The Laser-assisted Cold Spray process and deposit characterisation

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Abstract

Laser-assisted Cold Spray (LCS) is a new coating and fabrication process which combines the supersonic powder beam found in Cold Spray (CS) with laser heating of the deposition zone. LCS combines some advantages of CS: solid-state deposition, high build rate and the ability to deposit metals onto a range of substrates, with reduced operating costs which arise from not using a gas heater and replacing helium with nitrogen as the process gas. A system has been developed to impact metallic powder particles onto a substrate which is locally heated using a diode laser. A pyrometer and control system are used to record and maintain impact site temperature. In this study, < 50 µm powder particles are measured to be traveling at around 400 ms 1, and heated to temperatures between 450 °C and 900 °C when they impact the substrates. Build rates of up to 45 g min 1 were achieved for deposits with less than 1% porosity. Oxygen and nitrogen content in the deposits were measured to be less than 0.6 wt.% and 0.03 wt.% respectively.

Introduction

Recent trends in the area of Rapid Prototyping (RP) and Manufacturing (RM) have led to the development of processes which provide end users with metal components that are not simply form and fit prototypes, but allow functional load-bearing testing or actual in-service use [1]. Similar technologies are also used as a means of surface modification and coating of components for increased functionality. Many processes such laser Laser Engineered Net Shaping (LENS) [2] and Direct Metal Deposition (DMD) [3] are based on laser cladding and sintering, and use a laser to melt and fuse metallic powder into a net or near-net shape on a layer-by-layer basis.

Lasers can also be used with thermal spray processes such as high velocity oxy-fuel (HVOF) [4] or plasma spray [5]. This is typically either a surface preparation step to increase particle–substrate bonding and improve wear resistance [6], or more often as a post-processing step to modify microstructure [4], [7].

Drawbacks of high-temperature processes such as laser cladding and traditional thermal spray processes include deposit-substrate dilution, high thermally induced residual stresses, and as-solidified microstructures which lead to component distortion [8] and poor mechanical properties. There is also often a requirement for a high purity inert atmosphere to prevent oxidation during processing [9].

An increasing amount of work is focussing on the development of a non-melting deposition process, known as Cold Spray (CS) and related processes such as Cold Gas Dynamic Manufacturing (CGDM) [10]. In CS, powder is entrained in a supersonic gas jet which accelerates the particles to velocities above their ‘critical velocity’ required for deposition, often approaching 1000 ms 1 before they impact the substrate. On impact the particles undergo significant plastic deformation which leads to localised heating and flash welding at the interface [11]. This technique can be used to produce thick coatings and components [12]. However there are inherent problems associated with the ‘cold’ mechanism of deposition: high operating costs such as gas consumption and gas heating, inherent compressive residual stresses, and a reduction in bond strength and density when moving from depositing weaker materials to harder, stronger materials such as titanium alloys. While softer materials such as copper or aluminium can be deposited successfully using nitrogen, in order to increase deposition efficiency and deposit higher strength materials it is necessary to increase particle velocity by heating the process gas [13] and, in some cases, replacing nitrogen with helium. Gas heaters currently in use are up to 50 kW and are used to heat substantial amounts (> 2000 slpm) of gas to temperatures as high as 800 °C, which significantly increases both capital and running costs of the process.

Helium is an attractive choice for use as a process gas, as its high sonic velocity leads to increased particle acceleration [14]. In order to sustain sufficient mass flow rates of gas for particle acceleration, and to ensure high powder feedrates do not compromise nozzle performance, the diameter of the throat of the supersonic nozzles used is generally of the order of several millimetres. This, combined with pressures of up to 40 bar which are required to obtain the high speeds needed, means flow rates can easily approach 3000 standard litres per minute (slpm) [12]. Unfortunately this comes at a price; helium is around 30 times more expensive than nitrogen, and for a given operating pressure gas consumption is also higher [10]. This results in total gas cost being 80 times that for nitrogen, as shown in Fig. 1.

During CS, instantaneous, plastic deformation results in extremely localised heating of the order of one micron [11] around the particle surface. This means that although there is no increase in oxygen and nitrogen levels, hardness of the deposit is increased through cold working or strain-rate hardening [17]. It was suggested by Sakaki and Shimizu [15], modelled by and demonstrated by Dykhuizen and Neiser [16], that an increase in particle temperature can lead to an increase in deposition efficiency and possibly even a reduction in the critical deposition velocity as a result of particle softening [17]. However, increased gas temperatures heighten the risk of nozzle fouling when spraying low melting point metals such as aluminium [18], so a method of heating the metal particles without clogging the deposition nozzle would be essential. Increased deposition temperature may also help to overcome issues of low bond strength in CS coatings, which have been attributed to the extremely short timescales available for bonding to occur [11].

In Laser-assisted Cold Spray (LCS) impact temperatures are increased though laser heating of the deposition site. This avoids the problems associated with increased process gas temperatures, eliminates the need for gas heating or helium and aims to increase the range of materials which can be deposited. The amount of heat and the rate at which it is applied and removed from the impact zone affects both the rate of deposition and the properties of the material. Increasing particle temperature during traditional thermal spray processing usually increases oxide levels hence hardness [19]. However, during LCS, while there is a temperature increase the material remains non-molten. This means oxide and nitride levels remain comparable to those in CS, but one could expect to see an annealing effect if the deposited particles are cooled at a suitable rate to allow recovery.

LCS will enable the coating, and fabrication of near-net shape components with little or no melting, thus avoiding many of the thermal stressing, distortion, dilution and microstructural problems associated with many laser based technologies. The process was first illustrated in 2006 [20], where a laser was used to heat the deposition site in order to soften the substrate and particles, and allowing aluminium to be deposited using unheated nitrogen at around half the usual gas pressure of CS. The process also shows the capability of allowing deposition to be switched using the laser; when the laser is not on, deposition does not occur as particle velocity is below the CS critical velocity, which may benefit certain applications [21]. Recent work from another research group has been published describing a low-pressure (6 bar) Laser-assisted Cold Spray process for spraying composite metal oxide coatings, which uses similar principles but relies on a powerful laser (6 kW) and gas heating to between 445 °C and 650 °C [22]. Copper and nickel powders were each mixed with alumina and deposited onto low carbon steel at 2400 mm min 1 — while copper coatings were fully dense, nickel coatings showed porosity at lower temperatures and cracking at higher temperatures.

The aims of this paper are to provide an introduction to the LCS process, detail the particle velocities and temperatures under which deposition occurs, examine the effect of temperature on build rate and coating structure, and compare the coatings produced with those deposited using cold spray and HVOF. Although previous work has used LCS to deposit 316L stainless steel and aluminium, this work focuses on the deposition of titanium tracks and coatings. Titanium was chosen since it is a material which has high value applications in areas such as the biomedical and aerospace industry but which has proven difficult to deposit satisfactorily using CS [26], although recent work has shown coatings produced using a modified HVOF technique [28]. However, the majority of current thermal spray technologies such as plasma spraying involve melting the powder and therefore must be carried out in controlled atmospheres to avoid problems with oxidation [27]. Interstitial oxygen and nitrogen, while increasing the hardness of a coating, generally reduces its ductility and can have a detrimental influence on its cohesion [7]. Coatings with increased hardness and corresponding reduction in ductility means they are more susceptible to cracking which means they may not be suitable for certain applications.

Section snippets

LCS equipment

A schematic of the LCS system is shown in Fig. 2. A high pressure (10–30 bar) nitrogen gas supply is split and sent to a converging-diverging (de Laval) nozzle both directly and via a Praxair 1264HP high pressure powder feeder where metal powder particles are entrained. The two streams recombine and pass through the nozzle where they are accelerated to supersonic speeds. The high-velocity, powder-laden gas jet exits the nozzle and is directed towards a substrate. The powder stream impacts a

CFD

CFD was used to model:

  • Gas flow through the NDLV, HDLV, HMLN 200, HMLN 150 nozzles.

  • Particle velocity through the NDLV, HDLV and HMLN 200 nozzles at 20 bar.

CFD showed a difference of less than 50 ms 1 in gas or particle speeds between the nozzles studied — the NDLV nozzle was selected for further experiments, as it achieved the highest maximum gas velocity of 655 ms 1, and maximum particle velocity of 442 ms 1 for steel and 471 ms 1 for titanium particles, as shown in Fig. 6. It should be noted

Discussion

CFD and PIV were used to determine which of the available deposition nozzles would accelerate the powder particles to the highest velocities possible under conditions used for LCS. CFD predicted that even though LCS operates using unheated gas, the linear de Laval nozzle that had previously been designed for nitrogen as the working gas at 400 °C would give the best velocities from the nozzles available, and PIV verified these results. It was found that the titanium particles were accelerated to

Conclusion

LCS has been demonstrated as a viable method for the deposition of metallic coatings, most notably titanium. Oxide-free titanium coatings have been deposited without the use of gas heating at velocities around half of those required in cold spray. For impact velocities of approximately 400 ms 1, dense coatings were produced between 650 and 900 °C, well below the melting point of titanium (1668 °C). The coating structure and impurity content compare well with those observed for cold sprayed and

Acknowledgment

This research was supported by the Engineering & Physical Sciences Research Council.

References (31)

  • J. Mazumder

    Opt. Lasers Eng.

    (2000)
  • J. Pattison

    Int. J. Mach. Tools Manuf.

    (2007)
  • H. Assadi

    Acta Mater.

    (2003)
  • R. Morgan

    Mater. Lett.

    (2004)
  • M. Kulmala et al.

    Surf. Coat. Technol.

    (2008)
  • J. Kawakita et al.

    Surf. Coat. Technol.

    (2006)
  • T. Wohlers

    Wohlers Report

    (2007)
  • R.R. Unocic et al.

    Metall. Mater. Trans. B Proc. Metall. Mater. Proc. Sci.

    (2004)
  • T. Schnick et al.

    J. Therm. Spray Technol.

    (1999)
  • A. Ohmori et al.

    J. Therm. Spray Technol.

    (1993)
  • C. Coddet et al.

    ASM Int.

    (1999)
  • L. Pawlowski

    The Science and Engineering of Thermal Spray Coatings

    (2008)
  • R. Vilar

    J. Laser Appl.

    (1999)
  • K.H. Richter et al.

    Laser Cladding of the Titanium Alloy Ti6242 to Restore Damage Turbine Blades, International Congress on Applications of Lasers and Electro-Optics

    (2004)
  • Pattison, J., Ph.D Thesis: Cold Gas Dynamic Manufacturing, in Institute for Manufacturing, Department of Engineering....
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