Page 48 - Welding Robots Technology, System Issues, and Applications

2.1.3.2 Welding Speed Welding Technology 33
The effect of increasing the welding speed for the same current and voltage is to
reduce the heat input. The welding speed does not influence the electromagnetic
force and the arc pressure because they are dependent on the current. The weld
speed increase produces a decrease in the weld cross section area, and
consequently penetration depth (D) and weld width (W) also decrease, but the D/W
ratio has a weak dependence on travel speed [7]. These results suggest that the
travel speed does not influence the mechanisms involved in the weld pool
formation, it only influences the volume of melted material. Normal welding
speeds are from 100 to 500 mm/min depending on current, material type and plate
thickness.

2.1.3.3 Arc Length

The arc length is the distance between the electrode tip and the work-piece. The arc
length in GTAW is usually from 2 to 5 mm. If the arc length increases, the voltage
to maintain the arc stability must increase, but the heat input to work-piece
decreases due to radiation losses from the column of the arc. Consequently, weld
penetration and cross section area of melted material decrease with increasing arc
length.

2.1.3.4 Shielding Gases

Shielding gases are used in GTAW in order to prevent atmospheric contamination
of the weld metal. This contamination can produce porosity, weld cracking, scaling
and even change in the chemical composition of melted material. Besides shielding
gas also has a large influence on the stability of the electric arc. Gases with low
ionization potential facilitate the ignition of the electric arc and those with low
thermal conductivity tend to increase the arc stability.

Argon is the most used GTAW shielding gas. It has low ionization potential and is
heavier than air, providing an excellent shielding of the molten weld pool.
Furthermore it is less expensive than helium, the other inert shielding gas used in
the process. Argon is used in welding of carbon and stainless steels and low
thickness aluminum alloys components.

For welding thick aluminum work-pieces and other high-conductive materials,
such as copper alloys, helium is recommended because it has higher ionization
potential than argon, needing higher voltage for arc initiation and maintenance, but
producing higher heat-input. Helium or helium/argon (30-80% He) mixtures allow
increased welding speed and improved process tolerance.

Mixtures of argon with up to 5% of hydrogen are frequently used in welding of
austenitic stainless steels. Hydrogen increases arc-voltage and consequently heat-