Selection of welding process has to be done in conjunction with part design and materials selection. The primary selection criteria would be if the process delivers energy by contact or non-contact manner. Typical non-contact process that provide localized heating include laser welding and electron beam welding. Processes that provide broad-area non-contact heating include induction, convection, radiant heating. Convection and radiation are options for lower temperature heating as in soldering or plastics welding. Induction can be used for soldering, brazing, and forging. Most of the arc processes including TIG, MIG, and Plasma are usually defined as non-contact but they do indeed need an electrical connection to the part being welding for completing the circuit. In comparison, laser welding is truly a non-contact process which can provide targeted energy. Some not-so-common processes such as explosion welding could also be defined as a non-contact process.
Some of the non-contact processes also include adding filler alloy such as soldering, brazing, MIG, and TIG. Filler addition can be of two types: pre-placed filler or filler wire feed. Pre-placed filler provides greater complexity for part assembly and sometimes requires a special part/shim to be manufactured which is then consumed during the weld. Filler washers and rings are quite readily available for solders and brazes; in case of solder/braze addition of a flux will also have to considered unless the process is performed in a protective reducing atmosphere.
On the other hand, contact processes provide a different challenge since how the welding energy is supplied to the weld by direct contact. A good example is resistance welding where the electrodes comes in direct contact with the part. That is good and bad; good in the sense that the welding force pushes the parts together and removes any gaps that might be present, bad in the sense that electrodes have the opportunity to contaminate the surface or even stick to the part surface; welding is always a compromise. Similar issues are faced with other processes such as ultrasonic welding, friction welding, friction stir welding, and roll bonding.
In either case, once the process is selected, the user has to deal with the multitude of technologies/machines available to supply energy; the usual compromise is between price and performance. A basic power supply may be a fourth of the price of a high end unit with lots of features. If you need help at this stage, feel free to give us a call and we will guide you through the maze and help you make the optimum choice.