Jackson M.J. Micro and Nanomanufacturing

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432 Micro- and Nanomanufacturing

8000 1

7000 H

6000 i

5000 H

^ 4000 H

3000 H

2000 H

1000 H

-1000 J

Non-dimensioned hole length

^^—^ Nozzle diameter/Hole diameter = 2

^" - Nozzle diameter/Hole diameter = 4

^— - Nozzle diameter/Hole diameter = 8

Fig. 8.24. Shear stress distribution inside a narrow channel. The results are pre-

sented as function of the ratio of nozzle diameter-to-hole diameter,

DJ^/DH-

Note

that the unit of shear stress is N m"^

Laser Micro- and Nanofabrication 433

Shear stress distributions in the 53.6 |Lim diameter hole is com-

paratively smaller in magnitude than that experienced in the 429 |Lim

diameter hole. This is because the normal shock wave is larger than

the diameter of the hole, thus restricting the propagation of flow

downstream. The flow is forced to change direction at the entrance

to the hole, hence creating a separation point at the leading edge, as

shown in Figure 8.21b.

Shear stress distributions in a micro-channel behave differently

in comparison to a hole. There is little or no adverse pressure gradi-

ent acting against the flow direction in a channel. Fluctuations at the

channel entrance are similar to that of

hole (Figure 8.24).

The variation in pressure gradients with respect to the ratio be-

tween the nozzle and the kerf width is very important. It indicates

that if the kerf width is the same size as the nozzle, and assuming

that the flow is parallel, the width of the shock formed on the lead-

ing edge is approximately the same size as the kerf width (Figure

8.20a). The interaction of the shock end with the kerf edge creates

an expansion wave, thus leading to secondary oblique shock forma-

tions at some angle governed by the upstream flow Mach number. If

the kerf width is smaller than the shock width, then the expansion

wave emanating from the kerf edge is at an increased angle. The lar-

ger the shock angle, the greater the effect on flow properties, which

creates low-pressure gradients and considerably reduced shear

stresses. At some distance along the kerf length the value of shear

stress increases in magnitude; this is because the pressure gradient is

favorable to the exit of

the

kerf.

8.3.5 Stages of Surface Melting

There are a number of stages associated with surface melting. First,

a melt front wave forms and begins to penetrate the surface. The

pulse then finishes and no more energy can be put into the process.

The advancement of the wave front ceases, retracts and begins to

cool the material. Finally, re-solidification occurs; the volume that

was formerly melted has now solidified with the new microstructure.

However, there are problems such as different temperatures in and

around the mix that causes differences in surface tension that pro-

434 Micro- and Nanomanufacturing

motes mixing forces to take control within the melt. These are called

Marangoni forces and produce a texture on cooling the surface that

may be undesirable, thus causing residual stresses and cracking to

occur. The advantages of laser surface melting means that bulk

properties are unaffected, and in theory only a small surface depth is

affected.

8.3.6 Effects of Nanosecond Pulsed Microfabrication

If the shielding gas is oxygen then a re-cast layer is usually produced

when using nanosecond pulsed lasers. A thin oxide layer also ac-

companies the re-melt layer, which requires a secondary operation to

remove it. The process of using assist gases is described in detail by

Jackson et al.

[3-5].

Figure 8.25 shows the results of machining in

an oxygen-rich atmosphere.

Fig. 8.25. Trench machined in steel created using a nanosecond pulsed Nd:YAG

laser in an oxygen-rich gas atmosphere [5]

Laser Micro- and Nanofabrication 435

Helium tends to produce a highly pronounced re-cast layer that

is shown in Fig. 8.26. The reasons why it is so pronounced are ex-

plained by Jackson et al

[3-5].

Argon and air also tend to produce a

significant re-cast layer when used in a controlled atmosphere. Fig-

ures 8.27 and 8.28 show the effects of processing in argon and air

atmospheres when processing steel using a Nd:YAG nanosecond

pulsed laser.

Fig. 8.26. Trench machined in steel created using a nanosecond pulsed NdiYAG

laser in a helium-rich gas atmosphere [5]

Experiments were undertaken to find out what effects the assist

gas had on ablation or etch rates. It was found that adding an assist

gas reduced the etch rate

[3-5].

Also assist gas pressure greatly in-

fluences microfabrication using nanosecond pulsed lasers. Higher

gas pressures were also found to produce a low etch rate, mainly be-

cause at high gas pressure the molten material was prevented from

being expelled from the surface of

the

material.

436 Micro- and Nanomanufacturing

Fig. 8.27. Trench machined in steel created using a nanosecond pulsed NdiYAG

laser in an argon-rich gas atmosphere [5]

Fig. 8.28. Trench machined in steel created using a nanosecond pulsed Nd:YAG

laser in an air-rich gas atmosphere [5]

Laser Micro- and Nanofabrication 437

The effects of varying inlet gas pressure on machining rates are

shown in Figure 8.29. The Figure shows the average etch depth per

pulse achieved, when drilling a

mm thickness M2 tool steel plate.

It is evident that the etch rate achieved was considerably higher than

for no process assisted gas, which relies solely upon the vaporization

of molten material.

10 20 30 40 50

Laser intensity (GW/cm'^2)

•Air at 0 bar pressure — - Air at 8 bar pressure

Fig. 8.29. Variation of the machining etch rate as a function of gas pressure

438 Micro-and Nanomanufacturing

High-speed photography of plasma initiation was undertaken us-

ing an Imacon 468 high-speed camera (Figure 8.30). The images

shown were taken 240 ns after firing the laser and are negatively ex-

posed to show the plasma as a dark region.

OBLIQUE

NO 4 ^ SHOCK

SHOCK

NORMAL

SHOCK

^'^ (b) «^)

Fig. 8.30. Dynamic plasma field initiated using a 1064 nm laser wavelength at

different shielding gas pressures. The plasma was initiated 240 ns after firing the

laser beam at: (a) 0.5 bar gas pressure; (b) 4.0 bar gas pressure; and (c) 8.0 bar

gas pressure

The complete plasma life cycle was measured to be approxi-

mately 850 ns with no shielding gas. A significant change in the

plasma geometry is evident when changing the gas jet pressure. Par-

ticularly distinct regions of distortion can be seen at 4 and 8 bar

pressure. The distortions manifest themselves as a diamond shape at

the top of

the

plasma and a longitudinal expansion at the base. These

regions correspond to the positions achieved by the primary (nor-

mal) and the secondary (oblique) shock waves, as shown in Figure

8.20a.

Immediately after a shock, the molecular density of the jet in-

creases significantly, thus reducing the mean free path of molecules

due to compression and hence increases ionisation of the gases. This

complex plasma structure will severely disturb beam propagation

and affect the etch rates, as demonstrated in Figure 8.30.

Laser Micro- and Nanofabrication 439

Fig. 8.31. Mi orographic cross sections of laser machined M2 tool steel material

showing the results of laser-material interactions at different gas pressures: (a) 2

bar shielding gas pressure; (b) 4 bar shielding gas pressure; and (c) 8 bar shielding

gas pressure. The length-to-diameter ratio of the hole

(LH/DH)

was approximately

equal to 9

440 Micro- and Nanomanufacturing

The effect of the design of nozzles for laser micro-machining

processes is shown in Figures 8.31 and 8.32. The variation in

shielding gas pressure has a significant effect on the material re-

moval rate in the form of an etch rate, as shown in Figure 8.31.

At low shielding gas pressures, there is a pronounced removal of

material that results in the formation of a trench (Figure 8.3 la) in the

workpiece material. The micrograph shows a typical cross section

of a laser-machined material with a correspondent re-cast layer. The

re-cast layer is caused by the suppression of the molten material

from being released into the jet stream and subsequently vaporized.

This leads to the molten material being deposited at the sides of the

machined trench (Figure 8.31a). On further increasing the shielding

gas pressure, the formation of oblique and normal shock waves pre-

vents the molten material from being removed from the trench and

leads to the non-formation of a machined trench (Figures 8.31b and

8.31c). As a result of a reduction in the machining etch rate, the

amount of material re-deposited at the sides of the depression in the

center is very small compared to the low shielding gas pressure re-

gime. In all cases, the aspect ratio

{LH/DH) is 9.32.

The effect of the aspect ratio is shown in Figure 8.32. Low as-

pect ratios tend to produce the greatest amount of re-cast layers ow-

ing to the high shear stresses generated in the molten material (Fig-

ure 8.32a), while high aspect ratios tend to produce depressed re-cast

layers owing to the lack of shear stresses in the molten material.

Figure 8.23 shows the magnitude of shear stresses generated within

the melt pool using nozzles of different length when machining a 53

// m diameter trench in M2 tool steel. Figures

8.32a,

b, and, c, con-

firm the effect of variations in the aspect ratio of the nozzle at a

shielding gas pressure of 8 bar. The latter pressure was chosen in

order to illuminate the negative effects re-cast layers have on the re-

sultant morphology of micro-machined components.

Figures 8.31 and 8.32 show an interesting trend. The increase in

surface roughness as a function of nozzle length-to-diameter ratio

appears to show the advantages of using a high aspect ratio nozzle

with a low-pressure inlet shielding gas jet.

Laser Micro- and Nanofabrication

441

Fig. 8.32. Micrographic cross sections

laser machined

tool steel material

showing

the

results

laser-material interactions

different

LH/DH

ratios:

(a)

LH/DH

=4.6;

(b)

LH/DH

=9.3;

and (c)

LH/DH =

14. The shielding gas pressure was

equal

8 bar