The effect of process temperature and powder composition on microstructure and mechanical characteristics of low-pressure cold spraying aluminum-based coatings

The effect of operating gas temperature and powder type on microstructure and mechanical characteristics of cold spraying coatings deposited on EZ33A-T5 magnesium alloy was studied. Three aluminum-based cold spraying powder mixtures Al + Zn, Al + Al2O3 and Al + Zn + Al2O3 were used for the investigation. Deposition was performed using D423 low-pressure cold spray system at operating gas pressure of 1.0 MPa and different temperatures –300 °C, 450 °C, and 600 °C. The coatings microstructure was investigated with optical and scanning electron microscopy. Mechanical properties of the coatings were characterized through standard test methods for adhesion and cohesion strength, and standard test methods for Vickers hardness of thermal spray coatings. The results demonstrate that with increasing initial gas temperature at spraying nozzle inlet from 300 °C to 600 °C, an increase in the porosity of the coatings of all investigated powder mixtures can be observed. Microstructure characterization showed an increase in porosity from 2.3% to 4.1% for Al + Zn powder mixture, from 2.1% to 3.5% for Al + Al2O3 powder mixture, and from 2.5% to 5.6% for Al + Zn + Al2O3 powder mixture. The minimum porosity was obtained at 450 °C for all investigated powder mixtures. Adhesion and cohesion strength and microhardness of coatings were reach their maximum value at 450 °C. The best performance was obtained for Al + Al2O3 powder mixture: coating adhesion—31.9 MPa (was limited by the bonding strength of the glue), cohesion—93.5 MPa, microhardness—81 HV0.15. The influence of Al2O3 particles in the powder mixture on the above-mentioned parameters was also established. The results show that the presence of ceramic particles in powder mixtures can positively effect porosity level and mechanical characteristics.


Introduction
The use of magnesium alloys in modern technology is due to their low specific weight, high strength, excellent damping capacity, good fluidity for casting and other properties [1]. The largest consumers of magnesium alloys are the aviation and automotive industries, where weight reduction is one of the main tasks, which allows for significant reduction of greenhouse gas emissions and increase of vehicle fuel efficiency.
Low corrosion resistance is one of the main disadvantages of magnesium alloys [2]. Restoration of aircraft parts from magnesium alloys is quite a difficult task due to high sensitivity of these alloys to high temperatures, for which the traditional spray coating methods are associated with a number of limitations and technological problems.
There are different coating deposition methods which can be applied in order to obtain high characteristics of base material to protect it from corrosion, erosion, wear, ensure some specific properties of surfaces [3][4][5][6] or to increase the service life of parts [7]. Among all methods of thermal coating, cold spraying (CS) has a number of cold spraying on magnesium alloys will make it possible to find scientifically sound technological solutions to ensure reliable protection of aircraft parts from corrosion and mechanical damage.
Analysis of source literature on the selection of materials for CS method has shown that when using metalmatrix composites, their phase and structural composition in the coating undergoes no changes [37]. It was shown above that the addition of ceramic particles to the matrix base improves the coating properties and deposition efficiency. The adhesive strength, which in most cases is used as an optimization parameter, is as good as that of coatings obtained by other methods of thermal spraying [38].
Optimization of the microstructure of coatings, their physical and mechanical properties, and spraying modes are selected for each powder separately, in most cases experimentally. This process requires systematic changes in the spraying modes and coating analysis, which is time and resource-consuming. Despite a fairly large number of works devoted to cold spraying of aluminum-containing coatings on magnesium alloys, they are still isolated cases. The process of optimizing the quality of coatings for one (or more) response is individual for each powder material and substrate material, and depends on the specific conditions and parameters of spraying, the state of the surface layer of the substrate, etc.
Adding an appropriate amount of Al 2 O 3 into aluminum matrix to make metal-ceramic composition can improve coating performance. Numerous literatures are available on this direction; however, the most experiments were performed with in situ blended powder mixtures and commercially available metal-matrix composition has yet to be investigated. Moreover, according to our literature reviews, there are no comparative studies on different composition of protective and restorative coatings on magnesium alloy deposited using lowpressure cold spray system.
In the present study, the effect of cold spraying process temperature and aluminum-based powders composition on porosity, adhesion and cohesion strength, microhardness was examined. It is expected that the current research will provide greater detail regarding the effect of ceramic particles in initial powder mixture on coatings characteristics deposited on magnesium alloy. The results obtained can be used in the future to develop technological recommendations for spraying protective and restorative coatings on magnesium-alloy parts to increase their corrosion and other properties, repair already damaged surfaces, which ultimately will have a positive effect on the reliability and durability of products in general.

Materials
Commercially available aluminum-based powder mixtures Al + Zn, Al + Al 2 O 3 , Al + Zn + Al 2 O 3 (Dycomet Europe B.V., the Netherlands) were used as the powder material. These aluminum-based powders are used for D423 low-pressure cold spray system and recommended by equipment supplier for corrosion protection. Powder mixtures are mixed mechanically in mills by the powder supplier and are delivered in a ready-to-use state. Figure 1 illustrates the electron microscope micrographs of the powder mixtures used, and table 1 shows their composition and fraction size.
Mold casting EZ33A-T5 magnesium alloy was chosen as the substrate material. Nominal chemical composition of EZ33A-T5 cast magnesium alloy is shown in table 2 [39].
While processing magnesium alloys particular attention should be paid to material heating. Oxidizes at a rate dependent upon temperature and initial form and size. The EZ33A-T5 magnesium alloy in a form of billets, slabs, plate, sheet, bar are difficult to ignite but when heated in air to a temperature near the melting point (650°C ), can ignite and burn with intense heat and flame. Thin sheets can be ignited at temperatures near 510°C [40]. Before spraying, the substrate surface was subjected to sandblasting with 60 Grit aluminum oxide. Preparation was carried out with dry aluminum oxide under a pressure about 0.4 MPa at a distance from the nozzle to substrate of 80 mm and angle from 45°to 90° [41][42][43]. After that, the samples were blown with dry compressed air to remove dust and particles that could remain on the surface. Degreasing of the samples before spraying was performed with a solvent. The time after preparing the surface and deposition the coatings was no more than two hours. The average surface roughness of the substrates after sandblasting was about 160 μm.
Cylindrical samples with a diameter of 25.0 mm and a height of 38.0 mm were used to determine the cohesive strength of the coatings (figure 2(a)), and samples of a cylindrical shape with a diameter of 25.0 mm and a height of 30.0 mm were used for adhesive strength tests ( figure 2(b)). Plates with size of 25.0 mm × 50.0 mm × 1.5 mm were used to analyze of the microstructure and microhardness of the coatings (figure 2(c)).

Cold spray system and spraying parameters
The coating was sprayed using D423 low-pressure cold spray system (Dycomet Europe B.V., the Netherlands) at National Aerospace University 'Kharkiv Aviation Institute' (Kharkiv, Ukraine). Acceleration of gas flow with particles occurred using a standard supersonic Laval nozzle of D423 CS system with a critical cross section diameter of 2.7 mm and an outlet diameter of 6.0 mm. The powder injection point is located behind the critical cross section of the nozzle. Air was used as the working gas. Operating modes of spraying are shown in table 3.
The minimum value of stagnation temperature was chosen at 300°С, because with low-pressure CS, lower values are insufficient for spraying aluminum-based coatings. The maximum value of stagnation temperature and stagnation pressure for D423 CS system is 600°С and 1.0 MPa, respectively. The value of stand-of-distance and powder feed rate was chosen in accordance with the recommendations of the CS system manufacturer for deposition above mentioned powder mixtures.

Characterization of coatings
Metallographic studies (microstructures, coating layer thickness, coating surface condition) were performed using Selmi REM-106 scanning electron microscope with a magnification of up to 300,000x and Neophot 30 optical microscope with a magnification of up to 2000x. For microstructural characterization, cross-section of the samples was prepared using standard sample preparation techniques such as sandpaper grinding followed by fine polishing with diamond pastes until the surface was bright and free of scratches [44]. Microstructure studies were performed on non-etched samples.
To determine the bond strength of thermal spray coatings ASTM Standard C633-13 [45] was applied. When the adhesion strength of the coatings was investigated, a coating was applied to the end surface of one of the samples after preliminary preparation. The counter-sample with the same diameter was end-to-end connected to the coating using the adhesive. The criterion for bonding strength is the force at which the coating separates along the interface with the base metal during stretching, attributed to the surface area of the end face. The studies used an epoxy resin-based adhesive with a bonding strength of metal surfaces from 35 MPa to 40 MPa.
To measure the cohesive strength of coatings the tubular coating tensile (TCT) method was applied [46], which is a modification of ASTM Standard C633. A pair of cylindrical samples with central holes were tightened with a screw. The rod with samples was clamped in a lathe and the outer diameter of the tightened samples was machined. Then the surface was sandblasted and degreased. A coating with a thickness of 500 μm to 1500 μm was applied to the junction of two rotating samples. After spraying, the clamping screw and rod were removed, and the samples were fixed in the self-centering device of the tensile testing machine similar to the method of determining the adhesive strength.
The adhesion and cohesion strength σ was determined by the equation (1) where F is the breaking force; S is the cross-sectional area of the samples during tests for adhesive strength and the annular cross-sectional area of the coating when determining the cohesive strength.
The microhardness of obtained coatings was investigated on cross sections using a microhardness tester PMT-3 at a diamond tip indenter load of 150 g and 15 s loading time in accordance with ASTM Standard E92-17 [47]. Based on the measured area of the rectangle obtained on the screen by the intersection of lines tangent to the vertices of the print, the microhardness value is determined by the Vickers hardness test.

Results and discussions
3.1. Microstructure of coatings Analysis of the results of measures of the microstructure of cold spray coatings, subject to the spraying mode, structures with a wide range of parameters are implemented, including the packing density and the amount of deformation of individual particles in the layer. This difference in the structure of the sprayed coatings (and the adhesion strength as shown further) can most likely be explained by different levels of the impact packing effect during particle interaction. It is obvious that every particle fixed on the surface of the substrate is hit by incoming flow particles. The degree of particle concentration in the gas stream during cold spraying amounts from 10 −4 to 10 −5 [48], the number of such collisions is ∼10 3 K10 4 , which leads to a high degree of deformation of particles in the sprayed layer, their dense packing, and, as a result, to small porosity values.
To investigate the effect of air temperature at the nozzle inlet T 0 on the coating microstructure, powder mixtures were sprayed on EZ33A-T5 magnesium alloy substrate at temperatures 300°C, 450°C, and 600°C, with stagnation pressure of 1.0 MPa, and stand-of-distance of 10 mm. The porosity of obtained coatings was determined on microsections to obtain a microstructure at magnifications of ×500. ImageJ software was used to analyze and process the obtained images.
The analysis of coating microstructure led to the following results on the effect of air temperature Т 0 on coating porosity (figure 3).  Figure 4 illustrates the influence of stagnation temperature increase from 300°C to 600°C on coatings microstructure deposited at constant stagnation pressure of 1.0 MPa and stand-of-distance of 10 mm.
An increase in the gas temperature to 600°C for all the powder mixtures studied led to an increase in the porosity of the coatings. As noted earlier, an increase in temperature has a positive effect on the increase of spraying efficiency. Visually, an increase in the thickness of the coatings obtained at a gas temperature of 600°C was observed. At the same time, deterioration of porosity and physical and mechanical properties of the coatings was observed. This can be caused by significant overheating of both the powder and the substrate material, especially in the case of magnesium alloys. Most likely, for aluminum particles, the gas temperature of 600°C is too high at the spraying parameters used in the experiments.
An increase in the substrate temperature affects the increase in the adhesive strength of coatings by softening the substrate material, while reducing the stress required for sufficient plastic deformation [49]. This has a positive effect on the possibility of mechanical interlocking and the formation of metallurgical bonds [50].  However, significant overheating can negatively affect the spraying process, and therefore controlling this parameter is important in the practice of coating formation, especially for temperature-sensitive materials.

Adhesion and cohesive strength of coatings
Three aluminum-based powder mixtures Al + 25% Zn, Al + 25% Al 2 O 3 and Al + 25% Zn + 15% Al 2 O 3 were sprayed on prepared cylindrical samples of EZ33A-T5 magnesium alloy. Samples with coatings for the investigation of adhesive strength were prepared according to ASTM C633; a coating thickness of 0.35 mm was obtained after processing. The coated test specimen was subsequently glued to the uncoated specimen. Prior to experiments, the adhesive strength was determined by bonding uncoated samples, the value whereof amounted to 31.9 MPa.
Test specimens to investigate the cohesive strength of coatings were prepared according to the TCT method. Coatings with a thickness of 1.0 mm were applied to the joint surface of two samples with subsequent destruction in the universal testing machine. Figure 5 shows the coated samples before and after measuring adhesion and cohesion strength. Figure 6 illustrates the results of measuring adhesion strength of coatings. For all powder mixtures studied in this work, the highest values of adhesive strength were observed at a temperature of T 0 = 450°C. With increasing temperature up to 600°C, the bond strength of substrate coatings decreased for all studied powder mixtures. For powder mixtures Al + 25% Al 2 O 3 and Al + 25% Zn + 15% Al 2 O 3 , the maximum adhesive strength of coatings was limited by the strength limit of adhesive, the average value whereof in the work amounted to 31.9 MPa.
The lowest values of the adhesive strength of coatings were obtained for a powder mixture of Al + 25% Zn with a maximum of 15.2 MPa at an air temperature T 0 = 450°C. At a temperature of T 0 = 300°C the coating of a powder mixture Al + 25% Zn was not formed due to insufficient temperature-velocity parameters of particles.
Experimental results of cohesion strength of coatings for three powder mixtures at different temperatures T 0 are shown in figure 7.
The figure 7 illustrates that increase in the temperature T 0 leads to increase in cohesive strength, whereas cohesive strength demonstrates the largest values at a temperature of T 0 = 450°C for all three powder mixtures, as in the case of adhesive strength. Further increase in operating temperature to 600°C causes a decrease in  cohesive strength of all studied coatings. As it was said before, at a temperature of T 0 = 300°C the coating of a powder mixture Al + 25% Zn had not formed due to insufficient particles characteristics.

Microhardness of coatings
Aluminum-based coatings have been successfully applied to EZ33A-T5 magnesium alloy substrates using D423 CS system and air as the carrier gas. Figure 8 illustrates the results of measuring microhardness of coatings.
As it can be seen from figure 8, increasing the gas temperature from 300°C to 450°C has no significant effect on microhardness of all test powders. It is established that a further increase in the gas temperature to 600°C leads to a decrease in microhardness of the studied coatings, which can be explained by the increase in coating porosity. The lowest values of microhardness were obtained for coatings Al + 25% Zn (65 HV 0.15 ) in comparison with coatings Al + 25% Al 2 O 3 and Al + 25% Zn + 15% Al 2 O 3 − 81 HV 0,15 and 89 HV 0.15 , respectively, sprayed at air temperature of 500°C, air pressure of 1.0 MPa and distance of 10 mm.

Effect of gas temperature
Since the discovery of the CS phenomenon, the particle velocity has taken center stage in terms of its effect on the adhesive strength of coatings. The higher the particle velocity during the impact, the higher the probability of forming an area of strong deformation, removing the surface oxide layer from the contact surfaces, and forming adhesive bonds [50]. The authors of many papers associate an increase in adhesive strength with an increase in particle velocity during the collision [51][52][53][54].
Studying the effect of particle temperature on the spraying process and adhesive strength is more complex. However, it was found that increasing the particle temperature reduces the critical velocity required for deposition [55]. This decrease in the critical velocity has a positive effect on the larger values of the coefficient first proposed by Assadi et al, which reflects the ratio of the particle velocity to the values of the critical velocity of the sprayed material. The increase of this coefficient leads to the improvement in the quality of coatings:  adhesive and cohesive strength, microhardness, porosity, spraying efficiency, etc. The increase in the cohesive strength with increasing particle velocity is observed.
The dependence of the velocity of a powder particle on the velocity of the flow in which it is located was demonstrated even at the initial stages of studying the CS process [56][57][58]. The effect of the operating temperature of a gas and its type on the particle velocity can be explained by writing down the equation from a one-dimensional isentropic model for calculating the gas velocity in a supersonic nozzle [59]: where M is the local value of the Mach number in the nozzle, R is the specific gas constant, T is the local value of the gas temperature, k is the gas heat capacity ratio. It can be seen from this equation that higher values of the gas velocity and, accordingly, of the powder particles, can be obtained using a gas with a lower molecular weight (for example, He) and with an increase in the gas stagnation temperature [60].
The local gas temperature has a significant effect on the gas flow rate, as can be seen from equation (3). The ratio of the local gas temperature to the stagnation temperature can be represented as where Т 0 is the gas stagnation temperature.
The gas temperature at the nozzle inlet is one of the most important parameters of cold spraying, which largely determines the characteristics of coatings and process efficiency. The obtained results of microstructural analysis show that decrease in porosity of coatings sprayed at higher values of the initial temperature is a consequence of greater heating of powder particles in gas flow and higher values of velocity at the time of contact with the substrate. This leads to intense plastic deformation of the particles and filling of internal cavities of the coating. However, it should be noted that there is a certain increase limit of the initial gas temperature after which a negative impact on the quality of coatings may be observed.
The analysis of the microstructure of the coatings shows an increase in the degree of deformation of the particles with increasing gas temperature from 300°C to 600°C. Higher values of particle velocity lead to their greater plastic deformation and increase the contact area between the particles in the coating and the particles with the substrate.
This work establishes that at the initial inlet pressure of 1.0 MPa and the initial temperature of 600°C the values of microhardness, adhesive and cohesive strength are lower than the initial temperature T 0 equal to 450°C . This phenomenon can be attributed to significant oxidation of aluminum powder particles due to high temperature and intensive erosion of the sprayed layer with solid Al 2 O 3 particles [61]. The increase in porosity of coatings at a temperature of 600°C has a negative effect on the microhardness of coatings, as well as a decrease in the values of adhesion and cohesion.
As noted earlier, the porosity of coatings depends on the spraying modes and the properties of the powder material. The greater the kinetic energy of the particles during the collision, the more intense the plastic deformation, which is a necessary attribute of the coating formation process. When the particles reach critical velocity values, the adiabatic shear required for the formation of metallurgical bonds at the particle-substrate interface is observed.
With an increase in the operating temperature of the gas, an increase in the velocities of particles in the flow is observed. This has a positive effect on a sharp reduction in the coating porosity.
In addition, the stand-of-distance has a significant contribution to the value of the particle velocity at the moment of collision with the substrate. Therefore, the efficiency of the spraying process and the quality of coatings should be controlled taking into account the simultaneous influence of temperature, distance, and other spraying parameters.
In [62,63], it was shown that high temperatures led to an increase in the thickness of the oxide layer on the particle surface, negatively affecting the adhesive strength of coatings. However, an unequivocal conclusion regarding the effect of layer thickness on bond strength was not made. Therefore, it can be assumed that for operation at a gas temperature of 600°C it is necessary to increase the spraying distance and/or increase the nozzle traverse velocity.
An increase in the porosity of coatings at high initial gas temperatures negatively affects both the cohesive strength of coatings and the microhardness of coatings (see figures [6][7][8]. Similar conclusions were drawn in the papers [64,65].

Effect of ceramic particles
In this paper, Al + 25% Zn coatings demonstrated the lowest results compared to coatings containing Al 2 O 3 at gas temperature of 300°C and 450°C. From the analysis of previous papers, it was found that the addition of Al 2 O 3 particles to the metal powder plays an important role in the formation of a dense microstructure and larger values of mechanical properties of coatings [21]. The addition of ceramic particles to aluminum-based powder affects porosity, microhardness, adhesive and cohesive strength of coatings due to the hardening effect and higher values of plastic deformation of metal particles. The incoming Al 2 O 3 particles promote intensive impact pressing of the sprayed coating layer and prevent the rebound of aluminum particles. Comparison of the obtained coating microstructure from powders without ceramic particles with those therewith show that at the same gas temperature at the nozzle inlet coatings without a ceramic component have higher values of porosity.
Deposition of metal-matrix composites shows higher deposition efficiency and physical and mechanical properties compared to spraying of pure metals. The increase in adhesive strength can be attributed to the activation of the surface and the formation of micro-particles thereon. The increase in the adhesion strength of aluminum coatings is observed with an increase in the content of the ceramic component Al 2 O 3 in the coating, which is confirmed in the papers [66]. Lower values of adhesive strength were obtained for the Al+Zn powder composition, which contain no ceramic particles. The nature of sample failure for such coatings is observed along the coating-substrate interface. For samples with coatings made from powder mixtures of Al+Al 2 O 3 and Al+Zn+Al 2 O 3 , with the content of the ceramic component amounting to approximately 30%, failure is observed along the interface of the adhesive and coating, or the adhesive layer per se. This indicates that the adhesive strength of the coating is greater than the adhesive strength of the adhesive.
For powder mixtures of Al + Al 2 O 3 and Al+Zn+Al 2 O 3 at temperatures of 300°C and 450°C, the values of the adhesive strength of coatings exceeded the adhesive strength of the adhesive (31.9 MPa). Since the failure of the adhesive occurs earlier than the destruction of the coating-substrate interface, it is difficult to determine the real strength of the coatings.
The presence of ceramic particles in the powder mixture increases the roughness of pre-sprayed layers upon high-speed collision with the surface. The growth of micro-peaks has a positive effect on the amount of interlocking between powder particles in the coating, which in turn affects the value of the cohesive strength of coatings. It can be seen from the obtained results that the values of cohesive strength are higher than the values of adhesive strength for all powder mixtures analyzed, since the adhesion between homogeneous layers is stronger than heterogeneous ones. Similar results were obtained in [16,61,67]. An indisputable fact in the practice of applying coatings by thermal spray methods is that abrasive treatment of the surface of the substrate before spraying increases the mechanical interlocking of coatings with the substrate due to an increase in its roughness [68].
The Al+Zn coating is characterized by a dense structure and minimal porosity (<3%), as can be seen in figure 3. Al and Zn particles have sufficient velocity for plastic deformation, adhesion to the substrate and formation of a dense coating. Studies of the microstructure by scanning electron microscopy have shown that the deformation of particles is heterogeneous since the degree of deformation differs. Areas with more or less deformed particles are observed for all coatings. The particles located in the coating at the coating-substrate interface have a more flattened shape, and the coating area is characterized by minimal porosity. The degree of particle deformation decreases with distance from the interface, and the size and number of pores in the coating increases. Heterogeneity of deformation leads to bimodal grain structure of CS coatings, which is also noted in [69,70].
Compaction of the inner layers occurs due to the effect of tamping the previously formed layer with incoming powder particles. Undeformed Al 2 O 3 particles are clearly visible in the coating structure. The irregular shape of ceramic particles upon the collision with metal particles is the cause of formation of pores around Al 2 O 3 [71], which can be seen in micrographs of coatings.

Conclusions
(1) Microstructure, microhardness, adhesive and cohesive strength of aluminum-based coatings obtained by low-pressure cold spraying, largely depend on the values of the initial gas temperature at the nozzle inlet, which affects the gas flow rate in the nozzle and softening of powder particles in this flow.
(2) Intense plastic deformation of aluminum powder particles affects the compaction of coatings by filling or reducing pores in the coating, a stronger bond between deformed aluminum powder particles and Al 2 O 3 particles, as well as the substrate surface with improved physical and mechanical properties of coatings.
(3) A certain increase limit of the initial gas temperature may be observed after which the porosity of coatings increases and their quality deteriorates. The minimum porosity of Al + Zn coatings was 2.3% at a gas temperature of 300°C. For Al + Al 2 O 3 and Al + Zn + Al 2 O 3 coatings it was 1.7 and 2.0%, respectively, at a gas temperature of 450°C and constant other sputtering parameters.
(4) Values of microhardness, adhesive and cohesive strengths of coatings of aluminum-based powder mixtures with the addition of Al 2 O 3 particles are higher in comparison with the powders without Al 2 O 3 particles at the same values of the initial temperature. For coatings of Al + Zn, Al + Al 2 O 3 and Al + Zn + Al 2 O 3 powder mixtures, the maximum values of the above-mentioned characteristics were obtained at a gas temperature of 450°C and constant other spraying parameters: microhardness-65 HV 0.15 , 81 HV 0.15 , and 89 HV 0.15 , respectively; adhesive strength of 15.2 MPa, >31.9 MPa, and >31.9 MPa (glue bond strength limit), respectively; and cohesion strength of 59.8 MPa, 93.5 MPa, and 72.6 MPa, respectively. This can be attributed to the denser microstructure of coatings due to greater plastic deformation of metal particles and the hardening effect on account of hard ceramic particles.