Laser Assisted Mechanical Micromachining of Difficult-to- Machine Materials

Transcription

1 Laser Assisted Mechanical Micromachining of Difficult-to- Machine Materials Ramesh Singh Woodruff School of Mechanical Engineering Georgia Institute of Technology Atlanta, GA, USA Advisor: Shreyes N. Melkote

2 Introduction Need for 3-D microscale features in materials: Micromolding Optics MEMS Biomedical devices

3 Mechanical Micromachining Micro-cutting processes (e.g. micro-milling, micro-grooving) Can create 3-D features in a wide range of materials Micro-milling machine Friedrich et al. (996) Limitations for difficultto-machine materials (e.g. hardened steels, ceramics) Tool strength/stiffness Slow process Micro-tool 3

4 LAMM Laser assisted mechanical micromachining (LAMM) Integrates thermal softening with mechanical micro-cutting processes Thermal softening of workpiece results in low cutting forces Overcomes limitations of tool stiffness, bending strength and low material removal rate + = LAMM Laser heating Mechanical micromachining 4

5 LAMM Design & Fabrication Cutting Direction (Y-axis) Workpiece Feed (X-axis) Laser Source Tool Tool Dynamometer Optics X-Y Stages Focused Laser Beam X-Y Precision Stages Tool Post Workpiece Dynamometer Y- Z Stages Laser Characteristics: Yb fiber laser -3 W λ = 6 nm (invisible) Schematic of first generation LAMM setup for micro-grooving 5

6 Geometric Capabilities White light interferometery image of a µm peak to peak sinusoidal profile using W laser power with 7 µm spot size located at µm from the tool with mm wide tool on mold steel (H 3, 4 HRc) 6

7 Experimental Characterization Design for microgrooving experiments Full factorial (96 runs) with 3 replications Level Depth of Cut (µm) Laser Location (µm) Tool Width (µm) Cutting Speed (mm/min) Laser Power (W) Workpiece material - Mold steel, H-3 (4 HRc) Tool material WC coated with Ti-Al-N Tool width 3/5 µm Measured Response Cutting and Thrust forces Surface finish 7

8 Main Effects on Forces 35 Depth of cut Laser location Tool width Cutting speed Laser power 3 Cutting force Depth of cut Laser location Tool width Cutting speed Laser power 4 Thrust force 8 3 8

9 Response Forces 6 35 Force (N) % Drop Without Laser 5 Watt Laser Force (N) % Drop Without Laser 5 Watt Laser 5 Cutting Thrust Cutting Thrust Cutting forces for µm depth of cut with and without laser for 3 µm wide tool located at µm from the laser beam center and a cutting speed of mm/min Cutting forces for 5 µm depth of cut with and without laser for 3 µm wide tool located at µm from the laser beam center and a cutting speed of 5mm/min. 9

10 Thermal expansion of tool Machine deflection Tool deflection i o 9qv XV (3ri / ro ) ( X, y, z) = e e rdr 3 / i i 4λaπ r o Factors Affecting depth of Cut rv i ϖ r = r ϖ = v t / 4a dϖ u ϖ exp ϖ I X + + y 3 / ϖ = = 4ϖ V r i ϖ 87 Thermal expansion Tempearture ( o C) Distance along tool workpiece interface (µm) Temperature distribution at the tool-workpiece interface at µm depth of cut, mm/min cutting speed, W laser beam with 7µm spot size located at µm from the laser beam center Deflection (µm) δ x δ y 4 6 Load (N) Tool deflection

11 Measurement of Groove Profile 3-D plot of a typical micro-groove 5 Nominal depth of cut = µm 5 Height (µm) 5 7 µm Height (µm) 5 µm Distance (mm) Distance (mm) Nominal depth of cut = 5 µm Height (µm) µm Height (µm) 8 7 µm Distance (mm) Without laser Distance (mm) W laser

12 Main Effects on Roughness A typical 3-D surface plot of LAMM surface Depth of cut Laser location Tool width Cutting speed Laser power.95 Surface roughness

13 Heat Affected Zone Heat Heat Affected Zone Zone Heat Affected Zone Heat Affected Zone mm/min 5 5 µm 5 mm/min 5 5 µm Heat Zone Heat Affected Zone mm/min mm/min 5 µm Micrographs for pure laser scans (without cutting) at W laser power 3

14 LAMM -Summary A novel hybrid method to augment mechanical micromachining Important cutting and laser parameters identified Thermal expansion and machine deflection can affect the depth of cut and consequently cutting forces Dimensional accuracy improves significantly Lower laser power yields lower 3-D surface roughness values The laser scan speeds affect the extent of heat affected zone (HAZ) 4

15 Acknowledgements The Timken company, Canton, Ohio. 5