The Friction Stir Welding (FSW) technology has allowed assembling Aluminum alloys as well as all those alloys that cannot be easily welded with traditional welding methods. On the basis of this process they have developed a spot welding application, as possible replacement of riveting and spot welding.
The Friction Stir Spot Welding (FSSW) is a process that has brought notable improvements to the light construction industry. The FSSW process parameters constitute the fundamental terms of the joint implementation, as for instance the tool shape exerts great influence on the mechanical resistance of the weld joint owing to the thermal and mechanical gradients underwent by the material during the process. The importance of such phenomena depends not only on the process parameters (geometry, position, rotation of the tool, plunging depth and force applied to the tool), but also on the nature of the material.
The FSSW process consists of three phases (fig.1):
1. Plunging
2. Stirring
3. Drawing out
The process starts with a tool rotation. A slow plunging inside the machined material is carried out, compressing a pin in a spot before the shoulder gets in touch with the upper surface of the workpiece. Afterwards, the stirring phase allows mixing together the materials of the two pieces. Finally, once reached the desired plunging, the process stops and the tool is drawn out. When the tool plunges inside the sheet metal, the material interface undergoes a bending that is afterwards eliminated by the effect of the cutting efforts generated around the pin. The process conditions, tool plunging and its successive pause essentially determine heat generation and material plasticization. In this article we have studied the effects of the machining parameters (rotation speed and plunging duration) on the microstructure of the FSSW welding for aluminium AA6061-T4. The experimental results indicate that the microstructure and the mechanical properties notably vary according to machining parameters, significantly influencing the joint geometry. The figure 1 shows the sequence of the phases in which the tool gets in touch with the material, into which it successively plunges until when the shoulder compression on the material itself occurs, consequently obtaining the stirring of the two materials. As well known metal materials have a subtle oxide layer that forms on the surface, that oxide is broken when the material comes into contact with the pin of the rotating tool. The particles generated by its breakage are dispersed into the matrix of the material, which performs the metallurgical bonding between the two overlapping sheets of material. Besides, the interface of the oxide film is fragmented into a whole of discontinuous particles owing to the low deformation effort. The areas where the thermal recrystallization occurs, the thermo-mechanically altered zone and the part affected by heat are formed in sequence starting from the hole left by the pin. The process conditions, tool plunging and its successive pause, essentially determine heat generation and material plasticization.
Experimental campaign
The experimental campaign has been focused on the 6061-T4 aluminium sheets with 2 mm thickness. Two different configurations have been welded through Friction Stir Spot Welding. Such samples are indicated in figure 2. Besides, the welds have been analysed by using different tool geometry and with rotation speed of 1200, 1500, 1800 and 2100 rpm, with plunging times corresponding to 2, 3 and 4 s. The weld results are reported in figure 3.
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Analysis of results
The material flow during FSSW was observed by monitoring the distribution of tracing material, by examining the flow model in a weld and analysing the crystallographic structural variation inside the weld itself. The figure 4 shows the transport of the plasticized material in horizontal and vertical direction during the FSSW. When the welding is executed with a threaded pin, the material under the shoulder shows a movement towards the pin base direction (1), then spiral-shaped along the pin surface towards the bottom as result of the direction of the dragging force of the rotation (2). Once reached the extremity of the pin, the material is forced towards the top and the outside, then it comes back to the pin direction according to a helical rotation (3). The material transport process is repeated as the distances from the pin grow, thus creating a stirring zone. It has been demonstrated that the upper and lower sheet metal is incorporated at the pin root, before the stirred material moves towards the bottom, in particular when a threaded pin is used. A cylindrical smooth pin without threading limits the continuous flow of material in vertical direction and the formation of a stirring region around the pin. The experimental examination seems to suggest that the pin rotation is a fundamental prerequisite to develop the stirring zone in FSSW. The contribution of an intense plastic deformation and of an exposure to high temperature inside the stirring zone during FSSW results in the recrystallization and in a development of the structure of this zone, the dissolution and coarsening of precipitates around and in the stirring zone. On the basis of the microstructural characterization of the grains and of the precipitates, three distinct areas have been identified, stirring zone (SZ), thermo-mechanically altered zone (TMAZ), and thermally altered zone (HAZ), as shown in Figure 5a. They have carried out a metallographic examination that has allowed highlighting what has been described, with the goal of analysing the microstructure of the cross section of joints according to process parameters. Concerning traction tests, the experimental results have highlighted how the increase of the rotation speed for the different plunging times corresponds to a resistance rise. When the plunging time is equal to 2s, the percentage of resistance increase is quite small as the rotation speed grows. On the contrary, when the rotation is equal to 3 or 4s, the growth rate is higher than that of 2s. Nevertheless, we can observe how the rotation speed plays a determinant role in the traction test result. This study then points out that good results for welding have been attained with high rotation speed and long pause time. The welding microstructures can be subdivided into four regions, BM, HAZ, TMAZ and SZ, and both a dynamic recrystallazion and dissolution of precipitates into the welding are highlighted. The tool geometries remarkably determine the final welding result according to the process parameters. We remind, in fact, that the tool is the element at stake in that process. The right matching between tool and process parameters determines the final result of the welding.
