rus
Issue #3/2014
A.Filatov, A.Kishalov, N.Orlov, V.Serebryakov, V.Smirnov, M.Volkov
Laser Cleaning and Laser Shot Peening. Technologies for Surface Properties Improvement
Laser Cleaning and Laser Shot Peening. Technologies for Surface Properties Improvement
Wear resistance, fatigue strength, corrosion resistance, tightness of joints and fit strength – these properties of parts and joints in many respects determine the reliability and longevity of equipment and constructions. The results of application of laser technologies for the improvement of metal and nonmetal surface properties – laser cleaning and laser shot peening are considered.
Теги: corrosion resistance fatigue strength laser cleaning laser shot peening wear resistance износостойкость коррозионная стойкость лазерная очистка лазерный наклеп усталостная прочность
Break up of mechanism parts often starts from the surface. Therefore, if the quality of the part surface layer specified by designer is provided it will be possible to forecast reliability of the whole mechanism operation with confidence. Thus, solution of the problem of quality enhancement of mechanisms, equipment, constructions and devices lies within the range of provision of performance characteristics of the part surface layer. And this issue is related to the geometrics as well as physical and chemical properties of coatings and surface layer. Laser technologies and new options of their application in science and engineering are capable to improve the functional quality of part surface layer.
LASER CLEANING
When lasers have just appeared, instantly technologies paid attention to the capabilities connected with the use of "pure beam energy" as the cleansing agent. Pulse highly-concentrated light beam is able to heat thin surface layer of the material instantly in such manner that it can evaporate without noticeable action on the material which is located deeper. It should be noted that in many cases coating material does not even experience the thermal decomposition and contaminations accompanied with the formation and release of toxic or undesirable agents do not have time to occur. As a result, we have cleaning method which is unique by its properties and does not have all shortcomings inherent to the classic mechanical and chemical cleaning methods. It should be noted that manifestation of additional effects connected with the generation of thermo-elastic stresses and shock acoustic waves in near-surface layer increases the efficiency of cleaning process adding to the efforts on "lifting" contaminants from the surface.
For the most part, economical problems (instrument cost) and low process performance impeded the development of laser cleaning technologies. It is notable that laser cleaning methods have been successfully used for a long time in the restoration for the cleaning of museum valuables, in the area where the cost and performance do not play such defining role.
Occurrence of the reliable, compact and inexpensive laser sources – ytterbium fiber lasers with high coefficient of efficiency and long service life (up to 100 000 hours) – made it possible to jump to the new quality level for the laser cleaning technologies. Application of new laser radiation sources in the laser cleaning technologies provides meeting high up-to-date requirements to the production ecological compatibility. The companies which functions are connected solely with the development of equipment and laser cleaning methods operate in a number of advanced industrial countries.
The scheme of laser cleaning process (Fig. 1) is very simple – radiation of pulse laser is focused on the part surface. And the spot must have such size that the radiation power density would cause quick increase of the surface layer temperature during the pulse up to the temperature of its quick destruction (evaporation or sublimation). Approximate value of such power density is 107–1010 W/cm2. When focusing the radiation on the spot with the diameter of about 0.2 mm in order to reach such power density, use of pulse fiber laser with the mean power of just 10 W will be sufficient.
Currently, the systems of laser cleaning based on pulse fiber lasers by IPG Photonics have been developed. Backpack devices are the most compact and affordable class of laser cleaning equipment for the end users. German firm called Clean-Lasersysteme GmbH is the primary foreign producer of such compact laser cleaning systems. In Russia similar systems have been developed by NPP VOLO.
Use of Laser Cleaning Technology
The scope of tasks in the solution of which laser cleaning technologies are used includes the operations on pollutant removal and operations on the removal of coatings applied on the product during the production procedure. Laser cleaning is mostly used for:
removal of paint coatings;
removal of electrodeposited coatings;
removal of consumption pollutants;
removal of biological pollutants;
prior treatment of the surface before gluing or application of any functional coatings for the improvement of its adhesive properties;
removal of coolant-cutting fluids;
removal of corrosion residues;
removal of preservative coatings and combined pollutants.
Variety of laser cleaning applications produces the variety of laser devices intended for the solution of these tasks. In order to enter the market and get major distribution, similar laser devices must have several advantages. First of all, laser equipment intended for cleaning must have low cost. Secondly, it should provide low cost during its operation. Thirdly, it should demonstrate high reliability in operation. Fourthly, it should be compact, have simple configuration and must be easy to use.
NPP VOLO company (Fig. 2) managed to meet these conditions with its products – backpack equipment for laser cleaning. First of all, price of the backpack equipment intended for laser cleaning is approximately equal to the cost of annual usage of blasting equipment. Secondly, the laser cleaning equipment practically does not necessitate any operating expenses. Thirdly, average equipment life which defines the laser life cycle is more than 50 000 hours. Our units do not have wearing mechanical parts. Fourthly, total weight of such unit including the manipulator (hand tool) weight is not more than 15 kg.
Laser Cleaning Process
Figure 3 illustrates the results of laser cleaning of the surfaces of different materials using the laser backpack unit manufactured by NPP VOLO. In order to demonstrate the result visually the steel plate polluted with corrosion, scale chips and oil was selected. Lower area of the steel plate was cleaned by laser and upper area was specifically left uncleaned and polluted. Then, steel wire facing was applied on cleaned and untreated surfaces. The difference between the result of extremely unstable process of facing in the upper area and the result which meets the requirements in the lower area subjected to the prior laser cleaning is clearly seen. This experiment showed well the applicability of laser cleaning technology for the treatment of weld edge right before welding, for instance.
Turbine blade of gas-turbine engine (GTE) is shown in Fig. 4 before (on the left) and after laser cleaning (on the right). As the result of cleaning, surface consumption pollutants were removed. Fig. 5 demonstrates thread area of pump-compressor pipe (PCP) before (image on the left) and after laser cleaning (image on the right). PCP thread was polluted with the residues of coolant-cutting fluids and metal burrs and other workshop pollutants stuck to it after the thread cutting. Simple configuration of laser cleaning devices allows their easy integration into the production lines of enterprises.
Results of the laser cleaning of stone samples having biological and anthropogenic pollutants are given in Fig. 6 and 7. These are the samples of actual objects which were exposed to the open urban spaces of the Northern Capital for a long time. Their restoration cleaning did not require transfer of these monuments to the special workshops. And low power inputs and high ecological compatibility of laser equipment usage in combination with the backpack form of the unit allow equipping any municipal utility services with them.
Experience of laser cleaning using the equipment produced by NPP VOLO proved the main advantages of laser cleaning which are manifested with the respect to widespread sandblast cleaning. These advantages are:
ability to work practically in any premises and not in special boxes;
absence of noise;
contactless;
non-abrasiveness;
easy collection of cleaning products;
absence of additional operations on dust collection from the surface after cleaning;
obtaining of practically and chemically clean surface;
low power inputs;
absence of surface abrasive wear;
ability to clean thin materials;
high ecological compatibility;
capability of equipment integration into the production lines of enterprises.
LASER SHOT PEENING
Another technology which allows the considerable enhancement of performance characteristics of the surface as well as basic material of parts at the depth up to 1 mm was developed by NPP VOLO. This technology is called impact laser hardening or laser shot peening technology. The method of laser shot peening is known as one of the methods used for the increase of fatigue resistance, long-term strength, resistance against corrosion cracking of machine parts and constructions operated under the conditions of high temperatures and cyclic loads. If parts, components of machines and mechanisms are exposed to the cyclic alternating loads as well as considerable thermal loads, there is high probability of the formation of microcracks, growth of which can cause the part destruction. Surface hardening method allows decreasing the probability of microcracks formation.
Methods and Technologies of Shot Peening
Shot peening of metals and alloys refers to the change of structure and properties of metals and alloys by the action of plastic deformation at the temperatures which are lower than recrystallization temperature in the condition of "cold" surface. During the shot peening, the partial accumulation of deformation energy occurs in the metal; this energy is spent on the lattice distortion with the formation of predominantly-oriented textures and change of dislocations position. Shot peening is accompanied by the increase of strength, hardness and decrease of material ductility. After the treatment favorable system of compressing residual stresses is formed in the surface layer of part; this system is expressed by the increase of ultimate fatigue strength and sometimes wear resistance. Usually, shot peening is accomplished using the special equipment by means of rolling, bead-blasting treatment or percussive tools. In recent years the technology of laser surface impact hardening (laser shot peening) has shown great advantages in comparison with the traditional methods of metal and alloy working.
Principle of Laser Impact Hardening
Laser hardening of the surface is ensured by the shock wave with high amplitude which is generated in laser-produced plasma with the help of high-energy laser pulses. Hardening effect is reached at the expense of the mechanical deformation in cold condition produced by the shock wave and not by the thermal heating of the surface with laser radiation as in the case with laser quenching. In general case the surface of treated part (Fig. 8) is coated with the absorber layer (layer of paint or film which absorbs laser radiation well) which is non-transparent for laser radiation. Then, the layer of material which is transparent for laser radiation is added; it can be liquid, water (liquid can flow along the part surface, flow must be laminar without beads and visible whirls). Focused laser radiation is directed on the surface, and it passes through the transparent layer and interacts with the absorber layer. When interacting laser energy is adsorbed inside the absorber layer, its evaporation occurs and plasma torch is formed. Isolated plasma pressure grows quickly causing the occurrence of shock wave which goes to the part material through the residue of non-transparent absorber layer and outside through the layer of transparent material.
Absorber layer serves as the protection against direct contact of the part surface with plasma. As a rule, direct interaction of the part surface with plasma causes the formation of metal melt on the surface at the depth of 15-25 µm. Non-transparent absorber layer can be formed of different materials: dry or wet paint, black film, metal foil, all these materials are able to realize nominally-equal pressures.
Transparent layer isolates plasma from the atmosphere and serves as the reflector of acoustic wave in order to form the shock wave with higher power. Laminar water flow is the simplest transparent layer. It should be repeated that water is used not for cooling but for the isolation of plasma from easily-compressed atmosphere. In this case, pressure which is generated by plasma on the part surface increases by 10 times.
Shock wave spreading in the material causes the improvement of material properties. Plastic deformation caused by the shock wave during laser shot peening spreads much deeper than upon bead-blasting hardening. Therefore, compressing residual stresses are located at the larger depth of material. Residual stresses which are located deeper determine the improvement of the material properties. In case when there is great power of shock wave, distinctive imprints can be observed on the part surface (Fig. 9) after the treatment at the depth of several microns
Laser shot peening technology is mostly needed in engineering industries: aviation (engine-building, hardening of blades, hardening of different parts of airframe), automotive industry (hardening of engine components), electric-power industry (hardening of blades for combined cycle gas turbines), nuclear power engineering (treatment of weld joints and reactor walls including the restoration of working equipment; these measures prevent from the formation of cracks), oil, gas and fuel-producing industry (treatment of weld joints including pipes intended for installation in the area of arctic shelf). In medicine where prosthetic objects are incommensurably small in comparison with the engineering objects shot peening technology (treatment of prosthetic articulations) opens great prospects for the preservation and restoration of human health.
Advantages of Laser Shot Peening Technology
Application of laser hardening helps to avoid expenses connected with the replacement of mechanism parts during repair. The main advantages of laser shot peening are:
enhancement of the surface microhardness;
absence of parts deformation after the treatment and elimination of final operations;
increase of endurance limit and wear resistance of treated parts;
increase of component life cycle;
room temperature of treatment ensuring the absence of structural transformations in metals and alloys;
retention of the properties after heating during the process of operation;
ecological purity;
improvement of production standards;
absence of working gases;
absence of isothermal exposure and therefore decrease of power inputs;
improvement of sanitary and hygienic production conditions;
complete technology compliance with up-to-date requirements connected with the environment protection.
Conclusions
Technology of laser impact surface hardening (laser shot peening) occurred in the last century but it has started developing actively when the solid-state lasers were designed. Customer just needs to make right choice of the developer of end system. NPP VOLO company develops laser equipment for different industrial tasks of cleaning of the treated materials. VOLO designs laser equipment which implements the methods of improvement of surface layer in its work relying upon the world experience which was accumulated in this area as well as its own projects. Industrial enterprises have opportunity to upgrade their production integrating the laser equipment produced by VOLO into their production lines. ▪
LASER CLEANING
When lasers have just appeared, instantly technologies paid attention to the capabilities connected with the use of "pure beam energy" as the cleansing agent. Pulse highly-concentrated light beam is able to heat thin surface layer of the material instantly in such manner that it can evaporate without noticeable action on the material which is located deeper. It should be noted that in many cases coating material does not even experience the thermal decomposition and contaminations accompanied with the formation and release of toxic or undesirable agents do not have time to occur. As a result, we have cleaning method which is unique by its properties and does not have all shortcomings inherent to the classic mechanical and chemical cleaning methods. It should be noted that manifestation of additional effects connected with the generation of thermo-elastic stresses and shock acoustic waves in near-surface layer increases the efficiency of cleaning process adding to the efforts on "lifting" contaminants from the surface.
For the most part, economical problems (instrument cost) and low process performance impeded the development of laser cleaning technologies. It is notable that laser cleaning methods have been successfully used for a long time in the restoration for the cleaning of museum valuables, in the area where the cost and performance do not play such defining role.
Occurrence of the reliable, compact and inexpensive laser sources – ytterbium fiber lasers with high coefficient of efficiency and long service life (up to 100 000 hours) – made it possible to jump to the new quality level for the laser cleaning technologies. Application of new laser radiation sources in the laser cleaning technologies provides meeting high up-to-date requirements to the production ecological compatibility. The companies which functions are connected solely with the development of equipment and laser cleaning methods operate in a number of advanced industrial countries.
The scheme of laser cleaning process (Fig. 1) is very simple – radiation of pulse laser is focused on the part surface. And the spot must have such size that the radiation power density would cause quick increase of the surface layer temperature during the pulse up to the temperature of its quick destruction (evaporation or sublimation). Approximate value of such power density is 107–1010 W/cm2. When focusing the radiation on the spot with the diameter of about 0.2 mm in order to reach such power density, use of pulse fiber laser with the mean power of just 10 W will be sufficient.
Currently, the systems of laser cleaning based on pulse fiber lasers by IPG Photonics have been developed. Backpack devices are the most compact and affordable class of laser cleaning equipment for the end users. German firm called Clean-Lasersysteme GmbH is the primary foreign producer of such compact laser cleaning systems. In Russia similar systems have been developed by NPP VOLO.
Use of Laser Cleaning Technology
The scope of tasks in the solution of which laser cleaning technologies are used includes the operations on pollutant removal and operations on the removal of coatings applied on the product during the production procedure. Laser cleaning is mostly used for:
removal of paint coatings;
removal of electrodeposited coatings;
removal of consumption pollutants;
removal of biological pollutants;
prior treatment of the surface before gluing or application of any functional coatings for the improvement of its adhesive properties;
removal of coolant-cutting fluids;
removal of corrosion residues;
removal of preservative coatings and combined pollutants.
Variety of laser cleaning applications produces the variety of laser devices intended for the solution of these tasks. In order to enter the market and get major distribution, similar laser devices must have several advantages. First of all, laser equipment intended for cleaning must have low cost. Secondly, it should provide low cost during its operation. Thirdly, it should demonstrate high reliability in operation. Fourthly, it should be compact, have simple configuration and must be easy to use.
NPP VOLO company (Fig. 2) managed to meet these conditions with its products – backpack equipment for laser cleaning. First of all, price of the backpack equipment intended for laser cleaning is approximately equal to the cost of annual usage of blasting equipment. Secondly, the laser cleaning equipment practically does not necessitate any operating expenses. Thirdly, average equipment life which defines the laser life cycle is more than 50 000 hours. Our units do not have wearing mechanical parts. Fourthly, total weight of such unit including the manipulator (hand tool) weight is not more than 15 kg.
Laser Cleaning Process
Figure 3 illustrates the results of laser cleaning of the surfaces of different materials using the laser backpack unit manufactured by NPP VOLO. In order to demonstrate the result visually the steel plate polluted with corrosion, scale chips and oil was selected. Lower area of the steel plate was cleaned by laser and upper area was specifically left uncleaned and polluted. Then, steel wire facing was applied on cleaned and untreated surfaces. The difference between the result of extremely unstable process of facing in the upper area and the result which meets the requirements in the lower area subjected to the prior laser cleaning is clearly seen. This experiment showed well the applicability of laser cleaning technology for the treatment of weld edge right before welding, for instance.
Turbine blade of gas-turbine engine (GTE) is shown in Fig. 4 before (on the left) and after laser cleaning (on the right). As the result of cleaning, surface consumption pollutants were removed. Fig. 5 demonstrates thread area of pump-compressor pipe (PCP) before (image on the left) and after laser cleaning (image on the right). PCP thread was polluted with the residues of coolant-cutting fluids and metal burrs and other workshop pollutants stuck to it after the thread cutting. Simple configuration of laser cleaning devices allows their easy integration into the production lines of enterprises.
Results of the laser cleaning of stone samples having biological and anthropogenic pollutants are given in Fig. 6 and 7. These are the samples of actual objects which were exposed to the open urban spaces of the Northern Capital for a long time. Their restoration cleaning did not require transfer of these monuments to the special workshops. And low power inputs and high ecological compatibility of laser equipment usage in combination with the backpack form of the unit allow equipping any municipal utility services with them.
Experience of laser cleaning using the equipment produced by NPP VOLO proved the main advantages of laser cleaning which are manifested with the respect to widespread sandblast cleaning. These advantages are:
ability to work practically in any premises and not in special boxes;
absence of noise;
contactless;
non-abrasiveness;
easy collection of cleaning products;
absence of additional operations on dust collection from the surface after cleaning;
obtaining of practically and chemically clean surface;
low power inputs;
absence of surface abrasive wear;
ability to clean thin materials;
high ecological compatibility;
capability of equipment integration into the production lines of enterprises.
LASER SHOT PEENING
Another technology which allows the considerable enhancement of performance characteristics of the surface as well as basic material of parts at the depth up to 1 mm was developed by NPP VOLO. This technology is called impact laser hardening or laser shot peening technology. The method of laser shot peening is known as one of the methods used for the increase of fatigue resistance, long-term strength, resistance against corrosion cracking of machine parts and constructions operated under the conditions of high temperatures and cyclic loads. If parts, components of machines and mechanisms are exposed to the cyclic alternating loads as well as considerable thermal loads, there is high probability of the formation of microcracks, growth of which can cause the part destruction. Surface hardening method allows decreasing the probability of microcracks formation.
Methods and Technologies of Shot Peening
Shot peening of metals and alloys refers to the change of structure and properties of metals and alloys by the action of plastic deformation at the temperatures which are lower than recrystallization temperature in the condition of "cold" surface. During the shot peening, the partial accumulation of deformation energy occurs in the metal; this energy is spent on the lattice distortion with the formation of predominantly-oriented textures and change of dislocations position. Shot peening is accompanied by the increase of strength, hardness and decrease of material ductility. After the treatment favorable system of compressing residual stresses is formed in the surface layer of part; this system is expressed by the increase of ultimate fatigue strength and sometimes wear resistance. Usually, shot peening is accomplished using the special equipment by means of rolling, bead-blasting treatment or percussive tools. In recent years the technology of laser surface impact hardening (laser shot peening) has shown great advantages in comparison with the traditional methods of metal and alloy working.
Principle of Laser Impact Hardening
Laser hardening of the surface is ensured by the shock wave with high amplitude which is generated in laser-produced plasma with the help of high-energy laser pulses. Hardening effect is reached at the expense of the mechanical deformation in cold condition produced by the shock wave and not by the thermal heating of the surface with laser radiation as in the case with laser quenching. In general case the surface of treated part (Fig. 8) is coated with the absorber layer (layer of paint or film which absorbs laser radiation well) which is non-transparent for laser radiation. Then, the layer of material which is transparent for laser radiation is added; it can be liquid, water (liquid can flow along the part surface, flow must be laminar without beads and visible whirls). Focused laser radiation is directed on the surface, and it passes through the transparent layer and interacts with the absorber layer. When interacting laser energy is adsorbed inside the absorber layer, its evaporation occurs and plasma torch is formed. Isolated plasma pressure grows quickly causing the occurrence of shock wave which goes to the part material through the residue of non-transparent absorber layer and outside through the layer of transparent material.
Absorber layer serves as the protection against direct contact of the part surface with plasma. As a rule, direct interaction of the part surface with plasma causes the formation of metal melt on the surface at the depth of 15-25 µm. Non-transparent absorber layer can be formed of different materials: dry or wet paint, black film, metal foil, all these materials are able to realize nominally-equal pressures.
Transparent layer isolates plasma from the atmosphere and serves as the reflector of acoustic wave in order to form the shock wave with higher power. Laminar water flow is the simplest transparent layer. It should be repeated that water is used not for cooling but for the isolation of plasma from easily-compressed atmosphere. In this case, pressure which is generated by plasma on the part surface increases by 10 times.
Shock wave spreading in the material causes the improvement of material properties. Plastic deformation caused by the shock wave during laser shot peening spreads much deeper than upon bead-blasting hardening. Therefore, compressing residual stresses are located at the larger depth of material. Residual stresses which are located deeper determine the improvement of the material properties. In case when there is great power of shock wave, distinctive imprints can be observed on the part surface (Fig. 9) after the treatment at the depth of several microns
Laser shot peening technology is mostly needed in engineering industries: aviation (engine-building, hardening of blades, hardening of different parts of airframe), automotive industry (hardening of engine components), electric-power industry (hardening of blades for combined cycle gas turbines), nuclear power engineering (treatment of weld joints and reactor walls including the restoration of working equipment; these measures prevent from the formation of cracks), oil, gas and fuel-producing industry (treatment of weld joints including pipes intended for installation in the area of arctic shelf). In medicine where prosthetic objects are incommensurably small in comparison with the engineering objects shot peening technology (treatment of prosthetic articulations) opens great prospects for the preservation and restoration of human health.
Advantages of Laser Shot Peening Technology
Application of laser hardening helps to avoid expenses connected with the replacement of mechanism parts during repair. The main advantages of laser shot peening are:
enhancement of the surface microhardness;
absence of parts deformation after the treatment and elimination of final operations;
increase of endurance limit and wear resistance of treated parts;
increase of component life cycle;
room temperature of treatment ensuring the absence of structural transformations in metals and alloys;
retention of the properties after heating during the process of operation;
ecological purity;
improvement of production standards;
absence of working gases;
absence of isothermal exposure and therefore decrease of power inputs;
improvement of sanitary and hygienic production conditions;
complete technology compliance with up-to-date requirements connected with the environment protection.
Conclusions
Technology of laser impact surface hardening (laser shot peening) occurred in the last century but it has started developing actively when the solid-state lasers were designed. Customer just needs to make right choice of the developer of end system. NPP VOLO company develops laser equipment for different industrial tasks of cleaning of the treated materials. VOLO designs laser equipment which implements the methods of improvement of surface layer in its work relying upon the world experience which was accumulated in this area as well as its own projects. Industrial enterprises have opportunity to upgrade their production integrating the laser equipment produced by VOLO into their production lines. ▪
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