2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
77
resulted in both bending moment and torsional load resulting in shear stress. So, the
selection of tool material and deciding the optimum tool pin geometry is crucial for
efficient welding. Arora et al. [161] reported that tool pin having a smaller length
and larger diameter can withstand higher stress, torsional load, and undergoes less
wear as compared to a longer pin with a smaller diameter. The wear rate of the tool
is correlated with the stress generated due to the forces acting on the tool [162].
Tool welding force (F x ) reduces with the combined effect of an increase in rotational
speed along with a lower plunge depth at a fixed welding speed [163]. As tool rotation
increases, it leads to a higher heat generation rate that increases softening effect while
increasing plunge force resulting in higher friction force on the shoulder that leads
to an increase in wear rate. Increasing welding speed lowers the heat generation per
unit length leading to insufficient softening of the material and hence an increase
in traverse force [72]. Due to this temperature gradient the pin experiences low
temperature at the bottom portion leading to higher flow stress while it is deforming
the workpiece material towards its free end. This results in greater resistance to
the material motion and leads to a higher welding force. Tool shoulder is more
influential in heat production and softening than pin diameter. Therefore, a pin with
a larger length having a fixed pin diameter would need superior tool material for
better functioning without much tool pin wear.
Cui et al. [43] have observed that the tool materials for FSW of steels are an
important concern as it is hard to weld. It needs a specially designed tool with
superior mechanical properties. PcBN and W10–25%Re alloy are the two promising
materials that have good wear resistance, superior strength, and fracture toughness.
Tool material for steel with tapered threaded probe and concave shoulder is mostly
preferable for sound joint and to avoid high wear while processing. Also, the wear and
deformation characteristics of the tools are different for PcBN and WRe alloy. They
are used for welding distinct grades of steel. A tool associated with a chamfer pin
shows improved wear resistance than a helical pin. Also, increasing plunge depth or
providing a pilot hole can increase tool life. Park et al. [164] reported that the PcBN
tool mainly fails due to chemical reactions between the nitrogen, chromium, and
boron elements with iron during the FSW process and tungsten-based tools mainly
fail due to the inclusion of intermetallic in SZ. The wear of tools can be supervised
by proper optimization of input process parameters and the tool shape designs.
Selecting tool material for steel is a critical aspect of the FSW process. The tool
should withstand severe working conditions during the welding process. Also, the
tool material should not undergo any chemical reactions with the base material and
it should retain its high-temperature strength and toughness. The extent of tool wear
was investigated for the FSW process of HSLA-65 steel using a W-25%Re tool
material [43, 165].
Wang et al. [166] analyzed tool wear during FSW of Ti-6Al-4V. Three different
tool materials namely, W-1.1%La 2 O 3 and WC-8%Co and WC-11%Co were studied.
Tool wear classification was done by investigating the pin profile using photographic
method, weight loss measurement, and microscopic observations. Adhesive wear
was the main mechanism of tool wear because of the chemical affinity between
Ti-6Al-4V and WC-Co alloy. W-La 2 O 3 tool outperforms the rest two profiles in
77
resulted in both bending moment and torsional load resulting in shear stress. So, the
selection of tool material and deciding the optimum tool pin geometry is crucial for
efficient welding. Arora et al. [161] reported that tool pin having a smaller length
and larger diameter can withstand higher stress, torsional load, and undergoes less
wear as compared to a longer pin with a smaller diameter. The wear rate of the tool
is correlated with the stress generated due to the forces acting on the tool [162].
Tool welding force (F x ) reduces with the combined effect of an increase in rotational
speed along with a lower plunge depth at a fixed welding speed [163]. As tool rotation
increases, it leads to a higher heat generation rate that increases softening effect while
increasing plunge force resulting in higher friction force on the shoulder that leads
to an increase in wear rate. Increasing welding speed lowers the heat generation per
unit length leading to insufficient softening of the material and hence an increase
in traverse force [72]. Due to this temperature gradient the pin experiences low
temperature at the bottom portion leading to higher flow stress while it is deforming
the workpiece material towards its free end. This results in greater resistance to
the material motion and leads to a higher welding force. Tool shoulder is more
influential in heat production and softening than pin diameter. Therefore, a pin with
a larger length having a fixed pin diameter would need superior tool material for
better functioning without much tool pin wear.
Cui et al. [43] have observed that the tool materials for FSW of steels are an
important concern as it is hard to weld. It needs a specially designed tool with
superior mechanical properties. PcBN and W10–25%Re alloy are the two promising
materials that have good wear resistance, superior strength, and fracture toughness.
Tool material for steel with tapered threaded probe and concave shoulder is mostly
preferable for sound joint and to avoid high wear while processing. Also, the wear and
deformation characteristics of the tools are different for PcBN and WRe alloy. They
are used for welding distinct grades of steel. A tool associated with a chamfer pin
shows improved wear resistance than a helical pin. Also, increasing plunge depth or
providing a pilot hole can increase tool life. Park et al. [164] reported that the PcBN
tool mainly fails due to chemical reactions between the nitrogen, chromium, and
boron elements with iron during the FSW process and tungsten-based tools mainly
fail due to the inclusion of intermetallic in SZ. The wear of tools can be supervised
by proper optimization of input process parameters and the tool shape designs.
Selecting tool material for steel is a critical aspect of the FSW process. The tool
should withstand severe working conditions during the welding process. Also, the
tool material should not undergo any chemical reactions with the base material and
it should retain its high-temperature strength and toughness. The extent of tool wear
was investigated for the FSW process of HSLA-65 steel using a W-25%Re tool
material [43, 165].
Wang et al. [166] analyzed tool wear during FSW of Ti-6Al-4V. Three different
tool materials namely, W-1.1%La 2 O 3 and WC-8%Co and WC-11%Co were studied.
Tool wear classification was done by investigating the pin profile using photographic
method, weight loss measurement, and microscopic observations. Adhesive wear
was the main mechanism of tool wear because of the chemical affinity between
Ti-6Al-4V and WC-Co alloy. W-La 2 O 3 tool outperforms the rest two profiles in