Characterization of microstructure and mechanical properties of AL6063 using FSP Multipass

The need for low weight and high performance structural materials has revolutionized the technology and has led to the emergence of new processes and methodologies. Friction stir processing (FSP), based on the principle of friction stir welding, is an emerging solid state metal working process. This technique causes intense plastic deformation and high strain rates in the processed material, resulting in precise control of the microstructure through material mixing and densification. FSP process has been successfully used for achieving significant grain refinement and enhancement of surface properties. The present work focuses on the study of behavior of Aluminium cast alloy (Al-6063) processed by the friction stir processing technique. Samples of FSP-ed aluminium were examined and their microstructures, microhardness, Rockwell hardnesss and impact strength were studied and compared with base metal Al-6063. Hardness tester was employed to evaluate the interfacial bonding between the particles and matrix by indenting the hardness with the constant load and constant time. Impact test was employed to know the Impact Strength of samples against the Impact of Hammer.

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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 precipit