STATE OF THE ART AND APPLICATIONS OF LASER SURFACE TREATMENT

Transcription

1 STATE OF THE ART AND APPLICATIONS OF LASER SURFACE TREATMENT Kurt Schröder Institute of Nonconventional Processing, Forming and Laser Technology, Vienna University of Technology, A-1030 Vienna Abstract Laser surface treatment of metals shows many advantages compared to conventional techniques, e.g. a high flexibility with respect to the processed geometries or the possibility for a simple integration into existing production lines. Especially when only a small part of the surface of a workpiece shall be treated the laser process should be preferred. Because of the very high quenching rates, unique material properties can be achieved. There are many different techniques for obtaining modified surface properties by laser processing. e.g. remelting, transformation hardening, cladding, alloying and dispersing.. As laser sources CO 2 lasers or Nd:YAG laser with beam powers of several kw are used. Recently also the high power diode laser has become an interesting tool for surface treatment. In the following sections laser surface treatment processes will first be discussed in general. Afterwards some examples for processes investigated at the department within the last years will be given. Examples are remelting of the surfaces of barrel extruders, dispersing of TiC into Al substrates or annealing of X40 Cr 13 for wear reduction under low lubrication conditions and control of pulsating stress. The production of three dimensional structures with a blown powder cladding process will be demonstrated as well. 1. INTRODUCTION Surface treatment for increasing the thermal, wear and corrosion resistance of a workpiece gets more and more important. Improved surface properties are required e.g. for high performance tools in forming or cutting processes when both the endurance of the tool and the productivity are to be maximised. The same is true for engine parts in the automotive industry where both surfaces with high thermal or mechanical resistance and minimum weight of a component are required. Compared to several other surface treatment techniques like e.g. thermal spraying, overlay welding or physical vapour deposition (PVD), the laser provides some considerable advantages. Major advantages are the good control over the processed geometries, high quenching rates and small heat affected zone. In order to make the laser attractive for surface treatment processes, however, it is important to provide cheap laser power at the desired focal spot dimensions. Usually high power lasers are developed for cutting or welding applications which require by far smaller focal spot dimensions than surface treatment. The techniques, however, required for providing the good focusability of the laser beam result in high costs. When using such lasers for surface treatment additional optical systems for increasing the focal spot diameter must be used which even increase the costs of the total system. From this point of view it would be reasonable to develop lasers with a beam quality suitable for surface treatment. Fortunately with the development of high power diode lasers the possibility has evolved to provide cheap laser power at a very flexible focal spot geometry. It can be assumed that the developments within the field of high power diode lasers will foster the acceptance of industrial laser surface treatment in the near future

2 2. OVERVIEW OVER PROCESSES The aim of surface treatment processes is a modification of the material properties at the surface of a workpiece at maintaining the properties in the bulk of the workpiece. It can be distinguished between processes which achieve this requirement by adding material to surface layers or by just applying appropriate temperature gradients. When material is added to the surface layer, the amount of added material and the material properties of the involved substances usually give a complex system. Optimisation of such processes requires a good understanding of materials sciences. External control of the process is provided by varying the power density of the laser beam and the feed rate which both are responsible for the power input per volume and therefore for the melted volume per unit time. The additional materials usually are supported in the form of powders, gases, wires, ribbons or pastes. The most important processes with addition of material are: Alloying in alloying the added material forms an alloy with the material of the molten surface. The mixing of the different components is supported by a strong flow of the melt driven by surface tension gradients (Marangoni flow) and convection /1/. Typical elements for alloying are Ni, Cr, W, B, C or WC for steel and B, Ni, Si, Fe, Mn, Cr, Ti, Co, V, Mo, Ta, Cu, Zr, SiC, B 4 C or Si 3 N 4 for aluminium /2/. The surface properties are determined by the properties of the alloy. Cladding In cladding the mixing between added material and base material takes place only in a small transition region. In this case it is desired to maintain the properties of the added material at the surface of the workpiece. Dispersing Dispersing is used to embed hard particles with a high melting point in the surface layer of a workpiece. In dispersing only the base material is melted by the laser irradiation. Dispersing typically is used to introduce hard particles like TiC, WC, Cr 2 O 3, VC, B 4 C, SiC, BN, TiSi 2, TiO 2, TiB 2 in steel, aluminium or titanium base materials. In processes without added materials the changes of the surface properties are caused by temperature gradients, phase changes or mechanical influences. The most important processes are: Transformation hardening Transformation hardening is used for steel and cast iron. Heating by the laser above the austeniting temperature causes an α-γ transition of the material in the heated surface layer. Because of the high power density of the laser beam high temperature gradients are caused in the material which induce fast quenching when the laser beam moves ahead. This fast quenching causes the forming of hard surface layers consisting of martensite. Remelting In remelting the laser parameters are selected in a way that melting of the upper surface layer is occurring. There are several effects of the melting which can be utilised. For example inclusions can be vaporised or dissolved, the grain size can be adjusted and the hardness can be increased. Because of the flow of the produced melt the surface quality gets worse and usually additional processing is required. Annealing The annealing process is very similar to transformation hardening. In this process a material with a high martensite fraction is heated in order to dissolve a part of the martensite and thus to reduce the hardness. Shock hardening Shock hardening occurs when laser pulses with a duration in the ns range are applied. In this case shock waves are induced which cause a kind of mechanical deformation connected to an increase of the hardness. The involved mechanisms are therefore similar to those occurring during cold working. Figure 1 shows laser surface processes in relation to other laser processes used in production engineering. It can be seen that laser surface treatment requires a relatively high energy density (e.g. beam power) whereas the power density is much smaller that that used for the - 2 -

3 most important laser processes cutting and welding. The interaction time is determined by the required heating and quenching rates. It is higher than that used for welding or cutting. Figure 1: Overview over laser processes 3. COMPARISON OF LASER SOURCES FOR SURFACE TREATMENT When selecting a laser source for surface treatment several properties of the laser must be considered. In most cases lasers with rather high output powers (> 5 kw) at a low power density (< 10 5 W/cm 2 ) are required. In addition a high absorptivity of the laser beam at the processed surface is desirable. In Table 1 important laser sources for industrial applications are summarised. As far as the maximum output power is concerned, traditionally the CO 2 laser is the proper choice. CO 2 lasers are available with powers up to several 10 kw, although powers below 10 kw are sufficient for most commercial applications. They are still the cheapest laser sources when the investment costs per cw output power are considered. Drawbacks of the CO 2 laser are the poor absorptivity of most metals at the wavelength of the CO 2 laser (10.6 µm) and the lack of materials for beam guiding. For compensating the reflection losses therefore the rated output power of the CO 2 lasers must chosen accordingly. As no appropriate optical fibres are available mirrors must be used to guide the beam through free space. Because of these restrictions of the CO 2 laser the Nd:YAG laser is used as well for surface treatment applications. Especially for treating small parts the higher costs and the lower available output powers of Nd:YAG lasers compared to CO 2 lasers get less restrictive. In addition the absorptivity of most metals at the wavelength of the Nd:YAG laser (1.06 µm) is better than for the wavelength of the CO 2 laser so that less powerful Nd:YAG lasers often are sufficient. Also the simple beam handling with flexible fibres is a major advantage of the Nd:YAG laser. Recently also the pumping of Nd:YAG lasers with high power diode lasers has become interesting. This pumping technique provides a good beam quality and high efficiency at strongly simplified maintenance (no lamp changes required). A promising and relatively new laser type for surface treatment is the high power diode laser. This laser type shows a high efficiency and is very compact (see Figure 2 for a comparison between the sizes of CO 2 and diode lasers). The emission wavelength depends on the - 3 -

4 composition of the used semiconductor material and therefore can be influenced in the manufacturing process. It is therefore possible to build lasers emitting a wavelength which is well absorbed by metals, e.g. 808 nm (lasers at this wavelength are extensively used for pumping of solid state lasers and therefore are available at reasonable costs). In principle radiation at this wavelength could be guided by optical fibres albeit the incoupling of high beam powers is not simple because of the poor beam quality of this type of laser. Figure 2: Comparison between a CO 2 laser (left) and a high power diode laser (right) The poor beam quality, and thus the large focal spot size of the high power diode laser is the major disadvantage of this laser type compared to competing systems which makes it useful only for selected applications. In surface treatment, however, large focal spot sizes usually are required and therefore the diode laser is a promising candidate for many applications. Beam forming of the diode laser usually requires a small distance between the focusing optics and the processing area which is a problem when melt droplets are emitted by the process. For this reason the diode laser is best suited for transformation hardening where only solid phases are involved. Table 1: Comparison of high power laser sources Type of laser Wavelength (nm) Beam quality (Minimum focal spot diameter) Efficiency Investment Costs (EURO/kW) Costs (deprecations included) (EURO/kWh) CO 2 laser good fair Nd:YAG laser 1060 fair poor (flash lamp pumped) Nd:YAG laser 1060 fair... good good < 20 (diode pumped) Diode laser poor good Table 1 summarises the properties of high power laser beam sources. It can be seen that the CO 2 laser is still the cheapest beam source as far as the investment costs are concerned which is the reason why the CO 2 laser often is used although it shows some disadvantages as - 4 -

5 discussed above. When the costs considering deprecations are looked at, however, the differences between the different laser sources are smaller. In addition it must be pointed out that the prices of diode lasers are still going down which has an influence on the costs of both direct diode laser applications and applications of diode pumped solid state lasers. 4. EXAMPLES FOR LASER SURFACE TREATMENT 4.1 Hardening by remelting Remelting of steel surfaces with the laser beam can be applied to selectively increase the wear resistance. Barrel extruders show a very inhomogeneous stress and temperature distribution across their surface and therefore are a typical candidate for laser surface treatment. Hardening of barrel extruders consisting of the hot working steel X38 CrMoV 5 1 (W300) by remelting was demonstrated by using a 3 kw CO 2 laser and a segmented mirror (see Figure 3). The segmented mirror consisted of several cylindrical- parabolic mirror segments each producing a line focus of a length of 6 mm. These mirror segments were arranged in a way that all the individual focal lines coincided at the workpiece surface resulting in focal line of homogeneous power density /3/. In these experiments the hardness could locally be increased from 200 HV 1 to 650 HV 1. The following wear tests performed with samples of this material demonstrated that the rate of wear was reduced by a factor of 5 (DKI lamina abrasion test) as a result of the laser treatment. Figure 3: Remelting of a barrel extruder: Setup for remelting using a segemented mirror with 6 mm focal spot length (left). Micrograph of the remelted paths (beam power: 3 kw, feed rate: 1 mm/s, Argon flow rate: 10 l/min) (right) 4.2 Dispersing of TiC in an aluminium substrate Dispersing of hard particles into surfaces of aluminium parts provides a possibility to increase the wear resistance. Thereby it is desirable to obtain a high density and a good homogeneity of hard particles in the surface layer. Experiments were performed in order to find the optimum feed rates and powder flows for a high hard particle density. Figure 4 shows micrographs of the surface layers at two different feed rates. In both cases a TiC powder with a mean particle size of 60 µm was used

6 Figure 4: Dispersing of TiC into an Al substrate (shielding gas: argon at 12 l/min). Feed rates 20 mm/s (left) and 15 mm/s (right) 4.3 Annealing Annealing was applied to the fixture of the drill in the handpiece for tooth treatment (see Figure 5). This fixture must both reliably fix the drill in a defined position and provide the bearing at high rpm. Since the fixture rotates at about rotations per minute under low lubrication conditions, highest requirements with respect to hardness and wear resistance must be fulfilled. The used material X40Cr13 is a hardenable steel and could provide a hardness of 56 HRC. At this hardness, however, the springs for holding the drill would show a susceptibility to brittle fracture. For this reason formerly a hardness of only 51 HRC could be used for this piece. Application of laser treatment allowed to reduce the hardness in the region of the springs of the fixture to 49 HRC showing an increased lifetime of the fixture when applying alternating stress to the springs. The treatment was performed with Nd:YAG laser in a pulsed mode. Figure 5: Fixture for a drill for tooth treatment 4.4 The blown powder process for rapid prototyping The cladding process as explained above can also be used for building up three-dimensional structures. In this case the laser spot is kept very small and the powder flow is well adjusted to the focal spot position yielding narrow paths of deposited powder at a high powder depositing degree. By a proper control of the movement of the laser head complex three-dimensional structures can be built /4/, /5/. In experiments a Co-base alloy (see Table 2) has been used for demonstrating the blown powder process. A special processing head (Figure 6) guaranteed a good control over the focal spot position and the powder flow, resulting in thin walls (thickness about 1 mm) and a high powder deposition efficiency (more than 85 %). As the powder contains components which may be dangerous when inhaled, the processing system was enclosed by a hermetically - 6 -

7 sealed box. Powder deposition rates up to 30 g/min are reasonable where the surface quality continuously degrades when the deposition rate is increased (see Figure 7). Table 2: Material properties of the powder Base material: Cobalt-base-alloy (Sulzer-Metco 45C-NS) Composition: Co 25,5Cr 10,5Ni 7,5W 0,5C Density: 8500 kg/m 3 Hardness: HV(30) 350 ~ HRC 37 Figure 6: Experimental setup for the blown powder process: Processing head with the focusing optics and forming of the powder flow (left). Hermetically sealed processing box (right) In the course of the blown powder process it was possible to provide a tensile strength close to that of the solid base material (see Table 3). This is a clear advantage compared to the competing rapid prototyping, the selective laser sintering /6/. Table 3: Material properties of the prototyped structures Tensile strength R m (N/mm 2 ) Yield stress R p0.2 (N/mm 2 ) Elongation at fracture (%) Parallel ,8 11,4 Perpendicular 864, ,4 Figure 7 shows a prototyped cylindrical workpiece and a micrograph of a corresponding structure. It can be seen that almost no porosity is appearing which is one of the reasons for the good strength properties of the material. A problem of the process is still the poor surface quality of the prototyped walls. Obviously there is a strong correspondence between the powder flow rate and the surface roughness. For most applications, however, it is still necessary to perform a mechanical post- processing for improving the surface quality

8 Figure 7: Structure built up from a Co-base-alloy powder by the blown powder process: Prototyped sample with cylindrical symmetry (left). Micrograph of the prototyped wall at different powder flow rates (right). 5. FUTURE DEVELOPMENTS It can be assumed that future developments in the field of laser surface treatment will be driven by the cost development of appropriate laser sources. Cheap laser sources would support the introduction of laser processes to fields where they are not yet competitive. It can be expected that the costs of high power diode lasers will strongly reduce within the next years. Interesting applications of laser surface treatment may appear in the field of tool repair where welding overlays are used to replace the material in damaged regions. The low heat input and good control over the geometry are arguments for the laser process. It has even been demonstrated to use the same laser for ablating damaged regions and building up new material. LITERATURE 1. G. Tsotridis, H. Rother, E.D. Hondros, On modelling of Marangoni convection flows in simulated plasma disruptions, Fusion Engineering and Design, Vol. 15, pp (1991) 2. E. Beyer, K. Wissenbach, Oberflächenbehandlung mit Laserstrahlung (Laser in Technik und Forschung), Springer Verlag, Berlin, L. Migliore (editor), Laser materials processing, Marcel Decker Inc., New York, M. Resch, A. Kaplan, D. Schuöcker, Laser-assisted generating of three-dimensional parts by the blown powder process, Proc. of the SPIE: Gas Flow and Chemical Lasers, Vol. 4184, pp (2001) 5. A.F.H. Kaplan, M. Resch, Mathematical model of laser-assisted rapid prototyping, accepted for publication by Applied Physics Letters, EOS GmbH, Product information, Munich,