Nucleate pool boiling enhancement on Ti64 cold spray additive manufacturing coatings

This study elucidates the effectiveness of a metal additive manufacturing technique, cold spray deposition, used to prepare enhanced boiling surfaces consisting of Ti-6Al-4V (Ti64) coatings on aluminum substrates. The process was specially tuned to produce highly inhomogeneous porous Ti64 coatings. By adjusting key cold spraying parameters such as transfer speed of nozzle, temperature and powder feeder rate, reproducible rough coating fabrication was successfully achieved. Coated samples were systematically tested for heat transfer in a pool boiling apparatus using FC-72 as working fluid. Boiling curves of the enhanced cold sprayed surfaces were experimentally measured and compared to bare surfaces used as benchmarks to assess the enhancement. Data analysis showed that some of the tested surfaces exhibited significant enhancements in both Heat Transfer Coefficients and Critical Heat Flux: a maximum increase in the Critical Heat Flux of up to 61 %, and a nearly threefold increase in the Heat Transfer Coefficient at low heat flux, was obtained. By analysing the coatings with a variety of characterisation techniques (e.g. scanning electron microscopy, white light interferometry, etc.), it was found that this enhancement could be attributed to an increase in nucleation sites due to the superficial porosity and high roughness of the surfaces. However, excessively thick coatings may lead to a decrease in heat transfer performance as an excessively thickness of the coating will increase the conductive thermal resistance deteriorating the heat transfer enhancement at high heat fluxes. The best performance was achieved with the thinner coatings. The Ti64 coating developed using the cold spraying technique has high potential for industrial applications where effective boiling heat transfer performance is required. More importantly, the fabrication process has the capability of industrial production to rapidly cover large surface areas at relatively low cost, which is a significant advantage over subtractive manufacturing and other metal additive manufacturing methods.