25-08-2017, 09:32 PM
1458564148-Chapters.docx (Size: 179.96 KB / Downloads: 6)
INTRODUCTION
Laser cutting is basically a machining operation in which material removal is obtained by focusing a highly intense laser beam on the work piece. Laser cutting is gaining high attention by industries broadly due to the advantages characterized by cut quality and ease of processing.
Laser cutting is a process that is done by minimum wastage of material, least distortion of parts and minimum tool wear and tear. The industrial application of material processing with laser machining systems are mainly laser cutting (39%), laser welding (20%), laser labelling (16%), laser micromachining (8%), and laser drilling (4%) .
Two main types of lasers are used nowadays for metal processing; CO2 and Nd:YAG lasers. CO2 lasers have high efficiency(about 30%), higher beam quality with optimum Gaussian mode, higher depth of focus, smaller beam diameter to cut thick metals as high as 16 mm (for mild steel) and 12 mm (for Stainless steel). Nd:YAG lasers have the advantage of less floor space required, simple maintenance requirements, easy beam alignment etc.
Stainless steel is largely used in home, industry, hospitals, food processing, farming, aerospace, construction etc. The Austenitic grade stainless steel is mostly used. So cutting of these steels are required to produce different components.
In this study the material used in the simulation is AISI 304 of different thickness (1mm-3 mm). The parameters are Power, cutting speed. The simulation of cutting is carried out using laser heat source with Gaussian mode of power output. Phase change problem is tackled with enthalpy method.
1.1 PROBLEM STATEMENT
Cutting of thin stainless steel material using laser source has wide application in industries and other areas. Investigation of the parameters for successful cutting through experiments is costly and time consuming. Moreover these studies are limited to specific material type and process parameters. The temporal variation within the material is hard to know, so the micro structural changes.
These above problems prompt for simulation of the physical cutting process. Through simulation we can obtain the temperature distribution within the material.
1.2 OBJECTIVE
The objective of this study is to investigate the effect of Laser power, cutting speed, beam diameter on cut quality in terms of kerf width and to get different power, speed combinations for successful cutting for different thicknesses.
1.3 SCOPE
The material investigated in the simulation of cutting is of different thicknesses. (1mm- 3mm).
The parameters used are Laser power, cutting speed, beam diameter.
The factors to be studied are kerf width, temperature distribution within the material and different power-speed combinations for successful cutting.
CHAPTER-2
LITERATURE REVIEW
2.1 INTRODUCTION
Laser cutting is the increasingly used nonconventional method of machining i.e. for shaping and separating a work piece into segments of desired geometry. The cutting process is carried out by moving a laser beam along the surface of the work piece with constant distance thereby generating a narrow cut kerf. Laser cutting is mainly the application of heat through the laser beam on the work surface which is melted and removed (Fusion cutting) or directly evaporated (Evaporative cutting) to form the cut. In case of fusion cutting the material removal is supported by a gas jet impinging coaxially to the laser beam. The cutting gas accelerates the transformed material and ejects it from the kerf. The assistance of a reactive gas (Oxygen) may be taken to enhance the cutting by lessening the power required.
Laser Beam Machining (LBM) is a thermal process. The effectiveness of this process depends on thermal properties and, to a certain extent, the optical properties rather than the mechanical properties of the material to be machined. Therefore, materials that exhibit a high degree of brittleness, or hardness, and have favourable thermal properties, such as low thermal diffusivity and conductivity, are particularly well suited for laser machining. Since energy transfer between the laser and the material occurs through irradiation, no cutting forces are generated by the laser, leading to the absence of mechanically induced material damage, tool wear and machine vibration. Moreover, the material removal rate (MRR) for laser machining is not limited by constraints such as maximum tool force, built-up edge formation or tool chatter. LBM is a flexible process.
Figure 1: Basic principle of Laser Cutting
Based on interaction of the laser beam with the work piece and the role of assist gas in the material removal process, lasers can be used in several ways in the material removal processes during cutting. The four main approaches to cut the material using laser are evaporative laser cutting, fusion cutting, reactive fusion cutting, and controlled fracture technique. The selection of optimum technique and operation condition depends on the thermo-physical properties of the material, the thickness of the work piece, and the type of laser employed.
Figure 2: General schematic of a laser-cutting process with a coaxial gas jet to blow the molten material.
2.2: METHODS OF LASER CUTTING
Based upon the application of heat, the following laser cutting types are in maximum use in the industries and other areas.
2.2.1 Evaporative Laser Cutting
In evaporative laser cutting, the laser provides the latent heat until the material reaches the vaporization point and ablate into vapourstate. Since the materials removal is due to direct phase change to the vapour, the cut quality is extremely high with clean edges. The method is primarily suitable for the materials with low thermal conductivity and low heat of vaporization such as organic materials, cloth, paper, and polymers. Nonreactive gas jet may be used to reduce charring.
During laser vaporization cutting, the material is heated beyond its melting temperature and eventually vaporized. A process gas jet is used to blow the material vapour out of the kerf to avoid precipitation of the hot gaseous emissions on the work piece and to prevent them from condensation within the developing kerf.
Figure 3:Laser vaporization cutting
Typical materials that are cut by the vaporization method are acrylic, polymers, wood, paper, leather and some ceramics. This method has a high power requirement that depends on the thermal properties of the material. High power densities are obtained by appropriate adjustment of the laser radiation and focusing. For cutting of metals, laser vaporization cutting is the method with the lowest speed among other methods; however, it is suitable for very precise, complex cut geometries in thin work pieces.
2.2.2 Fusion cutting (Melt and Blow)
The laser fusion cutting process, also called inert gas melt shearing, is based on transformation of the material along the kerf into the molten state by heating with laser energy and the molten material blown out of the kerf by a high pressure inert gas jet. The laser beam is the only heat source during this cutting process and the high-pressure inert gas jet is responsible for melt ejection. The inert gas jet (mainly nitrogen or argon) is also responsible for shielding the heated material from the surrounding air as well as protecting the laser optics.
Figure 4:Laser fusion cutting
Laser fusion cutting is applicable to all metals especially stainless steels and other highly alloyed steels, aluminum and titanium alloys. A high quality cut edge is formed but the cutting speeds are relatively low in comparison with active gas cutting mechanisms. The advantage of this process is that the resulting cut edges are free of oxides and have the same corrosion resistance as the substrate. The cut edges may be welded without any post-cutting preparation. The main technical demand is to avoid adherent melt (dross attachment) at the bottom edges of the kerf. A high pressure (above 10 bar) is recommended to remove liquid that can adhere to the underside and solidify as dross.
2.2.3 Oxygen Assisted laser Cutting
The principle of laser oxygen cutting is that the focused laser beam heats the material in an oxidizing atmosphere and initiates an exothermic oxidation reaction of the oxygen with the material. The exothermic reaction supports the laser cutting process by providing additional heat input in the cutting zone resulting into higher cutting speeds compared to laser cutting with inert gases. The laser beam is responsible for igniting and stabilizing a burning process within the kerf, and the assist gas blows out the molten material from the cut zone and protects the laser optics.
Figure 5:Laser oxygen cutting
Laser oxygen cutting is applicable to mild steel and low-alloyed steel. The formation of the oxide layer on the cutting front increases the absorption of the laser radiation compared to absorption of a pure metallic melt. The oxides reduce the viscosity and surface tension of the melt and thereby simplify melt ejection. However, the resulting cut edges are oxidized . In case of Oxygen assisted high cutting velocity can be achieved for a given power level. But for stainless steels this method of cutting is not suitable due to the presence of Chromium. Formation of Chromium oxide prevents oxygen to pass through it.
Table 1: Chemical composition of AISI 304 Stainless Steel
Components
PREVIOUS STUDIES ON LASER CUTTING
Laser cutting since its inception has got increasing popularity due to various advantages over the conventional methods of cutting. Amongst various materials, steel has got maximum use in laser cutting applications. To have high rate of production and satisfying level of cut quality, optimum combination of laser and process parameters must be used because these parameters have high impact on the macroscopic and microscopic characteristics of finished parts like kerf width, HAZ. Proper selection of these parameters may avoid expensive redesign and subsequent finishing operation.
Various experiments were performed considering and analyzing the laser beam and process using advanced sensing and measuring equipment. These parameters are usually adjusted and tuned to provide the desired cut quality but it is much time consuming and heavy effort has to be given. Moreover if a different material is to be cut then the procedure has to be followed from the start. The findings of these experiments can’t be extrapolated for different part geometries. Important data such as time history of temperature, phase change, cooling rates can’t be obtained from experiments. But they are important for the evaluation of microstructural transformation during cutting.
2.4 HEAT TRANSFER MODELS OF LASER CUTTING
Early models of heat flow phenomena in laser cutting were based on the application of the solution of the classical heat conduction equation in integral or differential form using the concept of an instantaneous heat source for an infinite medium. The phenomena of melting, evaporation were not included in those simple analytical models.
A mathematical model gives the advantages of checking a physical model, estimating important parameters and calculating the effect of various parameters. But several assumptions are to be taken in order to simplify the model. In this way the solution deviates more from reality.
Keeping in view of the above, numerical simulation methods seem to be better prospects. Researches have been carried out since the seventies of the 20th century in the field of simulation of cutting, welding, surface treatments etc.Some of these are discussed below involving laser cutting from the several studies in the field of simulation.
In the case of laser welding one important factor comes into discussion is the depth of penetration. In their paper Mazumder and Steen have formulated a keyhole model for laser welding which was capable of calculating the necessary metallurgical data as well as the depth of penetration. From the result of the paper it was possible to calculate the conditions such as the maximum welding speed as a function of laser power or substrate thickness. But it does not allow for variable thermal properties, latent heat effects or chemical reaction within the substrate.
Using a high power laser and material removal technique by evaporation Kim et al formulated a two dimensional model for the transient analysis. Solid to vapour change occurred in one step. Computing time consumed were much more due to mesh regeneration after the evaporation front was established in terms of the nodal temperature reaching the evaporation temperature each time.
A one dimensional model using steady state analysis of the heat transfer mechanism during laser drilling was established by Yilbaset al. This model was capable of predicting the maximum temperature in the material, nucleation, the explosion process and the drilling efficiency.
Brockman et al developed a simple algorithm for calculation of the laser induced temperature field in a thin moving sheet and found an asymptotic formula for temperature at large distances from heating point. Cooling effects caused by gas flow and back irradiation are taken into account through the heat transfer coefficient. Due to small thickness of the sheet the averaging of temperature field and governing equation across the sheet was possible. A two dimensional Fourier transformation is applied to build the temperature field. This is applicable for every thin sheet and the convection coefficient value is in the range of 10 - 100 W /m2K for natural convection and is 100 – 5000 W /m2K for very intense flow (Blowing gas jet) conditions.
Figure 6: A thin sheet is moving with a velocity u along the x’-axis.
Aloke et al formulated a two dimensional model to estimate the dimensional accuracy of holes produced with laser cutting. The model used to calculate the size of hole and cut out disc of varying radii in steel plates with thickness 3.2 and 6.4 mm. It was assumed that, the heat transfer mostly occurs through conduction and thermal properties do not vary with temperature. The laser beam was taken as a line source and the temperature distribution over the thickness is considered uniform.
The solution to the two dimensional differential equation is:
SHAPE \* MERGEFORMAT
Where
( Theabsorptive factor
P ( Laser Power
V ( Cutting velocity
X ( Distance along direction of beam travel
R ( Radius of hole
K ( Conductivity
Figure 7: Cut-out solid disk with HAZ
The above equation (1) serve猠瑯⁰牥
es to predict the temperatures from the kerf end into the work piece material. The HAZ data is produced considering a temperature 873 K because according to Okerblom as cited by, the yield strength of mild steel is reduced to almost zero when the temperature is 873 K.
SHAPE \* MERGEFORMAT
Fig. 8 A schematic showing the laser cutting procedure
They concluded that, the kerf width is a major factor in achieving high dimensional accuracy compared with thermal deformation. Producing smaller kerf enhances the accuracy. Higher cutting speed and smaller spot size will result in closer tolerances. They also suggested that for thicker materials a model that considers heat flow in all three directions will provide better results.
2.5 SIMULATIONS ON LASER CUTTING
Yilbas et al investigated temperature and stress field developed around the cutting section in the hole cutting of thick mild steel sheet using finite element method. In the study laser cutting of hole of 15 mm diameter and 15 mm thickness is considered for mild steel. A transient heat transfer equation is solved numerically with the appropriate boundary conditions to predict the temperature field in the substrate material.
The phase change problem was analyzed using a nonlinear transient thermal analysis which was based on the enthalpy method. During phase change the latent heat evolution was tackled by enthalpy of the material. It was used as a function of temperature and incorporated in the energy equation.
They found that temperature rise is sharp in the vicinity of the cut edges.
Figure 9: (a) Temperature distribution along the x-axis when heat source is at two locations in the cut edges.
Figure 9: (b) Temperature gradient distributions along the x-axis
When heat source is at two locations in the cut edges
Arif and Yilbas simulated the laser cutting for three different materials namely Steel, Inconel 625 and Ti-6Al-4V alloy. Temperature as well as thermal stress fields is computed. They considered a constant cutting speed and a constant temperature heat source with a focused spot diameter which is assumed along the kerf surface at the cut edge resembling the heat source. They found that, the temperature along the cut edge builds up once the heat source reaches the end of the cut section (Figure 2.10). This was due to the radial heat transfer in the material.
Figure10: Temporal variation of the temperature at y axis location y=0.0125 m
SHAPE \* MERGEFORMAT
In the present study, laser hole cutting of thin AISI 304 stainless steel sheet is carried out for different thicknesses. Phase change phenomenon is addressed by the enthalpy method. Temperature distribution in the cut material is obtained to predict the possible thermal stresses. Simulations are carried out for different power velocity combinations to obtain the threshold values.