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Laser Surface Modification of Metalsand
XRD Characterisation
Presentation Outline
PhD Case Study
X-ray diffraction (XRD) characterisation
Residual stress calculation
Typical exam question
Laser surface modification
Material Processing Research Centre, Dublin city University
Laser Surface Modification
Material Processing Research Centre, Dublin city University
4
LASER - light amplification by stimulated emission of radiation: highly directional, coherent, and monochromatic beam of light
Laser in material processing can be used for many purposes i.e. cutting, surface modification
Several laser surface modification methods exist: Transformation hardening Laser alloying/cladding Glazing
Laser Surface Treatment
Material Processing Research Centre, Dublin city University
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Effects of Laser on materials
Material Processing Research Centre, Dublin city University
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Rofin DC-015, CO2 laser specifications: 10.6 µm wavelength Power capacity of 1500W Operates in both continuous and pulsed mode Pulse width ranges between 26µs to ~ 500ms
Laser System
Material Processing Research Centre, Dublin city University
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Laser system processing parameters
Material Processing Research Centre, Dublin city University
Power (W) 100 - 1500
Beam geometry Circle
Focus Surface
Spot size (mm) 0.09 – 1
Traverse speed (mm/min) upto 5000
Overlap (%) 10 - 30%
Assist gas Argon
Laser Mode TEM00
Operation Mode Cont./ Pulsed
Case Study: PhD Research
Material Processing Research Centre, Dublin city University
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One in four hundred people receive hip replacement surgeries in Ireland*
Up to 250,000 annual hip replacements surgeries in USA
Approximately 20% simply being replacements of failed implants
Success rate has significantly gone up but material life is low
Typical life of an artificial hip being 15 – 20 years
Patients undergo revision surgeries throughout their lifetime
One main challenge is developing a life long artificial hip replacement
Excessive wear debris and loosening of the implant are primary causes of failure
Improving tribological properties of the implant will greatly improve its lifetime
*http://www.wrongdiagnosis.com/h/hip_replacement/stats-country.htm#extrapwarning
Case Study: PhD Research
Material Processing Research Centre, Dublin city University
The aim of this study is to produce surface engineered implant alloy capable of having
improved tribological properties using high speed laser treatment
Using high speed laser treatment to achieve a rapid cooling rate
Rapid laser treatment can produce an amorphous structure
Aims of the Study
Material Processing Research Centre, Dublin city University
Advantages of laser surface engineering Superior bonding with the substrate
Simple oxidation elimination techniques
Improved depth control and reduced distortion
Little or no sample preparation required
Less time/ energy and material required compared
to convectional coating techniques
Typical Results
Increasing Energy
Topology and microstructure
LHS – Topology
RHS - cross-sectional
microstructure analysis
Effects of energy fluence
a) 524 J/cm2
b) 1048 J/cm2
c) 2096 J/cm2
Depth of processing
Overlapping
Homogeneity of treatment
Grain structure orientation
(a)
(b)
(c)
50 μm
50 μm
50 μm
100 μm
100 μm
100 μm
Microstructure Analysis
12Material Processing Research Centre, Dublin city University
SEM cross section micrographs of samples processed using the same energy fluence (1310 J/cm2): Titanium alloy Stainless steel
(a)
(a)
(b)
(b)
Meltpool and Roughness Analysis
MELTPOOL ANALYSIS ROUGHNESS ANALYSIS
13Material Processing Research Centre, Dublin city University
40 50 60 70 80 90 100 1100
10
20
30
40
50
60
70
80
90
100
7.9
12.6
15.7
20.4
23.6
Residence time (μs)
Mea
n de
pth
of p
roce
ssin
g (μ
m)
Irradiance (MW/cm2)
5.0E+06 1.0E+07 1.5E+07 2.0E+07 2.5E+070
2
4
6
8
10
12
14
50μs
67 μs
83 μs
100 μs
167 μs
Irradiance (W/cm2)
Rou
ghne
ss (
μm
)
Residence Time
Laser treatment of HVOF - WC-CoCr coatings
80 130 180 230 280 3301400
1500
1600
1700
1800
1900
2000
0.20.6
Peak Power (W)H
ardn
ess
(Hv)
Beam Spotsize (mm)
UNTREATED LASER TREATED
(a)
(b)
(c)
(a) & (b) Surface Topology
(c) Cross-sectional microstructure
X-ray Diffraction
Material Processing Research Centre, Dublin city University
X-rays are a form of electromagnetic radiation that have high energies and short
wavelengths (on the order of atomic spacings for solids)
X-ray diffraction occurs when waves encounter a series of regularly spaced
obstacle that:
(1) are capable of scattering the wave
(2) have spacings comparable in magnitude to the wavelength
X-rays diffraction can therefore be used for material characterisation of metal
Phase identification of metals
Determination of crystal structures
Residual stress measurements
X-ray Diffraction
Material Processing Research Centre, Dublin city University
Material Processing Research Centre, Dublin city University
Diffraction of x-rays by planes of atoms (A-A’) and (B-B’).
•Two parallel x-rays of wavelength λ impinging on a crystal surface at angle θ.
•Parallel to the surface is a row of crystal planes, separated by distance dhkl
• Assumptions: the same thing happen at the deeper planes reached by other penetrating X rays.
•From simple geometry, SQ=QT= dhkl sinθ which emerges as Bragg’s Law
•Interplanar spacing, dhkl
Material Processing Research Centre, Dublin city University
T= x-ray source, S = Specimen, C = detector, and O = axis.
X-ray diffractometer
Diffraction pattern
Material Processing Research Centre, Dublin city University
polycrystalline -iron
Stress Measurements
Material Processing Research Centre, Dublin city University
Stress MeasurementsX-ray diffraction can be used as a form of uniform stress measurement
When stress is applied lattice spacings change from stress free values
measuring the change in lattice position gives strain
Consider conventional stress measurement technique – electric resistance
Strain is measured by resistance caused by extension of the gauge
In x-ray method, the strain gauge is spacing of lattice planes
Applied stress is force per unit area – if the external force is removed the stress
disappears
Residual stress is the stress that persists in the absence of an external force
Residual stress causes fatigue crack resulting in failure of components
Material Processing Research Centre, Dublin city University
Stress MeasurementsX-ray stress measurement assumes uniaxial stress
Uniaxial stress considers stress in a single direction
Consider a rod of cross sectional area A stressed
elastically in tension by force F
Stress σ = F/A in y direction but none in x or z
direction
The stress σy produces a strain
If the bar is isotropic the strain is related by:
Material Processing Research Centre, Dublin city University
x-rays
XRD Stress MeasurementsBack reflection x-ray measurement is used to measure strain
using x-rays:
Residual stress measurements are given by:
Where,E – Young modulus
dn – spacing of planes parallel to the axis under stress
d0 - the spacing of same planes in absence of stress
ν – Poisson's ratio
Material Processing Research Centre, Dublin city University
Ti-6Al-4V XRD pattern
Material Processing Research Centre, Dublin city University
Questions
Material Processing Research Centre, Dublin city University
Q1. Figure 1 below shows the as-received XRD pattern for Ti-6Al-4V alloy:
Material Processing Research Centre, Dublin city University
Calculate the peak positions (2θ) for peak 1, 2, 3 and 4 given the following:• Cu Kα (λ = 1.5405 Å) radiation system used• Order of reflection, n = 1
Peak dhkl (Å)
1 2.555
2 2.341
3 2.243
4 1.7262
Q2. Subsequent to laser treatment, shift in peak positions were observed :
Material Processing Research Centre, Dublin city University
(b) Determine the dhkl (interplanar spacing) of the peaks given the following:
(c) Calculate the residual stress σy given that:• Young modulus of Ti-6Al-4V alloy, E = 113.8 GPa• Possion’s ratio, ν = 0.342
Peak 2θ
1 35.5
2 38
3 40
4 55
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Determination of Crystal Structures
W.D. Callister, Materials science and engineering an introduction, 5th Edition, Chp 3
Stress measurement using XRD
B.D. Cullity and S.R. Stock, Elements of X-ray Diffraction, 3rd Edition, Chapter 15
Case Study: PhD research
Online: Applied Physics A - Process mapping of laser surface modification of AISI
316L stainless steel for biomedical applications
Online: Int. Journal of Material Forming - Surface modification of HVOF thermal
sprayed WC-CoCr coatings by laser treatment
Additional Reading Material
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Applied Physics A: DOI 10.1007/s00339-010-5843-5 Process mapping of laser surface modification of AISI 316L stainless steel for biomedical
applications
Accepted 10 June 2010
Int. Journal of Material Forming: DOI 10.107/s12289-010-0891-0 Surface modification of HVOF thermal sprayed WC-CoCr coatings by laser treatment
Accepted 17 June 2010
Analysis of Microstructural changes during Pulsed CO2 Laser Surface Processing of AISI
316L Stainless Steel Accepted for publication – Advanced Materials Research (AMR)
Publications
Material Processing Research Centre, Dublin city University
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Question 1
Question 2
(b)
(c)
Formulas
Material Processing Research Centre, Dublin city University
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Question 1
Solutions
Material Processing Research Centre, Dublin city University
Peak dhkl (Å) θ 2θ
1 2.555 17.54 35.092 2.341 19.21 38.423 2.243 20.09 40.174 1.726 26.5 53.00
Question 2
Peak 2θ dhkl σy (GPa)
1 35.5 2.527 3.642 38.74 2.322 2.703 40.72 2.214 4.304 53.68 1.706 3.86