Electrodes, Surface Contact and Current Density

Electrodes
Typically made of copper alloys, electrodes actually have three separate functions: to conduct current to the workpieces being welded, to transmit the proper pressure or force to those workpieces to produce and forge a good weld, and to help dissipate heat from the area being welded. To ensure that all three of these functions are executed properly, it is important to regularly maintain the electrodes, keeping them clean and in good condition. A reprint of an RWMA chart describing various types of electrode materials and their different uses may be found on page 11-36 in the Appendices of the pdf or printed version of this manual.

Conducting Current
The first of these functions is purely electrical- fire weld current through the workpiece. Taking into account the relationship among current, voltage and resistance, it becomes important to pay attention to the type of electrodes used. For example, it wouldn't be wise to select electrodes made entirely from a high resistance material, since they would get so hot they'd melt before the current even had a chance to flow to the workpiece. It is also important to make sure that the electrodes are the right size for the application; proper electrode sizing is largely dependent on the amount of force being used on the workpieces.

Transmitting Force
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.

A key 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.

Surface 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.

By applying 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.

Force 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.

Without 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.

Cooling the Workpiece
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.

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 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
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 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).

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 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 good condition.

As proven 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.