ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 1 General CO

laser design.

Lasers. There are three basic types of CO

gas lasers: slow axial flow, transverse flow, and fast axial flow.

The slow axial flow laser is an older, proven design that has excellent mode, stability and pulsing capabilities. They

are available with powers up to approximately 800 W, allowing them to cut steels up to 6.5 mm (0.25 in.) thick. Figure 2

shows the typical design theory of a slow axial flow laser. The lasing action occurs by injecting laser gas at a pressure of

approximately 2.7 kPa (20 torr) into an evacuated glass tube. The tube has a rear mirror and an output coupler at either

end. A high-voltage glow discharge is then passed down the tube between an anode and cathode, causing lasing to occur.

This design, which can only create 70 W/m (20 W/ft) of tube length, requires a long resonator cavity to produce high

power levels.

Fig. 2 Slow axial flow CO

laser.

The transverse-flow laser was developed in response to the size versus power limitations of the slow axial flow

laser. Transverse-flow laser design allows power to be generated to approximately 25 kW. The transverse laser

accomplishes this by using a tangential blower to move a high volume of laser gas transversely across the anode-cathode

electrical path, as shown in Fig. 3. The basis of this design is that power is directly related to the volume of laser gas that

is excited at any one time. The drawbacks are that these lasers cannot be electronically pulsed for cutting very intricate

geometries, and the beam quality, or mode, is not ideal for high quality cutting applications.

Fig. 3 Transverse or cross-flow CO

laser.

The fast axial flow laser is the newest CO

type. It combines the high power of the transverse flow laser with the

beam quality and some of the pulsing capabilities of the slow axial flow laser. Its design is similar to the slow axial flow

laser, except that a tangential blower forces a large amount of laser gas axially down the resonator, as shown in Fig. 4.

This increases the available power to 700 W/m (210 W/ft) of active laser resonator. This laser type is currently being built

with power up to 3 kW, and is becoming increasingly popular as a cutting laser because of its increased power, excellent

beam quality, and pulsing capabilities.

Fig. 4 Typical fast axial flow CO

laser.

Both the slow and fast axial flow lasers can operate in either the continuous wave (CW) or electronically pulsed modes. In

CW operation, the laser operates at a continuous power level. This type of operation provides the highest cutting travel

speeds. Because of the high speeds required to cut thinner material above 3175 mm/min (125 in./min), a loss of accuracy

can occur on intricate parts because of motion system limitations. Overheating will also occur on any thickness if the part

is very complex. The solution to these problems is to pulse the laser electronically, thereby allowing intermittent high

peak powers (approximately two to eight times the peak CW power) and overall lower average powers, as shown in Fig.

5. This results in controlled heat input and higher accuracies, but at reduced feed rates, compared to CW operation.

Typical pulse rates for CO

laser cutting range from 100 Hz to 1000 Hz.

Fig. 5 Pulsed waveform.

The Nd-YAG is the second type of industrial laser. It is a solid-state laser in which the active medium is neodymium,

which is dissolved in a matrix of yttrium aluminum garnet. The resulting crystal is formed into a rod, which is excited by

external flash lamps using xenon or krypton. As the lamps flash, the light is absorbed by the rod, exciting the medium to

emit photons. This laser uses the same type of two-mirror resonator described earlier. The Nd-YAG laser is a pulse-only

laser with a cutting speed limited to about 762 mm/min (30 in./min). This laser also emits invisible infrared light, but with

a wavelength of 1.06 μm (41.7 μin.), compared to 10.6 μm (417 μin.) for a CO

laser. Because of this shorter wavelength,

the Nd-YAG beam is more readily absorbed by metals, and is therefore used to cut gold, silver, copper, platinum, and

other metals that would be very reflective to a CO

laser. The Nd-YAG laser also does a superb job of drilling and

trepanning small holes in metals and is typically used on aircraft jet engine parts made from high-temperature superalloys

and titanium. Figure 6 shows a typical Nd-YAG laser system design.

Fig. 6 Typical Nd-YAG laser design.

Laser Cutting

Gregg P. Simpson, Peerless Laser Processors Division, Peerless Saw Company; Thomas J. Culkin, Lumonics Materials Processing

Corporation

Competing Cutting Methods

The advantages and disadvantages of several conventional metal-shaping processes that compete with the laser are shown

in Table 1. While the laser does not displace any of these processes in terms of their special capabilities, laser cutting does

fill a very important void. For example, the laser is the ideal method for producing short-run or prototype blanked parts of

large, complex or small, intricate shapes (see Fig. 7). The choice to laser cut these parts is based on cost. The expense of

temporary tooling or edge finishing far exceeds the cost of laser cutting.

Table 1 Advantages and disadvantages of laser cutting versus traditional metal cutting methods

Cutting method Advantages

Disadvantages

Laser Good edge quality, good accuracy, small kerf, narrow HAZ,

no distortion, little noise; cuts nonmetals, cuts small and

complex shapes

High equipment cost, limited to under 13 mm

(0.5 in.) thick, slower feed rates over 6.4 mm

(0.25 in.); cuts single layer

Plasma arc Lower equipment costs, faster feed rates over 6.4 mm (0.25

in.); cuts over 13 mm (0.5 in.) thick

Lower accuracy, decreased edge quality, larger

kerf, wider HAZ, noisy, higher operating costs;

only cuts metal

Laser Good edge quality, good accuracy, lower scrap rate, no

distortion, small kerf, no tooling or tool wear, increased part

nesting; cuts complex shapes, cuts up to 13 mm (0.5 in.) thick,

cuts tempered materials and nonmetals

Higher equipment costs, lower process rate,

higher costs on larger part quantities

Nibbling (turret

punch press)

Good process rate, lower equipment costs, economical on

medium to high production runs

Lower edge quality, high tool wear, high tooling

costs, low accuracy, distortion, scrap; only cuts

10 mm (0.38 in.) thick

Laser Good edge quality, no tooling or dies, short setup times, rapid

and low-cost design changes, noncontact cutting; cuts complex

shapes and three-dimensional shapes, cuts tempered materials

Higher equipment costs, low rate on high

volumes

Punch press High volume rates; lower costs at high volumes; cuts over 13

mm (0.5 in.) thick

Greater tool fabrication time, higher tool costs

and maintenance, more setup time, poorer tool

design, part stresses, lower edge quality; only

cuts annealed steel

Laser High feed rate, economical on small and medium quantities;

cuts nonmetals and nonconductive metals

Lower edge quality, higher equipment costs,

thickness limitations

Wire electric

discharge

machining

Good edge quality, good accuracy, lower equipment costs,

cuts over 13 mm (0.5 in.) thick, cuts very fine and complex

shapes

Very slow on any thickness, fixturing

Laser Narrow kerf, faster feed rates, good accuracy, good edge

quality

Small HAZ, fumes; cuts limited materials

Abrasive water

jet

No HAZ, cuts up to 152 mm (6 in.) material thickness, no

distortion, cuts all materials

Abrasive disposal, noisy, high-pressure

plumbing, slow feed rates

Laser Flexibility, faster feed rate, short setup time

Small HAZ, accuracy, high equipment costs

Numerically

controlled

milling

No HAZ, good accuracy, good edge quality, low equipment

costs

Limited feed rate, high tool costs and

maintenance

Fig. 7 Typical laser-cut parts. (a) Ceramic. (b) Metal.

A second, broader example is the cutting of industrial-quality circular saw blades. Several years ago a major manufacturer

of such blades purchased a two-axis laser system to blank out the entire saw blade. These saw blades are being produced

in sizes ranging from 50 mm (2 in.) to 915 mm (36 in.) in diameter, with an average order quantity of 22 pieces. Because

of the small order quantity and the highly customized tooth profiles, many setups and operations were required to punch

the geometries. However, by using the laser, which cut the saw geometry in just one operation, the manufacturer was able

to reduce the cost on many items up to 30% over conventional punching, milling, turning, and blanking methods.

Laser cutting does have limitations for many applications. In the case of the heat-affected zone and fusion layer present in

some aerospace hardware, laser cutting can only be used to produce a semifinished part that requires further processing

using a different method.

Laser cutting advantages and disadvantages must be carefully considered for each particular application before deciding

whether use of the laser is advantageous. Because applications are too far-ranging to generalize, in-depth information

should be obtained from a laser manufacturer, systems house, laser job shop, or consultant.

Laser Cutting

Gregg P. Simpson, Peerless Laser Processors Division, Peerless Saw Company; Thomas J. Culkin, Lumonics Materials Processing

Corporation

General Cutting Principles

The mechanism for cutting steel with a laser is basically the same as cutting steel with an oxygen-fuel process in which

the fuel gas acts to heat the material so the oxygen can oxidize and react exothermically with the steel to produce the

cutting action. The oxygen also helps to sweep the molten material out of the kerf. In laser cutting, the fuel gas is replaced

with a laser beam focused to about 0.1 mm (0.004 in.), resulting in a power density of 1 MW/cm

(6.5 MW/in.

). It is this

characteristic that allows a 3.94 mm (0.155 in.) thick alloy steel chain saw bar to be cut out at 2.54 m/min (100 in./min)

with a 0.15 mm (0.006 in.) kerf width and without part distortion.

Laser cutting can also be accomplished on nonferrous and nonmetallic materials using assist gases. In the case of

nonferrous metals (aluminum, copper, brass, and bronze), which have high thermal conductivities, but do not react with

assist gases and are reflective to the laser beam, cutting occurs when the laser beam heats the material well above its

melting point, and an assist gas, such as air, argon, or helium, is used to sweep the molten material out of the cut. The

inert gases are used only when the cut edge of the material must be free of any impurities that would reduce its

serviceability in a very harsh environment.

In the case of nonmetallic materials with low thermal conductivities and high beam absorption, such as wood, cloth, and

paper, vaporization of the material upon cutting is nearly 100%. Because material vapors have a tendency to rise, an assist

gas, such as compressed air, is used at a low pressure to protect the focusing lens from the damaging vapors. Organic

materials such as plastics and woods are also cut with lower gas pressures. Acrylic plastic will yield a fine polished edge

if cut with an inert gas or compressed air at 70 kPa (10 psi) pressure. Table 2 summarizes the general purposes of assist

gases.

Table 2 Assist gas usage

Gas Major function

Usage

Oxygen Promotes chemical

reaction

Cutting ferrous metals

Argon, helium, and

nitrogen

Inhibit chemical

reaction

Cutting thin metal for oxide-free edge; cutting chemically reactive metals and

materials

Air and inert gases Remove excess by-

products

Lens protection, absorptive plume removal; cooling to clear hot gases away from

thermally sensitive materials; cutting nonmetals

Process Variables. There are seven basic parameters in the laser-cutting process: beam quality (mode), power (CW or

pulse), travel speed, assist gas, nozzles, focusing lens, and focal-point position. Slight changes in any one of these

parameters can yield significant changes in cut quality. To fine-tune excellent cut quality, it is recommended that only one

parameter at a time be varied, while the others remain constant.

Beam quality, or mode, is a very important parameter. A laser should be used with a TEM

or Gaussian mode,

which is ideal for cutting (Fig. 8a). A Gaussian mode has most of its energy in its center. Figures 8(b) and 8(c) illustrate

the power density versus the radial distance from the beam centerline for both TEM

and TEM

01*

modes. The TEM

mode can be focused to a smaller spot size than can TEM

01*

, and has greater energy density of power per unit area,

thereby increasing cutting efficiency. This TEM

mode decreases the heat input to the part, which allows faster cutting

speeds and a smaller heat-affected zone (HAZ). Because the beam can be focused to a smaller spot size, kerf width is also

reduced.

Fig. 8 (a) TEM

or Gaussian mode energy distribution. (b) TEM

power density (sharp tool). (c) TEM

01*

power

density (blunt tool).

Power plays a significant role in respect to both feed rates and thicknesses. For example, Fig. 9 shows that increasing

power increases speed and thickness for a given material, while holding all other parameters constant. Figure 9 also

shows that the maximum material thickness to be cut also increases with increasing power. When pulse cutting is used,

the average powers decrease, while the peak powers of each pulse increase from two to eight times the maximum CW

power. This reduction of average power reduces the feed rate by about 60 to 80%.

Fig. 9 Travel speed versus thickness for 600 W and 1250 W CO

lasers. Focused power at workpiece using 65

mm (2 in.) focal length lens. Oxygen assist gas at 350 kPa (50 psi). Carbon and alloy steels used.

An optimum travel speed exists that yields the best cut quality, while holding all other parameters constant. Because

feed rate is very material dependent, experimentation is needed in order to obtain the best results. However, for most

metals, the best cut is obtained at the maximum achievable speed, which also helps to minimize the HAZ. If the feed rate

is too low, burning will occur and a large HAZ with slag formation will be present. In nonmetals such as wood or cloth,

excessive charring will be present on the cut face. If the feed rate is too fast, the beam will climb out of the cut and only

partial cutting will occur.

Use of oxygen as the assist gas for cutting steel and stainless steel increases cutting speeds by 20 to 40% compared

to use of air. Air is used mostly to cut nonmetals because it helps reduce oxidation and burning. When cutting stainless

steel with an oxygen assist gas, slag or a fusion layer occurs, which is not tolerable in some applications. Although argon

or helium assist gases eliminate oxidation, they reduce travel speeds by up to 50%.

The gas nozzle design and standoff distance, which is the distance between the workpiece and the nozzle, can

significantly affect cut quality. A property designed nozzle will produce a laminar, high flow rate assist gas through the

cut. The laminar flow can be affected by the standoff distance. If the standoff distance is too great, the smoothly flowing

gas tends to break up. This disrupts the flow through the kerf and decreases the edge quality. Typical nozzle diameters are

1 to 2 mm (0.040 to 0.080 in.) with 0.5 to 3 mm (0.020 to 0.12 in.) standoff distances.

A focusing lens is used to focus the beam on the workpiece. This increases the power density of the beam. The lens is

used because the output beam of a laser is typically 11 to 21 mm ( to in.) in diameter and does not possess enough

energy per unit area to melt and vaporize materials. The lens can focus the beam to a spot size of 0.1 mm (0.004 in.) in

diameter. The same principle is in operation as when using a magnifying glass to focus sunlight on a piece of paper,

causing it to burn.

Lenses are classified by focal length, or the distance from the lens to the point at which the spot size is smallest (Fig. 10).

Typical lenses come with focal lengths that range from 38 to 254 mm (1.5 to 10 in.). Shorter focal length lenses, have

higher energy densities because they have a smaller spot size, as shown in Fig. 11. These lenses, however, have a limited

depth of field, or usable beam. Depth of field is the area of the focused beam that has enough energy density to process

materials. This spot size limitation can be offset by using a larger input beam on a longer focal length lens, as shown in

Fig. 12. Short lenses are typically used on reflective materials, such as aluminum, or on thin materials, for faster feed rates

and an improved surface finish. Longer focal length lenses are usually used on materials 6.4 mm (0.25 in.) or more thick

because of their greater depth of field, which produces a squarer cut and sharper geometry definition at the bottom of the

cut. Because of increased spot size, these lenses decrease feed rate, and overall surface finish quality, while increasing the

HAZ. Most lenses in use are between 64 and 127 mm (2.5 and 5.0 in.) in focal lengths.