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A Novel Coaxially Laser-Assisted (COLA) Cold Spray System

   

Allen C M, Marrocco T, McNutt P
TWI Ltd., Great Abington, Cambridge, CB21 6AL, United Kingdom.

Koivuluoto H, Latokartano J, Vuoristo P
Tampere University of Technology, Korkeakoulunkatu 6, FI-33720 Tampere, Finland.

Olsson R
Luleå University of Technology, SE-971 87 Luleå, Sweden.

Paper presented at ITSC 2015, 11-14 May 2015. Long Beach, CA, USA.

Abstract

Laser-assisted cold-spray has been recognized for over a decade as a technique capable of depositing high quality coatings. By laser heating (and hence softening) the surface being coated, deposition can occur at particle velocities lower than those normally associated with the cold spray process.

This can be used to increase deposition rate. However, it can also be used to facilitate the deposition of higher hardness material combinations, normally more out of the reach of the conventional cold spray process. Laser heating can also reduce the requirements of the process on gas usage and gas heating for a given combination, making it more costeffective.

In the work reported below, the capability of a novel co-axially laser-assisted system (COLA) to deposit higher hardness materials, relevant to a range of different industrial applications, has been evaluated. This system can be retrofitted to conventional cold spray equipment.

1. Introduction

Coating and deposition processes play important roles in key industries, improving the performance or aesthetics, adding value and function, or repairing and extending the service life of a wide range of components.

A number of coating processes have developed over the years to address the industrial needs of metallic parts. These include thermal spray methods (electric arc, flame, plasma, high velocity oxy-fuel (HVOF) etc.), arc cladding methods (e.g. submerged arc strip cladding) and laser cladding methods (using wires or powders). All these methods involve the partial or complete melting of the material being deposited, and sometimes the target substrate. As a result, these methods can result in oxidation, porosity, the introduction of solidification cracks and/or residual tensile stresses, and significant strength loss in the material being deposited and the underlying substrate.

Consequently, deposition methods avoiding material fusion can also be of interest for more demanding applications.

Examples include explosive cladding, friction surfacing and cold spray.

Cold spray is particularly well suited to the coating of parts with a range of different geometries and has access to a wide range of different metallic powder coatings. In cold spray, supersonic powder particle velocities and ‘low’ (e.g. <0.5-0.7Tm) temperatures are involved. As most of the process energy is kinetic, a certain critical velocity must be exceeded for deposition, via deformation of the interface between the impinging particle and the substrate, rupturing of any oxide films present and, with appropriate choice of materials and conditions, the formation of an adequate solid state weld [1].

The powder particles are injected in to a pressurized gas, passed through a nozzle designed to produce supersonic exit velocities (e.g. a de Laval nozzle). However, achieving sufficient velocity for higher hardness materials can still prove difficult, and this approach has to be combined with high inlet pressures (e.g. to 50bar), high gas temperatures (e.g. up to 1100ºC), and/or the use of helium (as opposed to air or nitrogen), in which the highest particle velocities can be reached. Having to resort to helium can prove prohibitively expensive for all but the most value-added applications.

Substrate heating is one approach that can be taken to reduce the critical velocity needed, and thus avoid helium usage. To this end, infra-red laser beam substrate heating has attracted a lot of interest, as infra-red lasers serve as directional, controllable heat sources, which can be readily targeted at the substrate at the same time as the particle/gas jet exiting the cold spray nozzle. As such, the benefits of laser-assisted cold spray have already been demonstrated by a number of different authors [2-7].

In the current work, the capabilities of a proprietary new system for coaxially laser-assisted cold spray of higher hardness powder/substrate combinations, referred to as COLA, have been evaluated. When formatting your paper, do not use Ventura or Corel Draw files.

2. Experimental Details

2. 1. Cold Spray Equipment

Spray trials were carried out using two conventional cold spray systems; a CGT Kinetiks 3000 system (with inlet gas pressure to 30bar) and a CGT Kinetiks 4000 system (to 40bar). To demonstrate the advantages of the new COLA equipment, all spraying experiments were carried out using conventional de Laval type nozzles, at stand-off distances typically in the range 40-50mm from the substrate, and using nitrogen with gas heating to ≤400ºC.

2. 2. Laser Equipment

Laser heating was carried out using proprietary laser equipment. This equipment was designed deliberately to retrofit to existing cold spray systems, i.e. no modifications to the existing cold spray apparatus were necessary.

2. 3. Process Monitoring and Control Equipment

Process monitoring was carried out during the spraying experiments. A temperature sensor signal was measured, that signal originating from the zone where particles were impacting on the laser beam heated substrate. Following the determination of suitable deposition conditions, closed loop feedback control equipment was developed, to maintain the temperature sensor signal at a suitable level, by adjusting laser beam power in real time during spraying.

Process monitoring was carried out during the spraying experiments. A temperature sensor signal was measured, that signal originating from the zone where particles were impacting on the laser beam heated substrate. Following the determination of suitable deposition conditions, closed loop feedback control equipment was developed, to maintain the temperature sensor signal at a suitable level, by adjusting laser beam power in real time during spraying.

2. 4. Materials

Seven different material combinations were evaluated as candidates for laser-assisted cold spraying with the COLA equipment. Although full details of these combinations cannot be released, these encompassed:

  • Two different Cu bronze alloys on steel (e.g. as wear and/or corrosion resistant coatings).
  • Two different Ni alloys on steel (also as wear and/or corrosion resistant coatings).
  • Peak aged Al alloy on peak aged Al alloy (e.g. for repair).
  • Ni alloy on Ni alloy (e.g. for repair).
  • Cu on to ceramic-coated steel (e.g. for electronics applications).

Substrates were typically flat, rectangular coupons, measuring 50 x 100 x 3-10mm thickness. These were manipulated robotically from side-to-side in front of a fixed spray apparatus.

In all cases, powders suitable for conventional cold spray were used, manufactured either by Sandvik Osprey (Cu) or TLS Technik. Particle size ranges were typically either -38 +10 μm or -25 + 5 μm, with spherical particle morphologies.

2. 5. Scope of Work

The energy density of the laser assistance used in the COLA equipment was checked as a function of laser stand-off. A suitable range of stand-offs was then selected, providing densities thought sufficient for substrate heating. This energy, the centre of the powder/gas jet and the centre of the field-ofview of the temperature sensor used for process monitoring were then aligned with each other. Temperature signals were then measured as a function of coupon type and coupon traverse speed, and conditions suitable for substrate heating determined.

Spray trials were then carried out using conventional cold spray conditions based largely on previous experience (spraying gas temperature and pressure, powder feeding rate etc.), and either with or without laser assistance.

In each trial the deposited materials were examined visually for signs of defects, e.g. cracks, spalling, surface breaking porosity, oxidation etc. Selected deposits were then crosssectioned and, following grinding and polishing, examined using a combination of optical microscopy and/or scanning electron microscopy (SEM). For coatings developed to offer corrosion resistance, open cell potential (OCP) testing was also carried out in a 3.5wt% NaCl solution. Conversely, for coatings where mechanical properties were more of interest, the hardness, oxygen and porosity contents of the coatings were measured, by Vickers micro-hardness (100g) indentation, chemical analysis and image analysis of scanning electron micrographs, respectively. Adhesion strength across the coating/substrate interface was also measured using a modified ASTM C633-01 test, as well as the cohesive strength of the coating itself, using a modified Tubular-Coating-Tensile (TCT) test [8].

3. Results

3. 1. Substrate Heating

Following the choice of a suitable working range of stand-off distances for the laser assistance (at or close to the focal plane of the proprietary laser design used), the spray nozzle was aligned with this assistance. Figure 1 shows a typical example of the time-based monitoring of a temperature signal from a sensor also aligned with the laser heated spray zone, as a coupon was tracked across the laser beam. This signal was relatively stable away from the coupon edges.

Figure 1: Example of time-based temperature signal from coupon tracking across laser beam.
Figure 1: Example of time-based temperature signal from coupon tracking across laser beam.

Analyzing temperature signals such as the example shown in Figure 1, Figure 2 shows one example of the variation in average temperature signals observed with traverse speed, from experiments on Ni alloy coupons. As would be anticipated, laser heat input increases as speed reduces, and the values of the temperature signals from the laser heated zone increase up to values of interest for laser assisted cold spray. The actual traverse speeds and the laser stand-off position used are not shown, as this information would reveal details of the proprietary laser equipment used.

Figure 2: Example of temperature signals received from Ni alloy coupons when tracking the laser beam across at different speeds.
Figure 2: Example of temperature signals received from Ni alloy coupons when tracking the laser beam across at different speeds.

Signals >400ºC were only recorded if laser heating was used.

3. 2. First-Round Spraying Trials

With suitable laser heating conditions established, cold spray trials were carried out with and without laser assistance. The microstructures of the coatings and their adherence to the
underlying substrates were then compared.

Figure 3 shows one example of the higher quality coatings achieved with laser heating, for Cu bronze tracks on steel.

Figure 3: Example of microstructures of Cu bronze cold sprayed tracks on steel: a. Without laser heating of the substrate. b. With laser heating.
Figure 3: Example of microstructures of Cu bronze cold sprayed tracks on steel: a. Without laser heating of the substrate. b. With laser heating.

Examples of poor adhesion to the substrate and poor cohesion within the coating are arrowed in Figure 3a. As Figure 3b shows, these defects appear absent in the laser assisted
coating.

A similar largely defect-free coating was achieved when spraying an Al alloy powder on to an Al alloy substrate with laser assistance, Figure 4.

Figure 4: Example of microstructure of Al alloy laser assisted cold sprayed coating on Al alloy.
Figure 4: Example of microstructure of Al alloy laser assisted cold sprayed coating on Al alloy.
Figure 5: Example of microstructures of Ni alloy coatings (sprayed on to steel substrates: not shown): a. Without laser heating of the substrate. b. With laser heating.
Figure 5: Example of microstructures of Ni alloy coatings (sprayed on to steel substrates: not shown): a. Without laser heating of the substrate. b. With laser heating.

As a third example, Figure 5 compares scanning electron micrographs of the microstructures of one of the Ni alloy coatings (deposited on steel), with and without laser assistance.

Similarly, when cold spraying a second Ni alloy on to a Ni alloy substrate, laser assistance also led to an apparent reduction in the number of defects in the coating. Nevertheless, the interface between the Ni alloy coating and the Ni alloy substrate appeared less well adhered than that between the Ni alloy coating shown in Figure 5 and its steel
substrate.

An analogous result was achieved when cold spraying Cu on to ceramic coated steel. The number of defects in the Cu coating itself appeared significantly reduced with laser assistance. However, laser assistance did not help that coating to adhere to the ceramic layer underneath.

3. 3. Second-Round Spraying Trials

With the COLA equipment confirmed as suitable for the spraying of certain powder/substrate combinations (e.g. Cu bronzes or Ni alloys on to steel, or peak aged Al alloy on to itself), further trials then took place on a subset of these combinations. These trials developed conditions suitable for the deposition of thicker coatings, more representative of industrial requirements.

Figures 6 and 7 show how the resultant coating qualities achieved, for Cu bronze coatings on to steel and Al alloy coatings on to Al alloy, are improved with suitable laser assistance.

Figure 6: Example of thicker Cu bronze cold sprayed coatings on steel with (left) and without (right) laser assistance.
Figure 6: Example of thicker Cu bronze cold sprayed coatings on steel with (left) and without (right) laser assistance.
Figure 7: Example of thicker Al alloy cold sprayed coatings on Al alloy with (left) and without (right) laser assistance. Coating thicknesses are typically between 1-2mm.
Figure 7: Example of thicker Al alloy cold sprayed coatings on Al alloy with (left) and without (right) laser assistance. Coating thicknesses are typically between 1-2mm.

3. 4. Process Control

Process control proved key to achieving consistently a coating quality of acceptable quality (e.g. Figure 6, left, and Figure 7, left). Dedicated control hardware and software were developed for this purpose.

Although details of the control hardware and software cannot be released, as the example in Figure 8 shows, with control disabled, the temperature signal during spraying increases to
>430ºC, as the coupon gradually heats up.

Figure 8: Example of temperature monitoring signal evolution without process control, during laser assisted spraying (side to side) of a rectangular steel coupon.
Figure 8: Example of temperature monitoring signal evolution without process control, during laser assisted spraying (side to side) of a rectangular steel coupon.

As Figure 8 shows, the temperature signal during spraying increases to >430ºC in this example, as the coupon gradually heats up.

Figure 9: (as Figure 8) but with process control enabled. Note different scales on axes.
Figure 9: (as Figure 8) but with process control enabled. Note different scales on axes.

With control enabled the temperature signal from the coupon remains within the 390-430ºC band required, as spraying proceeds, Figure 9. Control has been accomplished in this example using periodic reductions in laser power (lower line, arrowed).

3. 5. Coating Characteristics and Properties

The porosity and oxygen contents of the Cu bronze and Al alloy coatings shown in Figures 6 and 7 were measured and are summarized in Table 1.

Table 1: Porosity and oxygen contents of coatings.

CoatingLaser used?Porosity content, %Oxygen content, %
Cu bronze Yes 0.3 (10*) 0.16 (3*)
Cu bronze No ** 0.16 (3*)
Al alloy Yes 0.1 (10*) 0.18 (3*)
Al alloy No 0.5 (10*) 0.26 (3*)

* Mean of number of measurements shown in brackets.
** Not measured owing to poor coating quality.

As Table 1 shows, using laser assistance either maintained or reduced the porosity and oxygen contents of the coatings.

Figure 10 shows one example of the hardness surveys carried out through thickness, in this case from the Cu bronze coatings made with and without laser assistance. Laser assistance has hardened the substrate. The coating has softened slightly.

Figure 10: Vickers hardness values measured using 100g indenter load, through Cu bronze coating (negative distance values) on steel (positive values).
Figure 10: Vickers hardness values measured using 100g indenter load, through Cu bronze coating (negative distance values) on steel (positive values).

Estimates of the adhesive strength between the coating and the substrate, and the cohesive strength within the coating itself, are listed in Table 2 for these two coatings.

Table 2: Adhesive and cohesive coating strengths.

CoatingLaser used?Calculated mean adhesive strength, MPaCalculated mean cohesive strength, MPa
Cu bronze Yes 56.7 (4*) 29 (2*)
Cu bronze No ** 4 (2*)
Al alloy Yes 32 (2*) 177 (4*)
Al alloy No 8 (3*) 119 (5*)

* Mean of number of tests shown in brackets.
** Not measured owing to poor coating quality.

As Table 2 shows, laser assistance has increased both the adhesive and cohesive strengths.

Finally, Figure 11 shows the OCP test results for a second type of Cu bronze coating deposited onto steel, using a number of different laser assisted cold spray conditions.

Figure 11: Open circuit potential in 3.5wt% NaCl solution of Cu bronze coatings deposited on steel prepared, using different laser assisted cold spray conditions.
Figure 11: Open circuit potential in 3.5wt% NaCl solution of Cu bronze coatings deposited on steel prepared, using different laser assisted cold spray conditions.

Figure 11 suggests that coating barrier properties close to parent Cu can be achieved with appropriate spraying conditions.

4. Discussion

The work carried out has attempted to deposit higher hardness powder/substrate combinations. However, cold spray conditions which would normally be unsuitable (in nitrogen, with T≤400ºC) have been used deliberately, to accentuate any positive effects of the laser substrate heating employed.

The cold spray process is assisted using the COLA laser heating equipment under these conditions, for a number of materials combinations, including:

  • A Cu bronze deposited on to steel (Figure 3).
  • A Ni alloy deposited on to steel (Figure 4).
  • An Al alloy deposited on to an Al alloy (Figure 5).

In all these cases laser heating has improved both the adhesion of the coating to the substrate underneath, and the cohesion within the coating itself. Without this heating, lack of adhesion defects at the coating/substrate interfaces and voids within the coatings have been detected.

The capability of laser heating to avoid these defects becomes even more apparent as thicker coatings are deposited (Figures 6 and 7). Without heating, incidences of the spalling of coatings become evident.

In addition to reducing the number of defects in the coatings, laser assistance has also resulted in improvements in other coating characteristics and properties.

Estimates of coating porosity contents suggest that laser assistance can reduce porosity, perhaps at least five-fold.

Oxygen content measurements also suggest that the higher temperatures involved with laser heating do not lead to significant oxidation. Coatings with oxygen contents <0.2% have been achieved, and these levels are comparable with or actually lower than those found without heating.

Instead, the effect of the heating manifests itself as a slight softening of both the coating and/or the substrate immediately underneath it. However, a Cu bronze coating up to 81% as hard has still been achieved (Figure 10). A similar softening of the Al alloy coating was also measured, although a coating 84% as hard was still achieved.

The tensile test data given in Table 2 represents only a limited body of data, but suggests that the laser assistance from the COLA equipment also results in better adhering coatings (>30MPa adhesive strength) with better cohesive strength, than achieved without laser assistance.

As well as improvements in mechanical properties, laser assisted cold spray might also improve coating barrier properties. In this work, Cu bronze coatings can be produced that had similar OCP behavior to bulk Cu, suggesting dense coatings free of open-porosity. This indicates that an appropriately deposited coating might provide significantly better corrosion resistance than that achievable without laser heating.

Process monitoring (to determine a suitable processing temperature range) and closed-loop control (to then maintain the process within that range) have proven invaluable in the current work in achieving superior coating characteristics and properties. Temperature sensor signals from the processing zone appear sufficiently steady over time (Figure 1) that they can be used for control purposes, when appropriately filtered to minimize noise. The control developed has proven capable of holding the process to within +/-20ºC of a set value. This avoids process zone overheating, which can otherwise occur with a gradual buildup of heat (Figure 8), and is also useful as laser powers then quickly auto-adjust to different substrate heat sinking capacities.

Nevertheless, the approach taken in this work was less successful when tackling substrates with hardnesses >220HV0.1 approximately. As examples, sound coatings appeared possible, but lack of adhesion defects continued to be detected in spite of laser heating, when cold spraying:

  • A second Ni alloy on to itself.
  • Cu on to a ceramic coated steel.

These results suggest that the heat input available with the current COLA equipment design might limit the softening that can be achieved in these harder substrates.

5. Conclusions

Using controlled laser substrate heating in combination with the cold spray process improves the coating cohesion and coating to substrate adhesion of Cu bronze/steel, Ni alloy/steel and Al alloy/Al alloy materials combinations.

These improvements appear to feed through to reduced porosity contents, and higher adhesive and cohesive strengths. Furthermore, in spite of this heating, coating oxygen contents and coating hardnesses do not appear to be significantly altered.

Higher heat inputs appear necessary for the hardest substrates. These remain outside the current capability of the COLA equipment, but will be addressed by future developments.

6. Acknowledgments

This work has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 315157. Further information about the work can be found at www.cola-project.eu.

7. References

  1. Champagne, V. K. (Ed.), The Cold Spray Materials Deposition Process, 2007, Woodhead Publishing, ISBN 978-1-84569-181-3.
  2. Cockburn, A., Lupoi, R., Sparkes, M. and O'Neill, W., “Supersonic Laser Deposition of Corrosion and Wear Resistant Materials”, Proc. 37th International MATADOR 2012 Conference. pp. 387-391.
  3. Kulmala, M., Koivuluoto, H. and Vuoristo, P., "Influence of Laser Irradiation on Formation of Low-Pressure Cold Sprayed Coatings", Proc. Thermal Spray 2008: Thermal Spray Crossing Borders. pp. 950-955.
  4. Kulmala, M. and Vuoristo, P., "Influence of Process Conditions in Laser Assisted Low-Pressure Cold Spraying", Surface and Coatings Technology 202, 2008, pp. 4503-4508.
  5. P.S. Mohanty, WO Patent Application WO2011069101 A2.
  6. E. Calla and M. G. Jones, Patent US 8020509 B2.
  7. A. Cockburn, W. O'Neill, M. Sparkes, R. Lupoi, M. Bray, Patent Applications WO 2013061085 A1 and WO 2013061086 A1.
  8. Schmidt, T., Gartner, F., Assadi, H. and Kreye, H., "Development of a Generalized Parameter Window for Cold Spray Deposition", Acta Mat. 54, 2006, pp. 729-742.

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