physical model about laser impact on metals and alloys

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L. Lazov, N. Angelov, Physical <strong>model</strong> <strong>about</strong> <strong>laser</strong> <strong>impact</strong> on metals and alloysContemporary Materials I−2 (2010) Page 125 of 128which really participates in processing (structuralchanges, melting, vaporization in the unit volume).In certain references it is defined as a percentage ofthe total energy falling onto the material, necessaryfor concrete processing [1]. Under certain simplifications,if we ignore the conditions of working zone(convection, heat radiation and thermal conductivity)we assume that it is energy absorbed by thematerial.INTERACTION OF RADIATIONWITH A SAMPLEThe interaction of <strong>laser</strong> radiation with a sampleis connected with the level of absorption А%,which is defined as a percentage of energy absorbedby material from the total energy falling onto thematerial. Optical depth of penetration δ is definedwith absorption coefficient δ = 1/α. In this case wehave exponential decrease of radiation intensity withdepth z. For strong absorbing materials, such as metalsand alloys, the depth of penetration δ is lowerthan wavelength λ of falling radiation. Total energythat penetrates the material is absorbed and it doesnot depend on the thickness of the sample [1].The ratio of absorption according to [2] dependson: optical constants of material - coefficient of reflectionR, absorption index к / ; <strong>physical</strong> properties of <strong>laser</strong> radiation – wavelengthλ, polarization, power density q S ; chemical composition of surface – oxides, nitride,graphite layers and so far; topography of surface (roughness);as well as on the angle of radiation falling, temperatureand aggregate state of material which is under<strong>impact</strong>.The dependence of absorption ability А onwavelength λ for different materials at room temperatureand angle of falling 0 o (towards normal) to thesurface is presented in Fig. 2 [3].We can see from the graph that absorptionability (except for Аl) decreases with an increase ofwavelength. Differences in separate groups of metalwhich are under <strong>laser</strong> <strong>impact</strong> are clearly noticeable.Thus, for example, for precious metals (Ag, Cu) absorptiondecreases very fast for wavelengths fromvisible spectrum, and for transition metals (Fe, Mo)the absorption change is more smooth and expandsto the far infra-red range. Аl, which is a representativeof polyvalent metals, has specific change of absorptionwith medium maximum for wavelength λ =0,84 µm and almost for total range of wave lengthsit has low absorption [1]. We can see on the graphfor industry used <strong>laser</strong>s that there are great differencesconcerning the ratio of absorption for differentmetals. For example, iron (Fe) under <strong>impact</strong> fromCO 2 -<strong>laser</strong> and Nd:YAG <strong>laser</strong> has absorption capabilitywhich changes 4 times and for Аl this differenceis insignificant. If we compare the ratio of absorptionfor Аl under <strong>impact</strong> of diode <strong>laser</strong>s (λ = 0,808 -0,940 µm) and Nd:YAG <strong>laser</strong>s, the diode ones havepriority over others. The temperature of surfacewhich is under processing is very important for increasingabsorption of <strong>laser</strong> radiation falling on thematerial. This is easy to see in Fig. 3. If we reach themelting point then the absorption ability changes rapidly.This effect is noticed for Аl with different <strong>laser</strong>sources - CO 2 -<strong>laser</strong>, Nd:YAG <strong>laser</strong> and diode <strong>laser</strong>.For <strong>laser</strong>s with shorter wavelength that jump issignificant comparing to CO 2 -<strong>laser</strong>. Other variables,which <strong>impact</strong> absorption, are polarization and angleof falling of the <strong>laser</strong> beam. (For <strong>laser</strong> radiation <strong>impact</strong>s,we have clearly expressed maximum forBrewster’s angle, if electric vector E vibrates parallelto surface of falling. As for the <strong>impact</strong> with polarized<strong>laser</strong> radiation, if electric vector E vibratesnormally to plane of falling we observe a smooth increaseof absorption (see fig. 4). Nature of absorptionmaximum depends on wavelength λ. For λ = 10,6µm the maximum is narrow and has a value of <strong>about</strong>80%, and for visible range it is wide and <strong>about</strong> 30%.The above effects are valid at low power density thatdoes not allow creation of plasma–cloud over theprocessing surface.Figure 2. How absorption ability А depends onwavelength λ for different metals and steel at roomtemperature and angle of falling along normally to thesurface
L. Lazov, N. Angelov, Physical <strong>model</strong> <strong>about</strong> <strong>laser</strong> <strong>impact</strong> on metals and alloysContemporary Materials I−2 (2010) Page 126 of 128<strong>impact</strong>, the heat propagates to short distances indepth of the product and also on its surface. The finalresult is insignificant heat <strong>impact</strong>ed zone as wellas lack of convection losses.100%8060λ = 10,6 μmA40λ = 1,064μmFigure 3. Graphics of dependence of absorption A ontemperature T for Аl at different wavelengths of <strong>laser</strong>radiation20λ = 0,532 μmTHEORY OF THERMAL CONDUCTIVITYFOR METALSConcerning the <strong>impact</strong> with power densityover certain threshold, when absorbed energy of processingzone is bigger than that and is scattered as aresult of heat conductivity, we see that materialstarts to melt. The size of melting zone depends onparameters of processing, absorbed energy per unitlength, but also on geometry and properties of thesample. For the purpose of marking, we need absorbedenergy to generate such structural or phasechanges in surface layer to obtain the quality of markingwe need. Optimization of processing consists ofselection of such parameters of processing (functionof <strong>physical</strong> processes) which should cause appropriatetemperature fields in material, ensuring desiredgeometry and quality of marking zone. By performingpreliminary numeric calculation for temperaturefields at <strong>laser</strong> <strong>impact</strong> over certain materials, whenwe notice the factors which influence processing, wemay predict and optimize the final results. In therange of optical absorption of <strong>laser</strong> radiation in themetal, photon-electron interaction occurs, which resultsin absorption of electromagnetic energy, whichis transformed into heat energy. Propagation of heatin the material is described by phenomenon of thermalconductivity.Laser marking is known by its specific verysmall range of <strong>impact</strong> (several by ten µm), big powerdensity and short duration of <strong>impact</strong>. High temperaturegradients for warming and cooling of theworking zone are observed. Concerning duration of00 20 40θ, oFigure 4. Graphs of the dependence of absorbance A ofthe angle of incidence θ of <strong>laser</strong> radiation for Al atdifferent wavelengthsPenetrated in metal, <strong>laser</strong> radiation is completelyabsorbed by free electrons in thin layer underthe surface with depth 0,1 1,0 m. That is the reasonfor their energy to increase and intensity of hitsbetween them to grow. For the duration (time11t ~ 10 s ) from the beginning of the <strong>impact</strong>, theyhave delivered a very small part of that energy tocrystal lattice. During that time strong overheatingof electronic gas occurs ( Te Ti, where Teis thetemperature of electronic gas and T iis the temperatureof crystal lattice). During that time, intensity ofhits between free electrons and ions of crystal latticeincreases as well as a stream of energy from electron9gas to the lattice. For time t rel~ 10 s from the beginningof the <strong>impact</strong>, the temperature difference T T e T ibecomes minimum and the state inworking volume has a characterization of sharedtemperature of metal T . The time t relis defined asthe time of relaxation.As explained already, the main part of transmissionof heat in the metal at the <strong>laser</strong> <strong>impact</strong> occursfrom electron conductivity. This means that heatprocesses of processing with <strong>laser</strong> radiation havethe same <strong>physical</strong> nature as with the <strong>impact</strong> with traditionalsources of heat. That gives the basis to de-6080
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physical model about laser impact on metals and alloys

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