The use of holographic optics in laser additive layer manufacture. Prof John R Tyrer Dept of Mechanical & Manufacturing Engineering

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1 The use of holographic optics in laser additive layer manufacture Prof John R Tyrer Dept of Mechanical & Manufacturing Engineering

2 Traditional Laser Beam Problems Shape Intensity Beam intensity distribution.... A B A B Heat gradient in workpeice

$Diffractive Optical Elements!$ designed profile! Shape! Intensity!

3 Diffractive Optical Elements! Reconstruct the beam intensity into any custom designed profile! Shape! Intensity! Reconstruction efficiency of >95%! No moving parts! Shape, Intensity and focus from a single element! Large depth of field! Rapid switching of irradiance distributions - a machine tool! Highly customized intensity profiles! In this case providing uniform temperatures! Even cooling rates

4 Laser hardening of tool steel Process parameters 1~4kW at 100~1000 mm/min Results Hardness values >700Hv Case depths 0.3~0.8 mm 4

5 Advantages of using a DOE! Allows the beam distribution to be optimised for the process! Rapid beam distribution change switch optics >10 sec DOE 28 4 Gaussian No melt pool! No maintenance 2! Increased depth of field! ± 30 mm with DOE (Top Hat)! ± 12 mm with equivalent Gaussian lens Datum 0-2 Burn through -28 5

6 For a general heat source, take the Green s function Solution to distributed = problem Sum of { Distribution function X Solution to point problem } Rosenthal, D. Mathematical modelling of heat distribution during welding and cutting, Welding Journal Research Supplement, 1941 V20 pp 220s-234s

7 Iterative design of diffractive optic shape

8 Autogenous laser welding with Rugby posts! Rugby post irradiance distribution! Shallow melt phase (bead on plate) conduction welding! Energy density of 140 MJ/m 2 yielded penetration of approximately 63%, (1.25 x 1.25 mm, Rugby post DOE

9 Top view of welded surface of our very 1 st diffractive laser weld

10 Heat flow predictions Gaussian v Rugby Posts

11 Comparison of model and structure

12 Temperature Gradient Affects The temperature gradient in the material causes undesired affects on the deposited profile.! Excess heat at central region of deposition leading to temperature gradients within the deposition! Segregation of alloy constituents! Excess heat input to process; boiling/porosity! Material transport doming deposition profile! Opportunity for inter track porosity! Uneven deposition surface leading to overlapping of tracks! Microstructure changes as previous tracks are reheated! Unstable foundations for the build-up of multiple layers; poor inter-track fusion/entrapment Deposition Profile

13 Material Transport Gaussian Beam High Thermal gradient Modified Beam Controlled Thermal Gradient

$Gaussian beam provided by refractive cutting head, below focus to match the nominal beam$

14 High Speed Video of Inconel 625 onto Mild Steel! 1000 Fps! Self illumination with neutral density filter! 1.2kW CO 2! Gaussian beam provided by refractive cutting head, below focus to match the nominal beam width Half Pipe Gaussian Beam Size 3x5mm 3mm Ø Powder Track Thickness (mm) Power (W) Traverse Speed (mm/s) Energy Density (MJ/m 2 ) 82 42

15 Material Transport

Fronts move from centre out to edges, as expected with

16 Stirring Direction and Velocities! Frames at 1ms intervals! Show front to be moving at approximately 0.1m/s! Fronts move from centre out to edges, as expected with buoyancy flow! Half Pipe fronts moving backwards at approximately 0.02m/ s

17 Deposition Profile! Reconstructing the beam intensity has reduced material transfer within the deposition formation! DOE has produced flatter wider deposition 300µ m

18 Temperature Gradient Affects The temperature gradient in the material causes undesired affects on the microstructure of the deposition Excess heat at central region of deposition! Minimum temperature of melting point! Excess heat input to process! Distortion! Effects on bulk material! Degradation of alloy constituents! Boiling - porosity! Segregation Uneven cooling! Large columnar grain growth! Reduced corrosion resistance! Reduced mechanical properties! Radial growth directions! Formation of residual stresses! Cracking Deposition Microstructure >>T m T m

19 Substrate Heat Transfer! Heat retained in substrate from previous depositions! Alter temperatures reached and cooling rates! Errors in the microstructures formed

Fe C Mn P S N C i r Co M o Ti Al Si - - - - - - - - - Substrate Bal 0.15-0.20 Powder 5ma x 0.10 max 0.60-0.

20 Manipulation of Grain size E E 2mm 2mm Beam irradiance profiles for (a) Gaussian irradiance distribution and (b) Off Set Rugby Post DOE irradiance distribution. Fe C Mn P S N C i r Co M o Ti Al Si Substrate Bal Powder 5ma x 0.10 max max ma x 0.05 max max ma x ma x ma x 0.50 max Nb K max Gaussian Offset Rugby Post DOE

$! Add on to SEM Electron Back Scatter Diffraction!$ $Scattered electrons from atomic planes diffract together!$

21 ! Add on to SEM Electron Back Scatter Diffraction! Electrons hit tilted sample! Scattered electrons from atomic planes diffract together! Electron fluoresce on phosphor screen and imaged by camera! Patterns indexed by computer to give phase and grain orientation

travel Gaussian deposition showing larger columnar grains DOE showing finer grains, on the right hand side, lower

22 Inverse pole figure and grain boundary maps of the deposition of nickel!!! Section below! Corrosion Resistance! Strength! Fracture Toughness! travel Gaussian deposition showing larger columnar grains DOE showing finer grains, on the right hand side, lower rugby post. A reduced grain size leads to improved properties Controlling weld pool temperatures! 40% Reduction in grain size! Reduction of low angle boundaries! Indication of reduced residual stress 200!m

23 Gaussian and Rugby Post DOE powder Nickel deposit on Mild Steel (a) (b) (c) (d) (e) (f) Analysis for (a) the Gaussian and (b) Rugby Post DOE with corresponding and EDS maps for (c&e) nickel and (d&f) iron

24 Crystal Orientations! Both deposits show a random distribution of grain orientations! Peak of twin boundaries in Gaussian deposition! Formation of twin boundaries in Gaussian deposition, this will have little effect on material properties

25 Grain Size Refinement

26 Grain Size Refinement! DOE has halved the grain size throughout the majority of deposition volume! 28% reduction in maximum grain size! 51% Reduction in mean grain size Percentage of Deposition Gaussian Distribution Grain Area (!m 2 ) Uniform Pedestal Distribution 50% ~<1,600 ~<800 90% ~<6,900 ~<2, % <8,470 <6,080 Mean 3,070 1,500

27 Laser processing of nylon powder using 3x5mm holographic element Irradiance Distribution for 3x5mm Uniform Optic 3 layers build, 1mm thick of 5 side by side runs Microtome section 5 side by side runs

28 Conclusions!!!!!!!!! Microstructure control induced through beam shape design Reduced thermal gradients allowed consistent temperature in pool Cooling rated controlled reducing grain size Removal of excessive temperatures likely to prevent alloy degradation 51% reduction in mean grain size Improvements in mechanical and corrosion properties High speed video has shown significant stirring in a Gaussian beam deposition A DOE reconstructed beam has greatly reduced the material transport in the deposition process Removing the stirring had produced a deposition with reduced doming of the surface 300µ m

Conclusions! Powder run trials resulted in consistent and fully bonded layers. Overall part density was increased, this fully dense material was close to parent material mechanical properties.

29 Conclusions! Powder run trials resulted in consistent and fully bonded layers. Overall part density was increased, this fully dense material was close to parent material mechanical properties.! Oxidisation is controlled by environmental shielding, uniform interaction zone temperature profile maintains material properties.! With the use of HOE s the beam profile can be changed to match that of the thermal conduction in the material to produce a uniform heat distribution within the interaction zone.! HOE s offer significant improvements, concerning the controllability of the temperature gradient across the weld section allowing a more uniform penetration across the width of the beam instead of a current concentration towards the centre of the beam path.! Another note worthy advantage of the system is the weld edge resolution, as the beam has a clearly definable rectangular edge.! Near net shape engineeringly useful capability in plastic is a real objective which is now achievable

30 Acknowledgements! This presentation is the summary of many peoples work including;! Dr Sara Noden, Dr Rebecca Higginson, John Jones, Matt Gibson, Dr James Kell, Daniel Lloyd, Darrell Bunyan John Tyrer Professor of Optical Instrumentation School of Mechanical and Manufacturing Engineering Loughborough University Loughborough Leicestershire LE11 3TU

related to the welding of aluminium are due to its high thermal conductivity, high

related to the welding of aluminium are due to its high thermal conductivity, high Chapter 7 COMPARISON FSW WELD WITH TIG WELD 7.0 Introduction Aluminium welding still represents a critical operation due to its complexity and the high level of defect that can be produced in the joint.