Characterization of microstructure and mechanical properties of AL6063 using FSP Multipass

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Chapter 1.2 FSP Parameters
FSP/FSW involves complex material movement and plastic deformation. Welding parameters, tool geometry, and joint design exert significant effect on the material flow pattern and temperature distribution, thereby influencing the microstructural evolution of material. A few major factors affecting FSP process, such as tool geometry, welding parameters, joint design are addressed.
1.2.1 Tool geometry
Tool geometry is the most influential aspect of process development. The tool geometry plays a critical role in material flow and in turn governs the traverse rate at which FSW can be conducted. An FSP tool consists of a shoulder and a pin as shown schematically in Fig 1.11. As mentioned earlier, the tool has two primary functions: (a) localized heating, and (b) material flow. In the initial stage of tool plunge, the heating results primarily from the friction between pin and workpiece. Some additional heating results from deformation of material. The tool is plunged till the shoulder touches the workpiece. The friction between the shoulder and workpiece results in the biggest component of heating. From the heating aspect, the relative size of pin and shoulder is important, and the other design features are not critical. The shoulder also provides confinement for the heated volume of material. The second function of the tool is to 'stir' and 'move' the material. The uniformity of microstructure and properties as well as process loads are governed by the tool design. Generally a concave shoulder and threaded cylindrical pins are used.
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1.2.2 Processing parameters
For FSP, two parameters are very important: tool rotation rate (v, rpm) in clockwise or counterclockwise direction and tool traverse speed (n, mm/min) along the line of joint. The rotation of tool results in stirring and mixing of material around the rotating pin and the translation of tool moves the stirred material from the front to the back of the pin and finishes welding process. Higher tool rotation rates generate higher temperature because of higher friction heating and result in more intense stirring and mixing of material as will be discussed later. However, it should be noted that frictional coupling of tool surface with workpiece is going to govern the heating. So, a monotonic increase in heating with increasing tool rotation rate is not expected as the coefficient of friction at interface will change with increasing tool rotation rate.
1.2.3 Tool Tilt angle
In addition to the tool rotation rate and traverse speed, another important process parameter is the angle of spindle or tool tilt with respect to the workpiece surface. A suitable tilt of the spindle towards trailing direction ensures that the shoulder of the tool holds the stirred material by threaded pin and move material efficiently from the front to the back of the pin. Further, the insertion depth of pin into the workpieces (also called target depth) is important for producing sound appearance with smooth tool shoulders. The insertion depth of pin is associated with the pin height. When the insertion depth is too shallow, the shoulder of tool does not contact the original workpiece surface. Thus, rotating shoulder cannot move the stirred material efficiently from the front to the back of the pin, resulting in generation of welds with inner channel or surface groove. When the insertion depth is too deep, the shoulder of tool plunges into the workpiece creating excessive flash. In this case, a significantly concave appearance is produced, leading to local thinning of the processed plate. It should be noted that the recent development of 'scrolled' tool shoulder allows FSP with 0' tool tilt.
1.3 Process modelling
FSP results in intense plastic deformation and temperature increase within and around the stirred zone. This results in significant microstructural evolution, including grain size, grain boundary character, dissolution and coarsening of precipitates, breakup and redistribution of dispersoids, and texture. An understanding of mechanical and thermal processes during FSP is needed for optimizing process parameters and controlling microstructure and properties of welds.
1.3.1. Metal flow
The material flow during friction stir welding is quite complex depending on the tool geometry, process parameters, and material to be welded. It is of practical importance to understand the material flow characteristics for optimal tool design and obtain high structural efficiency welds. This has led to numerous investigations on material flow behavior during FSP. A number of approaches, such as tracer technique by marker, welding of dissimilar alloys/metals, have been used to visualize material flow pattern in FSP. In addition, some computational methods including FEA have been also used to model the material flow.
1.3.2. Temperature distribution
FSP results in intense plastic deformation around rotating tool and friction between tool and workpieces. Both these factors contribute to the temperature increase within and around the stirred zone. Since the temperature distribution within and around the stirred zone directly influences the microstructure of the welds, such as grain size, grain boundary character, coarsening and dissolution of precipitates, and resultant mechanical properties of the welds, it is important to obtain information about temperature distribution during FSP. However, temperature measurements within the stirred zone are very difficult due to the intense plastic deformation produced by the rotation and translation of tool. Therefore, the maximum temperatures within the stirred zone during FSP have been either estimated from the microstructure of the processed zone or recorded by embedding thermocouple in the regions adjacent to the rotating pin.
1.4 Microstructural evolution
The contribution of intense plastic deformation and high-temperature exposure within the stirred zone during FSP results in recrystallization and development of texture within the stirred zone and precipitate dissolution and coarsening within and around the stirred zone. Based on microstructural characterization of grains and precipitates, three distinct zones, stirred (nugget) zone, thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ), have been identified as shown in Fig. 1.12. The microstructural changes in various zone have significant effect on post process mechanical properties. Therefore, the microstructural evolution during FSP has been studied by a number of investigators.