Defect features, texture and mechanical properties of friction stir welded lap joints of 2A97 Al-Li alloy thin sheets

Highlights

• Hook defect size mainly varies with the pin length of stirring tool.

• The proportion of LAGBs and substructured grains increases from NZ to TMAZ.

• Weld zones, especially the NZ, have the much lower texture intensity than the BM.

• Lap shear failure load and fracture location of joints is relative to the hook defects.

Abstract

1.4 mm 2A97 Al-Li alloy thin sheets were welded by friction stir lap welding using the stirring tools with different pin length at different rotational speeds. The influence of pin length and rotational speed on the defect features and mechanical properties of lap joints were investigated in detail. Microstructure observation shows that the hook defect geometry and size mainly varies with the pin length instead of the rotational speed. The size of hook defects on both the advancing side (AS) and the retreating side (RS) increased with increasing the pin length, leading to the effective sheet thickness decreased accordingly. Electron backscatter diffraction analysis reveals that the weld zones, especially the nugget zone (NZ), have the much lower texture intensity than the base metal. Some new texture components are formed in the thermo-mechanical affected zone (TMAZ) and the NZ of joint. Lap shear test results show that the failure load of joints generally decreases with increasing the pin length and the rotational speed. The joints failed during the lap shear tests at three locations: the lap interface, the RS of the top sheet and the AS of the bottom sheet. The fracture locations are mainly determined by the hook defects.

Introduction

Al-Li alloys have been widely used in various structural components in the aeronautics and astronautics devices due to their high specific strength and superior comprehensive performances [1], [2]. As the third generation of Al-Li alloys, 2A97 Al-Li alloy has excellent corrosion resistance, superior damage tolerance and high resistance to fatigue crack growth [3]. Moreover, these benefits promote the broad application of 2A97 Al-Li alloy in the field of military industries and transportation. When 2A97 Al-Li alloy is used in industries, joining or welding is an indispensable process for assembling parts and components into units. For aluminum alloys, friction stir welding (FSW) has many benefits and is a preferred welding method compared with mechanical or fusion welding [4], [5]. Lots of fusion welding defects, such as solidification cracks [6], interfacial brittle compounds and gas porosity [7], do not occur in FSW joints due to the lower process temperature than the melting temperature of the welded materials [8]. In addition, the mechanical properties of FSW joints can also reach the similar strength as mechanical joining but with the lower weight and shorter manufacturing period [9].

To date, many scholars have made systematic study on friction stir butt welding of aluminum alloys, copper alloy and magnesium alloys, and got abundant achievements [10], [11]. Besides friction stir butt welding, another important and widely used type of FSW is friction stir lap welding (FSLW). Lap joints are widely used in the assembly of parts in car manufacturing, electrical industries and aircraft structures [12]. In addition, the joining mechanisms of butt welding and lap welding are different owing to the different correlations between the stirring tool and the interface. Generally, the typical microstructure feature for friction stir lap joints is the existence of a geometrical defect, called ‘hook’, originating at the interface of the two welded sheets [13]. Owing to the penetration of tool into the bottom sheet during FSLW, the material moves upward from the lower sheet to the upper one, which leads to the upward bending of sheet interface and the formation of hook defect [14]. Unlike friction stir butt joints, the surface oxide film of sheets remains in the intermediate region of lap joints with a nearly continuous film mode instead of being broken and dispersed particles [15]. The continuous residue of the oxide film will prevent the effective metallurgical bonding and lead to the formation of hook defect, which reduces the mechanical properties of lap joints. Moreover, the weld formation and the relationship between welding conditions and joint performance can also learn from two other solid welding techniques, ultrasonic spot welding and friction stir spot welding [16], [17].

At present, the defect feature during the FSLW has been investigated in the joints of medium and thick plates [12], [13], [15]. The results demonstrated that the welding parameters, such as welding speed and rotational speed, played an important role in the formation of defects. However, for the thin sheets, the influencing factors of defects and the correlation between defects and mechanical properties of joints are still unknown. Theoretically, the effects of defects on the mechanical properties of lap joints even become much greater than medium and thick plates. For FSLW joints of thin sheets, more weld features need to be investigated and analyzed experimentally.

For 2A97 Al-Li alloy, the reinforcing mechanism, microstructure evolution, corrosion behavior and mechanical properties have been investigated extensively. The investigation indicates that 2A97 Al-Li alloy suffer from the disadvantages of high level of texture [18]. In theory, the mechanical properties of joints will be influenced by not only defect but also texture [19]. The strong texture leads to the anisotropy of mechanics [20], which would affect the use of joints. As a regular stirring process, FSLW may affect the formation and distribution of texture in joints due to the complex thermal-mechanical coupling action [21]. Wang et al. [22] investigated the texture evolution of AA5052-O and AA6061-T6 butt joints by EBSD. The results indicated that {001} 〈100〉 cube texture and {123} 〈634〉 S texture in the BM gradually transformed into {111} 〈1(−)12(−)〉 A1shear texture due to the presence of shear deformation. However, there are few public reports on the texture evolution in the FSLW joint of Al-Li alloys, especially 2A97 Al-Li alloy thin sheets with high level of texture.

In order to promote the application of FSLW in the joining of thin sheets, it is necessary to investigate the effects of FSLW process parameters on the formation and scale change of hook defect. Meanwhile, the texture evolution in the lap joints of 2A97 Al-Li alloy thin sheets should be investigated in detailed. In the study, we aim to produce FSLW joints of 2A97 Al-Li alloy thin sheets. The hook defect distribution, texture evolution and mechanical properties of joints were characterized, respectively. The effects of pin length and rotational speed on the distribution and size of hook defects were investigated in detail. The study contributes to the understanding of the mechanical properties of FSLW joints of thin sheets from the defects and texture evolution aspects.