Page 81 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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shoulder. The tool rotation speed directly influences the heat generated in the
process because the mechanical power input to the tool is given by Equation 2.8,
where P is the power, M the torque and ȍ is the angular speed of the tool.

P M : 2.8

The mechanical power input is dissipated mainly by thermal losses because plastic
work may be neglected. Heat generated in the process is also influenced by the
plunge force because it affects the torque. Heat generated increases with increasing
tool rotation speed and tool plunge force. Travel speed influences the heat input per
unit weld length (specific heat input), affecting metal flow around the probe.
Specific heat input decreases with increasing travel speed, which reduces material
softening in the vicinity of the probe, making plastic flow more difficult. High
travel speeds may cause defects, such as cavities. For low tool rotation speed, low
plunge force and high travel speed external defects may form for welds in some
aluminum alloys. The increase of the plunge force moves defects to the interior of
the weld [52]. The ratio tool rotation speed vs travel speed is sometimes used to
distinguish between hot welds, having high ratio, and cold welds, with low ratio.
Hot welds are less sensitive to defect formation but may exhibit more significant
changes in microstructure and mechanical properties than cold welds in aluminum
alloys.

Other relevant parameters are the time of indentation of the tool and the tool
shoulder angle, besides the other geometric characteristics of the tool referred in
the previous section. The time of indentation of the tool is the period between the
instant the tool contacts the work-piece and the instant the tool begins moving
along the joint. During this period generated heat spreads in the vicinity of the
probe, softening material and stabilizing material flow around the probe. If this
period is too short defects can appear in the initial part of the weld. Time can range
usually from 5 to 30 s. The tool shoulder angle allows a gradual increase of the
pressure on the top surface of the plates being welded and helps to direct the
material flow. Tool angles up to 3º are common.

2.5.4 Process Variants

In the last few years several variants of FSW process have been developed. One of
these variants is thermal assisted FSW in which a heat source is applied in the joint
before the FSW tool, in order to preheat and soften the material [53]. This reduces
welding forces, welding power and tool wear and increases travel speed. This
variant can be useful in welding of steels and other high strength materials.

Another variant is spot FSW, developed for lap joints, that produces spot welds
having higher mechanical strength than those produced by resistance spot welding.
Robotic applications of this process are being developed [54].