Laser surface modification
PhD Case Study
X-ray diffraction (XRD)
characterisation
Residual stress calculation
Typical exam question
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
4
5
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
6
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
7
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
9
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
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
Topology and microstructure
I
n
c
r
e
a
s
i
n
g
E
n
e
r
g
y
LHS – Topology
RHS - cross-sectional
100 μm
50 μm
microstructure analysis
Effects of energy fluence
a) 524 J/cm2
b) 1048 J/cm2
c) 2096 J/cm2
100 μm
50 μm
Depth of processing
Overlapping
Homogeneity of treatment
Grain structure orientation
100 μm
50 μm
SEM cross section micrographs of samples processed using the same energy fluence (1310 J/cm2):
Titanium alloy
Stainless steel
12
14
90
12
80
10
70
Irradiance
(MW/cm2)
60
50
7.9
40
12.6
30
15.7
20
20.4
10
23.6
0
40
60
80
Residence time (μs)
100
Roughness (μm)
Mean depth of processing (μm)
100
8
Residence
Time
50μs
6
67 μs
4
83 μs
100 μs
2
167 μs
0
5.0E+06
1.5E+07
2.5E+07
Irradiance (W/cm2)
13
Surface Topology
Cross-sectional
microstructure
2000
Hardness (Hv)
1900
Beam Spotsize (mm)
1800
0.2
0.6
1700
1600
1500
1400
80
180
280
Peak Power (W)
380
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
•
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.
Diffraction of x-rays by planes of atoms (A-A’) and (B-B’).
•
From simple geometry,
SQ=QT= dhkl sinθ which
emerges as Bragg’s Law
•
Interplanar spacing, dhkl
T= x-ray source, S = Specimen, C = detector, and O = axis.
polycrystalline -iron
X-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
X-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:
x-rays
Back 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
Q1. Figure 1 below shows the as-received XRD pattern for Ti-6Al-4V alloy:
Peak
dhkl (Å)
1
2.555
2
2.341
3
2.243
4
1.7262
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
Q2. Subsequent to laser treatment, shift in peak positions were observed :
Peak
2θ
1
35.5
2
38
3
40
4
55
(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
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
28
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)
29
Question 1
Question 2
(b)
(c)
30
Question 1
Peak
1
2
3
4
dhkl (Å)
2.555
2.341
2.243
1.726
θ
17.54
19.21
20.09
26.5
2θ
35.09
38.42
40.17
53.00
Question 2
Peak
1
2
3
4
2θ
35.5
38.74
40.72
53.68
dhkl
2.527
2.322
2.214
1.706
σy (GPa)
3.64
2.70
4.30
3.86
31