shielding-gas-weld-4.html
AGA CW Handbook A4_32130-UK
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5 purpose of shielding gas the
arc
in gas
arc
welding
can be divided into three parts: the ca- thode, the anode and the
arc
plasma region. in mag
welding
, where the filler material forms the positive electrode (anode), the cathode region in the work piece is formed of one or several cathode spots. oxidising gas is required to stabilise these cathode spots, which otherwise tend to wander on the surface of the work piece causing spatter and an uneven weld. 1.2.3 carbon
dioxide
or oxygen? it is most often more economical to use carbon
dioxide
instead of oxygen as the oxidising component of the shielding gas. one of the benefits is better geometry and appearance of the weld compared to argon-oxygen mixtures. this is caused by the differences in the fluidity of the molten pool due to the surface tension and amount of oxidation. when carbon
dioxide
is used instead of oxygen, oxidati- on and slag formation are reduced, which has a beneficial effect on the appearance of the weld and the amount of required post-weld finishing work. another benefit of carbon
dioxide
as the oxidising component is bet- ter penetration, in particular side penetration. this is primarily caused by a higher
arc
voltage and energy transfer and the higher
arc
pressu- re caused by carbon
dioxide
compared to argon-oxygen mixtures. 1.2.4 helium helium (he) is an inert gas, as is argon. helium is used in combination with argon, with an addition of few per cent of carbon
dioxide
or oxy- gen, in the shielding gases for mag
welding
of stainless steels. pure helium or helium-argon mixtures are used as shielding gases in tig and mig
welding
. compared to argon, helium provides wider penetration and higher wel- ding speed due to higher
arc
energy. when helium is used,
welding
is more sensitive to changes in the
arc
length, and the
arc
is more difficult to ignite in tig
welding
compared to argon. the figure illustrates the effect of the carbon
dioxide
content in a shielding gas on material transfer and typical penetration in
welding
structural steels in the spray
arc
region. increase in the carbon
dioxide
content of the gas causes high
arc
pres- sure, improving the shielding effect of the gas and increasing the side penetration, while also increasing the size of the weld bead and the amount of spatter, surface slag and
welding
fume. ar+ ar+ ar+ ar+ 100% co 2 2% co 2 5% co 2 10% co 2 20% co 2 the effect of the carbon
dioxide
content in a shielding gas on mag
welding
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