Keywords

1 Introduction

In the cold spray (CS) coating process, the feedstock powder is sprayed with the help of a nozzle to get a high impact velocity or jet towards the substrate upon which coating is to be performed. With this coating process technology number of metals, composite materials, polymers, and ceramics can be coated based upon the applications and requirements. Nowadays, this coating technology is growing in the manufacturing industry by creating a dedicated branch in additive manufacturing called cold spray additive manufacturing, solid-state powder-based deposition process [1,2,3], and cold gas dynamic manufacturing [4].

Decrease of particle size to the nano-level, domination of surface roughness observed parallelly [5], residual stresses increases upon grinding in nanostructured coatings [6]. Powder material heat treatment increases the efficiency of deposition about two times and decreases the porosity about four times [7]. Cladded powders can be developed by covering the alumina particles with a nickel layer using the hydrogen reduction method [8]. Using CS technology compact nanocrystalline grains produced due to impact, moiré pattern was observed due to preserved tamping effect produced by microstructure, and there was no size change found in nano-sized grains [9]. The feasible and simple way for determining particle distribution of cold sprayed coated composites found to be the Weibull distribution method [10].

Using crystal orientation of particles on the coating surface with the help of molecular dynamics simulation, the shape of particles at the bottom edge is found of square-shape, rectangle-shape, and hexagon-shape [11]. The aerosol deposition (AD) method is also a type of CS coating process technique, in which ceramic coating formation and bonding were analyzed practically as well as using CFD simulations [12]. The molecular dynamic simulation gives the results with asymmetric particle deformation at the interface that occurred during cold gas spray [13]. The particle size of 40 µm founds gives high impact velocity as compared to the 20, 60, 80, and 100 µm particle sizes [14].

CS coated surface has compressive residual stresses because of deformation and particle impact at high velocity [15]. The newly developed model based on Johnson-Cook (JC) model can effectively predict the stresses and deformed shape of particles [16]. In CS process technology, the temperature of the powder particles also depends upon the injector length, the higher the injector length, the lower will be the temperature of particles and vice-versa [17]. The CS coating process gives high thermal conductivity and sensible adhesion strength [18]. The surface roughness of sintered alumina in comparison with alumina found to be lower [19] which affects the porosity, hardness, and density in outer and middle regions due to grain size reduction. The wear rate increases with an increase in the load [20]. Laser remelting decreases the porosity of thermal ceramic coatings, a significant effect as compare to as-sprayed coating exhibits upon various mechanical properties like microhardness, elastic modulus, etc. [21]. Crater formation found negligible upon hard metal substrates [22]. CS coating using composite powder gives low porosity and high hardness and fracture-toughness [23].

CS coatings with low temperature (500 °C) give the best corrosion resistance. Post deposition heat treatment improves bonding and porosity [24]. In comparison with uncoated material with aluminium and chromium coated, aluminium coated found to be better oxidation resistance at high working temperature environment [25]. Pulse plasma nitriding is not sufficient to improve corrosion resistance [26]. The pre-heat temperature in CS coating technology improves the first layer deposition which improves the bonding strength [27].

Critical velocity and coating quality improve with improving the impact velocity well above the critical velocity. In the presence of oxide, the particles jetting and flattening decrease [28]. Velocity or impact velocity affects the CS coating to an extent [27]. Iron-based amorphous/nanocrystalline composite coatings exhibit better corrosion resistance as compared to galvanized steel with improved corrosion resistance and microhardness than steel substrate [28, 29]. Copper and copper-titanium dioxide coating using low-pressure CS coating technique give metallic and amorphous structure, respectively [30].

This research aims to improve the impact velocity and temperature of powder particles by improving the geometrical parameters like standoff distance and powder shape and size range in CS coating process technology. This analysis was carried out with constant parameters which include propelling gas mixture, particle size range, pressure conditions, temperature conditions at inlet and outlet sections, etc. The best-suited shape of powder particles and standoff distance based upon the simulation results was discussed.

2 Model, Material, and Methodology

2.1 Geometry

The geometry required for the simulation of the CS process technology should have the same arrangement as that of the experimental model. The geometry used in this simulation process had been drawn using SolidWorks software, which consists of an injector-nozzle arrangement, the substrate has been placed in front of the nozzle outlet on the same axis as the nozzle, and the arrangement is made known in Fig. 1. The nozzle used in this work is of the circular cross-sectional area with diameters of 19 mm, 3 mm, and 7 mm at inlet, throat, and outlet, respectively. The length of the convergent-divergent nozzle has been taken as 230 mm with 55 mm convergent part and 175 mm divergent part. The barrel has been placed at the inlet of the convergent part of diameter 19 mm with 15 mm length. The injector has been positioned at the inlet of the nozzle of 2.5 mm diameter and 30 mm in length. A circular plate shape substrate, with 6 mm thickness and 60 mm diameter, has been placed in front of the nozzle outlet. All the parts of the geometry have been designed to simulate the work in 2D and using symmetry to reduce the computational time.

Fig. 1
A diagram of a nozzle presents the dimensions of the nozzle barrel, pressure gas inlet, particle injector, convergent and divergent sections, outlet, and steel substrate.

Geometry with standoff distance (X = 15 mm, 25 mm, 35 mm, and 45 mm)

Full size image

2.2 Computational Field and Boundary Condition

The computational field with boundary conditions with all the phases and domains has been demonstrated in Fig. 2. The geometry has been converted into several elements and nodes of quadrilaterals. The CFD analysis has been carried out using standoff distances of 15 mm, 25 mm, 35 mm, and 45 mm having 90,579, 92,499, 93,174, and 90,578 nodes, respectively. The geometry with 25 mm of standoff distance has been employed for the analysis of different particle shapes and size range. The CFD simulation meshing has been shown in Fig. 3, the chief regions which include the symmetry, the outlet of the nozzle including the substrate, throat area and inlet of the nozzle had been displayed separately via Fig. 3a–d, respectively.

Fig. 2
A diagram indicates the sections of pressure gas inlet, particle inlet, the wall of nozzle, the axis of symmetry, fluid phase, substrate, solid phase, far off field, and pressure outlet.

Boundary conditions for CS coating CFD simulation

Full size image
Fig. 3
A set of 4 diagrams with layers of mesh. Diagram a presents the structure of the nozzle, b presents the nozzle outlet, c presents the section of the throat, and d exhibits the section of particle and gas inlet.

CFD simulation mesh: a meshing of geometry, b meshing at nozzle outlet/near the substrate, c meshing at the nozzle throat, and d meshing at the particle and gas inlet

Full size image

2.3 Governing Equations and Material Properties

The gas phase is governed by ideal gas law by considering the compressibility effect. The gas flow embraces the continuity equations, energy equations, and momentum equations predominant to the flow of gases. The gases used in this simulation process have a mixture of nitrogen and helium, and the mixture of nitrogen and helium has been taken in the ratio of 4:6 (N2: He). The powder particles have been kept as copper metal with different shapes and sizes, having a density of 8978 kg/m3 and specific heat (Cp) of 381 J/Kg K. The shape of powder particles has been taken as spherical and non-spherical (SF = 0.8) (crushed sandstone shape) [31] with a size range of 15–60 µm using the rosin-rammler method with the mean particle size of 30 µm and injected with the temperature of 300 K. The substrate material has been taken as steel at the temperature of 300 K, with density, specific heat (Cp), and the thermal conductivity of 8030 kg/m3, 502.48 J/kg K, and 16.27 W/m K, respectively.

2.4 Numerical Simulation

The CFD analysis, employing the ANSYS software, has been carried out to escalate the role of particle shape and size and the effect of standoff distance before impact. The temperature and pressure of the propelling gas at the gas inlet are 675 K and 15 bar, whilst at particle, inlet has been taken as 325 K and 1 bar.

The simulation of CS process technology has been carried out using the pressure-based, axisymmetric model. The numerical simulation model has been taken as the two-equation realizable k-ɛ model with standard wall function, as this model is the most realistic as compared to other available models.

3 Outcomes and Discussion

Interpreting to the prior research work [14, 27], the pressure taken was 3–7 MPa to obtain the optimal impact velocity of particles having a diameter of 10 µm. Whereas, in the present study, the pressure is taken as 15 bar or 1.5 MPa with varying particle size and shape of powder particles. Consequently, this study implies the idea of shape effect on impact velocity accompanied by particle size. Secondly, the effect of particle shape on temperature and impact velocity is investigated.

3.1 Effect of Shape and Size on Impact Velocity and Temperature

The impact velocity and temperature simulation analysis for two different shapes of particles and size range upon impact velocity and temperature have been shown by Figs. 4 and 5. The simulation results are plotted in Figs. 6 and 9, and the velocity and temperature of the spherical-shaped particle found higher as compare to non-spherical, which shows that spherical shape of powder particles will certainly affect the coating quality in terms of porosity, hardness, and bond strength as compare to non-spherical shape powder particles, because of the velocity and temperature difference and shape difference. The impact velocity and temp. of the particles before impact have been indicated by the data label in Figs. 6 and 7 in which the velocities of spherical and non-spherical particles just before impact upon the substrate have been recorded as 1259.03 and 1211.87 m/s. The final impact velocity of spherical and non-spherical particles was 410.806 and 395.233 m/s. The temperature of spherical and non-spherical particles is 671.06 K and 621.81 K at the substrate. This indicates that the spherical particles strike with high velocity and temperature as compare to non-spherical ones. All the simulations of different particle shapes have been performed using geometry with a 25 mm standoff distance.

Fig. 4
A temperature contour diagram near the substrate depicts multiple lines shaded in different colours according to a gradient scale ranging from 3.000 e + 02 to 6.750 e + 02.

Temperature contour near the substrate

Full size image
Fig. 5
A velocity contour diagram near the substrate indicates multiple lines shaded in different colors according to a gradient scale ranging from the minimum value of 0.000 e + 00 to a maximum value of 1.024 e + 04.

Velocity contour near the substrate

Full size image
Fig. 6
A line graph of velocity versus stand-off distance plots trends for spherical and non-spherical particles. Both lines descend in a concave downward manner.

Effect of particle shape on velocity

Full size image
Fig. 7
A dot plot of temperature versus stand-off distance plots trends for spherical and non-spherical particles. Both plots have a very small variation and progress horizontally.

Effect of particle shape on temperature

Full size image

3.2 Effect of Standoff Distance on Impact Velocity and Temperature

The analysis of standoff distance upon impact velocity and temperature of powder particles gives the results as shown in Figs. 8 and 9, in which the velocities 1234.84 m/s, 1259.03 m/s, 1325.06 m/s, and 1110.85 m/s were found for 15 mm, 25 mm, 35 mm, and 45 mm standoff distance, respectively. The final impact velocity of spherical and non-spherical particles was 403.806 m/s, 4010.806, 443.216, and 381.448 m/s for 15, 25, 35, and 45 mm standoff distances, respectively, upon which the particle gets impacted on the substrate surface. The temperature behaves in ascending order with that of standoff distance. The velocity before impact indicates the difference and found to be best with 35 mm standoff distance and lowest with 45 mm standoff distance. Considering velocity and temperature 35 mm standoff distance found efficient for CS coating.

Fig. 8
A line chart of velocity versus stand-off distance plots lines for 15, 25, 35, and 45 millimeters. All the lines start from the same point between 2100 and 2400 meters per second and descend in a concave downward manner.

Effect of standoff distance on velocity

Full size image
Fig. 9
A line chart of temperature versus stand-off distance plots lines for 15, 25, 35, and 45 millimeters. All the lines descend in a concave downward manner.

Effect of standoff distance on temperature

Full size image

4 Conclusion

The research findings to analyze the effect of particle shape and size on impact velocity and temperature and also the role of standoff distance in CS process technology indicates that:

  1. 1.

    The impact velocity and temperature of spherical-shaped particles are high as compare to the non-spherical shape (SF = 0.8) of particles.

  2. 2.

    The spherical shape of powder particles with a standoff distance of 35 mm is optimum to use in CS coating.

  3. 3.

    The standoff distance higher than and lower than that of findings, affect the coating in terms of its properties, porosity, bond strength, and efficiency.