Effect of tool pin profile on microstructure and mechanical properties of AL6063 in Friction stir processing

Friction stir processing (FSP) is a solid state process in which a non consumable stirring (rotating) tool is plunged into a work piece up to half thickness, which causes intense plastic deformation, material mixing, and thermal exposure, resulting in refinement of micro structural properties, enhancement of mechanical properties, and homogeneity of the processed (nugget) zone. The FSP technique has been successfully used for producing fine-grained structure and surface composite, modifying the microstructure of materials, synthesizing composites like metal-metal composites.
The use of FSP generates significant frictional heating and intense plastic deformation, thereby resulting in the occurrence of dynamic recrystallization in the stirred zone (SZ). Although there is still a controversy about the grain-refinement mechanism in the SZ, it is generally believed that the grain refinement is due to dynamic recrystallization. Therefore, the factors influencing the nucleation and growth of the dynamic recrystallization will determine the resultant grain microstructure in the SZ. It has been demonstrated that the FSP parameters, tool geometry, material chemistry, workpiece temperature, vertical pressure, and active cooling exert a significant effect on the size of the recrystallized grains in the SZ.

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CHAPTER 3
EXPERIMENTATION
Friction stir processing (FSP) was carried out using a CNC vertical milling machine set up fitted with a specially designed FSP tool which stirred in vertical axis or perpendicular to workpiece. The base metal (i.e. Al 6063) was fixed in a specially designed fixture that was held on the bed of milling machine. The FSP tool was rotated on the surface of base material. The processed samples was characterized for optical microscopy, micro hardness and impact test. The results were also obtained for base metal (6063Al) for comparison. The worn tracks of the samples were investigated under scanning electron microscope. The details of various equipments, techniques and procedures used in the study are as following.
3.1. Process parameters and procedure All the parameters kept constant during process for all four tool profile as given table 3.1. Table 3.2 show plate dimensions used for FSP with each tool pin profile.
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3.3 Equipments
3.3.1 CNC vertical milling machine
CNC vertical milling machine used for friction stir processing of aluminium 6063 sample plates. After selection of the entire machine parameters next step is experimentation. So for FSP CNC vertical milling machine selected which having pneumatic tool holder. In the CNC vertical milling the spindle axis is vertically oriented. Milling cutters are held in the spindle and rotate on its axis, position of tool is also vertical which is held in pneumatic chuck. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. There are two subcategories of vertical mills: the bed mill and the turret mill. The Operating system of such machines is a closed loop system and functions on feedback. These machines have developed from the basic NC (NUMERIC CONTROL) machines. A computerized form of NC machines is known as CNC machines. A set of instructions (called a program) is used to guide the machine for desired operations.
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Also, various other codes are used. A CNC machine is operated by a single operator called a programmer. This machine is capable of performing various operations automatically and economically.
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While reading this article, along with collecting other information about milling machines, it is crucial for one to understand the computerized form of such machines. In tool holding mechanism, improvement on CAT Tooling is BT Tooling, which looks similar and can easily be confused with CAT tooling. Like CAT Tooling, BT Tooling comes in a range of sizes and uses the same body taper. However, BT tooling is symmetrical about the spindle axis, which CAT tooling is not. This gives BT tooling greater stability and balance at high speeds. One other subtle difference between these two tool holders is the thread used to hold the pull stud. CAT Tooling is all Imperial thread and BT Tooling is all Metric thread. Note that this affects the pull stud only, it does not affect the tool that they can hold, both types of tooling are sold to accept both Imperial and metric sized tools.
3.3.2 Tool holder
Basic function of tool holder is to holds the tool in desired axis in CNC VMC machine. In experimentations pneumatic tool was used. An improvement on mechanical Tooling is pneumatic Tooling, which looks similar and can easily be confused with pneumatic tooling. Like mechanical tooling, pneumatic tooling comes in a range of sizes and uses the same body taper. However, tooling is symmetrical about the spindle axis. This gives tooling greater stability and balance at high speeds. In these experimentations pneumatic tooling used which save much time than mechanical tooling during changing of tools. Following diagram shows simple tool holder used for holding the cylindrical tools.
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3.3.3 Fixture The fixture for holding the base plate, while carrying out FSP was designed in house and fabricated at Dhiman Industries, Bathinda. The fixture consisted of