Nonmetallic Joining Processes Since the joining requirements for the SPS primarily involve nonmetallic materials, processes for the joining of non-metals (or plastics) generally fall into four common categories: hot gas welding, friction welding, heated tool welding (contact welding), and ultrasonic welding. Of these four categories, only contact welding and ultrasonic welding appear feasible for application to SPS construction. Heated tool welding is essentially a process which could be used to activate "green" tape graphite composite during layup fabrication. The joint is held at activation temperature under slight pressure (with forming rollers or dies, for example), and cooled to maintain the shape and provide the desired joint. Although heated tool welding can possibly be used for activating thermosetting materials (e.g., most epoxies and polyimides), its primary use has been in the joining of thermoplastic materials (e.g., polysulfones) for commercial uses. The heated tool welding is highly successful for joining thin flexible sheets or films (e.g., polyamides). Ultrasonic welding is a fairly new method used for joining plastic parts, and is particularly useful for joining thermoplastic materials. This technique is a thermal bonding method that welds parts with frictional heat generated in the joint by mechanical vibrations at ultrasonic frequency. Typical commercial ultrasonic welding systems for plastics converts 60 Hz electrical energy into 20,000 Hz mechanical energy. Welding between joints is very rapid (fractions of a second), and therefore does not degrade the strength of material adjacent to the weld. Joint design is very critical in bringing the material to melting temperature by optimizing the vibratory energy. One of the many advantages of ultrasonic welding is that completely automatic joining systems can be utilized. A particular advantage for ultrasonic welding on SPS is that metals as well as nonmetals, can be joined efficiently with the process. Metallic Joining Processes As presently conceived, the SPS reference configurations contain a minimum of metallic materials, primarily aluminum alloy for the antenna waveguides. Candidate joining processes for joining aluminum include resistance welding, ultrasonic welding, and perhaps electron beam welding. As previously mentioned, much work has been devoted in the past to developing electron beam welding techniques and equipment for in-space fabrication. One major advantage for using electron beam welding is that the process requires a vacuum environment (such as encountered in space) for its operation. The major disadvantage (for SPS application)
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