Laser Machining of Structural Ceramics: An Integrated Experimental & Numerical Approach for Surface Finish

Hitesh D. Vora and Narendra B. Dahotre

Laboratory for Laser Materials Processing & Synthesis, Materials Science and Engineering, University of North Texas, Denton, Texas, USA

Background

Motivation

Numerical Study

Funding: National Science Foundation (CMMI 1010494)

Schematic of laser-material interaction

Conventional machining techniques (grinding) • Unacceptable tool wear & insufficient accuracy • Mechanical or/and thermal damage • Lower material removal rate or machining time • Higher operating costs

Potential solution: Laser Machining • Innovative and potential tool for bulk material

removal and shaping of structural ceramics • Non-contact process - eliminates tool wear • Efficient, reliable, cost effective solution to

fabricate complex structures at large scales

Laser-Material Interaction

Current research aims at presenting the state of

the art in the field of laser machining of

alumina and emphasizes on experimental and computational approaches in understanding physical nature of the complex phenomena.

Need: Obtaining desired surface finish at much higher material removal rate

Solution: Better understanding of various physical phenomena (heat transfer & fluid flow) and its influence on the evolution of surface finish during laser machining of ceramics

Objective

All explained physical phenomena happened • within the small interval of time • very difficult to observed physically

• Solution→ finite element method

Experimental Study

• Laser power density in Gaussian distribution

Governing Equations

A COMSOL® multiphysics based two-step numerical model coupled with heat transfer and fluid flow was developed

2

2

2 2exp

4

.

locx

DP

EAP

w

p

g

• Recoil pressure (Pr) at the evaporating surface depends on the incident laser energy density and is given by the following equation

• Navier-Stokes equations was used to model the movement of the liquid under the action of the recoil pressure

Results & Discussions

Heat conduction

Surface melting

Surface Vaporization

Plume formation

Recoil pressure

Liquid pile-up

Rapid Solidification

Ongoing efforts include the extension of one-dimensional numerical model into two- and three-dimensional with inclusion of effects of multiple laser pulses on the resulting surface morphology during laser machining of alumina

Defense and space exploration Thermal protection systems in exhaust cones, insulating tiles for space shuttle, ceramic coatings: engine components, and windshield glass of many airplanes

Refractory High Temperature Strength, high wear resistance, high hardness and durability. Used in refractory products such as furnace liners, crucibles, structural insulation

Machining and fabrication Excellent hardness & heat

resistance properties ideal for drilling, shaping, grinding and

forming metal work pieces

Reference: Google images Reference: Google images

Reference: Google images Reference: Google images

a) prediction of solid, liquid & vapor interface by Level-set method,

b) prediction of crater and melt pool dimensions, c) flow of molten material due to various

boundary conditions, d) prediction of surface profile after solidification

𝑃𝑟 = 𝑃𝑔

1.69

𝐿𝑣

𝑘. 𝑇𝑠𝑀𝑣. 𝐿𝑣

1 + 2.2𝑘. 𝑇𝑠

𝑀𝑣 . 𝐿𝑣

2 𝑜𝑛𝑙𝑦 𝑤ℎ𝑒𝑛 𝑇 ≥ 𝑇𝑣

Test

Average

Energy

Density

Laser

pulses

Numerically

calculated Rt

Experimentally

measured Rt

standard

deviation

Difference

# (J/m2) (1/s) (m) (m) (%)

1 3.5x106 1 170.3 174.1 2.81 3.49

2 3.5x107 10 465.8 490 5.92 5.19

3 7.1x107 20 559.6 604 3.80 7.93

4 10.6x107 30 967.5 898 5.16 7.18

5 14.1x107 40 1030.96 1010 4.84 2.03

6 17.7x107 50 1870.06 1920 3.89 2.67

Experimentally measured and numerically predicted and surface roughness

• Material lost due to evaporation causes an increase in crater depth

• Liquid expulsion created by the recoil pressure increase the pile-up height

• After a critical crater depth (260 m), recoil pressure was insufficient to eject the liquid material out of the crater

• Hence, liquid material solidified inside the crater wall leading to formation of typical tear drop shape topography

• Surface roughness increased with increasing pulse rate (1, 10, 20, 30, 40, and 50) or the increase in average laser energy density

• Process model can be used as a handy tool for the process engineers to configure the process variables to obtain the specified quality characteristics (surface finish & machining rates)

Acknowledgement

Path to multidimensional model