Requirement of precision components, complicated design, stringent standards and testing, unusual size of workpiece, restrictions in conventional machining processes has lead to development of advanced machining processes shortly AMP. In past few years, there were several AMP’s developed such as electric discharge machining (EDM), electron beam machining, electrochemical machining, chemical machining processes (CMP), ultrasonic machining (USM), and jet machining processes abrasive jet machining, water jet machining, laser beam machining etc. Each of these AMP’s has its limitations in workpiece material, shape etc. But LBM is the one of the AMP’s where almost all material can be processed. One of major advantage of LBM is its ability to machine both conductive and non-conductive materials.
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Laser beam machining (LBM) is one of the most widely used thermal energy based non-contact type advance machining process which can be applied for almost whole range of materials. Laser beam is focused for melting and vaporizing the unwanted material from the parent material. As of now the major application on LBM is profile cutting of geometrically complex part and making miniature holes in sheetmetal.
HISTORY OF LASER:
In 1917, it was Albert Einstein who first told the world about the process called “Stimulated Emission” which makes the laser possible. In 1957, Gordon Gould, a Columbia University student designed the first laser device in his laboratory. However the first working laser (ruby laser) was found on 16th of May, 1960 by Dr. Theodore Maiman. This demonstration of ruby laser acted as entry door to this field. Till then lots and lots of researches have been done and various lasers were found. Some of major contributions and highlights are “Gas laser” which used helium and neon gases by Ali Javan during 1960, semi conductor laser by Gunther Fenner in 1962, CO2 laser by Kumar Patel on 1964, Nd-YAG laser by Geusic in 1964 etc. The first excimer laser was demonstrated in 1970 by Basov et ah and it was liquid xenon which was excited with a pulsed electron beam. The first commercial application of continuous wave CO2 lasers was made during 1967 by Western Electric and the first successful industrial application of laser cutting was die-board slotting. Whereas now, lasers have been into many areas such as aerospace, die and mold manufacturing, biomechanical devices, automotive, electric, and electronic industries etc. Likewise many studies have been made to find many such lasers and also studies were done to improve / optimize the working parameters of the lasers.
FUNDAMENTALS OF LASER:
Light Amplification by Stimulated Emission of Radiation – LASER is a device which produces a monochromatic light beam where all the waves are coherent. LASER consists of four primary components:
- Active medium: It contains atoms whose electrons are excited to higher energy levels by an energy source. They are solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs.
- Excitation Mechanism: Excitation mechanisms pump energy into the active medium. Three basic method of excitation are optical, electrical or chemical.
- High Reflectance Mirror
- Partially Transmissive Mirror
The major principles of Laser are stimulation, amplification and population inversion.
When energy is applied to a laser active medium electrons are raised to an unstable energy level then spontaneously decay to a lower relatively long-lived metastable state. There is possibility to pump large amounts of energy since electrons in this state will not spontaneously return to their ground energy level; thus we can obtain a population inversion in which most of the atoms are in a metastable state. Lasing action is initiated by an electron after achieving population inversion. If the photon released is of exactly the right wavelength it will stimulate an atom in a metastable state to emit a photon of the same wavelength (Stimulated Emission). Large amount of these stimulated photons will be lost when they interfere with the sides of the lasing active medium. However if the photons travel parallel to the long axis of the optical cavity they will continue to stimulate emissions of photons having the same wavelengths which combine coherently until they reach the mirrored ends of the optical cavity. This stimulated emission continues as the beam strikes the 100% reflective mirror and gets reversed to strike against the partially reflecting mirror. A small portion of the coherent light is released while the rest is reflected back through the lasing medium to continue the process of stimulating photons.
Types of Laser:
There are several types of lasers available based on active medium (solid, liquid or gas), types of gases used, types of crystals used, and mode of operation (continuous wave, pulsed, q-switched) etc. But only few lasers are used for industrial application called as material processing lasers. The commonly used lasers in industries are: CO2 Gas lasers; Nd-YAG – solid state laser and Excimer laser.
CO2 lasers tend to be high powered (up to 3 kW) and are used in the continuous-wave mode. The Nd-YAG lasers are used in the pulsed mode and can achieve peak powers of 7-10 kW.
Mechanics of Laser:
The mechanism of material removal during laser beam machining includes four different stages such as:
- Warm up
- Chemical degradation / plasma shielding
The material is heated above its melting point when a high energy laser beam is focused on workpiece surface. The melted or vaporized material is then removed by using high pressure assist gas. Unlike other processes, LBM is a thermal process and the effectiveness depends on thermal properties of the material rather than its mechanical properties. This is the major characteristics for which hard-to-machine material such as titanium alloys, super nickel alloys etc and highly brittle material such as glass, ceramics etc can be processed by LBM.
NEEDS OF LASER IN AEROSPCE INSUSTRY:
As mentioned earlier, lasers are employed much more in material processing industries than other industries. And current scenario of material processing, application of lasers plays a vital role in aerospace industry. The following are some of key points which explain the reason behind this:
- Cooling holes: Aerospace engineering technology is growing rapidly and the components are exposed to ever increasing exhaust and combustion temperatures. Hence cooling is very much required in order to withstand those high temperatures. One of method is to have cooling holes on TBC (thermal barrier coated) layers. Modern aircraft has nearly 100,000 such cooling holes which are made by laser drilling operation.
- Airframe weight: One of major criteria of any aircraft is the weight. Many improvements and experiments are going on to lessen the weight of airframe structure. Also it enhances low fuel consumption. High strength aluminum alloys are used for this process. It is found that almost 80% of material used in commercial aircraft and 50% material used in military aircraft is of this aluminum alloy. Laser cutting is one of highly recommended for processing this material.
- Reduce cost: In this present scenario, cost reduction plays an important role in any industry. With increasing price of petroleum, one of strategy is to reduce total manufacturing cost, labor cost in particular. CNC controlled motorized laser can be used for drilling and cutting purposes which reduces the manufacturing cost.
MAJOR LBM PROCESSES USED IN AEROSPACE INDUSTRY:
There are two major LBM processes employed in aerospace industry namely, laser drilling and laser cutting. In this report I have made some investigation on two case studies, one for laser drilling and another for laser cutting.
In laser drilling process is a thermal process which incorporates high energy laser beam which is focused on particular area where the material gets vaporized to form holes on workpiece. There are two types of laser drilling process, percussion laser drilling and trepan laser drilling.
Percussion laser drilling:
Percussion drilling is drilling where it directly punches the workpiece material where there is no relative movement of laser or workpiece. Thus the processing time is much less when compared to trepan drilling process. Also for drilling 100,000 holes in aircraft components such as turbine blades, airfoil vanes etc, percussion drilling is recommended.
This drilling involves cutting around circumference of the hole. Thus it takes more time than percussion drilling since it has to go around the circumference to make a hole on workpiece material.
Laser cutting process involves basic principle of mechanics of laser i.e. high energy laser beam is focused to particular area on the workpiece where the material is melted above its melting point. Then the molten material is removed by coaxial assist gas jet or induced vapor pressure thus forming the cut kerf. There are three types of laser cutting processes, laser fusion cutting, laser flame cutting and sublimation cutting.
Laser fusion cutting:
In this process inert gas such as nitrogen, argon etc is used as assist gas. This process fully depends on the energy of laser beam which is used for high alloyed steels.
Laser flame cutting:
Oxygen is used as assist gas in this process and it is widely used for low alloyed steels. This process receives some amount of energy from exothermic reaction of the workpiece material. Also the laser power is lower when compared to laser fusion cutting.
The material is molten by absorbed laser energy until it partially evaporates. This requires high power densities with much slower speeds than other to cutting processes.
CASE STUDY # 1: LASER DRILLING OF MULTILAYER AEROSPACE MATERIAL
As mentioned earlier, aerospace components requires thousands of cooling holes to dissipate the heat of combustion and exhaust gas. These components are given a coating called TBC – thermal barrier coating to protect the components from direct exposure to the corrosive environments. Thus the objective of this case study is to investigate and optimize the optimum parameters related to quality of drilled holes on a TBC, thermal barrier coated material. Some of such parameters are mentioned in below figure.
The experiments are made using Nd-YAG JK704 laser with lens of focal length: 120mm. Oxygen is used as assist gas. Previous studies by Corcoran et al identified oxygen as the suitable assist gas for this process. The below chart shows the specification for JK704 laser system.
Experiments are carried out using Rene 80 substrate coated with TBC. Here yttria stabilized zirconia is used as TBC and it is bonded with the substrate material by means of plasma sprayed MCrAlY bind coat. The material thickness is about 3.6mm.
The material composition of Rene 80 is: Ni= 60.0%, Cr= 14.0%, Co= 9.5%, Ti= 5.0%, Mo= 4.0%, W= 4.0%, Al= 3.0%, C= 0.17%, B= 0.015% and Zr= 0.03%
One-at-a time approach is the traditional approach for conducting laser drilled experiments. In this approach only one parameter is changed keeping all others as constant. But this is time consuming and most importantly interactions of parameters are not considered which may lead to wrong results. Thus Taguchi type approach is considered where orthogonal array was designed to reduce the number of experiments required. From 108, the experiments are reduced to 18. Holes were drilled on Rene 80 substrate surface by percussion laser drilling using the laser system as mentioned earlier. The mean diameters were recorded using profilometer. The following output responses were recorded:
- Remelt layer thickness;
- Microcracking depth and
- Spalling – Delamination of TBC.
After recording the values as mentioned in the experimental procedure, a chart was plotted which gives the direct comparison of results of 18 experiments.
The Taguchi analysis gives the output responses of Remelt layer thickness and microcracking depth which can be then compared with the OEM (Original equipment manufacturer) vane airfoil standards. This is to check whether the attained values are within the mentioned values mentioned in the standards. As per OEM standards, the remelt layer thickness < 0.279mm and microcracking in base metal and remelt layer < 0.203mm. Using the results from chart-3, S/N signal-to-noise ratio is calculated. The transformation of these results to S/N ratio helps to identify the optimum parameters. Thus by averaging the results of S/N ratio for each parameter level, the parameter effects plot can be obtained.
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Higher S/N ratio is preferred in S/N analysis. By using the results from Chart – 4 i.e. parameter effects plot, the optimum parameters can be obtained for laser drilling of Rene 80 sample. These optimum parameters are chosen in such a way that the remelt layer thickness, microcracking and Delamination of TBC are minimized.
Based on derived optimum parameter table, microcracking confirmation tests are carried out to validate the findings. It was found that the laser drilled holes posses very little microcracking at these optimum parameters (Table – 3). Three iterations were made, say A, B and C and micrographs were obtained.
Conclusion of case study #1:
The parameters considered for this study are pulse energy, pulse width, pulse shape, TBC density and the gas pressure. The investigation and results of above case study provides the following conclusions:
- Pulse energy: High pulse energy reduces the level of microcracking and low pulse energy reduces the level of melting of remelt layer thickness. It is also found that interaction occurs between pulse energy and pulse width.
- Pulse width: The severity of microcracking and Delamination i.e. the spalling is reduced considerably when shorter pulses are used.
- Pulse shape: Pulse shape do not have adherent effect on remelt layer thickness whereas in order to minimize delamination, a ramp-up pulse is recommended and to minimize microcracking, a treble pulse is recommended.
- TBC density: The TBC density has very little effect on the remelt layer thickness, however high density TBC yielded least delamination of bond coat and the TBC used.
- Gas pressure: Higher the gas pressure is better the output parameters. Gas pressure of 70 psi was found to reduce all the three output response parameters – remelt layer thickness, microcracking and delamination.
Some pictures showing the laser drilled holes on aircraft turbine component.
CASE STUDY #2: LASER CUTTING OF AEROSPACE ALUMINIUM:
One of primary goal of aerospace industry is to lessen the weight of airframe structure. This has advantage of saving fuel consumption, and hence the cost. High strength aluminum alloys were used for these applications and laser cutting is one of process which is recommended for processing high strength aluminum but there are some challenges which has limited conventional laser cutting for this application.
Thus, the objective of this case study is to investigate the challenges of conventional laser cutting and study the proposed solution to overcome these challenges.
Conventional laser cutting:
The usage of conventional laser cutting is limited to process aluminum alloys used in aerospace application is because of two major reasons,
- It produces cuts with poor surface finish and
- Large heat affected zone (HAZ) is created.
These poor machining characteristics of laser cutting decrease the fatigue life of components which is very essential for aerospace applications.
The mechanisms of laser cutting process is, when a high energy laser beam is focused on the workpiece the material gets melted and vaporized and then a assist gas jet is used to drag the molten material away from the workpiece material. If these dragging requirements are not more pronounced, then the molten material may remain in the cutting edge of the workpiece which yields to very poor quality cut and also large HAZ is generated in the cut edge. These all combine to affect the overall mechanical performance of the workpiece material.
The best method to improve the dragging efficiency is to increase the gas pressure of assist gas jet. This is required in order to establish a laminar boundary layer between assist gas jet and the molten material. The dragging of molten material is more efficient as a result viscous and pressure drag of the assist gas jet which are the driving force of removal of molten material from the workpiece. Gas pressure cannot be increased beyond 2 bar in conventional laser cutting as it uses converging coaxial cutting head. Any pressure more than 2 bar yields to more aerodynamic interactions. Due to this MSD mach shock disk, a normal shock wave is created which produces serious degradation in gas jet and reduces the dragging capacity.
Proposed laser cutting:
The drawbacks of conventional laser cutting can be addressed by using
- Converging – diverging (supersonic) nozzle instead of coaxial nozzle
- Different geometrical configuration of assist gas jet such that it is in off-axis with the laser beam.
Using converging- diverging nozzle, we can produce a fully expanded free-jet where MSD can be avoided. Thus we can produce cut of superior finish and also the HAZ is significantly reduced.
The experimental setup used for proposed laser cutting is as follows:
- Laser used: CO2 slab laser (Rofin DC 035)
- Output power: 3.5 KW
- Mode of operation: CW, continuous wave mode
- Nozzle: Self designed cutting head (supersonic head)
Experiments were conducted using conventional and supersonic cutting head and results were recorded.
Comparison of results:
The workpiece material used fro this experiment is 2024-T3, high strength aluminum alloy. The results were recorded and compared to conventional laser cutting. It was found that the finish was superior and HAZ was reduced than conventional laser cutting.
A Challenge in proposed solution:
One of major drawback in proposed solution is that, for cutting complex contours the supersonic cutting head has to be changed frequently to remain tangent to the cutting direction. This drawback can be overcome by using a motorized off-axis nozzle controlled by a CNC controller.
Conclusion of case study# 2:
The following are the conclusion derived from above case study is that by using a cutting head supersonic, off-axis (non coaxial) nozzle can process high strength aluminum alloys with
- An excellent and superior finish.
- Negligible heat affected zone, HAZ can be obtained.
- Production rates can be increased since cutting speed is increased.
- By using a CNC controlled motorized off-axis cutting head, parts with complex contours can be processed.
- Labor costs can be reduced as laser is fully automated.
In this report, a brief discussion about the two major laser beam machining used in aerospace industry, laser drilling and laser cutting were discussed. Also two case studies related to respective process were studied and results were mentioned. LBM is widely used in aerospace applications and more and more researches are going on to improve the current laser technology and many new trends & directions were forecasted in this aerospace engineering field.
- A.Corcoran, L.Sexton, B.Seaman, G.Bryne, The laser drilling of multi layer aerospace material systems, Journal of material processing technology (2002)
- A.Riveiro, F.Quintero, J.Pou, F.Lusquinos, R.Comesana, J.del Val, M.Boutinguiza and R.Soto, Laser cutting of aerospace aluminum
- Avanish Kumar Dubey, Vinod Yadava, Laser beam machining-A review, International Journal of Machine Tools & Manufacture, 2008
- The Fascinating world of sheetmetal, Dr.Hubert Bitzel & Johanna Burchertt
- F. Dausinger, B.G. Teubner, Strahlwerkzeug Laser: Energieeinkopplung und Prozesseffektivität, Stuttgart, 1995
- Lecture notes by Prof. A.Senthil kumar, Mechanical Engineering department, National University of Singapore, Singapore.
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