Laser surface treatment of grey cast iron for automotive applications

Transcription

1 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 221 Laser surface treatment of grey cast iron for automotive applications V. Ocelík, P. N. Tang, M. C. de Boer, U. O. B. de Oliveira & J. Th. M. De Hosson Department of Applied Physics, Materials Science Center and The Netherlands Institute for Metals Research, University of Groningen, The Netherlands Abstract The surface of pearlitic grey cast iron was treated using a 2 kw Nd:YAG laser beam with the final aim to improve its surface properties, mainly for automotive applications. Two kinds of laser surface treatments were experimentally applied. In the laser surface hardening approach the surface of cast iron was hardened simply by passing a laser beam at travel speeds 1-50 mm/s keeping the surface temperature constant in the irradiated area. Detailed structural analysis of the layer modified by the laser treatment was performed by optical and scanning electron microscopy, EDS and orientation imaging microscopy with the electron back scattered diffraction and microhardness profiles were measured. Conclusions concerning the mechanism and kinetics of pearlite austenite transformation during the laser surface hardening were drawn. The size of the transformed zone was calculated using a three dimensional model for a Gaussian beam distribution in accordance with direct observations of laser beam energy densities. In the laser surface cladding experiments a thick Co-based alloy coating was created on the cast iron surface using so-called side-cladding set-up when the powder of additional material is locally molten together with a small amount of substrate and then rapidly solidified creating a coating with the metallurgical bond to the substrate. A gradual change of a single processing parameter was used for an appropriate experimental analysis and statistical correlation study between main processing parameters and geometrical characteristics of an individual laser track. Keywords: grey cast iron, laser surface hardening, laser cladding. WIT Transactions on Engineering Sciences, Vol 49, 2005 WIT Press

2 222 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 1 Introduction The grey cast iron contains a high percentage of flake graphite therefore it has special properties such as excellent machinability, the ability to resist galling and excellent vibration damping. As demands for high performance and durability in automotive applications are continuously advancing further attempts are required to improve the strength, hardness and wear resistance of engine parts by hardening or coating of cast iron surface. Notably, the surface modification of cast irons by high-energy laser beam has been studied extensively in [1-2]. In the present work we will show the possibilities to improve a cast iron surface through the surface temperature controlled laser surface hardening and laser cladding with Co-based coating. 2 Experimental The composition of the base cast iron material, usually used to cast diesel engine heads, is shown in Table 1. EuTroLoy16006 Co based alloy powder with 28 wt% Cr, 4.5 wt% W and 1.1wt% C was used in laser cladding experiments. Table 1: Chemical composition of the cast iron material. Element Fe C Si Mn P S Cr Cu or Ni wt% bulk <0.01 < kW Nd:YAG laser Rofin-Sinar CW20 working in a continuous mode was used. Argon gas was applied as shielding gas in both types of experiments: laser surface hardening and laser cladding. The optical pyrometer Maurer KTR 1075 monitoring the radiation in a wavelength region µm was used for the laser hardening process in a temperature range of 400 ºC to 1500 ºC. The pyrometer lens system with focal length of 500 mm collects the radiation from a spot size of 1.8 mm in diameter when aligned perpendicular to the surface. To study the effects of the principal laser processing parameters on the laser tracks, we used a concept of graded experiment [3]. This approach is based on a continuous increase of one process parameter while others are kept fixed. The computer control allows operating the laser power P during the laser hardening and laser cladding experiments via a linear change of laser lamps pumping power inside an interval of W. The change of the laser power inside a processing zone (an area comparable with laser beam size) has to be reasonably small. In all our experiments the entire laser track length has been chosen in such a way that the change of laser power inside the processing zone was always smaller than 10 W. In the case of laser cladding experiments, Sulzer Metco Twin 10C powder feeding system was used to deliver Co-base powder to the cladding nozzle using argon as a carrier gas. Optical microscopy, Scanning Electron Microscopy (SEM Philips XL30 FEG), EDS (EDAX) and XRD techniques were used to study a microstructure of treated and non-treated materials WIT Press WIT Transactions on Engineering Sciences, Vol 49,

3 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII Results and discussion 3.1 Laser surface hardening: microstructural observations and hardening mechanisms In the laser surface hardening experiments on cast iron a temperature range between ºC (Fe α Fe γ transformation) and 1150 ºC (cast iron melting point) is of our interest. The laser power was gradually increased whereas the laser beam moved over the substrate with constant velocity. The optical pyrometer monitored the surface temperature at the center of laser beam spot. The 200 mm focus length lens was applied to diverge the laser beam at two different laser defocus distances resulting in two different laser beam diameters. According to laser beam intensity measurements by the Primes GmbH BeamMonitor system, the defocusing of a top-hat distributed Nd:YAG beam results in a Gaussian beam energy distribution with laser spot size 2r b = 5.4 mm for defocus d = +30 mm and in a spot size 2r b = 2.7 mm for defocus d = +15 mm. The laser beam scanning speeds of 0.5, 2, 10, 15, 25 and 50 mm/s were monitored with these two laser beam diameters. The required temperature range for hardening was reached only at scanning speeds of v = 0.5, 2, 10 and 15 mm/s for a spot size of 5.4 mm. The highest scan speeds were too high to reach the desired temperature range even at the highest laser power 1750 W. The scanning speeds v = 5, 15, 30 and 50 mm/s resulted in the laser tracks with desired temperature range for 2.7 mm laser beam spot size, keeping the laser power still inside a controllable interval W. Table 2 shows the observed relation between processing parameters and the laser power required to reach the corresponding temperatures. Table 2: Required laser power to reach the desired surface temperatures for different laser beam spots and scanning speeds during the graded laser surface hardening of cast iron. Beam spot Beam velocity [mm] [mm/s] 1000 ºC 1075 ºC 1150 ºC W 425 W 450 W W 1093 W 1215 W W 1338 W 1535 W W 296 W 338 W W 633 W 1011 W W 1160 W 1293 W W 1483 W 1500 W In total 24 perpendicular cross-sections were made on eight different laser tracks to investigate the size of the transformed zone and microstructures. Figure 1 shows the characteristic microstructure of GG25 cast iron material observed with optical microscopy and the shape of the transformed area on the cross section perpendicular to the laser beam. It should be noted, that a circular shape of the transformed area observed on Fig. 1b is determined mainly by the 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 49,

4 224 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII Gaussian distribution of laser beam power density and by the circular shape of laser beam spot. Figure 1: a) Characteristic microstructure of GG25 cast iron with pearlitic matrix, graphite flakes and MnS inclusions; b) The shape of hardening area transverse to the laser beam direction. The former induces a higher laser power density for the points, which are closer to the center of laser track and the latter even sharpens this influence, because the points at a larger distance from the center will spend a shorter time under the laser beam. Nevertheless, a circular shape of a hardened zone will probably be not preferable in real applications. A solution of this problem lies in a change of the shape of the laser beam to a rectangular one, together with a top-hat distribution of power density in the transversal direction to the scanning direction. The linear relationships between the measured surface temperatures and the depth as well as the width of transformed area were observed for all scanning speeds and both laser spot sizes. Figure 2: SEM micrographs of the laser treated zone. a) Non-dissolved cementite lamellae near the boundary (line TB) between the transformed (upper left corner) and non-transformed pearlitic zone. b) Chemical heterogeneity (pearlite ghosts) in the fully transformed zone near the surface. WIT Transactions on Engineering Sciences, Vol 49, 2005 WIT Press

5 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 225 From the microstructural point of view the demarcation line between the austenitized zone (AZ) and the non-transformed substrate material is quite sharp, as Fig. 2a clearly demonstrates. The basic structure of the AZ consists of martensite with retained austenite and undissolved graphite flakes. In the whole austenitized area fine strips are observed that originate from the cementite lamellae. These strips are of two kinds. Close to the outer boundary of the AZ, where the peak temperature was relatively low and the thermal cycle short, the cementite plates continue from the original pearlite grains into the austenitized matrix (Fig. 2a). The thickness of the plates remains apparently unchanged. In this case the strips represent the phase heterogeneity residual cementite lamellae remain undissolved during the heating period. Far from the outer boundary the cementite lamellae disappear but traces of the original pearlitic structure are still apparent (Fig. 2b). They are due to chemical heterogeneity enhanced carbon content whereupon the cementite plates have just dissolved (pearlite ghosts [2]). Such a microstructure is characteristic for so called direct austenitic transformation kinetics [2], realized at high heating rates when cementite plates did not take place at the initial stage of transformation: Fe α( + Fe 3C) Fe γ( + Fe 3C), (1) which starts at temperatures above A 3. After that, non-dissolved cementite lamellae and low-carbon austenite matrix coexist and with increasing temperature the carbide rapidly dissolves giving rise to a carbon concentration gradient in the austenite. On some cross-sections with higher surface temperature it is possible to observe a small, remelted region near the surface and center of the laser track with completely dissolved graphite flakes and the ledeburite microstructure. The hardness profiles measured on laser track cross-sections in horizontal and vertical directions shown values from 600 to 1000 HV2 with quite a large scatter caused by a local present of soft graphite phase. There are a couple of appropriate models for the calculation of temperature field T(t,y,z) under a Gaussian scanning laser beam during laser surface hardening, when only a conduction heat transfer is assumed in a semi-infinite substrate and thermal properties of the substrate are temperature independent. In our calculations we used the Ashby-Easterling approximation [4]: ( ) 2 2 AP / v 1 z+ z0 y T T0 = exp + (2) 1/2 2 πλ( tt ( + t0) 4a t t+ t 0 when laser beam with power P moves in the x direction with velocity v. T 0 is the initial temperature of the substrate, A is the substrate absorption, λ is the thermal 2 conductivity, a is the thermal diffusivity of the substrate and t0 = rb /4a. Latent heat effects are neglected as well as losses due to radiation or surface convection. The depth of the transformation zone observed for the different laser power P and different beam velocities provides a possibility to calibrate absorption coefficient A = 0.5±0.05, assuming that peak temperature near the 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 49,

6 226 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII bottom of transformed area reaches 912 ºC, which is A 3 temperature. Such a calibration gives more reliable values for the whole temperature field, than a calibration that uses the measured surface temperature by pyrometry. This due to the size of area from which pyrometer integrates the signal, which is almost comparable to the size of laser beam spot. Figure 3 shows an example of the temperature profiles calculated for two different laser beam velocities. From the time differentiation of Eq.2 it is possible to calculate heating and cooling rates at any point inside the transformed zone. Depth mm 0.4 Depth mm Temp K time s Temp K time s Figure 3: 3D representation of temperature time field in the center of laser beam for two beam velocities: v = 2.5 (left) and v = 10 mm/s. Nevertheless, the model described by Eq.2 is not applicable when a surface of real metallic parts should be hardened. In such a case, there is a possibility to control the laser surface hardening process using an appropriate change of instantaneous laser power P. We proved such an approach using the LomPocPro PID regulation system, with carefully tuned regulation parameters. Surface temperature in the laser spot was kept at T = 950, 975 and 1000 ºC for three different laser beam velocities v = 2.5, 5 and 10 mm/s with the precision ±8 ºC resulting in a regular shape of transformed zone also in the vicinity of irregular shapes or edges. 3.2 Laser cladding Laser cladding is another possibility to improve the surface properties of cast iron. The process can be described as an addition of one material by cladding on the surface of the substrate, where the heat source is a high power laser beam. The resulting thickness of the clad is typically from 50 µm to 2 mm in one single step. Once a thicker protection layer is needed, the process can be applied again. Cobalt base alloys have been already reported as very good materials for wear and heat resistant coatings on steel substrates [5]. Therefore a Co-base alloy was selected to study the side laser cladding processing conditions on the cast iron substrate. Here, we took the advantage of grade method again. The laser power was continuously increased during building of single laser track from 630 to 1750 W at all combinations of five different levels of powder feeding rates: WIT Transactions on Engineering Sciences, Vol 49, 2005 WIT Press

7 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 227 F = mg/s and five laser beam scanning speeds S = mm/s. In brief, 25 laser tracks were performed to explore the whole processing window. Figure 4: A typical laser track cross-section with its main geometric characteristics: clad height H, clad width W, clad area A c, molten area A m and clad angle α. Figure 4 shows a typical laser track cross-section together with the definition of its main geometric characteristics. From these characteristics, two important quantities may be evaluated: dilution D = A m /(A c +A m ), which quantifies the relative amount of the substrate material that has been molten during cladding process and the clad angle α, which is in practice required to be large enough so as to avoid porosity creation in the coating, while overlapping individual laser tracks. All 25 laser tracks were cross-sectioned in five cuts defined five different laser power inputs, which provide a total of 125 data points corresponding to different combinations of S, F and P values. It has been shown that a systematic investigation of the laser cladding process can be based on so-called laser processing combined parameters [6]. Two fundamental parameters are important: the amount of the powder per unit length F/S and the heat input per unit length P/S. Figure 5 shows the laser track crosssection map for various P and F/S and clearly demonstrates some relevant influence of the side laser cladding process parameters on the single clad track geometry. From a practical point of view it is useful to search for the best combined parameter that correlates with the characteristic behavior of an individual laser track. This approach was already successfully applied in the study of coaxial laser cladding process [7]. The detailed statistical analysis of the whole set of our data points revealed that in accordance with the coaxial laser cladding process three main characteristics are linearly dependent on the same combined parameters, i.e. always with a correlation coefficient larger then 0.9; namely laser clad height H is proportional to F/S, laser clad width W is proportional to P/S 1/2 and clad area A c linearly depending on P 1/2 F/S. On the other side combined parameters, which control the behavior of clad angle α and dilution D during the side cladding, differ somewhat from those found for coaxial cladding [7]. These were found to be F/(P*S) 1/2 for clad angle and log(p*(s/f) 1/2 ) for dilution, again with correlation coefficients larger then 0.9. Figure 6 represents our so-called processing parameters map that summarizes our experimental data and statistical relations between processing parameters and laser track characteristics. In this map a more simple relation α ~ S/F valid for 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 49,

8 228 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII coaxial cladding set-up was used. This relation works also in side cladding setup, but with a slightly smaller correlation coefficient (0.89). Two different pairs of dilution hyperbolae are depicted. One is satisfactory for single laser track cladding (D = 5-30 %) and the other for the formation of a coating by a simple overlap of individual laser tracks (D = %). Thus the optimal laser clad parameters area for the creation of a continuous coating is on the right side of vertical line for α = 100º and between two hyperbolae for appropriate dilutions. Figure 5: 4 Laser track cross-section map for side cladding of Co-based alloy on cast iron substrate. Combined F/S parameter on the horizontal axis characterizes the amount of fed material per unit length. Conclusions The possibilities of temperature controlled laser surface hardening and laser cladding with a 2 kw continuous Nd:YAG laser beam on pearlitic grey cast iron was investigated with the aim to improve its surface properties for automotive applications. The main conclusions are presented here: After hardening of grey cast iron by Nd-YAG laser beam with a Gaussian energy distribution and scanning speed from 2 to 50 mm/s there are several different microstructural regions found in the transformed zone, which consist mainly of martensite, retained austenite, remaining cementite lamellae and non-dissolved graphite flakes. The hardened depth varies from 300 to 700 µm; WIT Transactions on Engineering Sciences, Vol 49, 2005 WIT Press

9 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 229 Figure 6: Processing window for side cladding in P vs. S/F representation. The points correspond to the analyzed laser cross-sections. Vertical solid line determines the clad angle condition required for continuous coating by single tracks overlap. Two pairs of solid hyperbolae terminate the areas of allowed dilutions for single-track cladding and for creating a coating. Dashed curves are isolines with denoted values for clad areas. Observation of the remaining cementite lamellae and ghost pearlitic structure indicates that the pearlite to austenite transformation at high heating rates takes place according to a direct mechanism: Feα( + Fe3C) Feγ( + Fe3C) ; Microhardness values have increased by laser surface hardening from 250 HV of the base material to HV inside the transformed zone. The microhardness depth profile corresponds to the change in microstructure of transformed area; PID regulation of the input laser power is sufficient to control the surface temperature during laser surface hardening, but the optimal processing parameters should be tuned for each application separately. The maximum surface temperature should not exceed ~ ºC because otherwise local surface melting occurs; A rectangular laser beam shape with a top hat energy density distribution in the direction perpendicular to the beam propagation is the best choice for laser surface hardening; Calculations based on Ashby-Easterling approximation for a Gaussian laser beam distribution showed that the cooling rates in the transformed zone was 2005 WIT Press WIT Transactions on Engineering Sciences, Vol 49,

10 230 Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII approximately 500 to 6000 K/s at depths of mm and laser beam velocities from 2 to 50 mm/s; A wide processing window is available for side laser cladding of Stellite 6 type Co-based alloy on cast iron substrate. Relatively strong statistical correlations were found between the main laser processing parameters and main characteristics of single laser track, which allows the construction of a so-called processing map. Acknowledgement The Netherlands Institute for Metals Research is acknowledged for financial support. References [1] Gadag S.P., Srinivasan M.N. & Mordike B.L., Effect of laser processing parameters on the structure of ductile iron. Materials Science and Engineering A, A196, pp , [2] Zárubová N., Kraus V. & Čermák J., Mechanisms of phase transformations during laser treatment of grey cast iron. Journal of Materials Science, 27, pp , [3] Felde I., Kálazi Z., Vero B., Réti T., Králik Gy. & Szabados O., An experimental design technique for the approximation of process parameters in laser surface hardening, in: T.S. Sudarshan, M. Jeandin (Eds.), Surface Modification Technologies XIV, ASM International, Metals Par, Ohio and IOM Communications UK, 2001, pp [4] Ashby M.F. & Easterling K.E., The Transformation hardening of steel surfaces by laser beams I. Hypo-autectoid steels. Acta metall., 32, pp , [5] De Mol Van Otterloo & De Hosson J.Th.M., Microstructural features and mechanical properties of a cobalt-based laser coating, Acta mater., 45, pp , [6] Felde I., Reti T., Zoltan K., Costa L., Colaço R., Vilar R. & Verö B., A Simple Technique to Estimate the Processing Window for Laser Clad Coatings. Proceedings from the 1st International Surface Engineering Congress and the 13th IFHTSE Congress, 7-10 October 2002, Columbus, Ohio, ASM International 2003, p [7] De Oliveira U., Ocelík V. & De Hosson J. Th. M., Analysis of coaxial laser cladding processing conditions. Surface & Coatings Technology, in press. WIT Transactions on Engineering Sciences, Vol 49, 2005 WIT Press