Surface Engineering of Metals - Principles, Equipment and Technologies Part 6 doc

Fig 3.15 Typical correlations of the absorption coefficient for metals (continuous curves) and oxides (dashed curves): a) with temperature for pure and oxidized steel; b) with temperature for the majority of refractory metals and oxides; c) with wavelength of laser radiation incident on the heated material (From Burakowski, T., et al [11] With permission.) – deposition of an absorbing coating which in most cases also fulfills the role of an anti-reflection coating This can be a paint coating in the form of paints, varnishes and colloids, powder, paste, electrodeposit, chemical or any other coating which exhibits good absorption of radiation Absorption coatings are most often produced from saline compositions (e.g., magnesium and zinc phosphates), metal oxides (e.g., zinc, titanium, silicon, chromium, iron, aluminum), non-metals, sulfides, carbides (e.g., silicon and molybdenum carbides), graphite, soot, blackening, etc They increase the absorption rate of the surface by even up to 90% at lower temperatures and up © 1999 by CRC Press LLC to 50% in the higher temperature range (approx 1500ºC) As an example, paints of the gouache type increase absorption rates by 60 to 90% [25−27], magnesium zinc Zn3(PO4)2 or magnesium phosphate coatings Mn3(PO4)2 up to 55 to 90%, iron sulfide coatings (Fe2S3) up to 20 to 40%, graphite coatings up to 40 to 70% [11, 23−27] An interesting development is that of exothermic oxide-metal coatings (e.g., FeO+Al, FeO+Si) which, besides raising the absorption rate of radiation, also cause the emission of heat This is achieved by a exothermic reaction between the components of the coating, initiated by laser radiation Such coatings may, within certain limits, intensify the effectiveness of laser heating, conducive to a decrease of the power density of the beam, thus to using lower power lasers With appropriately selected composition, the coating may also prevent its constituents from migrating to the substrate [24] It is also possible to employ combinations of the above methods When selecting the method of enhancing the absorption rate of the surface, one should take into consideration that increasing surface roughness causes a deterioration its three-dimensional quality It does not, however, cause changes of the chemical composition of the metal’s heated zone which usually, although to a negligible degree, takes place in the case of absorbing coatings Unless remelting of the substrate material takes place, such coatings either not cause deterioration or cause only insignificant deterioration of surface quality Good absorbing coatings should, moreover, feature good adhesion to the substrate, stability of thermophysical properties, homogeneity of properties and of thickness, as well as ease of deposition and stripping [24] The thickness of such coatings usually does not exceed 0.1 mm Since laser heating is usually carried out in air, a protective atmosphere is introduced, usually nitrogen or argon at (0.5 to 1.0)∞105 Pa pressure [5, 11] This is done to protect against oxidation of the substrate which was not preoxidized, if it is detrimental to the technological process, but predominantly to protect the laser lenses against radiation 3.4.3 Depth of penetration of photons into the metal The initial phase of laser heating consists of absorption of photons which are not reflected from the surface by free or bound electrons of the heated material Because of the high frequency of laser radiation, exceeding many times the frequencies of electromagnetic radiation used for induction heating, the depth of penetration of photons into the heated material is small It can be determined from a formula analogous to that used for induction heating [7]: (3.4) where: d - depth of penetration of laser radiation into the heated material, m; ρ - resistivity (reciprocal of conductivity) of heated material, m; - â 1999 by CRC Press LLC relative magnetic permeability of heated material; f - frequency of changes of electromagnetic field, bound to laser radiation wavelength by the correlation: f = c/ λ , where c - velocity of light (wavelength of µm corresponds to a frequency of 3.33∞10-15 Hz) According to equation (3.4), the radiation of molecular CO lasers penetrates the material deeper than that of Nd-YAG lasers To simplify the problem it can be said that photons from infrared radiation penetrate materials that are not transparent to this range of frequencies to a depth not exceeding 10-6 to 10-5 cm (1 to 10 µm), i.e., a depth which is comparable to the wavelength of the incident radiation [6, 7] Photons are absorbed by free or bound electrons of the heated material and cause a rise of their energy Electrons with higher energy interact with the crystalline lattice and with other electrons and cause passing of energy deeper into the heated material which is manifest by a rise of temperature From the zone heated by the penetrating photons the heat is therefore passed on to the colder zones of the material by means of heat conduction at a rate which depends on the conductivity coefficient of the given material which is significantly higher for metals than for non-metals Usually materials with high thermal conductivity are characterized by high electrical conductivity and, at the same time, high reflectivity These are properties which characterize metals For non-metals there is a generally opposite rule For this reason, the intensity of conducting heat into the material from the directly heated surface of the metal is greater than for an absorptive coating deposited on the metal substrate Assuming that the diameter of the laser spot is 20 mm and the depth of photon penetration is approximately 10 µm, the volume of material affected by photon penetration is approximately mm In this volume there occurs absorption of 0.5 to 10 kW of radiant power and its transformation into heat energy Such great powers evolved in such small volumes cause almost instantaneous heating of that volume of material The time of energy transfer by electron collisions is approximately 10 -12 to 10-14 s, while the time of energy transfer to the crystalline lattice is approximately 10-11 to 10-12s [6] For normal and gigantic laser pulse durations of 10 -3 to 10 -7s it is safe to assume that during one pulse, electrons which absorbed photons collide many times with other electrons and with the lattice For this reason it can be assumed that radiant energy is instantly transformed into heat energy at the site where radiation is absorbed Thus it is possible to obtain uncommonly short heating times for laser heating, ranging from 10 -1 to 10-8 s [4, 27, 29] Transfer of heat from heated spots by penetration of photons (limited to a layer of a fraction of a micrometer to several micrometers at the most) to the remaining portion of the load is significantly slower and equal to the rate of heat conduction by the given material In laser heating, the highest temperature is attained by the thin subsurface layer in the zone of incidence of the laser beam (i.e., in the laser spot); in stricter terms, in the zone of the greatest power density Surface temperature distribution approximately corresponds to the distribution of © 1999 by CRC Press LLC Fig 3.16 Schematic representation of the interaction of laser radiation with material power density in the cross-section of the beam at the site of the laser spot [30] Heat accumulated in the surface layer heated by laser radiation (over 90% of the total heat evolved) is passed on concentrically into the material by way of heat conduction Thus, cooling rates are very high, compared to rates of heating Only a small portion of the heat delivered to the material (max 10%) is given off by radiation of the material’s surface layer (Fig 3.16) The efficiency of laser heating is very low This efficiency depends on the type and design of the laser heater and on the absorption coefficient of the heated surface On the average, the efficiency of heating a solid by laser pulses ranges from to 3%, while that of continuously operating gas lasers ranges from to 7% 3.4.4 Laser heating stages For given values of exposure time t and absorbed power density q = q 0A, the heated material, e.g., steel, may be subjected to successive heating stages to a higher temperature [11, 30] with a rise of q and t Each such stage includes the previous heating stage (Fig 3.18) Stage Heating to temperature Tmax