An improved process has been devised for making closed channels (e.g., coolant channels) in a heat-exchanger liner or a similar metal object. The process involves the following steps: First, grooves destined to become closed channels are machined into the object from one surface. Next, a ceramic filler material is troweled into the grooves, made flush with the surface, and allowed to harden by evaporation of water. Next, vacuum plasma spraying (VPS) is used to deposit metal on the exposed metal and filler surface areas to cover the grooves and thereby form closed channels (see figure). Finally, the filler material is removed from the channels. This process satisfies a need for making the channels at lower cost and in less time than is possible by an older process in which the grooves are filled with wax, then a thin layer of copper is electroplated on the exposed metal and wax surface areas, then a thick nickel structural jacket is deposited on the copper.
The success of the process depends on the proper choice of the filler material. A ceramic is attractive for this purpose because, unlike wax, it can withstand the VPS process temperature. The filler is required to be sufficiently fine-grained to enable the deposited metal to have a sufficiently smooth surface [32 µin. (0.8 µm) in the original application for which the process was devised]. The filler is also required to be removable after VPS.
The ceramic filler materials found to be most suitable are mixtures of silica powder with water and various proportions of sodium silicate or colloidal silica, which serve as binders. A third binder made of a cellulose ether compound can be used in combination with either of the other two binders to tailor the viscosity of the overall mixture for both troweling the filler into the channels and removing the filler after VPS.
It is desirable to facilitate and accelerate post-VPS removal of the filler by providing flow paths for flushing the filler out of the channels. For this purpose, before the filler material is troweled into the channels, a yarn or rope made of a polymeric material is placed in the bottom of each groove, extending beyond the ends of the groove. The polymeric material should be either one that decomposes and shrinks at the high VPS process temperature or else one that can be pulled out after hardening of the filler or after VPS. After VPS, the filler can be flushed out of the completed channels; for example, by alternating flows of pressurized water and a hot NaOH solution.
This work was done by D. Andy Hissam and Frank Zimmerman of Marshall Space Flight Center and William M. Davis, Phillip Krotz, Yoon K. Liaw, Peter Oelgoetz, and Heather Sanders of Rockwell International Corp. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Manufacturing/Fabrication category, or circle no. 123 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).