Laser Surface Treatment Processes

Laser Surface Treatment Processes “Beauty is only skin deep, but it is only the skin you see” PATIL S B Assistant Professor-Mechanical Engineering College of Engineering, Pune-411 005 [email protected]

Short term Refresher course On “ Modern Trends in Machining and Materials” at VJTI, MUMBAI

Why surface treatment?  Service life Oxidation resistance Corrosion resistance Wear resistance Hardness Tribological properties High temperature hardness Strength Erosion resistance Fatigue life 11/28/09

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Common advantages of laser surface treatment processes  Chemical cleanliness  Controlled thermal penetration and therefore distortion  Controlled thermal profile and therefore shape and location of heat affected region  Less after machining, if any, is required  Remote non contact processing is usually possible  Relatively easy to automate 11/28/09

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Laser surface treatment processes 

Laser transformation hardening

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Laser surface melting

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Laser surface alloying

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Laser cladding

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Particle injection

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Laser surface texturing

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Laser stripping

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AND MANY--- MANY MORE!

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Regions of laser power density and pulse duration

Shock hardening

108

1

10-2

106

Drilling

104

106 Energy density (J/mm2)

Welding

Glazin g

104 Power density (W/mm2)

102

Cutting Cladd ing

102

C Trans fo harde n

1

10-2 10-8

10-6

10-4

rmati

on

ing

10-2

1

102

Interaction time (Sec.)

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Laser transformation hardening -Selective hardening for wear resistance Also to change metallurgical and mechanical properties Practical uses of laser treatment 1. Increase in hardness 2. Increase in strength 3. Reduced friction 4. Improved wear resistance 5. Increase in fatigue life 6. Creation of surface carbide 7. Creation of unique geometrical wear pattern 8. Tempering

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Laser hardening features

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Principle of laser transformation hardening

Conventional hardening Heating to a temperature above α -γ transformation (750°to 900°C) depending on the carbon content where the soft pearlite phase transforms to austenite and carbon particle dissolves. Upon subsequent cooling, the austenite transforms to martensite.

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Laser transformation hardening 3 Heating rapidly (10 K/sec) to a temperature between a critical solid-state transformation temp. and melting temp. The large volume of adjacent material acts as an efficient sink, which 3 cools (10 K/sec) the surface rapidly.

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Principle of laser hardening-0.35%C

1600

Beam

Liquid Liquid + Austenite 1130

1200 Austenite

Martensite 800

Base material 400

Ferrite + Cementite

Fe 0.35

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Austenite + Cementite 723

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Wt % C

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TTT Diagram

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TTT Diagram for 0.8%C steel °C 800

Austenite

700 600 Fine Pearlite R. C. 30-40

500

Coarse Pearlite R. C. 5-20

Feathery Bainite R. C. 40-50

400

Acicular Bainite R. C. 50-60

300 MS

200 100

Martensite R. C. 65-70

1

2

4

8

Seconds

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15

30

1

2

Mf 4

8

Minutes

15 30

1

2

4

8

15

Hours

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Principal process variables of laser hardening 1.

2.

3.

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Material properties - Composition - Geometry - Absorptivity Beam properties - Wavelength - Power - Power density - Beam interaction time Process properties - Process gas - Coverage of large areas

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Scheme of specimen preparation

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100 % MIRROR FOR BEAM DEFLECTION

Nd: YAG PULSED LASER

Z

3-AXIS GANTRY TYPE MACHINE TOOL SYSTEM

X

Y

Fig. 4.2 The schematic set-up showing Nd: YAG laser system with 3-axis gantry

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Thermal modeling

The general governing equation of heat conduction of transient behavior in cylindrical co-ordinate is given as 1 ∂  ∂T  1 ∂  ∂T  ∂  ∂T  ∂T  k r  + 2  k  +  k  + q = ρ .c p r ∂r  ∂r  r ∂θ  ∂θ  ∂z  ∂z  ∂t

As per the assumption neglecting the effect of internal heat generation on the temperature distribution the equation for axis-symmetric heat conduction reduces to

∂ 2T 1 ∂ T ∂ 2T 1 ∂ T + + 2 = 2 r ∂ r α ∂t ∂r ∂z

Where

α=

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k ρ.c p

is thermal diffusivity

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*From equation (3.16) T ( r , z, t ) = Tm axe

 r2   −  4α t   

2 2 2  z2 + a2 Q f  4α t  −4αz t − ( z4α+ta )  2 2  + z + a erfc T ( r , z, t ) = z e − e  2 αt    k π    



2



 −  4rα t     z  − ( z ) erfc   e      2 α t   

---(3.26)

The above equation can be used to calculate temperature at any point and can be further reduced by using repeated integrals of error function to  z2 + a2 2Q f αt   z  ierfc T (r , z , t ) =  − ierfc  2 αt k   2 αt  

 r2 

 − 4αt  '  e    

---(3.27)

Equation (3.27) is the final equation to calculate the temperature at the desired location.

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*Material composition

%C

% Si

% Mn

%S

%P

3.35

2.48

0.84

0.068

0.068

With graphite Flakes in a pearlitic matrix

Material microstructure

Pearlite

Graphite flakes

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Metallographic analysis Retained austenite and undissolved graphite

Coarse martensite

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Confirmation experiment

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Microstructures for various degrees of overlaps-1 0% Degree of overlap

10% Degree of overlap

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Microstructures for various degrees of overlaps-2 20% Degree of overlap

30% Degree of overlap

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Microstructures for various degrees of overlaps-3 40% Degree of overlap

50% Degree of overlap

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Microhardness distribution along surface for various degrees of overlaps-1

Hardness distribution for 10% overlap

Hardness (Hv)

800 600 400 200 2650

2450

2250

2050

1850

1650

1450

1250

1050

850

650

450

250

50

85 0 10 50 12 50 14 50 16 50 18 50 20 50 22 50 24 50 26 50

65 0

0 45 0

50

800 700 600 500 400 300 200 100 0 25 0

Microhardness (Hv)

Hardness Distribution for 0% overlap

Distance (Microns)

Distance (Microns)

800 700 600 500 400 300 200 100 0 50 25 0 45 0 65 0 85 0 10 50 12 50 14 50 16 50 18 50 20 50 22 50 24 50 26 50

Hadrness (Hv)

Hardness distribution for 20% overlap

Distance (Microns)

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Microhardness distribution along surface for various degrees of overlaps-2

Hardness distribution at 40% overlap 1000 Hardness (Hv)

800 600 400 200

85 0 10 50 12 50 14 50 16 50 18 50 20 50 22 50 24 50 26 50

65 0

45 0

50 25 0

20 50 22 50 24 50 26 50

18 50

16 50

14 50

12 50

85 0 10 50

65 0

0 45 0

50

800 700 600 500 400 300 200 100 0 25 0

Hardness (Hv)

Hardness distribution for 30% overlap

Distance (Microns)

Distance (Microns)

85 0 10 50 12 50 14 50 16 50 18 50 20 50 22 50 24 50 26 50

65 0

45 0

700 600 500 400 300 200 100 0 50 25 0

Hardness (Hv)

Hardness distribution at 50% overlap

Distance (Microns)

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Comparison of theoretical & actual case depth Com parision of theoratical & experim ental case depth 600

Theoratical case depth Experimental case depth

Case depth (microns)

500 400 300 200 100 0 890

890

890

905

905

905

918

918

918

Beam pow er (w atts)

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Conclusion-1  Microstructure analysis reveals that laser hardening of Cast Iron using Nd: YAG laser is quite feasible.  The transformation of austenite to martensite during rapid cooling cycle is diffusion less and depends on the cooling (quenching) rate.

 No abnormal metallurgical change is witnessed during the process.  The process parameters like laser power density, spot size, beam energy distribution as well as the thermo physical properties of the material affect the process, independently.

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Conclusion-2  The increase in surface hardness is common in the range of 600 to 700 Hv from 240 Hv.  During overlapping of spots tempering effect is observed. For 50% overlap using optimum parameters typically ranges in between 500 to 550 Hv, however it is much more consistent and uniform.  The experimental results fairly match with the theoretical.  The wear characteristic of Cast Iron is improved during laser hardening.

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Laser surface melting  Similar experimental arrangement as transformation hardening, except a focused or near focused beam is used  Characteristics -Moderate to rapid solidification rates producing fine near homogenous structures. -Little thermal penetration, resulting less distortion. -Surface finishes of around 25 µm are fairly easily obtained, reduces need of post processing. -Process flexibility

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Common material for laser melting-1  Cast iron Inhomogeneous structure of ferrite and graphite (flakes, sphere etc) changes to graphite to cementite and austenite to martensite Carbon dissolution- Increased hardness - increased wear resistance

 Stainless steel Production of fine austenitic and martensitic structures Improved corrosion resistance

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Common material for laser melting-2  Tool and special steels Hardening through solution treatment –Carbide dissolution - Controlled quench rates for fine dispersion of carbides Fine carbide dispersion with high hot hardness 

Tendency to crack at higher hardness Preheat requirement -500° C-Low carbon steel -650° C-0.7 wt% C steel -700° C-Tool steel

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Laser surface alloying  Similar to laser melting except another material is injected into melt pool

 Characteristics -Fine and homogenous microstructure -Minimal segregation -Some surface alloys requires rapid quench rates e.g. Fe-Cr-C-Mn -Varied surface thickness-1 to 2000µm -Loss of more volatile components can be expected 11/28/09

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Laser alloying/cladding principle

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Laser surface alloying-Applications Material

Alloying element

Titanium

Alloying with carbon or nitrogen Production of hard carbide or nitrides

Cast iron

Alloying with Cr, Si or C Cheap C. I. into exotic irons

Steel

Alloying with Cr Improves corrosion resistance

Aluminium

Surface hardening by alloying with Si, C, N & Ni

Superalloys

Alloying with Cr

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Laser alloying advantages 

Permits precise selection of area to be modified



Requires a very small amount of modifier alloy



Results in extremely rapid heating and cooling of the surface



Produces wide variety of chemical and microstructural states outside of typical phase diagrams



Produces no distinct bond-line; will not delaminate



Requires little or no surface preparation for certain applications



Produces minimal hazardous waste



Performed remotely with robotics and fiber-optics



Performed at rates between 20-50 sq. ft./h

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Laser alloying of D2 steel-1

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Laser alloying of D2 steel-2

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Laser alloying-applications

Copper/stainless steel cryogenic valves

Aluminum engine block

4340 Steel moulds Titanium impeller wear

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Alloying of Al cylinder liner with Si

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Alloying of Al cylinder liner with Si

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Laser alloying-General applications 

Corrosion Protection



Superior alloy, refined grain size



Pumps, cylinders, rollers and die-casting dies



Wear Resistance



Surface metal matrix composites using SiC, WC, TiC, TiB2, Al2O3, etc.



Surface modification of dissimilar materials



Durable non-skid Surfaces



Protection from hydrogen embrittlement

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Laser cladding  To overlay one metal with another to form a sound interfacial bond or weld without diluting metal with substrate material.  Methods -Laser cladding with pre placed powder -Blown powder laser cladding

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Laser cladding Advantages 

Can produce denser coatings with little or no porosity, finer surface finishes, more consistent layer thicknesses, and more precise clad placement, than traditional thermal spray techniques.



Is inherently a low heat input process, resulting in low dilution, fine microstructures, small heat affected zones (HAZ), and low distortion.



Helps reduce processing time.



In specific applications, laser cladding may restore parts to their original dimensions without secondary operations.



Improves upon the materials inherent susceptibility to corrosion, wear and oxidation.

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Laser cladding- Industries and Applications 

Aerospace



Automotive



Marine



Oil and gas industries



Power generation



Restoration of bits, dies, industrial blades, and motor casings



Remanufacture of engine components



Hard facing

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Laser cladding- Common clad materials  Carbon Steels  Carbide Composites  Cobalt-base Superalloys  Stainless Steels  Titanium Alloys  Nickel-base Superalloys

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Cladding typical data

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Cladding with fiber coupled laser

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Cladding with fiber coupled laser

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Cladding with fiber coupled laser

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Cladding with diode laser

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Cladding with diode laser

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Cladding with fiber coupled laser

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Particle injection  Similar to cladding process by blown powder except that the particles projected into molten pool do not melt entirely

- Improved hardness and wear resistance due to reduced friction

- Hardening of Al & its alloys with TiC, SiC, WC or Al2O3 particles

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Laser surface texturing  A chopped laser beam is used to make a regular patterns or small pits or dimples

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LST Concept  This technology is based on a pulsating laser beam that, by a material ablation process, generates thousands of micro pores or dimples in one of the mating surfaces. All parameters are highly controlled and can be optimised for each application if required.  Typically the density of micro-pores is 50% of the surface area

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LST Advantages           

Reduces metal to metal contact Could facilitate speed / performance increase Reduces friction by up 75% Wear resistance can be increased 6 fold in extreme cases Improves component life & reliability Longer life in lubricant starvation situations Improves seizures resistance 2 fold Reduces power consumption Allows increased service periods or down sizing Reduces maintenance costs Heat generation can be reduced by 30%

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LST-Applications

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LST-Other possible applications            

Mechanical seals Duo-cone Seals Roller bearing thrust ribs Thrust Bearings Thrust collars/washers Water pump seals Plain & hydrodynamic bearings Piston rings & other engine components Surfaces lubricated by water or non flammable solutions High temperature surfaces lubricated by ATF or other low viscosity lubricants Gas Seals in turbines Helps reduce fretting corrosion

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LST applications in Seal failure LST can help prevent or minimise the most common causes of mechanical seal failure          

Wear due to frequent start-ups Fluctuations in process pressure Abrasive media Fluid vaporizes or exceeds flash point Fluid attacks / corrodes / degrades seal components Cavitations breaks up faces Mixed lubrication Debris builds up around seal Media crystallisation Dry running

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Laser stripping • Paint • Scale

LASER STRIPPING PORCESS: The laser is programmed to maximize absorption of the laser beam by the contaminated (paint, scale) material. This typically results in the substrate material (metal, plastic, ceramic), reflect most of the laser energy leaving a clean stripped surface. Due to their high reflection factor, metallic surfaces are especially suitable for laser cleaning.

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Laser stripping process

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Advantages of Laser Stripping         

Improved Quality Less labour intensive Improved throughput No secondary waste Lower health risk Low noise and dust Easy to automate Lower consumable and disposal costs PVDF coating removal Application

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Fe-Fe3C Phase Diagram

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