Contents
Chapter 2 Literature Review
2.3 Literature-2: Experimental Investigation
2.3.1 Process Parameters
2.3.1.1 Tool Geometry
The tool pin dimensions are critical to produce sound weld [Dequing and Shuhua, 2004]. Scialpi et al. [2007] studied the influence of shoulder geometry on microstructure and mechanical properties of friction stir welded 6082 aluminum alloy. They found that the tool with fillet and cavity (TFC) crown was the best in terms of crown quality and tool with fillet and scroll (TFS) and TFC showed a higher strength and elongation. Fujii et al. [2006]
reported that for 1050-H24 material a columnar tool without threads was the best in terms of mechanical properties. The tool shape was not significant for 6061-T6. The triangular prism tool and the tool having column with threads were suitable for high and medium rotational speeds. The tool shape was not significant for lower rotational speeds. Boz and Kurt [2004]
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concluded that screw type tool with higher pitch acted like a drill rather than a stirrer and propelled the weld metals outwards. As a result of this weld metal was accumulated towards the stirrer shoulder and therefore the welding process could not be affected. The best bonding was obtained with lower pitched stirrers but it had no effect on mechanical and metallographic properties. The square cross-section stirrer showed poor mechanical and metallographic properties.
Elangovan and other researchers [Elangovan et al., 2008; Elangovan and Balasubramanian, 2008a; Elangovan and Balasubramanian, 2008b] studied the effects of tool pin profile on the formation of FSP/FSW zone in 6061and 2219 Al alloy. They found that square pin profiled tools produced defect-free region. The joints produced by square pin tool showed superior tensile properties. The tool shape was not significant for lower rotational speeds. The square cross-section stirrer showed poor mechanical and metallographic properties. Also in friction stir spot welds of 6061 Al alloy the tensile shear strength increased with increasing probe length [Tozaki et al., 2007], while the cross-tension strength was not affected significantly by probe length. Kumar and Kailas [Kumar and Kailas, 2008b] observed that the pin transfers the material layer by layer, while the shoulder transfers the material by bulk. They also observed onion ring pattern in the weld zone. It is formed due to the combined effect of geometric nature of the pin driven material flow and vertical movement of the material due to shoulder interaction. The FSW of T-joints requires a large amount of heat input due to the necessary forging action aimed to fulfill the joint fillets [Acerra et al., 2010]. A slight increase of tool shoulder diameter permit to obtain sound welds even with larger tool feed rate [Acerra et al., 2010].
D’Urso and Giardini [2010] found that threaded tool was effective compared to plane tool for AA6022-T4 aluminum alloy lap joints. Leal et al. [2011] found that the tools with conical cavity shoulders require higher spindle torques than flat shoulder tools. It had lesser effect on microstructure, hardness and the formation of defects compared to the rotational speed. Bigger tool provides higher joint resistance [Donati et al., 2009], but sometimes high temperatures may be reached in T-joints of AA6082-T6 aluminium alloy. Leal et al. [2008]
found welds with excellent appearance but some reduction in thickness when used tool shoulder with conical cavity. Whereas scrolled tool shoulder gave less smooth weld without reduction in thickness. In case of scrolled tool shoulder, the amount of material moved from
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the advancing to the retreating side of the tool was much higher than that of conical cavity tool.
Out of different considered tool pin geometries in previous studies straight cylindrical tools produced weakest joints compared to other tool geometries [Elangovan and Balasubramanian, 2007; Shanmuga Sundaram and Murugan, 2010]. But due to its lowest fabrication cost and best self cleaning phenomena (increases the tool life and its pin profile effectiveness) it is the most widely used type of TG. Apart from basic straight cylindrical tool pin, many researchers used different types of tool pin geometries in order to investigate their effects on joint properties. These are tapered cylindrical [Liu et al., 2012; Imam et al., 2013; Sharma et al., 2012; Elangovan and Balasubramanian, 2007; Elangovan and Balasubramanian, 2008b], threaded [Boz and Kurt, 2004; Elangovan and Balasubramanian, 2007; Elangovan and Balasubramanian, 2008b; Rajakumar et al., 2011b; Silva et al., 2011;
Liang et al., 2012; Hao et al., 2013], conical threaded [Zhang et al., 2012], square [Elangovan and Balasubramanian, 2007; Elangovan and Balasubramanian, 2008b;
Heidarzadeh et al., 2012; Imam et al., 2013], triangular [Elangovan and Balasubramanian, 2007; Elangovan and Balasubramanian, 2008b], tapered cylinder with grooves [Shanmuga Sundaram and Murugan, 2010], tapered square [Shanmuga Sundaram and Murugan, 2010;
Imam et al., 2013], tapered hexagon [Shanmuga Sundaram and Murugan, 2010], paddle shaped [Shanmuga Sundaram and Murugan, 2010] tool geometry. Among the above used tool pin geometries straight and tapered cylindrical tools are the simplest ones. Then in fabrication complexity and economy point of view the next simple tool pin geometry is threaded cylindrical tools. Rajakumar et al. [Rajakumar et al., 2010; Rajakumar et al., 2011a; Rajakumar et al., 2011b; Rajakumar and Balasubramanian, 2012] found defective joints for a PnD of less than 2.5 mm and greater than 7.5 mm. The images of various FSW tools used by different researchers are shown in Fig. 2.3.
De et al. [2014] studied the tool durability while joining 1018 C-Mn steel by FSW process. They observed an enhancement in tool durability with an increase in either the tool shoulder diameter or the tool RPM. This is due to the enhancement in workpiece temperature. While in other hand a higher tool traverse rate reduces peak temperature, increases stresses on the tool leading in the reduction of tool durability. Similar work was done by DebRoy et al. [2014] during FSW of an aluminium alloy. The tool durability maps
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developed by them are useful in analyzing how the thicker plates and higher tool traverse rates affect adversely to the tool durability. In their review article Rai
about different FSW tool materials and tool geometries (which inclu
shoulder and pin) used for soft as well as hard workpiece material. They have also reviewed the tool wear and deformation during the FSW process. The material adhesion phenomena of various pin geometries are analyzed by Mehta
adhesion, polygonal tool pins lose its functionality and experience severe stresses under specific conditions. But the permanent adhesion of the plasticized material can be avoided by reducing the number of sides of the polygon
[a: Ghidini et al., 2005; b: Gaafer et al., 2010 2009; f:Hui-jie and Hui-jie, 2009; g: Miles
2012; l: Sakthivel et al., 2009; m:
22
developed by them are useful in analyzing how the thicker plates and higher tool traverse rates affect adversely to the tool durability. In their review article Rai et al. [2011] discussed about different FSW tool materials and tool geometries (which includes both the tool shoulder and pin) used for soft as well as hard workpiece material. They have also reviewed the tool wear and deformation during the FSW process. The material adhesion phenomena of various pin geometries are analyzed by Mehta et al. [2014]. They observed that due to adhesion, polygonal tool pins lose its functionality and experience severe stresses under specific conditions. But the permanent adhesion of the plasticized material can be avoided by reducing the number of sides of the polygonal pins
Figure 2.3 FSW tool geometries
, 2010; c: Balasubramanian, 2008; d: Arora et al., 2010; e Miles et al., 2009; h: Thomas et al., 2005; i: Aval et al., 2011 m: Forcellese et al., 2012; n: D'Urso et al., 2009; o: Scialpi
developed by them are useful in analyzing how the thicker plates and higher tool traverse [2011] discussed des both the tool shoulder and pin) used for soft as well as hard workpiece material. They have also reviewed the tool wear and deformation during the FSW process. The material adhesion phenomena ]. They observed that due to adhesion, polygonal tool pins lose its functionality and experience severe stresses under specific conditions. But the permanent adhesion of the plasticized material can be avoided
e:Lammlein et al., , 2011; j,k:-Zhang et al.,
et al., 2007]
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