2. PhD Case Study X-ray diffraction (XRD) characterisation Residual stress calculation Typical exam question Laser surface modification

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 4

5. 5

6. Rofin DC-015, CO 2 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

7. 7 Power (W)100 - 1500 Beam geometryCircle FocusSurface Spot size (mm)0.09 – 1 Traverse speed (mm/min)upto 5000 Overlap (%)10 - 30% Assist gasArgon Laser ModeTEM 00 Operation ModeCont./ Pulsed

9. 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

10. 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

11. Increasing EnergyIncreasing Energy Topology and microstructure LHS – Topology RHS - cross-sectional microstructure analysis Effects of energy fluence a)524 J/cm 2 b)1048 J/cm 2 c)2096 J/cm 2 Depth of processing Overlapping Homogeneity of treatment Grain structure orientation 50 μm 100 μm

12. 12 SEM cross section micrographs of samples processed using the same energy fluence (1310 J/cm 2 ): Titanium alloy Stainless steel

13. 13

14. Surface Topology Cross-sectional microstructure

16. 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

17. 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 d hkl Assumptions: the same thing happen at the deeper planes reached by other penetrating X rays. From simple geometry, SQ=QT= d hkl sinθ which emerges as Bragg’s Law Interplanar spacing, d hkl

18. T= x-ray source, S = Specimen, C = detector, and O = axis.

19. polycrystalline  -iron

21. 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

22. 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 σ y produces a strain If the bar is isotropic the strain is related by: x-rays

23. Back reflection x-ray measurement is used to measure strain using x-rays: Residual stress measurements are given by: Where, E – Young modulus d n – spacing of planes parallel to the axis under stress d 0 - the spacing of same planes in absence of stress ν – Poisson's ratio

26. Q1. Figure 1 below shows the as-received XRD pattern for Ti-6Al-4V alloy: 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 Peakd hkl (Å) 12.555 22.341 32.243 41.7262

27. Q2. Subsequent to laser treatment, shift in peak positions were observed : (b)Determine the d hkl (interplanar spacing) of the peaks given the following: (c)Calculate the residual stress σ y given that: Young modulus of Ti-6Al-4V alloy, E = 113.8 GPa Possion’s ratio, ν = 0.342 Peak2θ 135.5 238 340 455

28. Determination of Crystal Structures W.D. Callister, Materials science and engineering an introduction, 5 th Edition, Chp 3 Stress measurement using XRD B.D. Cullity and S.R. Stock, Elements of X-ray Diffraction, 3 rd 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

29. 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 CO 2 Laser Surface Processing of AISI 316L Stainless Steel  Accepted for publication – Advanced Materials Research (AMR) 29

30. Question 1 Question 2 (b) (c) 30

31. Question 1 31 Peakd hkl (Å)θ2θ 12.55517.5435.09 22.34119.2138.42 32.24320.0940.17 41.72626.553.00 Question 2 Peak2θd hkl σ y (GPa) 135.52.5273.64 238.742.3222.70 340.722.2144.30 453.681.7063.86