Laser Machining of Carbon Fibre Reinforced Composites and FE Modelling Professor Lin Li Laser Processing Research Centre The University of Manchester Email: lin.li@manchester.ac.uk In collaboration with SIMTech Singapore
Introduction Material composition: carbon fibre resin matrix Material Structure: Laminated with designed fibre orientation Properties of CFRP composite: High specific strength (strength/density) High specific modulus (modulus/density) Application of CFRP composites: Aerospace Automotive Marine Sports goods Infrastructure in weight-critical components. Airbus and Boeing have announced two new aircraft the A350 and the B787 with composite content anticipated to be over 30% and 50% respectively*. Example of CFRP woven cloth Relative structural efficiency 3 2 1 Aluminum (7075-T6) Static Fatigue Titanium (Ti-6Al- Static 4V) Fatigue Carbon / Epoxy (AS / 3501-6) Static Fatigue * Hexcel Corporation annual report 2004 Relative efficiency of aircraft materials* 0 * F.C Campbell Manufacturing processes for advanced composites Elsevier UK pp28
Machining of Composites Trimming and routing after curing for smooth edge Drilling holes for fastener and acoustic damping (a single aircraft often requires many thousands of drilled holes in various composite parts*) Half of manufacturing process is dedicated to machining composite parts** *http://www.compositesworld.com/hpc/issues/2004/november/636 ** http://www.mmsonline.com/articles/120501.html
Summary of Techniques for Machining CFRP Techniques Advantages/disadvantages Development Mechanical Abrasive waterjet Ultrasonic Laser Well developed process Mechanical force induced delamination Tool wear and dust Low temperature process no heat damage to matrix One jet head for all material and thickness no tool exchange High noise and abrasive slurry are potential hazardous to operator and environment Best machining quality Slow process Non-contact process and no tool wearing Easy for automation HAZ and charring Tool design Machining strategy Increase pressure (700 MPa) Combining with mechanical machining Combining with mechanical machining Under development Reduce charring/haz Combining with other technologies
Challenges of Laser Machining of Composites Thermal properties of fibres and matrix materials*: Material Conductivity (W/m/K) Heat capacity (JKg -1 K -1 ) Vaporization/ Decomposition temperature ( o C) Heat of Vaporization (J g -1 ) Density (gcm -3 ) Polymer 0.2 1200 350-500 1000 1.25 Graphite 50.00 710 3300 43000 1.85 Factors affect laser machining quality Challenges High differences of thermal conductivities Matrix recession charring heat High differences of vaporisation temperature affected zone Laser beam cutting direction and fibre orientation parallel perpendicular v v * V. Tagliaferri A. Di Ilio I. Crivelli Visconti Laser cutting of fibre reinforced polysters Composite 16(14) 317-325 (1985)
Work at Manchester/Singapore 1. Laser machining CFRP composite using a 1 kw single mode fibre laser DPSS lasers (IR green and UV) CO 2 laser and excimer laser. 2. Examination of machining results 3. FE Modelling. This presentation: Fibre laser cutting and 355 nm DPSS laser cutting
Fibre Laser Cutting 1 2 500 µm 200 µm 3 200 µm 4 20 µm
Fibre Laser Cutting 1200 1000 Kerf Width at Beam Entry Kerf Width at Beam Exit Fibre Pull Out 800 600 400 200 0 20 30 45 50 55 60 Cutting Speed (mm/sec) 900 W laser power
Effect of Gas N 2 O 2 He
Water Assisted Laser Cutting 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 Power: 155Watt Water Assisted No Water 0 Kerf Width Depth of Cut HAZ
Water Assisted Laser Cutting 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 Power: 270Watt Water Assisted No Water 0 Kerf Width Depth of Cut HAZ
Water Assisted Laser Cutting 900 W 50 mm/s No water With water
355 nm DPSS Laser Cutting Laser: Wavelength: 355 nm Maximum power: 10 W Pulse frequency: 40 khz Beam size: 20~25 µm Material: carbon fibre reinforced plastic (CFRP) composites Thickness: 0.3~7 mm Atmosphere: Air Mirror 1 Galvanometer Z Y X Laser beam Galvanometer Mirror 2 F-theta lens Workpiece
Drilling of Composite: effects of scanning speed V= 50 mm/s V=200 mm/s V=800 mm/s Hole size: 2 mm. Sample thickness 0.3 mm HAZ reducing with increase scanning speed. Number of passer required to drill through increase with scanning speed. Time of drilling through decreasing with scanning speed. Number of passes required to drill through 50 45 40 35 30 25 20 15 10 5 0 burning drilling through without serious damage not drilling throug h -100 400 900 Scanning speed (mm/s) 1.2 1 0.8 0.6 0.4 0.2 0 Time of drilling through (s)
Drilling of Composite: effects of laser beam scanning spacings Hole size 2mm. Sample thickness 1 mm 100 40 90 2-ring 36 Laser beam path: spacing 1-ring 2-ring 3-ring Number of passes required to drill through 80 70 60 50 40 30 20 10 0 32 28 24 3-ring 20 16 12 2-ring 8 4 3-ring 0 0 0.1 0.2 0.3 Time of drilling through (s) Laser bream trace spacing (mm) Material removal rate is higher using 3-ring than 2-ring beam paths Optimum beam spacing is 100 µm which is great than effective beam size 35 µm
Material Removal Mechanism: Fibre ejection by heat conduction Laser beam trace T M1 M2 Laser machining of conventional material 1 2 3 Laser machining of CFRP M1 M2 Fibres are chopped into small pieces. Heat is conducted into M1and M2. Heat is constrained in M1 and M2. Surrounding polymer matrix are heat up to high temperature. Polymer matrix loss its holding power. Fibres are ejected. Fibre redeposited on sample surface
Laser Machined CFRP Composites No serious HAZ and delamination free are observed
Bearing Strength of Drilled Composites 7 12 6 5 5.3 kn 10 8 Load (kn) 4 3 Load (kn) 6 4 2 1 0 Laser drilled Mechanical drilled 0 0.5 1 1.5 2 2.5 2 0 0 2 4 Displacement (mm) Displacement (mm) Mechanical drilled Laser drilled
Finite Element Analysis of Heat Transfer Finite Element Analysis of Heat Transfer during laser machining CFRP during laser machining CFRP Governing equations for heat conduction: ( ) ( )( ) ( ) ( ) ( )( )!! "!! # $ % + & ' ( ) * + = % + & ' ( ) * + = R Q t n t n T K n t t n T C R Q t n t n T K n t t n T C f f f m m m 1 1 ) ( - - 1. Transient irradiated surface ; If the interaction surface is on the matrix. ; If the interaction surface is on the fibre. 2. The remaining solid ( ) ( )! " # $ % & ' ' ' ' = ' ' n t n T K n t t n T C c c c ( ( )( ) ( )( )!! "! # $ % % = & % % = & n T K R Q t n T K R Q t f m 1 1 ; If the interaction surface is on the matrix. ; If the interaction surface is on the fibre. Under boundary conditions of: I. T(n0)=293K II. T( t)=293k III. At laser irradiated surface: IV. At all other surfaces: ( )! " = " # # T T h n T K s c
Modelling Considerations 120 100 320 º C 80 424 º C Weight% 60 40 500 º C N 2 Fibre diameter: 7 µm Spacing: 1 µm 20 0 715 º C 0 200 400 600 800 1000 1200 Temperature ( C) Thermal Gravimetric Analysis polymer decomposes at 424 C. Carbon burns at 882 C Air
Laser Beam Reflectivity Spectrum 30 25 Reflectivity(%) 20 15 10 5 0 200 244 288 332 376 420 464 508 552 596 640 684 728 772 816 860 904 948 992 1036 1080 Wavelength(nm)
Material Properties Property Fibre Epoxy Volume Fraction Density(kg/m 3 ) Thermal conductivity in ambient temperature(w.m -1.K -1 ) 60% 1800 50 40% 1200 0.1 Specific Heat(J.kg -1.K -1 ) Decomposition Temperature (K) 710 1153 1884 698
Effect of Cutting Speed on Heat Affected Zone 400 mm/s (24 m/min) 800 mm/s (48 m/min) Cut geometry pre-determined
Effect of Cutting Speed on Heat Affected Zone 50 mm/s 200 mm/s 800 mm/s Cut geometry calculated
Comparison of FEA and Experiment Results Heat Affected Zone Ablation depth 50 80 HAZ (! m) µ 40 30 20 10 FEA EXP Ablation Depth (! m) µ 70 60 50 40 30 20 10 FEA EXP 0 0 500 1000 Scanning Speed (mm/s) 0 0 500 1000 Scanning Speed (mm/s) The HAZ overestimation observed from FEA can be explained by thermal conductivity of fibres decrease considerably at high temperatures. Effect of speed on heat transfer mechanisms: Low speed: long heating time leads to low heat loss rate and more material removal. High speed: heat input decreasing results in heat loss rate increase and less material removal.
Simulation path of the laser beam Effects of Laser Beam Scanning Spacings Computation domain (Red section) Beam Path Far Side Previously Cut Groove Far Side Block End 75 µm 75 µm Spacing Beam Path Beam Path Far Side Block End 100 µm Block End 150 µm 100 µm spacing 150 µm spacing
Effects of Laser Beam Scanning Spacings 200 µm spacing
Removal Depth 30 25 Exp. FEA 20 Removal Depth (µm) 15 10 5 0 75 100 150 Beam Scanning Spacing (µm)
Summary Minimum HAZ and delamination free are achieved by using UV laser machining of CFRP composites. Bearing strengths of machined CFRP composites are similar between laser and mechanical drilled samples. Introduced New Material removal mechanism: combination of laser ablation and heat conduction. Process speed could be increased by optimising laser scanning spaces. FEA prediction showed that the HAZ and ablation depth were more sensitive to lower speed as compared to higher ranges. FEA predicted that at 150µm distance i.e. 5 times the active beam spot the chip formation of fibres takes place.
Acknowledgements The University of Manchester: Dr. S. Marimuthu Mr. R. Negarestani Dr. M. Sheikh Dr. P. Mativenga Singapore (SIMTech) Dr. Zhongli Li Mr. Pau Loong Chu Dr. Hongyu Zheng Dr. Gnian Cher Lim