Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84 ; DOI: 10.3221/IGF-ESIS.22.08 70 Moreover, the footprint of the cold spray beam is very narrow typically around 5 mm diameter due to small size of the nozzle (10-15 mm 2 ) and spray distance (5-25 mm), yielding a high-density particle beam, results in precise control over the area of deposition over the substrate surface. This process is similar to a micro shot peening and hence the coatings are produced with compressive stresses, rather than tensile stresses, which results in dense and ultra thick (5-50 mm) coatings without adhesion failure. The low temperature formation of coating leads to oxides and other inclusions -free coatings with wrought-like microstructure [2]. C OLD S PRAY ( CS) SYSTEM he CS system can be designed in either portable or manual and robotic or fixed systems. The gasses having aerodynamic properties are generally used to propel the powder particles, as: 1) Helium 2) Nitrogen 3) Mixture of He and N 2 4) Dry air (79% N 2 - 21% O 2 ) The main components of CS system includes [4]:  Powder feeder (powders used are in the range of 1 to 50 μm in diameter)  Source of a compressed gas  Gas heater to preheat the gas, to compensate for the cooling due to rapid expansion in nozzle  Supersonic nozzle ( Delaval nozzle)  Spraying chamber with a motion system  System for monitoring and controlling spray parameters (to measure and control the gas temperature and pressure) T YPES OF CS SYSTEM uring the practical development of cold spray technology, two methods of injecting the spray materials into the nozzle were patented, leading to what is known today as high pressure cold spray (HPCS) and low pressure cold spray (LPCS) system. The two main distinctions of these two systems are; the utilisation of 5-10 bars pressure gas in LPCS instead of 25-30 bars in HPCS and the radial injection of powder in LPCS instead of axial injection in HPCS [1, 4]. Low Pressure Cold Spray (LPCS) In low-pressure cold spray the accelerating gas, usually air or nitrogen, at relatively low pressure (5-10 bar) and preheated ( up to 550 o C), within the gas heater to optimize its aerodynamic properties, and then forced through a ‘DeLaval’ nozzle. At the diverging side of the nozzle, the heated gas is accelerated to about in the range of 300 to 600 m/s. Solid powder particles are radially introduced downstream of the throat section of the supersonic nozzle and accelerated toward the substrate as shown in the Fig.1of the LPCS system. The feedstock particles are effectively drawn in from the powder feeder by Venturi effect, i.e. by keeping the static pressure within the nozzle below the atmospheric pressure. This is achieved if the ratio of the cross-sectional area of the supersonic nozzle at the powder entry point, Ai (m 2 ) to that of the throat (A*) satisfies following equation: Ai/A*≥1.3Po+0.8 where; Po = gas pressure at the nozzle inlet (MPa) [4]. Due to the elimination of the need of a high pressure delivery system in LPCS, there is improvement in its operational safety, system is more portable, flexible in automation, and spraying cost also reduced significantly than a HPCS system, but the deposition efficiency with this system typically do not exceed 50%. Also in this system the powder particles does not pass through the throat, hence wear of the nozzle walls occurs only in the supersonic portion of the nozzle and, this ensures a longer service life of the nozzle. Additionally, a LPCS system is more compatible with a number of system modifications. High Pressure Cold Spray (HPCS) In high-pressure cold spray, the accelerating gas helium or nitrogen at high pressure (25-30 bar) is preheated (up to 1000 ° C) to optimise its aerodynamic properties (not to increase particle temperature) and then forced through a converging- diverging ‘DeLaval’ nozzle. At the nozzle, the expansion of the gas produces the conversion of enthalpy into kinetic energy, which accelerates the gas flow to supersonic regime (1200 m/s) while reducing its temperature. The solid powder feedstock particles mix with the propellant gas in the pre-chamber zone and are then axially fed into the gas stream, upstream of the converging section of the nozzle at a higher pressure than the accelerating gas to prevent backflow of the carrier gas to the powder feeder as shown in Fig. 2. The accelerated solid particles (600 to 1200 m/s) impact the substrate D