Material flow and microstructural evolution during friction stir spot welding of AZ31 magnesium alloy

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Abstract

Material flow and local texture evolution during friction stir spot welding (FSSW) of AZ31 magnesium alloy was characterized by varying tool rotation rates. Texture at various locations of the welded region was measured using electron backscatter diffraction (EBSD). Material flow is significantly influenced by tool rotation rate with a conical step spiral pin tool, and FSSW introduces a unique basal fiber texture in the welded region. Results indicate that local texture evolution is dominated by shear deformation through material flow. The tool shoulder applies both shear and compressive deformation to the upper region material; however, the rotating pin introduces only shear deformation to the adjacent material. As the tool rotation rate increases, the effect of both tool shoulder and pin becomes more prominent by introducing a higher degree of basal pole tilt with respect to the initial rolling texture at the periphery of the pin, but less tilt in the upper region beneath the tool shoulder undersurface. The equiaxed fine grain structure in the stir zone appears to result from the twinning-induced dynamic recrystallization and discontinuous dynamic recrystallization.

Introduction

Friction stir spot welding (FSSW) is a solid-state welding technique that is a derivative of friction stir welding, which was developed by TWI, UK in 1991 as a novel method for joining aluminum alloys [1]. As schematically shown in Fig. 1, FSSW incorporates a tool consisting of a shoulder with a protruding pin which is rotated and plunged into the workpiece to a pre-determined depth. After the weld is made, the pin is retracted, leaving a keyhole. The frictional heat generated at the tool-workpiece interface softens the surrounding material, and the rotating pin causes material to flow around it. The forging pressure applied by the tool shoulder and the mixing of the plasticized material results in the formation of a stir zone [2], [3].

Detailed material flow during FSSW is quite complex and not fully understood. Studies so far have shown that tool geometry, welding parameters and material to be welded exert significant influence on the consequent material flow pattern. Su et al. [4] analyzed material flow during FSSW of AA 5754/AA 6111 using a threaded pin tool and indicated a downward material flow in the location close to the pin periphery and an upward material flow within the stir zone away from the pin periphery. Tozaki et al. [5] also provided a similar schematic illustration of material flow during spot welding of dissimilar AA 2017 and AA 5052. Su et al. [6] and Gerlich et al. [7] addressed detailed material flow during dissimilar aluminum alloy spot welding as material underneath the shoulder moved towards the pin root, then moved downward via the pin threads and discharged from the bottom of the threads to the stir zone in a helical vertical rotational flow. Yang et al. [8] studied the material flow during FSSW of AZ31 magnesium alloy using copper foil as a tracer and proposed that material release from the pin provided the intrinsic driving force for the downward motion of the plasticized material.

Magnesium alloys often exhibit strong basal texture components during shear deformation, such as rolling and extrusion [9], [10], [11], [12], [13], [14], [15]. Many researchers have studied texture generation during friction stir welding and processing of magnesium and aluminum alloys, and have linked texture evolution in the stir zone to shear deformation [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Field et al. [16] and Sato et al. [17] pointed out that shear deformation led mainly to material flow along the pin column surface. Park et al. [19] addressed texture in the stir zone of friction stir welded AZ61 magnesium alloy and reported components of a strong accumulation of the basal planes along the surface of the welding pin tool. Woo et al. [25] also reported similar texture evolution in AZ31 magnesium alloy using neutron diffraction and indicated a 90-degree rotation of the basal plane in the stir zone compared to the initial rolled texture. Very recently, Yuan et al. [29] indicated that the synergy of shear deformation and compressive deformation led to texture development in the stir zone of friction stir processed AZ31 magnesium alloy through the interaction of horizontal and vertical material flow. Texture evolution during FSSW has gained more and more interest [30], [31], [32], [33], [34], as it plays an important role in numerical modeling of material flow and mechanical behavior of materials, especially for hexagonal close packed metals, for which limited slip systems are available for room temperature deformation. Microstructural evolution during FSSW of magnesium alloys has not been reported yet. The current paper aims to expand the understanding of the effect of material flow on texture evolution during FSSW of a magnesium alloy and mechanisms of grain structure evolution.

Section snippets

Experimental procedure

Rolled AZ31 Magnesium alloy sheets of 1.25 mm thickness were used for this study. The sheets were sheared to dimensions of 38.1 mm × 25.4 mm. Friction stir spot welds were produced using a RoboStir™ spot welder (Friction Stir Link Inc., Waukusha, WI) with an axial load capacity of 22.2 kN and spindle rotation rates up to 3000 rpm. During the welding, axial force, torque, and time were data logged. The spot welder was operated under position control mode, and the tool rotation rate was varied from 1000 

Cross-section and hooking features of spot welds

Fig. 2 shows the macroscopic cross-sections of friction stir spot welded AZ31 under various tool rotation rates. The friction stir spot welded AZ31 exhibits a profile of the conical pin with a 62° deviation of pin cutting edge to the horizontal line. Though a step spiral pin tool was used, the morphology of the step feature was not retained because the workpiece material stuck to the tool during retraction. Since oxide often exists on the faying surfaces of metals, and FSSW of AZ31 magnesium

Material flow during friction stir spot welding

Material flow induces the formation of hooking and can be partially represented by the morphology of hooking features and further oxide film distribution. During FSSW, material undergoes severe plastic deformation and thermal cycles. As the tool plunges into the sheet material, soft material is transported down by the synergy of the featured pin and tool shoulder, and then it is released and flows upward to form the overall stir zone [4], [6]. This process continues until the tool retracts.

Conclusions

The material flow during friction stir spot welding of AZ31 magnesium alloy is significantly influenced by tool rotation rate. The material under the pin steps and at the periphery of the keyhole flows in a vertically inclined path as a result of material release from both the horizontal and vertical planes. As the tool rotation rate increases, the boundary between the stir zone and TMAZ curves upward and inward, and the stir zone localizes. Friction stir spot welding introduces an intense

Acknowledgments

The authors gratefully acknowledge the support of the National Science Foundation through grant NSF-EEC-0531019.

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