Friction-stir dissimilar welding of aluminium alloy to high strength steels: Mechanical properties and their relation to microstructure

Abstract

The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy.

Introduction

Energy and environmental issues in transportation systems have a strong impact on material selection and on the development of joining techniques [1], [2], [3], [4], [5], [6], [7]. The incorporation of light-weight materials in many structures (e.g. automotive, shipbuilding and aerospace) allow a reduction of weight and consequently fuel consumption. In this regard, dissimilar joints between light-weight materials such as aluminium alloys (Al alloy) and steels are receiving increased interest both in science and industrial application [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, the substitution of one material for another is not straightforward and highly efficient products require appropriate joining processes.

Dissimilar fusion welding between Al alloy and steels is a challenge in welding control because of the large differences in melting temperature and physical and mechanical properties of the alloys involved. The process often results in complex weld pool shapes, inhomogeneous solidification microstructures and segregations in addition, the extremely low solubility of Fe in Al leads to the formation of brittle and excessive Al-rich Fe_xAl_y intermetallic phases [21], [22] which are detrimental for the mechanical properties of the joint [8], [9], [10], [11], [12], [20], [22].

Friction stir welding (FSW) is based on extreme plastic deformation in the solid-state where no associated bulk melting is involved [23], [24], [25], [26]. At early stages of the process development, FSW appears especially attractive for joining Al alloys and other light-weight materials like Mg alloys [24], [25], [26], [27], [28], [29], [30], [31]. This is connected with two main reasons:

• the process prevents melting and solidification, minimising residual stresses, cracking, porosity and loss of volatile solutes;

• the plastic deformation (stirring) of such light-weight materials (e.g. Al and Mg alloys) can be realized using relatively simple welding tools (e.g. made of tool steel).

For an application of the FSW process on steels and titanium, an optimisation of tool material and geometry is needed; continuing attempts can be found in the literature [32], [33], [34], [35], [36]. However, FSW application for these materials is still limited by the cost of effective available tools [37].

In the case of dissimilar joints, FSW appears as a suitable and promising process to minimise problems related to materials incompatible with respect to melting temperatures and the formation of brittle intermetallic phases [27], [38]. Recent studies have shown that welds between dissimilar materials such as Mg alloys to Al alloys [39], [40], [41], [42], [43], [44], Mg alloys to steels [45], [46], [47], Al alloys to Titanium [48] and Al alloys to steels [13], [14], [15], [16], [17], [18], [19], [20] can be produced by FSW. The high quality of dissimilar welds produced between Al or Mg alloys to steels or titanium in a butt-joint configuration is associated to the smart idea of positioning the tool pin centre shifted towards the Al or Mg alloys (fixed on the retreating side) [14], [15], [16], [17], [18], [19], [20], [48] or fixing Al or Mg alloys on the top side in the case of lap joints [13], [45], [46], [47]. Thus, the tool barely makes contact with the steel and minimum tool wear occurs, improving the cost efficiency of the process.

In the present study, two grades of high strength steel (HSS) with significantly different microstructure, and strengths were selected to be joined to AA6181-T4 Al alloy by FSW. In order to access the influence of the distinct HSS base material on joint efficiency, the joints were produced by applying the same welding parameters and by shifting the tool pin centre towards the Al alloy. Early investigations were conducted in one of these joints and presented elsewhere [14]. Those studies [14] were focussed on the analysis of the complex material flow based on microstructural observations applying SEM-EBSD technique. Here, we discuss the influence of distinct HSS base material on the joint efficiency and microstructure formation. We show that independent of the HSS chosen the joint efficiency is determined by the heat-affected zone of the Al alloy, which controls the mechanical properties of the joints.