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The term "micro welding" refers to the precise placement connective metals and the application of energy to form welds less than 100 micrometers wide. Usually performed under a microscope for accuracy, the actual welding process in some cases exactly mimics normal welding, simply on a much smaller scale. In most high-volume industrial settings, micro welding is automated, requiring little skill on the part of the operator, but some specialty or irregular welds require a highly-trained technician with a very steady hand. Since it is such an exact, complicated process involving specialized tools, most jobs are outsourced to specialists.
Resistance micro welding is the most basic method, and is used to join flat plates of various metals. The two plates are slightly overlapped, and weld electrodes are placed on either side of the overlap. An electrical current is applied to heat the metal to a molten state, and the electrodes channel that current from one to the other, through the overlap. As the metal cools, it forges together to form a permanent bond. The resistance method is used to join tiny seams where the joint must be perfectly flat, such as in medical implants.
Flash micro welding is used for tiny butt joints, such as wires or jewelry, and is done by heating the end of each piece with an electrical current and clamping them together. The process produces a steep temperature gradient along the axis of the piece, so it allows a wide variety of materials and shapes to be joined without deforming. The most versatile of all micro welding operations, it does produce sparks, so is not suitable for flammable or temperature-sensitive materials.
Arc micro welding is used to join two parts of any shape, usually with the help of a filler material that is placed along the weld site to act like glue. An electrical current is applied to an electrode that is guided along the weld site, which heats the base metal and the filler metal until they mix to form a new metal. As the site cools, the parts and the filler are inextricably combined. This method is used to make precise repairs in metal molds and tools, especially when pieces have chipped off or worn down and need to be built back up.
Laser micro welding is normally what is used in automated industrial settings. The laser can be as tightly-controlled as necessary, and can reach into awkward places that other welding methods cannot, heating the metal almost instantly, and requiring no filler material or pressure to create the perfect joint every time. The laser method can also be used to engrave metals, although shiny surfaces may need to be pre-treated to avoid reflecting the laser light.