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Short Circuit Transfer for GMAW

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On June 14, 2001, Bill Polhemus wrote:

I am reviewing a set of notes on an existing structural drawing
(I like to look at what other engineers have felt were important
to include in the general notes. I nearly always learn
something).

I have come across one that has me puzzled: "Short circuit
transfer for the Gas Metal Arc Welding process is not
permitted."

-------- end of partial quote --------

Bill,

Gas shielded metal arc welding (GMAW) is a versatile process
that has three basic modes of operation, or metal transfer
across the arc:  short circuiting, globular, and spray.  Because
GMAW uses no flux, there is little if any slag on the finished
weld.  This is an important advantage, especially in a
production environment with automatic or robotic equipment. 
When used as a manual process, GMAW has the advantage of extreme
versatility.  Without changing the size of the welding wire, a
skilled welder can switch from 16 gauge sheet metal, using short
circuiting transfer, to heavy plate, using spray transfer,
merely by adjusting the voltage and wire feed rate.  Glubular
transfer and spray transfer work well on heavy steel, but not
for all welding positions.  Short circuiting transfer works well
for all-position welding, but it can be tricky on heavy steel,
especially for inexperienced welders.

With most manual welding processes, as the welder develops his
skill through experience, a weld that looks better generally is
better.  With short circuiting GMAW, however, this may not be
true.  Unlike with other processes, with short circuiting GMAW
it is deceptively easy to lay down a weld that looks good but
really isn't, and the most likely failure mode is lack of fusion
between the weld metal and the base metal.  Furthermore, when an
unskilled GMAW welder has trouble with the appearance of his
weld, the natural response may be to turn down the heat, which
increases the likelihood of poor or no fusion.  All of this is
reflected in the AWS D1.1 welding code, which requires
qualification testing of all welding procedures for short
circuiting GMAW.

It is possible to weld heavy steel with short circuiting GMAW,
but the required welder proficiency tends to be higher than for
other processes, and the productivity tends to be lower.  Just
by changing the welding wire, the same equipment can be used for
gas shielded flux cored arc welding (FCAW-G), which is an
excellent and productive all-position process for heavy steel. 
With FCAW-G, welds that look better generally are better.

By prohibiting the use of short circuiting GMAW for heavy
structural steel, the specifier reduces the hassle of
implementing a QA/QC program to verify the performance of a
contractor who may not know what he is doing, since he
apparently lacks the good sense to pick a better process for
heavy structural steel.

My opinions here are based on first hand experience.  I started
welding in 1972, and I maintain current certifications that are
recognized by building officials throughout the state of
Washington.  My most dramatic weld test coupon failures have
been with short circuiting GMAW.

Best regards,

John V. Loscheider, P.E., S.E.
Loscheider Engineering Company
Renton, Washington
425-255-0126

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