Contents
Chapter 2 Literature Review
2.2 Process parameters
Material movement and plastic deformation involved in FSW process is relatively complex (Mishra and Ma, 2005). Welding parameters contribute significantly in governing material flow and temperature distribution in the weld, thereby influencing the microstructural evolution of material. In this section, published research articles are reviewed with the objective to figure out the most influencing process parameters in FSW process.
2.2.1 Tool rotational speed and welding speed
The working principle of FSW process includes plunging of a rotating tool up to a predefined depth called as the plunging depth and then traversing the plunged tool in rotating condition along the line of welding for achieving the desired weld. The heat required for plasticization of material is achieved through the frictional heat generated between the mating surfaces of tool and work piece. This heat is again aided by adiabatic heating due to plastic deformation of the material during the stirring action of the tool inside the work piece material. Thus, tool rotational speed is an important parameter in governing the heat generation or in other words temperature distribution in the weld.
Apart from the tool rotational speed, welding speed governs the amount of heat deposited per unit weld length. This in turn regulates the temperature distribution along
Literature Review
the weld line which contributes towards microstructural evolution of the joints. Many researchers contribute towards investigation of effect of tool rotational speed, welding speed and tool geometry on FSW of different materials and produced suitable welding window for the selection of tool rotational speed for successful FSW.
Effects of tool rotational speed and welding speed on FSW of aluminum bronze joints were investigated by Zoeram et al. (2017). Use of lower tool rotational speed and welding speed was suggested to affect the joint mechanical and microstructural properties due to insufficient heat input. Joining of carbon nano tubes reinforced aluminum-copper-magnesium alloy was attempted by Zhao et al. (2017). The joints properties were evaluated under constant tool rotational speed with welding speed. With moderate range of welding speed the joints were reported to attain 87% of joint efficiency. Fracture and fatigue properties of friction stir welded samples were investigated with varying tool rotational speed and welding speed in the work by Moghadam and Farhangdoost (2016). The experiments were conducted over 2024 aluminum alloy and it was concluded that combination of high tool rotational speed and welding speed results in deterioration of fracture toughness of the joints. Defect formation, material flow behaviour and microstructural evolution of AA5456 aluminum alloy in FSW process was investigated in the work presented by Shirazi et al. (2015).
The investigation results leads to the impression that combination of high tool rotational speed and welding speed results in turbulence in the material flow and leads to formation of internal defects. Effect of tool rotational speed and welding speed on UTS of the joints were investigated in the work by Liu et al. (2012). Results produced reflect a nonlinear behaviour of UTS with these two process parameters with increase up to a certain level of parameters and then following a decreasing trend. The effect of tool rotational speed and welding speed on vertical force was captured by Arora et al. (2010). The conclusions show an increase in vertical force with increase in welding speed and decrease in vertical force with increase in tool rotational speed. The effect of tool rotational speed and welding speed on peak temperature of welds was reported in Rajamanickam et al.
(2009). The investigation reports that with increase in tool rotational speed peak temperature of the joins increases. However, effect of welding speed on peak temperature of the welds was reported to have less significant. Similar conclusive statement was also observed from the investigation carried out by Rezaei et al. (2011) during FSW of AA7075 aluminum alloy. Effect of welding speed at constant tool
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rotational speed on average grain size at the NZ was investigated by Cavaliere et al.
(2008). The investigation reported to observe decrease in grain size with increase in welding speed. Similar trend of decrease in NZ grain size with increase in welding speed was also reported by Zhang et al. (2015) in friction stir welded samples of high strength Al-Zn-Mg-Cu aluminum alloy. On the contrary Liu et al. (2013) observed increase in NZ grain size with increase in welding speed on FSW of AA6061 aluminum alloy. Effect of tool rotational speed and welding speed was also investigated for average NZ grain size by Sharma et al. (2012) for FSW of AA7039 aluminum alloy. The grain size at the NZ was found to have increasing trend with increase in tool rotational speed. Similar conclusion on size of grains at NZ of friction stir welded joints of AA7020 aluminum alloy was reported in Gafer et al. (2010) and Al-Jarrah et al. (2014). Mechanical properties of the joints viz. UTS, yield strength and percentage of elongation of the welded joints for AA2014 aluminum alloy were measured against various combination of tool rotational speed and welding speed by Aydin et al. (2012). All of these properties reported to display a nonlinear behavior with the process parameters. Similar nonlinear behaviour of mechanical properties of the joints was also reported in the work presented by Sakthivel et al. (2009) and Liu et al. (2011). Combination of high tool rotational speed and low welding speed were reported to result in internal defects in the welded samples that deteriorate the weld qualities. Similar nonlinear behaviour of UTS and percentage of elongation of the joints were reported by Hou et al. (2014) and Xu et al.
(2009) for friction stir welded AA6061 and AA2219 aluminum alloys respectively. On the other hand Lim et al. (2004) reported to observe a linear behaviour of UTS and percentage elongation of the joints of AA6061 aluminum alloy with increase in tool rotational speed and welding speed. The effect of tool rotational speed and welding speed on micro hardness of welded samples of AA6082 aluminum alloy was investigated by Wan et al. (2014). The results showed that with increase in welding speed hardness of the joints followed an increasing trend. However, similar decisive conclusion was not reported with tool rotational speed. Efficiency of the joints produced from AA6061 aluminum alloy was reported to increase with increase in tool rotational speed and decrease in welding speed in the work reported by Cui et al. (2013).
The survey of literature on effect of tool rotational speed and welding speed on FSW process revealed that these two parameters have significant influence over the
Literature Review
process. These two parameters govern the process to achieve desired mechanical and microstructural properties of the joints.
2.2.2 Tool geometry
The tool geometry plays a critical role in material flow and in turn governs the traverse rate at which FSW can be conducted. An FSW tool consists of a shoulder and a pin as shown schematically in Fig. 2.1. The FSW 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. Additional heating results from deformation of material during the plasticization. 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 is governed by the tool design. Different researchers have carried out research to evaluate effect of tool geometry on FSW process and available literature are reviewed and presented as follows.
Fig. 2.1 Schematic representation of a conventional FSW tool
Tool geometry of FSW tool includes tool pin length, tool pin diameter, tool pin profile, shoulder diameter, shoulder profile. The parameters associated with the tool are schematically represented in Fig. 2.1. Out of various tool pin profiles developed in FSW process the most primitive one is the cylindrical profile. Various studies reported successful joining of different similar and dissimilar materials with this profile. Better mechanical properties, appreciable joint efficiency and microstructural properties were reported to observe with cylindrical pin profile in FSW process over various materials
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(Mehta et al., 2017; Fujii et al., 2006; Boz and Kurt, 2004). Higher surface area and better mechanical contact with the plasticized materials around the tool pin during the welding process makes this profile one of the most suitable option for FSW process. The modification on the straight cylindrical pin is obtained through producing threads on the pin. Various threaded profiles has been investigated and reported in literature and many have confirmed successful joining with square thread profile (Imam et al., 2013;
Elangovan and Balasubramanian, 2008; Sundaram and Murugan, 2010).Apart from the profile of the pin the other parameter associated with the tool that highly influence the FSW process is the geometry that includes shoulder diameter and shoulder profiles. The process of FSW process was investigated over various ranges and design of shoulder diameter. The common conclusions made in each contribution is the influence of shoulder diameter is maximum on FSW process (Scialpi et al., 2007; Leal et al., 2011;
Rajakumar et al., 2011; Liu et al., 2008; Mehta et al., 2011). Shoulder diameter is mainly responsible for frictional heat generation between the rotating tool and the rigidly clamped workpiece material. This frictional heat softens the material around the tool and the stirring action of the tool help in plasticization of the materials. Thus contribution of shoulder diameter is observed to be more in heat generation during the process after tool rotational speed and welding speed. However, effect of pin diameter is found to be insignificant as reported by various researchers (Tozaki et al., 2007; Sharma et al., 2012;
Mehta et al., 2017; DebRoy et al., 2012). The other parameter associated with FSW tool is the tool pin length. However, the available articles effect of tool pin length reported to be negligible compared to other parameters (Elangovan and Balasubramanian, 2008;
Mishra and Ma, 2005; Gibson et al., 2014; Garg et al., 2017, Mehta and Badheka, 2016).
The survey on effect of tool geometry on FSW process provides the insight that straight cylindrical and square tool pin profile results in better mechanical properties compared to other tool pin profiles. Apart from the tool pin profile, shoulder diameter of the tool also plays significant role in FSW process. However, higher shoulder diameter might results in deterioration of joint quality. On the other hand tool pin diameter does not seem to have significant influence on FSW process.