Summer 2015 (contents):

1. Follow-up Force

2. Laser Welding of Plastics

3. Resistance Welding Electrode Cooling


Follow-up Force

One of the key factors in resistance welding ( Spring 2015 Newsletter) is the welding force (the other two are current/energy and time).  Welding force is applied through the electrodes and the force brings the parts to be welded into close contact.  As the name implies, resistance welding depends on resistance to current flow to generate heat required for welding.  This resistance can come from bulk resistance and contact resistance.  While bulk resistance is function of material properties, contact resistance - resistance across the interface between two surfaces in contact - can be influenced by welding force; higher welding force leads to lower contact resistance and vice-versa.  Both bulk and contact resistance change during the welding cycle.

At the start of the welding cycle, the top electrode comes down and applies pressure on the parts to be welded, which are squeezed between the top and bottom electrode.  Pressure applied based on welding force and electrode area will determine the starting contact resistance.  Once the welding pressure is stabilized, current starts to flow and the parts begin to soften and come into intimate contact at which point the contact resistance drops to practically zero and should remain so through the remainder of the welding cycle.  This is where it can get tricky.  The heating and softening happens in a matter of milliseconds and the mechanical system that is applying the welding force may not be able to move down quickly enough, i.e. follow-up, to maintain the welding force.  Under such circumstances, the effective force decreases and causes a rapid increase in contact resistance.  By now the welding current is at its peak and a combination of high current and high resistance can cause abnormally high heat generation at the interface. Rapid heating can cause uncontrolled melting and results in weld spatter ( Winter 2011 Newsletter)) being spit out of the weld; such an effect can be exacerbated during projection welding and cross-wire welding which provide greater opportunity for weld spatter.  One way to avoid spatter and provide a wider process window is to have a weld head that moves quickly and is able to provide sufficient follow-up force.  As a welding engineer, you should be diligent in working towards improving follow-up force characteristics by using an upper electrode assembly with as low moving mass as possible, a piston/cylinder assembly that has low friction, and a diaphragm/spring that can provide rapid acceleration.  As always, may the force be with you.


Laser Welding Plastics

Plastics that soften and melt on heating (thermoplastics) can be welded with lasers.  As with welding metals, laser light proves to be a very useful non-contact energy source for welding plastics.  There are however some key differences between metals and plastics.  As opposed to metals, plastics are very poor conductors of heat and it is difficult to grow a controlled weld pool.  Plastics also have the potential of decomposing at temperature spikes much above their melting point.  Given the limitations, the common option available is lap welding with laser light shining through the plastic on the top which has to be transparent and the bottom plastic which has to be opaque.  If the plastic on the bottom in also transparent, then an absorbing layer is applied to the interface; can be either carbon black or other proprietary materials.  In addition, the two plastics have to be pushed into intimate contact during the welding process.

Most of the development of this lap welding process, also known as TTIR (Through Transmission Infrared), has been done using conventional welding lasers and direct diode lasers that send out energy near the 1 micron wavelength.  Plastics that are visibly clear are also almost always transparent to the 1 micron wavelength and can be used for lap welding with the aid of absorbers.  However there are applications where clear plastics have to be welded but additives are not acceptable and would have been a limitation for laser welding.  To overcome this limitation, laser sources have been developed that produce light in the 2 micron range which has increased absorptivity in clear plastics, though not 100% since energy still has to reach the interface through the top layer.  The 2 micron laser light can now be focused on the weld interface where its absorption is high enough to generate heat and make a weld without the need for additives; yet another successful application for the ever expanding universe of laser welding.


Resistance Weld Electrode Cooling

Since the welding electrodes are in intimate contact with the work-piece, the electrode tips can get quite hot during high volume production.  In addition to conductive cooling through the body of the electrode and holders, the user often has to resort to additional water cooling; piped through passageways drilled into the holder and electrode.  Even though heat extracted by the cooling water is energy lost, cooling of electrode tips provides many benefits.  Maintaining electrode tips at close to room temperature reduces tip flaring, tip cracking, accumulation of contaminants on the tip, and electrode sticking.  A consistent tip temperature also reduces the need to compensate for changes in tip temperature by adjusting weld parameters to avoid overheating.  Cooled tips can also help in shaping the weld nugget in resistive materials by allowing the nugget to grow along the weld interface but restricting nugget growth towards the surface; limiting such growth helps reduce excessive indentation.  Care should be taken to avoid cooling water temperature being too cold which can lead to precipitation of moisture on tip surface; could have detrimental effect on steels susceptible for hydrogen embrittlement.