Excerpt from the Proceedings of the COMSOL Conference 2010 Boston COMSOL Assisted Simulation of Laser Engraving Hamidreza Karbasi Conestoga College Institute of Technology and Advanced Learning School of Engineering and Information Technology 299 Doon Valley Dr., Kitchener, Ontario, N2G 4M4, Canada [email protected] Abstract: Laser engraving is becoming an customers who are testing laser engraving on the appealing option for those applications where new and expensive materials. The software previously Electrical Discharge Machining would also be an effective tool for the (EDM) was the only choice because of its development and evaluation of new engraving advantages with practicability, time and cost. techniques with an eye towards reducing wear- Laser engraving technology removes material in and-tear on the actual laser engraving machine a layer-by-layer fashion and the thickness of and the elimination of a destructive testing layers is usually in the range of a few microns procedure. and it is depending on several parameters such as the specimen material, laser power, Transverse Keywords: Laser engraving, Laser parameters, Electromagnetic Mode (TEM), and beam COMSOL modeling, material removal. traverse speed. The crucial first step in the setup process is to find out the depth of material which 1. Introduction will be removed in each layer and is traditionally done off-line by trial-and-error technique. This is As industrial laser applications continue to a lengthy process in which the operator must grow with the development of new laser alter the laser parameters such as power and technologies, laser engraving has become one of traverse speed gradually and use a microscope to the most attractive areas among these examine the quality and measure the depth of technological advancements. The advantage of removed material. For new materials, which may laser engraving is apparent over some be very expensive, this process could take weeks conventional machining methods such as EDM and become very costly. as it is more efficient and economical. As laser The main purpose of this research project was to engraving is a process of removing layers from develop a proof of concept software that could objects through vaporization, one of the most simulate the geometry of engraved surface and important parameters of the process is in estimate the depth and width of an engraved determining the thickness of the layers. The groove and its associated laser parameters. common technique for obtaining the proper For simulation purposes, COMSOL was used to thickness and its associated laser parameters is simulate the moving laser beam as a source of an iterative-based process in which the laser heat over physical domain made of different parameters are changed individually and the materials. Through interaction modeling of resulting engraving is examined to determine selected materials with the laser beam in TEM01 accurate depth and width. To achieve the best mode, the temperature distribution was engraving results, this trial-and-error process can determined. The temperature is the key take from several days to several weeks. parameter to determining the geometry of an In laser engraving, the process inputs fall into engraved groove. As the laser heated the material two groups, fixed and variable (controllable). and the temperature passed the vaporization Some of the fixed inputs include laser optics, point at high power intensity, the density of the wavelength, beam profile, ambient temperature, material would switch to the density of air to and specimen material and dimension. Some of form the groove. The simulation results were the controllable inputs include laser power, pulse validated by several experiments. frequency, and traverse speed. Obviously, the The simulation software can be improved to desired process output is good quality engraving include more parameters and become a basis for characteristics such as smooth edges, good a powerful laser engraving simulation package. surface finishing, and creating clear and fine The benefits of such a package would be big details. saving in setup time and material costs for The objectives of this research were: y To generate a computer simulation of heat transfer such as conduction and convection. laser engraving for a simple line This understanding can be obtained through a y To take a fundamental step towards reliable simulation. As stated in the objectives simulating a complete layer and list, the simulation will determine the subsequently laser engraving of a 3D temperature distributions through a physical object domain while it is exposed to a moving laser beam with known parameters. This will increase Some benefits of a computer simulation such understanding of laser heat flow in material as this are as follows: processing which in turn will help in prediction y The software can be used to fine-tune and process improvement. the laser parameters such as speed, The heat conduction principle is considered power, and pulse frequency to ensure and solved as the main physics for this problem. quality engraving. The depth of The speed of conduction depends on the material engraving will be predicted and as a density, specific heat capacity and thermal result the thickness of layers will be conductivity, the difference in temperatures, and determined. the shape of the conductor. y Identifying more effective parameters involved in the process of laser ΔQ ⎛ ΔT ⎞ ∂T −= kA⎜ ⎟ ρ CP ).( =∇−∇+ QTk engraving. Δt ⎝ Δx ⎠ ∂t y A tool for evaluation of new engraving (1) techniques without wasting actual where: machine time. These new techniques Q is heat may enable good quality engraving on t is time curves surfaces, eliminate side effect of k is thermal conductivity segmentation for engraving over large ρ is density areas, and improve the continuity of C is specific heat capacity engraving when using Q-switched p A is the cross section of the material lasers by tuning repetition rate. T is the temperature in difference y To assess laser engraving impacts on x is the distance between the heat sources new materials (e.g., thermal stress generated during laser engraving can Equation (1) is considered to be solved for its lead to microcracking which in turn transient response under these conditions: affects mechanical properties and may • Moving laser beam at 50 mm/s along a induce corrosion in metals). 40mm-long straight line 1 • 100-micron laser spot considered as a In this research, COMSOL Multiphysics is used for simulation purposes to mathematically moving heat flux boundary condition model appropriate heat transfer physics. with Q=0 COMSOL is a Finite Element Analysis (FEA) • TEM01 considered for heat flux software package that allows the user to develop distribution over the laser spot 3D models with associated boundary conditions. • Heat flux will be the multiplication of The material properties and equations can be laser intensity by material absorptivity defined within the program to yield an accurate coefficient as Pxy*Absorptivity representation of the problem. Using post- • Absorptivity will be a function of processing function of COMSOL, the data can temperature that yields a new value as be returned for visual representations which help the material melts to assess the results. Heat flux will switch ON and OFF at given frequency (a true q-switching model needs the understanding of the relationship between the 2. Physics duty cycle and pulse peak power) The design of laser material processing requires an understanding of laser theory and 3. Problem Statement The problem was defined to estimate the • Temperature (much higher absorptivity at maximum depth and width of engraving. The melting state) parameters for this problem are as follows: • Surface oxidation thickness (acting as • Hermit-Gaussian laser beam working at anti-reflection coating) TEM01 mode • Angle of incidence • Beam diameter = 0.1mm • Material and surface roughness • laser power = 73W (roughness increases diffuse reflection) • laser beam velocity = 50mm/s • Substrate materials are Steel Din 1.2767, Aluminum, Copper, and Brass 5. COMSOL Modeling • Ambient temperature of 300K The heat transfer module of COMSOL is used to solve Equation (1) for its transient 4. Material Properties2 response. The 3D model of the physical domain can be developed in COMSOL and then the In Table 1, the most important properties of boundary conditions need to be defined the materials for our test purposes are listed, carefully. The position of laser path which including melting and vaporizing temperatures, defines the trajectory of moving heat source must and absorptivity percentages in the solid and be given to the software. For our case this path is liquid (melting) states. Notice the high a straight line. The next step in COMSOL absorptivity for steel and aluminum and low modeling is to mesh the physical domain and absorptivity for copper: a tremendous increase in specify finer mesh size wherever a higher absorptivity percentages takes place when accuracy is required. Solving and post temperature passes the melting point. processing are the last two steps. In laser material processing we are dealing Table 1. Materials Absorptivity at 1064 nm with a multiphysics environment which Mat. ρ- Tm Tv Abso- Abso- COMSOL is capable of creating. The complete kg/m^ (K) (K) Solid Mel. problem involves the physics of optics, 3 electromagnetic wave (RF), heat transfer, and Steel 7850 1808 3003 0.36 0.9 electro-thermal interaction. For this round of modeling, the laser beam is simulated as a Al. 2700 933 2333 0.2 0.8 moving heat source over the physical domain Cop. 8700 1356 2903 0.1 0.35 which can be defined by Equation (1) if the right hand side of the equation is set to zero and the moving heat flux is considered as the boundary When the laser beam strikes the material only condition along the defined path. For further a portion of laser energy is absorbed and the rest modeling, physic optics can be brought into the is diffusely reflected.