1. Temperature Control in Resistance Welding
2. Laser Soldering
3. Nitrogen Shielding Gas?
Temperature Control in Resistance Welding
One of the unique aspects of resistance welding (Spring 2015 Newsletter) is that the weld interface is not visible and it is difficult to figure out if the interface has become hot enough to form a weld. In contrast, other processes such as TIG, MIG, Laser, and E-beam, the weld puddle provides visible feedback. During weld development in resistance welding, the user has to conduct trials to estimate the interface temperature based on weld strength results. Such development is complicated by the fact that bulk resistance in metals increases with temperature which can lead to a weld temperature run-off ultimately leading weld spatter, or in worst case, a giant flash. Such an experience can leave a novice user all shaken up while pondering the next move.
Fortunately, power supplies with feedback controls allow for an approximate control of temperature (without actually measuring the temperature) at the weld interface by proper programming of the weld pulse. In the current control mode, temperature run off can be controlled by introducing a downslope at the end of the pulse; a gradual reduction in current compensates for the increase in bulk resistance, and can produce conditions where the weld temperature remains stable for long enough time to form a weld. Another option is to use voltage mode which maintains programmed voltage across the electrodes while the welding current changes as required. Voltage mode is most suitable for controlling weld temperature as the process is self-compensating; as the bulk resistance increases, the weld current decreases to maintain programmed voltage by following Ohm’s law (V=IR), and prevents run-off by limiting heat generated by resistive heating (H=I2Rt). Temperature control in voltage mode can be further tweaked by introducing a down slope at the end of the pulse. One can also use the power control mode that essentially splits the difference between voltage and current control modes; power control comes in handy when welding high volume production with tungsten/moly electrodes that can get hot over time. Some advanced power supplies also allow for a combo mode where the initial portion of the pulse is in voltage mode and the latter portion is in current mode; switching between modes is triggered by preset current and/or time limits. In addition to the good old Ohm’s law, other factors such as part size, electrode conductivity, and changes in contact area affect the interface temperature. A good understanding of various control modes and their effect on weld temperature can help you design a robust process without unnecessary fireworks.
Laser soldering is a non-contact process that uses a beam of laser light to produce the heat required to form the solder bond. A beam of light is aimed towards the solder location to heat the two parts to be soldered. After a programmed amount of time, the solder is introduced, usually in the form of wire feed but can also be in the form or solder drop. Flux introduced through the wire or in the paste goes to work to remove any surface oxides and contaminants. In the meantime, the solder starts to melt and flows to wet both parts to be soldered. At the end of the cycle, the laser is turned off and the solder cools to form a bond.
Like any process, laser soldering has its challenges. In contrast to using a soldering iron, where heat is transferred through physical contact between the soldering iron tip and the parts, the laser generates heat by absorption of the laser light at the surface. Since laser energy absorption depends on surface characteristics, the user has to make sure the surface color and texture is absorptive and consistent. In contrast to the soldering iron, where maximum temperature is controlled by tip temperature, laser soldering can cause excessive rise in temperature if the part surface is darker than usual. Too high a temperature rise can cause internal cracking in the underlying circuit boards which may not be visible. On the other hand, a surface that is more reflective than normal can cause under-heating and a potentially a cold solder joint. In addition to heating, incident laser direction is also an important factor since user has to worry not only about energy absorption but also about reflection (a large fraction of laser energy is actually reflected) since the reflected energy can damage highly absorptive plastic housings in the vicinity. The user has to carefully gauge the pros and cons of this process before selecting it over other options.
Nitrogen Shielding Gas?
Nitrogen does not come to mind when you think of inert shielding gases; conventional wisdom suggests Argon or Helium. The main goal of using an inert shielding gas to protect the molten metal from oxygen contamination. Usually Nitrogen is also considered reactive enough to interfere with the welding process and is best avoided, especially with arc welding processes such a TIG/MIG/Plasma where the arc is hot enough to ionize the gas molecules. Fortunately, during laser welding with YAG or fiber lasers, shielding gas ionization is not a major issue. Secondly, and strangely enough, molten Aluminum does not react with Nitrogen. Combination of the two factors allows for use of Nitrogen as shielding gas during laser welding of aluminum. With the rise of the Li-ion battery industry, laser welding of Aluminum is increasingly common, and using Nitrogen instead of Argon can significantly reduce manufacturing costs. Just make sure that the Nitrogen is dry as any moisture contamination will produce porosities in Aluminum welds.