Introduction
Recently, laser beam machines have been widely used for cutting a metal. In the case where the metal is made of nonferrous metals such as aluminum, brass, stainless steel, the machining condition for executing the cutting operation is modified when piercing is executed at the cutting start point, and when the cutting operation after that is executed. For example, an oxygen gas is used as an assist gas in piercing; on the other hand, an inert gas such as nitrogen gas is used as an assist gas in cutting.
Laser-beam machining (LBM) is accomplished by precisely manipulating a beam of coherent light to vaporize unwanted material. LBM is particularly suited to making accurately placed holes. It can be used to perform precision micromachining on all microelectronic substrates such as ceramic, silicon, diamond, and graphite. Examples of microelectronic micromachining include cutting, scribing & drilling all substrates, trimming any hybrid resistors, patterning displays of glass or plastic and trace cutting on semiconductor wafers and chips.
A laser beam machine can make quality of machining almost constant in such manner that a first reflecting means in a first beam guide portion is moved and driven so as to maintain a length of an optical path of laser beam to be almost constant in spite of a moved position of a machining head. According to the invention, a second reflecting means for catching laser beam in a second beam guide portion is located at a position facing the first reflecting means, thereby relatively shortening a length of the optical path between the first and second reflecting means and shortening the whole length of the optical path, and maintaining quality of machining with laser beam good.
Lasers can be used to cut, drill, weld and mark. LBM is particularly suitable for making accurately placed holes. A schematic of laser beam machining is shown in Figure below:
Fig : (a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM.
General Applications of Lasers in Manufacturing
How Laser Beam works
The solid-state laser utilizes a single crystal rod with parallel, flat ends. Both ends have reflective surfaces. A high-intensity light source, or flash tube surrounds the crystal. When power is supplied by the PFN (pulse-forming network), an intense pulse of light (photons) will be released through one end of the crystal rod. The light being released is of single wavelength, thus allowing for minimum divergence.
One hundred percent of the laser light will be reflected off the rear mirror and thirty to fifty percent will pass through the front mirror, continuing on through the shutter assembly to the angled mirror and down through the focusing lens to the workpiece.
The laser light beam is coherent and has a high energy content. When focused on a surface, laser light creates the heat used for welding, cutting and drilling.
The workpiece and the laser beam are manipulated by means of robotics. The laser beam can be adjusted to varying sizes and heat intensity from .004 to .040 inches. The smaller size is used for cutting, drilling and welding and the larger, for heat treating.
Types of Laser
The types of laser used in LBM are basically the carbon dioxide (CO2) gas lasers. Lasers produce collimated monochromatic light with constant wavelength. In the laser beam, all of the light rays are parallel, which allows the light not to diffuse quickly like normal light. The light produced by the laser has significantly less power than a normal white light, but it can be highly focused, thus delivering a significantly higher light intensity and respectively temperature in a very localized area.
Lasers are being used for a variety of industrial applications, including heat treatment, welding, and measurement, as well as a number of cutting operations such as drilling, slitting, slotting, an marking operations. Drilling small-diameter holes is possible, down to 0.025 mm. For larger holes, the laser beam is controlled to cut the outline of the hole.
The range of work materials that can be machined by LBM is virtually unlimited including metals with high hardness and strength, soft metals, ceramics, glass, plastics, rubber, cloth, and wood.
LBM can be used for 2D or 3D workspace. The LBM machines typically have a laser mounted, and the beam is directed to the end of the arm using mirrors. Mirrors are often cooled (water is common) because of high laser powers.
Laser cutting
Different types of lasers are available for manufacturing operations which are as follows:
· CO2 (pulsed or continuous wave): It is a gas laser that emits light in the infrared region. It can provide up to 25 kW in continuous-wave mode.
· Nd:YAG: Neodymium-doped Yttrium-Aluminium-Garnet (Y3Al5O12) laser is a solid-state laser which can deliver light through a fibre-optic cable. It can provide up to 50 kW power in pulsed mode and 1 kW in continuous-wave mode.
Below is a simple representation of how a CO2 laser beam is generated.
Types of LBM
1- Laser Drilling
Drilling is one of the most important and successful applications of industrial lasers. Laser hole drilling in ceramic, silicon and polymer substrates is widely used in electronics industry. Laser drilling of metals is used to produce tiny orifices for nozzles, cooling channels in air turbine blades, via drilling of circuit board, etc.
Holes less than 0.25mm in diameter are difficult to drill mechanically, laser drilling offers good choices for small hole drilling, especially for hard and brittle materials, such as ceramics and gemstones. Large holes can be drilled by trepanning, i.e., by overlappingly drilling the circumference of a circle to form a large hole. High throughput of hole drilling are realized by mask projection and automation.
Table 3.1 compared laser drilling with electrical discharge machining (EDM) and traditional mechanical drilling. EDM is limited to electrically conductive materials, while drill wear and breakage is a big concern in mechanical drilling. Laser drilling is effective for small hole drilling, they can be flexibly automated.
| | Mechanical Drilling | Laser Drilling |
| Advantages | Large diameter, large depth, low equipment cost | High throughput, no drill wear/breakage, noncontact, small HAZ, wide range of materials, low operating cost |
| Disadvantages | Drill wear/breakage, low throughput, difficult to drill small holes, limited materials | Hole taper, limited depth and diameter, recast layer |
Comparison of laser drilling and mechanical drilling
Laser drilled holes usually have tapers, in example the hole is not perfectly straight. Also a redepotion area may exist around the hole, because laser drilling is realized through violent phase change, the material becomes melted, then ablated, then cool down and become solid state again. Redeposition is serious for long pulses (pulse duration > 10 nanosecond. It was found that tapering and redeposition can be lowered by suitably choose shorter wavelengths and pulse durations.
How Laser Drilling works
In laser drilling, a short laser pulse with high power density feeds energy into the workpiece extremely quickly, causing the material to melt and vaporize. The greater the pulse energy is, the more material is melted and vaporized. Vaporization causes the material volume in the drilled hole to increase suddenly, creating high pressure. The vapor pressure expels the molten material from the hole. Spatter and vapor shoot upward in the direction of the processing optics. Once the laser beam breaks through to the other side, the spatter and vapor exit through the bottom. To prevent damage to the processing optics, manufacturers design the machines so that there is a large distance between the optics and the workpiece. A coaxial gas flow can also be used to shield the optics from spatter.
2- Laser Cutting
Laser cutting is a technology that uses a laser to cut materials, and is usually used in industrial manufacturing. Laser cutting works by directing the output of a high power laser, by computer, at the material to be cut. The material then either melts, burns, vaporizes away, or is blown away by a jet of gas,[1] leaving an edge with a high quality surface finish. Industrial laser cutters are used to cut flat-sheet material as well as structural and piping materials.
Laser cutting machines can accurately produce complex exterior contours. The laser beam is typically 0.2 mm (0.008 in) diameter at the cutting surface with a power of 1000 to 2000 watts.
Laser cutting can be complementary to the CNC/Turret process. The CNC/Turret process can produce internal features such as holes readily whereas the laser cutting process can produce external complex features easily.
Laser cutting takes direct input in the form of electronic data from a CAD drawing to produce flat form parts of great complexity. With 3-axis control, the laser cutting process can profile parts after they have been formed on the CNC/Turret process.
Lasers work best on materials such as carbon steel or stainless steels. Metals such as aluminum and copper alloys are more difficult to cut due to their ability to reflect the light as well as absorb and conduct heat. This requires lasers that are more powerful.
How Laser Cutting Works
When a reactive gas such as oxygen is used, it also delivers additional exothermic energy through chemical reaction between the assist gas and the molten material. This chemical reaction produces additional energy that enhances the cutting process. This extra energy can be beneficial to cut thick sections of materials, however, catastrophic oxidation must be prevented to ensure final cutting quality. We see the efficiency and overall quality of laser machining is strongly dependent on the interaction of the gas jet with the workpiece. Also as we mentioned in section 3.1, inert gases are used in laser machining when oxidation need to be reduced.
In this section we give a brief review on the recent progress in other aspects of gas jet effects, focusing on nozzle design, jet alignment, effects of pressure and gas purity. Then we present a relative detail description on the effects of shock structure and standoff distance on laser cut quality.
3- Laser Beam Welding
Laser Beam Welding (LBW) is a modern welding process; it is a high energy beam process that continues to expand into modern industries and new applications because of its many advantages like deep weld penetration and minimizing heat inputs. The turnby the manufacturers to automate the welding processes has also caused to the expansion in using high technology like the use of laser and computers to improve the product quality through more accurate control of welding processes.
Operation
Like electron beam welding, laser beam welding has high power density (on the order of 1 Megawatt/cm²(MW)) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece.
LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.
A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced.
Materials that may be cut
A wide range of materials may be cut using a laser beam, however care must be taken in choosing the correct type of laser. Below is a table outlining the suitability of both CO2 and Nd:YAG lasers for materials likely to be encountered in the course of an average manufacturing business.
| MATERIAL | CO2 | Nd:YAG | BOTH |
| Mild and Carbon Steel | | | 5 |
| Stainless Steel | | | 4/5 |
| Alloy Steel | | | 4/5 |
| Tool Steel | | | 5 |
| Aluminium Alloys | 2/3 | 4/5 | |
| Copper Alloys | 1 | 3 | |
| Titanium | | | 4 |
| Plastics | 5 | 0/1 | |
| Rubber | 4 | 1 | |
| Paper (Gasket ETC) | 5 | ¾ | |
| Ceramics | | | 3/4 |
0=Impossible/Dangerous
5=Excellent
Safety on Laser Machining
Even though there are advantages from using the laser machining processes in industry and technology, laser is one of the most dangerous tools that can kill users. Here is a list of the danger concerns that users should be aware of:
- Working on Laser machines while the door open may cause damages in the eyes and burn hands. Therefore, a user should close the door of the machine before starts working.
- Lasers produce coherent light which when looked at appears to the eye to have come from a very distant source. Consequently, the image formed on the retina by a laser source is always incredibly small and therefore of very high power density.
- If a laser product is being used to process product, for example cutting, welding and surface treatments, there may also be chemical toxicity risk to address. The processing of organic materials such as thermoplastics is a particular risk that needs careful assessment in the context of local exhaust extraction (LEV) and personal protective equipment (PPE) provision.
- The wavelengths of emitted radiation is determined by and a characteristic of the chemical composition of the ‘lasing’ medium. For example, carbon dioxide lasers emit in the far infrared at a wavelength of 10.6 microns. Some media are capable of being made to ‘lase’ at several wavelengths, organic dye lasers being one such example.
- Additionally, since laser action is essentially an inefficient process, most lasers of class 3B and above will have significant electrical power needs, often at high voltage and three phase. Electrical safety, especially during maintenance and repair, is therefore a significant risk that needs to be adequately controlled by manufacturers and employers that use laser products.
Advantage of laser beam machining
· No limit to cutting path as the laser point can move any path.
· The process is stress less allowing very fragile materials to be laser cut without any support.
· Very hard and abrasive material can be cut.
· Sticky materials are also can be cut by this process.
· It is a cost effective and flexible process.
· High accuracy parts can be machined.
· No cutting lubricants required
· No tool wear
· Narrow heat effected zone
· Faster process
· Useful with a variety of material : metals, composites, plastics, and ceramics
· More precise
Limitations of laser beam machining
· Uneconomic on high volumes compared to stamping
· Limitations on thickness due to taper
· High capital cost
· High maintenance cost
· Assist or cover gas required
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