardness at the WZ was found to decrease by increasing the tool rotational speed and/or reducing the welding speed. Tensile tests results demonstrate that, for the as welded specimens, the tensile fracture took place on A356 side where the hardness was minimal. While for PWHTed specimens, the fracture took place at the WZ. Increasing the tool rotational speed reducing both tensile and yield strengths, but increases the ductility of the joint.     

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.     

Process forces during friction stir welding of aluminium alloys

Abstract

Flow and consolidation of the material under the tool shoulder and subsequent nugget formation are among the least understood aspects of friction stir welding and processing (FSW/P). Welding parameters and tool profile impact the process forces acting on the tool. This work is an observational study of the process forces associated with bead on plate runs on two aluminium alloys, 6061 and F357. Polar plots of the resultant forces acting on the tool spindle are analysed and correlated to the process parameters. The dependence of the nugget's width with various heat indices is evaluated.     

Effect of tool rotation rate on microstructure and mechanical properties of friction stir welded copper

Abstract

Defect free copper welds were achieved by friction stir welding (FSW) carried out at a constant welding speed of 100 mm min^?1. The influence of tool rotation rate on microstructure, mechanical properties and fracture location was investigated. As the tool rotation rate increased, the grains of nugget zone grew significantly, the thermomechanically affected zone became indistinct and the grain size increased, but the effect of tool rotation rate on the grain size of heat affected zone was limited. Both ultimate tensile strength (UTS) and elongation increased first and then decreased with increasing rotation rate and the UTS achieved a highest value of 282 MPa at the rotation rate of 400 rev min^?1 together with the welding speed of 100 mm min^?1, which was on the level of the base metal. The fracture occurred at the cavity defect on the advancing side of the joint when the FSW was performed at a low tool rotation rate, while it occurred on the retreating side when the tool rotation rate was relatively high.     

Microstructural analysis of stir zone of Al alloy produced by friction stir spot welding

Abstract

Friction stir spot welding (FSSW), which was recently developed as a spot joining technique, has been applied to a lap joint of Al alloy 6061 sheets, 1 mm in thickness, to clarify its microstructural features of the joint. A nugget shaped stir zone having finer grain size was observed around the exit hole of the probe. Crystallographic texture analyses using EBSD method suggested that the material flow occurred along the rotating direction of the FSSW tool in the wide region including the stir zone. In the periphery of the nugget shaped stir zone, which was characterised by finer grain size than the stir zone interior, no inclusions or precipitates were found on the SEM scale. A softened region was formed around the joint centre, which could be explained as resulting from dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSSW.     

Resistance and friction stir spot welding of DP600: a comparative study

Abstract

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel method to spot welding sheet metal and has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn coated DP600 AHSS (1·2 mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process. Results show a correlation found among microstructure, failure loads, energy requirements and bonded area for both spot welding processes.     

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.     

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.     

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.     

Fracture toughness of structural aluminium alloy thick plate joints by friction stir welding

Abstract

The fracture toughness in a friction stir welded joint of thick plates of structural aluminium alloy type A5083-O is investigated. A joint between two 25 mm thick plates is fabricated by one sided, one pass friction stir welding. The Charpy impact energy and critical crack tip opening displacement (CTOD) in the friction stir weld are much higher than those in the base metal or heat affected zone, whereas mechanical properties such as stress–strain curve and Vickers hardness are not conspicuously different. The effects of the microstructure on crack initiation and propagation are studied in order to clarify the difference in fracture toughness between the stir zone and base metal. The analyses of the fracture resistance curves and the diameters of dimples in the fracture surface after both tensile and bending tests show that the fine grained microstructure in the stir zone helps to increase ductile crack initiation and propagation resistance. It is found that the high fracture toughness value in the stir zone is affected by the fine grained microstructure in friction stir welds.     

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.     

Friction stir welding of pure titanium lap joint

Abstract

The effects of welding parameters on friction stir welding of pure titanium lap joint were investigated together with the microstructural characteristics of the sound joint. Three kinds of welding defects were found under the condition of tool load control, namely, the groove-like defect, the inner cavity defect and the overheating rough surface and tool penetration defect with increasing heat input. The tool plunge depth control effectively increased the lap width compared with the tool load control, so the sound joints fractured in the base metal were acquired at 250 rev min^–1–75 mm min^–1 and 200 rev min^–1–50 mm min^–1. The sound joint consisted of the thermomechanically affected zone, the stir zone, the lap zone and the top layer. The microstructure was fined obviously after welding, and finer grains were observed in the lap zone and top layer.     

Friction Skew-stir welding of lap joints in 5083–0 aluminium

Abstract

A variant of the friction stir welding technique developed at TWI, called Skew-stir (tradename) was applied to the welding of lap joints in Al alloy AA5083–0. This technique differs from the conventional method in that the axis of the tool is given a slight inclination, or skew, to that of the machine spindle. It is particularly advantageous in instances where a wide weld region is required, such as lap joints in which the interface is perpendicular to the machine axis. The microstructures and mechanical properties of welds made using both a conventional pin type probe and the rotary Skew-stir technique with an A-Skew (tradename) probe were studied. The joints made using the Skew-stir technique sustained significantly higher tensile loads and had longer fatigue lives compared with those made using the conventional pin type probe.     

The effect of cryogenic CO2 cooling on corrosion behaviour of friction stir welded AA2024-T351

Abstract

Cryogenic cooling with CO₂ was applied during friction stir welding of AA2024-T351 in order to reduce the temperature increase during welding, and thus improve the corrosion resistance of the weld. The effect of cryogenic cooling on corrosion susceptibility was investigated with gel visualisation, immersion tests and local electrochemical measurements. The most susceptible area for both uncooled and cooled welds was in the heat-affected zone (HAZ) region, which showed intergranular attack. Cryogenic cooling had no detectable influence on the degree of anodic reactivity in the weld region. However, it did decrease the width of the reactive HAZ.     

Effect of initial base metal temper on mechanical properties in AA7050 friction stir welds

Abstract

The effect of initial base metal temper on mechanical properties in AA7050 friction stir welds was investigated. AA7050 plates, 6·4 mm thick, with three different heat treatment conditions (T7451, T62 and W), were friction stir welded using nearly identical welding parameters, followed by post-weld aging approximating a T7451 heat treatment. The microstructure, transverse hardness profiles and transverse tensile properties were characterised for these three welds. Experimental results show that preweld heat treatment conditions of AA7050 base metal have significant effect on the mechanical properties of the friction stir welds. Friction stir welding of AA7050 in the W condition, followed by post-weld aging, can change the fracture location from HAZ to weld nugget and increase tensile and yield strengths and elongation in transverse tension, relative to welding in T62 or T7451 conditions.     

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.     

Study of friction stir joining of thin aluminium sheets in lap joint configuration

Abstract

Thinning in friction stir lap joints and its relation with the process variables was investigated. Friction stir welds were made on 1 mm thick AA6111 aluminium alloy sheets in order to study the effects of rotation rate, traverse speed, plunge depth, tilt angle and pin height on faying surface defects. Sheet thinning on the advancing and retreating sides was quantified and the lap shear strength of the joints was evaluated. A decrease in the pitch of the runs at constant rotation rate increased the sheet thinning and reduced the joint strength in a linear manner. Process pitch and pin height were found to be the most critical factors in determining the faying surface lift up. A pin of the same height as the sheet thickness resulted in maximum lap shear strength. Tool tilt did not show a significant effect on the sheet thinning.     

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.     

New technological methods and designs of stir head in resistance friction stir welding

Abstract

This paper proposes a new welding method, resistance friction stir welding (RFSW), which provides a novel method to solve some of the challenges of friction stir welding high melting point of ferrous and non-ferrous metals. The RFSW is a new hybrid welding technology, which utilises the advantages of both resistance welding and friction welding. The technological principle of RFSW uses the compound effects of resistance heating and friction heating; the integration methods are discussed and analysed in this paper. It is believed that RFSW is an innovative, practical welding technique. The national patents for invention have already been applied in China.     

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.