| Project Team | Institution | Role | ||
| Dr Nick Birbilis | Monash University | Program Leader | ||
| Dr Daniel Fabijanic | Deakin University | Research Fellow | ||
| Mr York (Joe) Han | Monash University | Postgraduate Student | ||
| Mr Shuo Li | Monash University | Postgraduate Student | ||
| Professor Julio Soria | Monash University | Associate Investigator | ||
| Dr Kevin Spencer | The University of Queensland | Research Fellow | ||
| Mr Qiang Wang | The University of Queensland | Postgraduate Student | ||
| Assoc Professor Ming-Xing Zhang | The University of Queensland | Associate Investigator | ||
 |
|
|
|
|
Executive Summary
Characterisation of cold spray coatings: The bonding process of deformation leading to metallurgical bonding has been confirmed but more work is required to understand the intermixing at the interface. The residual stresses measured using neutron diffraction depend primarily on the coating material, and not on the process parameters. The physical properties of various cold spray coatings are similar to those of an equivalent bulk material. The use of mixed powder size distributions and reinforcement with Al2O3 significantly improves coating properties such as bond strength and porosity.
Stainless steel-Al2O3 coatings: Using the same approach as in the Al coatings of using powder size mixtures and Al2O3 reinforcement results in substantially improved density, wear and corrosion resistance.
Numerical simulation of the cold spray process: The Centre now has the capability of doing 1-D isentropic simulations of particle accelerations within a sonic cold spray nozzle, and this is now used for process optimisation. A 2-D computational fluid dynamics model of a particle in a supersonic gas stream impinging on a substrate has been finalised. Good agreement between the model and particle velocimetry experiments has been observed. This can now be used to understand the interaction between particle size and gas stream conditions.
Characterisation of cold spray coatings: The bonding process of deformation leading to metallurgical bonding has been confirmed but more work is required to understand the intermixing at the interface. The residual stresses measured using neutron diffraction depend primarily on the coating material, and not on the process parameters. The physical properties of various cold spray coatings are similar to those of an equivalent bulk material. The use of mixed powder size distributions and reinforcement with Al2O3 significantly improves coating properties such as bond strength and porosity.
Stainless steel-Al2O3 coatings: Using the same approach as in the Al coatings of using powder size mixtures and Al2O3 reinforcement results in substantially improved density, wear and corrosion resistance.
Numerical simulation of the cold spray process: The Centre now has the capability of doing 1-D isentropic simulations of particle accelerations within a sonic cold spray nozzle, and this is now used for process optimisation. A 2-D computational fluid dynamics model of a particle in a supersonic gas stream impinging on a substrate has been finalised. Good agreement between the model and particle velocimetry experiments has been observed. This can now be used to understand the interaction between particle size and gas stream conditions.
Project Aims/Targets
Cold spray is a high-rate solid state deposition process used to spray coatings and preforms using metal or metal/ceramic powder mixtures as the feedstock. The powder is entrained in a high-speed gas stream, and the powder is accelerated via drag above a critical velocity of 500-1000 m/s, where it forms a bond with a metallic substrate on impact. The bonding process is enabled through plastic deformation of the powder and removal of surface oxides to enable conformal metal/metal contact.
The cold spray deposition process operates at suitably low temperatures for coating light metal substrates, and may also be used to produce freeform shapes. The aims of the project are to obtain a better understanding of the process, and to develop new coating methods to improve the wear and corrosion resistance of light metal substrates. This has involved: (1) characterisation of the structure and performance of cold spray coatings to better understand the bonding process, (2) applications of cold spray coatings to expand the range of applications of light metals, (3) the use of the cold spray process in novel ways such as through the heat treatment of coatings and the fabrication of composite freeforms, and (4) the use of gas dynamic simulations to better understand the cold spray process, to enable more efficient process control.
The work in the Centre has been applied to two different types of cold spray processes - Kinetic Metallization and Cold Spray.
Cold Spray
Cold spray is the generic name used for a variety of similar deposition processes, but strictly speaking it refers to processes where the driving gas is accelerated to supersonic velocity (Mach 2 – Mach 3), and the particle size ranges from 1-50μm. Due to the high volume of gas consumed, nitrogen is typically used as the process gas. Some collaborative experiments have been conducted using the equipment housed in the CSIRO laboratories in Clayton.
Kinetic Metallization
Kinetic Metallization (KM) is a cold spray variant that accelerates the process gas to sonic speed (Mach 1). This results in greatly reduced gas consumption. Smaller powder sizes are needed with KM than with supersonic cold spray, ~5-15μm, and helium is most often used as the process gas to offset the Mach 1 speed limitation. This equipment is housed within the centre at the University of Queensland.
Cold spray is a high-rate solid state deposition process used to spray coatings and preforms using metal or metal/ceramic powder mixtures as the feedstock. The powder is entrained in a high-speed gas stream, and the powder is accelerated via drag above a critical velocity of 500-1000 m/s, where it forms a bond with a metallic substrate on impact. The bonding process is enabled through plastic deformation of the powder and removal of surface oxides to enable conformal metal/metal contact.
The cold spray deposition process operates at suitably low temperatures for coating light metal substrates, and may also be used to produce freeform shapes. The aims of the project are to obtain a better understanding of the process, and to develop new coating methods to improve the wear and corrosion resistance of light metal substrates. This has involved: (1) characterisation of the structure and performance of cold spray coatings to better understand the bonding process, (2) applications of cold spray coatings to expand the range of applications of light metals, (3) the use of the cold spray process in novel ways such as through the heat treatment of coatings and the fabrication of composite freeforms, and (4) the use of gas dynamic simulations to better understand the cold spray process, to enable more efficient process control.
The work in the Centre has been applied to two different types of cold spray processes - Kinetic Metallization and Cold Spray.
Cold Spray
Cold spray is the generic name used for a variety of similar deposition processes, but strictly speaking it refers to processes where the driving gas is accelerated to supersonic velocity (Mach 2 – Mach 3), and the particle size ranges from 1-50μm. Due to the high volume of gas consumed, nitrogen is typically used as the process gas. Some collaborative experiments have been conducted using the equipment housed in the CSIRO laboratories in Clayton.
Kinetic Metallization
Kinetic Metallization (KM) is a cold spray variant that accelerates the process gas to sonic speed (Mach 1). This results in greatly reduced gas consumption. Smaller powder sizes are needed with KM than with supersonic cold spray, ~5-15μm, and helium is most often used as the process gas to offset the Mach 1 speed limitation. This equipment is housed within the centre at the University of Queensland.
Characterisation of Cold Spray Coatings
A variety of different metal coatings were deposited on various metal substrates to characterise the bonding process in cold spray. Detailed characterisation of the coating/substrate interface at Monash has shown that the extent of the deformed zone is much larger for Al and Mg substrates than for Cu and Fe substrates. An example of the deformed structure of a copper coating is shown in Figure B26.
A variety of different metal coatings were deposited on various metal substrates to characterise the bonding process in cold spray. Detailed characterisation of the coating/substrate interface at Monash has shown that the extent of the deformed zone is much larger for Al and Mg substrates than for Cu and Fe substrates. An example of the deformed structure of a copper coating is shown in Figure B26.
In Al and Mg substrates a recrystallized zone was identified; however, the extent of recrystallization within the coating is still not clear. Of particular interest, HAADF/STEM analysis revealed the interfaces are more complex than previously understood, and an intermixing zone ~0.1-1μm between the coating and substrate materials was observed. The nature of this intermixing process will be investigated further.
It has been long understood that cold spray results in compressive residual surface stresses, similar to shot peening. Several early experiments in the Centre suggested the residual stresses could be significant in some cases. Neutron diffraction was used to measure and construct depth profiles of the residual stresses in the coating and substrate for Cu/Cu, Cu/Al, Al/Al and Al/Cu coating/substrate combinations. The measurements were done at the OPAL reactor at ANSTO’s Bragg Institute, in collaboration with Dr. Vladimir Luzin. They showed that in the Cu coatings, the residual stresses are large regardless of the substrate material, and in the Al coatings the residual stresses are small (Figure B27). Based on these results a simple framework was established for predicting the residual stress levels in cold spray coatings. This relates the kinetic energy of the particle in-flight to the amount of plastic strain on impact. Further internal stress measurements are to be carried out to extend this work. This is an important area of investigation since the residual stress at the surface and interface determines the bond strength, spall resistance and fatigue resistance of the coating.
It has been long understood that cold spray results in compressive residual surface stresses, similar to shot peening. Several early experiments in the Centre suggested the residual stresses could be significant in some cases. Neutron diffraction was used to measure and construct depth profiles of the residual stresses in the coating and substrate for Cu/Cu, Cu/Al, Al/Al and Al/Cu coating/substrate combinations. The measurements were done at the OPAL reactor at ANSTO’s Bragg Institute, in collaboration with Dr. Vladimir Luzin. They showed that in the Cu coatings, the residual stresses are large regardless of the substrate material, and in the Al coatings the residual stresses are small (Figure B27). Based on these results a simple framework was established for predicting the residual stress levels in cold spray coatings. This relates the kinetic energy of the particle in-flight to the amount of plastic strain on impact. Further internal stress measurements are to be carried out to extend this work. This is an important area of investigation since the residual stress at the surface and interface determines the bond strength, spall resistance and fatigue resistance of the coating.
The elastic modulus and thermal expansion coefficient of the cold spray coatings were very close to those of an equivalent bulk material. This suggests a higher proportion of metallic interparticle bonding in cold spray than in thermal spray coatings. Neutron diffraction measurements of the bulk oxide content of the coatings was found to be the same as that of the feedstock powders, which shows there is no oxidation during the cold spray process. This further confirms their suitability for light metal substrates.
It has long been known by powder metallurgists that mixtures of particle size distributions can be used to achieve higher densities in powder metallurgy compacts, but until now this idea has not been applied to cold spray coatings. It is difficult to obtain dense cold spray coatings of Al due to its low density and tenacious oxide layer, and experiments with mixed powder size distributions in cold spray coatings yielded coatings with nearly 100% theoretical density, as well as improved bond strength and corrosion resistance. The bond strength of the coatings was too high to measure using conventional techniques, so a new shear test was developed. Testing of Al coatings sprayed on Mg substrates have shown that in some cases the bond strength is even stronger than the magnesium substrate material. This work was extended to Al-Al2O3 composite coatings, Figure B28 and the type of failure was observed to change with the Al2O3 volume fraction from adhesive failure at the interface to cohesive failure within the coatings.
It has long been known by powder metallurgists that mixtures of particle size distributions can be used to achieve higher densities in powder metallurgy compacts, but until now this idea has not been applied to cold spray coatings. It is difficult to obtain dense cold spray coatings of Al due to its low density and tenacious oxide layer, and experiments with mixed powder size distributions in cold spray coatings yielded coatings with nearly 100% theoretical density, as well as improved bond strength and corrosion resistance. The bond strength of the coatings was too high to measure using conventional techniques, so a new shear test was developed. Testing of Al coatings sprayed on Mg substrates have shown that in some cases the bond strength is even stronger than the magnesium substrate material. This work was extended to Al-Al2O3 composite coatings, Figure B28 and the type of failure was observed to change with the Al2O3 volume fraction from adhesive failure at the interface to cohesive failure within the coatings.
Stainless Steel-Al2O3 Composite Coatings
Type 316 Stainless steel coatings are difficult to cold spray because of the hardness and high rate of work hardening of the powder. Such coatings could be used to clad magnesium parts in contact with steel to avoid galvanic effects, but they must have a low level of porosity. Experience with Al coatings has shown the benefits of using particle size mixtures to improve coating density and bond strength, and in the case of stainless steel coatings this yielded a significant improvement in coating performance. In the first stage of the project, coatings cold sprayed using optimised particle size mixtures showed similar corrosion resistance to bulk 316 stainless steel and less than 1% porosity.
For the second stage of the project, composite coatings of stainless steel reinforced with Al2O3 were sprayed and their corrosion and wear resistance was assessed. The corrosion resistance of the composite coatings was similar to that of bulk 316 stainless steel, while the wear resistance improved by as much as 75%. The wear process transitioned from adhesive wear in the unreinforced coatings, to abrasive wear and high-temperature oxidation in the Al2O3 – reinforced coatings (Figure B29).
Type 316 Stainless steel coatings are difficult to cold spray because of the hardness and high rate of work hardening of the powder. Such coatings could be used to clad magnesium parts in contact with steel to avoid galvanic effects, but they must have a low level of porosity. Experience with Al coatings has shown the benefits of using particle size mixtures to improve coating density and bond strength, and in the case of stainless steel coatings this yielded a significant improvement in coating performance. In the first stage of the project, coatings cold sprayed using optimised particle size mixtures showed similar corrosion resistance to bulk 316 stainless steel and less than 1% porosity.
For the second stage of the project, composite coatings of stainless steel reinforced with Al2O3 were sprayed and their corrosion and wear resistance was assessed. The corrosion resistance of the composite coatings was similar to that of bulk 316 stainless steel, while the wear resistance improved by as much as 75%. The wear process transitioned from adhesive wear in the unreinforced coatings, to abrasive wear and high-temperature oxidation in the Al2O3 – reinforced coatings (Figure B29).
Numerical Simulation of the Cold Spray Process
Two different approaches were taken to simulate the cold spray process. The first was to construct a 1‑D isentropic simulation of a particle accelerating through the nozzle of a KM system (sonic velocity). This has been applied as a means of understanding the effects of process parameters such as particle density, particle size, gas type, temperature and pressure, on the exit speed of the particles. Use of the model makes it much simpler to identify conditions where the process is optimised.
Two different approaches were taken to simulate the cold spray process. The first was to construct a 1‑D isentropic simulation of a particle accelerating through the nozzle of a KM system (sonic velocity). This has been applied as a means of understanding the effects of process parameters such as particle density, particle size, gas type, temperature and pressure, on the exit speed of the particles. Use of the model makes it much simpler to identify conditions where the process is optimised.
The second approach taken has been the numerical simulation of a particle in a supersonic gas stream impinging on a substrate. An axisymmetric 2-D gas dynamic simulation was constructed using the commercial code FLUENT. The domain boundaries have been optimised and the model results compare favourably to experimental measurements on CSIRO’s cold spray equipment using particle velocimetry (Figure B30). The model is able to capture the interaction of the particle with the shock wave, as the gas jet impinges on the substrate. It has been found that particles with a higher Stokes number ~22 are unaffected when crossing the shock front, while those with a lower Stokes number ~1 have their velocity reduced. This information can be used for optimising the combination of gas conditions and particle size in the cold spray process.