The second function of the electrodes is mechanical. The amount of force
needed to make a good weld varies, depending on the type of metal being
welded and other factors, but a general figure would be about 600-800
lbs. Because electrodes are typically on the small side- roughly from
about the size of an acorn to the size of a plum, it is also important
to choose electrodes that are able to withstand the force needed to
make a good weld.
point to understand is that force and resistance have an inverse relationship:
more force will result in less resistance, and vice-versa. The equation
has to do with surface contact, which refers to the specific area on
the workpieces touched by the electrodes.
contact will be covered further in the next section, but the following
example will begin to illustrate this relationship: if you examine your
fingertip under a magnifying glass, what first appears to be a smooth
surface is actually a mass of rough-looking ridges and bumps. The same
is true of electrodes and workpieces. The tips of the electrodes and
the surfaces of the workpieces may look to be smooth and in good condition,
but in reality their surfaces are quite rough, especially if the electrodes
are old and worn or if the workpieces are dirty.
pressure to these rough surfaces, any microscopic inconsistencies (e.g.,
dirt or grease on the workpiece and/or pits and cracks in the electrodes)
are compressed and the surface actually evens out. This results in improved
(increased) surface contact between the electrode tips and the workpiece,
and between the workpieces themselves. When the surface contact is increased,
current can flow more readily from the tips through the workpieces,
which means that the resistance has been lowered.
also is what helps to keep the weld intact as it's being formed. As
the current generates heat, the workpiece metal begins to melt. A good
analogy to this process is a child eating a popsicle on a hot summer
day. When the popsicle melts, it doesn't remain on the stick-- it drips
everywhere. When metal melts it wants to do the same thing, however
because it's molten metal and not a runny popsicle, it doesn't simply
drip. It explodes out of the workpiece. This is why proper weld force
is so important: it literally forces the molten metal to stay put, so
it can then cool to form a weld nugget.
sufficient force, the metal will do what it wants to do, which is what
causes expulsion. Expulsion is nothing more than little pieces of molten
metal exploding out of the weld because they're not being properly held
in. The problem with expulsion is that all the metal flying out of the
weld is metal that's not going in to the weld; a weld cannot be made
stronger by removing metal from it. Determining the proper amount of
force is entirely application dependent. The RMWA can be contacted for
additional recommendations and guidelines.
Electrodes get considerably hot with 10-20 KA or more repeatedly flowing
under hundreds of pounds of force. Although most welders have an internal
water cooling system that allows water to circulate through the tips
of the electrodes while welds are being made, a common problem is a
lost, damaged or improperly sized cooling water tube. Without anything
to cool off the tips, heat can quickly build up to the point where the
electrodes will eventually weld to the workpieces. To correct this problem,
the water tube should be placed so that the incoming cold water strikes
the hottest part of the tip first, as shown in figure 1-2.
The ultimate goal of the weld process is for the weld current to generate
sufficient heat between the workpieces being welded so that the metal
will melt, fuse together and form a weld nugget. For this to happen,
the surface contact must be maximized. The following experiment may
sound silly, but proves an important point: take a piece of Scotch tape
and stick it to a clean piece of paper. Assuming that the tape was clean
beforehand, it probably sticks very well. Now sprinkle some salt on
the piece of paper. Stick another piece of tape to the paper with the
salt on it. Depending on how much salt is there, the tape probably sticks
somewhat to not at all. Lastly, stick a third piece of tape to some
carpeting, then pull it off. Now try to stick that same tape to the
paper. The third piece probably doesn't stick at all.
the electrodes to the tape and the workpiece to the paper. The clean
tape sticks best to the clean paper, just like well-maintained, clean
electrodes have the best contact with a clean workpiece. The tape sticks
so-so to the paper with the salt on it, just like electrodes will have
a so-so contact with the workpiece if it's dirty, greasy, etc. Lastly,
the tape that has been stuck to the carpet and then restuck to the paper
probably doesn't stick well at all, just like worn or pitted electrodes
don't have very good contact with the workpiece. By maximizing the surface
contact, current density is increased. Both of these factors play key
roles in ensuring that enough heat is generated to reach that ultimate
goal of forming a weld nugget.
Current density describes how much current is being delivered to a specific
area. In other words, it describes the concentration of the current
in a small area of the workpiece- namely, the area where the weld is.
To calculate current density, the amperage (how much current) is divided
by the surface area (area of contact between the electrode and the workpiece).
As a rule, the smaller the surface area, the denser the current. When
the current is denser, the surface area gets hotter and the metal melts
faster. Consequently, a current density that is too high for the application
may cause expulsion. In contrast, a larger surface area delivers a less
dense current. If the current density is too low for the application,
there may be cold welds or perhaps no welds at all.
shape and overall condition of the electrodes affect the surface area
in contact. Small pieces missing from the tips of the electrodes (pitting)
will result in an increased current density due to the decreased surface
area. The same amount of current fired through a smaller surface area
may cause little hot spots that expel molten metal (expulsion), and/or
may result in undersized weld nuggets. Conversely, if the electrode
tips mushroom and get bigger, the current density is lower. For example,
suppose that there are 6-mm round tips on a welder. The area of each
tip is about 28 mm2. (The area of a circle is pr2: 32*3.14 "28). Suppose
the tips deliver 10 kA to a workpiece. Current density equals the amperage
divided by the surface area, so the current density will be 0.36 kA,
or 36 Amps for every millimeter squared of surface (10 kA/28 mm2 = 0.36
kA/mm2). What happens if the tips mushroom to measure 7-mm (about 0.040
inches greater in diameter)? Although one millimeter doesn't seem like
a significant increase, consider what happens to the current density:
The 7-mm tips now have a surface area of about 38 mm2 (3.52*3.14 "38).
Dividing the amperage by the surface area results in 0.26 kA or 26 Amps
for every millimeter squared of surface. The difference between 36 Amps
per mm2 and 26 Amps per mm2 is a rather significant 28% reduction in
current density! (36 Amps - 26 Amps = 10 Amps difference; 10 Amps is
27.78% of 36 Amps).
the electrodes to mushroom only one millimeter bigger, over a quarter
of the current density has been lost, even though the same amount of
current is passing through the tips. Imagine the size of the loss if
they've mushroomed 2, 3, even 4 millimeters! A constant current control
or a weld stepper may be used to regulate the amount of current used,
but a controller or stepper does not track the change in surface area.
So, even though the current is regulated, the current density is overlooked.
Unfortunately, inadequate current density usually produces inadequate
welds. Following proper preventive maintenance schedules can help ensure
sufficient current density by ensuring that the electrodes remain in
in the example above, it is crucial to have the proper current density
at the area where the weld is to be made. Depending on the materials
being welded, however, 'proper' current density is actually a range,
rather than one specific amount. Welding engineers call this range the
weld lobe. Each parameter involved in making the weld (current, voltage,
resistance, etc.) has its own range, or lobe. Quality welds are made
when the weld process stays within the lobe. The next chapter will discuss
weld lobes and tolerancing, which is a way to ensure that the weld process
does not fall outside of the lobe.