14 ADVANCED MATERIALS & PROCESSES | MARCH 2024 SURFACE ANALYSIS AND COATING EXPOSURE TESTING Anyone who has ever stayed in a condo on the beach has likely noticed that the outdoor air conditioner condenser unit looked rusted, with good reason, as coastal environments are tough on materials. The combination of high-intensity sun, rain, heat, humidity, and seawater can quickly deteriorate an improperly protected metallic or composite surface. When it comes to a reusable rocket, any deterioration of the protective coating system could jeopardize hardware that is planned to be launched multiple times. A small corrosion pit, for example, could act as a fatigue crack initiation site. Much of the author’s work is focused on the analysis of advanced surface preparation of substrates and testing of protective coatings for the materials used in the construction of rockets. Surface analysis is performed using a scanning electron microscope (SEM) in conjunction with energy dispersive x-ray spectroscopy (EDS). Other common analytical methods include x-ray fluorescence (XRF) and Fourier transform infrared (FTIR) analysis. Corrosion testing utilizes a combination of indoor laboratory (i.e., salt spray) and outdoor exposure testing to qualify pretreatments and coating systems. Such testing has played a key role as the world looks to replace non- environmentally friendly cadmium (Cd) plating and hexavalent chromium (Cr+6) conversion coatings Indeed, significant research is being performed at NASA’s Marshall Space Flight Center in additive and digital manufacturing, and the application of those techniques to various materials engineering challenges. FRICTION STIR WELDING Another key technology for modern manufacturing of lightweight rocket tank structures fabricated from aluminum alloys is friction stir welding (FSW). FSW is a relatively straightforward joining technique outlined in Fig. 3 that transforms metals “from a solid state into a ‘plasticlike’ state, and then mechanically stirs the materials to- gether under pressure to form a welded joint”[4]. Following the technique’s invention in 1991, FSW was adopted by NASA for joining the external fuel tank of the Space Shuttle. The technique allowed NASA to eliminate fusion welds, whose shortcomings were problematic from a mechanical integrity standpoint. The benefit of FSW is that it can be used to join two aluminum sections without melting. Also, it is well suited for lightweight aluminum alloys, such as Al-Li 2195. The FSW process has been further refined in recent years to provide aluminum joints that are mechanically sound and suitable for the stresses encountered over multiple flight cycles. A typical microstructure resulting from the FSW process is shown in Fig. 4. REDUCE REUSE RECYCLE Imagine designing and building a fancy car with all the bells and whistles, only to take it on a long trip and then discard it at the destination. That’s a bit of a stretch, but not too far off from the Saturn V launch system used for the Apollo program. Only the capsule came back to Earth and it was not reusable (Fig. 5). Today’s orbital launch rockets incorporate advanced computerized design and modeling, unique manufacturing methods, and coating systems to protect the underlying materials. This allows the rockets to be used multiple times (Fig. 6). They must withstand the repeated mechanical and thermal stresses of launch, reentry, and landing. Because orbital launches occur oceanside for safety reasons, corrosion also comes into play for hardware that is out in the elements awaiting launch, landing on a platform at sea, or being transferred from one coastal location to another. Other hardware, such as reusable composite payload fairings that parachute into the ocean following reentry through the Earth’s atmosphere, must be able to withstand brief immersion in seawater. This is where materials analysis and simulated exposure testing help design for reusability. Fig. 3 — Schematic of friction stir welding[3]. Fig. 4 — A transverse metallographic cross-section showing the typical microstructure of a friction stir welded joint. Etched with Keller’s reagent. Fig. 5 — Apollo capsule.
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