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Seawater corrosion behavior of laser surface modified Inconel 625 alloy
MATERIALS SCIEWCE & ENGINEERING ELSEVIER
A
Materials Scienceand EngineeringA206 (1996) 138-149
Seawater corrosion behavior of laser surface modified Inconel 625 alloy K.P. Cooper”*, P. Slebodnickb, E.D. Thomasb “Physical Metalhrrgy Branch, Materials bEnviromnental Effects Branch, Materials
Science and Technology Science and Technology
Division, Division,
Naval Research Laboratory, Naval Research Laboratory,
W’clshir~gtor~, DC 20375-5343, Wmhington, DC 2037.54343,
USA USA
Received 30 May 1995;in revised form 28 July 1995
Abstract Seawater corrosion behavior of laser surface processed Inconel 625 alloy was investigated. Three laser surface modified samples were prepared, one by laser meltin g, and the other two by laser melt/particle injection processing with tungsten carbide (WC) and titanium carbide (TIC) particles, respectively. Particle injection involved embedding the carbide particles into a laser melted surface and resulted in a metal matrix-particulate composite surface layer which was both hard and wear-resistant. While Inconel 625 is a corrosion resistant alloy suitable for marine applications, and WC and TiC are generally inert to chemical attack, results from this study showed that laser surface modification produced microstructures that were susceptible to seawater corrosion to varying degrees. Nominal corrosion was observed in the dendritic structure produced by laser melting of the alloy surface. In the particle injected samples, the WC particulate phase in contact with the Inconel alloy matrix showed different kinds of attack, while the TiC particulate phase showed none. In both particle injected samples, resolidification of the Inconel alloy melt produced significant departures in composition and microstructure from those of the base alloy. Eutectic and dendritic carbides and, in WC, interphase carbides were some of the resolidification byproducts that formed in the matrix surrounding the particulate. Alloyed with solute elements from the base alloy, each product phase contributed to unique forms of corrosion. A qualitative analysis of the corrosion behavior of the injected samples showed that corrosive damage was more severe in the WC injected sample than in the TiC injected sample and in the laser melted sample. This paper describes the processing, microstructural and compositional characterization, and seawater corrosion behavior of the laser surface modified samples, and attempts to explain the observations as a consequence of the formation of galvanic cells. Keywords:
Corrosion; Inconel 625
1. Introduction The use of lasers to enhance the wear resistance of metals has been well documented [I]. Laser surface modification processes make use of several hardening and strengthening mechanisms, such as transformation hardening, solid solution strengthening, precipitation hardening, and dispersoid strengthening. Depending upon the type of alloy (hardenable, precipitation, etc.) and the laser technique (heat treating, melting, alloying, cladding), one or more of the above mechanisms can contribute to improved wear resistance. The same microstructural modifications that improve wear resistance can affect corrosion behavior which this paper attempts to explore. * Corresponding author.
Laser melt/particle injection processing involves the formation of hard metal matrix-carbide conlposite surface layers [2]. Typically, the modified layer contains about 50%-60% by volume of carbide particulate. To retain continuity of properties such as corrosion resistance across the interface and a strong metallurgical bond throughout to prevent delamination, the sur-
rounding
matrix is desirably of the same composition
and microstructure
as the base alloy. Wear and friction
studies [3,4] on these samples have demonstrated 2-5 times improvement in wear resistance. These studies concluded that particulate reinforcement was the predominant mechanism in enhancing hardness and wear resistance.
The main thrust in studying laser melt/particle injection processing of Inconel 625 alloy was to improve the Published by Elsevier ScienceS.A. SSDI 0921-5093(95)10013-x
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Table 1 Properties of carbide particulates and Inconel 625 Material
WC
TiC
Inconel 625
Composition (wt.‘%) C) Melting pt. (“C) Density (g cm -‘) --AF(1500”C) (kcal mol-‘) Microhardness (kg mm - ‘) Crystal structure Stability temperature (“C) Electrical resistivity (~0 cm)
6.13 2720 15.63 8.4 1780 Hexagonal 2600 17
IO-20 3147 4.93 39.5 2988 FCC 3140 60-250
0.1 1427 8.44 295 FCC 0.0129
wear resistance of a relatively soft alloy without sacrificing its useful properties. Inconel 625 is a high strength, corrosion resistant superalloy widely used in the chemical industry for its resistance to a variety of aqueous corrodents [5]. The presence of MO makes it resistant to pitting [5]. It is extensively used in the marine environment, especially by the United States Navy. Crum [6] reports for Inconel 625 the highest pitting resistance in stagnant seawater and hot seawater applications such as heat exchangers. This research work was undertaken to study the corrosion behavior of carbide particle injected Inconel 625 alloy samples in particular, and to extend the understanding to metal matrix-particulate composite surfaces in general. Carbide particulates studied were WC and TIC because of their very high hardness and wear resistance. Unlike Sic, which dissociates easily at high temperatures, WC and TIC are quite stable at the temperatures encountered in the laser melt pool. However, compared with Sic, which has an extremely high electrical resistivity of 2 x lo6 1tQ cm - ‘, WC and TiC are fairly conductive [7]. This would give some importance to the possibility of the formation of galvanic cells between the carbide particulate and the surrounding Inconel matrix. Electrical resistivity and other relevant properties of WC, TIC and Inconel 625 are compared in Table 1. While their melting points are not far apart, there are some differences in the characteristics of WC and TiC which may influence their response to laser processing, microstructural development, wear and corrosion. While WC is a line compound, TiC has a wide solubility range, which may mean the absence or presence of compositional gradients within the carbide phase which can affect corrosion behavior. Their densities differ, especially with respect to Inconel 625, which may affect particle injection processing. Their crystal structures differ, which may influence microstructural morphology and hence, wear and corrosion. Their free energies of formation ( - AF) differ, which may influence their high temperature stability and hence, microstructural development, wear and corrosion behavior. Their microhardness values differ, which may affect the wear behavior.
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Corrosion of composite surfaces becomes important because galvanic coupling is inevitable owing to the presence of two dissimilar materials. 111 addition, traditional techniques of corrosion protection using inhibitive coatings are not feasible in a wear type of application. A cursory glance at the recent literature on laser surface processing [8] shows increased interest in depositing composite coatings containing various volume fractions of hard particulates. It is hoped that the observations and analysis reported in this paper will help in developing sound particulate-containing coatings, whether laser-deposited or deposited by other directed-energy beam sources. Most of the reported research work on the applications of laser surface modification has focused on enhancing the wear resistance of metals, and to a lesser extent, on enhancing the corrosion resistance [9]. Very few studies have involved characterizing the modified surfaces for both wear and corrosion resistance, and none, to our knowledge, has involved laser deposited metal matrix composite surface layers. Hardfacing alloys that improve wear resistance, such as Tribaloy and Stellites, are usually applied using thermal spray techniques. If porosity or channels are not avoided during deposition, or if dilution with the substrate occurs, then these coatings can become susceptible to various forms of corrosion. This is not necessarily detrimental because, historically, most wear applications have not involved reactive environments. But, with the rapid advancement in technology, the need to fabricate components that perform successfully in both high wear and corrosive environments has arisen. The laser deposited metal matrix-particulate composites, discussed herein, were an attempt to meet this very technological need. A review of the literature on corrosion behavior of laser modified samples showed that most of the work involved developing various laser surface modification techniques and discussing possible mechanisms for improved corrosion resistance in specific alloy systems. Corrosion resistance is improved either by rapidly melting the surface and redistributing the constituents, or by alloying the surface with suitable alloying elements, or by cladding the surface with a corrosion resistant alloy. McCafferty and Moore [lo] demonstrated that laser melting removed or redistributed sulfur inclusions and redistributed Cr in Cr-depleted grain boundaries in stainless steels; it homogenized aluminum bronze; and it removed residual porosity in plasma-sprayed coatings. Reitz and Rawers [ll] laser melted zirconium alloys and found a ten- to IOO-fold improvement in corrosion resistance with the corrosion rate depending upon the type of microstructural transformation. In laser melted Fe-aluminum bronzes, Draper [12] reported stifling of anodic dissolution due to removal of local galvanic action and due to the formation of a
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Table 2 Nominal composition of Inconel 625 (wt.%) Ni
Cr
MO
Fe
Nb
Si
C
62.0
21.5
9.0
4.0
4.0
0.5
0.05
passive, homogeneous film. Liu et al. [13] reported enhancing pitting resistance in laser melted 2014 aluminum alloy exposed to 1 M NaCl. Mazumder and Singh [14] laser alloyed 1016 steel with a mixture of Cr and Ni and reported spontaneous passivation and corrosion resistance approaching that of 304 stainless steel. Occasionally, the act of improving corrosion resistance can also result in an improvement in hardness and wear resistance or vice versa. Almeida et al [15] laser alloyed aluminum alloys with Cr and found an improvement in pitting corrosion resistance as well as a 2-3 times increase in surface hardness. In their study on comparing the properties of coatings produced by laser cladding and conventional methods, Oberlander and Lugscheider [16] report achieving high wear and corrosion resistance in mild steel laser clad with NiCr-Nb-B and Ni-Cr-Ta-B alloy powders. Most of the corrosion studies have concentrated on reporting the results of the response of the samples to various corrosion tests. Very few studies have attempted an analysis of the changes in the microstructure, if any, caused by exposure to the reactive environment. This paper discusses the various kinds of corrosive attack observed in the microstructure after exposure to seawater flow and the extent to which they occur.
2. Experimental 2.1. Laser processing und metnllograplzy
Laser melting involves melting a metal surface by irradiating it with a laser beam. Subsequent solidification, which is rapid, produces a fine-scale microstructure which can have unique properties. Laser melt/particle injection processing involves injecting particulate material into the melt pool formed by a laser beam. For wear resistance, the particles are usually high melting point carbides which do not fuse but which mix intimately with the molten base alloy. Upon solidification, the carbide/melt mixture forms an in situ metal matrix/carbide particulate composite surface layer. Wide tracks of such layers are produced by translating the sample below an oscillating laser beam. Details of beam characteristics, particle feeding, and processing conditions are given elsewhere [2,17]. A high power (15 kW), continuous wave, CO, laser was used in this study. The substrate was hot-rolled and annealed Inconel 625 alloy of nominal composition given in
Table 2 obtained from Into Alloys, Bloomington, DE and the particulates were 45 to 150 [ml WC and TiC of 99.5% and 98.5% purity, respectively, obtained from Consolidated Astronautics, Smithtown, NY. The laser processed samples were sectioned and polished with diamond paste to a 1 Ann finish. To reveal the microstructure, the samples were chemically etched in a mixture of 80 ml HCl, 13 ml HF and 7 ml HNO,. This etchant highlights dendritic and grain boundary segregation in some Ni-based superalloys, The etched samples were examined in the scanning electron microscope (SEM) and evaluated for dendritic segregation, morphology and distribution of the injected carbide, metal matrix microstructure, and microstructural gradients within the injected layer. For comparison, the base metal microstructure was also determined. Crystal structures and compositions of the resolidification byproducts were determined by transmission electron microscopy (TEM) and composition variations among the resolidification products by Auger electron spectroscopy (AES). 2.2. Corrosion studies
Seawater exposure tests were performed on all three samples simultaneously. The samples were sectioned across the laser treated surface and these cross-sections were polished with diamond paste to a 1 Fuji finish. All three polished samples were thoroughly cleaned in ethanol and suspended in test cells in a flow-through trough shown in Fig. 1. Natural, unfiltered seawater from the Naval Research Laboratory, Marine Corrosion Facility, Key West, FL seawater system was used throughout the test duration. Seawater parameters were: temperature 19-32 “C; salinity 35-39 g kg- ‘; pH 7.8-8.3; dissolved O2 4.2-6.9 mg 1-l; and resistivity 15.4-20.2 .Q cm. Seawater parametric trends for a typical year are given in Fig. 2. Except for the variation
Fig. 1. Seawatercorrosiontest apparatusshowing samples suspended in a flow-through trough.
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salinity 40.0
35.0
30.0
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25.0
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20.0
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in temperature, which is seasonal, all other parameters are fairly constant. All test cells had once-through seawater flow which was continually refreshed to maintain seawater parameters. After six months of exposure all samples were removed from the trough and ultrasonically cleaned of any adhering sludge with distilled water followed by cleaning with ethanol. The ascleaned samples were examined in the SEM and the cross-section microstructures were evaluated for the type and extent of corrosion.
3. Results 3.1. Micuostructul~e
The laser surface melted sample shows a segregated structure with dendrites evolving from the substrate and extending to the free surface. An example of the dendritic structure is shown in the SEM micrograph in Fig. 3. There is no obvious evidence of any other transformation product, but it is believed that microscopic solute carbides make up most of the interdendritic space. By contrast, a particle injected microstructure is a composite of carbide particulate and metal matrix. A typical cross-sectional view of the particle injected surface layer is shown in the SEM micrograph in Fig. 4. In this macrograph the WC particulate phase, which appears light, is surrounded by the Inconel alloy matrix, which appears gray. The WC is uniformly distributed throughout the injected layer and occupies about 50% of the total volume. No through channels or porosity appear in the injected
layer which is about 1.5 cm wide and 0.15 cm thick. The free surface appears rough and, to prepare it for wear application, it is made flat by a simple grinding step. A similar macrostructure formed in the TiC injected sample, although it was difficult to drive the lighter TIC into the heavier Inconel melt. None of the laser processed samples developed a heat affected zone between the fusion layer and the substrate. This is not surprising since Inconel 625 has excellent high temperature stability. The substrate microstructure consisted of small equiaxed grains with very fine carbides dotting the grain boundaries and with occasional carbide stringers within the grains formed during the working process. Close examination of the injected layer revealed two distinct microstructures in each sample shown in the SEM micrographs in Figs. 5 and 6. Conveniently, in the SEM, W-containing phases appear light, Ti-containing phases appear dark and the inconel appears gray. Fig.
Fig. 3. Example of the dendritic structure in Intone1 625 produced by laser surface mdting.
142
Fig. 4. Typical cross-sectional view of particle injected surface layer. Substrate is Inconel 625 alloy, particulates are WC.
5(a) shows products with a “Chinese-script” type morphology in the Inconel matrix (marked as A in the micrograph) and as an interphase zone surrounding the WC particulate phase (B). Also observed, but not evident in Fig. 5(a), were phases with a “lamellar” morphology arranged within the interdendritic regions of the metal matrix [18]. TEM identified the script-type product as W-rich M,C carbide and the lamellar phase as a metastable Cr-rich M,C carbide [19]. Since they appear predominantly in the inter-dendritic regions, these carbides are termed eutectic carbides. Fig. 5(b) shows “blocky”, cellular-dendritic phases randomly distributed within the metal matrix (A) and as an interphase zone surrounding the WC particulate phase (B). AES analysis identified the cellular-dendritic phase as rich in W, Ni, Cr, MO and Nb (see Fig. 6(a)) and the Inconel matrix as having only a trace of W (see Fig. 6(b)) [20]. Since the cellular-dendritic phase appears
equiaxed, has W as the major ingredient, and since WC is non-cubic, the cellular-dendritic phase is likely to be a MC type carbide. Fig. 7(a) shows a structure with a “lamellar” morphology in interdendritic regions of the Inconel matrix (A) and a smooth and featureless surface on the TiC particulate (B). TEM analysis identified the lamellar phase as MC type carbide, containing Ti [19]. Since they appear in the inter-dendritic regions, this carbide is termed eutectic carbide. Fig. 7(b) shows a network of dendritic products randomly distributed within the metal matrix (A) and a smooth and featureless surface on the TiC particulate (B), AES analysis identified the AESSURW U/F l/31/88 ARM 3 AC0iME:?, tllll, FILE:84,19~4~1 lncanel - Laser ParticleInjected SCALEFACTOR, OFFSEMl,B56, 1826,667t COUHWSEC
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any localized corrosion at that interface. In the laser melted sample modest amounts of corrosion occurred in the interdendritic regions in the solidified surface. This result is shown in the SEM micrographs in Fig. 9. The higher magnification micrograph in Fig. 9(b) clearly shows some structure in the regions between the dendrites. Prior to seawater exposure, no structure was apparent on the as-polished sample. SEM micrographs showing a variety of corrosion response to seawater of WC injected Inconel 625 samples appear in Figs. 10 and 11. Examples of corrosion behavior in the base region of the injected layer are given in Fig. 10. In this region, where the carbide dissolution rate is modest and where only eutectic carbides appear, WC particles show significant corrosion damage across most of their cross-section, while the surrounding interphase zone made up of script-type carbide (A in Fig. 10(a)), and the metal matrix (B) show none. The severity of attack in the WC particulate is clearly evident in Fig. 10(c) which is a close-up view of a carbide/matrix interface in Fig. 10(b). Within the corroded WC particulate a faceted structure appears. Examples of corrosion behavior in the top region of the
(4
(b) Fig. 7. Microstructures obtained in TiC injected Inconel 625. (a) Near the base of the injected layer, TiC particulate surrounded by dendritic Inconel alloy matrix with lamellar carbides within interdendritic regions. (b) Near the top of the injected layer, network of dendritic carbides distributed randomly within Inconel matrix.
dendritic phase as rich in Ti, Nb and MO (see Fig. 8(a)) and the Inconel matrix as having only a trace of Ti (see Fig. 8(b)) [20]. Since the dendritic phase has orthogonal symmetry, has Ti as the major element, and since TiC is cubic, the dendritic phase is probably cubic MC type carbide. While the cellular-dendritic and dendritic carbides are found primarily in the upper half of the solidified melt pool, the eutectic carbides are found throughout the injected layer. Presence of the eutectic carbides is not easily evident in Figs. 5(b) and 7(b) because dendritic carbides dominate the microstructure. In all samples the injected carbide particulate interior appears smooth and shows no alteration from the processing. Table 3 summarizes the microstructural analysis. Because of the fine-scale of the microstructure, quantitative analysis of the various phases present could not be done using standard methods such as energy dispersive spectroscopy.
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3.2. Corrosion behavior
As expected, in all three samples, there is no evidence of seawater corrosion in the homogenized Inconel 625 alloy substrate. Also, continuity of corrosion properties across the interface between the substrate and the solidified surface layer was demonstrated by the absence of
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Table 3 Summary of microstructural analysis and corrosion results Laser process
Microstructure
Corrosion observed
Melting
Ni-rich dendrites and interdendritic solute-rich structure Ni-rich dendrites, interdendritic Cr-rich lamellar carbide, and W-rich scripttype carbides in metal matrix and as interphase zone around particulate In addition to structure in lower half, W-, Ni-, Cr-, Moand Nb-rich cellular dendritic carbides in metal matrix and as interphase zone around particulate
Within interdendritic soluterich structure
WC injection (lower half of injected layer)
WC injection (upper half)
TiC injection (lower half) TiC injection (upper half)
the interdendritic
Along particulate/interphase zone interface; within interdendritic regions of the cellular-dendritic carbides; and adjacent to eutectic carbides in the interdendritic regions of the Ni-rich dendrites At eutectic carbide sites in interdendritic regions of the Ni-rich dendrites Within interdendritic regions of the dendritic carbides
Ni-rich dendrites, and Ti-rich interdendritic eutectic carbides In addition to structure in lower half, Ti-, Nb- and Morich dendritic carbides in metal matrix
injected layer are given in Fig. 11. In this region, where the carbide dissolution rate is high and where cellulardendritic carbides appear, the WC particles appear unharmed, except along their interface with the interphase zone (A in Fig. 11(a)), while the surrounding matrix shows pockets of aggressive corrosion (B). The type and extent of attack in the Inconel matrix are clearly seen in Fig. 11 (c) which is a close-up view of a carbide/matrix interface in Fig. 1 l(b). The presence of polishing marks (see Fig. 11(a)) on the WC particulate demonstrates that these remain unaffected in this region of the injected layer. The corrosion attack in the matrix phase appears to be mainly in the interdendritic regions (A in Fig. 1l(c)) of the Ni-rich dendrites, where eutectic carbides are found, as well as in the interdendritic regions (B) of the cellular-dendritic carbides. SEM micrographs showing corrosion response to seawater of TIC injected Inconel 625 samples appear in Figs. 12 and 13. An example of the corrosion behavior in the base region of the injected layer is given in Fig. 12(a). In this region, where the carbide dissolution rate is modest and where only eutectic carbides appear, the injected TiC particles are smooth and show no corrosion damage (A) while the surrounding matrix shows mild etching (B). The extent to which the Inconel matrix has been attacked is seen in Fig. 12(b), which is a close-up view of the matrix in Fig. 12(a). The corrosion appears to be at the eutectic carbide sites (A) within
Within particulate phase
regions
of the Ni-rich
den-
drites. An example of the corrosion behavior in the top region of the injected layer is given in Fig. 13(a). In this region, where the carbide dissolution rate is high and where a network of dendritic carbides appear, the TiC particles again appear smooth and unaffected (A) while the surrounding matrix shows pockets of moderate corrosion (B). The extent to which the Inconel matrix is attacked is clearly seen in Fig. 13(b) which is a close-up view of the matrix in Fig. 13(a). The corrosion attack appears to be mainly at the dendritic carbide-metal matrix interfaces (A). In all cases, the TiC particle/matrix interface appears unaffected. Table 3 summarizes the seawater corrosion results and compares them to the observed microstructures.
4. Discussion 4.1. Microst~uctwal nnnlysis Surface melting of a metal alloy sample by a laser beam or by any other directed-energy heat source results in a structure which is rapidly solidified and which usually consists of columnar grains originating from the substrate which is the heat sink. Within the grains are finely spaced dendrites. In the laser-melted sample, the dendritic structure indicates a general loss of homogeneity which existed in the as-received condition. The extent of inhomogeneity or segregation in the solidified
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structure affects the corrosion behavior. With the introduction of the particulate phase into the melt pool, not only is there segregation from solidification, but also microstructural modification due to the presence of the particulate and from particulate-melt interactions. Laser melt/particle injection processing by design forms a particulate composite surface containing unaltered particles surrounded by a metal matrix identical to the substrate in chemistry and microstructure. But some degree of microstructural and compositional modification is inevitable due to the high melt temperatures and due to interaction between the particulate phase and the liquid metal. This interaction is in the form of carbide/melt mixing brought about by temperature gradient-driven fluid flow and results in partial dissolution of the carbide particulate into the melt. The dissolution rate is higher when the particulate is finer, when the particulate volume is greater, and when the laser-metal interaction times are longer. For this work these variables were held constant. Being closer to the laser beam, the top of the melt pool is hotter and the dissolution rate is greater compared with the base of the melt pool which is away from the laser beam. The results of carbide dissolution and the extreme
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(cl Fig. 10. Micrographs showing corrosion response of WC injected Inconei 625 sample near the base of the injected layer. (a,b) Corrosive attack within the particulate phase and an unaffected matrix. (c) High magnification view of the particulate-matrix interface in (b) showing the nature and degree of corrosion in the particulate phase.
@I Fig. 9. Micrographs showing corrosion in the laser melted sample. (a) Modest amounts of corrosion in the interdendrific regions of the solidified layer. (b) Higher magnification view of (a) showing etching of the areas between the dendrites.
thermal gradients in the melt pool are the differing microstructures observed within the injected layer. Dissolution of the carbide results in enrichment of the Ni-base alloy melt with carbon and W or Ti ions depending upon the injected species. Dissolution is a surface phenomenon as evidenced by the rounding of the angular carbide particles. Lack of any obvious structure within the sectioned and polished carbide particles suggests that the interior of the particles was
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unaffected during processing. Fused or porous particles are rarely seen in the injected layer. Where the dissolution rate is modest, solidification of the carbide enriched Inconel melt begins with the formation of Ni-rich dendrites and ends with the formation of eutectic carbides in the inter-dendritic regions. In the WC injected sample, an additional interphase zone, made up
(a)
(b) Fig. 12. Micrographs showing corrosion response of Inconel 625 sample near the base of the injected layer. attack within the interdendritic regions of the dendritic matrix. (b) High magnification view of matrix in (a) nature of corrosion.
(b)
(Cl Fig. 11. Micrographs showing corrosion response of WC injected Inconel 625 sample near the top of the injected layer. (a) and (b) Corrosive attack along interface between particulate and interphase zone and within inter-cellular regions of the randomly distributed cellular-dendritic carbides. (c) High magnification view of the particulate-matrix interface in (b) showing the nature and degree of corrosion.
TiC injected (a) Corrosive Inconel alloy showing the
of script-type carbide, forms around the particulate surface towards the end of solidification. These carbides are mildly alloyed with elements from the base alloy. Where the dissolution rate is high, the dendritic carbides solidify first as randomly distributed products within the melt, followed by the formation of the Ni-rich dendrites and ending with the solidification of eutectic carbides in the interdendritic regions. In the WC injected sample, an additional cellular-dendritic interphase zone, similar in characteristics to the randomly distributed cellular-dendritic carbides, forms on the particulate surface during the early stages of solidification. The dendritic carbides are heavily alloyed with elements from the base alloy. In summary, the microstructure of carbide injected samples consists of carbide particulates surrounded by a highly modified metal matrix. While the injected WC particles exhibit extensive surface modification, the injected TiC particles exhibit only a smooth and rounded surface. This may be due to the fact that WC is less stable than TiC and dissociates and dissolves to a greater extent into the melt. As shown in Table 1, the free energy of formation ( - AF) of WC at 1500 “C is one-fifth that of Tic.
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4.2. Corrosion response
The corrosion results make it clear that the inherent seawater corrosion resistance of solutionized and homogenized Inconel 625 is compromised to different degrees following surface modification with lasers. Regardless of the type of laser surface modification, melting or particle injection, the loss of corrosion resistance appears to occur by mechanisms involving the formation of galvanic cells. Because of the extremely fine and complex nature of the solidification microconstituents, meaningful line profile analyses demonstrating elemental redistribution were not possible. Evidence for sensitization was indirect. The presence of various carbide phases, formed during solidification and rich in chromium and other alloying elements as determined by TEM and AES, demonstrated depletion of alloying elements from the adjacent metal phase. By the same reasoning, meaningful elemental analysis to identify the corrosion species after the corrosion tests was not feasible and, in any event, is beyond the scope of this work. In the absence of direct measurements of composition profiles, some of the discussion on elemental redistribu-
(4
(b) Fig. 13. Micrographs showing corrosion response of TiC injected Inconel 625 sample near the top of the injected layer. (a) Corrosive attack within the interdendritic regions of the randomly distributed dendritic carbides. (b) High magnification view of matrix in (a) showing the nature of corrosion.
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tion and the formation of pseudogalvanic cells, which follows, is conjecture. Solidification after laser melting results in partitioning out of solution of useful alloying elements, such as chromium. Chromium, the primary alloying element in Inconel 625, is added to promote resistance to environmental degradation. Its role is to passivate the external surface of the alloy by forming Cr,03 which is the primary reason for resistance to corrosion. The loss of Cr from solution would result in loss of corrosion resistance. In the laser-melted sample, the segregated microstructure means that the alloy is no longer homogeneous as it was in the as-received condition. The partitioned alloying elements combine with the residual carbon and form carbides which precipitate out in the interdendritic regions and along grain boundaries. Once useful alloying elements such as Cr and MO are tied up as carbides, the interdendritic and grain boundary regions are depleted of these elements removing their passivation protection locally. Galvanic microcells form between these carbides and the depleted region around them. Areas adjacent to these carbide precipitates, which are depleted of alloying elements, are selectively attacked. It must be added that in this sample, the extent of corrosion is modest, considering that the sample was exposed to seawater for six months. The corrosion behavior observed in particulate injected samples is complex compared to the laser-melted sample. It is also different from that observed in cemented carbides. Corrosion in cemented carbides generally occurs as surface depletion of the binder phase with the result that at the surface region only a carbide particulate skeleton remains [21]. Both WC-Ni and Tic-Ni cemented carbides are moderately resistant to seawater and are lightly attacked by it [21]. This is because of minimal alloying of the carbide with Ni. As mentioned earlier, the addition of the particulate phase using lasers results in vigorous melt/particle interaction and a much more complicated microstructure. Corrosion was observed to occur primarily in two regions for both WC and TiC injected samples. Within the interdendritic regions of the Ni-rich dendrites where soluterich eutectic carbides reside and in the metal matrix adjacent to the heavily alloyed cellular-dendritic and dendritic carbides. Depletion of alloying elements from solution would be more severe in carbide injected samples than in the simple laser melted samples because extra carbon is available as a result of the dissolution of the carbide particulate phase in the laser melt pool. This extra carbon ties up more of the alloying elements from the base alloy. Enriched with alloying elements, the eutectic and dendritic carbides form strong galvanic couples with the surrounding Inconel alloy matrix phase. Corrosion was found to occur in the solute-depleted metal adjacent to the eutectic and cellular-dendritic carbides in the WC injected sample, and adjacent
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to the dendritic carbides in the TiC injected sample. In the TiC injected sample, the eutectic MC carbide in the interdendritic regions itself corroded. It is conceivable that partition of alloying elements such as Cr out of solution did not occur during the formation of the Ti-rich lamellar carbide, and that this carbide became anodic relative to the metal around it.. Owing to the fine scale of the microstructure the composition of these phases could not be ascertained. In the WC injected sample, an additional third region showed corrosion. Corrosion occurred as a result of galvanic couples between the WC particulate and the matrix surrounding it. This corrosion behavior was unusual. In the lower half of the solidified melt pool, while the metal matrix phase and the script-type interphase zone were unaffected by corrosion, the WC particulate exhibited massive erosive type of corrosion. In the galvanic cell between the WC particulate and the interphase zone, the WC particulate became the sacrificial anode. No data is available on the position of WC and other alloyed carbides in the electrochemical series to confirm this observation. It is postulated that the script-type M& carbide is more noble than the WC particulate. Exposure to seawater of WC particles alone showed no evidence of surface corrosion, faceted or otherwise, which supports the notion that in the injected sample the WC particulate corroded via a galvanic cell mechanism. In the upper half of the solidified melt pool, corrosion also occurred at the interface between the WC particulate and the cellulardendritic interphase zone on the particle surface. Corrosion was confined to this interface, while the rest of the carbide particulate and the interphase zone were unaffected. The exact reason for this localized, crevice-type of corrosion is not known. It is surmised that, as the cellular-dendritic interphase zone nucleated and grew around the particulate, a thin film of liquid metal may have wicked behind the growth front by capillary forces. Being highly depleted of solute elements, this film formed a galvanic cell with its carbide neighbors and corroded. Due to its fine scale, a chemical analysis of the particulate/interphase zone interface could not be done, but close examination of the high magnification micrograph in Fig. 14 shows discontinuous alloy films around the particulate (A) and within the interphase zone (B). Crevice and pitting corrosion have been reported in particulate composite materials exposed to seawater. In these materials corrosion occurs because of the existence of crevices, pores and other flaws between the reinforcement and the metal matrix. Most of the reported work has been done on Sic/Al alloy composites with some on SiC/Cu and TiC/Cu alloy systems [22]. In these materials corrosion occurs in the region adjacent to the particulate phase where flaws can occur. No corrosion was observed within the car-
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Fig. 14. Micrograph showing detail of the WC particulate/interphase zone interface.
bide particulate phase itself. Most particulate composites are fabricated by the powder metallurgy route which involves sintering as the final step. The temperatures are thus not high enough to cause significant interaction between the particulate phase and the metal matrix. Hence, there are not many reaction products, such as alloyed carbides, present in the metal matrix. Since there is little change in composition and microstructure, corrosion within the metal matrix is rare. If the composites were made via a solidification route, such as melt infiltration, and if the temperatures were high enough to initiate particulate/melt reactions, then some of the corrosion results described in this work, such as interdendritic corrosion, can be expected. A qualitative comparison of the degree of corrosion observed in the carbide injected samples, reveals that corrosion occurred to a greater degree in the WC injected sample than in the TiC injected sample. Tafel test results indicated a higher corrosion rate (mpy) for the WC injected surface than for the TiC injected surface [23]. In fact, a comparison of the microstructures after seawater exposure shows mild corrosion of comparable degree in the laser melted and the TiC injected samples. There are several reasons for this observation. Many sites are available for corrosion in the WC injected layer than in the TiC injected layer. This is because WC is less stable than TiC and forms a variety of resolidification products. Secondly, AES has shown that more alloying elements are partitioned off from the Inconel 625 matrix by the resolidification byproducts in WC injected sample than in the TiC injected sample. In fact, Cr was removed from the matrix in the WC injected sample but not in the TiC injected sample. Depleted of corrosion control alloying elements, the corrosion rate is greater in the WC injected sample than in the TiC injected sample. Thirdly, WC is more conducting than TiC and would be more prone to forming galvanic couples with Incone1 625 and with the solidification by-products.
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4.3. Con‘osiorz Contl~ol
Acknowledgment
Segregated microstructures can be eliminated by subjecting the laser processed sample to a homogenization treatment. Corrosion is avoided by putting back into solution the useful alloying elements found in the segregated regions. The laser can itself be used to perform the heat treatment by suitably adjusting its parameters. Another technique to consider is depositing composite coatings by avoiding or minimizing the formation of byproducts due to solidification reactions. This can be done by laser fusing a mixture of Inconel 625 alloy powder and carbide particulate applied either as a slurry or as a lightly compacted chip to the surface. Laser power can be controlled to prevent incursions into the very high temperature regimes which can cause harmful dissolution and alloying. In extreme cases it may be necessary to coat the particulate material with an electrical insulator prior to deposition. The insulator restricts the flow of galvanic current thereby decoupling the particulate from the metal matrix.
The authors would like to thank Mr. Keith Lucas of the Key West Facility of the Naval Research Laboratory for conducting the seawater corrosion tests.
5. Conclusions Seawater corrosion studies of laser surface modified Inconel 625 alloy samples revealed several interesting results. The inherent corrosion resistance of the normally homogeneous alloy is lowered due to several microstructural changes caused by laser processing. Segregation, however fine, from resolidification of the laser melted surface is a source of corrosion. Alloying from dissolution of the particulate species during particle injection results in the formation of a variety of phases with different morphologies and compositions. Eutectic carbides in the interdendritic regions of the metal matrix, cellular-dendritic and dendritic carbides within the metal matrix, and interphase zones around the particulate are some of the solidification byproducts. Each carbide phase is a cause of depletion of useful alloying elements from the metal matrix and the formation of galvanic microcells between it and the adjacent metal. Corrosion usually occurred in the solute-depleted metal. In the case of WC, corrosion also occurred within the particulate itself. In the case of TIC, corrosion’ also occurred at the eutectic carbide sites. To minimize seawater corrosion of laser modified surfaces, the microstructures have to be homogenized and, while adding particulate material, dissolution and alloying has to be avoided.
References [l] K.P. Cooper. in P.J. Blau (ed.), AS&I Ha/ldbook. Friction, Lubricnfioiz n17d U/ear Tech/zology, Vol. 18, ASM, Materials Park, OH, 1992, pp. 861-872. PI K.P. Cooper, in G. Sepold (ed.), Laser Beam Surjhce Treating and Coarirzg, Vol 957. SPIE, Bellingham, WA, 1988, pp. 42-54. [31 J.D. Ayers, Weor, 97 (1984) 249. [41 K.P. Cooper, J. Vat. Sci Techol. A, 4 (1986) 2857. [51 D.L. Graver, in D. Benjamin and C.W. Kirkpatrick (eds.), Metals Handbook; Properties and Selection] Staiidess Steels, Tool Materials and Special Purpose Mttals, 9th edn., Vol. 3, ASM,
Metals Park, OH, 1980, pp. 171-174.
161J.R. Crum, in L.J. Korb and D.L. Olson (eds.),
Metals
Hand-
Corrosioq 9th edn., Vol. 13, ASM, Metals Park, OH, 1987. pp. 653-657. [71 J.H. Westbrook and E.R. Stover, in I.E. Campbell and E.M. Sherwood (eds.), High Temperature Materials and Techology, Wiley, New York, 1967, p. 324. PI P. Denny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994. t91 C.W. Draper and P. Mazzoldi (eds.), Laser Surface Treatment oj Metals, Martinus Nijhoff, Dordrecht, 1986. t101 E. McCafferty and P.G. Moore, Laser Surface Treatment of Metals, Martinus Nijhoff, Dordrecht, 1986, pp. 263-296. [I 11W. Reitz and J. Rawers, in J. Mazumder and K.N. Mukherjee (eds.), Laser Materials Processing 111, TMS, Warrandale, PA, 1989, pp. 21-35. VI C.W. Draper, High Ternp Materials and Processes, 6 (1984) 213. [I31 Z. Liu, M. McMahon, K.G. Watkins, W.M. Steen, R.M. Vilar and M.G.S. Ferreira, in P. Deqny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994: pp. 975-984. [14] 3. Mazumder and J. Singh, in C.W. Draper and P. Mazzoldi (eds.), Laser Surface Treatment of Metals, Martinus Nijhoff, Dordrecht, 1986, pp. 297-308. PI A. Almeida, R. Vilar, R. Li, M.G.S. Ferreira, K.G. Watkins and W.M. Steen, in P. Denny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994, pp. 903-912. [I61 B.C. Oberlander and E. Lugscheider, Materials Sci. Technol., 8 (1992) 657. u71 K.P. Cooper and J.D. Ayers, in F.D. Seaman (ed.), Lose) Surface Modification, AWS, Miami, FL, 1988, pp. 115-131. 1181K.P. Cooper and P. Slebodnick, J. Laser Appl., 1 (1989) 21. [I91 K.P. Cooper and A. Singh, NRL, unpublished results. PO1K.P. Cooper and R.A. Bayles, NRL, unpublished results. Pll H.S. Kalish; in L.J. Korb and D.L. Olson (eds.), Metals Handbook, Corrosion, 9th edn., Vol. 13, ASM, Metals Park, OH, 1987, pp. 846-858. PI D.M. Aylor, in L.J. Korb and D.L. Olson (eds.), Metals Handbook. Corrosion, 9th edn., Vol. 13, ASM, Metals Park, OH, 1987, pp. 859-863. ~31 K.P. Cooper, P. Slebodnick and E.D. Thomas, NRL, to be published. book,
A
Materials Scienceand EngineeringA206 (1996) 138-149
Seawater corrosion behavior of laser surface modified Inconel 625 alloy K.P. Cooper”*, P. Slebodnickb, E.D. Thomasb “Physical Metalhrrgy Branch, Materials bEnviromnental Effects Branch, Materials
Science and Technology Science and Technology
Division, Division,
Naval Research Laboratory, Naval Research Laboratory,
W’clshir~gtor~, DC 20375-5343, Wmhington, DC 2037.54343,
USA USA
Received 30 May 1995;in revised form 28 July 1995
Abstract Seawater corrosion behavior of laser surface processed Inconel 625 alloy was investigated. Three laser surface modified samples were prepared, one by laser meltin g, and the other two by laser melt/particle injection processing with tungsten carbide (WC) and titanium carbide (TIC) particles, respectively. Particle injection involved embedding the carbide particles into a laser melted surface and resulted in a metal matrix-particulate composite surface layer which was both hard and wear-resistant. While Inconel 625 is a corrosion resistant alloy suitable for marine applications, and WC and TiC are generally inert to chemical attack, results from this study showed that laser surface modification produced microstructures that were susceptible to seawater corrosion to varying degrees. Nominal corrosion was observed in the dendritic structure produced by laser melting of the alloy surface. In the particle injected samples, the WC particulate phase in contact with the Inconel alloy matrix showed different kinds of attack, while the TiC particulate phase showed none. In both particle injected samples, resolidification of the Inconel alloy melt produced significant departures in composition and microstructure from those of the base alloy. Eutectic and dendritic carbides and, in WC, interphase carbides were some of the resolidification byproducts that formed in the matrix surrounding the particulate. Alloyed with solute elements from the base alloy, each product phase contributed to unique forms of corrosion. A qualitative analysis of the corrosion behavior of the injected samples showed that corrosive damage was more severe in the WC injected sample than in the TiC injected sample and in the laser melted sample. This paper describes the processing, microstructural and compositional characterization, and seawater corrosion behavior of the laser surface modified samples, and attempts to explain the observations as a consequence of the formation of galvanic cells. Keywords:
Corrosion; Inconel 625
1. Introduction The use of lasers to enhance the wear resistance of metals has been well documented [I]. Laser surface modification processes make use of several hardening and strengthening mechanisms, such as transformation hardening, solid solution strengthening, precipitation hardening, and dispersoid strengthening. Depending upon the type of alloy (hardenable, precipitation, etc.) and the laser technique (heat treating, melting, alloying, cladding), one or more of the above mechanisms can contribute to improved wear resistance. The same microstructural modifications that improve wear resistance can affect corrosion behavior which this paper attempts to explore. * Corresponding author.
Laser melt/particle injection processing involves the formation of hard metal matrix-carbide conlposite surface layers [2]. Typically, the modified layer contains about 50%-60% by volume of carbide particulate. To retain continuity of properties such as corrosion resistance across the interface and a strong metallurgical bond throughout to prevent delamination, the sur-
rounding
matrix is desirably of the same composition
and microstructure
as the base alloy. Wear and friction
studies [3,4] on these samples have demonstrated 2-5 times improvement in wear resistance. These studies concluded that particulate reinforcement was the predominant mechanism in enhancing hardness and wear resistance.
The main thrust in studying laser melt/particle injection processing of Inconel 625 alloy was to improve the Published by Elsevier ScienceS.A. SSDI 0921-5093(95)10013-x
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Table 1 Properties of carbide particulates and Inconel 625 Material
WC
TiC
Inconel 625
Composition (wt.‘%) C) Melting pt. (“C) Density (g cm -‘) --AF(1500”C) (kcal mol-‘) Microhardness (kg mm - ‘) Crystal structure Stability temperature (“C) Electrical resistivity (~0 cm)
6.13 2720 15.63 8.4 1780 Hexagonal 2600 17
IO-20 3147 4.93 39.5 2988 FCC 3140 60-250
0.1 1427 8.44 295 FCC 0.0129
wear resistance of a relatively soft alloy without sacrificing its useful properties. Inconel 625 is a high strength, corrosion resistant superalloy widely used in the chemical industry for its resistance to a variety of aqueous corrodents [5]. The presence of MO makes it resistant to pitting [5]. It is extensively used in the marine environment, especially by the United States Navy. Crum [6] reports for Inconel 625 the highest pitting resistance in stagnant seawater and hot seawater applications such as heat exchangers. This research work was undertaken to study the corrosion behavior of carbide particle injected Inconel 625 alloy samples in particular, and to extend the understanding to metal matrix-particulate composite surfaces in general. Carbide particulates studied were WC and TIC because of their very high hardness and wear resistance. Unlike Sic, which dissociates easily at high temperatures, WC and TIC are quite stable at the temperatures encountered in the laser melt pool. However, compared with Sic, which has an extremely high electrical resistivity of 2 x lo6 1tQ cm - ‘, WC and TiC are fairly conductive [7]. This would give some importance to the possibility of the formation of galvanic cells between the carbide particulate and the surrounding Inconel matrix. Electrical resistivity and other relevant properties of WC, TIC and Inconel 625 are compared in Table 1. While their melting points are not far apart, there are some differences in the characteristics of WC and TiC which may influence their response to laser processing, microstructural development, wear and corrosion. While WC is a line compound, TiC has a wide solubility range, which may mean the absence or presence of compositional gradients within the carbide phase which can affect corrosion behavior. Their densities differ, especially with respect to Inconel 625, which may affect particle injection processing. Their crystal structures differ, which may influence microstructural morphology and hence, wear and corrosion. Their free energies of formation ( - AF) differ, which may influence their high temperature stability and hence, microstructural development, wear and corrosion behavior. Their microhardness values differ, which may affect the wear behavior.
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Corrosion of composite surfaces becomes important because galvanic coupling is inevitable owing to the presence of two dissimilar materials. 111 addition, traditional techniques of corrosion protection using inhibitive coatings are not feasible in a wear type of application. A cursory glance at the recent literature on laser surface processing [8] shows increased interest in depositing composite coatings containing various volume fractions of hard particulates. It is hoped that the observations and analysis reported in this paper will help in developing sound particulate-containing coatings, whether laser-deposited or deposited by other directed-energy beam sources. Most of the reported research work on the applications of laser surface modification has focused on enhancing the wear resistance of metals, and to a lesser extent, on enhancing the corrosion resistance [9]. Very few studies have involved characterizing the modified surfaces for both wear and corrosion resistance, and none, to our knowledge, has involved laser deposited metal matrix composite surface layers. Hardfacing alloys that improve wear resistance, such as Tribaloy and Stellites, are usually applied using thermal spray techniques. If porosity or channels are not avoided during deposition, or if dilution with the substrate occurs, then these coatings can become susceptible to various forms of corrosion. This is not necessarily detrimental because, historically, most wear applications have not involved reactive environments. But, with the rapid advancement in technology, the need to fabricate components that perform successfully in both high wear and corrosive environments has arisen. The laser deposited metal matrix-particulate composites, discussed herein, were an attempt to meet this very technological need. A review of the literature on corrosion behavior of laser modified samples showed that most of the work involved developing various laser surface modification techniques and discussing possible mechanisms for improved corrosion resistance in specific alloy systems. Corrosion resistance is improved either by rapidly melting the surface and redistributing the constituents, or by alloying the surface with suitable alloying elements, or by cladding the surface with a corrosion resistant alloy. McCafferty and Moore [lo] demonstrated that laser melting removed or redistributed sulfur inclusions and redistributed Cr in Cr-depleted grain boundaries in stainless steels; it homogenized aluminum bronze; and it removed residual porosity in plasma-sprayed coatings. Reitz and Rawers [ll] laser melted zirconium alloys and found a ten- to IOO-fold improvement in corrosion resistance with the corrosion rate depending upon the type of microstructural transformation. In laser melted Fe-aluminum bronzes, Draper [12] reported stifling of anodic dissolution due to removal of local galvanic action and due to the formation of a
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Table 2 Nominal composition of Inconel 625 (wt.%) Ni
Cr
MO
Fe
Nb
Si
C
62.0
21.5
9.0
4.0
4.0
0.5
0.05
passive, homogeneous film. Liu et al. [13] reported enhancing pitting resistance in laser melted 2014 aluminum alloy exposed to 1 M NaCl. Mazumder and Singh [14] laser alloyed 1016 steel with a mixture of Cr and Ni and reported spontaneous passivation and corrosion resistance approaching that of 304 stainless steel. Occasionally, the act of improving corrosion resistance can also result in an improvement in hardness and wear resistance or vice versa. Almeida et al [15] laser alloyed aluminum alloys with Cr and found an improvement in pitting corrosion resistance as well as a 2-3 times increase in surface hardness. In their study on comparing the properties of coatings produced by laser cladding and conventional methods, Oberlander and Lugscheider [16] report achieving high wear and corrosion resistance in mild steel laser clad with NiCr-Nb-B and Ni-Cr-Ta-B alloy powders. Most of the corrosion studies have concentrated on reporting the results of the response of the samples to various corrosion tests. Very few studies have attempted an analysis of the changes in the microstructure, if any, caused by exposure to the reactive environment. This paper discusses the various kinds of corrosive attack observed in the microstructure after exposure to seawater flow and the extent to which they occur.
2. Experimental 2.1. Laser processing und metnllograplzy
Laser melting involves melting a metal surface by irradiating it with a laser beam. Subsequent solidification, which is rapid, produces a fine-scale microstructure which can have unique properties. Laser melt/particle injection processing involves injecting particulate material into the melt pool formed by a laser beam. For wear resistance, the particles are usually high melting point carbides which do not fuse but which mix intimately with the molten base alloy. Upon solidification, the carbide/melt mixture forms an in situ metal matrix/carbide particulate composite surface layer. Wide tracks of such layers are produced by translating the sample below an oscillating laser beam. Details of beam characteristics, particle feeding, and processing conditions are given elsewhere [2,17]. A high power (15 kW), continuous wave, CO, laser was used in this study. The substrate was hot-rolled and annealed Inconel 625 alloy of nominal composition given in
Table 2 obtained from Into Alloys, Bloomington, DE and the particulates were 45 to 150 [ml WC and TiC of 99.5% and 98.5% purity, respectively, obtained from Consolidated Astronautics, Smithtown, NY. The laser processed samples were sectioned and polished with diamond paste to a 1 Ann finish. To reveal the microstructure, the samples were chemically etched in a mixture of 80 ml HCl, 13 ml HF and 7 ml HNO,. This etchant highlights dendritic and grain boundary segregation in some Ni-based superalloys, The etched samples were examined in the scanning electron microscope (SEM) and evaluated for dendritic segregation, morphology and distribution of the injected carbide, metal matrix microstructure, and microstructural gradients within the injected layer. For comparison, the base metal microstructure was also determined. Crystal structures and compositions of the resolidification byproducts were determined by transmission electron microscopy (TEM) and composition variations among the resolidification products by Auger electron spectroscopy (AES). 2.2. Corrosion studies
Seawater exposure tests were performed on all three samples simultaneously. The samples were sectioned across the laser treated surface and these cross-sections were polished with diamond paste to a 1 Fuji finish. All three polished samples were thoroughly cleaned in ethanol and suspended in test cells in a flow-through trough shown in Fig. 1. Natural, unfiltered seawater from the Naval Research Laboratory, Marine Corrosion Facility, Key West, FL seawater system was used throughout the test duration. Seawater parameters were: temperature 19-32 “C; salinity 35-39 g kg- ‘; pH 7.8-8.3; dissolved O2 4.2-6.9 mg 1-l; and resistivity 15.4-20.2 .Q cm. Seawater parametric trends for a typical year are given in Fig. 2. Except for the variation
Fig. 1. Seawatercorrosiontest apparatusshowing samples suspended in a flow-through trough.
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salinity 40.0
35.0
30.0
2 2
25.0
3
2
20.0
0
15.0 3
PH 10.0
5.0
‘k “.‘.“‘.‘,(...t.._... ..._..... .. .._.........._._.._._.._,, .,,.,,.,_,__,__,,,_,,_,,_,_,,,,, . _,, \-\, “.,i;i‘-,““,-“-,““..,.~,-,‘~,,““.~ -._. ___,I__.,\ _,_._,,. \,\,,>_\ ,_,, \,<.\\,:._,A,,p,-~+~ir.~
_...... . .,. ,_,,-\,,*,* ._,., -,,---*.
dissolved 02 0.0
Fig. 2. Plot showing seawater parametric trends for a typical calendar year.
in temperature, which is seasonal, all other parameters are fairly constant. All test cells had once-through seawater flow which was continually refreshed to maintain seawater parameters. After six months of exposure all samples were removed from the trough and ultrasonically cleaned of any adhering sludge with distilled water followed by cleaning with ethanol. The ascleaned samples were examined in the SEM and the cross-section microstructures were evaluated for the type and extent of corrosion.
3. Results 3.1. Micuostructul~e
The laser surface melted sample shows a segregated structure with dendrites evolving from the substrate and extending to the free surface. An example of the dendritic structure is shown in the SEM micrograph in Fig. 3. There is no obvious evidence of any other transformation product, but it is believed that microscopic solute carbides make up most of the interdendritic space. By contrast, a particle injected microstructure is a composite of carbide particulate and metal matrix. A typical cross-sectional view of the particle injected surface layer is shown in the SEM micrograph in Fig. 4. In this macrograph the WC particulate phase, which appears light, is surrounded by the Inconel alloy matrix, which appears gray. The WC is uniformly distributed throughout the injected layer and occupies about 50% of the total volume. No through channels or porosity appear in the injected
layer which is about 1.5 cm wide and 0.15 cm thick. The free surface appears rough and, to prepare it for wear application, it is made flat by a simple grinding step. A similar macrostructure formed in the TiC injected sample, although it was difficult to drive the lighter TIC into the heavier Inconel melt. None of the laser processed samples developed a heat affected zone between the fusion layer and the substrate. This is not surprising since Inconel 625 has excellent high temperature stability. The substrate microstructure consisted of small equiaxed grains with very fine carbides dotting the grain boundaries and with occasional carbide stringers within the grains formed during the working process. Close examination of the injected layer revealed two distinct microstructures in each sample shown in the SEM micrographs in Figs. 5 and 6. Conveniently, in the SEM, W-containing phases appear light, Ti-containing phases appear dark and the inconel appears gray. Fig.
Fig. 3. Example of the dendritic structure in Intone1 625 produced by laser surface mdting.
142
Fig. 4. Typical cross-sectional view of particle injected surface layer. Substrate is Inconel 625 alloy, particulates are WC.
5(a) shows products with a “Chinese-script” type morphology in the Inconel matrix (marked as A in the micrograph) and as an interphase zone surrounding the WC particulate phase (B). Also observed, but not evident in Fig. 5(a), were phases with a “lamellar” morphology arranged within the interdendritic regions of the metal matrix [18]. TEM identified the script-type product as W-rich M,C carbide and the lamellar phase as a metastable Cr-rich M,C carbide [19]. Since they appear predominantly in the inter-dendritic regions, these carbides are termed eutectic carbides. Fig. 5(b) shows “blocky”, cellular-dendritic phases randomly distributed within the metal matrix (A) and as an interphase zone surrounding the WC particulate phase (B). AES analysis identified the cellular-dendritic phase as rich in W, Ni, Cr, MO and Nb (see Fig. 6(a)) and the Inconel matrix as having only a trace of W (see Fig. 6(b)) [20]. Since the cellular-dendritic phase appears
equiaxed, has W as the major ingredient, and since WC is non-cubic, the cellular-dendritic phase is likely to be a MC type carbide. Fig. 7(a) shows a structure with a “lamellar” morphology in interdendritic regions of the Inconel matrix (A) and a smooth and featureless surface on the TiC particulate (B). TEM analysis identified the lamellar phase as MC type carbide, containing Ti [19]. Since they appear in the inter-dendritic regions, this carbide is termed eutectic carbide. Fig. 7(b) shows a network of dendritic products randomly distributed within the metal matrix (A) and a smooth and featureless surface on the TiC particulate (B), AES analysis identified the AESSURW U/F l/31/88 ARM 3 AC0iME:?, tllll, FILE:84,19~4~1 lncanel - Laser ParticleInjected SCALEFACTOR, OFFSEMl,B56, 1826,667t COUHWSEC
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(b) Fig. 5. Microstructures obtained in WC injected Inconel 625. (a) Near the base of the injected layer, WC particulate surrounded by dendritic Inconel alloy matrix with “script-type” carbides within matrix phase and around particulate phase. (b) Near the top of the injected layer, “blocky” cellular-dendritic carbides distributed randomly within Incone1 matrix and as an interphase zone around particulate surface.
B
(b)
,
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,
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Fig. 6. AES analysis of phases in WC injected Inconcl 625. (a) Spectrum showing randomly distributed and interphase cellular-dendritic carbides as rich in W, Ni, Cr, MO and Nb. (b) Spectrum showing Inconel alloy matrix as having a trace of W.
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any localized corrosion at that interface. In the laser melted sample modest amounts of corrosion occurred in the interdendritic regions in the solidified surface. This result is shown in the SEM micrographs in Fig. 9. The higher magnification micrograph in Fig. 9(b) clearly shows some structure in the regions between the dendrites. Prior to seawater exposure, no structure was apparent on the as-polished sample. SEM micrographs showing a variety of corrosion response to seawater of WC injected Inconel 625 samples appear in Figs. 10 and 11. Examples of corrosion behavior in the base region of the injected layer are given in Fig. 10. In this region, where the carbide dissolution rate is modest and where only eutectic carbides appear, WC particles show significant corrosion damage across most of their cross-section, while the surrounding interphase zone made up of script-type carbide (A in Fig. 10(a)), and the metal matrix (B) show none. The severity of attack in the WC particulate is clearly evident in Fig. 10(c) which is a close-up view of a carbide/matrix interface in Fig. 10(b). Within the corroded WC particulate a faceted structure appears. Examples of corrosion behavior in the top region of the
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(b) Fig. 7. Microstructures obtained in TiC injected Inconel 625. (a) Near the base of the injected layer, TiC particulate surrounded by dendritic Inconel alloy matrix with lamellar carbides within interdendritic regions. (b) Near the top of the injected layer, network of dendritic carbides distributed randomly within Inconel matrix.
dendritic phase as rich in Ti, Nb and MO (see Fig. 8(a)) and the Inconel matrix as having only a trace of Ti (see Fig. 8(b)) [20]. Since the dendritic phase has orthogonal symmetry, has Ti as the major element, and since TiC is cubic, the dendritic phase is probably cubic MC type carbide. While the cellular-dendritic and dendritic carbides are found primarily in the upper half of the solidified melt pool, the eutectic carbides are found throughout the injected layer. Presence of the eutectic carbides is not easily evident in Figs. 5(b) and 7(b) because dendritic carbides dominate the microstructure. In all samples the injected carbide particulate interior appears smooth and shows no alteration from the processing. Table 3 summarizes the microstructural analysis. Because of the fine-scale of the microstructure, quantitative analysis of the various phases present could not be done using standard methods such as energy dispersive spectroscopy.
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AESSURUEY U/F l/30/88 AREA3 AC0TW2,72 llIH, FILE: 85,4,5,4 lnconel - Laser Particle injected SCALEFACTOhOFFSET=l37,9& 0,000 i: COUWSEC -I ’
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3.2. Corrosion behavior
As expected, in all three samples, there is no evidence of seawater corrosion in the homogenized Inconel 625 alloy substrate. Also, continuity of corrosion properties across the interface between the substrate and the solidified surface layer was demonstrated by the absence of
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Fig. 8. AES analysis of phases in TiC injected Inconel 625. (a) Spectrum showing dendritic carbides as rich in Ti, Nb and MO. (b) Spectrum showing Inconel alloy matrix as having a trace of Ti.
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Table 3 Summary of microstructural analysis and corrosion results Laser process
Microstructure
Corrosion observed
Melting
Ni-rich dendrites and interdendritic solute-rich structure Ni-rich dendrites, interdendritic Cr-rich lamellar carbide, and W-rich scripttype carbides in metal matrix and as interphase zone around particulate In addition to structure in lower half, W-, Ni-, Cr-, Moand Nb-rich cellular dendritic carbides in metal matrix and as interphase zone around particulate
Within interdendritic soluterich structure
WC injection (lower half of injected layer)
WC injection (upper half)
TiC injection (lower half) TiC injection (upper half)
the interdendritic
Along particulate/interphase zone interface; within interdendritic regions of the cellular-dendritic carbides; and adjacent to eutectic carbides in the interdendritic regions of the Ni-rich dendrites At eutectic carbide sites in interdendritic regions of the Ni-rich dendrites Within interdendritic regions of the dendritic carbides
Ni-rich dendrites, and Ti-rich interdendritic eutectic carbides In addition to structure in lower half, Ti-, Nb- and Morich dendritic carbides in metal matrix
injected layer are given in Fig. 11. In this region, where the carbide dissolution rate is high and where cellulardendritic carbides appear, the WC particles appear unharmed, except along their interface with the interphase zone (A in Fig. 11(a)), while the surrounding matrix shows pockets of aggressive corrosion (B). The type and extent of attack in the Inconel matrix are clearly seen in Fig. 11 (c) which is a close-up view of a carbide/matrix interface in Fig. 1 l(b). The presence of polishing marks (see Fig. 11(a)) on the WC particulate demonstrates that these remain unaffected in this region of the injected layer. The corrosion attack in the matrix phase appears to be mainly in the interdendritic regions (A in Fig. 1l(c)) of the Ni-rich dendrites, where eutectic carbides are found, as well as in the interdendritic regions (B) of the cellular-dendritic carbides. SEM micrographs showing corrosion response to seawater of TIC injected Inconel 625 samples appear in Figs. 12 and 13. An example of the corrosion behavior in the base region of the injected layer is given in Fig. 12(a). In this region, where the carbide dissolution rate is modest and where only eutectic carbides appear, the injected TiC particles are smooth and show no corrosion damage (A) while the surrounding matrix shows mild etching (B). The extent to which the Inconel matrix has been attacked is seen in Fig. 12(b), which is a close-up view of the matrix in Fig. 12(a). The corrosion appears to be at the eutectic carbide sites (A) within
Within particulate phase
regions
of the Ni-rich
den-
drites. An example of the corrosion behavior in the top region of the injected layer is given in Fig. 13(a). In this region, where the carbide dissolution rate is high and where a network of dendritic carbides appear, the TiC particles again appear smooth and unaffected (A) while the surrounding matrix shows pockets of moderate corrosion (B). The extent to which the Inconel matrix is attacked is clearly seen in Fig. 13(b) which is a close-up view of the matrix in Fig. 13(a). The corrosion attack appears to be mainly at the dendritic carbide-metal matrix interfaces (A). In all cases, the TiC particle/matrix interface appears unaffected. Table 3 summarizes the seawater corrosion results and compares them to the observed microstructures.
4. Discussion 4.1. Microst~uctwal nnnlysis Surface melting of a metal alloy sample by a laser beam or by any other directed-energy heat source results in a structure which is rapidly solidified and which usually consists of columnar grains originating from the substrate which is the heat sink. Within the grains are finely spaced dendrites. In the laser-melted sample, the dendritic structure indicates a general loss of homogeneity which existed in the as-received condition. The extent of inhomogeneity or segregation in the solidified
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structure affects the corrosion behavior. With the introduction of the particulate phase into the melt pool, not only is there segregation from solidification, but also microstructural modification due to the presence of the particulate and from particulate-melt interactions. Laser melt/particle injection processing by design forms a particulate composite surface containing unaltered particles surrounded by a metal matrix identical to the substrate in chemistry and microstructure. But some degree of microstructural and compositional modification is inevitable due to the high melt temperatures and due to interaction between the particulate phase and the liquid metal. This interaction is in the form of carbide/melt mixing brought about by temperature gradient-driven fluid flow and results in partial dissolution of the carbide particulate into the melt. The dissolution rate is higher when the particulate is finer, when the particulate volume is greater, and when the laser-metal interaction times are longer. For this work these variables were held constant. Being closer to the laser beam, the top of the melt pool is hotter and the dissolution rate is greater compared with the base of the melt pool which is away from the laser beam. The results of carbide dissolution and the extreme
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04
(b)
(cl Fig. 10. Micrographs showing corrosion response of WC injected Inconei 625 sample near the base of the injected layer. (a,b) Corrosive attack within the particulate phase and an unaffected matrix. (c) High magnification view of the particulate-matrix interface in (b) showing the nature and degree of corrosion in the particulate phase.
@I Fig. 9. Micrographs showing corrosion in the laser melted sample. (a) Modest amounts of corrosion in the interdendrific regions of the solidified layer. (b) Higher magnification view of (a) showing etching of the areas between the dendrites.
thermal gradients in the melt pool are the differing microstructures observed within the injected layer. Dissolution of the carbide results in enrichment of the Ni-base alloy melt with carbon and W or Ti ions depending upon the injected species. Dissolution is a surface phenomenon as evidenced by the rounding of the angular carbide particles. Lack of any obvious structure within the sectioned and polished carbide particles suggests that the interior of the particles was
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unaffected during processing. Fused or porous particles are rarely seen in the injected layer. Where the dissolution rate is modest, solidification of the carbide enriched Inconel melt begins with the formation of Ni-rich dendrites and ends with the formation of eutectic carbides in the inter-dendritic regions. In the WC injected sample, an additional interphase zone, made up
(a)
(b) Fig. 12. Micrographs showing corrosion response of Inconel 625 sample near the base of the injected layer. attack within the interdendritic regions of the dendritic matrix. (b) High magnification view of matrix in (a) nature of corrosion.
(b)
(Cl Fig. 11. Micrographs showing corrosion response of WC injected Inconel 625 sample near the top of the injected layer. (a) and (b) Corrosive attack along interface between particulate and interphase zone and within inter-cellular regions of the randomly distributed cellular-dendritic carbides. (c) High magnification view of the particulate-matrix interface in (b) showing the nature and degree of corrosion.
TiC injected (a) Corrosive Inconel alloy showing the
of script-type carbide, forms around the particulate surface towards the end of solidification. These carbides are mildly alloyed with elements from the base alloy. Where the dissolution rate is high, the dendritic carbides solidify first as randomly distributed products within the melt, followed by the formation of the Ni-rich dendrites and ending with the solidification of eutectic carbides in the interdendritic regions. In the WC injected sample, an additional cellular-dendritic interphase zone, similar in characteristics to the randomly distributed cellular-dendritic carbides, forms on the particulate surface during the early stages of solidification. The dendritic carbides are heavily alloyed with elements from the base alloy. In summary, the microstructure of carbide injected samples consists of carbide particulates surrounded by a highly modified metal matrix. While the injected WC particles exhibit extensive surface modification, the injected TiC particles exhibit only a smooth and rounded surface. This may be due to the fact that WC is less stable than TiC and dissociates and dissolves to a greater extent into the melt. As shown in Table 1, the free energy of formation ( - AF) of WC at 1500 “C is one-fifth that of Tic.
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4.2. Corrosion response
The corrosion results make it clear that the inherent seawater corrosion resistance of solutionized and homogenized Inconel 625 is compromised to different degrees following surface modification with lasers. Regardless of the type of laser surface modification, melting or particle injection, the loss of corrosion resistance appears to occur by mechanisms involving the formation of galvanic cells. Because of the extremely fine and complex nature of the solidification microconstituents, meaningful line profile analyses demonstrating elemental redistribution were not possible. Evidence for sensitization was indirect. The presence of various carbide phases, formed during solidification and rich in chromium and other alloying elements as determined by TEM and AES, demonstrated depletion of alloying elements from the adjacent metal phase. By the same reasoning, meaningful elemental analysis to identify the corrosion species after the corrosion tests was not feasible and, in any event, is beyond the scope of this work. In the absence of direct measurements of composition profiles, some of the discussion on elemental redistribu-
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(b) Fig. 13. Micrographs showing corrosion response of TiC injected Inconel 625 sample near the top of the injected layer. (a) Corrosive attack within the interdendritic regions of the randomly distributed dendritic carbides. (b) High magnification view of matrix in (a) showing the nature of corrosion.
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tion and the formation of pseudogalvanic cells, which follows, is conjecture. Solidification after laser melting results in partitioning out of solution of useful alloying elements, such as chromium. Chromium, the primary alloying element in Inconel 625, is added to promote resistance to environmental degradation. Its role is to passivate the external surface of the alloy by forming Cr,03 which is the primary reason for resistance to corrosion. The loss of Cr from solution would result in loss of corrosion resistance. In the laser-melted sample, the segregated microstructure means that the alloy is no longer homogeneous as it was in the as-received condition. The partitioned alloying elements combine with the residual carbon and form carbides which precipitate out in the interdendritic regions and along grain boundaries. Once useful alloying elements such as Cr and MO are tied up as carbides, the interdendritic and grain boundary regions are depleted of these elements removing their passivation protection locally. Galvanic microcells form between these carbides and the depleted region around them. Areas adjacent to these carbide precipitates, which are depleted of alloying elements, are selectively attacked. It must be added that in this sample, the extent of corrosion is modest, considering that the sample was exposed to seawater for six months. The corrosion behavior observed in particulate injected samples is complex compared to the laser-melted sample. It is also different from that observed in cemented carbides. Corrosion in cemented carbides generally occurs as surface depletion of the binder phase with the result that at the surface region only a carbide particulate skeleton remains [21]. Both WC-Ni and Tic-Ni cemented carbides are moderately resistant to seawater and are lightly attacked by it [21]. This is because of minimal alloying of the carbide with Ni. As mentioned earlier, the addition of the particulate phase using lasers results in vigorous melt/particle interaction and a much more complicated microstructure. Corrosion was observed to occur primarily in two regions for both WC and TiC injected samples. Within the interdendritic regions of the Ni-rich dendrites where soluterich eutectic carbides reside and in the metal matrix adjacent to the heavily alloyed cellular-dendritic and dendritic carbides. Depletion of alloying elements from solution would be more severe in carbide injected samples than in the simple laser melted samples because extra carbon is available as a result of the dissolution of the carbide particulate phase in the laser melt pool. This extra carbon ties up more of the alloying elements from the base alloy. Enriched with alloying elements, the eutectic and dendritic carbides form strong galvanic couples with the surrounding Inconel alloy matrix phase. Corrosion was found to occur in the solute-depleted metal adjacent to the eutectic and cellular-dendritic carbides in the WC injected sample, and adjacent
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to the dendritic carbides in the TiC injected sample. In the TiC injected sample, the eutectic MC carbide in the interdendritic regions itself corroded. It is conceivable that partition of alloying elements such as Cr out of solution did not occur during the formation of the Ti-rich lamellar carbide, and that this carbide became anodic relative to the metal around it.. Owing to the fine scale of the microstructure the composition of these phases could not be ascertained. In the WC injected sample, an additional third region showed corrosion. Corrosion occurred as a result of galvanic couples between the WC particulate and the matrix surrounding it. This corrosion behavior was unusual. In the lower half of the solidified melt pool, while the metal matrix phase and the script-type interphase zone were unaffected by corrosion, the WC particulate exhibited massive erosive type of corrosion. In the galvanic cell between the WC particulate and the interphase zone, the WC particulate became the sacrificial anode. No data is available on the position of WC and other alloyed carbides in the electrochemical series to confirm this observation. It is postulated that the script-type M& carbide is more noble than the WC particulate. Exposure to seawater of WC particles alone showed no evidence of surface corrosion, faceted or otherwise, which supports the notion that in the injected sample the WC particulate corroded via a galvanic cell mechanism. In the upper half of the solidified melt pool, corrosion also occurred at the interface between the WC particulate and the cellulardendritic interphase zone on the particle surface. Corrosion was confined to this interface, while the rest of the carbide particulate and the interphase zone were unaffected. The exact reason for this localized, crevice-type of corrosion is not known. It is surmised that, as the cellular-dendritic interphase zone nucleated and grew around the particulate, a thin film of liquid metal may have wicked behind the growth front by capillary forces. Being highly depleted of solute elements, this film formed a galvanic cell with its carbide neighbors and corroded. Due to its fine scale, a chemical analysis of the particulate/interphase zone interface could not be done, but close examination of the high magnification micrograph in Fig. 14 shows discontinuous alloy films around the particulate (A) and within the interphase zone (B). Crevice and pitting corrosion have been reported in particulate composite materials exposed to seawater. In these materials corrosion occurs because of the existence of crevices, pores and other flaws between the reinforcement and the metal matrix. Most of the reported work has been done on Sic/Al alloy composites with some on SiC/Cu and TiC/Cu alloy systems [22]. In these materials corrosion occurs in the region adjacent to the particulate phase where flaws can occur. No corrosion was observed within the car-
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Fig. 14. Micrograph showing detail of the WC particulate/interphase zone interface.
bide particulate phase itself. Most particulate composites are fabricated by the powder metallurgy route which involves sintering as the final step. The temperatures are thus not high enough to cause significant interaction between the particulate phase and the metal matrix. Hence, there are not many reaction products, such as alloyed carbides, present in the metal matrix. Since there is little change in composition and microstructure, corrosion within the metal matrix is rare. If the composites were made via a solidification route, such as melt infiltration, and if the temperatures were high enough to initiate particulate/melt reactions, then some of the corrosion results described in this work, such as interdendritic corrosion, can be expected. A qualitative comparison of the degree of corrosion observed in the carbide injected samples, reveals that corrosion occurred to a greater degree in the WC injected sample than in the TiC injected sample. Tafel test results indicated a higher corrosion rate (mpy) for the WC injected surface than for the TiC injected surface [23]. In fact, a comparison of the microstructures after seawater exposure shows mild corrosion of comparable degree in the laser melted and the TiC injected samples. There are several reasons for this observation. Many sites are available for corrosion in the WC injected layer than in the TiC injected layer. This is because WC is less stable than TiC and forms a variety of resolidification products. Secondly, AES has shown that more alloying elements are partitioned off from the Inconel 625 matrix by the resolidification byproducts in WC injected sample than in the TiC injected sample. In fact, Cr was removed from the matrix in the WC injected sample but not in the TiC injected sample. Depleted of corrosion control alloying elements, the corrosion rate is greater in the WC injected sample than in the TiC injected sample. Thirdly, WC is more conducting than TiC and would be more prone to forming galvanic couples with Incone1 625 and with the solidification by-products.
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4.3. Con‘osiorz Contl~ol
Acknowledgment
Segregated microstructures can be eliminated by subjecting the laser processed sample to a homogenization treatment. Corrosion is avoided by putting back into solution the useful alloying elements found in the segregated regions. The laser can itself be used to perform the heat treatment by suitably adjusting its parameters. Another technique to consider is depositing composite coatings by avoiding or minimizing the formation of byproducts due to solidification reactions. This can be done by laser fusing a mixture of Inconel 625 alloy powder and carbide particulate applied either as a slurry or as a lightly compacted chip to the surface. Laser power can be controlled to prevent incursions into the very high temperature regimes which can cause harmful dissolution and alloying. In extreme cases it may be necessary to coat the particulate material with an electrical insulator prior to deposition. The insulator restricts the flow of galvanic current thereby decoupling the particulate from the metal matrix.
The authors would like to thank Mr. Keith Lucas of the Key West Facility of the Naval Research Laboratory for conducting the seawater corrosion tests.
5. Conclusions Seawater corrosion studies of laser surface modified Inconel 625 alloy samples revealed several interesting results. The inherent corrosion resistance of the normally homogeneous alloy is lowered due to several microstructural changes caused by laser processing. Segregation, however fine, from resolidification of the laser melted surface is a source of corrosion. Alloying from dissolution of the particulate species during particle injection results in the formation of a variety of phases with different morphologies and compositions. Eutectic carbides in the interdendritic regions of the metal matrix, cellular-dendritic and dendritic carbides within the metal matrix, and interphase zones around the particulate are some of the solidification byproducts. Each carbide phase is a cause of depletion of useful alloying elements from the metal matrix and the formation of galvanic microcells between it and the adjacent metal. Corrosion usually occurred in the solute-depleted metal. In the case of WC, corrosion also occurred within the particulate itself. In the case of TIC, corrosion’ also occurred at the eutectic carbide sites. To minimize seawater corrosion of laser modified surfaces, the microstructures have to be homogenized and, while adding particulate material, dissolution and alloying has to be avoided.
References [l] K.P. Cooper. in P.J. Blau (ed.), AS&I Ha/ldbook. Friction, Lubricnfioiz n17d U/ear Tech/zology, Vol. 18, ASM, Materials Park, OH, 1992, pp. 861-872. PI K.P. Cooper, in G. Sepold (ed.), Laser Beam Surjhce Treating and Coarirzg, Vol 957. SPIE, Bellingham, WA, 1988, pp. 42-54. [31 J.D. Ayers, Weor, 97 (1984) 249. [41 K.P. Cooper, J. Vat. Sci Techol. A, 4 (1986) 2857. [51 D.L. Graver, in D. Benjamin and C.W. Kirkpatrick (eds.), Metals Handbook; Properties and Selection] Staiidess Steels, Tool Materials and Special Purpose Mttals, 9th edn., Vol. 3, ASM,
Metals Park, OH, 1980, pp. 171-174.
161J.R. Crum, in L.J. Korb and D.L. Olson (eds.),
Metals
Hand-
Corrosioq 9th edn., Vol. 13, ASM, Metals Park, OH, 1987. pp. 653-657. [71 J.H. Westbrook and E.R. Stover, in I.E. Campbell and E.M. Sherwood (eds.), High Temperature Materials and Techology, Wiley, New York, 1967, p. 324. PI P. Denny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994. t91 C.W. Draper and P. Mazzoldi (eds.), Laser Surface Treatment oj Metals, Martinus Nijhoff, Dordrecht, 1986. t101 E. McCafferty and P.G. Moore, Laser Surface Treatment of Metals, Martinus Nijhoff, Dordrecht, 1986, pp. 263-296. [I 11W. Reitz and J. Rawers, in J. Mazumder and K.N. Mukherjee (eds.), Laser Materials Processing 111, TMS, Warrandale, PA, 1989, pp. 21-35. VI C.W. Draper, High Ternp Materials and Processes, 6 (1984) 213. [I31 Z. Liu, M. McMahon, K.G. Watkins, W.M. Steen, R.M. Vilar and M.G.S. Ferreira, in P. Deqny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994: pp. 975-984. [14] 3. Mazumder and J. Singh, in C.W. Draper and P. Mazzoldi (eds.), Laser Surface Treatment of Metals, Martinus Nijhoff, Dordrecht, 1986, pp. 297-308. PI A. Almeida, R. Vilar, R. Li, M.G.S. Ferreira, K.G. Watkins and W.M. Steen, in P. Denny, I. Miyamoto and B.L. Mordike (eds.), Laser Materials Processing, Vol. 77, ICALEO 93, LIA, Orlando, FL, 1994, pp. 903-912. [I61 B.C. Oberlander and E. Lugscheider, Materials Sci. Technol., 8 (1992) 657. u71 K.P. Cooper and J.D. Ayers, in F.D. Seaman (ed.), Lose) Surface Modification, AWS, Miami, FL, 1988, pp. 115-131. 1181K.P. Cooper and P. Slebodnick, J. Laser Appl., 1 (1989) 21. [I91 K.P. Cooper and A. Singh, NRL, unpublished results. PO1K.P. Cooper and R.A. Bayles, NRL, unpublished results. Pll H.S. Kalish; in L.J. Korb and D.L. Olson (eds.), Metals Handbook, Corrosion, 9th edn., Vol. 13, ASM, Metals Park, OH, 1987, pp. 846-858. PI D.M. Aylor, in L.J. Korb and D.L. Olson (eds.), Metals Handbook. Corrosion, 9th edn., Vol. 13, ASM, Metals Park, OH, 1987, pp. 859-863. ~31 K.P. Cooper, P. Slebodnick and E.D. Thomas, NRL, to be published. book,