Centre for Industrial Photonics

Laser-assisted Cold Spray

Schematic showing LCS process

Introduction

Laser-assisted cold spray (LCS) is a metal deposition technique which is being developed in order to allow the deposition of metal coatings and 3-D structures onto a variety of substrates. The LCS process has been developed from the Cold Gas Dynamic Manufacturing process and aims to capitalise on the advantages of cold pray while mitigating some of the limitations by removing the need for gas heating and helium as a process gas.

Aims

• develop a system for the deposition of metal and metal ceramic composite coatings by laser assisted deposition.
• demonstrate the ability to deposit coatings using cold nitrogen as the accelerating gas.
• establish the process parameters required for successful deposition of a variety of materials
• develop the capability to deposit industrially relevant coatings
• pursue collaborative partnerships in order to benchmark the process against current deposition technologies
• establish that coatings meet industrial requirements through chemical,  mechanical and microstructural characterisation

The process

LCS has been developed in order to widen the range of materials which can be deposited using nitrogen and eliminate the need for gas heating, dramatically reducing process costs and increasing the range of applications for which cold spray (CS) may prove attractive.

In LCS, powder is accelerated in a supersonic gas jet before it impacts the substrate. A laser heats the deposition site to between 30 and 80% of particle melting point. This significantly reduces particle strength so that, on impact the particles undergo significant plastic deformation which leads to localized heating and flash welding at the interface. The use of a laser allows impact velocities around half those used in CS (< 500 ms^-1). Reducing the required velocity allows cold nitrogen to be used as the process, gas reducing expenditure from £12 per minute for helium to around 10 pence per minute and reduces power consumption through the elimination of the gas heater. This reduction in operating costs means that LCS may be suitable for many applications for which CS has proved too costly, allowing the fully solid process route found in cold spray to find a use in a broad range of applications.

The LCS System

Schematic of LCS system	Schematic of De laval nozzle used in LCS

The LCS system consists of a high pressure (10-30 bar) nitrogen gas supply which is split and sent to a converging-diverging, (de Laval) nozzle both directly and via a high pressure powder feeder, where metal powder particles are entrained within it. 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 region of the substrate which is simultaneously illuminated by a 980 nm, 1 kW diode laser. Laser power is controlled by a closed loop feedback system which uses an infra-red pyrometer to monitor deposition zone temperature.

Snapshot of deposition in progress

Coatings

Coatings have been deposited using aluminium, 316L stainless steel, an Al-Ti mixed powder and titanium. Titanium has been chosen since it is a high value material which has applications in areas such as the biomedical industry but which has not yet been deposited commercially using CS. Present deposition technologies such as plasma spraying involve melting the powder and therefore must be carried out in controlled atmospheres to avoid problems with oxidation. Coatings produced so far compare well with CS with coatings of similar or superior structures being deposited at lower process costs.

• Dense, low oxide content titanium coatings have been deposited using particle velocities below 500 ms^-1.

LCS titanium coating deposited onto 1mm titanium plate without distortion

• Etched cross- sections show coating structure in LCS is similar to CS where spherical particles have undergone a large degree of deformation on impact with the substrate

Cold sprayed Al	LCS deposited Al

• Observed porosity compares favorably with CS with less than 1 % porosity observed in titanium LCS coatings

Cold sprayed Ti 3% porosity	LCS deposited Ti 0.5% porosity

• Coatings can be deposited under conditions which results in significantly lower gas and operating costs

Cold Sprayed Al-Ti 20 bar He 300 °C ~£12/min gas costs	LCS deposited Al-Ti 15 bar N2, room temp ~10p/min gas costs

Applications

In addition to work on titanium coatings for biomedical applications, work is currently underway to develop the capability to deposit hard coatings using the process. Current deposition technologies build up coatings from molten material, solidification shrinkage leaves the coatings with high residual stresses which can lead to distortion of thin sections. The use of LCS will enable coatings to be deposited onto thin sections which are difficult to process with current technologies by avoiding solidification shrinkage.

Publications

• Cockburn, A., Bray, M., and O'Neill, W. Laser assisted cold spray: the effect of temperature and velocity on coating characteristics, LAMP 2009, Kobe, Japan, 29 June - 2 July 2009.
• Bray, M., Cockburn, A. and O'Neill, W. The laser-assisted cold spray process and deposit characterisation Surface and Coatings Technology, 203 (19). pp. 2851-2857. ISSN 0257-8972
• Cockburn, A., Bray, M., and O’Neil, W. The laser-assisted cold spray process,The Laser User, Issue 53, Winter 2008.
• Cockburn, A., Bray, M., and O’Neil, W. LCS can reduce process costs and increase application range, in: Industrial Laser Solutions, November 2008.
• Cockburn, A., Bray, M. and O'Neill, W: The Laser-assisted Cold Spray process and deposit characterisation, in: Surface Coatings & Technology 203 (19) pp. 2851-2857.
• Bray, M. and Cockburn, A. and O'Neill, W. (2008) Recent developments of the laser-assisted cold spray process and deposit characterisation In: 3rd Pacific International Conference on Application of Lasers and Optics (PICALO), 16 - 18 April 2008, Beijing, China.
• Bray, M., Cockburn, A. and O'Neill, W. Recent developments of the laser-assisted cold spray process In: 9th National Conference on Rapid Design, Prototyping & Manufacture, 13 June 2008, Lancaster University.