4 An Application from a Defect—A Friction …
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making a concave weld line and thinning of the plate [36]. The excessive flash lowers
the product quality and necessitates surface smoothing operations.
Root flaws Root flaws occur because of very hot processing conditions under a
constant load. As a result, material gets plasticized severely and the pin penetrates
deeper into the material to maintain the constant load. This consequently leads to the
adherence of the plasticized material with the backing plate [33]. This defect leads
to poor weld quality.
By lowering ω and increasing v, the excessive heat generation can be controlled.
The decrease in PD and also by reducing α, heat generation can be further reduced
to avoid the above-mentioned defects.
4.2.2 Defects Generated Due to Insufficient Heat Generation
Insufficient heat results from very low ω and very high v. The low value of F z also
leads to a reduction in heat generation. The low amount of heat cannot plasticize the
material to its desired value, and hence affects the material mixing and formation of
bonds. This can lead to defects like void, kissing bond, tunnel or wormhole defects,
which are described below.
Lack of fill or void Due to insufficient heat generation, the material is not plasticized
properly, and as a result, the tool is not able to mix the material to diffuse them
uniformly. This results in a lack of fill or voids on the surface of the weld line [36].
This reduces the joint strength.
Kissing bond The reactive materials like Al, Mg form oxide in the presence of air,
and because of insufficient heat generation, oxide layers break into fragments due to
improper mixing and fail to form a bond with the material. The presence of the oxide
fragments in the diffusion zone weakens the bond. This defect is called a kissing
bond defect [36, 37].
Tunnel or wormhole In the absence of required heat generation, and not having
sufficient plunge force from the tool shoulder, the material underneath the shoulder
and around the pin does not get mixed properly, and hence cannot fill the void created
by the pin at the trailing edge. This leads to the formation of tunnel-like void. The
tunnel defect usually occurs inside the weld line and is bigger than the voids that
are normally seen on the surface. The presence of a tunnel severely reduces the joint
strength [37].
By increasing the value of ω and reducing v, the heat generation can be controlled
and the required amount of heat can be obtained for sound welding. Increasing PD
and also by increasing α, contributes further to generate more heat. These parameters
thus should be chosen accordingly to avoid any defects formed due to insufficient
heat generation.
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making a concave weld line and thinning of the plate [36]. The excessive flash lowers
the product quality and necessitates surface smoothing operations.
Root flaws Root flaws occur because of very hot processing conditions under a
constant load. As a result, material gets plasticized severely and the pin penetrates
deeper into the material to maintain the constant load. This consequently leads to the
adherence of the plasticized material with the backing plate [33]. This defect leads
to poor weld quality.
By lowering ω and increasing v, the excessive heat generation can be controlled.
The decrease in PD and also by reducing α, heat generation can be further reduced
to avoid the above-mentioned defects.
4.2.2 Defects Generated Due to Insufficient Heat Generation
Insufficient heat results from very low ω and very high v. The low value of F z also
leads to a reduction in heat generation. The low amount of heat cannot plasticize the
material to its desired value, and hence affects the material mixing and formation of
bonds. This can lead to defects like void, kissing bond, tunnel or wormhole defects,
which are described below.
Lack of fill or void Due to insufficient heat generation, the material is not plasticized
properly, and as a result, the tool is not able to mix the material to diffuse them
uniformly. This results in a lack of fill or voids on the surface of the weld line [36].
This reduces the joint strength.
Kissing bond The reactive materials like Al, Mg form oxide in the presence of air,
and because of insufficient heat generation, oxide layers break into fragments due to
improper mixing and fail to form a bond with the material. The presence of the oxide
fragments in the diffusion zone weakens the bond. This defect is called a kissing
bond defect [36, 37].
Tunnel or wormhole In the absence of required heat generation, and not having
sufficient plunge force from the tool shoulder, the material underneath the shoulder
and around the pin does not get mixed properly, and hence cannot fill the void created
by the pin at the trailing edge. This leads to the formation of tunnel-like void. The
tunnel defect usually occurs inside the weld line and is bigger than the voids that
are normally seen on the surface. The presence of a tunnel severely reduces the joint
strength [37].
By increasing the value of ω and reducing v, the heat generation can be controlled
and the required amount of heat can be obtained for sound welding. Increasing PD
and also by increasing α, contributes further to generate more heat. These parameters
thus should be chosen accordingly to avoid any defects formed due to insufficient
heat generation.