Surface Engineering Part 3

Dr Zhu Liu

Corrosion and Protection CentreSchool of MaterialsThe University of ManchesterMATS 64532: Surface Engineering and Materials Design

Contents Lasers Interaction of laser beam with materials Laser surface engineering Application examples

Laser PrinciplesWhat is a laser?

LASER: Light Amplification by Stimulated Emission of Radiation

Features of laser beam Coherent electromagnetic radiation Monochromaticity (single wavelength) Coherency High radiance and low beam divergence Ability to focus to a small spot size

Types of lasers: pulsed and continuous-wave (CW)

Typical Industrial Lasers for Surface Treatment

Laser Beam Focusingdmin: minimum focus spot size:

D: beam diameter: laser radiation wavelengthf: focal lengthFor example: a CO2 laser beam (=10.6 m), D=38 mm, f=150 mm

dmin = 102 mFocusDefocusLensLaser beamDf

Interaction of Laser Beam with Materials

Depending on laser beam wavelength and materials, there are two beam/material interaction mechanisms:

Photothermal process

Photochemical processLASER BEAM OPTICAL ENERGY

Laser wavelength: infrared or far-infraredMaterials: metals, ceramics, glass Absorption: Interaction between the photons and electrons Vibration of electrons Vibration of lattice Temperature rise

Laser beam optical energy: it becomes thermal energy when it interacts with materials.Photo-thermal process

Photo-chemical Process Laser wavelength: short wavelength - UV range with high photon energy Materials: polymer

Laser beam/polymer interaction: Laser wavelengths in the UV range capable of breaking bonds of polymeric materials during processing, without generating much thermal effect.

Influencing factors on AbsorptionWavelength [1]Surface roughness [2]Temperature [3]

Laser Materials Processing The first public demonstration of laser materials processing

In May, 1967, the TWI, in England, successfully cut through tool steel (1/10 inch) using a CO2 laser. James Bond film, Goldfinger, 1964, filmed in England

Laser Materials ProcessingMaterials RemovalCutting, drilling, Marking/Engraving, Slotting/Grooving, Scribing, Trimming, Micro-machining, Cleaning JoiningWelding, Soldering, Brazing, Glue hardening, Epoxy curing Surface TreatmentTransformation hardening, Shot-peening, Melting, Alloying, Cladding, Annealing, Particle injection, L-PVD, L-CVD, L-sol-gel

Laser Surface Treatment Non-melting processes Laser transformation hardening Laser annealing Laser shock peening Melting processes Laser melting Laser alloying Laser cladding Other laser processes Laser physical vapour deposition Laser chemical vapour deposition Laser gel-gelLog (interaction time), D/v, s

Range of laser processes mapped against power density per unit time [4]Log(power density)

Processing SchematicsWorkpieceCNC tableLensBeam guidanceLASERvScanningdirection

Laser Power (W): P Laser Beam Size (mm): DScanning Velocity (mm/s): v

Power density: 4P/pD2Interaction time, D/v

Characteristics of LSE Rapid heating Rapid cooling Microstructure Refinement of microstructures Extended solubility of an element in another Formation of non-equilibrium phases, like amorphous Properties Corrosion, wear and etc.

TimeTemperatureCooling rate: 104 1011 K/s

Laser Transformation HardeningTraverse directionLaser beamSubstrateAc1Ac3

Laser Transformation Hardening for Large Area Coverage Overlapping effect

Softer and harder regions due to back tempering Beneficial effect on wear performanceRe-heated region back tempering

*Features of Laser Transformation Hardening Advantages:

Selective hardeningBetter fatigue propertiesSelf-quenchingLess thermal distortion than othersSuitable for complex geometryNo post-treatment required

Limitations:

High capital cost and low efficiency for large area treatment

LTH Application Examples

Controller shaft (100Cr6, 0.45mm hardened depth. 850Hv) [5] Application ExamplesLaser hardening of a steel bottle-openerLaser hardening of saw toothEnhanced wear performance: Bearing Wear areas Seal areas Gears Cams Engine components Cutting, bending, forming edges

Laser Surface Melting Laser inputabcde Solidification microstructure Dendritic structure

Cooling rate controls microstructural# size; the faster the cooling rate, the finer the microstructure is.

Laser Surface melting - LSMThick-layer melting:Melt depth: 20 200 mCooling rate: 104 106 K/sThin-film melting:Melt depth: 1 20 mCooling rate: 107 1011 K/sApplicationsSurface protection Wear Corrosion Other propertiesPre-treatment Anodising of aluminium alloysPost-treatment Weld-decay HVOF coatings Refinement of microstructures Extended solubility of an element in another Formation of non-equilibrium phases, like amorphousClassification of LSM

LSM Application ExamplesAISI 304 austenitic stainless steel

Poor pitting corrosion resistance - compositional heterogeneity such as an MnS inclusion, segregates or precipitates.

LSM: significant improve pitting corrosion resistance

Main reasons: 1) dissolution/refinement of large scale sulphide inclusions due to rapid cooling rate cooling rate - < 1m;2) the formation of duplexaustenite/ferrite structure. S has a higher solubility in ferritethan in austenite.

Post-treatment: Laser melting of weldmentsExample 1: Weld decay of austenitic stainless steel Example 2: Friction stir weldment of aluminium alloysApplying thick-layer melting to HAZs to eliminate sensitisation, to restore corrosion resistance of austenitic stainless steels, using high-power CO2, Nd:YAG or HPDL. Applying thin-film melting to HAZs and TMZs to eliminate various precipitations, to restore corrosion resistance of Al alloys, using a high-power Excimer laser.

Example 3: Laser de-sensitised 316 austenite stainless steel:

Poor IGC resistance sensitised microstructure: Cr rich M23C6 and Cr-depleted regions along the grain boundaries. LSM modifies the microstructure and eliminate IGC.

i) dissolution of M23C6 carbides during melting;ii) no re-occurrence of M23C6 carbides after solidificationiii) homogenisation of Cr-depleted regions.* Importance: laser welding of stainless steels does not introduce weld decay

LSM Application ExamplesExample 4: LSM of AA2014 and AA2024 alloys

As-receivedCO2 laser-melted

MaterialsMain second phasesAs-receivedCO2 LSMAA 2014-T6Al2CuAl2Cu

AA 2024-T3Al2CuMgAl2CuMg, Al2Cu

Cu in -Al solution after LSM0.14 2.6 wt.%(depending on cooling rate)

LSM Application Examples

Why?Results of corrosion tests

LSM Application ExamplesPotential+Al2Cu (2014) - Al solutionAl2CuMg (2024)-AA 2024: Al2CuMg is anodic to -Al solid solutionAA 2014: Al2Cu is cathodic to -Al solid solution Increased Cu in -Al solution shifts corrosion potential to more positive direction AA 2024: reduce resistance to pitting corrosion AA 2014: improve resistance to pitting corrosion Increase of Cu

LSM Application Examples CO2 LSM reduces pitting corrosion resistance of AA2024-T351 alloy

Thinking of how we can completely eliminate second phases

Cooling rate determined by type of lasers

Excimer laser: UV wavelength, ns pulse width

Expected:

Higher cooling rate Elimination of 2nd phase particles No-precipitation after solidification

AA2024-T351Excimer LSM of AA2024Excimer Laser Surface Melting of Aerospace AA2024-T351 and AA2050-T8 alloys Thin-film melting Melt depth: 510 m No visible inter-metallics within the melt layer observedXRD analysis showing complete dissolution of second-phase intermetallics after a certain number of laser pulses per area.

AA2024-T351 in deareated 3.5% NaCl solution:

Significant reduction of passive current density (Ipass);

2. The higher the number of laser pulses, the lower the Ipass.Excimer Laser Surface Melting of Aerospace AA2024-T351 and AA2050-T8 alloys

AA2050-T8 in deareated 3.5% NaCl solution:

Laser-treated surface exhibited passive behaviorSignificant reduction of passive current density (Ipass);The higher the number of laser pulses, the lower the Ipass.

Excimer LSM of Magnesium alloys Electrochemical potentials

As-received Excimer LSM Excimer LSM Significant improvement of corrosion performance by Excimer LSMExcimer LSM of Magnesium alloys

Example 1: Pre-treatment of AA2024 alloy for anodising

Example 2: Post-treatment of HVOF MMC coatings

Potential Applications of LSM

Anodising Problems (due to the presence of 2nd phase particles)

1. Defects within the anodised layer, affecting corrosion performance

2. Reduction of anodising efficiency Pre-treatment for Anodizing of AA2024

Anodising at 12 V in 0.56 M H2SO4 Improvement:

Elimination of defectsIncreasing anodising efficiencyPre-treatment for Anodizing of AA2024

Anodising of LSM AA2024:Reduction of IpassIncreasing number of pulses results in lower Ipass, and more positive pitting potentialNo delaminationPre-treatment for Anodizing of AA2024

TreatmentEcorr (V)Epit (V)Passive current density (A/cm2)As-received-0.70-0.510.1E-5AnodisingAs-received-0.76-0.610.2E-710-pulses-0.74-0.600.8E-925-pulses-0.60-0.370.3E-950-pulses-0.56-0.310.2E-9

Laser modification of HVOF MMC coatingsFully melting: Pore-free, Fusion-bond, New phases at interfacePartially melting (HT) via diffusion: New phases at interface60WC/Co-base alloy

Corrosion resistance Wear resistance Immersion testLaser modification of HVOF MMC coatings

Example: Sealing (Re-melting) of Ceramic TBCs for gasturbine blades:

Plasma sprayed coatings: pores, micro-cracks, rough surface

Before laser treatmentAfter laser treatment

Laser Surface Alloying

Process as shown below: Metal film A + Metal substrate B

Produce a NEW material, which is different from A and B. Reference 10

Two types of laser surface alloying processes

1) Thin film alloying:Laser: short pulse, short wavelength, such as Excimer laserThin film: < 1 m, such as: 50 nm Pd (or Ni) on Ti foilMixing mechanism: diffusion of mass transfer

2) Thick layer alloying:Laser: continuous-wave, IR wavelength, such as CO2 and YAG.Thick layer: 0.1 to a few mmsMixing mechanism: convective (due to Marangoni effect)

CO2 laser (CW, Power of kW, interaction time >50 s)Homogenous alloying distribution by convection

Pulsed laser (such as Nd:YAG laser, Excimer laser with pulse width of 20 ns)Concentration profile along the depth by liquid state diffusion mainly. Reference 10Two types of laser surface alloying processes

Mixing Mechanism Marangoni EffectThermal gradient intensive convection (thermo-capillary flow) rapid homogenisation within the melt pool

It suggests that the convection speed is several orders of magnitude higher than the scanning speed, leading to extremely rapid homogenisation.

Selection of Materials for LSAThermal Properties between alloying elements and substrate

Difference of melting temperatures between alloying element and substrate materialVaporisation temperatureVapour pressureThermal conductivity

Examples:Difficult case:alloying Zn into Cu

Features of Laser Surface AlloyingAdvantages:Most of elements can be added;Chemical composition of alloying layer is fully controllable;Extended solubility;Localised treatment;Fine microstructure and less segregation;

Gas alloying: much thicker compared to PVD, CVD or other conventional methods.

Limitations:Some loss of the volatile elements;Variation of alloying element distribution

Example: Excimer laser micro-alloyingTi foils (50 m thickness) employed as electron-transparent window materials in the electron beam treatment of flue gasesEnvironment: a complex mixture of ions, radicals, and excited species, and sulphuric acid at 80 C.Currently, Ni, Pd (15 500 nm) can be deposited by PVD to prolong life time, but with a problem of detachment during services. Possible solution: Laser surface alloying of Ni or Pd into Ti foils, without thermal distortion.

Laser selected to be the best-suited: Excimer laser (248 nm, 20 ns pulse width)

Ti foilNi alloyedPd alloyedAfter immersion tests in H2SO4 at 80CExcimer laser micro-alloying: significant improvement of corrosion resistance in H2SO4 at 80C no spallation occurred No thermal distortion No vacuum required

MaterialWeight loss, gm-2d-1Ti foil45.1 + 0.50Ni alloyed0.62 + 0.02Pd alloyed0.42 + 0.01

Different form of LSALaser gas alloying TiN on Ti6Al4V and Ti for improvement of corrosion resistance and wear resistance

Treated depth: 200 mhttp://www.opticsjournal.net/OEPNNews.htm?id=PT111108000072z6C9E

Laser CladdingA1A2HAZDilution (Dl): percentage of clad contamination by substrate materials.

Limited and controllable heat input: Low dilution Minimal thermal distortion Minimal heat affected zones (HAZs) Bonding strength Fusion bond No spallation Coating microstructure No porosity No cracks Fine microstructure Homogeneous elemental distribution (less segregation) Process High process stability Robust, easily automated process High reliability in production Advantages of Laser Cladding

Applications of Laser CladdingSurface protection against:

Wear Corrosion Thermal

To prevent critical systems breakdown, and increase product lifecycle.Repairing (re-conditioning):

Damaged partsWornCorrodedMechanically- damaged

To salvage the partsManufacturing 3-D components

Laser Cladding 1) Cladding of high performance materials onto low cost substrates

Example 1: Laser cladding of 50Nb-50Ti alloy on the tip of airfoils (Ti-6A1-4V alloy) - excellent oxidation resistance at 1000C in air;

Example 2: Laser cladding for extending the solid solubility limits of rare-earth additions (e.g. yttrium, rhenium hafnium and cerium) in nickel-based superalloys to improve their resistance to oxidation at elevated temperatures.

Example 3: Laser cladding of Al, or Al alloys on Mg for significantly enhances the corrosion resistance, along with wear resistance.

Laser cladding of WC-Co MMC for wear protection

Laser cladding of print roller shaftSubstrate material....................................................Mild Carbon SteelCladding material ............................................Hoganas C22 HastelloyLaser type................................................... Coherent Highlight 4000LLaser power................................................................................. 4 kWSpot size ........................................................................... 0.5 x 12 mmTravel Speed ........................................................................0.6 m/minPowder Feed Rate .................................................................. 25 g/minStep Size...................................................................................... 8 mmClad Thickness ......................................................................... 0.5 mmRef.: Cladding with High Power Diode Lasers, http://www.coherent.com/downloads/CladdingWithHPDDL_WhitepaperFinal.pdf

Laser Cladding of Shaft JournalSubstrate material ......................................................High Alloy SteelCladding material .............................................................. Inconel 625Laser type ................................................... Coherent Highlight 1000FLaser power ................................................................................. 1 kWSpot Size......................................................................................2 mmProcess Speed .....................................................................0.75 m/minRef.: Cladding with High Power Diode Lasers, http://www.coherent.com/downloads/CladdingWithHPDDL_WhitepaperFinal.pdf

Laser Cladding of Roller Teeth Mock Up

Substrate material ..................................................... Mild carbon steelCladding material .............................................Deloro 60 clad powderLaser type ................................................... Coherent Highlight 1000FLaser Power................................................................................. 1 kWSpot Size......................................................................................2 mmProcess Speed .....................................................................0.35 m/minClad Thickness ........................................................................1.2 mmRef.: Cladding with High Power Diode Lasers, http://www.coherent.com/downloads/CladdingWithHPDDL_WhitepaperFinal.pdf

A layer of Inconel 622 deposited onto a carbon steel A210 boiler tube, which was then bent 1D and 4D without cracking. Laser Cladding of Boiler Tubes Ref.: Laser Cladding, http://www.alspi.com/lasercladding.htm

Laser Cladding Application Examples

2) Repairing (re-generating, re-conditioning) of damaged (corroded), high value components

Conventional methods: arc welding, flame spraying.Disadvantages: high thermal input, distortion, post-deposition machining.

Laser cladding technique:

Well-controlled, high accuracy (no post machining), low thermal input less distortion (less dimensional change)

Restoration ofdamaged blisksRestoring of the airfoil profileReconditioning of worn bores

Video Shows on Laser Cladding 1. Laser Cladding Multi Axis

http://www.youtube.com/watch?v=zAOsqDc6fgk&feature=related

Laser Cladding wire feeding, The University of Manchester.E:\Wire feed Laser Deposition.wmv

3. Laser Cladding

http://www.youtube.com/watch?v=Yrg86Gqmdc8&NR=1

Laser-assisted sprayed coating with defects-free and metallurgical bondThermal sprayed coating with defects Defects-free Metallurgical bond Higher processing efficiencyLaser-assisted Thermal Spray

Shot Peening A method of cold working metals in which compressive stresses are induced in metal surface layer by the impingement of a stream of shot at high velocity under controlled conditions

How does it work?

Laser Shock Peening

Mechanism:Generation of plasma within a overlayPressure on surface (GPa) shock wave plastic deformation - compressive residual stressNo melting

Laser Shock Peening To generate 1-3 mm residual compressive stress zones To improve fatigue lives of components

Applications Improvement in resistance to stress corrosion-cracking, Improvement in fatigue properties: Aircraft construction: fastener holes; aircraft gas turbine components

Advantages of LST over Conventional techniques for Corrosion Protection Dense surface layer with no porosity (cf. spraying )Fusion bond in laser cladding (cf. spraying, roll bonded, electrodeposition)Localised treatment with less HAZ than diffusion techniqueNo vacuum required (cf. electron beam, PVD, CVD)No fundamental restriction on component shape Novel, superior microstructure, such as microcrystalline or amorphous)Possibility of limited areas to be treatedChemical cleanlinessRemote, non-contact, easy to automate

Applicability of LST in Industries Large area coverage

Disadvantages: Low efficiencyAdvantages: Localised treatment with minimum HAZ

Economic considerations

High capital investmentCase-by-case consideration

Lack of technology transferLack of experienced personnel

Comparison between Laser and Other (Conventional) Techniques Laser shot peening and shot peeningLaser cladding and thermal sprayLaser alloying and ion implantationLaser CVD (pyrolytic and photolytic) and CVD

In terms of:

Adherence of coating and substrateFeatures of coatings (density, porosity, microstructure)Treated depthLevel of compressive stressLevel of solid solubility

Laser Assisted PVD Pulsed Laser Deposition Mechanism: photon interaction to create an ejected plume of material from any target. The ejection of material to occur due to rapid explosion of the target surface due to super-heating.Target: Just about anything! (metals, semiconductors, polymers ).Laser: Typically excimer (UV, 10 nanosecond pulses)Vacuum: Atmospheres to ultrahigh vacuum

Thermal PVDLaser PVDproduces a vapor composition dependent on the vapor pressures of elements in the target materialthe laser-induced expulsion produces a plume of material with stoichiometry similar to the target. It is generally easier to obtain the desired film stoichiometry for multi-element materials using PLD than with other deposition technologies. Faster and higher quality coatings

TiO2 Structure and Applications Anatase absorbs light at 380-390 nm (4-10% of solar spectrum)*J. Lian et al. Thin Solid Films 516 (2008) 33943398*K. HASHIMOTO et al. Jpn. J. Appl. Phys., Vol. 44, No. 12 (2005)*U. Diebold, Sur. Sci. Rep., 48(2003) 53-229 Laser/Sol-gel Technique

Photo-catalysis in TiO2 During UV excitation**A.L. Linsebigler et al. Chem. Rev., 1995, 95 (3), 735-758Eventually, powerful redox radicals further interact with organic contaminates by decomposing them into CO2 and other volatile species.Oxygen in AirWater/Moisture in Air

Mechanism of Disinfection of Bacteria by TiO2

Improving Efficiency of TiO2 Control e-/h+ recombination Red Shift into visible spectra1,2 1. P. Periyat et. al. J. Sol-Gel Sci. Tech. 43 (2007) 299-3042. Shah, et al J. Appl. Phys. Lett. 83, 4143 (2003) Ag2+ + 2e- Ag0

TiO2

Ag+2 - -+ ++ +Ag+2Additives, dopants into TiO2; Rare Earths (La, Ce, Nd) Noble metal ions (Ag, Au, Pt) Non-metal anions (N, C, S) Noble metals are inert, can sustain for longer time, Au, Ag and Pt nanoparticles are found to red shift the absorption of pure TiO2 into the visible spectrum. Ag ions/Ag nanoparticles interact with protein, leading to inactivation of protein and death of bacterial cells.

Comparison of various techniques for TiO2 film generation

Our new technique Sol-gel/Laser-induced Technique (SGLIT) Sol-gel TiO2 films amorphous Excimer laser - to crystallise sol-gel TiO2 films into anatase (or rutile)Conventional Sol-gel techniquesStep 1: sol-gel TiO2 filmStep 2: furnace annealingStep 3: Ag ion adsorptionStep 4: UV illumination Our SGLIT technique:Step 1: sol-gel TiO2 film Step 2: Ag ion adsorptionStep 3: Laser irradiation

Ag-TiO2 films

Experimental ProcedureTiO2 coated glassMask - raw beamXY tableStage 1: Preparation of TiO2 sol-gel coatings on glassStage 2: Adsorption of Ag ionsStage 3: Excimer laser processing of coatingsExcimer laser operating parameters

FilmFluence, mJ/cm2Repetition rate, Hz Number of pulsesAg-TiO285-1001550-200

Characterisation and TestsMaterials Characterization XRD phase analysis SEM/EDX film morphology/elemental analysis XPS Ag state TEM more detailed micro/nanostructural analysisAnti-bacteria Tests (S. Aureus and E.Coli): under UV illumination under normal light under dark room

XRD Structural Analysis Sol-gel Ag-TiO2: amorphous Excimer laser crystallize Ag-TiO2 film into: single anatase Anatase with minor Rutile, depending on laser operating conditions.TiO2 filmsAg-TiO2 films Ag stabilised anatase -TiO2 No information on Ag

FilmStructureAs-driedamorphousPure TiO2 (Laser)Mainly Anatase, + minor RutileAg-TiO2 (Laser)Only AnataseAg-TiO2 (Furnace+UV)Only Anatase

XPS Chemical Analysis of Ag NPs on TiO2 50 pulses @ 85 mJ/cm2200 pulses @ 85 mJ/cm2 Formation of Ag in metallic state, indicating Excimer laser irradiation capable of converting Ag ions into metallic Ag (not Ag oxide)

Grain size and Ag nanoparticle size Grain size of TiO2 (Laser-treated) > Grain size of TiO2 (Furnace-treated)

Particle size of Ag (Laser-treated) < Particle size of Ag (Furnace + UV treated)

Surface Morphology

Amorphous TiO2 matrixAnatase TiO2 matrixAg NpsMesoporous, rough surface with Ag NPs uniformly distributing on the surface. Smooth, and pore-free after furnace sintering at 700C for 1 hr and immersion in Ag ions, and adsorbed Ag ions being irradiated by UV lamp for 4-5 hrs. Laser treatmentFurnace treatment

After LaserAfter Laser Irradiation @ 85mJ/cm2, 50 pulses Hexagonal and cubic Ag NPs were generated after laser irradiation. Nanocrystallised depth is 150 nm out of 350 nm in total thickness.FCC AgTEM Analysis nanostructure details

UV-Visible SpectroscopyPhoto-absorption studiesAg-TiO2 (Furnace)Ag-TiO2 (Laser)Ag-TiO2 (as-dried) Laser-treated surface exhibits a red-shift and a strong absorption peak at 518 nm, which is believed to be due to the surface plasmonic effect from Ag nanoparticles. Furnace-treated did show a small peak at 410 nm.

Anti-Bacterial Drop Test Two types of bacteria are tested.Gram-positive bacterium: a thick, multilayered cell wall.Typical example: Staphylococcus Aureus (S. Aureus).

2. Gram-negative bacterium: Gram negative cell walls are more complex than Gram positive cell walls, both structurally and chemically. Typical example: Escherichia coli (E.Coli)

Under UV (365 nm) light IlluminationInitial concentration used to plate: 2x104 CFU/ml Laser-prepared Ag-TiO2 killed E.Coli up to 100% after 30 min, while the furnace-annealed partially kill the bacteria with constant remain. Gram Negative Bacteria - E. Coli

Under Dark and Normal Light conditionsDark Room Under both conditions, laser-prepared Ag-TiO2 killed 100% E.Coli after 60 min, while the furnace-treated failed to kill completely.Normal LightGram Negative Bacteria - E. Coli

Under Dark and Normal Light Conditions Laser-prepared Ag-TiO2 killed S. Aureus up to 100% after 60 min, under both dark and normal light conditions. Gram Positive Bacteria - S. Aureus

ConclusionsExcimer laser irradiation has two functions of 1) nano-crystallisation; and 2) reduction of Ag ions, being one-step process of generating Ag-TiO2 films.Film characteristics: meso-porous, rough surface morphology with enlarged surface area, anatase, uniform distribution of Ag NPs.Laser-generated Ag-TiO2 showed red-shift in solar spectrum and enhanced photo-absorption.Laser-generated Ag-TiO2 has improved efficiency of killing both E.Coli and S. Aureus bacteria compared with furnace-treated Ag-TiO2.

References Roessler, D. M., Industrial Laser Handbook, 1986, pp16-30.Steen, W.M., Industrial Laser Handbook, 1986.Duley, W.W., Laser Processing and Analysis of Materials, Plenum Press, New York, 1983.Steen, W. M., Laser Materials Processing, Spring-Verlag, 1993.Haag, M. and Rudlaff, T., Assessment of Different High Power Diode Lasers for Materials Processing, SPIE Vol. 3097, Europto, 1997, p583-591.Williams, K., Presentation, Loughbourg University.Chong, P. H., Liu, Z, et.al, Large Area Laser Surface Treatment of 2014 Aluminium Alloys, Applied Surface Science, 2002.Conde, A., et al., Corrosion Behaviour of Steels after laser Surface Melting, Materials and Design, 21, 2000, p441-445.Kumar, S and Banerjee, M. K., De-sensitisation of Type 316 Stainless Steel by Laser Surface Melting, Anti-Corrosion Methods and Materials, Vol.47, 2000, pp.20-25.Draper, C. W. and Poate, J. M., Laser Surface Alloying, International Metal Reviews, Vol.30 (2), 1985, p85-108.

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