This paper aims to clarify the relationship between a)mean particle velocity at a specific point in a flow field, in which spherical powder with specific diameter range is seeded, measured by cross-correlation particle image velocimetry(PIV)and b)corresponding mean particle diameter, using one-dimensional model. The target spray methods are High Velocity Oxy-Fuel(HVOF)thermal spray and Warm Spray with WC-12wt%Co as powder material. Two distributions of particle diameter and two porosity values of particle are tested. The particle velocity at 0.2m downstream of the barrel exit along the center line is calculated using a principle of cross-correlation PIV along the following steps; 1)twenty particles are distributed within the interrogation and search regions using random numbers for the first frame, 2)velocity of all particle diameters are calculated by one-dimensional model, 3)new particle locations after a specific time period Δ t is obtained for the second frame, 4) brightness distributions for the two frames are obtained, 5)mean distance of particles' movement in the downstream direction,Δ x, is obtained by cross-correlation of two brightness distributions, 6)particles' velocity in the interrogation region is calculated as Δ x/ Δ t. Then, Δ x/ Δ t is compared with the particle velocity calculated by one-dimensional analysis for four mean diameters;number-/length-/area-/volume-averaged diameters. The calculated results show that the arithmetic mean value of lengthaveraged and area-averaged diameters provides closest velocity corresponding to the particle velocity obtained by crosscorrelation PIV method. However, we need to keep in mind that if the particle temperature reaches the melting point of binder material of powder, the particle will disintegrate into smaller pieces, and that the percentage of smaller particles will increase while flying in the gas flow, affecting the experimental value measured by cross-correlation PIV.
This paper proposes application of ultrasonic washing to the evaluation of the strength of cold spray coatings.The ultrasonic washing test exploits the phenomenon that when a polished surface of a cold sprayed coating is subjected to ultrasonic washing,powder particles that are not well bonded come off the surface while leaving those that are well-bonded in the surface.In this study,polished surfaces of copper coatings,fabricated at spray gas temperatures of 200-800℃,were ultrasonically washed in water for various times up to 30 minutes,and the apparent(surface)porosity due to particle detachment was determined by image analysis.For all specimens,the surface porosity increased with increasing ultrasonic washing time but the increase leveled off after about 15 minutes.The final surface porosity at 30 minutes,which was as high as 9% at 200℃,decreased drastically with increasing gas temperature,with values of the coatings sprayed 600 and 800℃ reduced to well below 1%.The final porosity was found to inversely correlate with the tensile strength of the coatings.The ultrasonic washing test may provide an effective means for rapid evaluation of cold spray coatings.