Typical weld parameters for producing a continuous seam weld in 0.54 mm. thick aluminum alloy sheet lap joints are 24,000 amperes current, 8 spot welds per cm., 20 milliseconds time per weld, 245 kilograms weld force, producing a continuous weld at 1.7 cm per second. One big problem with resistance welding resulting from the required high weld currents is to provide sufficient cooling to the weld electrodes; this problem requires additional study before adapting resistance welding equipment to the space environment for SPS construction tasks. Ultrasonic welding is another promising candidate for joining metallic materials on SPS. In fact, since nonmetallic materials (such as graphite composites with thermoplastic matrices) can also be joined using ultrasonic weld techniques, the possibility of having a single joining process applicable to all materials on SPS is extremely attractive. As defined previously, ultrasonic welding occurs by applying high frequency vibratory energy to the joint while the joint is under moderate low static force. Although the precise mechanism by which ultrasonic welds are produced has not been completely established, it is theorized that very local slip between the workpieces to be joined expell foreign matter, permitting intimate metal to metal contact and joining at temperatures below the normal melting point of the material. The delivered vibratory energy degrades to moderate heat energy which must be absorbed by the workpiece, but apparently the heat plays no significant role in the joining process. The power required to spot weld two metal sheets together is generally given as (Ref.: Welding Handbook): For a 0.051 cm. thick aluminum alloy having a Vickers hardness of 40, approximately 110 joules of electrical energy is required to produce one spot weld by ultrasonic welding techniques. Continuous seam welds are produced by overlapping spot welds for the length of weld required. Summary of Candidate Joining Processes A review of various candidate joining processes for use on SPS construction indicate the following:
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