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2.1 Heat transfer in friction stir welding
2.1.1 Experimental investigation
Experimental investigation of FSW is imperative for development of process in all respect to adept in manufacturing industry. Therefore, different types of experimental setup is developed to weld similar and dissimilar material in different joint configuration. Joint configurations for FSW include square butt [42], T-butt [43], lap [43], multiple lap [43], T- lap [43], pipe joint [44] etc. Most commercial FSW applications use simple butt joint configurations and alternative designs such as T-sections and corner welds are very rarely
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considered. Martin et al [45] developed the welding mechanism which consisted of a rotating pin located in a non-rotating shoulder shaped to the internal corner of the plates to be welded. The shaped shoulder contains the stirred material and slides over the surface of the material during T joint welding. Lammlein et al. [46] conducted experimental studies on pipe joining through FSW. They found that pipe butt joint can be done successfully with full penetration but there were some issues that emerged relating to the tool geometry, secondary heating, system eccentricity and the method of internal support and tool disengagement. In order to produce good penetration, the tool offsets about 6 mm with respect to weld interface. However, the key issues facing in conducting experiment on FSW machine include design of a proper fixture to hold the workpiece, position it correctly with respect to the FSW tool, and support it during welding operation and measurement of forces acting on the workpiece. Baghel [47] developed a fixture that accommodates both backing plate and metal plate to be welded. Fratini et al. [48] also prepared a fixture to overcome the shortcomings of FSW of titanium alloys with particular attention to the choice of the materials and to the cooling systems, both under the back plate and around the tool.
A premature failure of the welding tool can lead to unacceptable weld joint quality and loss of welding productivity. Real time monitoring of thermal cycles, torque and traverse force experienced by the tool in the proximity of the tool are requisite for appropriate design of friction stir welding (FSW) process. Temperature measurements have been successfully made by embedding thermocouples into the weld zone. However, the temperature profile has been captured until the thermocouples were consumed in the stirred zone. In some cases, the thermocouples continue to collect data even after the weld tool has passed. Tang et al. [23] measured the temperature distribution during FSW of Aluminum as a function of the distance from the centerline of the weld, the rotational speed and traversing speed. Edwards et al. [49] the first to use this method of embedding thermocouples to experimentally measure the temperatures in the weld zone during FSW of Titanium.
Also, monitoring the process through the measurement of forces and torques during welding and which can also be used as input parameter to validate numerical models.
Threadgill et al. [50] and Luo et al. [51] demonstrated how these can be measured indirectly by monitoring electrical control signals to the motor provided they have been adequately
Chapter 2 calibrated. In early work by Midling et al. [52] the heat input was determined from the size of the HAZ. This work showed that different shoulder materials produced different heat inputs when welding separately 7108 and AA6082-T6 aluminium alloy. A more sophisticated method reported by Johnson et al. [53-54] used a dynamometer to measure the forces. While this method is far more accurate, care needs to be taken not to overheat or overload the equipment. Another problem with this technique is that the equipment is prohibitively expensive. Reynolds et al. [55] measured the traversing force with the hydraulic cylinder pressure and the torque by the pressure drop across a hydraulic motor.
The torque measurement is reasonably accurate provided the efficiency of the motor is taken into account. Another approach used by Lienert et al. [56] used strain gauges on the tool to measure the welding forces and torques.
Material selection for tool and tool design deeply affect the performance of tools, weld quality and cost. In FSW, the tool remains under severe heating condition during welding and significant wear may result if the tool material has low yield strength at high temperature. Stresses experienced by the tool are dependent on the strength of the workpiece at high temperatures common under the FSW conditions. FSW has been applied to metals with moderate melting points. Initially, FSW was applied primarily to aluminum alloys, which could be easily welded due to the relatively low softening temperatures of these alloys (such as copper, lead, zinc, and magnesium). Steel tools have also been used for the joining similar and dissimilar in different configuration [57-59]. But tool wear was found at high temperature strain condition, which suggests that process parameters can be adjusted to increase tool life. Prado et al. [60] analysis change in shape of tool due to severe tool wear during FSW of Al 6061 z 20% Al2O3 MMC. They suggest that the need for threads in the tools is not required because the tools continued to produce good quality welds even after the threading had worn out and tool had obtained a smooth shape.
In contrast, for a number of years it was difficult to weld ferrous alloys and other high softening-temperature metals due to the lack of suitable tool materials. Until recently, there were no tool materials that would stand up to the high stresses and temperatures necessary for FSW of materials with higher melting points, such as steels, stainless steels, and nickel-base alloys. In 1998, tungsten alloys and polycrystalline cubic boron nitride
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(PCBN) were developed to create FSW tools for use in steel, stainless steel, titanium alloys, and nickel-base alloys. Weinberger et al. [61] analyses the weldability, microstructure and mechanical properties of friction stir welded steel 15-5PH using tungsten based tools. They produced good quality welds on martensitic precipitation hardened steels using a W25Re alloy tool. Park et al. [62] studied the fundamentals of pcBN tool wear during FSW of five types of ferritic, duplex, and austenitic stainless steels. Their results suggest that with increase in the flow stress causes the severe tool wear during FSW of austenitic stainless steels, which results in greater nitrogen pickup in austenitic stainless steel FS welds.
Whereas, Seighalani et al. [63] examine the effect of tool material, tool geometry, tilt angle, tool rotational speed, welding speed, and axial force on the weld quality of titanium alloys.
Due to the excessive erosion, tool material and geometry play the main roles in FSW of titanium alloys. Properties of the resultant welds have been shown to be outstanding.
Although some issues remain (primarily limited tool life with tungsten base tools), FSW has been demonstrated as a technically and economically feasible process in high-temperature materials [64-65]. Zhang et al. [66] studied FSW of commercially pure titanium using a pcBN tool and observed severe tool wear. The debris from the tool reacted with titanium alloy to form titanium borides; both titanium borides and pcBN debris contributed to the grain refinement as well as increase in surface hardness. This situation suggests that a pcBN tool can be utilized for FSW of titanium and its alloys high quality welding.