Global mechanical tensioning for the management of residual stresses in welds

Abstract

The general principles behind the global mechanical tensioning technique for controlling weld residual stresses are examined using a finite element model to follow their evolution throughout the welding process. While we focus specifically on friction stir welding, the tool is represented simply as a heat source. As a result, the findings have relevance to a wide range of welding processes. For aluminium alloy friction stir welds, the maximum longitudinal weld stresses have been reported to fall approximately linearly to zero under mechanical tensioning to a level around 40% of the yield stress. Under larger tensioning levels, the weld stress becomes increasingly compressive. This behaviour is explained in terms of the reduction in compressive plastic straining ahead, and an increase in tensile plastic straining behind, the heat source as the tensioning level is increased. Finally, it is shown that tensioning during welding is much more effective than post-weld tensioning.

Introduction

Friction stir welding (FSW) is being used increasingly in a wide range of applications, particularly for joining aluminium alloys. The basic process has been described extensively elsewhere [1], but in essence involves a rotating tool consisting of a cylindrical shoulder and pin. The tool is plunged into the weld line until the shoulder is in contact with the plate surface. Once the material is sufficiently hot from frictional heating and plastic work, the tool traverses along the weld line and the hot plasticised material is extruded past the rotating pin, while constrained between the shoulder and backing bar, so as to form a joint behind the pin. Because FSW is a solid state welding method, it is particularly suited to joining high strength aluminium alloys that were previously considered unweldable using fusion techniques [1]. Although the process alleviates many of the metallurgical problems associated with fusion welding, such as liquation and solidification cracking, friction stir welds can still suffer from significant levels of residual stress, which are often similar in magnitude to those seen in fusion welds [2], [3], [4]. In general terms, the residual stresses arise from plastic misfit strains introduced as a result of the steep gradients in temperature that are generated local to the heat source as the tool advances [5]. As tensile residual stresses in welded structures produced from high strength Al-alloys can have a negative impact on service life [4], [6], [7], it is highly desirable to reduce their level as far as possible. One approach, that can be adopted to mitigate weld residual stresses, is to use weld tensioning methods to engineer the local stress state during welding by controlling the plastic misfit strains generated by the thermal field.

In welding the maximum tensile residual stresses are typically found on, or either side of, the weld line. These arise during cooling of the weld as a result of the compressive plastic misfit generated as the material expands and softens ahead of the heat source [5]. It has long been known that tensioning techniques can reduce residual stresses and the concomitant tendency for distortion (e.g. [8]). In practice, a large number of techniques have been proposed for controlling residual stresses in welding, including both thermal and mechanical tensioning methods. One of the earliest reported applications was by Greene and Holzbaur [9], who in 1946 used superimposed temperature gradients to achieve reduced residual stresses in longitudinal butt welds in ship hull structures. Local induction heating has also been investigated for residual stress improvement [10]. Michaleris and Sun [11] and Dull et al. [12] have applied thermal tensioning to reduce buckling distortion, whilst Dong et al. [13] developed an in-process thermal-stretching technique for effectively mitigating residual stresses and distortion on repair welding of aluminium panels. In addition, Barber et al. [14], van der Aa et al. [15] and Williams and co-workers [16], have applied local cooling, with either solid or liquid CO₂ trailing the heat source, as a means of creating dynamically controlled low residual stress and distortion free welds. Several mechanical tensioning systems have also been proposed. Yang et al. [17], [18] have mechanically compressed the weld on cooling using a pair of rollers on both sides of the weld line, reducing both residual stress and buckling distortion. Finally, preliminary work by Williams et al. [8] has shown that the application of global, or far field, mechanical tensioning externally during the welding process can greatly reduce the tensile residual stresses in FSWs. In global mechanical tensioning a load is applied uniformly along opposite ends of the plates prior to clamping the parts for welding (see Fig. 1), so that a uniform tensile stress is maintained in the two butted plates parallel to the weld line. The clamping and tensioning loads are then released after the friction stir welding tool has traversed along the join line forming a weld. Perhaps counter-intuitively, Williams et al. found that high levels of mechanical tensioning parallel to the welding direction can actually reverse the state of stress, so that compressive longitudinal residual stresses are found in the weld region [8].

While a plethora of techniques for influencing the generation of plastic misfit strains and the resultant residual stresses during welding have emerged, at present very little work has been carried out into quantifying the effectiveness of these methods, or establishing their basic principles. This is in part because, to date, little modelling work has been carried out to study the underlying interactions between the externally imposed thermal or mechanical loads and the transient welding stresses. This paper aims to help fill this gap and thereby clarify the mechanism of mechanical tensioning. To this end we have focused on the mechanical tensioning of aluminium alloy friction stir welds. A simplified finite element model of the process has been developed and validated using experimental residual stress data available in the literature measured by X-ray synchrotron diffraction [8], [19], [20], [21], for mechanically tensioned AA7449 and AA2024 FSW's. While modelling work has been carried out on the basic FSW process previously (e.g. [22], [23], [24], [25], [26], [27], [28], [29], [30]), few studies have focused on residual stress prediction [26], [29], [30]. The study by Preston et al. [30] did look at thermal tensioning, but none have looked in detail at the effect of mechanical tensioning on FSWs. Although this paper examines friction stir welding, because the development of residual stresses have been taken to be governed solely by the heat input, the general conclusions regarding the mechanisms of stress relief by mechanical tensioning are applicable to other welding processes.