Chapter 29 Shielded Metal Arc Welding 797
Copyright Goodheart-Willcox Co., Inc.
• Incomplete fusion is a weld discontinuity in
which blending does not occur between the
weld metal and joint faces or adjoining weld
beads. This reduced fusion results when the
base material or previously deposited weld
metal is not melted during the welding process.
Incomplete fusion can occur at any location
within the weld joint and occurs in both fi llet
and groove welds. It often is found on one leg
of a fi llet weld. Incomplete fusion is caused
by an incorrect welding angle that unevenly
distributes heat between the two sides of a joint.
It also may be caused by oxides or other foreign
material on the surface of the base material.
Proper cleaning, correct welding techniques,
and adequate amperage go a long way toward
achieving excellent fusion.
• Incomplete joint penetration is the failure of
the fi ller metal or base metal to fi ll the weld
root completely, Figure 29-34. Common causes
are a bad groove weld design or a fi t-up that
is unsuitable for the welding conditions.
Incomplete joint penetration can occur if the root
face dimensions are too large, the root opening
is too small, or the included angle of a groove
weld is too narrow. An incorrect electrode
angle or holding the arc too long also results in
insuffi cient penetration of the base metal.
• Unacceptable weld profi les, or poorly shaped
beads, occur when too much or too little fi ller
metal is added to a weld or when the heat is
directed unevenly toward one side of the weld
joint. These discontinuities include improper
contour, undercut, and overlap. Many result in
stress concentrations within the weld. Groove
welds should be slightly convex, rising a little
above the surface of the metal. Fillet welds should
generally be slightly concave, blending smoothly
into each leg of the joint. Undercut is a groove
melted into the base metal adjacent the weld toe,
or weld root, and left unfi lled by weld metal. The
usual cause of undercut is excessive travel speed.
See Figure 29-35. Overlap is a protrusion and
notch in the weld metal beyond the weld toe,
or weld root, which results in a concentration of
internal stresses. Overlap is caused by incorrect
welding technique or insuffi cient current.
Leonid Eremeychuk/Shutterstock.com
Figure 29-33. The uniform porosity of this weld is
likely a result of poor technique.
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Figure 29-34. A fillet weld with incomplete joint
penetration results from failure to incorporate an
adequate amount of the base metal into the bead.
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Figure 29-35. This weld has an undercut on the
top edge and exhibits excessive weld spatter.