Tensile properties of friction stir welded joints of 2024 aluminum alloys in different heat-treated-state

Abstract

In this study, the different heat-treated-state 2024 Al-alloys were friction stir welded. The tensile properties of the joints have a tendency to increase with the precipitation hardening of the base material. The peak tensile properties have been obtained in the T6 (100 °C – 10 h) joint. It is observed that the weld zone is strengthened by the friction stir welding process for the 2024-O joint. The fracture regions are detected near the nugget for W joint, the interface between the nugget and the thermomechanically affected zone for T4 and T6 joints and base material for O joint.

Introduction

The welding of aluminum and its alloys has always represented a great challenge for designers and technologists. Aluminum alloys, especially heat-treatable aluminum alloys, are difficult to join by fusion welding techniques, since some welding defects such as crack and porosity are easily formed in the weld during the solidification of the welding pool [1]. Moreover, the conventional techniques often lead to significant strength deterioration in the joint because of phase transformations and softening induced in the alloy.

Friction stir welding (FSW) is a new and promising welding process that can produce low-cost and high-quality joints of heat-treatable aluminum alloys. FSW is well suited for joining aluminum alloys, especially for those usually considered as unweldable such as 2xxx and 7xxx series [2], [3]. Although this joining process is a relatively recent development, it has emerged in many areas of applications like the aircraft, aerospace, railway, marine and automotive industries [4]. FSW is a solid phase welding process in which the metal to be welded is not melted during the welding [1], [5]. This technique involves a non-consumable, cylindrical, rotating tool (usually hardened steel) which moves between the seam of two butted plates and stirs them together. The effect of friction stir welding on the material is both on heat flow and plastic strain. The heat is generated by friction between the tool shoulder and the top of the sheets. However, plastic flow occurs by the rotation of the pin tool. These thermo-mechanical conditions fluctuate within the joint and create a heterogeneous microstructure across the weld. The weld bead across the section shows a region of deep deformation that is often referred to as the nugget zone (NZ) [6], [7]. This zone shows a very fine equiaxed grain structure with high angle boundaries. The temperature reached in the NZ is sufficiently high to encourage the dissolution of the hardening precipitates [2], [3], [8], [9]. The zone to be adjacent to the NZ is referred to as thermomechanically affected zone (TMAZ). Although this zone has not been directly subjected to pin or shoulder action, it has experienced a severe thermo-mechanical alteration due to internal shear stresses. The combination of deformation and high temperature result in low hardness because of the coarsening of the strengthening precipitates [3], [10]. In addition, the dislocation density is relatively high in the grains. In the heat affected zone (HAZ) the hardening precipitates can dissolve or coarsen, and the hardness may show a significant decrease [3], [8], [9] as compared to the base material (BM).

Precipitate strengthened alloys show a decrease of mechanical properties in the weld zone because of the dissolution and growth of strengthening precipitates during the welding thermal cycle [9], [11], [12]. The weld joints produced by FSW have been reported to have joint tensile properties of 80–100% of the base material [13], [14]. Many studies have focused on the mechanical properties of FSW joints of heat-treatable aluminum alloys such as 2017-T351 [13], 2195-T8 [15], 7020-T6 [16], 6061 [3], [17], 6063 [8], [10] and 6082 [18]. The effects of the welding parameters on the FSW characteristics, such as microstructural evolution in the joints [2], [3], [8], [9], [12], [17], [19], [20], [21], thermal cycle histories during welding [22], [23], residual stress distributions in the joints [24], [25], [26], and material flow behavior in the welds [15], [23], [27], [28], [29] have been investigated by many researchers. Only a small number of studies have been involved with the effects of the base material conditions on the FSW behavior, especially the effects of heat treatment states of the same types of aluminum alloys [30].

The present investigation is focused on the tensile properties of friction stir welded joints of different heat-treated 2024 aluminum alloys (W, T4, T6 (100 °C – 10 h), T6 (190 °C – 10 h), and O tempers). The emphasis is placed on the microstructure, microhardness and tensile properties and fracture locations of the joints.