Comparison between plasma arc cutting and laser cutting.

Ilii, Sanda ; Munteanu, Adriana

1. INTRODUCTION

The machining technologies based on the thermal effect of plasma and those based on the laser beam have an important place in the field of unconventional technologies. At present, plasma arc machining (PAM) and laser beam machining (LBM) represents the most modern machining technologies used in the industry domains such as: machines manufacturing, electronics, aeronautics etc., due to their advantages, to the fact that they allow machining of the high-alloy refractory and stainless steels till exotic metals with maximum of productivity, to the automation capacity, to the low expenses towards traditional techniques, and also due to the quality obtained of the surface material.

In the machine manufacturing industry, the plasma, as a "tool", is used especially in cutting operations, coating, welding, melting, and assistance of the mechanical processing operations such as turning, threading, drilling, grooving etc., in order to improve the machinability of various materials (Marinescu et alt., 2000); (McGeough, 2002).

Laser beam machining process is used in cutting operations, drilling, engraving, welding, heat treating, cladding, vapor deposition, tracing etc.

2. OPERATING PRINCIPLE

2.1 Plasma Arc Cutting

The plasma/ion beam machining is based on the thermal or chemical effects generated in the contact zones between ions or plasma and the accessible surfaces of the workpiece (Slatineanu et alt., 2004). The electric energy is used to form the jet of plasma in the presence of a plasmogen gas. The plasmogen gas (primary gas) must accomplish the following conditions: it ensures protection to the incandescent electrode against the oxidation process; to be neutral toward the material of the workpiece. The mono-atomic inert gases, witch are today the most used to produce thermal plasma (Argon, Helium, Azoth, air etc.), accomplish these conditions. Since in the process the thermal energy is delivered, the material in solid state is heated, melted and then boiled. The interatomic material bond-breaking is realized by thermal way (Nemchinsky & Severance, 2006).

In accordance with the Welding Handbook, "the plasma arc cutting process severs metal by using a constricted arc to melt a localized area of a workpiece, removing the molten material with a high-velocity jet of ionized gas issuing from the constricting nozzle".

In today's industry it is a widely used process to cut different metals and, combined with computer numerically controlled (CNC) machines, to cut alloy plate into desired shapes and prepare weld angles.

Normally, a plasma arc cutting system has a power supply, an arc starting circuit, and a torch. The power source and arc starter circuit are connected to the cutting torch through leads and cables that supply proper gas flow, electrical current flow, and high frequency to the torch to start and maintain the process. The arc and the plasma stream are focused by a very narrow nozzle orifice (Nemchinsky & Severance, 2006).

2.2 Laser cutting

In the case of laser cutting, a gas jet propagating in vertical direction is used to blow out the molten material that is produced due to heating of the workpiece by the laser beam at the momentary end of the curve. This process is usually called 'laser fusion cutting' or 'gas assisted laser cutting'. If the assisting gas jet has a considerable content of oxygen, the gas jet does not only have a mechanical effect as blowing out liquid material, but leads also to an additional heating of the workpiece due to the exothermic reaction of oxygen, for instance with steel. In this case, with the same laser power, an improved thickness can be cut or a higher processing speed can be obtained.

3. COMPARISON BETWEEN PAC AND LBM

The first and most important issues to consider are those of cost and material application, influence of cutting on the material quality in the edge zone and productivity.

[FIGURE 1 OMITTED]

Regarding the accuracy obtained, previous work (Marinescu et alt., 2000) shows that laser cutting machines are more accurate than plasma cutting machines. Also, towards LBM, in the case of PAC machining, its applications to many parts shapes are severally limited.

As we can observe in table 1, grater productivity can be obtained using the plasma for cutting instead of using lasers. Plasma cutting requires electrically conductive material. Lasers, in contrast, because they use highly focused light, can cut both electrical conductive and non-conductive materials.

Thus, a very large range of materials could be machined using the laser beam: mild and carbon steel, stainless steel, alloy steel, tool steel, aluminum alloys, cooper alloys, titanium, plastics, rubber, ceramics, composites, wood, glass etc.; however care must be taken in choosing the correct type of laser. Laser cutters are often used on exotic metals and minerals that would quickly dull or damage conventional blade or wire cutters. These included such items as hardened ceramics and precious gemstones. Laser cutting is an excellent choice for cutting Acrylic/Perspex. The cut edge results appear smooth and at 90[degrees] angle to surface.

PAC process was imposed especially due to the fact that it allows the cutting of high-alloy refractory and stainless steels with maximum of productivity, through the automation capacity, through the low expenses towards traditional techniques, and also due to the quality of the cut and low thickness of the heat affected zone (HAZ), within 1.50 mm.

Previous work (Sommer, 2000) on PAC and LBM processes shows minimal differences between PAC and laser cutting as regards to heat distortion and heat affected zone.

Also, regarding metal removal rates, only PAC process can compare with conventional machining, having the greater removal rate (about 300 in3/hr.) while LBM is so low in material removal rate (about 0.0004 [in.sup.3]/hr.) that it should be considered only for special applications where no better process can produce the part.

Analyzing the possible surface damage, both PAM and LBM processes presents damage on the machined surface caused by the intense heat generated by the processes. Such damage generally must be removed by subsequent operations, machining, grinding, or honing for example (Weller, 1984).

Regarding the dimensional control issue, LBM process can produce parts with tolerance quality comparable to conventional milling (0.03 mm.), while PAM process cannot be controlled to close dimensions and its use is restructed to simple roughing cuts (1,27 mm.) (Weller, 1984).

Before buying a PAC and Lasers machine, purchase & running costs, along with long term maintenance must carefully be considered. Referring to workload & Cost, in many instances, a laser has to run for 20+ hours a day to justify its existence and the amount of workload has to be carefully considered. A PAC machine can often be an ideal alternative tool with flexibility and low costs requiring a much lower utilization.

Another factor that must be considered is the space issue. Lasers need a very large floor area due to the additional equipment which is essential for its operation.

The following disadvantages must be taking into considerations before choosing one of the PAM or LBM processes. PAC machining case: edges rounding, as a result of a low power of plasma generator; high coefficient of roughness on one of the cutting surfaces, as a result of turbulence effects; appearance of burs and drops as a result of a too high cutting speed; appearance of thermal affected zone in witch high stresses can be developed and cracks can appear; radiations emissions and noise.

There are a few disadvantages on laser cutting, also. The material being cut gets very hot, so in narrow areas, thermal expansion may be a problem. Distortion can be caused by oxygen, which is sometimes used as an assist gas, because it puts stress into the cut edge of some materials; this is typically a problem in dense patterns of holes. Lasers also require high energy, making them costly to run. Laser cutting produces a recast layer in the kerf that may be undesirable in some applications. Lasers are not very effective on metals such as aluminum and copper alloys due to their ability to reflect light as well as absorb and conduct heat. Neither are lasers appropriate to use on crystal, glass and other transparent materials.

4. CONCLUSION

This paper summarizes the PAC and LBM processes performances. The theoretical comparison study was made taking into consideration the following issues: material applications, productivity, cost and influence of cutting on the material quality in the edge zone. After analyzing the specific technical literature that we had access to, the authors came to the conclusion that laser beam machines offer more accuracy and precision. On the other hand, PAC machines have a significant advantage over laser machines in terms of acquisition cost, maintenance and productivity. However, both laser and plasma arc cutting machines offers good results depending on the type of material being cut and of the results needed.

5. REFERENCES

Chaussier, J.F.; Gredey, D.; Bermer, D.; Vannes, A.B.; Rigolet, M. (1995). La decoupe par procedes <<haute energies Laser--Jet d'eau--Plasma--Elecro--erosion, Mechanical Industries Technical Centre (CETIM), ISBN 2-85400-356-X

Marinescu, N.I.; Nanu, D.; Lacatus, E. et.alt. (2000). Manufacturing processes, Optoelectronics National Institute, Bucharest, Romania

McGeough, J.A. (2002) Micromachining of Engineering Materials, New York--Basel: Marcel Dekker, Inc.

Mihaila, I.V. (1999). Non-conventional Technologies, West Printery, ISBN 973-9329-52-7, Oradea, Romania

Nemchinsky, V.A.; Severance, W.S. (2006). What we know and what we do not know about plasma arc cutting. Journal of Physics D: Applied Physics, Vol. 39, No. 22, November 2006, R423-R438

Slatineanu, L.; Nagit, Gh.; Dodun, O. et.alt. (2004). Nontraditional manufacturing processes, Tehnica-Info Publishing, ISBN 9975-63-164-9, Kishinev

Sommer, C. (2000). Non-Traditional Machining HandBook, Unit 1: Fundamentals of Non-Traditional Machining, Advance Publishing. Ing., ISBN 1-57537-325-4, Houston

Weller, E.J. (1984). Nontraditional Machining Processes, Society of Manufacturing Engineers, ISBN 0-87263-133-8 Dearborn (Michigan, USA)

Tab. 1. PAC and LBM process performances (Mihaila, 1999)

Machining Productivity Specific
 type (maximum) productivity
 Q [[mm.sup.3]] [[mm.sup.3]/min]

PAC [10.sup.5] [10.sup.3]
LBM 10 10

Machining Precision Roughness
 type (max. value) (min. value)
 [mm.sup.3]/minKW] [[micro]m]

PAC 0,5 25
LBM 0,01 1,6

Table 2. Precision of process (Marinescu, 2000)

 C[O.sub.2] Laser
Criteria Plasma Cutting cutting

Minimum size of the 0.006", depending on 0.002"
cutting slit cutting speed
(kerf width)

Cut surface Cut surface will show a striated structure
appearance

Degree of cut edges Good; occasionally Fair, will
to completely will demonstrate demonstrate
parallel conical edges non-parallel cut
 edges with some
 frequency

Processing tolerance Approximately Approximately 0.02"
 0.002"

Degree of burring on Only partial burring occurs
the cut

Thermal stress of Deformation, tempering and structural
material changes may occur in the material

Forces acting on Gas pressure poses problems with thin
material in direction workpieces, distance cannot be maintained
of gas or
water jet during
processing

Table 3. Materials applications (Weller, 1984).

 Material PAC LBM

Metals Aluminum A B
 and Steel A B
Alloys Super Alloys A B
 Titanium B B
 Refractory C C

 Material PAC LBM

Non- Ceramic D A
metals Plastic C B
 Glass D B

A--Good application; C--Poor; B--Fair; D--Inapplicable.