Stresses In Friction Stir Welding Research Articles

The effect of hardening laws and thermal softening on modeling residual stresses in FSW of aluminum alloy 2024-T3

In the present paper, a numerical model consisting of a heat transfer analysis based on the Thermal Pseudo Mechanical (TPM) model for heat generation, and a sequentially coupled quasi-static stress analysis with a built-in metallurgical softening model was implemented in ABAQUS. Both isotropic and kinematic rules of hardening were used in order to study the effect of the hardening law on the residual stresses as well as on the final yield stress. This numerical model was then applied in two different cases. Firstly, a very simple 1D Satoh test was modeled. Different combinations of either isotropic or kinematic hardening together with the metallurgical softening model were applied in order to give a first impression of the tendencies in residual stresses in friction stir welds when choosing different hardening and softening behaviors. Secondly, real friction stir butt welding of aluminum alloy 2024-T3 were simulated and compared with experimentally obtained results for both temperatures and residual stresses (using the slitting method). The comparisons showed good agreement regarding temperatures whereas the residual stress comparisons indicated different sensitivities for the cold and hot welding conditions toward the choice of hardening rules and especially whether including the softening model or not.

Neutron diffraction measurements of residual stresses in friction stir welding: a review

Significant amounts of residual stresses are often generated during welding and result in critical degradation of the structural integrity and performance of components. Neutron diffraction has become a well established technique for the determination of residual stresses in welds because of the unique deep penetration, three‐dimensional mapping capability, and volume averaged bulk measurements characteristic of the scattering neutron beam. Friction stir welding has gained prominence in recent years. The authors reviewed a number of neutron diffraction measurements of residual stresses in friction stir welds and highlighted examples addressing how the microstructures and residual stresses are correlated with each other. An example of in situ neutron diffraction measurement result shows the evolution of the residual stresses during welding.

Efficacy of active cooling for controlling residual stresses in friction stir welds

This paper investigates the effect of active cooling on the development of welding stresses during friction stir welding by finite element modelling. The primary aim was to see if cooling powers, corresponding to those currently obtainable using liquid CO2 cooling systems applied in practically feasible locations could bring about a significant reduction in weld residual stress. Various cooling strategies were examined. The simulations revealed that a large reduction in residual stress can be obtained, particularly at the weld line, depending on the size, power and positioning of the cooling sinks. A heat sink placed as close as possible behind the heat source had the greatest effect on reducing the build up of tensile stresses. All the approaches tended to reduce the weld centreline stress more effectively than those at the heat affected zone/thermomechanically affected zone boundary (i.e. below the edge of the tool shoulder).

Inverse eigenstrain analysis of residual stresses in friction stir welds

This paper presents the results of eigenstrain analysis in a friction stir welded 12mm-thick 12%Cr steel plate. The finite element models are established for inverse eigenstrain analysis for three different model cases. As the region containing the eigenstrain distribution expands, the accuracy of the residual strain reconstruction improves. If the eigenstrain is allowed to be present along the entirety of the sample, good agreement can be achieved. It is also noting that eigenstrain (permanent plastic strain) represents the consequence of numerous inelastic processes occurring during to welding. The results suggest that significant inelastic deformation of the plate takes place even at larger distances from the weld. This conclusion requires validation by independent means, experimental and/or modelling.

Influence of time on residual stresses in friction stir welds in agehardenable 7xxx aluminium alloys

Friction stir welding (FSW) can be used to weld 7xxx series wrought aluminium alloys. As these materials are age-hardenable, their properties can change with time. This work used the neutron diffraction technique to measure the residual stresses around a FSW in a 7xxx alloy and to determine how these residual stresses changed with time. It was found that the residual stresses associated with the weld nugget decreased, while those associated with the heat-affected zone increased with time. This is in contrast to strength and hardness values that increased in all regions of the weld with time.

Residual Stresses of Friction Stir Welded 2024-T4 Joints

Friction stir welding (FSW) is a solid-state joining technique which can produce high-quality joints efficiently. The residual stresses in FSW are generated due to the effect of both the uneven temperature field and of the tool force, which is different from that in fusion welding. In this study the residual stresses of 3mm-thick 2024-T4 aluminum alloy FSW joints have been investigated by using the Hole-drilling method. To reduce the influence of drilling upon the experimental results, annealed stress-free 2024 aluminum alloy plates were drilled; the relieved strains were measured and were subtracted from the total strains measured from the joints. The results showed that the longitudinal residual stresses in the joint were much larger than the transverse residual stresses; high longitudinal tensile residual stresses were concentrated near the tool shoulder direct affected zone and asymmetrically distributed at the different sides of the weld line; i-e, high at the advancing side and relatively low at the retreating side. Outside the tool shoulder direct affected zone, the longitudinal residual stresses decreased rapidly and became compressive residual stresses away from the weld line; the peak of the longitudinal residual stresses was 164.5MPa.The mechanism of the generation of the residual stresses was analyzed preliminarily.

The simulation of residual stresses in friction stir welds

A numerical model is developed to investigate energy dissipations and residual stress distributions in friction stir welds. Results indicate that the maximum longitudinal residual stress can be increased with the increase of the translational velocity of the pin. But the variation of the angular velocity of the pin does not significantly affect the residual stress distributions. Energy dissipation in friction is increased with the increase of the angular velocity of the pin. However, with the increase of the translational velocity of the pin the plastic dissipation of energy is increased and the frictional dissipation is decreased.

Evaluation of Surface Residual Stresses in Friction Stir Welds Due to Laser and Shot Peening

The effects of laser, and shot peening on the residual stresses in friction stir welds (FSW) has been investigated. The surface residual stresses were measured at five different locations across the weld in order to produce an adequate residual stress profile. The residual stresses before and after sectioning the coupon from the welded plate were also measured, and the effect of coupon size on the residual stress relaxation was determined and characterized. Measurements indicate that residual stresses were not uniform along the welded plate, and large variation in stress magnitude could be exhibited at various locations along the FSW plate. Sectioning resulted in significant residual stress relaxation in the longitudinal direction attributed to the large change in dimensions in this direction. Overall, laser and shot peening resulted in a significant reduction in tensile residual stresses at the surface of the specimens.

FE Modelling of Mechanical Tensioning for Controlling Residual Stresses in Friction Stir Welds

Although Friction Stir Welding (FSW) avoids many of the problems encountered when fusion welding high strength Al-alloys, it can still result in substantial residual stresses that have a detrimental impact on service life. An FE model has been developed to investigate the effectives of the mechanical tensioning technique for controlling residual stresses in FSWs. The model purely considered the heat input and the mechanical effects of the tool were ignored. Variables, such as tensioning level, heat input, and plate geometry, have been studied. Good general agreement was found between modelling results and residual stress measurements, justifying the assumption that the stress development is dominated by the thermal field. The results showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~ 50% of the room temperature yield stress, tensile were replaced by compressive residual stresses within the weld.

Geometry Effects when Controlling Residual Stresses in Friction Stir Welds by Mechanical Tensioning

Finite element modelling has proved to be an effective tool for the investigation of trends effected by changing welding conditions. This is especially important in mechanical tensioning of friction stir welds because of the large number of parameters involved. In this paper, an FE model is used to examine the effectiveness of the mechanical tensioning technique for controlling residual stresses in FSWs by the investigation of trends caused by changes to the welding parameters. Comparisons between different geometries, traverse speeds, and welding off-axis angle all produced consistent results, and showed that the peak stresses are most strongly influenced by both the local tensioning and heat input, and not by the more global welding conditions. The results also showed a progressive decrease in the residual stresses for increasing tensioning levels and, although affected by the heat input, a relatively low sensitivity to the welding variables. At tensioning levels greater than ~50% of the room temperature yield stress, tensile stresses were replaced by compressive residual stresses within the weld.

A variational model for shear stress in friction stir welding based on the weld shape in transverse sections

A model is proposed to interpret observed shapes of transverse cross sectional macrostructures in aluminium friction stir welds. The model applies calculus of variations to minimize a functional corresponding to the work done to shear the material rotating around the tool, thus allowing calculation of an average value for the shear stress on the outer shearing surface around the carousel, as a function of the radial position only and independent of the angular position on that surface. The main point is that there is valuable information to be obtained from observing the geometry of the outer shape of the carousel. Thus, a formula is given to calculate the distribution of the average shear stress at the interface between the rotating material and the nearly stationary base material, on the basis of the observed weld shape.

Numerical simulation of transient temperature and residual stresses in friction stir welding of 304L stainless steel

Three-dimensional nonlinear thermal and thermo-mechanical numerical simulations are conducted for the friction stir welding (FSW) of 304L stainless steel. The finite element analysis code—WELDSIM, developed by the authors specifically for welding simulation, was used. Two welding cases with tool rotational speeds of 300 and 500 rpm are analyzed. The objective is to study the variation of transient temperature and residual stress in a friction stir welded plate of 304L stainless steel. Based on the experimental records of transient temperature at several specific locations during the friction stir welding process for the 304L stainless steel, an inverse analysis method for thermal numerical simulation is developed. After the transient temperature field is determined, the residual stresses in the welded plate are then calculated using a three-dimensional elastic–plastic thermo-mechanical simulation. The effect of fixture release after the welding on the residual stresses is also studied. Comparison with the residual stress fields measured by the neutron diffraction technique shows that the results from the present numerical simulation have good agreement with the test data.