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Laser surfacing of brass with Ni–Cr–Al–Mo–Fe using various laser processing parameters
Materials Science and Engineering A325 (2002) 365– 374 www.elsevier.com/locate/msea
Laser surfacing of brass with Ni–Cr–Al–Mo–Fe using various laser processing parameters K.F. Tam a, F.T. Cheng a,*, H.C. Man b a
Department of Applied Physics, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, People’s Republic of China b Department of Manufacturing Engineering, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, People’s Republic of China Received 22 February 2001; received in revised form 2 May 2001
Abstract Brass (Cu–38%Zn) specimens were laser surface modified with Ni– Cr– Al– Mo– Fe (Ni– 10%Cr– 7%Al– 5%Mo– 5%Fe–1%B) using a 2 kW CW Nd:YAG laser. Successful laser surfacing was achieved with a power density around 70 W mm − 2, a scanning speed in the range 15–35 mm s − 1, and a 50% overlap of adjacent tracks. The hardness of the alloyed layer increased as the dilution ratio decreased. The cavitation erosion resistance, defined as the reciprocal of the mean erosion rate, of the modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C was improved by a factor in the range 2.47– 4.60 compared with that of brass, and correlated well with the hardness. The improvement in the cavitation erosion resistance was attributed to the formation of hard boride phases in a ductile Ni-rich matrix. On the other hand, there was no significant improvement in the corrosion resistance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser surfacing; Cavitation erosion; Brass; Ni– Cr– Al– Mo– Fe; Corrosion
1. Introduction Cavitation refers to the nucleation and growth of cavities or bubbles in a fast-flowing or vibrating liquid when the local pressure in the liquid drops below a certain level. These bubbles will implode or collapse when they are transported to a high-pressure region. The collapse of bubbles is accompanied by the emission of shock waves and micro-jets, which will exert stress pulses on a solid surface in the vicinity. Repetitions of such pressure pulses on the solid surface will eventually result in fatigue failure and material loss, and such a mode of damage is known as cavitation erosion [1]. Thus, cavitation erosion is a common problem faced by designers for pumps and other hydraulic machinery. Cavitation erosion might be visualized as a random dynamic micro-indentation process. In this sense, the resistance to cavitation erosion is closely related to the mechanical properties of the material under consider* Corresponding author. Tel.: +852-27665691; Fax: 23337629. E-mail address: [email protected] (F.T. Cheng).
+ 852-
ation. Brass, an inexpensive alloy commonly used in marine applications, is rather corrosion resistant, but much less so with respect to cavitation erosion because of its low mechanical strength. In view of the fact that cavitation erosion is a surface phenomenon, surface modification is a viable solution to improve the cavitation erosion resistance since it may be employed to modify the surface properties while retaining those of the bulk, with the consumption of only a small amount of precious materials. Among the different techniques available, laser surface modification is unique in being able to form a surface layer of novel alloys with a fine microstructure and with strong bonding to the substrate. It is the aim of the present study to laser alloy brass (Cu –38%Zn) with Ni –Cr –Al –Mo –Fe. Ni –Cr –Al – Mo –Fe is a new type of self-bonding powder that possesses a number of advantages with respect to processing and properties. It can be easily preplaced on a brass substrate by thermal spraying, thus greatly facilitating laser surface alloying in the subsequent step. The melting point of Ni –Cr –Al –Mo –Fe is only slightly higher than that of brass, and this ensures simultaneous
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melting and mixing of the powder and a thin layer of the substrate. Ni forms a solid solution with Cu in any proportions, and this results in a good interfacial bonding. The other elements present are expected to enhance the mechanical strength and the corrosion resistance. Reports on studies of laser surface modification of copper alloys are scarce in the literature, especially those devoted to enhancing cavitation erosion resistance [2] or wear resistance [3]. This is not unexpected as laser surface modification of copper alloys presents a certain degree of difficulty owing to high reflectivity and thermal conductivity [4], and successful treatments require a careful selection of the processing parameters [5]. It is expected that with a proper choice of laser processing parameters, an alloyed layer that bonds well to the substrate, may be produced and the cavitation
erosion resistance of the brass substrate may be significantly improved.
2. Experimental details
2.1. Specimen preparation The nominal compositions of brass Cu–38%Zn and the alloying powder Ni–Cr –Al –Mo –Fe are presented in Table 1. The as-received brass specimens were cut with dimensions 30 mm× 15 mm ×8 mm, while the particle size of Ni–Cr –Al –Mo –Fe alloy powder ranged from − 140 to + 350 mesh (B 105 and \40 mm). The alloying powder was preplaced on the brass substrate with an oxyacetylene thermal spraying gun
Fig. 1. Metallographic cross-section of laser-modified track with power density 76 W mm − 2 and scanning speed 15 mm s − 1.
Table 1 Nominal compositions of brass and Ni–Cr–Al–Mo–Fe powder Material
Cu
Zn
Pb
Brass Ni–Cr–Al–Mo–Fe alloy powder
Balance
38
1.5
Ni
Cr
Al
Mo
Fe
B
Balance
10
7
5
5
1
Table 2 Summary of laser power density, scanning speed, precursor coating thickness (t), melt depth (d) and dilution ratio of the laser-modified layers Specimen
Laser power density (W mm−2)
Scanning speed (mm s−1)
Precursor coating thickness (t) (mm)
Melt depth (d) (mm)
Dilution ratio (DR)a (%)
Ni–Cr–Al–Mo–Fe-1 Ni–Cr–Al–Mo–Fe-2 Ni–Cr–Al–Mo–Fe-3 Ni–Cr–Al–Mo–Fe-4 Ni–Cr–Al–Mo–Fe-5
76 76 76 64 64
15 25 35 15 25
230 230 230 230 230
371.0 280.5 248.5 273.3 242.2
38 18 7 16 5
a
n
coating thickness (t) ×100%. melt depth (d)
Dilution ratio (DR) = 1−
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ferric chloride solution (25g FeCl3, 25 ml HCl and 100 ml H20) for 5 s. The average melt depth was determined using image analysis and the dilution ratio (DR) was calculated, and shown in Table 2. Optical microscopy (Nikon MICROPHOT-FXA) and scanning electron microscopy (SEM) (Model S250; Cambridge) were employed to analyze the microstructure of the lasermodified layer, using the etched specimens. The chemical compositions and the phases formed in the surface of the laser-modified specimens were investigated by energy-dispersive X-ray analysis (EDX) and X-ray diffractometry (Model PW3710; Philips), using Cu Ka generated at 40 kV and 35 mA as the radiation source, with a nickel monochromatic filter. The hardness profiles of the laser-modified specimens were measured along the depth of the cross-section of the melt zone, using a Vickers hardness tester (Buehler Micromet II) with a load of 200 g and a loading time of 15 s. The surface morphology of the eroded specimens was also examined by SEM. Fig. 2. Set-up of the cavitation erosion test in unattachment mode.
2.4. Ca6itation erosion test (Model SPH-2/h; Shanghai Welding and Cutting Tool Works). The as-received brass specimens were sand blasted and degreased prior to flame spraying, in order to enhance adhesion between the sprayed powder and the substrate. Moreover, a neutral flame was used for minimizing the oxidizing effect. The average thickness of the preplaced coating was measured using a digital micrometer. A coating thickness of 230 mm was used.
2.2. Laser processing parameters Preliminary trial runs indicated that successful single tracks could be obtained with a laser power density in the neighborhood of 76 W mm − 2 and a scanning speed in the neighborhood of 15 mm s − 1. The metallographic cross-section of a trial run is shown in Fig. 1. In the present study, the laser processing parameters were varied from this setting in order to investigate the characteristics of the specimens with different laser processing parameters so as to optimize the effect of power density and scanning speed. The power density was varied from 64 to 76 W mm − 2 with a beam diameter of 5 mm, and a scanning speed from 15 to 35 mm s − 1. Table 2 summarizes the processing parameters used in successful surfacing. Laser surfacing was achieved by overlapping of adjacent tracks, with an overlapping ratio of 50%, and argon was used as the shielding gas.
The cavitation erosion test was carried out conforming to the ASTM Standard G32-92 [6], using an ultrasonic vibratory cavitation facility (Sonicator Ultrasonic Liquid Processor Model XL2020, 550 W; Heat System Inc., USA) in the unattachment mode. In the unattachment mode, the specimen was held stationary at a distance of 1 mm below the horn, instead of mounting the specimen directly to the ultrasonic horn, as shown in Fig. 2. The vibratory studs were made of super duplex stainless steel (S32760) and the surface of the specimen was polished with 1 mm diamond paste in order to provide a constant surface roughness. A 3.5 wt.% NaCl solution (pH 6) was used as the cavitating medium and contained in an open container of 1000 ml. In order to keep the solution at a constant temperature of 239 1 °C, a circulation system with controllable temperature (Cole and Palmer, Polystat) was used. The vibratory frequency and the peak-to-peak amplitude used were 20 kHz and 60 mm, respectively. The eroded specimen was weighed after each 30 min in a 4-h cavitation erosion test using an electronic balance (METTLER AT balance). The erosion loss of materials was expressed in terms of the mean depth of erosion (MDE) and the mean erosion rate (MER), as calculated according to the following equations: MDE (mm) =
DW 10zA
2.3. Microstructural and metallographic analysis
and
After laser irradiation, the laser-modified specimens were cross-sectioned, polished and etched with acidic
MER (mm h − 1)=
DW 10zADt
(1)
(2)
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where DW is the weight loss (mg), Dt is the time interval (h), A is the eroded area (cm2) and z is the density of the modified layer (g cm − 3). In addition, the cavitation erosion resistance Re is defined as the reciprocal of MER. Re (h mm − 1)=(MER) − 1
(3)
2.5. Potentiodynamic polarization test The potentiodynamic polarization test was per-
formed with an EG&G PARC 273 corrosion system according to the ASTM Standard G5-94 [7], in 3.5 wt.% NaCl solution (pH 6), at 23 °C. A saturated calomel electrode (0.244 V versus SHE at 25 °C) was used as the reference electrode and two parallel graphite rods, for current measurement. All data were recorded after an initial delay of 10 min to let the specimen reach a steady state. The electrode potential was increased from 100 mV below the corrosion potential at a scan rate of 1 mV s − 1 until an anodic current density of 5 mA cm − 2 was reached.
Fig. 3. Microstructure of (a) Ni –Cr–Al– Mo– Fe-1, (b) Ni – Cr– Al– Mo– Fe-2, and (c) Ni – Cr– Al– Mo– Fe-3, and microcrack in (d) Ni –Cr–Al– Mo– Fe-3, (e) Ni –Cr–Al–Mo–Fe-4 and (f) Ni –Cr–Al–Mo– Fe-5.
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Fig. 4. Hardness profiles along the melt depth of the cross-section of various specimens.
3. Results and discussion
3.1. Microstructural and metallographic analysis The laser scanning speed significantly affects the microstructure, dilution ratio and composition of the laser-treated layer since it determines the laser–material interaction time and the rate of resolidification. To investigate the effect of scanning speed, the power density was kept constant at 76 W mm − 2 while the scanning speed was varied in the range 15– 35 mm s − 1 for Ni–Cr – Al –Mo – Fe-1, Ni– Cr –Al – Mo – Fe-2 and Ni –Cr –Al –Mo – Fe-3. For Ni – Cr – Al – Mo – Fe-4 and Ni –Cr –Al –Mo – Fe-5, the power density was set at 64 W mm − 2 and the scanning speed was varied from 15 to 25 mm s − 1. After the remelting of the preplaced Ni– Cr – Al – Mo –Fe layer on brass by laser irradiation, no pores were present, and the remelted layer bonded well to the brass substrate with metallurgical bond. The optical micrographs of laser-modified specimens Ni–Cr – Al – Mo –Fe-1, Ni– Cr – Al – Mo – Fe-2, Ni–Cr – Al –Mo –Fe3, Ni –Cr –Al –Mo – Fe-4 and Ni– Cr – Al – Mo –Fe-5 are shown in Fig. 3. From the optical micrograph of Ni– Cr – Al – Mo – Fe1 (Fig. 3a), it could be observed that, owing to track overlapping for surfacing, the profile of the melt layer was quite different from that of the single track profile in Fig. 1. The surface layer could be divided into black regions (copper-rich) and white regions (nickel-rich). The black region was selectively etched, revealing a dendritic microstructure, while the white region was shiny. For Ni – Cr –Al – Mo – Fe-2 (Fig. 3b), the melt depth was smaller than that in Ni– Cr – Al – Mo – Fe-1 owing to a higher scanning speed. The dilution ratio of Ni–
Cr –Al –Mo –Fe-2 was 18%, while that of Ni– Cr – Al – Mo –Fe-1 was 38%. Moreover, the microstructure of Ni –Cr –Al –Mo –Fe-2 was more homogeneous and had a higher percentage of alloying elements, as indicated by an increase of the portion of white region. The black region in Ni–Cr –Al –Mo – Fe-2 had a finer dendritic structure than that in Ni–Cr –Al –Mo –Fe-1. This could be attributed to the higher scanning speed used, resulting in a smaller melt pool and a higher self-quenching rate. Ni –Cr – Al – Mo –Fe-3 (Fig. 3c) had a homogeneous microstructure and contained mainly the alloying elements. The scanning speed corresponding to Ni–Cr – Al –Mo –Fe-3 was 35 mm s − 1 and was higher than the other two. As a result of the higher self-quenching rate, the melt layer had microcracks (Fig. 3d) that were induced by thermal stress.
Fig. 5. Relation between the average hardness and the scanning speed with different processing power densities.
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g-NiCr were much stronger, as a result of the higher scanning speed.
3.4. Compositional profiles
Fig. 6. Variation of the average hardness with DR.
Ni –Cr –Al –Mo – Fe-4 and Ni– Cr – Al – Mo –Fe-5 showed a similar structure as Ni– Cr – Al –Mo –Fe-2 and Ni –Cr –Al – Mo – Fe-3. They were treated with a lower power density, and a lower scanning speed of 15 and 25 mm s − 1, respectively. The longer interaction time for Ni –Cr – Al – Mo – Fe-4 and Ni– Cr – Al –Mo – Fe-5 compensated for the lower power density. Moreover, better homogeneity of the microstructure together with a low dilution ratio was obtained. In addition, no microcracks were found in Ni– Cr – Al – Mo –Fe-5.
3.2. Hardness profiles The hardness profiles along the melt depth of the laser-modified specimens are shown in Fig. 4a,b. From the graphs, it could be observed that the hardness of the modified layers (except Ni– Cr – Al –Mo – Fe –5) rose from the surface to a maximum at the center of the modified layer and then dropped to the hardness of the brass substrate. All of the specimens exhibited a substantial increase in their hardness, and the hardness profile was essentially determined by the distribution of the hard borides. For specimen processed with different scanning speeds under the same power density, the hardness achieved increased with the scanning speed (Fig. 5). The hardness Hv0.2 of the modified layer was related to the DR, which could be regarded as the combined effect of the laser processing parameters (Fig. 6). 3.3. X-Ray diffraction analysis
The compositional profiles of the specimens laser surface modified with Ni–Cr –Al –Mo –Fe are shown in Fig. 8. For Ni–Cr –Al –Mo –Fe-1, the alloying elements were evenly distributed near the surface of the melt layer but were more concentrated near the interface between the melt layer and the substrate. The hardness profile was also consistent with this observation. As the scanning speed increased, the compositional profiles showed a relatively more homogeneous distribution of the alloying elements. Due to the lower dilution ratio in Ni –Cr –Al –Mo – Fe-3 (DR, 8%), the melt layer had a high nickel, chromium, and molybdenum content. This agreed with the micrograph of Ni–Cr –Al – Mo – Fe-3 that showed a homogeneous microstructure and a large proportion of white region, which was identified to be the Ni-rich phases. In addition, Zn was retained in all the laser-modified specimens, albeit its low boiling point ( 907 °C). The short interaction time did not provide sufficient energy for the latent heat required for vaporization. The compositional profile of Ni–Cr – Al – Mo –Fe-4 was similar to that of Ni–Cr –Al –Mo –Fe-1. The alloying elements were more concentrated at the interface between the melt layer and the substrate. Compared with the other specimens, these two specimens were subjected to a longer interaction time, which resulted in more laser energy absorbed and a larger melt pool. Therefore, the intermixing was more difficult due to the high dilution ratio. At first, the alloying elements were at the surface of the melt layer and flowed to intermix with the substrate by convection. The alloying elements were resolidified at the bottom of the melt pool due the high thermal conductivity of the substrate, leading to enrichment of the alloying elements at the interface. The compositional profile of Ni–Cr –Al –Mo –Fe-5 was similar to that of Ni–Cr –Al –Mo –Fe-2. As the scanning speed increased to 25 mm s − 1, the compositional profile along the modified layer of Ni–Cr –Al –Mo –Fe5 became more homogeneous, due to a low dilution ratio and a shorter interaction time. Therefore, more alloying elements were present in the modified layer and hence a higher hardness was achieved (Fig. 4b).
3.5. Ca6itation erosion resistance The X-ray diffraction spectra of the specimens laser surface modified with Ni– Cr – Al – Mo – Fe are shown in Fig. 7. All of the laser-modified layers were mainly composed of Cu, Ni, (Cu,Ni) and g-NiCr, and a small amount of secondary phases CrB, Fe2B, Ni3B and Ni4Mo [8]. For Ni –Cr – Al – Mo – Fe-3 and Ni–Cr –Al – Mo – Fe-5, the intensities of the peaks (Cu,Ni) and
Fig. 9 shows the graph of cumulative mean depth of erosion as a function of time for the as-received and laser surface modified specimens eroded in 3.5 wt.% NaCl solution (pH 6) at 23 °C. The cavitation erosion resistance Re of all the laser surface modified specimens was improved relative to that of the brass substrate
K.F. Tam et al. / Materials Science and Engineering A325 (2002) 365–374
(Table 3). The cavitation erosion resistance of the modified specimens increased from 2.47 to 4.60 times that of the substrate. The graph in Fig. 10 clearly indicated that Re was mainly determined by the hardness of the modified layer [9,10].
3.6. Ca6itation damage mechanism The SEM micrographs of Ni – Cr – Al – Mo – Fe-3 sub-
371
jected to the 4-h cavitation erosion test are given in Fig. 11. The morphology of the cavitated surface that consisted of undulations and craters indicated that the damage mechanism of the surface was ductile fracture. The SEM picture of the nickel-rich region is shown in Fig. 11a and that of the copper-rich region is shown in Fig. 11b, together with the chemical composition determined by EDX. The copper-rich region obviously was more severely damaged.
Fig. 7. X-Ray diffraction spectra of (a) Ni –Cr–Al–Mo–Fe-1, (b) Ni – Cr– Al– Mo– Fe-2, (c) Ni – Cr– Al– Mo– Fe-3, (d) Ni – Cr– Al– Mo–Fe-4 and (e) Ni – Cr– Al– Mo– Fe-5.
372
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Fig. 8. Chemical compositional profiles of various laser surface modified specimens: (a) Ni – Cr– Al– Mo– Fe-1, (b) Ni – Cr– Al– Mo–Fe-2, (c) Ni–Cr– Al–Mo– Fe-3, (d) Ni –Cr–Al– Mo– Fe-4 and (e) Ni – Cr– Al– Mo– Fe-5.
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Fig. 10. Variation of the normalized Re with the average hardness. Fig. 9. Cumulative mean depth of erosion MDE as a function of time for the as-received and laser surface modified specimens eroded in 3.5 wt.% NaCl solution (pH 6) at 23 °C.
3.7. Electrochemical corrosion beha6iour The potentiodynamic polarization curves of the asreceived and laser surface modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C are shown in Fig. 12. The corrosion potential Ecorr and the corrosion current density Icorr extracted from the curves are presented in Table 3. The values of Ecorr were quite close to that of the as-received specimen, while there was in general a reduction in Icorr. However, it could be observed that there was a shift of the polarization curves towards higher current densities, especially for Ni–Cr – Al –Mo –Fe-3, which had cracks in the surface layer. Moreover, some of the specimens did not passivate. It might thus be concluded that laser surface alloying with Ni –Cr –Al –Mo – Fe could not bring an improvement in corrosion resistance, perhaps due to a low Cr content in the matrix of the modified layer.
Fig. 11. Microstructure of brass laser surface modified with Ni –Cr– Al– Mo– Fe (Ni– Cr– Al– Mo– Fe-3): (a) nickel-rich region (SEM), and (b) copper-rich region (SEM).
Table 3 Cavitation erosion resistance Re and corrosion parameters of as-received and laser surface modified specimens Specimen
Average hardness (Hv0.2)
Re (h mm−1)
Normalized Re
Ecorr (mV)
Icorr (mA cm−2)
Epit (mV)
As-received brass Ni–Cr–Al–Mo–Fe-1 Ni–Cr–Al–Mo–Fe-2 Ni–Cr–Al–Mo–Fe-3 Ni–Cr–Al–Mo–Fe-4 Ni–Cr–Al–Mo–Fe-5
110 152 176 198 184 256
13.40 33.02 41.20 56.28 52.58 61.48
1.00 2.47 3.08 4.21 3.93 4.60
−238 −236 −239 −249 −229 −245
1.0 0.15 0.1 0.1 0.25 0.2
−173 −177 −191 / / /
374
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Fig. 12. Potentiodynamic polarization curves of the as-received and laser surface modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C.
4. Conclusions Brass (Cu–3.8%Zn) substrate was successfully laser alloyed with Ni– Cr – Al – Mo – Fe to a depth of 250– 350 mm using a laser power density of around 70 W mm − 2, a scanning speed from 15 to 35 mm s − 1, and a 50% overlap between adjacent tracks. The hardness of the modified layers was increased from 110Hv0.2 of the substrate to the range 152– 256Hv0.2. The hardness increased as the dilution ratio decreased. The cavitation erosion resistance Re in 3.5 wt.% NaCl solution (pH 6) of the modified specimens was improved by a factor of 2.47– 4.60 compared with that of the substrate, depending on the processing parameters used. The improvement was mainly due to the formation of hard boride phases in a ductile Ni-rich matrix. The cavitation erosion resistance was found to correlate positively with the hardness of the modified layer. No significant improvement in corrosion resistance in 3.5 wt.% NaCl solution (pH 6) was achieved.
Acknowledgements The authors would like to acknowledge the Research
Committee of the Hong Kong Polytechnic University for the provision of a research grant (number G-V710).
References [1] A. Karimi, J.L. Martin, Int. Metals Rev. 31 (1) (1986) 1 – 26. [2] C.W. Draper, High Temp. Mater. Process 6 (3-4) (1984) 213 – 224. [3] K. Nakata, K. Tomoto, F. Matsuda, Trans. Jpn. Welding Res. Inst. 25 (1) (1996) 37 – 41. [4] G. Dehm, B. Medres, L. Shepeleva, C. Scheu, M. Bamberger, B.L. Mordike, S. Mordike, G. Ryk, G. Halperin, I. Etsion, Wear 225 – 229 (1999) 18 – 26. [5] Z. Liu, K.G. Watkins, W.M. Steen, N. Pirch, A. Gasser, P.G. Hatherley, Proceedings of ICALEO’98, Orlando, FL, Vol. 85 (1), 1998, pp. 94 – 103. [6] ASTM 032-92, Annual Book of ASTM Standards, Vol. 03.02, ASTM, Philadelphia, PA, 1994. [7] ASTM 05-94, Annual Book of ASTM Standards, Vol. 03.02, ASTM, Philadelphia, PA, 1994. [8] S.D. De la Torre, D. Oleszak, A. Kakitsuji, K. Miyamoto, H. Miyamoto, R. Martinez-S., F. Almeraya-C, A. Martinez-V., D. Rios-J., Mater. Sci. Eng. A276 (2000) 226 – 235. [9] J.Z. Lichtman, E.R. Weingram. In: J.W. Holl, G.M. Wood (Eds.), Symposium on Cavitatation: Research, Facilities and Technology, American Society of Mechanical Engineers, New York, 1964, p. 185. [10] A. Laird, J.M. Hobbs, National Engineering Laboratory Report No. 496, East Kilbride, 1971.
Laser surfacing of brass with Ni–Cr–Al–Mo–Fe using various laser processing parameters K.F. Tam a, F.T. Cheng a,*, H.C. Man b a
Department of Applied Physics, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, People’s Republic of China b Department of Manufacturing Engineering, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, People’s Republic of China Received 22 February 2001; received in revised form 2 May 2001
Abstract Brass (Cu–38%Zn) specimens were laser surface modified with Ni– Cr– Al– Mo– Fe (Ni– 10%Cr– 7%Al– 5%Mo– 5%Fe–1%B) using a 2 kW CW Nd:YAG laser. Successful laser surfacing was achieved with a power density around 70 W mm − 2, a scanning speed in the range 15–35 mm s − 1, and a 50% overlap of adjacent tracks. The hardness of the alloyed layer increased as the dilution ratio decreased. The cavitation erosion resistance, defined as the reciprocal of the mean erosion rate, of the modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C was improved by a factor in the range 2.47– 4.60 compared with that of brass, and correlated well with the hardness. The improvement in the cavitation erosion resistance was attributed to the formation of hard boride phases in a ductile Ni-rich matrix. On the other hand, there was no significant improvement in the corrosion resistance. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser surfacing; Cavitation erosion; Brass; Ni– Cr– Al– Mo– Fe; Corrosion
1. Introduction Cavitation refers to the nucleation and growth of cavities or bubbles in a fast-flowing or vibrating liquid when the local pressure in the liquid drops below a certain level. These bubbles will implode or collapse when they are transported to a high-pressure region. The collapse of bubbles is accompanied by the emission of shock waves and micro-jets, which will exert stress pulses on a solid surface in the vicinity. Repetitions of such pressure pulses on the solid surface will eventually result in fatigue failure and material loss, and such a mode of damage is known as cavitation erosion [1]. Thus, cavitation erosion is a common problem faced by designers for pumps and other hydraulic machinery. Cavitation erosion might be visualized as a random dynamic micro-indentation process. In this sense, the resistance to cavitation erosion is closely related to the mechanical properties of the material under consider* Corresponding author. Tel.: +852-27665691; Fax: 23337629. E-mail address: [email protected] (F.T. Cheng).
+ 852-
ation. Brass, an inexpensive alloy commonly used in marine applications, is rather corrosion resistant, but much less so with respect to cavitation erosion because of its low mechanical strength. In view of the fact that cavitation erosion is a surface phenomenon, surface modification is a viable solution to improve the cavitation erosion resistance since it may be employed to modify the surface properties while retaining those of the bulk, with the consumption of only a small amount of precious materials. Among the different techniques available, laser surface modification is unique in being able to form a surface layer of novel alloys with a fine microstructure and with strong bonding to the substrate. It is the aim of the present study to laser alloy brass (Cu –38%Zn) with Ni –Cr –Al –Mo –Fe. Ni –Cr –Al – Mo –Fe is a new type of self-bonding powder that possesses a number of advantages with respect to processing and properties. It can be easily preplaced on a brass substrate by thermal spraying, thus greatly facilitating laser surface alloying in the subsequent step. The melting point of Ni –Cr –Al –Mo –Fe is only slightly higher than that of brass, and this ensures simultaneous
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melting and mixing of the powder and a thin layer of the substrate. Ni forms a solid solution with Cu in any proportions, and this results in a good interfacial bonding. The other elements present are expected to enhance the mechanical strength and the corrosion resistance. Reports on studies of laser surface modification of copper alloys are scarce in the literature, especially those devoted to enhancing cavitation erosion resistance [2] or wear resistance [3]. This is not unexpected as laser surface modification of copper alloys presents a certain degree of difficulty owing to high reflectivity and thermal conductivity [4], and successful treatments require a careful selection of the processing parameters [5]. It is expected that with a proper choice of laser processing parameters, an alloyed layer that bonds well to the substrate, may be produced and the cavitation
erosion resistance of the brass substrate may be significantly improved.
2. Experimental details
2.1. Specimen preparation The nominal compositions of brass Cu–38%Zn and the alloying powder Ni–Cr –Al –Mo –Fe are presented in Table 1. The as-received brass specimens were cut with dimensions 30 mm× 15 mm ×8 mm, while the particle size of Ni–Cr –Al –Mo –Fe alloy powder ranged from − 140 to + 350 mesh (B 105 and \40 mm). The alloying powder was preplaced on the brass substrate with an oxyacetylene thermal spraying gun
Fig. 1. Metallographic cross-section of laser-modified track with power density 76 W mm − 2 and scanning speed 15 mm s − 1.
Table 1 Nominal compositions of brass and Ni–Cr–Al–Mo–Fe powder Material
Cu
Zn
Pb
Brass Ni–Cr–Al–Mo–Fe alloy powder
Balance
38
1.5
Ni
Cr
Al
Mo
Fe
B
Balance
10
7
5
5
1
Table 2 Summary of laser power density, scanning speed, precursor coating thickness (t), melt depth (d) and dilution ratio of the laser-modified layers Specimen
Laser power density (W mm−2)
Scanning speed (mm s−1)
Precursor coating thickness (t) (mm)
Melt depth (d) (mm)
Dilution ratio (DR)a (%)
Ni–Cr–Al–Mo–Fe-1 Ni–Cr–Al–Mo–Fe-2 Ni–Cr–Al–Mo–Fe-3 Ni–Cr–Al–Mo–Fe-4 Ni–Cr–Al–Mo–Fe-5
76 76 76 64 64
15 25 35 15 25
230 230 230 230 230
371.0 280.5 248.5 273.3 242.2
38 18 7 16 5
a
n
coating thickness (t) ×100%. melt depth (d)
Dilution ratio (DR) = 1−
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ferric chloride solution (25g FeCl3, 25 ml HCl and 100 ml H20) for 5 s. The average melt depth was determined using image analysis and the dilution ratio (DR) was calculated, and shown in Table 2. Optical microscopy (Nikon MICROPHOT-FXA) and scanning electron microscopy (SEM) (Model S250; Cambridge) were employed to analyze the microstructure of the lasermodified layer, using the etched specimens. The chemical compositions and the phases formed in the surface of the laser-modified specimens were investigated by energy-dispersive X-ray analysis (EDX) and X-ray diffractometry (Model PW3710; Philips), using Cu Ka generated at 40 kV and 35 mA as the radiation source, with a nickel monochromatic filter. The hardness profiles of the laser-modified specimens were measured along the depth of the cross-section of the melt zone, using a Vickers hardness tester (Buehler Micromet II) with a load of 200 g and a loading time of 15 s. The surface morphology of the eroded specimens was also examined by SEM. Fig. 2. Set-up of the cavitation erosion test in unattachment mode.
2.4. Ca6itation erosion test (Model SPH-2/h; Shanghai Welding and Cutting Tool Works). The as-received brass specimens were sand blasted and degreased prior to flame spraying, in order to enhance adhesion between the sprayed powder and the substrate. Moreover, a neutral flame was used for minimizing the oxidizing effect. The average thickness of the preplaced coating was measured using a digital micrometer. A coating thickness of 230 mm was used.
2.2. Laser processing parameters Preliminary trial runs indicated that successful single tracks could be obtained with a laser power density in the neighborhood of 76 W mm − 2 and a scanning speed in the neighborhood of 15 mm s − 1. The metallographic cross-section of a trial run is shown in Fig. 1. In the present study, the laser processing parameters were varied from this setting in order to investigate the characteristics of the specimens with different laser processing parameters so as to optimize the effect of power density and scanning speed. The power density was varied from 64 to 76 W mm − 2 with a beam diameter of 5 mm, and a scanning speed from 15 to 35 mm s − 1. Table 2 summarizes the processing parameters used in successful surfacing. Laser surfacing was achieved by overlapping of adjacent tracks, with an overlapping ratio of 50%, and argon was used as the shielding gas.
The cavitation erosion test was carried out conforming to the ASTM Standard G32-92 [6], using an ultrasonic vibratory cavitation facility (Sonicator Ultrasonic Liquid Processor Model XL2020, 550 W; Heat System Inc., USA) in the unattachment mode. In the unattachment mode, the specimen was held stationary at a distance of 1 mm below the horn, instead of mounting the specimen directly to the ultrasonic horn, as shown in Fig. 2. The vibratory studs were made of super duplex stainless steel (S32760) and the surface of the specimen was polished with 1 mm diamond paste in order to provide a constant surface roughness. A 3.5 wt.% NaCl solution (pH 6) was used as the cavitating medium and contained in an open container of 1000 ml. In order to keep the solution at a constant temperature of 239 1 °C, a circulation system with controllable temperature (Cole and Palmer, Polystat) was used. The vibratory frequency and the peak-to-peak amplitude used were 20 kHz and 60 mm, respectively. The eroded specimen was weighed after each 30 min in a 4-h cavitation erosion test using an electronic balance (METTLER AT balance). The erosion loss of materials was expressed in terms of the mean depth of erosion (MDE) and the mean erosion rate (MER), as calculated according to the following equations: MDE (mm) =
DW 10zA
2.3. Microstructural and metallographic analysis
and
After laser irradiation, the laser-modified specimens were cross-sectioned, polished and etched with acidic
MER (mm h − 1)=
DW 10zADt
(1)
(2)
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where DW is the weight loss (mg), Dt is the time interval (h), A is the eroded area (cm2) and z is the density of the modified layer (g cm − 3). In addition, the cavitation erosion resistance Re is defined as the reciprocal of MER. Re (h mm − 1)=(MER) − 1
(3)
2.5. Potentiodynamic polarization test The potentiodynamic polarization test was per-
formed with an EG&G PARC 273 corrosion system according to the ASTM Standard G5-94 [7], in 3.5 wt.% NaCl solution (pH 6), at 23 °C. A saturated calomel electrode (0.244 V versus SHE at 25 °C) was used as the reference electrode and two parallel graphite rods, for current measurement. All data were recorded after an initial delay of 10 min to let the specimen reach a steady state. The electrode potential was increased from 100 mV below the corrosion potential at a scan rate of 1 mV s − 1 until an anodic current density of 5 mA cm − 2 was reached.
Fig. 3. Microstructure of (a) Ni –Cr–Al– Mo– Fe-1, (b) Ni – Cr– Al– Mo– Fe-2, and (c) Ni – Cr– Al– Mo– Fe-3, and microcrack in (d) Ni –Cr–Al– Mo– Fe-3, (e) Ni –Cr–Al–Mo–Fe-4 and (f) Ni –Cr–Al–Mo– Fe-5.
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Fig. 4. Hardness profiles along the melt depth of the cross-section of various specimens.
3. Results and discussion
3.1. Microstructural and metallographic analysis The laser scanning speed significantly affects the microstructure, dilution ratio and composition of the laser-treated layer since it determines the laser–material interaction time and the rate of resolidification. To investigate the effect of scanning speed, the power density was kept constant at 76 W mm − 2 while the scanning speed was varied in the range 15– 35 mm s − 1 for Ni–Cr – Al –Mo – Fe-1, Ni– Cr –Al – Mo – Fe-2 and Ni –Cr –Al –Mo – Fe-3. For Ni – Cr – Al – Mo – Fe-4 and Ni –Cr –Al –Mo – Fe-5, the power density was set at 64 W mm − 2 and the scanning speed was varied from 15 to 25 mm s − 1. After the remelting of the preplaced Ni– Cr – Al – Mo –Fe layer on brass by laser irradiation, no pores were present, and the remelted layer bonded well to the brass substrate with metallurgical bond. The optical micrographs of laser-modified specimens Ni–Cr – Al – Mo –Fe-1, Ni– Cr – Al – Mo – Fe-2, Ni–Cr – Al –Mo –Fe3, Ni –Cr –Al –Mo – Fe-4 and Ni– Cr – Al – Mo –Fe-5 are shown in Fig. 3. From the optical micrograph of Ni– Cr – Al – Mo – Fe1 (Fig. 3a), it could be observed that, owing to track overlapping for surfacing, the profile of the melt layer was quite different from that of the single track profile in Fig. 1. The surface layer could be divided into black regions (copper-rich) and white regions (nickel-rich). The black region was selectively etched, revealing a dendritic microstructure, while the white region was shiny. For Ni – Cr –Al – Mo – Fe-2 (Fig. 3b), the melt depth was smaller than that in Ni– Cr – Al – Mo – Fe-1 owing to a higher scanning speed. The dilution ratio of Ni–
Cr –Al –Mo –Fe-2 was 18%, while that of Ni– Cr – Al – Mo –Fe-1 was 38%. Moreover, the microstructure of Ni –Cr –Al –Mo –Fe-2 was more homogeneous and had a higher percentage of alloying elements, as indicated by an increase of the portion of white region. The black region in Ni–Cr –Al –Mo – Fe-2 had a finer dendritic structure than that in Ni–Cr –Al –Mo –Fe-1. This could be attributed to the higher scanning speed used, resulting in a smaller melt pool and a higher self-quenching rate. Ni –Cr – Al – Mo –Fe-3 (Fig. 3c) had a homogeneous microstructure and contained mainly the alloying elements. The scanning speed corresponding to Ni–Cr – Al –Mo –Fe-3 was 35 mm s − 1 and was higher than the other two. As a result of the higher self-quenching rate, the melt layer had microcracks (Fig. 3d) that were induced by thermal stress.
Fig. 5. Relation between the average hardness and the scanning speed with different processing power densities.
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g-NiCr were much stronger, as a result of the higher scanning speed.
3.4. Compositional profiles
Fig. 6. Variation of the average hardness with DR.
Ni –Cr –Al –Mo – Fe-4 and Ni– Cr – Al – Mo –Fe-5 showed a similar structure as Ni– Cr – Al –Mo –Fe-2 and Ni –Cr –Al – Mo – Fe-3. They were treated with a lower power density, and a lower scanning speed of 15 and 25 mm s − 1, respectively. The longer interaction time for Ni –Cr – Al – Mo – Fe-4 and Ni– Cr – Al –Mo – Fe-5 compensated for the lower power density. Moreover, better homogeneity of the microstructure together with a low dilution ratio was obtained. In addition, no microcracks were found in Ni– Cr – Al – Mo –Fe-5.
3.2. Hardness profiles The hardness profiles along the melt depth of the laser-modified specimens are shown in Fig. 4a,b. From the graphs, it could be observed that the hardness of the modified layers (except Ni– Cr – Al –Mo – Fe –5) rose from the surface to a maximum at the center of the modified layer and then dropped to the hardness of the brass substrate. All of the specimens exhibited a substantial increase in their hardness, and the hardness profile was essentially determined by the distribution of the hard borides. For specimen processed with different scanning speeds under the same power density, the hardness achieved increased with the scanning speed (Fig. 5). The hardness Hv0.2 of the modified layer was related to the DR, which could be regarded as the combined effect of the laser processing parameters (Fig. 6). 3.3. X-Ray diffraction analysis
The compositional profiles of the specimens laser surface modified with Ni–Cr –Al –Mo –Fe are shown in Fig. 8. For Ni–Cr –Al –Mo –Fe-1, the alloying elements were evenly distributed near the surface of the melt layer but were more concentrated near the interface between the melt layer and the substrate. The hardness profile was also consistent with this observation. As the scanning speed increased, the compositional profiles showed a relatively more homogeneous distribution of the alloying elements. Due to the lower dilution ratio in Ni –Cr –Al –Mo – Fe-3 (DR, 8%), the melt layer had a high nickel, chromium, and molybdenum content. This agreed with the micrograph of Ni–Cr –Al – Mo – Fe-3 that showed a homogeneous microstructure and a large proportion of white region, which was identified to be the Ni-rich phases. In addition, Zn was retained in all the laser-modified specimens, albeit its low boiling point ( 907 °C). The short interaction time did not provide sufficient energy for the latent heat required for vaporization. The compositional profile of Ni–Cr – Al – Mo –Fe-4 was similar to that of Ni–Cr –Al –Mo –Fe-1. The alloying elements were more concentrated at the interface between the melt layer and the substrate. Compared with the other specimens, these two specimens were subjected to a longer interaction time, which resulted in more laser energy absorbed and a larger melt pool. Therefore, the intermixing was more difficult due to the high dilution ratio. At first, the alloying elements were at the surface of the melt layer and flowed to intermix with the substrate by convection. The alloying elements were resolidified at the bottom of the melt pool due the high thermal conductivity of the substrate, leading to enrichment of the alloying elements at the interface. The compositional profile of Ni–Cr –Al –Mo –Fe-5 was similar to that of Ni–Cr –Al –Mo –Fe-2. As the scanning speed increased to 25 mm s − 1, the compositional profile along the modified layer of Ni–Cr –Al –Mo –Fe5 became more homogeneous, due to a low dilution ratio and a shorter interaction time. Therefore, more alloying elements were present in the modified layer and hence a higher hardness was achieved (Fig. 4b).
3.5. Ca6itation erosion resistance The X-ray diffraction spectra of the specimens laser surface modified with Ni– Cr – Al – Mo – Fe are shown in Fig. 7. All of the laser-modified layers were mainly composed of Cu, Ni, (Cu,Ni) and g-NiCr, and a small amount of secondary phases CrB, Fe2B, Ni3B and Ni4Mo [8]. For Ni –Cr – Al – Mo – Fe-3 and Ni–Cr –Al – Mo – Fe-5, the intensities of the peaks (Cu,Ni) and
Fig. 9 shows the graph of cumulative mean depth of erosion as a function of time for the as-received and laser surface modified specimens eroded in 3.5 wt.% NaCl solution (pH 6) at 23 °C. The cavitation erosion resistance Re of all the laser surface modified specimens was improved relative to that of the brass substrate
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(Table 3). The cavitation erosion resistance of the modified specimens increased from 2.47 to 4.60 times that of the substrate. The graph in Fig. 10 clearly indicated that Re was mainly determined by the hardness of the modified layer [9,10].
3.6. Ca6itation damage mechanism The SEM micrographs of Ni – Cr – Al – Mo – Fe-3 sub-
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jected to the 4-h cavitation erosion test are given in Fig. 11. The morphology of the cavitated surface that consisted of undulations and craters indicated that the damage mechanism of the surface was ductile fracture. The SEM picture of the nickel-rich region is shown in Fig. 11a and that of the copper-rich region is shown in Fig. 11b, together with the chemical composition determined by EDX. The copper-rich region obviously was more severely damaged.
Fig. 7. X-Ray diffraction spectra of (a) Ni –Cr–Al–Mo–Fe-1, (b) Ni – Cr– Al– Mo– Fe-2, (c) Ni – Cr– Al– Mo– Fe-3, (d) Ni – Cr– Al– Mo–Fe-4 and (e) Ni – Cr– Al– Mo– Fe-5.
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Fig. 8. Chemical compositional profiles of various laser surface modified specimens: (a) Ni – Cr– Al– Mo– Fe-1, (b) Ni – Cr– Al– Mo–Fe-2, (c) Ni–Cr– Al–Mo– Fe-3, (d) Ni –Cr–Al– Mo– Fe-4 and (e) Ni – Cr– Al– Mo– Fe-5.
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Fig. 10. Variation of the normalized Re with the average hardness. Fig. 9. Cumulative mean depth of erosion MDE as a function of time for the as-received and laser surface modified specimens eroded in 3.5 wt.% NaCl solution (pH 6) at 23 °C.
3.7. Electrochemical corrosion beha6iour The potentiodynamic polarization curves of the asreceived and laser surface modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C are shown in Fig. 12. The corrosion potential Ecorr and the corrosion current density Icorr extracted from the curves are presented in Table 3. The values of Ecorr were quite close to that of the as-received specimen, while there was in general a reduction in Icorr. However, it could be observed that there was a shift of the polarization curves towards higher current densities, especially for Ni–Cr – Al –Mo –Fe-3, which had cracks in the surface layer. Moreover, some of the specimens did not passivate. It might thus be concluded that laser surface alloying with Ni –Cr –Al –Mo – Fe could not bring an improvement in corrosion resistance, perhaps due to a low Cr content in the matrix of the modified layer.
Fig. 11. Microstructure of brass laser surface modified with Ni –Cr– Al– Mo– Fe (Ni– Cr– Al– Mo– Fe-3): (a) nickel-rich region (SEM), and (b) copper-rich region (SEM).
Table 3 Cavitation erosion resistance Re and corrosion parameters of as-received and laser surface modified specimens Specimen
Average hardness (Hv0.2)
Re (h mm−1)
Normalized Re
Ecorr (mV)
Icorr (mA cm−2)
Epit (mV)
As-received brass Ni–Cr–Al–Mo–Fe-1 Ni–Cr–Al–Mo–Fe-2 Ni–Cr–Al–Mo–Fe-3 Ni–Cr–Al–Mo–Fe-4 Ni–Cr–Al–Mo–Fe-5
110 152 176 198 184 256
13.40 33.02 41.20 56.28 52.58 61.48
1.00 2.47 3.08 4.21 3.93 4.60
−238 −236 −239 −249 −229 −245
1.0 0.15 0.1 0.1 0.25 0.2
−173 −177 −191 / / /
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Fig. 12. Potentiodynamic polarization curves of the as-received and laser surface modified specimens in 3.5 wt.% NaCl solution (pH 6) at 23 °C.
4. Conclusions Brass (Cu–3.8%Zn) substrate was successfully laser alloyed with Ni– Cr – Al – Mo – Fe to a depth of 250– 350 mm using a laser power density of around 70 W mm − 2, a scanning speed from 15 to 35 mm s − 1, and a 50% overlap between adjacent tracks. The hardness of the modified layers was increased from 110Hv0.2 of the substrate to the range 152– 256Hv0.2. The hardness increased as the dilution ratio decreased. The cavitation erosion resistance Re in 3.5 wt.% NaCl solution (pH 6) of the modified specimens was improved by a factor of 2.47– 4.60 compared with that of the substrate, depending on the processing parameters used. The improvement was mainly due to the formation of hard boride phases in a ductile Ni-rich matrix. The cavitation erosion resistance was found to correlate positively with the hardness of the modified layer. No significant improvement in corrosion resistance in 3.5 wt.% NaCl solution (pH 6) was achieved.
Acknowledgements The authors would like to acknowledge the Research
Committee of the Hong Kong Polytechnic University for the provision of a research grant (number G-V710).
References [1] A. Karimi, J.L. Martin, Int. Metals Rev. 31 (1) (1986) 1 – 26. [2] C.W. Draper, High Temp. Mater. Process 6 (3-4) (1984) 213 – 224. [3] K. Nakata, K. Tomoto, F. Matsuda, Trans. Jpn. Welding Res. Inst. 25 (1) (1996) 37 – 41. [4] G. Dehm, B. Medres, L. Shepeleva, C. Scheu, M. Bamberger, B.L. Mordike, S. Mordike, G. Ryk, G. Halperin, I. Etsion, Wear 225 – 229 (1999) 18 – 26. [5] Z. Liu, K.G. Watkins, W.M. Steen, N. Pirch, A. Gasser, P.G. Hatherley, Proceedings of ICALEO’98, Orlando, FL, Vol. 85 (1), 1998, pp. 94 – 103. [6] ASTM 032-92, Annual Book of ASTM Standards, Vol. 03.02, ASTM, Philadelphia, PA, 1994. [7] ASTM 05-94, Annual Book of ASTM Standards, Vol. 03.02, ASTM, Philadelphia, PA, 1994. [8] S.D. De la Torre, D. Oleszak, A. Kakitsuji, K. Miyamoto, H. Miyamoto, R. Martinez-S., F. Almeraya-C, A. Martinez-V., D. Rios-J., Mater. Sci. Eng. A276 (2000) 226 – 235. [9] J.Z. Lichtman, E.R. Weingram. In: J.W. Holl, G.M. Wood (Eds.), Symposium on Cavitatation: Research, Facilities and Technology, American Society of Mechanical Engineers, New York, 1964, p. 185. [10] A. Laird, J.M. Hobbs, National Engineering Laboratory Report No. 496, East Kilbride, 1971.