5 Welding of Dissimilar Metals—Challenges and a Way Forward …
183
configuration, and inability to weld metals with low ductility and toughness, in ultrasonic welding and friction welding methods, respectively, reduce their applications.
FSW has been a potential alternative to these methods, in case of joining dissimilar
materials. This is because of the advantages which have been discussed previously.
5.5 Mechanism of Joining Dissimilar Materials in FSW
IMC formation and mechanical interlocking are the two mechanisms governing FSW.
The IMC formation refers to diffusion-controlled process. In FSW, the IMCs is
formed below the melting temperature of the base metals. During the joining of
dissimilar combination of metals in both configurations such as lap and butt, the
number of IMCs are directly influenced by the kinetics and rate of diffusion of
atom. Both types of IMCs and their thickness influence the joint quality [40]. The
diffusion-controlled process involves three stages: (a) diffusion of atoms in between
the workpieces, (b) formation of solid solutions at the IMC, and (c) growth in its
thickness. The first stage depends upon the diffusion coefficients of the workpieces
[41]. The second stage refers to the formation of solid solutions which is the mixture
of the two workpieces. The growth in the thickness of IMC initiates as the solid
solution in second stage reaches a saturated level in third stage. This thickness S
varies as depicted in Eqs. 5.1 and 5.2 [41, 42].
S = K t
x
(5.1)
K = K 0 exp
−Q
RT
(5.2)
where t is the time of reaction at a specific temperature and pressure which does
change as per the Eqs. (5.1) and (5.2) [41, 42]. Similarly, Q, R, and T refer to the
activation energy in J/mol, universal gas constant in J/K mol, and temperature in K,
respectively. K and K 0 are a constant and pre-exponential factor.
The mechanical interlocking phenomenon in FSWwhich is observed in case of
lap configuration occurs because of the formation of sawtooth and it can be explained
as shown in Fig. 5.13. In the process, metal-1 is considered as the softer material as
compared to metal-2.
It begins with the plunging of the pin into the workpieces (metal-1 and metal2), deforming them plastically. This is followed by rubbing of metal-2 leading to
formation of metal debris which gets projected in the metal-1. This occurs because
of the rotating pin. These debris have the sawtooth shape, as shown in Fig. 5.13. With
further movement of the tool, bonding takes place. The third step: The direction of
metal-1 fills the gap between an activated surface and the rotating pin is shown in
Fig. 5.13. The activated metal-2 surface draws the metal-1 that travels over it and
develops a metallic bond between both the metals. One of such profiles developed
during welding of Al-steel using FSW is shown in Fig. 5.14, where the sawtooth
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configuration, and inability to weld metals with low ductility and toughness, in ultrasonic welding and friction welding methods, respectively, reduce their applications.
FSW has been a potential alternative to these methods, in case of joining dissimilar
materials. This is because of the advantages which have been discussed previously.
5.5 Mechanism of Joining Dissimilar Materials in FSW
IMC formation and mechanical interlocking are the two mechanisms governing FSW.
The IMC formation refers to diffusion-controlled process. In FSW, the IMCs is
formed below the melting temperature of the base metals. During the joining of
dissimilar combination of metals in both configurations such as lap and butt, the
number of IMCs are directly influenced by the kinetics and rate of diffusion of
atom. Both types of IMCs and their thickness influence the joint quality [40]. The
diffusion-controlled process involves three stages: (a) diffusion of atoms in between
the workpieces, (b) formation of solid solutions at the IMC, and (c) growth in its
thickness. The first stage depends upon the diffusion coefficients of the workpieces
[41]. The second stage refers to the formation of solid solutions which is the mixture
of the two workpieces. The growth in the thickness of IMC initiates as the solid
solution in second stage reaches a saturated level in third stage. This thickness S
varies as depicted in Eqs. 5.1 and 5.2 [41, 42].
S = K t
x
(5.1)
K = K 0 exp
−Q
RT
(5.2)
where t is the time of reaction at a specific temperature and pressure which does
change as per the Eqs. (5.1) and (5.2) [41, 42]. Similarly, Q, R, and T refer to the
activation energy in J/mol, universal gas constant in J/K mol, and temperature in K,
respectively. K and K 0 are a constant and pre-exponential factor.
The mechanical interlocking phenomenon in FSWwhich is observed in case of
lap configuration occurs because of the formation of sawtooth and it can be explained
as shown in Fig. 5.13. In the process, metal-1 is considered as the softer material as
compared to metal-2.
It begins with the plunging of the pin into the workpieces (metal-1 and metal2), deforming them plastically. This is followed by rubbing of metal-2 leading to
formation of metal debris which gets projected in the metal-1. This occurs because
of the rotating pin. These debris have the sawtooth shape, as shown in Fig. 5.13. With
further movement of the tool, bonding takes place. The third step: The direction of
metal-1 fills the gap between an activated surface and the rotating pin is shown in
Fig. 5.13. The activated metal-2 surface draws the metal-1 that travels over it and
develops a metallic bond between both the metals. One of such profiles developed
during welding of Al-steel using FSW is shown in Fig. 5.14, where the sawtooth