Nozzle clogging in cold spray of aluminum 6061 through 304 stainless steel split nozzles: Mechanisms, observations, and quantitative measurements

Diverging section nozzle clogging is an issue in high pressure cold spray that can limit spray-ability and process parameter selection for certain powder-nozzle material combinations. This phenomenon is not well understood or well-characterized due to the difficulty and cost required for experimentation. An experimental framework for quantitative evaluation and analysis of nozzle clogging in cold spray is presented for aluminum 6061 powder particles in 304 stainless steel split nozzles. Backscatter scanning electron microscopy was used to characterize nozzle wall deposits at various locations in the diverging section of the split nozzles and compared for different particle size distributions, powder flow times, and for a misaligned injector tube. Experimental data are presented and mechanisms for initiation and progression of nozzle clogging in cold spray are proposed. Particles impact the nozzle wall which leaves residual particle material adhered to the nozzle wall. Subsequent impacts lead to additional transferred particle material and the formation of agglomerates on the nozzle wall. There was a measured difference where a particle size distribution of 20–37µ m showed less overall deposited material and different agglomerate geometry in the latter half of the diverging section when compared to distributions with larger particle sizes. A misaligned injector showed more overall deposited material near-throat. Average particle speeds were measured and analyzed. Dynamic impact models were built and loaded with gas-dynamics model predictions at three different diverging section locations and three different impact angles. The impact models were used to explain percent area results related to particle size and injector offset.