The Process of Cold Spraying

Discuss about the Microstructural Characterisation of Titanium.

This research paper is about the use of cold praying for titanium metal during 3D printing of car engines. Cold gas spaying involves the modification of the substrate surface to give certain engineering benefits which the substrate cannot provide alone. 3D printing is a technique in which there is joining or solidification of a material by computer control to yield an object of three-dimensional. 3D printing is generally a process of making 3-dimension car engine of the solid type from a digital file. This 3D printed car engine can be acquired through additive processes where the car engine is produced by laying down successive layers of titanium deposits through the process of cold spraying until the car engine is created.

Every layer is noted as a thinly sliced horizontal cross-section of the final car engine. Over the past, there has been growth in the techniques of additive manufacturing have changed rapidly the way of designing, developing and manufacturing new objects such as car engines. This research paper specifically discusses the cold spraying as a method of additive manufacturing of car engines (Alhulaifi, 2013). Cold spraying involved a high rate process of deposition of minerals in which particles of powder are accelerated in a compressed supersonic jet of gas at high velocities, after the impact, the substrate formerly deposited, distort and bond to the surface plastically. The major components of cold spraying system include straying chamber, supersonic nozzle, gas heater, the source of compressed air, powder feeder, and parameters for controlling and monitoring system (Alkhimov, 2010).

The high-velocity gas jet is generated by the application of diverging-converging de Lava nozzle. This procedure is also referred to as a cold gas dynamic spray since the procedure uses the principle of the gas dynamic of nitrogen or helium as an accelerating gas.

In this procedure, a gas of high pressure which may be either nitrogen or helium is supplied and used in acceleration of the powder of particles. The figure below shows the schematics o the process of cold gas dynamic spraying:

A powder feeder of high pressure is used to subject the power in a high stream of pressure of gas. The major component of the divergent-convergent nozzle is the Laval nozzle which is involved in the supersonic acceleration of the gas and provides the particles with the velocity needed to deposit into the car engines. In the nozzle, there are two parts, namely the divergent section and the convergent section. There are two categories of equipment for cold spraying that are commercially available, these include a low-pressure system of cold spraying and high-pressure system of cold spraying. In the process of cold spraying, the particles and gas temperatures continue beneath the temperature of melting of the materials of spray and hence the particles are in solid form, and the formation of coating takes place as a result of the impact of the particles’ kinetic energy (Choudhuri, 2009).

The High-Velocity Gas Jet

The major component of cold spraying is the convergent-divergent nozzle. The gas of high pressure is projected into the back of section for convergent as shown in the figure below:

The nature of compression of the gas enables the operation of the nozzle. At nozzle’s throat, the gas attains sonic condition when it attained the optimum possible rate of mass flow. The section of divergent, the gas continues accelerating to high velocities. As the gas acceleration in the segment of divergent of the nozzle, there is a decrease in pressure and temperature from their initial constant values. There is the acceleration of the particles as they pass through the nozzle acquiring the supersonic gas’ kinetic energy (Douchy, 2010).

The particles velocities in the process of cold spraying are evaluated by the gas pressure, gas temperature, and gas used. An appropriate way of improving the velocity of the gas is using a gas with the low weight of the molecules like helium or increasing the temperature of the gas. Usually, the particles’ velocities escalates with the decrease in the particles size and the low-density particles under similar conditions will attain higher velocity. Nevertheless, this high velocity of minute particles does not essentially promote the cold spray decomposition because of blow shocks exterior to the nozzle (Guan-Jun, 2013).

There is the occurrence of shockwaves during the interaction amongst the particles with the substrate which in this case is the car engine. These shockwaves are due to the supersonic velocity adjustment to the conditions downstream. During the impact of the molecules of gas on the substrate (car engine), there is a variation in momentum and energy and the waves of pressure from an ordinary shock wave. Since the flow of gas is perpendicular to the substrate, the angle of deflection is larger compared to the optimum angle of deflection for an oblique shockwave. This results in the reduction in the gas velocity and also that of the particles entrained. In case the particles are too heavy or too large, there will be no acceleration from the nozzle, however, there will be the deceleration in the zone of bow shock (Helfritch, 2010).

In case the particles are too light and small, they will obtain a high velocity but when they reach the zone of the bow shock, they will be decelerated. By increasing the distance of standoff, there can be a decrease in the effect of the zone of the bow shock and the performance of decomposition can be improved. In case there is an increase in standoff beyond the maximum distance, the efficiency of decomposition is reduced due to the reduced particles velocity (Jodoin, 2009).

Cold Spray of Titanium Deposits in 3D Printing of Car Engines

The process of cold spraying of aluminium and copper has been explored extensively in the past, currently, there has been an increasing curiosity to the engineering and scientific communities regarding the exploration of titanium as well as its alloys. This section evaluates the process of cold spraying of titanium deposits in 3D printing in car engines. There is a layer of barrier in titanium which offers a huge potential for applications related to corrosion resistant such as in car engines. This is the major reason why the titanium is used in the 3D printing of car engines. Titanium is also characterized by bio-inertness such that the coatings of titanium can be utilized in acetabular cup of the vertebral prosthesis and femoral component (Jodoin, 2010).

The probable difficulties of titanium cold spraying are as a result of crystal structure, high oxygen reactivity, and high critical velocity. However, the oxygen reactivity with titanium at high temperatures makes it an issue when straying titanium at an advanced temperature of the gas process. The deposits of cold sprayed titanium can be prepared from both powders of spherical feedstock and powder of angular feedstock. The powder of spherical titanium is generated through the process of plasma atomization or inert gas atomization and the powder of angular feedstock titanium is generated from the procedure of dihydride-hydride. In the process of hydride-dehydride, the titanium is hydrated initially through heating in the hydrogen environment, there is crushing of the compound of brittle titanium hydride to the varieties of sizes desired which is then dehydride in a vacuum through heating (Julio, 2011).

In the process of gas atomization, the molten of titanium metal bath is floated in a water shell cooled with titanium to prevent pollution, and the metal that is molten streams passed the nozzle which is then separated by the use of inert gas torches of plasma atomized and melt the powder in a vacuum. The ability of the spherical powder to flow portrays a behaviour that is superior over powder of angular titanium, which is appropriate for continuous feeding of powder in numerous processes of spraying. However, the route of spraying for the powder of spherical titanium results into a quite expensive end product since a kilogram of the power of spherical titanium is more expensive compared to the angular powder with ranges of the size appropriate from cold spraying the car engine (Meng, 2011).

The deposition efficiency of the cold spraying titanium metal used in 3D printing in car engines can be determined by the ration of the mass of particles of titanium sprayed during 3D printing in car engines to the bonded particles’ mass. The efficiency of deposition of titanium metal on the car engine depends on the parameters of the process and the particles and substrate properties. It has been estimated that the efficiency of deposition of titanium particles of sizes 21 μm on the surface of the car engine substrate is 85 percent which means that 85 percent of the particles sprayed in the surface of the car engine is deposited on the substrate (Moridi, 2014).

Deposition Efficiency of Cold Sprayed Titanium Metal in 3D Printing of Car Engines

The deposit was made by the use of helium as a gas for propelling at the temperature of 400 to 400^oC and pressure of 2.1 to 2.8 MPa. The temperature and velocity of the particles increase lead to in a higher proficiency of titanium deposition when the velocity of the particles is kept constant. Higher temperatures of the particles could promote the zone of adiabatic shear instability and because of the titanium’s low-temperature conductivity, there will be a higher zone of plastic deformation. The coating deposition during cold spraying is critically determined by the speed of the particle and there occurs a velocity of the particles known as critical velocity. Any velocity beyond critical velocity will lead to successful deposition (Normand, 2009).

The estimation on critical velocity can be done from the knowledge of the distribution of particles size, particles velocity, and deposition efficiency and for cold spraying titanium particles of 25μm during 3D printing in car engines, the critical velocity is approximately 690m/s for angular titanium powder. The temperature and velocity of the titanium particles are the two significant factors in the formation of deposit during cold spraying for titanium metal in car engines. Beyond the critical velocity limit, any further increase in the velocity of the particles will result in the reduced porosity of the deposit (Papyrin, 2009).

Cold spraying has high ultra-thick deposits build up potential and the engine manufactured for rapid prototyping for automotive. Titanium which is a high strength material is time intensive and difficult hence there is an abundant concern in generating thick deposit of titanium with suitable mechanical features. The post deposition treatment by the use of heat is applied in minimize internal faults and porosity and to increase the mechanical features of the sprayed titanium during 3D printing of car engines. The technique of heat recovery adopted in the titanium metal 3D printing of car is vacuum heat treatment. Heat treatment of the deposits of titanium leads to grain growth, recrystallization, as well as recovery of the car engine (Pattison, 2009).

Some of the advantages of using cold spraying for titanium metal during 3D printing of car engines include rapid exploration of the coating composition, creation of stress-relieving cracks through the thickness, creation of layers porosity for reduced thermal conductivity, production of a finer-scale two-phase microstructure, the addition of chemical energy to assist in the deposition. However, there are numerous challenges faced when using the cold spraying process for titanium metal during 3D printing of car engine, these challenges are discussed below. One of the challenges facing the use of cold spraying is the lower rate of deposition. When making the sprayed coating solution, it is noted that with 50% deposition efficiency, the rate of deposition is normally lower compared to the powder spray (Rokni, 2017).

Critical Velocity of the Particles for Cold Spraying Titanium Metal in 3D Printing of Car Engines

This means that there will be higher deposition efficiency when using powder spray in titanium metal 3D printing in car engines compared to using col spraying. For a particular quantity of titanium particles, there are five times more materials injected into the torch to process a certain quantity of powder compared to the injection of powder. There is also a challenge of shorter standoff distance when using the cold spraying method. The spray solution needs a shorter distance of standoff from the exit torch to the substrate which in this case is the titanium. This imposes a challenge when coating parts of the car engine that have a complex shape, especially where the torch cannot be made close such as turbine vane cascades and doubles (Sanyangare, 2010).

It has been noted that the distance of standoff required ranges between 4 cm and 8cm in an ideal situation normally half the distance used in the powder spray. In general, longer distance of standoff is possible with more energetic precursors and larger power torches. Another challenge faced when using the cold spraying is that very new composition is a new challenge since every precursor is a new challenge. The physical state sequence that is experienced by the solution precursor affects the ultimate size of particle delivered which is specific to the particular process. There is also a challenge of finding a highly-molarity precursor that is affordable (Schucknecht, 2011).

In case of titanium, an expensive precursor which is composed of ethanol and isopropoxide is used in which the oxide weight fraction is approximately 10%. Also in powder spray, there is some selective loss of a single element in relation to another despite not being a frequent occurring case. It is a feature of this process that within a specific composition an individual can vary the ratios of the component rapidly but with every new composition result into the fresh challenge of looking for an appropriate precursor and making it function. It is normally important to add constituents to the precursor for improvement in the viscosity (Spencer, 2010).

This is also lack of proper diagnostics of the particles for solution spray. The injected droplet in the spray solution normally has a dimension of approximately 20 microns, nevertheless, the ceramic form melted in which the coating is generated is normally is the range of size of one digit micron. The measurement of velocity and temperatures of the particles has not been attained in any instrument. This measurement is very challenging as a result of the huge number of minute particles involved. There have been reported the formation of globular pores and microcracks since the spray coatings are normally produced through melt-quenched splats. This process classically introduces cracks and pores in the coating microstructure which normally results in deterioration of the quality of coating from bulk properties (Torrell, 2014).

Conclusion

Another challenge is that the preferential evaporation of the elements of metals of which the pressure of vapour is high. The ability of dopant introduction and proper composition of the metallic elements are significant to optimization of performance. Elements with higher pressure of vapour have a tendency of evaporating more than other elements. This property is critical for particles with minute sizes hence it becomes more complex when the distribution of the powder size is under consideration (Xiong, 2011).

Conclusion

This research paper discusses the use of cold praying for titanium metal during 3D printing of car engines. This 3D printed car engine can be acquired through additive processes where the car engine is produced by laying down successive layers of titanium deposits through the process of cold spraying until the car engine is created. Cold gas spaying involves the modification of the substrate surface to give certain engineering benefits which the substrate cannot provide alone. 3D printing can be defined as a process by which a material is solidified or joined through computer control to yield an abject of three-dimensional. Titanium which is a high strength material is time intensive and difficult hence there is an abundant concern in generating thick deposit of titanium with suitable mechanical features.

The advantages of cold spraying faster exploration of the coating composition, creation of stress relieving cracks, generation of layered porosity for lowered thermal conductivity, the addition of chemical energy to assist in decomposition, and also the production of fine microstructure with high strength of fracture.  Some of the benefits of cold spraying include reduced cost of repairing a product, the extended lifespan of the substrate, variety of substrate materials, and a extensive range of materials for coating. The challenges faced by the cold prying when used in the 3D printing of car engines include lack of proper diagnostics of the particles for solution spray, every composition poses a fresh challenge, shorter distance of standoff, and lower rates of deposition.

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