Dr. Livingstone, I Presume WELDWISE 2400 - Manual (Page 14)

0428-INS-400 Rev. E
1-6
Surface Contact
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.
Compare 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 re-
stuck 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
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.
The size, 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 mm
2 . (The area of
a circle is πr 2 : 3 2 *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 mm
2 = 0.36 KA/mm 2 ). 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 mm
2 (3.5 2 *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 mm 2 and 26
Amps per mm 2 is a rather significant 28% reduction in current density! (36 Amps – 26 Amps = 10
Amps difference; 10 Amps is 27.78% of 36 Amps).
By allowing 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