Solidification in Spray Forming
P.S. GRANT
Solidification in spray forming takes place in two distinct steps: typically half...
Figure 3(c) shows an EBSD orientation map for the
same microstructure, indicating that the primary Si
network consisted of...
by atomization as they move away from the point of
atomization under the action of the cold, accelerating
atomizing gas.[6...
at the point of deposition is given by the integration of
the distribution of droplet solid fractions in Figure 4.
For the...
and grain multiplication in spray forming may be
assessed.
Table I shows that do was comparatively insensitive to
the sign...
diameters in Table I, Figure 9 suggests that since the Si
volume fraction was measured to be constant for all the
billets ...
and assuming a typical value of a = 10)5
m2
s)1
for
metals, then tT
eq < 0.1 s.[1,19]
Large droplets with a
temperature Ti...
compositions that are too enriched. Figure 13(b) shows
that solid and liquid compositions will then equilibrate
to Ceq
S a...
meters in length. Consequently, it can be concluded
safely that liquid persists in the spray-formed billet
during and afte...
Solidification drives the further growth of solid grains
until all liquid is consumed.
Despite the refined microstructures p...
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Spray forming

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Spray forming

  1. 1. Solidification in Spray Forming P.S. GRANT Solidification in spray forming takes place in two distinct steps: typically half of the alloy latent heat is removed rapidly from the droplet spray created by gas atomization; the droplets are then constituted into a billet at deposition where the remaining liquid fraction solidifies relatively slowly. However, within the droplet spray, individual droplets have different thermal and solidification histories and depositing droplets may be solid, mushy, or liquid. Despite many studies of solidification behavior in spray forming, uncertainties and some misconceptions re- main on how the solidification conditions in the spray and billet interact to give rise to the characteristic spray-formed microstructure comprising refined, polygonal/equiaxed primary grains with low levels of microsegregation. This article presents a simple numerical model for the spray-formed grain size arising from the deposition of the various droplets in the spray and combines insights provided by the model with previous investigations of the phenomena occurring during and immediately after deposition to propose a comprehensive description of the important solidification behavior during spray forming. Remelting, grain multiplication, thermal and elemental equilibration, and microstructural coarsening are proposed to play a critical role in the evolution of the spray-formed microstructure. DOI: 10.1007/s11661-006-9015-3 Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION THE spray forming process is an advanced casting process for the manufacture of billet materials and is shown schematically in Figure 1. Spray forming com- prises the sequential steps of (1) the continuous gas atomization of a melt stream to produce a spray of 10- to 500-lm-diameter alloy droplets, (2) droplet cooling at typically 102 to 104 Ks)1 and acceleration to 50 to 100 ms)1 under the action of the atomizing gas, (3) the deposition of droplets at the growing spray-formed billet surface, and (4) the relatively slow cooling at 0.1 to 10 Ks)1 and solidification of any residual liquid in the spray-formed billet.[1] As well as potential near-net- shape benefits, the primary advantage of the spray forming process is the ability to manufacture alloy compositions that are problematical in conventional processes such as ingot casting, direct chill casting, and powder metallurgy. Commercial examples of alloys manufactured by spray forming include Al-Si based alloys for cylinder liners,[2] high speed and speciality steel billets,[3] Si-Al alloys for thermal management applications,[4] and Al-Nd alloys for sputtering targets.[5] The disadvantages of spray forming include as-sprayed porosity that requires closing by hot isostatic pressing or other downstream processes, and losses because not all droplets created by atomization end up in the spray- formed billet. These aspects and some of the underlying process physics have been reviewed in Reference 1. Despite many investigations of the relationship between the spray forming conditions, the solidification conditions arising, and the as-sprayed microstructure,[7–16] some uncertainties remain in how the underlying process physics and mechanics give rise to the advantageous spray-formed microstructure that typically comprises the following: (1) equiaxed/polygonal grains of diameter typically in the range 20 to 50 lm and the complete absence of columnar/dendritic morphologies, (2) high levels of microstructural homogeneity and low levels of microsegregation regardless of posi- tion in the billet, and (3) the ability to produce this characteristic spray-formed microstructure in all engineering alloy systems. For example, Figure 2(a) shows a 26 kg Al-5.3 Mg-1.2 Li-0.28 Zr alloy billet spray formed at Oxford University, and the corresponding as-spray-formed microstructure is shown in the electron backscattered diffraction (EBSD) orientation map in Figure 2(b). The as-spray-formed grain size was ~15 lm and the poros- ity was <1 area pct. In a further example, Figure 3(a) shows an optical micrograph of a chill-cast Si-30 wt pct Al alloy comprising coarse and highly defective primary Si and interprimary a-Al/Si eutectic with significant shrinkage porosity associated with the wide freezing range of ~650 °C. In contrast, Figure 3(b) shows the microstructure of a spray-formed billet of the same alloy. The microstructure now comprised a more glob- ular, refined network of primary Si continuously pen- etrated by a-Al arising from a fully divorced eutectic.[17] P.S. GRANT, Cookson Professor of Materials, is with the Depart- ment of Materials, Oxford University, Oxford OX1 3PH, United Kingdom. Contact e-mail: patrick.grant@materials.ox.ac.uk This article is based on a presentation made in the symposium entitled ‘‘Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunf ’’, which occurred March 13–15, 2006, in the TMS Spring meeting in San Antonio, TX, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee. Article published online February 2, 2007. 1520—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
  2. 2. Figure 3(c) shows an EBSD orientation map for the same microstructure, indicating that the primary Si network consisted of randomly oriented Si grains with an average grain size of ~1 lm. These Si-rich alloys offer an attractive combination of relatively high thermal conductivity, low thermal expansion, and low density for thermal management applications.[4,17] This paper presents a numerical model for the spray- formed grain size and combines insights provided by the model with previous investigations of the phenomena occurring during and immediately after deposition to propose a comprehensive description of important solidification behavior during spray forming. II. A MODEL FOR THE SPRAY-FORMED GRAIN SIZE Under a wide range of atomizing conditions for different alloys, the volumetric or mass mean particle diameter dV has been related to operating variables by the widely quoted formula[18] dV D ¼ b nð1 þ RÞ We !n ½1Š where D is the melt delivery nozzle diameter, n is the ratio of molten metal to gas kinematic viscosity, R is the ratio of molten metal to atomizing gas mass flow rates, and b and n are experimental constants. Reported values of b and n for a wide range of materials and atomizer geometries are typically 4 to 5 · 10)6 and 0.5, respec- tively.[1] The probability density Pi of droplet diameters di about the volumetric mean dV approximates to a log- normal distribution with a standard deviation r given by Pi ¼ 1 r ffiffiffiffiffiffi 2p p exp À ln di À ln dV 2r2 ! ½2Š where the droplet diameter interval Ddi has a midpoint di, n is the number of droplet intervals, and Xn i¼1 PiDdi ¼ 1 ½3Š Assuming dV and r are 100 lm and 0.7 ln (lm), respectively,[9] Figure 4 shows the resulting log-normal droplet diameter distribution and is typical of those seen experimentally. Previous investigations of droplet dynamic and ther- mal behavior have used numerical models to predict the solid fraction of the different droplet diameters created Fig. 2—(a) A typical 26-kg Al-5Mg-1.2Li-0.28Zr billet spray formed at Oxford University and (b) an Al-5Mg-1.2Li-0.28Zr as-sprayed microstructural orientation map obtained by EBSD showing noncol- umnar/dendritic equiaxed polygonal grains characteristic of the spray forming process. Fig. 1—Schematic of the spray forming process for the manufacture of billets. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, JULY 2007—1521
  3. 3. by atomization as they move away from the point of atomization under the action of the cold, accelerating atomizing gas.[6,7,8] An example of these type of calcu- lations is shown in Figure 5 for Al-4 wt pct Cu droplet diameters of 10, 50, 120, and 200 lm. For these droplets depositing at an axial distance of 0.4 m, diameters less than 50 lm were predicted to be fully solid; droplets with a diameter greater than ~200 lm were fully liquid; and droplets with intermediate diameters were mushy with a variation in droplet solid fraction dependent upon droplet diameter. Using these calculations, the boundaries between droplet diameters that were solid, mushy, or liquid are shown in Figure 4. While the details and simplifying assumptions of the various modeling studies of droplet flight and simultaneous solidification in spray forming may vary, all show a spread of droplet solid fractions at deposition. Assuming that the variation of mushy droplet solid fraction is linear with the logarithm of droplet diameter, as shown in Figure 6, and ignoring any equilibration of droplet temperatures or solid fractions (later discussion), the mass-averaged spray solid fraction fS Fig. 3—(a) Optical micrograph of chill-cast Si-30 wt pct Al, (b) opti- cal micrograph of as-spray-formed Si-30 wt pct Al, and (c) EBSD orientation map of (b). Fig. 4—A typical log-normal droplet diameter probability density distribution on a mass or volume basis obtained by gas atomization, with superimposed assumed boundaries between solid, mushy, and liquid droplets at the point of deposition during spray forming. Fig. 5—Calculated variation of Al-4 wt pct Cu droplet solid fraction as a function of droplet diameter and axial distance from the point of atomization, indicating that at an axial distance of 0.4 m, the spray comprises a mixture of solid, mushy, and liquid droplets. 1522—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
  4. 4. at the point of deposition is given by the integration of the distribution of droplet solid fractions in Figure 4. For the assumed distribution of droplet diameters and solid, mushy, and liquid droplets in Figures 4 and 6, fS is 0.5, which is in broad agreement with previous studies (e.g., References 6, 9 and 16). Figure 7 shows the droplet diameter distribution of Figure 4 replotted in terms of a normalized number distribution, together with the same superimposed boundaries between solid, mushy, and liquid droplets. Figure 7 shows that the number mean of the distribu- tion dN was 23 lm and that, while the majority of droplets by mass or volume arriving at the point of deposition were either liquid or mushy, in terms of the number of droplets, the distribution was dominated by the large number of solid droplets. The number of solid particles NV,i in the droplet diameter interval Ddi is given by NV ;i ¼ 6 p PiDdi d3 i for fi > 0 ½4Š wherefi is the droplet solid fraction and, since the probability distribution function in Figure 4 has an integrated area of unity, the total number of solid and mushy particles per unit volume NV is simply given by NV ¼ Xn i¼1 NV ;i ½5Š Assuming in the first instance that (1) each solid or mushy particle in the spray nucleates/becomes one ‘‘embryonic’’ grain in the billet top surface immediately after deposition and (2) there is no significant additional nucleation in the liquid fraction of the billet top surface because of little/no undercooling and an excess of nuclei arising from the deposition of a large number of solid/ mushy droplets,[19] then the average embryonic grain diameter in the billet top surface do can be estimated from do % fs NV !1 3 ½6Š A similar approach has been presented to estimate the number of subgrains formed in a depositing droplet during rapid solidification.[20] Table I shows the calculated variation of fS and do using Eqs. [2] through [6] as a function of assumed deposition conditions described by the particle size distribution (given by dV and r and where r was varied independently of dV in this case but recognizing some interdependency has been suggested[18] ) and the diam- eters for fully solid (df = 1) and fully liquid (df = 0) droplets. Two other variables were introduced into the calculations in Table I: (1) a critical droplet diameter d* below which droplets were assumed to be too small and to have insuffi- cient axial momentum to intersect the billet sur- face, but instead were seeded in the gas flow, swept around the billet, and removed as overspray; and (2) a grain multiplication factor G to account for there to be more than one nuclei per solid or mushy droplet, and to account for the breakup of mushy and remelted droplets on impact with the billet surface so that droplets may nucleate more than one embryonic grain in the mushy zone. Unfortunately, there is little guidance in the litera- ture as to the possible values for G in the spray forming process, and so the approach taken here was to investigate the sensitivity of the calculated embry- onic grain size to a wide variation in G from 1 to 20. In this way, the importance of multiple nuclei per droplet Fig. 6—Assumed variation of droplet solid fraction fi with diameter di, for fi = 1 at di = 50 lm and fi = 0 at di = 200 lm. Fig. 7—The log-normal droplet diameter probability density distri- bution on a number basis for the data in Fig. 4, with the same superimposed boundaries between solid, mushy, and liquid droplets at the point of deposition during spray forming. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, JULY 2007—1523
  5. 5. and grain multiplication in spray forming may be assessed. Table I shows that do was comparatively insensitive to the significant changes in the assumed deposition conditions and that while some of the model assump- tions were somewhat arbitrary, they made only a relatively small difference to the calculated embryonic grain diameter. For example, Figure 8 shows the effect of the grain multiplication factor G on the calculated embryonic grain diameter. As the multiplication factor increased from 1 to 5, the calculated grain diameter decreased from 31 to 18 lm, and reduced further to 11 lm for a grain multiplication factor of 20. Embry- onic grain diameters cannot be readily compared with final grain diameters from microstructural analysis, because, as described later, grain coarsening in the liquid/mushy state is expected. Nonetheless, calculated embryonic grain diameters in Table I of 11 to 58 lm, even when significant grain multiplication effects or low spray solid fractions were assumed, are consistent with final spray-formed grain diameters.[1] While castings may show grain size variations from a few or tens of microns to millimeters within a billet, the comparatively low sensitivity of predicted and experimental grain diameters to strong variations in the condition of depositing droplets in spray forming arises from two factors. (1) Figure 7 shows that regardless of specific spray forming conditions, it is likely there will always be a surfeit of small solid particles in the spray, which, while contributing little to the billet mass, dominate grain nucleation behavior in the billet top surface region. (2) Equation [6] shows that even when large changes in the number of solid particles per unit volume in the spray may be realized, the resulting grain size has only a relatively weak cube root dependency on the number of solid particles. In order to provide experimental support for the grain size model, the effect of the number of solid particles per unit volume on the microstructural scale of Si-30 wt pct Al billets was investigated by the simultaneous coinjec- tion of solid Si-30 wt pct Al particles during spray forming. The coinjected solid particles were entrained continuously into the droplet spray and deposited at the billet surface as if they had originated from atomization directly. In this way, the number of solid particles was controlled independently of the melt mass flow rate and atomizing conditions. The coinjection of overspray has become standard commercial practice to improve spray forming process yield and productivity.[21] The experi- ments were carried out at Sandvik Osprey (Neath, United Kingdom), and although efforts were made to keep spray forming conditions constant as the ratio of particle mass flow rate to liquid alloy mass flow rate was altered, some small changes had to made to the proprietary spray forming conditions to control billet shape, porosity, etc. However, these changes were slight compared with the changes in coinjection mass flow rate. The typical microstructure of spray-formed Si-30 wt pct Al has been shown in Figures 3(b) and (c). Figure 9 shows the variation of the inverse of the a-Al/Si interface length per unit area obtained by quantitative image analysis of many digitized optical micrographs taken from the billet centers as a function of the ratio of solid Si-30 wt pct Al particle to liquid Si- 30 wt pct Al mass flow rates. As the particle mass flow rate increased, there was a progressive and marked microstructural refinement, described by the reduction in the inverse of interface length per unit area. Consis- tent with the trends of the calculated embryonic grain Fig. 8—Variation of calculated average embryonic grain diameter do as a function of assumed grain multiplication factor G. Table I. Calculated Variation in fS and do Using Equations [2] through [6] as a Function of Assumed Deposition Condi- tions Described by the Particle Size Distribution (dV, r); the Diameters for Fully Solid (df = 1) and Fully Liquid (df = 0) Droplets; the Critical Droplet Diameter for Deposition d* ; and the Grain Multiplication Factor G dV (lm) r (ln (lm)) df = 1 (lm) df = 0 (lm) d* (lm) G fS do (lm) 100 0.7 50 200 0 1 0.5 31 100 0.7 50 200 10 1 0.5 32 100 0.7 50 200 20 1 0.5 37 100 0.6 50 200 10 1 0.5 38 100 0.5 50 200 10 1 0.5 44 100 0.8 50 200 10 1 0.5 28 150 0.7 50 200 10 1 0.31 40 250 0.7 50 300 10 1 0.21 58 80 0.7 50 200 10 1 0.61 28 100 0.7 30 200 10 1 0.39 29 100 0.7 80 200 10 1 0.62 34 100 0.7 50 150 10 1 0.43 30 100 0.7 50 300 10 1 0.59 34 100 0.7 50 200 10 2 0.5 24 100 0.7 50 200 10 5 0.5 18 100 0.7 50 200 10 7.5 0.5 16 100 0.7 50 200 10 10 0.5 14 100 0.7 50 200 10 15 0.5 12 100 0.7 50 200 10 20 0.5 11 1524—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
  6. 6. diameters in Table I, Figure 9 suggests that since the Si volume fraction was measured to be constant for all the billets investigated, co-injection directly increased the number and decreased the final size of Si particles in the mushy top surface region of the billet. Further evidence for the manipulation of spray- formed grain size by direct control of the number density of nuclei in the spray is provided in References 22 and 23 where deliberate partial oxidation of Al alloy droplets was used to create a fine dispersion of highly numerous but negligible volume oxide particles that significantly reduced the as spray-formed grain size. III. REMELTING AND EQUILIBRATION The essentially geometric approach to understanding spray-formed grain diameter described above does not account for thermal and compositional equilibration effects that must occur as droplets with a wide range of thermal conditions impact onto the billet surface that has a temperature Teq and solid fraction feq. Outside thermal transients at the beginning and end of deposi- tion, Teq and feq should remain approximately constant as the billet forms.[1] The variation of Teq and feq with spray forming parameters has been investigated using a range of techniques including pyrometry,[11] thermal imaging[24] and modeling.[6,10,14–16] Figure 10 shows the final grain size of spray-formed 2000 series Al alloy billets versus billet top surface solid fraction at that point measured by plunging a thermocouple into the billet top surface during spray forming under a range of conditions.[11] A wide range of solid fractions from 0.3 to 0.95 but a narrow range of grain diameters of 24 to 11 lm were measured, with grain diameter decreasing progressively with increasing top surface solid fraction. The study confirmed that there must be a significant liquid fraction on the billet top surface throughout the spray forming of good quality billets.[11] The spray forming conditions required to ensure that the billet top surface always contains liquid have been investigated analytically[25] and, in some studies, optimum conditions for acceptable porosity, grain size, yield, etc. identified.[16] The contin- uous presence of a liquid fraction on the billet top surface distinguishes spray forming as a semicontinuous casting process distinct from incremental spray deposi- tion processes such as arc[26,27] or plasma[28] spraying, where discrete droplet deposition events are preserved in the final microstructure. When droplets arrive at the mushy billet top surface, their velocities are typically in the range 50 to 100 ms)1 depending upon spray forming conditions,[8,29] and the following phenomena can occur: solid particles bounce from the billet surface and are lost as overspray;[30] substantially liquid droplets splash at the billet surface, generating a host of smaller secondary droplets that then either redeposit onto the surface or are lost as overspray;[31,32] or mushy/liquid droplets rapidly spread and are incorporated into the billet. Droplet spreading on impact with a solid or mushy surface under spray forming conditions is rapid,[32–34] and lateral spreading of a fully liquid droplet occurs within a few microseconds.[35] Solidification is relatively slow and restricts droplet spreading only in the final stages.[33] Once spreading is complete, droplets will equilibrate to the billet average surface temperature Teq. The time for thermal equilibration tT eq can be estimated from tT eq % x2 a ½7Š where x is a representative equilibration distance and a is the thermal diffusivity. For x of up to several hundreds of microns for the largest depositing droplets Fig. 9—Si/a-Al interface length per unit area in spray-formed Si-30 wt pct Al as a function of the ratio of the coinjected Si-30 wt pct Al particulate flow rate to liquid Si-30 wt pct Al flow rate during spray forming of billets. Fig. 10—The final grain size of spray-formed 2000 series Al alloy billets versus billet top surface solid fraction measured by plung- ing of thermocouples into the billet top surface during spray forming.[11] METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, JULY 2007—1525
  7. 7. and assuming a typical value of a = 10)5 m2 s)1 for metals, then tT eq < 0.1 s.[1,19] Large droplets with a temperature Ti > Teq will be quenched on deposition; solid particles with Ti < Teq will be reheated, and because the billet top surface temperature must be above the alloy solidus temperature,[11] droplets with Ti < Teq must partially remelt.[19] For mushy droplets, an abrupt change in droplet cooling rate from the spray of typically 102 to 104 Ks)1 to that in the billet of typically 0.1 to 10 Ks)1 will destabilize the dendrite/cell network growing within the droplet and promote dendrite/cell network breakup.[36] Accompanying thermal equilibration will be equili- bration to the billet solid fraction feq. Figure 11 shows schematically the changes in solid fraction during the spray forming process as a function of axial distance from the point of droplet atomization. The range of droplet solid fractions depositing at the billet surface equilibrate in the region of the billet top surface, labeled the equilibration zone, and the billet solid fraction (ignoring any radial variations across the billet) then gradually increases as the billet is slowly withdrawn to maintain a constant spray distance. Figure 11 also indicates the smallest solid droplets seeded in the gas flow, larger solid particles undergoing reheating and remelting, and mushy particles undergoing thermal shock and breakup (grain multiplication). As-spray-formed microstructures such as those shown in Figure 2(b) indicate that while up to typically half of the impacting droplet spray mass is solid in the form of a fine-scale cellular/dendritic morphology within discrete droplets such as that shown for the 80-lm-diameter Ni superalloy IN718 droplet in Figure 12, there are no remnants of the droplet dendritic or cellular microstruc- ture in the billet. Therefore, the conditions during and immediately after deposition involving partial remelting, thermal/mechanical destabilization, and the presence of both a significant liquid fraction and a relatively flat temperature gradient must facilitate the rapid spheroi- dization of remnant solid fragments in an attempt to minimize solid/liquid interfacial area. This process is shown schematically in Figure 11. IV. MICROSEGREGATION Only thermal and corresponding solid fraction equil- ibration has been considered. However, impacting mushy droplets with different solid fractions will contain residual liquid of varying composition. Similarly, par- tially remelted solid particles will release relatively concentrated liquid as last to solidify regions remelt. The time for equilibration of the liquid concentration tC eq in the equilibration zone can be estimated from tC eq ¼ x2 DL ½8Š where DL is the coefficient of diffusion for solute atoms in the liquid. Again, assuming x is up to several hundred microns and DL = 10)8 m2 s)1 , then tC eq is typically of the order of seconds. The processes of thermal and compositional equilibration are shown in the schematic equilibrium phase diagram of Figure 13 for a binary alloy of composition C0, a billet top surface temperature of Teq, and corresponding solid and liquid concentrations at equilibrium of Ceq S and Ceq L , respectively. Impacting mushy droplets 1 and 2 are assumed to impact the billet surface with temperatures T1 > Teq and T2 < Teq, respectively. The equilibration is shown as two distinct thermal and compositional steps in (a) and (b), respectively, because of the differing timescales estimated from Eqs. [7] and [8]. On thermal equilibration in Figure 13(a) from T1 to Teq, droplet 1 has solid and liquid compositions that are too dilute with respect to Teq; while droplet 2 equilibrates from T2 to Teq and has solid and liquid Fig. 11—Schematic representation of the changes in solid fraction during the spray forming process and the accompanying change in microstructure prior to and after droplet deposition. Fig. 12—Backscatter electron micrograph of a gas-atomized IN718 Ni superalloy droplet cross section showing microsegregation of Nb in the fine-scale interdendritic regions. 1526—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
  8. 8. compositions that are too enriched. Figure 13(b) shows that solid and liquid compositions will then equilibrate to Ceq S and Ceq L , respectively. The extent to which this microscale compositional equilibration is achieved depends upon the local solidification time but is expected in the manufacture of commercial scale billets. Equilibration of liquid compositions will neces- sarily involve remelting and homogenization as con- centrated, relatively hot liquid is diluted by mixing with cooler, remelted solid of lower solute concentration. Figure 14 shows a histogram of the maximum to minimum variation in the concentration of alloying elements Cr, Fe, Nb, Mo, Ti, and Al in three Ni superalloy IN718 overspray particles of diameter 25, 100, and 120 lm of the type shown in Figure 12, as well as the corresponding IN718 ring spray formed at the same time. Concentration variations were measured under identical electron probe microanalysis conditions where the incident electron beam was moved in 2-lm steps across a number of grain/dendrite boundary regions, and this procedure was repeated many times on randomly selected areas of the microstructures. The Ni superalloy IN718 with nominal composition in Table II is notoriously susceptible to Nb micro- and macrosegregation in conventional processing, and the potential of spray forming to reduce Nb and other segregation problems has been studied extensively.[37–40] Figure 14 shows that as the particle diameter decreased and the cooling rate increased, the extent of alloying element microsegregation decreased. For all particles, Nb showed the greatest tendency for microsegregation. However, all the overspray particles showed greater Nb microsegregation than the as-spray-formed micro- structure, despite the much slower cooling rate of the spray-formed ring. This reduced microsegregation can now be understood to arise from the large amounts of reheating, remelting, mixing, and equilibration occur- ring in the equilibration zone at the ring surface during spray forming. V. MACROSEGREGATION AND COARSENING In the production of billets on a commercial basis, billet sizes can be up to 600-mm diameter and several (a) (b) Fig. 13—Equilibration of mushy droplets with temperatures T1 and T2, impacting a billet top surface with temperature Teq: (a) tempera- ture and (b) composition. µ µ µ Fig. 14—Maximum to minimum concentration variations for alloy- ing elements Cr, Fe, Nb, Mo, Ti, and Al in three Ni superalloy IN718 overspray particles of diameter 25, 100, and 120 lm and the corresponding spray-formed Ni superalloy ring manufactured at the same time. Table II. Nominal Composition of Ni Superalloy IN718 Element Ni C Cr Mo Al Ti B Nb Fe Wt pct 52.5 0.04 19 3.0 0.5 0.9 0.02 5.1 18.5 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 38A, JULY 2007—1527
  9. 9. meters in length. Consequently, it can be concluded safely that liquid persists in the spray-formed billet during and after spray forming for many minutes. Supporting evidence for the persistence of billet liquid is given by macrosegregation profiles that can develop in spray-formed billets under some conditions.[12] In Ref- erence 12, Cu macrosegregation on the length scale of a billet was investigated by chemical analysis, and mea- sured macrosegregation profiles conformed to an inverse segregation, shrinkage-related mechanism dependent upon the persistence of interconnected liquid through- out the billet.[12] Spheroidized solid arising from droplet impact, remelting, and equilibration described previously was shown schematically in Figure 11 as isolated islands of solid surrounded by continuous liquid in a relatively flat temperature gradient. Although the liquid fraction is continuously reducing as heat continues to be extracted from the billet, primarily by convection to the atomizing gas,[41] with such a high density of solid particles, diffusion-controlled coarsening of the solid may be expected to occur. The kinetics of coarsening of spray- formed microstructures has been shown to follow typical diffusion-controlled growth kinetics at solid fractions of up to ~0.7[42,43] with d3 À d3 0 ¼ KðTÞt ½9Š where d is the grain diameter at time t, d0 is the starting grain size at t = 0, and K(T) is the temperature (and solid fraction) dependent coarsening constant. In a variety of Al- and Ni-based spray-formed alloy systems, K(T) increased with solid fraction as interdiffusion distances decreased, until feq ~ 0.7 at which point solid/ solid impingement became significant and K(T) re- duced.[43] Second-phase particles formed during solidi- fication have been suggested to play an important role in inhibiting grain growth at higher solid fractions where the thickness of liquid films delineating grains approached the second-phase particle size.[42] Coarsen- ing constants at feq = 0.7 were measured to be in the range K(T) = 600 to 1400 and 150 to 200 · 10)18 m3 s)1 for Al- and Ni-based alloys, respectively.[43] Assuming that on the basis of Table I for an Al alloy that d0 = 20 lm and K(T) = 600 · 10)18 m3 s)1 from exper- iment,[43] then Eq. [9] indicates an increase in grain diameter to 30 lm after 30 seconds and to 40 lm after 100 seconds. These times are short compared with billet solidification times. Consequently, coarsening can be expected to contribute significantly to the final spray- formed grain size. Similarly to the remelting and equilibration behavior previously described, because coarsening involves the transport of solute by diffusion, it can also be expected to play a role in reducing microsegregation. The developing spheroidal/polygonal spray-formed grain structure will allow flow of liquid to feed solidification shrinkage in the spray-formed billet much better than comparable columnar/dendritic morpholog- ies expected in conventionally cast billets of similar size, and because approximately half of the alloy latent heat has been removed prior to deposition, spray forming is able to produce cast billets in compositions considered problematical by conventional casting on account of their large freezing ranges and tendency to shrinkage- induced macrosegregation and defects. The Si-30 wt pct Al alloy shown in Figure 3(c) is an excellent example of a cast structure that cannot be made easily (if at all) by other casting routes because the alloy has a freezing range of ~650 °C. More recently, in a modern imple- mentation of the original idea to use spray forming to produce thin strip directly,[44] spray forming is being used to remove a significant fraction of latent heat prior to twin roll casting in order to make wrought alloys with relatively wide freezing ranges more ‘‘castable.’’[45] A similar approach is being explored for the ‘‘nucleated casting’’ of large diameter superalloy billets in which spray forming is used to create a high number fraction of solid particles in difficult to process Ni superalloys, before the droplet spray that retains a high mass fraction of liquid is directed continuously into a water- cooled mold.[46] VI. CONCLUSIONS Because of the broad range of droplet sizes produced by atomization, there is always a wide range of droplet thermal conditions within the spray at deposition. A simple model has been described to relate the distribu- tion of droplet diameters and solid fraction at deposition to the embryonic grain diameter in the billet top surface region. Calculated embryonic grain diameters of 11 to 58 lm are in good agreement with experimental mea- surements of final grain diameters in a range of alloys. Model predictions are relatively insensitive to assumed deposition conditions, because the embryonic grain diameter is determined primarily by the large number of fully solid particles always present in the spray. Although these particles contribute little in terms of volume to the final billet, they are critical in determining the distribution of embryonic grain diameters. The large thermal mass associated with the larger liquid droplets must be sufficient to partially remelt the solid component of the spray on deposition so that the billet top surface is mushy. The reheating and remelting processes that must occur for a large number of depositing solid and mushy droplets destabilizes their dendritic/cellular microstructure, causing it to break up and providing a grain multiplication effect. The resulting high number of small solid fragments in the mushy billet top surface and the corresponding small interdiffusion distances promote rapid thermal and compositional equilibration that helps to reduce microsegregation. All remnants of the droplet microstructure are removed and the solid component in the billet top surface region spheroidizes in the relatively flat temperature gradient to reduce solid/liquid interfacial area. In the subsequent relatively slow cooling of the still forming billet, the spheroidized solid coarsens quickly in the early stages due to small interdiffusion distances and highly mobile liquid films between embryonic grains. 1528—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
  10. 10. Solidification drives the further growth of solid grains until all liquid is consumed. Despite the refined microstructures produced by spray forming, it cannot be described as a rapid solidification process because solidification occurs in two distinct steps, of which only the in-flight solidi- fication of droplets is rapid and is experienced by only about half of the spray mass. The key phenomena that distinguish spray forming from other solidifica- tion processes are the enormous amounts of remelting, mixing, and equilibration that occur in the billet top surface. 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