Inter-particle bonding in cold spray deposition of a gas-atomised and a solution heat-treated Al 6061 powder
The heat treatment of a number of gas-atomised aluminium alloys prior to cold spraying recently showed that the resultant microstructural modification was accompanied by an improvement in deposition; however, the relationship between the microstructural homogenisation occurring after recrystallisation and the increase in deposition efficiency and particle–particle bonding had not been investigated. In this study, Al 6061 gas-atomised feedstock powder, before and after solution heat treatment, was cold sprayed and these materials were characterised using electron backscatter diffraction and transmission electron microscopy. The solution heat-treated Al 6061 powder showed large stress-free grains as opposed to the as-atomised feedstock powder which exhibited smaller grains with the presence of dislocations. The coating produced from as-received powder exhibited a homogeneous distribution of misorientation and lattice defects throughout the particles, whereas the coating produced from solution heat-treated powder showed a strain concentration in the interfacial zones. This was attributed to the partial dissolution and the clustering of solute atoms, allowing the aluminium matrix to deform around the newly formed precipitates.
Cold gas dynamic spray is a well-established materials processing technique that is based on the kinetic energy of incident particles to deposit coatings. Cold spray uses metallic powders accelerated by a supersonic gas through a convergent–diverging nozzle (also called De Laval) and the particles impact onto a surface, and the combined plastic deformation of the feedstock material and the substrate forms a coating [1, 2, 3]. Cold spray is a solid-state process, where every sprayed material is kept below its melting temperature and therefore any deleterious effect of the temperature during deposition is avoided [4, 5].
Gas-atomised powders are the most common type of powder used in the recent cold spray applications, and its microstructure was extensively examined recently in relation to cold spray. Due to the nature of the gas atomisation process and being closely related to the high cooling rates (10−4 to 10−8 K s−1 ) reached by the melt during solidification, the powders typically exhibit a cellular/dendritic microstructure [7, 8]. Attributed to several mechanisms , the solute atoms are rejected from the matrix during the dendrite growth  forming a brittle intermetallic cellular network in a variety of materials, from Fe-based alloys  to Al alloys [12, 13]. The presence of an inconsistent intermetallic network can affect the formability of the powder and has been shown to affect any post-deposition ageing heat treatment . To solve this issue, the heat treatment of the powder prior to spraying was recently considered and performed on several gas-atomised aluminium alloys, such as Al–Zn–Mg, Al–Mg–Si and Al–Cu alloys [15, 16, 17]. Based on the diffusion of elements throughout the matrix during a solution heat treatment cycle above the solvus curve, the solute atoms segregation was reduced, and the non-uniformly distributed alloying elements were mostly dissolved in the aluminium solid solution. The homogenisation of the microstructure examined was accompanied by an increase in deposition efficiency and a stronger bonding between particles as well as with the substrate [15, 16].
It is generally accepted that the accumulation of dislocations during severe plastic deformation is responsible for various grain structure formations observed in cold-sprayed deposits: from elongated grains (200 nm to 1 μm) to ultrafine grains (50–200 nm) [5, 9] in the interfacial regions. The dislocation piling into arrays of dislocation walls during the high strain rate plastic deformation and the rotation of the lattice along the shear leads to the formation of elongated grains . Subsequently, these elongated grains are subdivided into smaller subgrains due to further accumulation of dislocations. In order to accommodate the intensive plastic deformation in the impact area, the newly formed subgrain rotate and form equiaxed ultrafine grains at the particle–particle interface. More recently, the interaction of powder constituted of a second phase was studied and the interaction of the second phase with the dislocations during deposition was determined to be linked with the formation of ultrafine grains . Although nano-sized grains were also observed in Ti cold spray coatings in the particle interior, the formation of ultrafine grains in the particle–particle interface was ascertained in many cases  and was showed to contribute to the metallurgical bonding .
The grain structure of a cold spray coating is an indicator of its deposition mechanism, and Electron Backscatter Diffraction (EBSD) was proven to be a useful tool in order to observe the microstructure of the deposits [14, 18, 21]. Although the indexing of grains at the particle–particle interface revealed itself challenging due to the heavy lattice deformation occurring in this area, EBSD scans of cold spray coatings provide a various range of data which can be interpreted. Low-angle and high-angle grain boundaries distribution [18, 19] as well as local average misorientation  are crucial to determine the strain localisation or the dislocation density.
Although the microstructure of cold spray deposits was widely studied, the effect of a heat treatment of the powder on its deposition on a substrate is yet to be observed on a microstructural level. The improvement of deposition was already proved in previous studies without further explanation. The grain structure alteration of the particle of powder and as well as a potential stress release during heat treatment are two factors which matter during cold spray deposition due to the solid-state nature of the process.
In this study, a solution heat treatment was performed on an Al 6061 powder and its effect was analysed on a microstructural level using SEM and EDS. The particles grain structure and its size distribution before and after heat treatment were also characterised using EBSD, and the resulting coatings were also analysed to understand the various deposition mechanisms occurring cold spray deposition. The misorientation and the grain boundaries density were compared in order to evaluate the effect of the heat treatment on the particle plastic deformation.
A spherical gas-atomised Al 6061 powder (LPW, Runcorn, UK) was the feedstock material of this study. The chemical composition of the powder was 1.00 wt% Mg, 0.65 wt% Si, 0.26 wt% Cr, 0.22 wt% Cu, 0.1 wt% Mn, 0.07 wt% Fe and 0.01 wt% Zn, the remainder being Al. The substrates used for the cold spray experiments were 3-mm-thick 6061-T6 aluminium alloy plates (30 × 100 mm). The composition of the substrate is 0.99 wt% Mg, 0.66 wt% Si, 0.16 wt% Cr, 0.31 wt% Cu, 0.08% Mn, 0.25% Fe and 0.01 wt% Zn, the remainder being Al. T6 corresponds to the heat treatment applied to the material, which is in that case a solution heat treatment followed by an artificial ageing. The particle size analysis was performed using laser diffractometry (Laser Mastersizer 3000, Malvern Instruments, Malvern, UK) where the metallic powder was dispersed in air.
Solution heat treatment and quenching of the powder feedstock
The solution heat treatment of the metallic powder was performed under controlled conditions due to its explosivity. In the presence of an ignition source and oxygen, various metal powders can blow up if they are at a minimum temperature called ignition temperature. In the solution heat treatment cycle used to homogenise the powder, Al 6061 was kept at 530 °C for 4 h. The temperature was chosen following the standard T6 (solution heat treatment + artificial ageing) conditions . In order to avoid any fire hazard, 120 g of the feedstock powder was sealed in a quartz tube prior to their heat treatment in order to prevent any exposure to oxygen during the following quench. A 10-mPa vacuum was created in the vial using a diffusion pump before the sealing was performed using an oxy-propane flame, closing the top of the vial. The dimensions of the tube after sealing were as follows: outer diameter 14 mm, length ~ 100 mm and 2 mm wall thickness. The quartz tube was then inserted in the oven and kept at 530 °C for 4 h. Subsequently, the tube containing the powder was quenched into water for 2 min until it reached room temperature. In this case, the solid solution obtained during the heating cycle is quenched to be maintained in this state and to limit the diffusion during cooling. Once at room temperature, the vial was opened, and powder collected for further analysis and experiments. The powder heat treatment and its cold spray deposition were performed the same day in order to prevent any ageing from occurring.
Cold spray deposition
The coating deposition was performed using a bespoke high-pressure cold spray system at the University of Nottingham. The rig setup was described in more detail in a previous publication . The coatings were deposited onto Al 6061 substrates at He gas pressure of 2.9 MPa at room temperature. He rather than N2 was chosen as a carrier gas in order to increase particle velocities. As no gas preheating was used, the use of He was considered necessary for the particles to reach critical velocity. Due to its monoatomic nature, the speed of sound is about three times higher for He than N2 leading to higher particles velocity. A commercial high-pressure powder feeder (Praxair 1264 HP, Indianapolis, IN, USA) was used at a pressure 0.1 MPa higher than the main pressure. A hardened stainless nozzle was used for the experiments, designed with an area expansion ratio of 8 and a divergent length of 150 mm. The nozzle was held stationary while the substrates were maintained on a x–y table. The substrate was located at 20 mm from the nozzle exit and four passes were sprayed onto the substrate by using a transverse speed of 60 mm/s in order to obtain the required thickness for an analysis of the coating deposition. An overlap of 2 mm was set to deposit a coating homogeneous in thickness. A 1-mm-thick coating was deposited using the heat-treated powder, whereas a 300-µm-thick coating was deposited using the feedstock powder.
The morphology of the powder was observed using a scanning electron microscopy (JEOL JSM 6490LV, JEOL Ltd., Japan) operating at 10 kV. The cross section of the feedstock powder and the deposits were evaluated using a field-emission gun SEM (7100F, JEOL Ltd., Japan) at an accelerating voltage of 15 kV. The cold-sprayed samples were sectioned using a precision saw at a cutting speed of 0.005 mm/s to avoid any damage on the coating during the operation. To examine the coatings in the transmission electron microscope (TEM), thin lamellae were prepared using a focused ion beam (FIB) lift-out method. A FEI Quanta 200 3D Dual Beam FIB-SEM was employed for the preparation to achieve a sample thickness of ~ 150 nm after thinning using a gallium beam at 20 kV. Samples were characterised using bright-field imaging, meaning that the bright area appearing in the micrographs correspond to transmitted area, whereas darker areas correspond to diffracted regions. Each sample was tilted in order to obtain images with minimal amounts of diffraction artefacts. The powders and cross-sectioned cold spray deposits were mounted in epoxy resin. Powders were subsequently ground with P1200 silicon carbide paper, whereas cold spray deposits were ground using P240, P400, P800 and P1200 grinding paper. Once ground, samples were polished using 6 and 1 μm diamond paste. Final polishing was performed using a colloidal silica (0.06 μm) suspension. The microhardness analysis was performed using an MMT-7 Vickers Microhardness instrument (Buehler, IL, USA). Each sample underwent eight measurements. A 10-gf load was applied for 10 s for mounted particles and coatings in cross section. The porosity of the samples was measured on 10 random fields obtained in the backscattered electron mode (BSE) at 500× magnification. ImageJ (US National Institute of Mental Health, MD) was used for that matter, and the operator used thresholding to separate pores from microstructural artefacts.
Electron backscatter diffraction
EBSD was performed on a FEG SEM (7100F, JEOL Ltd., Japan) at an accelerating voltage of 15 kV. EBSD scans were carried out using AZtechHKL software (Oxford Instruments, UK). The data obtained were post-processed using the HKL Channel 5 Tango software (Oxford Instruments, UK). A noise reduction of a factor 3 was carried out, corresponding to the examination of each non-indexed pixel. If the non-indexed pixel has more than a specified number of indexed neighbours (in this case 6), it will be replaced by the most common neighbouring orientation [24, 25]. High-angle grain boundaries (> 15°) and low-angle boundaries (2°–15°) are, respectively, represented on the Euler maps by black lines and white lines. Misorientation lines were also plotted to measure the orientation variation across particles. Point-to-point curves indicate the profiles of the orientation fluctuations between neighbouring points, whereas point-to-origin curves indicate the profiles of the orientation between all points and a reference point. Moreover, the local misorientation maps were plotted using AZtechHKL software. The average misorientation between each measurement and its eight neighbours were calculated, excluding grain boundaries higher than 5° in angle. Typically used to localise strain variations, a colour gradient is used from blue corresponding to the absence of misorientation to a yellow/green colour indicating a high misorientation.
Solution heat treatment of the feedstock powder
In order to get a more thorough understanding of the effect of the heat treatment on the powder particle microstructure, the grain distribution and orientation was observed using EBSD.
The Euler scans were here chosen to be presented as they reveal the orientation of the grains on a 3-axis system, more detailed than the Inverse Pole Figure. On those pictures, low-angle grain boundaries (LAGB) and high-angle grain boundaries (HAGB) are added to the scans in order to understand further the effect of the heat treatment. The feedstock Al 6061 presents a wide number of LAGB at its extremities and especially in the bottom-left part. By contrast, the heat-treated powder particle is mainly constituted of HAGB, presenting only one major LAGB in the centre of the particle. The distinction between LAGB and HAGB is crucial for understanding EBSD data, since LAGB are generally described as an array of dislocations , whereas HAGB are considered typical of newly formed grains, such as recrystallised grains . Although XRD can measure microstrains through analysis of diffraction peaks, it was not used in this study as XRD offered little information concerning any precipitate phase. The phases were not detected due to a low solute atoms content, and only aluminium peaks were observed.
Cold spray deposition
The coating deposited using the solution heat-treated powder (Fig. 8b) presents a homogeneous microstructure, already observed in the heat-treated particles. The nanometric precipitates seen using EDS and SEM are also present in the coating microstructure. An intimate bonding is observed between the majority of the particles, indication of a better cohesion in the coating.
Overall, the feedstock and the solution heat-treated powder exhibited two different behaviours. A better deposition was observed in the case of the heat-treated powder, revealing an intimate bonding with limited porosity, measured at about 0.3 ± 0.13%; on the other side, a much higher porosity is noticed in the coating produced from feedstock powder, reaching around 1.3 ± 0.43%. The gaps surrounding most of the particles are a clear indicator of a particle impinging on a previously deposited material which is deforming but not forming a sufficiently strong bond to create a pore-free coating.
The microhardness of both powders prior to the process was measured as well as a value in the centre or the resulting coatings. The feedstock Al 6061 powder was measured at 105.3 ± 3.2 HV, whereas the hardness of the particles post-heat treatment was evaluated at 82.5 ± 5.6, 24 h after spraying. The coatings hardness were also measured 24 h after spraying, where the coating deposited using feedstock Al 6061 powder was measured at 135 ± 6.2 and the one sprayed using the preprocessed powder resulted in a coating at 123.5 ± 3.6 HV. The hardness values highlight the work-hardening occurring during deposition.
The homogenisation of powder microstructure through solution heat treatment in recent studies [15, 16, 17] showed a major impact on cold spray deposition. Microstructural modification of gas-atomised aluminium alloy powder by homogenisation resulted in a reduced porosity, higher deposition efficiency upon cold spraying, and a different dislocation and grain boundary size distribution. This section will cover the reasons leading to these behaviours based on the analysis of the microstructure of the powder and the coating. First, the microstructural modification of the powder particles will be discussed followed by a discussion on the bonding phenomena taking place in these two coatings.
The Al 6061 powder was homogenised by raising the temperature of the material above the solvus curve, as expected in a solution heat treatment process. The microstructural modification observed in previous studies on Al7075  and Al2024  was also noticed in the present study, accompanied by the formation of submicron precipitates after the heat treatment. While the formation of precipitates goes potentially against the goal of this project, the dissolution of the intermetallic network was believed to be the crucial parameter in order to increase the quality of cold spray deposition by giving the aluminium matrix the possibility to deform around the nanometric-scaled precipitates. The dissolution of grain boundaries precipitates during solution heat treatment of gas-atomised aluminium alloy powder 6061 was recently investigated on a chemical composition level. It was confirmed that the number of secondary phases was greatly reduced, especially the proportion of Mg2Si, which drastically decreased after the first minutes of heat treatment . Although various treatments time were used, the complete homogenisation of the microstructure was never achieved, the dissolution of some phases being overlapped by the formation of others during extended heat treatment times. Two different coatings microstructure resulted from the spraying of the feedstock and the heat-treated powder. As cold spray is based on the plastic deformation of the incident particles, porosity can be used as a deformation and bonding indicator. Junction voids and gaps between particles are used to assess the optimum deposition conditions for a material. Often depending on spraying parameters, it can be corrected by increasing the gas temperature or pressure. In this study, the two coatings (Fig. 8) exhibit a major difference in porosity (highlighted by the increase in thickness for the same spraying conditions), although having been sprayed in the same conditions, which suggests an important modification in the particles’ deposition behaviour.
The feedstock powder was found to contain a certain amount of LAGB (Fig. 7), located mostly towards the edge of the particle. LAGBs is often assumed to be composed of an array of dislocations, their presence in the powder is a potential indicator of residual stresses in the droplet post-atomisation [19, 29, 30]. In comparison, the solution heat-treated powder has a negligible amount of LAGB, coherent with the formation of the stress-free grains post-heat treatment. The reduction of LAGB in the heat-treated powder was attributed to the recrystallisation taking place during the heat treatment. At the heat treatment temperature, the material recovers and recrystallises where new defects-free grains are formed. The equiaxed shape of the grain in the solution heat-treated powder is also an indicator of recrystallisation .
In this study, EBSD results [the band contrast (Fig. 9) and the local average misorientation (Fig. 13)] of the coating deposited from feedstock particle reveal that deformation and strain are distributed homogeneously across the whole particle. The large misorientation was observed in between each HAGB (Fig. 12) where various misorientation between 1° and 10° were detected in between different measured points. The presence of a wide distribution of LAGB in the centre of the particle is also an indication of heavy deformation in the particle interior, LAGB being often considered as an array of dislocations. On the other hand, LAGB in the coating deposited using the heat-treated powder appear to be distributed mainly at the particle interface. The local average misorientation data, typically used to measure the local strain variations, exhibits a gradient from blue (low misorientation) to green (high misorientation) from the centre of the heat-treated particle to its extremities. This is in contrast with the homogeneous strain distribution observed in the coating with the feedstock powder.
During the high strain rate impact (106–9 s−1) the localised temperature rise combined with the severe plastic deformation (SPD) results in phenomena such as adiabatic shear instability and dynamic recrystallisation which has been showed to play a significant role in particle bonding [31, 34]. The microstructure of the resulting coating is representative of the thermomechanical phenomena taking place during impact. Three different regions are commonly observed in a typical cold spray deposit. The first one, corresponding to the particle interiors, is relatively lightly deformed which can be easily observed in the present coating deposited using the solution heat-treated powder (Fig. 9) where the centre of the particle presents a bright colour indicating a high quality of Kikuchi bands and thus a relatively low deformation. The two following regions are located closer the particle–particle interface, where various mechanisms occur due to the extensive shear deformation. The accumulation of dislocations in the interfacial area at temperature and strain exceeding certain values area then leads to the formation of elongated subgrains corresponding to the second region in a cold spray deposit microstructure. This phenomenon is subsequently followed by a subdivision of those subgrains into a third region. This grain refinement in the interfacial area was observed in various alloys using TEM and also shown using EBSD and the grain boundary distribution in aluminium deposits [13, 35, 36]. In that sense, the role of dislocations in the cold spray bonding mechanisms is critical, and their ability to accumulate and lead to this dynamic recrystallisation and the formation of subgrains is a prerequisite for successful bonding. The presence of a clear strain concentration in the particle–particle interfacial regions combined with the reduced porosity suggests that this phenomenon is responsible for an intimate bond between the particles. In this study, the limited indexing in the interfacial area does not provide sufficient information to observe the recrystallisation and give an accurate measurement of the subgrain formation in this area. Nonetheless, the difference in strain gradient at the interfacial particle boundaries at the feedstock and the heat-treated coatings can be used at an indicator of the localised deformation in each particle.
A recent study showed that the presence of secondary submicrometric phases in metallic powder can act as nucleation sites for recrystallisation across the whole particle during cold spray deposition . The accumulation of dislocations around the secondary phases led to dynamic recrystallisation inside the particle, which reduced the number of dislocations in the particle–particle interfacial area due to a more homogeneous deformation throughout the particle. This phenomenon of localised dislocation accumulation has been also observed in cases of severe plastic deformation of various materials during processes such as equal channel angular pressing  or accumulative roll bonding . The interaction of dislocation with precipitates and second-phase particles was thoroughly studied using TEM, where a high dislocation density was observed in their surrounding area . Dynamic recrystallisation and grain refinement were thus observed in the vicinity of the particles due to the dislocation accumulation in these regions.
The solution heat-treated powder, appearing free of dislocations/lattice defects and exhibiting a larger grain structure compared to the feedstock powder, showed a decrease in porosity once deposited (Fig. 8) as well as a large increase in coating thickness. The proportion of precipitates in the particles microstructure was recently quantified as a function of heat treatment time . It was confirmed that a thermal processing was reducing the quantity of the majority of the precipitates that can be found in as-atomised particles. Despite any notable difference in the precipitate density between the heat-treated powder compared to the feedstock powder (Fig. 3), the microstructure appears very different: the dissolution of solute elements, and the clustering of some others resulted in the loss of the cellular microstructure of a typical gas-atomised Al alloy powder. This modification is believed to alter the cold spray deposition of the particles. The discontinuous lines of precipitates surrounded by a soft solid solution of aluminium are favourable during deformation, as the aluminium matrix can deform around the precipitates during deposition. The aluminium particle has the possibility to deform more homogeneously, resulting in a more localised deformation at the particle–particle interface, where most of the severe plastic deformation occurs.
On the other hand, the inhomogeneous microstructure exhibited by the feedstock particles does not allow the particles to deform very well, requiring much more peening from the incident particles to form the splat shape needed for cold spray deposition. As the brittle intermetallic network remains in the particle microstructure, the strain is not only localised in the particles’ exterior, but also highly present around the interdendritic region, illustrated by the local misorientation distribution (Fig. 13).
The TEM images of both coatings’ interface do not allow us to quantify this deformation as the dynamic recrystallisation appears relatively limited, but the decrease in porosity and the increase in coating deposition highlight the improvements offered by the preprocessing of the powder.
The feedstock powder underwent a recrystallisation process during the heat treatment, leading to the formation of larger equiaxed grains post-process. An absence of LAGB prior to spraying was also observed in the heat-treated particles.
A more intimate bonding was observed between the particles in the coating deposited using the solution heat-treated powder, whereas gaps and inter-particles voids were characterised in the coating sprayed using the feedstock powder.
The coating deposited from feedstock powder exhibited a homogeneous distribution of strain and misorientations across the whole deposit as revealed by the misorientation lines and the local average misorientation graph, correlated with the cellular microstructure exhibited by the particles as-atomised.
The coating deposited from solution heat-treated powder appeared to be relatively free of strain in the particle interiors whereas the particle extremities exhibited a large number of misorientation. This was attributed to the partial dissolution and the clustering of solute atoms, allowing the aluminium matrix to deform around the newly formed precipitates.
This study provides an understanding of the benefits of the heat treatment of aluminium alloy powder prior to cold spray. It gives the possibility of considering an extended use of this process in industry using larger scale facilities as an improvement of the resulting coatings properties for similar process parameters is highly beneficial to any application. Not only providing improvement on a microstructural level as showed in this study, the heat treatment of feedstock powder offers large benefits for structural repair. Solution heat treatment is a typical heat treatment performed to reach peak strength in aluminium alloys but requiring temperatures too high to be used for repair. By solutioning the powder prior to the spray run, it is believed that a low-temperature ageing would be enough to develop a homogeneous distribution of fine precipitates, reaching optimum properties suitable for repair applications.
This work was supported by the Engineering and Physical Sciences Research Council [Grant Number EP/M50810X/1]; in the form of a CASE Ph.D. studentship and industrial funding from TWI via the National Structural Integrity Research Foundation (NSIRC). The authors also acknowledge support from Ms Heidi De Villiers Lovelock and Dr. Henry Begg at TWI for editorial work and valuable technical discussion, Mr. Rory Screaton at the University of Nottingham for conducting the cold spray experiments, Dr. Nigel Neate for performing the TEM characterisation and Mr. Kamaal Al-Hamdani for providing samples for experiments.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All the data used for this study have been included in the document.
- 6.Shiwen H, Yong L, Sheng G (2009) Cooling rate calculation of non-equilibrium aluminum alloy powders prepared by gas atomization. Rare Metal Mater Eng 38:353–356Google Scholar
- 7.Kong CJ (2004) Microstructural evolution in AlSn-based gas atomised powder and thermally sprayed coatings. University of NottinghamGoogle Scholar
- 17.Nutt PS, Rokni R (2017) Structure/property relations for CS-5056 Al vs wrought. In: CSATGoogle Scholar
- 22.ASM International. Handbook Committee (1991) Heat treating of aluminium alloys. In: ASM handbook: heat treating, ASM International, pp 841–850Google Scholar
- 28.Hatch JE (1984) Microstructure of alloys. In: Hatch JE (ed) Aluminium properties and physical metallurgy. ASM International, Russell, pp 58–104Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.