Longevity ARCMATE 205 - Manual (Page 29)

160/205 MIG Welder/ARC Welder
Page 28 of 37
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held the same. As the carbon dioxide content increases over 20%, spray transfer GMAW
becomes increasingly problematic, especially with smaller electrode diameters.
Argon is also commonly mixed with other gases, oxygen, helium, hydrogen, and nitrogen. The
addition of up to 5% oxygen (like the higher concentrations of carbon dioxide mentioned above)
can be helpful in welding stainless steel, however, in most applications carbon dioxide is
preferred. Increased oxygen makes the shielding gas oxidize the electrode, which can lead to
porosity in the deposit if the electrode does not contain sufficient deoxidizers. Excessive oxygen,
especially when used in application for which it is not prescribed, can lead to brittleness in the
heat affected zone. Argon-helium mixtures are extremely inert, and can be used on nonferrous
materials. A helium concentration of 50%–75% raises the required voltage and increases the heat
in the arc, due to helium's higher ionization temperature. Hydrogen is sometimes added to argon
in small concentrations (up to about 5%) for welding nickel and thick stainless steel material. In
higher concentrations (up to 25% hydrogen), it may be used for welding conductive materials
such as copper. However, it should not be used on steel, aluminum or magnesium because it can
cause porosity and hydrogen embrittlement. Additionally, nitrogen is sometimes added to argon
to a concentration of 25%–50% for welding copper, but the use of nitrogen, especially in North
America, is limited.
Shielding gas mixtures of three or more gases are also available. Mixtures of argon, carbon
dioxide and oxygen are marketed for welding steels. Other mixtures add a small amount of
helium to argon-oxygen combinations, these mixtures are claimed to allow higher arc voltages
and welding speed. Helium is also sometimes used as the base gas, with small amounts of argon
and carbon dioxide added. However, because it is less dense than air, helium is less effective in
shielding the weld than argon– which is denser than air. It also can lead to arc stability and
penetration issues and increased spatter, due to the much more energetic arc plasma. Helium is
also more expensive than other shielding gases. Other specialized and often proprietary gas
mixtures claim even greater benefits for specific applications.
The desirable rate of gas flow depends primarily on weld geometry, speed, current, the type of
gas, and the metal transfer mode being utilized. Welding flat surfaces requires higher flow than
welding grooved materials, since the gas is dispersed more quickly. Faster welding speeds, in
general, mean that more gas needs to be supplied to provide adequate coverage. Additionally,
higher current requires greater flow, and generally, more helium is required to provide adequate
coverage than argon. Perhaps most importantly, the four primary variations of GMAW have
differing shielding gas flow requirements—for the small weld pools of the short circuiting and
pulsed spray modes, about 10 L/min (20 ft³/h) is generally suitable, while for globular transfer,
around 15 L/min (30 ft³/h) is preferred. The spray transfer variation normally requires more
because of its higher heat input and thus larger weld pool; along the lines of 20–25 L/min (40–
50 ft³/h).