- Email: [email protected]
Microstructures of laser-clad nickel-based hardfacing alloys
Surface and Coatings Technology 106 (1998) 183–192
Microstructures of laser-clad nickel-based hardfacing alloys L.C. Lim a,b,*, Qian Ming a, Z.D. Chen c a Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Institute of Materials Research and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c Gintic Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Avenue, Singapore 630875, Singapore Received 8 July 1997; accepted 11 May 1998
Abstract Distinctive microstructures of laser-clad passes with three nickel-based hardfacing alloys were observed. The boride-hardened Colmonoy 6 consisted of a structure of c-nickel dendrites and interdentritic eutectics of c-nickel, nickel borides and nickel silicides; carbide-hardened Colmonoy 88 consisted of mixed carbides, while WC-strengthened AI-1236 consisted of partially dissolved WC particles embedded in a similar microstructure to Colmonoy 6. For a given laser beam intensity, the microstructures of the clad layers are significantly influenced by the powder feed rate (PFR) employed and, to a lesser extent, the translation speed ( TS) of the laser beam. A cellular-to-dendritic transition for Colmonoy 6, a morphology change of the mixed carbide from coarse leaflike to fine, equiaxed but angular shape for Colmonoy 88, and a lesser degree of dissolution of WC particles for AI-1236 occurred with increasing PFR. At sufficiently high TS, the nucleation and growth of the mixed carbide phases in the Colmonoy 88 layer can be suppressed, especially when the PFR is low. The effect of PFR and TS on the clad microstructure can be understood in terms of the specific energy input (E ) and the specific heat energy (E ) available, respectively. © 1998 Elsevier Science S.A. S h Keywords: Hardfacing; Laser cladding; Nickel-based alloy
1. Introduction Powder hardfacing has long been a practice for improving the wear resistance of materials. In the hardfacing layers, some hard phases are generally incorporated in certain metal-based matrices to withstand wear. Borides and carbides are common hard phases in current nickel-based hardfacing alloys. They are either added in the form of composite powders or precipitate during processing. Welding and thermal spraying have been two of the most commonly used hardfacing techniques, in which plasma-arc welding, oxyfuel powder spray and plasmaarc powder spray are adaptable to powder application [1,2]. In recent years, powder hardfacing by means of lasers, or laser cladding, has gained increasing popularity [3–6 ]. As laser beams feature a high power density, a low interaction time and easy adaptation to automation [3],
* Corresponding author. Fax: +65 779 1459; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 52 5 - 8
they are an ideal energy source for the hardfacing of materials, in that rapid heating and cooling of localized regions of the part can be effected. The low-energy input produces a small heat-affected zone (HAZ ), minimal dilution and minimum distortion of the workpiece, while improved deposition control results in accurate profiling of the clad surface, minimization of the waste of expensive cladding materials, and reduced machining. The rapid solidification effects produce refined and novel microstructures, and hence improved properties [3,4]. Despite the above, until now no systematic study has been reported on the effects of materials and process parameters on the microstructure of laser-clad nickelbased hardfacing layers. In this work, typical microstructures of three nickelbased hardfacing alloys, laser-clad under different conditions, will be presented. They are Colmonoy 6, Colmonoy 88 and AI-1236 alloy powders, each representing an alloy strengthened by different kinds of hard phase, i.e. borides for Colmonoy 6, carbides for Colmonoy 88 and added WC particles for AI-1236. The effects of the cladding parameters and the thermal effect due to multiple passes will also be presented.
184
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
2. Experimental The samples for microstructural study were laser-clad passes on a normalized low-carbon steel (AISI 1020). They were produced by the blown powder laser cladding technique with a TRUMPF 3 kW CW CO laser system. 2 Three kinds of commercial nickel-based hardfacing powders, namely boride-hardened Colmonoy 6, carbidehardened Colmonoy 88 and WC-strengthened composite AI-1236, were used for the laser cladding. Their chemical compositions and particle sizes are given in Table 1. The power of the laser beam was set at 1.65 kW, and the beam diameter at 4 mm. The powder feed rate (PFR) was varied from 7.66 to 16.28 g min−1 for Colmonoy 6, 10.8 to 20.4 g min−1 for Colmonoy 88 and 7.44 to 13.16 g min−1 for AI-1236, respectively, each corresponding to a range of 0.5–0.9 rpm of the powder hopper. The translation speed (TS ) of the laser beam ranged from 350 to 600 mm min−1. Microstructural examination was performed mainly on the cross-sections of single clad passes. In addition, microstructural study on multipass samples was also performed for comparison purposes. For the latter, the metallographic samples were prepared by grinding off the top surface layer to a sufficient depth. After grinding and polishing the specimens to a mirror finish, they were etched electrolytically with a 10% aqueous solution of chromic acid to reveal the microstructure, and examined under an optical microscope and in an SEM. Energy dispersive X-ray (EDX ) analysis was performed to identify the phases observed in the clad materials. X-ray diffraction ( XRD) analysis was also performed on the multipass clad layers.
constituents, as shown in Fig. 1. EDX anlaysis revealed the dendrites to be c-nickel, which also contains Cr, Fe and Si. Relatively coarse columnar dendrites were observed near the bond line and near the surface of the clad passes of all the samples examined, while equiaxed dendrites were observed in the clad material away from the bond line. The substrate beneath the clad layer had a martensite structure. The phases between the c-nickel dendrites are eutectic phases (Figs. 1 and 2). EDX analysis revealed that the relatively larger, darker component of the eutectic is boride, while the lighter component between the phases is c-nickel. The borides are largely nickel borides, but EDX analysis indicates that chromium and iron are also present [7]. The interdendritic eutectics are therefore largely the binary eutectic of c-nickel and nickel boride. EDX analysis also revealed that the Fe content in the borides decreases the further away they are located from the bond line. The Fe content also decreases with increasing PFR, which can be attributed to a reduced degree of dilution as the PFR is increased [8]. In between the c-nickel dendrites and the binary eutectic are finer structures in the form of fibres or
3. Results 3.1. Colmonoy 6 3.1.1. Single clad pass The microstructure of single clad passes of Colmonoy 6 features primary dendrite phases and interdendritic
Fig. 1. Columnar dendrites in Colmonoy 6 single clad pass (PFR=11.9 g min−1, TS=450 mm min−1).
Table 1 Chemical compositions (wt.%) and particle size of the hardfacing powders used Element
Colmonoy 6
Colmonoy 88
AI-1236
C Si Cr W B Co Fe Others Ni Particle size (mm)
0.57 4.19 13.64 – 3.04 0.14 4.24 <0.50 Balance 80 (+40/−40)
0.76 4.12 15.69 16.22 3.09 0.02 3.81 <0.20 Balance 70 (+30/−40)
1.58 2.80 9.56 25.92 2.15 – 2.86 – Balance 100 (+20/−30) for WC particles, 60 (+30/−20) for nickel-based powder
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Fig. 2. Equiaxed dendrites and interdendritic eutectics in a Colmonoy 6 clad pass produced with PFR=11.9 g min−1 and TS= 450 mm min−1.
lamellae which are rich in Ni and Si ( Fig. 2). As the structure is very fine, EDX analysis is unable to distinguish the individual phases. It can be deduced from the process of solidification that this constituent is probably the ternary or quaternary eutectic of the Ni–Cr–Fe–B–Si system studied. It should be mentioned that instead of the dendritic structure, a cellular-like substructure was observed in samples clad at low PFRs. For samples clad with an intermediate PFR, a transition from the cellular to dendritic structure was observed. An example of such is given in Fig. 3. Fig. 2, taken from a sample clad with a high PFR, represents the final dendritic state of the transition. 3.1.2. Multipass clad layer The multipass clad layer has a banded appearance to the naked eye. Under the SEM, the overlapped (OL) region appears brighter, while the darker bands between them are the clad layer not overlapped by subsequent passes. The latter is hereafter referred to as the pseudosingle-pass (PSP) region. In the OL region, dendrites, binary eutectics and scattered ternary and quatenary eutectics were observed (Fig. 4(a)). Although similar microstructures were also observed near both edges of the PSP region, a cellularlike microstructure was noted in its central portion (Fig. 4(b)). Microhardness measurements further revealed that both regions were of comparable hardness, namely 690 HV (at a load of 200 g) in the OL region and 687 HV in the PSP region. The results of the XRD analysis are shown in Fig. 5(a). The fcc c-nickel peaks are most prominent. As c-nickel is a solid solution, its XRD peaks were displaced slightly relative to those of nickel. XRD anlaysis revealed two major kinds of borides,
185
(a)
(b)
Fig. 3. (a) Cellular-like microstructure in a Colmonoy 6 clad pass produced with PFR=7.7 g min−1 and TS=350 mm min−1. (b) Transition from cellular to dendritic structure in another sample produced with PFR=11.9 g min−1 and TS=600 mm min−1.
i.e. (Ni,Fe) B and Cr B . The former has an orthorhom3 5 3 bic crystal structure, while the latter a tetragonal crystal structure [9]. These borides with c-nickel formed the interdendritic eutectics. Nickel silicide (Ni Si) was also 3 detected in the XRD analysis, which agrees with the results of the microstructural study described above. 3.2. Colmonoy 88 3.2.1. Single clad pass For Colmonoy 88, characteristic hard phases were prominent in the clad samples. EDX analysis revealed that these phases were mixed carbides of tungsten, chromium, nickel and iron, i.e. ( W,Cr,Ni,Fe) C . x y Typical examples of such hard phases are given in Fig. 6. For passes clad with high PFRs (e.g. 20.4 g min−1), which produced a high hardness value, the hard phases are angular (rectangular or quadrangular in most cases)
186
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(b)
Fig. 4. Microstructures of Colmonoy 6 multipass clad layer: (a) overlapped (OL) region, (b) pseudo-single-pass (PSP) region (PFR=16.3 g min−1, TS=600 mm min−1).
but fairly equiaxed in shape and are compactly dispersed in the alloy matrix ( Fig. 6(a)). For passes clad with low PFRs (e.g. 10.8 g min−1), corresponding to a low hardness value [8], the hard phases are coarse and of a leaflike morphology (Fig. 6(c)). For passes clad with medium PFRs (eg. 16.0 g min−1), which produced a medium hardness value, a mixture of angular and leaflike carbides was observed (Fig. 6(b)). EDX analysis further revealed that the angular carbides contain higher W but less Fe than the leaf-like carbides. For the sample clad with a PFR of 10.8 g min−1 and a TS of 600 mm min−1, which produced the lowest hardness [10], the microstructure had a granular appearance and a band of carbides, as shown in Fig. 7. The band was approximately located in the middle of the clad layer and ran parallel to the bond line (Fig. 7(a)). Some leaf-like carbides were fairly well developed within the band, while small angular carbide nuclei could be detected outside the band. Fig. 7(b) shows a leaf-like
carbide in its initial stage of formation, while Fig. 7(c) shows freshly nucleated granular carbides in the same sample. The extremely fine dendritic structures surrounding the carbides in both figures are typical quenched structures, suggesting that the carbides must have nucleated from within the melt. The above observation indicates that the mixed carbide is the first phase to form during solidification of the molten powder. Under certain cladding conditions which favour the growth of carbides, such carbides could evolve into a leaf-like morphology. Under optimum conditions, such as with a high PFR, the growth of the carbides could be restricted, giving rise to finely dispersed angular carbides in the microstructure. Other than the mixed carbide phases, the remaining constituent, hereafter referrred to as the matrix, is largely a nickel-based metal with a microstructure similar to that of Colmonoy 6, consisting of c-nickel dendrites, binary eutectics and ternary eutectics of c-nickel and nickel borides and/or silicides. One major difference is that the dendrites and eutectics are much finer in the present case. Fig. 8 gives the percentage of the hard mixed carbide phases as a function of the PFR and the TS of the laser beam. Note that a smaller amount of the mixed carbide phase was observed as the PFR was decreased and the TS of the laser beam was increased. We will discuss this later. 3.2.2. Multipass clad layer Coarse angular hard phases were observed in the microstructure of the multipass clad layer in both PSP and OL regions produced with a high PFR of 20.4 g min−1 [7]. Nevertheless, the number of angular hard phases appeared to be less, and they tended to line up in the OL region. Furthermore, different hardness values were recorded for the two regions. The average microhardness for the PSP regions is 936 HV, while it is 889 HV for the OL region. In other words, the OL region is softer than the PSP region. XRD anlaysis, shown in Fig. 5(b), confirmed the presence of c-nickel, mixed borides ((Ni,Fe) B) 3 and nickel silicide (Ni Si) in the multipass clad layer 3 of Colmonoy 88. The peaks which resemble the WC peaks are those of the mixed carbide phase, i.e. ( W,Cr,Ni,Fe) C . x y 3.3. AI-1236 3.3.1. Single clad pass A typical microstructure of a laser-clad AI-1236 layer is shown in Fig. 9. The added hard phases, tungsten carbide ( WC ) particles with a microhardness of around 2600 HV, stand proud in the nickel-based alloy matrix of a similar microstructure to that of Colmonoy 88. Partial dissolution of the tungsten carbide also
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
187
Fig. 5. XRD spectra of multipass clad specimens: (a) Colmonoy 6, (b) Colmonoy 88, (c) AI-1236.
occurred in the laser-clad microstructure. Fig. 10 shows the details of the dissolution products, which consist of primary carbides and eutectic carbides in the vicinity of the partially melted WC phase. Similar observations were also reported by Cooper [3]. EDX analysis revealed that both the primary carbides and the component carbide of the eutectic carbides are the mixed carbide, i.e. ( W,Cr,Ni,Fe) C [7]. In general, the degree of carx y
bide dissolution increases with decreasing PFR and with increasing TS of the laser beam, which also leads to a reduced amount of WC particles in the clad layer, as shown in Fig. 11. 3.3.2. Multipass clad layer Similar observations were also made on multipass clad samples. In the PSP region, eutectic and primary
188
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(a)
(b)
(b)
(c)
(c)
Fig. 6. Microstructurs of Colmonoy 88 clad passes of different hardness values: (a) high hardness (PFR=30.4 g min−1, TS=400 mm min−1), (b) medium hardness (PFR=13.2 g min−1, TS=400 mm min−1), (c) low hardness (PFR=10.8 g min−1, TS=350 mm min−1).
Fig. 7. (a) Granular structure with a band of carbides in a Colmonoy 88 clad pass produced with the lowest PFR and the highest TS. (b, c) Details of the carbides within the band. Note that the freshly nucleated carbides in (c) are surrounded by a quenched structure (PFR=10.8 g min−1, TS=600 mm min−1).
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Fig. 8. Area percentage of mixed carbides in Colmonoy 88 clad samples.
189
Fig. 11. Area percentage of WC particles in AI-1236 clad samples.
Fig. 9. Distribution of tungsten carbide particles (white) in an AI-1236 clad pass (PFR=10.5 g min−1, TS=350 mm min−1).
carbides were found to scatter in the complex nickelbased matrix (Fig. 12). In the OL region, a new eutectic structure was observed ( Fig. 13). It can be seen from Fig. 13(a) that the lighter and darker constituents radiate out in colonies. The lighter constituent is identified as ( W,Cr,Ni,Fe) C , having a composition similar to that x y of the eutectic carbide, and the darker constituent can be identified as c-nickel. The two form a binary eutectic, which is a prominent constituent in the OL region. Fig. 13(b) shows another case of the eutectic structure. As one moves away from the OL region, the amount of the said binary eutectic decreases accordingly, and eventually disappears. The XRD spectrum of AI-1236 clad samples is shown in Fig. 5(c). The dominant phases detected are tungsten carbides ( WC ), mixed carbides (( W,Cr,Ni,Fe) C ), x y c-nickel, nickel silicide (Ni Si) and borides 3 ((Ni,Fe) B). 3
Fig. 10. Eutectic carbides and primary carbides in an AI-1236 clad pass (PFR=13.2 g min−1, TS=450 mm min−1).
Fig. 12. A partially melted WC particle and adjacent eutectic carbides in the PSP region of an AI-1236 multipass clad sample (PFR=13.2 g min−1, TS=450 mm min−1).
190
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(b)
Fig. 13. Microstructure of an AI-1236 multipass clad sample: (a) partially melted WC particles, coarse eutectic carbides and colonies of fine eutectic carbides further away; (b) primary and fine eutectic carbides in a nickel-based braze matrix (PFR=13.2 g min−1, TS=450 mm min−1).
4. Discussion 4.1. Effect of power flow rate (PFR) 4.1.1. Colmonoy 6 The microstructure of the laser-clad layers with nickelbased hardfacing powders is directly related to the cladding parameters used, such as the laser power, powder feed rate (PFR) and the translation speed ( TS) of the laser beam. In the present work, the power of the laser was fixed and the effects of the latter two parameters were investigated. At a fixed TS, the change in the microstructure of clad samples with PFR can be seen more clearly in terms of the specific energy input E (J g−1), i.e. the S energy available per unit mass of powder (E is given S
by P/PFR, where P is the power of the laser beam) [11]. Note that E is independent of the TS. S Figs. 2 and 3 show that the microstructure of Colmonoy 6 clad samples varies from a cellular to a dendritic morphology with increasing PFR, equivalent to decreasing E . This is because a lower E will lead to S S a lower molten-clad mass temperature, which in turn will lead to a colder substrate surface, and hence a higher cooling rate for the ensuing solidification process. Note that dendritic solidification is promoted by a low G/R ratio, where G is the temperature gradient and R is the growth rate [12–14]. Note also that G is roughly linearly related to the cooling rate, but R is a strong function of the degree of supercooling, and hence the cooling rate. Therefore, with increasing PFR, or decreasing E , the G/R ratio decreases accordingly, and a S transition from cellular to dendritic solidification is thus expected, as observed in the present work. Furthermore, at a sufficiently high cooling rate, such as with a high PFR (equivalent to a low E ), one would S expect the nucleation rate to increase accordingly. This explains the smaller primary dendrite spacing and the resultant finer microstructure of Colmonoy 6 samples clad at high PFRs. The dendritic micostructure in the OL region as opposed to the cellular microstructure in the PSP region of the multipass clad layer in samples clad with Colmonoy 6 can also be explained based on the coolingrate effect. Note that due to the substrate heating effect of earlier passes, the cooling rate experienced by the PSP region of the multipass clad layer is expected to be lower than in the case of a single clad pass. On the other hand, the OL regions would experience a much higher cooling rate due to a larger area of heat sink in contact with such a region. It should also be noted that the heat inputs, and hence the PFR, also affect the extent of dilution, and hence the composition of the molten pool of clad material [8]. This in turn would also affect the solidification behaviour of the clad material. However, judging from the similar dendritic structure observed near the bond line and the surface of the clad material, one can conclude that the cooling rate plays a more significant role in the solidification behaviour of the clad material. 4.1.2. Colomony 88 and AI-1236 The PFR also affects the morphology of the mixed carbides in Colmonoy 88 clad samples. As the PFR increases (or E decreases), the morphology of the S carbides varies from a coarse, leaf-like appearance to numerous, fine angular carbides. This is a result of the high carbide nucleation rate in the layers clad with high PFRs (equivalent to low E values). Note that a higher S amount of Fe in the molten metal, a result of a higher degree of dilution, also brings about a morphological change in the mixed carbides in that the formation and
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
growth of leaf-like carbides is favoured. This is most evident in samples clad with low PFRs (equivalent to high E ) and a high TS. S The finer dendrite and eutectics in the matrix of Colmonoy 88 clad passes can be attributed to the formation of numerous primary mixed carbides in the molten melt which provide heterogeneous sites for the nucleation of the c-nickel phase. For AI-1236, the extent of dissolution of WC particles at a given TS is also controlled by PFR. A high PFR (equivalent to low E ) produces a lesser degree of S dissolution, and hence a high amount of unmelted WC particles in the clad layer [7]. This in turn gives a higher hardness value. Due to the presence of heterogeneous nucleation sites in the Colmonoy 88 and AI-1236 clad layers, the difference between the microstructures of multipass clad layers and single clad pass layers appears to be less obvious. However, due to the higher cooling rate experienced by the OL region, and hence the shorter time available for phase transformation, a smaller amount of angular hard phases was observed in this region in the case of the Colmonoy 88 clad multipass layer, leading to a lower hardness value. On the other hand, different eutectic phases and structures were observed in the OL region of AI-1236 clad multipass layers. It is possible that such a solidification microstructure is a result of nonequilibrium cooling due to the high cooling rate experienced by the material in the OL region. More work is required to ascertain whether this is actually the case.
4.2. Effect of the translation speed of the laser beam As regards the translation speed ( TS) of the laser beam, a better way to see its effect is to make use of the concept of specific heat energy E , defined as h E =gP/vd (J mm−2), where g is the net efficiency, P is h f the power of the laser beam, v is the the TS, and d is f the focused spot diameter of the laser beam [11]. Incomplete transformation of the dendritic carbides in Colmonoy 88 samples clad at the lowest PFR of 10.8 g min−1 clearly shows the effect of this ( Fig. 7). Note that at high TS, and hence low E , nucleation and h subsequent growth of the mixed carbides may be suppressed, resulting in a smaller amount of resultant carbides and a low hardness value of the clad layer. Nonetheless, the effect of TS, and hence E , on microh structures is not as prominent as that of PFR, and hence E . For all three alloy powders examined, the S effect of TS on the microstructure of the nickel-based matrix phase, i.e. c-nickel dendrites and interdendritic eutectics, is less significant as compared to that on the hard carbide phases [7].
191
5. Conclusions (1) Microstructurally, Colmonoy 6 consists largely of c-nickel dendrites and interdendritic eutectics of c-nickel, nickel borides and nickel silicides. Colmonoy 88 consists of mixed carbides, while AI-1236 consists of coarse partially melted WC particles and finer mixed carbides, embedded in a similar microstructure to Colmonoy 6. (2) At a given laser beam intensity, the microstructures of the laser-clad passes of the three hardfacing alloys are influenced significantly by the powder feed rate (PFR) and, to a lesser extent, the translation speed ( TS ) of the laser beam. (3) The effect of PFR on the microstructures of the clad passes can be understood in the light of the specific energy input E (J g−1). For Colmonoy 6, the laser-clad S microstructure undergoes a cellular-to-dendritic transition as the PFR is increased, corresponding to a smaller specific energy input. This in turn will result in a lower molten-clad mass temperature, and hence a lower substrate surface temperature, and correspondingly, a higher cooling rate, and hence a higher G/R ratio. The higher cooling rate produced by a higher PFR also gives rise to a change in the microstructure from coarse to fine dendrites, resulting in a higher hardness of the resultant clad pass. For Colmonoy 88, with increasing PFR, the morphology of the hard phases varies from a few coarse leaf-like particles to numerous fine and angular particles, which also yields a higher hardness value. The formation of leaf-like mixed carbides is also promoted by the presence of Fe in the clad metal, an effect of dilution. For AI-1236, increasing PFR produces a lesser degree of dissolution of the WC particles and hence a high hardness value of the clad pass. (4) The effect of TS is not as obvious as that of the PFR over the range of cladding parameters studied in the present work. In general, at a given PFR, as the TS of the laser beam is increased, the specific heat energy (E ) decreases accordingly. At sufficiently high TS (and h hence low E ), the nucleation and growth of the carbide h phases is suppressed, leading to a reduced number of hard phases and a lower hardness value for Colmonoy 88. (5) In multipass clad layers, the overlapped (OL) regions generally experience a higher cooling rate than the pseudo-single-pass (PSP) region. This in turn could promote dendritic solidification in the Colmonoy 6 clad layer, a sluggish phase transformation of primary mixed carbides in Colmonoy 88, and the possible formation of nonequilibrium eutectic phases in AI-1236 clad layers.
Acknowledgement The laser equipment time provided by the Gintic Institute of Manufacturing Technology at Nanyang
192
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Technological University for the present project is greatly appreciated.
References [1] J.R. Davis, in: ASM Handbook, 10th ed., vol. 6, ASM International, Metals Park, OH, 1993, pp. 789–829. [2] B.A. Kushner, E.R. Novinski, in: ASM Handbook, 10th ed., vol. 18, ASM International, Metals Park, OH, 1992, pp. 829–833. [3] K.P. Cooper, in: ASM Handbook, 10th ed., vol. 18, ASM International, Metals Park, OH, 1992, pp. 861–872. [4] C. Williams, in: R.C. Crafer, P.J. Oakley ( Eds.), Laser Processing in Manufacturing, Chapman and Hall, London, 1993, pp. 141–161.
[5] W.M. Steen, Laser Material Processing, Springer, London, 1991. [6 ] J.M. Yellup, Surf. Coat. Technol. 71 (1995) 121–128. [7] Qian Ming, MEng thesis, National University of Singapore, 1996. [8] Qian Ming, L.C. Lim, Z.D. Chen, Surf. Coat. Technol., in press. [9] Powder Diffraction File: PDF-2 database (computer file), JCPDS International Centre for Diffraction Data, Newton Square, PA, 1994. [10] M. Qian, L.C. Lim, Z.D. Chen, W.L. Chen, J. Mater. Process. Technol. 63 (1997) 590–593. [11] A.G. Grigoryants, Basics of Laser Material Processing (P.S. Ivanov, Trans.), Mir, Moscow, 1994. [12] K.A. Jackson, J.P. Hunt, Acta Metall. 13 (1965) 1212. [13] G.J. Davies, Solidification and Casting, Applied Science Publishers, Barking, 1973. [14] M.C. Flemings, Solidification Processing, McGraw-Hill, New York, 1974.
Microstructures of laser-clad nickel-based hardfacing alloys L.C. Lim a,b,*, Qian Ming a, Z.D. Chen c a Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Institute of Materials Research and Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c Gintic Institute of Manufacturing Technology, Nanyang Technological University, Nanyang Avenue, Singapore 630875, Singapore Received 8 July 1997; accepted 11 May 1998
Abstract Distinctive microstructures of laser-clad passes with three nickel-based hardfacing alloys were observed. The boride-hardened Colmonoy 6 consisted of a structure of c-nickel dendrites and interdentritic eutectics of c-nickel, nickel borides and nickel silicides; carbide-hardened Colmonoy 88 consisted of mixed carbides, while WC-strengthened AI-1236 consisted of partially dissolved WC particles embedded in a similar microstructure to Colmonoy 6. For a given laser beam intensity, the microstructures of the clad layers are significantly influenced by the powder feed rate (PFR) employed and, to a lesser extent, the translation speed ( TS) of the laser beam. A cellular-to-dendritic transition for Colmonoy 6, a morphology change of the mixed carbide from coarse leaflike to fine, equiaxed but angular shape for Colmonoy 88, and a lesser degree of dissolution of WC particles for AI-1236 occurred with increasing PFR. At sufficiently high TS, the nucleation and growth of the mixed carbide phases in the Colmonoy 88 layer can be suppressed, especially when the PFR is low. The effect of PFR and TS on the clad microstructure can be understood in terms of the specific energy input (E ) and the specific heat energy (E ) available, respectively. © 1998 Elsevier Science S.A. S h Keywords: Hardfacing; Laser cladding; Nickel-based alloy
1. Introduction Powder hardfacing has long been a practice for improving the wear resistance of materials. In the hardfacing layers, some hard phases are generally incorporated in certain metal-based matrices to withstand wear. Borides and carbides are common hard phases in current nickel-based hardfacing alloys. They are either added in the form of composite powders or precipitate during processing. Welding and thermal spraying have been two of the most commonly used hardfacing techniques, in which plasma-arc welding, oxyfuel powder spray and plasmaarc powder spray are adaptable to powder application [1,2]. In recent years, powder hardfacing by means of lasers, or laser cladding, has gained increasing popularity [3–6 ]. As laser beams feature a high power density, a low interaction time and easy adaptation to automation [3],
* Corresponding author. Fax: +65 779 1459; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 52 5 - 8
they are an ideal energy source for the hardfacing of materials, in that rapid heating and cooling of localized regions of the part can be effected. The low-energy input produces a small heat-affected zone (HAZ ), minimal dilution and minimum distortion of the workpiece, while improved deposition control results in accurate profiling of the clad surface, minimization of the waste of expensive cladding materials, and reduced machining. The rapid solidification effects produce refined and novel microstructures, and hence improved properties [3,4]. Despite the above, until now no systematic study has been reported on the effects of materials and process parameters on the microstructure of laser-clad nickelbased hardfacing layers. In this work, typical microstructures of three nickelbased hardfacing alloys, laser-clad under different conditions, will be presented. They are Colmonoy 6, Colmonoy 88 and AI-1236 alloy powders, each representing an alloy strengthened by different kinds of hard phase, i.e. borides for Colmonoy 6, carbides for Colmonoy 88 and added WC particles for AI-1236. The effects of the cladding parameters and the thermal effect due to multiple passes will also be presented.
184
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
2. Experimental The samples for microstructural study were laser-clad passes on a normalized low-carbon steel (AISI 1020). They were produced by the blown powder laser cladding technique with a TRUMPF 3 kW CW CO laser system. 2 Three kinds of commercial nickel-based hardfacing powders, namely boride-hardened Colmonoy 6, carbidehardened Colmonoy 88 and WC-strengthened composite AI-1236, were used for the laser cladding. Their chemical compositions and particle sizes are given in Table 1. The power of the laser beam was set at 1.65 kW, and the beam diameter at 4 mm. The powder feed rate (PFR) was varied from 7.66 to 16.28 g min−1 for Colmonoy 6, 10.8 to 20.4 g min−1 for Colmonoy 88 and 7.44 to 13.16 g min−1 for AI-1236, respectively, each corresponding to a range of 0.5–0.9 rpm of the powder hopper. The translation speed (TS ) of the laser beam ranged from 350 to 600 mm min−1. Microstructural examination was performed mainly on the cross-sections of single clad passes. In addition, microstructural study on multipass samples was also performed for comparison purposes. For the latter, the metallographic samples were prepared by grinding off the top surface layer to a sufficient depth. After grinding and polishing the specimens to a mirror finish, they were etched electrolytically with a 10% aqueous solution of chromic acid to reveal the microstructure, and examined under an optical microscope and in an SEM. Energy dispersive X-ray (EDX ) analysis was performed to identify the phases observed in the clad materials. X-ray diffraction ( XRD) analysis was also performed on the multipass clad layers.
constituents, as shown in Fig. 1. EDX anlaysis revealed the dendrites to be c-nickel, which also contains Cr, Fe and Si. Relatively coarse columnar dendrites were observed near the bond line and near the surface of the clad passes of all the samples examined, while equiaxed dendrites were observed in the clad material away from the bond line. The substrate beneath the clad layer had a martensite structure. The phases between the c-nickel dendrites are eutectic phases (Figs. 1 and 2). EDX analysis revealed that the relatively larger, darker component of the eutectic is boride, while the lighter component between the phases is c-nickel. The borides are largely nickel borides, but EDX analysis indicates that chromium and iron are also present [7]. The interdendritic eutectics are therefore largely the binary eutectic of c-nickel and nickel boride. EDX analysis also revealed that the Fe content in the borides decreases the further away they are located from the bond line. The Fe content also decreases with increasing PFR, which can be attributed to a reduced degree of dilution as the PFR is increased [8]. In between the c-nickel dendrites and the binary eutectic are finer structures in the form of fibres or
3. Results 3.1. Colmonoy 6 3.1.1. Single clad pass The microstructure of single clad passes of Colmonoy 6 features primary dendrite phases and interdendritic
Fig. 1. Columnar dendrites in Colmonoy 6 single clad pass (PFR=11.9 g min−1, TS=450 mm min−1).
Table 1 Chemical compositions (wt.%) and particle size of the hardfacing powders used Element
Colmonoy 6
Colmonoy 88
AI-1236
C Si Cr W B Co Fe Others Ni Particle size (mm)
0.57 4.19 13.64 – 3.04 0.14 4.24 <0.50 Balance 80 (+40/−40)
0.76 4.12 15.69 16.22 3.09 0.02 3.81 <0.20 Balance 70 (+30/−40)
1.58 2.80 9.56 25.92 2.15 – 2.86 – Balance 100 (+20/−30) for WC particles, 60 (+30/−20) for nickel-based powder
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Fig. 2. Equiaxed dendrites and interdendritic eutectics in a Colmonoy 6 clad pass produced with PFR=11.9 g min−1 and TS= 450 mm min−1.
lamellae which are rich in Ni and Si ( Fig. 2). As the structure is very fine, EDX analysis is unable to distinguish the individual phases. It can be deduced from the process of solidification that this constituent is probably the ternary or quaternary eutectic of the Ni–Cr–Fe–B–Si system studied. It should be mentioned that instead of the dendritic structure, a cellular-like substructure was observed in samples clad at low PFRs. For samples clad with an intermediate PFR, a transition from the cellular to dendritic structure was observed. An example of such is given in Fig. 3. Fig. 2, taken from a sample clad with a high PFR, represents the final dendritic state of the transition. 3.1.2. Multipass clad layer The multipass clad layer has a banded appearance to the naked eye. Under the SEM, the overlapped (OL) region appears brighter, while the darker bands between them are the clad layer not overlapped by subsequent passes. The latter is hereafter referred to as the pseudosingle-pass (PSP) region. In the OL region, dendrites, binary eutectics and scattered ternary and quatenary eutectics were observed (Fig. 4(a)). Although similar microstructures were also observed near both edges of the PSP region, a cellularlike microstructure was noted in its central portion (Fig. 4(b)). Microhardness measurements further revealed that both regions were of comparable hardness, namely 690 HV (at a load of 200 g) in the OL region and 687 HV in the PSP region. The results of the XRD analysis are shown in Fig. 5(a). The fcc c-nickel peaks are most prominent. As c-nickel is a solid solution, its XRD peaks were displaced slightly relative to those of nickel. XRD anlaysis revealed two major kinds of borides,
185
(a)
(b)
Fig. 3. (a) Cellular-like microstructure in a Colmonoy 6 clad pass produced with PFR=7.7 g min−1 and TS=350 mm min−1. (b) Transition from cellular to dendritic structure in another sample produced with PFR=11.9 g min−1 and TS=600 mm min−1.
i.e. (Ni,Fe) B and Cr B . The former has an orthorhom3 5 3 bic crystal structure, while the latter a tetragonal crystal structure [9]. These borides with c-nickel formed the interdendritic eutectics. Nickel silicide (Ni Si) was also 3 detected in the XRD analysis, which agrees with the results of the microstructural study described above. 3.2. Colmonoy 88 3.2.1. Single clad pass For Colmonoy 88, characteristic hard phases were prominent in the clad samples. EDX analysis revealed that these phases were mixed carbides of tungsten, chromium, nickel and iron, i.e. ( W,Cr,Ni,Fe) C . x y Typical examples of such hard phases are given in Fig. 6. For passes clad with high PFRs (e.g. 20.4 g min−1), which produced a high hardness value, the hard phases are angular (rectangular or quadrangular in most cases)
186
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(b)
Fig. 4. Microstructures of Colmonoy 6 multipass clad layer: (a) overlapped (OL) region, (b) pseudo-single-pass (PSP) region (PFR=16.3 g min−1, TS=600 mm min−1).
but fairly equiaxed in shape and are compactly dispersed in the alloy matrix ( Fig. 6(a)). For passes clad with low PFRs (e.g. 10.8 g min−1), corresponding to a low hardness value [8], the hard phases are coarse and of a leaflike morphology (Fig. 6(c)). For passes clad with medium PFRs (eg. 16.0 g min−1), which produced a medium hardness value, a mixture of angular and leaflike carbides was observed (Fig. 6(b)). EDX analysis further revealed that the angular carbides contain higher W but less Fe than the leaf-like carbides. For the sample clad with a PFR of 10.8 g min−1 and a TS of 600 mm min−1, which produced the lowest hardness [10], the microstructure had a granular appearance and a band of carbides, as shown in Fig. 7. The band was approximately located in the middle of the clad layer and ran parallel to the bond line (Fig. 7(a)). Some leaf-like carbides were fairly well developed within the band, while small angular carbide nuclei could be detected outside the band. Fig. 7(b) shows a leaf-like
carbide in its initial stage of formation, while Fig. 7(c) shows freshly nucleated granular carbides in the same sample. The extremely fine dendritic structures surrounding the carbides in both figures are typical quenched structures, suggesting that the carbides must have nucleated from within the melt. The above observation indicates that the mixed carbide is the first phase to form during solidification of the molten powder. Under certain cladding conditions which favour the growth of carbides, such carbides could evolve into a leaf-like morphology. Under optimum conditions, such as with a high PFR, the growth of the carbides could be restricted, giving rise to finely dispersed angular carbides in the microstructure. Other than the mixed carbide phases, the remaining constituent, hereafter referrred to as the matrix, is largely a nickel-based metal with a microstructure similar to that of Colmonoy 6, consisting of c-nickel dendrites, binary eutectics and ternary eutectics of c-nickel and nickel borides and/or silicides. One major difference is that the dendrites and eutectics are much finer in the present case. Fig. 8 gives the percentage of the hard mixed carbide phases as a function of the PFR and the TS of the laser beam. Note that a smaller amount of the mixed carbide phase was observed as the PFR was decreased and the TS of the laser beam was increased. We will discuss this later. 3.2.2. Multipass clad layer Coarse angular hard phases were observed in the microstructure of the multipass clad layer in both PSP and OL regions produced with a high PFR of 20.4 g min−1 [7]. Nevertheless, the number of angular hard phases appeared to be less, and they tended to line up in the OL region. Furthermore, different hardness values were recorded for the two regions. The average microhardness for the PSP regions is 936 HV, while it is 889 HV for the OL region. In other words, the OL region is softer than the PSP region. XRD anlaysis, shown in Fig. 5(b), confirmed the presence of c-nickel, mixed borides ((Ni,Fe) B) 3 and nickel silicide (Ni Si) in the multipass clad layer 3 of Colmonoy 88. The peaks which resemble the WC peaks are those of the mixed carbide phase, i.e. ( W,Cr,Ni,Fe) C . x y 3.3. AI-1236 3.3.1. Single clad pass A typical microstructure of a laser-clad AI-1236 layer is shown in Fig. 9. The added hard phases, tungsten carbide ( WC ) particles with a microhardness of around 2600 HV, stand proud in the nickel-based alloy matrix of a similar microstructure to that of Colmonoy 88. Partial dissolution of the tungsten carbide also
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
187
Fig. 5. XRD spectra of multipass clad specimens: (a) Colmonoy 6, (b) Colmonoy 88, (c) AI-1236.
occurred in the laser-clad microstructure. Fig. 10 shows the details of the dissolution products, which consist of primary carbides and eutectic carbides in the vicinity of the partially melted WC phase. Similar observations were also reported by Cooper [3]. EDX analysis revealed that both the primary carbides and the component carbide of the eutectic carbides are the mixed carbide, i.e. ( W,Cr,Ni,Fe) C [7]. In general, the degree of carx y
bide dissolution increases with decreasing PFR and with increasing TS of the laser beam, which also leads to a reduced amount of WC particles in the clad layer, as shown in Fig. 11. 3.3.2. Multipass clad layer Similar observations were also made on multipass clad samples. In the PSP region, eutectic and primary
188
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(a)
(b)
(b)
(c)
(c)
Fig. 6. Microstructurs of Colmonoy 88 clad passes of different hardness values: (a) high hardness (PFR=30.4 g min−1, TS=400 mm min−1), (b) medium hardness (PFR=13.2 g min−1, TS=400 mm min−1), (c) low hardness (PFR=10.8 g min−1, TS=350 mm min−1).
Fig. 7. (a) Granular structure with a band of carbides in a Colmonoy 88 clad pass produced with the lowest PFR and the highest TS. (b, c) Details of the carbides within the band. Note that the freshly nucleated carbides in (c) are surrounded by a quenched structure (PFR=10.8 g min−1, TS=600 mm min−1).
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Fig. 8. Area percentage of mixed carbides in Colmonoy 88 clad samples.
189
Fig. 11. Area percentage of WC particles in AI-1236 clad samples.
Fig. 9. Distribution of tungsten carbide particles (white) in an AI-1236 clad pass (PFR=10.5 g min−1, TS=350 mm min−1).
carbides were found to scatter in the complex nickelbased matrix (Fig. 12). In the OL region, a new eutectic structure was observed ( Fig. 13). It can be seen from Fig. 13(a) that the lighter and darker constituents radiate out in colonies. The lighter constituent is identified as ( W,Cr,Ni,Fe) C , having a composition similar to that x y of the eutectic carbide, and the darker constituent can be identified as c-nickel. The two form a binary eutectic, which is a prominent constituent in the OL region. Fig. 13(b) shows another case of the eutectic structure. As one moves away from the OL region, the amount of the said binary eutectic decreases accordingly, and eventually disappears. The XRD spectrum of AI-1236 clad samples is shown in Fig. 5(c). The dominant phases detected are tungsten carbides ( WC ), mixed carbides (( W,Cr,Ni,Fe) C ), x y c-nickel, nickel silicide (Ni Si) and borides 3 ((Ni,Fe) B). 3
Fig. 10. Eutectic carbides and primary carbides in an AI-1236 clad pass (PFR=13.2 g min−1, TS=450 mm min−1).
Fig. 12. A partially melted WC particle and adjacent eutectic carbides in the PSP region of an AI-1236 multipass clad sample (PFR=13.2 g min−1, TS=450 mm min−1).
190
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
(a)
(b)
Fig. 13. Microstructure of an AI-1236 multipass clad sample: (a) partially melted WC particles, coarse eutectic carbides and colonies of fine eutectic carbides further away; (b) primary and fine eutectic carbides in a nickel-based braze matrix (PFR=13.2 g min−1, TS=450 mm min−1).
4. Discussion 4.1. Effect of power flow rate (PFR) 4.1.1. Colmonoy 6 The microstructure of the laser-clad layers with nickelbased hardfacing powders is directly related to the cladding parameters used, such as the laser power, powder feed rate (PFR) and the translation speed ( TS) of the laser beam. In the present work, the power of the laser was fixed and the effects of the latter two parameters were investigated. At a fixed TS, the change in the microstructure of clad samples with PFR can be seen more clearly in terms of the specific energy input E (J g−1), i.e. the S energy available per unit mass of powder (E is given S
by P/PFR, where P is the power of the laser beam) [11]. Note that E is independent of the TS. S Figs. 2 and 3 show that the microstructure of Colmonoy 6 clad samples varies from a cellular to a dendritic morphology with increasing PFR, equivalent to decreasing E . This is because a lower E will lead to S S a lower molten-clad mass temperature, which in turn will lead to a colder substrate surface, and hence a higher cooling rate for the ensuing solidification process. Note that dendritic solidification is promoted by a low G/R ratio, where G is the temperature gradient and R is the growth rate [12–14]. Note also that G is roughly linearly related to the cooling rate, but R is a strong function of the degree of supercooling, and hence the cooling rate. Therefore, with increasing PFR, or decreasing E , the G/R ratio decreases accordingly, and a S transition from cellular to dendritic solidification is thus expected, as observed in the present work. Furthermore, at a sufficiently high cooling rate, such as with a high PFR (equivalent to a low E ), one would S expect the nucleation rate to increase accordingly. This explains the smaller primary dendrite spacing and the resultant finer microstructure of Colmonoy 6 samples clad at high PFRs. The dendritic micostructure in the OL region as opposed to the cellular microstructure in the PSP region of the multipass clad layer in samples clad with Colmonoy 6 can also be explained based on the coolingrate effect. Note that due to the substrate heating effect of earlier passes, the cooling rate experienced by the PSP region of the multipass clad layer is expected to be lower than in the case of a single clad pass. On the other hand, the OL regions would experience a much higher cooling rate due to a larger area of heat sink in contact with such a region. It should also be noted that the heat inputs, and hence the PFR, also affect the extent of dilution, and hence the composition of the molten pool of clad material [8]. This in turn would also affect the solidification behaviour of the clad material. However, judging from the similar dendritic structure observed near the bond line and the surface of the clad material, one can conclude that the cooling rate plays a more significant role in the solidification behaviour of the clad material. 4.1.2. Colomony 88 and AI-1236 The PFR also affects the morphology of the mixed carbides in Colmonoy 88 clad samples. As the PFR increases (or E decreases), the morphology of the S carbides varies from a coarse, leaf-like appearance to numerous, fine angular carbides. This is a result of the high carbide nucleation rate in the layers clad with high PFRs (equivalent to low E values). Note that a higher S amount of Fe in the molten metal, a result of a higher degree of dilution, also brings about a morphological change in the mixed carbides in that the formation and
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
growth of leaf-like carbides is favoured. This is most evident in samples clad with low PFRs (equivalent to high E ) and a high TS. S The finer dendrite and eutectics in the matrix of Colmonoy 88 clad passes can be attributed to the formation of numerous primary mixed carbides in the molten melt which provide heterogeneous sites for the nucleation of the c-nickel phase. For AI-1236, the extent of dissolution of WC particles at a given TS is also controlled by PFR. A high PFR (equivalent to low E ) produces a lesser degree of S dissolution, and hence a high amount of unmelted WC particles in the clad layer [7]. This in turn gives a higher hardness value. Due to the presence of heterogeneous nucleation sites in the Colmonoy 88 and AI-1236 clad layers, the difference between the microstructures of multipass clad layers and single clad pass layers appears to be less obvious. However, due to the higher cooling rate experienced by the OL region, and hence the shorter time available for phase transformation, a smaller amount of angular hard phases was observed in this region in the case of the Colmonoy 88 clad multipass layer, leading to a lower hardness value. On the other hand, different eutectic phases and structures were observed in the OL region of AI-1236 clad multipass layers. It is possible that such a solidification microstructure is a result of nonequilibrium cooling due to the high cooling rate experienced by the material in the OL region. More work is required to ascertain whether this is actually the case.
4.2. Effect of the translation speed of the laser beam As regards the translation speed ( TS) of the laser beam, a better way to see its effect is to make use of the concept of specific heat energy E , defined as h E =gP/vd (J mm−2), where g is the net efficiency, P is h f the power of the laser beam, v is the the TS, and d is f the focused spot diameter of the laser beam [11]. Incomplete transformation of the dendritic carbides in Colmonoy 88 samples clad at the lowest PFR of 10.8 g min−1 clearly shows the effect of this ( Fig. 7). Note that at high TS, and hence low E , nucleation and h subsequent growth of the mixed carbides may be suppressed, resulting in a smaller amount of resultant carbides and a low hardness value of the clad layer. Nonetheless, the effect of TS, and hence E , on microh structures is not as prominent as that of PFR, and hence E . For all three alloy powders examined, the S effect of TS on the microstructure of the nickel-based matrix phase, i.e. c-nickel dendrites and interdendritic eutectics, is less significant as compared to that on the hard carbide phases [7].
191
5. Conclusions (1) Microstructurally, Colmonoy 6 consists largely of c-nickel dendrites and interdendritic eutectics of c-nickel, nickel borides and nickel silicides. Colmonoy 88 consists of mixed carbides, while AI-1236 consists of coarse partially melted WC particles and finer mixed carbides, embedded in a similar microstructure to Colmonoy 6. (2) At a given laser beam intensity, the microstructures of the laser-clad passes of the three hardfacing alloys are influenced significantly by the powder feed rate (PFR) and, to a lesser extent, the translation speed ( TS ) of the laser beam. (3) The effect of PFR on the microstructures of the clad passes can be understood in the light of the specific energy input E (J g−1). For Colmonoy 6, the laser-clad S microstructure undergoes a cellular-to-dendritic transition as the PFR is increased, corresponding to a smaller specific energy input. This in turn will result in a lower molten-clad mass temperature, and hence a lower substrate surface temperature, and correspondingly, a higher cooling rate, and hence a higher G/R ratio. The higher cooling rate produced by a higher PFR also gives rise to a change in the microstructure from coarse to fine dendrites, resulting in a higher hardness of the resultant clad pass. For Colmonoy 88, with increasing PFR, the morphology of the hard phases varies from a few coarse leaf-like particles to numerous fine and angular particles, which also yields a higher hardness value. The formation of leaf-like mixed carbides is also promoted by the presence of Fe in the clad metal, an effect of dilution. For AI-1236, increasing PFR produces a lesser degree of dissolution of the WC particles and hence a high hardness value of the clad pass. (4) The effect of TS is not as obvious as that of the PFR over the range of cladding parameters studied in the present work. In general, at a given PFR, as the TS of the laser beam is increased, the specific heat energy (E ) decreases accordingly. At sufficiently high TS (and h hence low E ), the nucleation and growth of the carbide h phases is suppressed, leading to a reduced number of hard phases and a lower hardness value for Colmonoy 88. (5) In multipass clad layers, the overlapped (OL) regions generally experience a higher cooling rate than the pseudo-single-pass (PSP) region. This in turn could promote dendritic solidification in the Colmonoy 6 clad layer, a sluggish phase transformation of primary mixed carbides in Colmonoy 88, and the possible formation of nonequilibrium eutectic phases in AI-1236 clad layers.
Acknowledgement The laser equipment time provided by the Gintic Institute of Manufacturing Technology at Nanyang
192
L.C. Lim et al. / Surface and Coatings Technology 106 (1998) 183–192
Technological University for the present project is greatly appreciated.
References [1] J.R. Davis, in: ASM Handbook, 10th ed., vol. 6, ASM International, Metals Park, OH, 1993, pp. 789–829. [2] B.A. Kushner, E.R. Novinski, in: ASM Handbook, 10th ed., vol. 18, ASM International, Metals Park, OH, 1992, pp. 829–833. [3] K.P. Cooper, in: ASM Handbook, 10th ed., vol. 18, ASM International, Metals Park, OH, 1992, pp. 861–872. [4] C. Williams, in: R.C. Crafer, P.J. Oakley ( Eds.), Laser Processing in Manufacturing, Chapman and Hall, London, 1993, pp. 141–161.
[5] W.M. Steen, Laser Material Processing, Springer, London, 1991. [6 ] J.M. Yellup, Surf. Coat. Technol. 71 (1995) 121–128. [7] Qian Ming, MEng thesis, National University of Singapore, 1996. [8] Qian Ming, L.C. Lim, Z.D. Chen, Surf. Coat. Technol., in press. [9] Powder Diffraction File: PDF-2 database (computer file), JCPDS International Centre for Diffraction Data, Newton Square, PA, 1994. [10] M. Qian, L.C. Lim, Z.D. Chen, W.L. Chen, J. Mater. Process. Technol. 63 (1997) 590–593. [11] A.G. Grigoryants, Basics of Laser Material Processing (P.S. Ivanov, Trans.), Mir, Moscow, 1994. [12] K.A. Jackson, J.P. Hunt, Acta Metall. 13 (1965) 1212. [13] G.J. Davies, Solidification and Casting, Applied Science Publishers, Barking, 1973. [14] M.C. Flemings, Solidification Processing, McGraw-Hill, New York, 1974.