Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Abstract

Stationary shoulder friction stir lap welding (SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side (RS) and the advancing side (AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.

Introduction

As a solid-state joining technique, friction stir welding (FSW) was invented in 1991 [1]. After decades of development, various metals in different welding structures have been successfully joined by using FSW [2], as it avoids key defects normally found in fusion welding, and creates joints with excellent mechanical properties.

In conventional FSW, a tool consisting of a probe and a shoulder was commonly used. In general, the diameter of the shoulder is about three times bigger than that of the probe. However, this type of tool is associated with several issues. One is the significant through-thickness temperature gradient because the heat generated by the shoulder is much higher than that by the probe, with the peak temperature developing at the top surface, which affects joint microstructure and properties [3]. Another issue is the generation of the flash and arc corrugation because some plastic material moves out of the weld [4].

As a consequence, stationary shoulder friction stir welding (SSFSW) was developed, where the tool is characterized by a rotating probe and a non-rotating shoulder [5]. The probe contributes to the total frictional heat, allowing for a nearly linear heat input in the joint thickness direction. The non-rotating shoulder constrains the plastic material to move out of the weld and eliminates the flash and arc corrugation to form a smooth weld surface [6]. In addition, significant decrease in frictional heat produces narrow thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ) than conventional FSW, which improves joint quality.

In some applications, lap structure has been successfully carried out by FSW and SSFSW, and these techniques were classified as friction stir lap welding (FSLW) [7] and stationary shoulder friction stir lap welding (SSFSLW) [8]. FSLW has been employed for joining similar and dissimilar alloys [[9], [10], [11]]. As a characteristic feature of the lap joint, the formation of hook defect is ineluctable, and is always related to welding parameters.

Zhou et al. [12] reported that with increasing rotation speed, the hook height increases. Liu et al. [13] identified that the hook increases constantly in size with increasing rotation speed, and decreases with increasing welding speed. Lin et al. [14] reported that the size of the hook strongly affects joint strength. Badarinarayan et al. [15] found that as the distance from the tip of the hook to the weld surface decreases, mechanical properties reduce, because increasing hook size cuts down the effective bearing thickness. Salari et al. [16] concluded that hook height was a quality criterion for the joints. Yue et al. [17] found that the hook is a weak zone, and the crack initiates at the hook tip.

The attention of the hook formation has acquired from researchers over the years. Yin et al. [18] proposed that the driving force resulting from the shoulder and probe penetration forces the lower workpiece material to move upwards, thereby creating the hook. Dubourg et al. [19] suggested that the hook defect originates from the upward bending of the interface between the upper and lower workpieces.

In general, the temperature and strain of the interface affect directly the hook morphology during welding, which in return impact joint strength. As reported by Yang et al. [20], the suppression of hook defects can be achieved by reducing the welding heat input. Li et al. [8] found that SSFSLW reduces the hook height and increases the bond width and mechanical properties of joints due to 30% reduction in heat input provided by SSFSLW in comparison to FSW.

As the main factor affecting hook, further understanding of the interface behavior is necessary. Yet, there are few studies that illustrate the variations of the interface temperature and strain during SSFSLW. Besides, the published papers on SSFSLW rarely reported in detail the effect of the welding parameters on the morphology and distribution of alclad layers in weld zone. Thus, SSFSLW was applied to join 2024 aluminum alloy using different welding speeds in the present study. The morphology and distribution of alclad layers were investigated. In addition, a coupled Eulerian-Lagrangian (CEL) model of SSFWLW using the ABAQUS software was developed to understand the material flow behavior, and the variations in temperature field and strain on the bonding interface.