Effects of flux additions on inclusion removal andmicrostructure in electron beam button meltingof Udimet 720M. Halali, M. McLean, and D. R. F. West

An investigation has been made of the effects of additions of flux on the removal of non-metallic inclusions and onthe macrostructure and microstructure developed during the electron beam button melting of alloy Udimet 720.The fluxes used were based on the alumina–lime–silica system and were added either prior to the start of the dripmelting procedure or after drip melting into the copper crucible had commenced. Flux additions led to a decrease inthe sulphur content of the alloy and to a decrease in inclusion content in the bulk of the button. The distribution ofinclusions that collected on the upper surface of the buttons was dramatically modified by flux additions whichdistributed them to the perimeter of the top surface rather than collecting them into a central ‘raft’ as occurs in theabsence of the flux. The flux additions also led to a macroscopic change in the shape of the button, giving a concavetop surface profile instead of the normal convex form and to a smooth, rather than rough, surface at thebutton/mould interface. The presence of a molten flux layer between the button and the crucible wall lowered thethe heat transfer coefficient giving rise to dendritic growth instead of the fine equiaxed chill zone found in buttonsmelted without flux. MST/4428

At the time the work was carried out the authors were in the Department of Materials, Imperial College of Science, T echnologyand Medicine, Prince Consort Road, L ondon SW7 2BP. Dr Halali is now in the Department of Materials Science andEngineering, Sharif University, Azadi Street, T ehran, Iran. Manuscript received 6 August 1999; accepted 2 December 1999.£ 2000 IoM Communications L td.

molten flux layer between the copper crucible and theIntroduction button in modifying the heat transfer coefficient and hence

the final solidification microstructure (e.g. columnar versusequiaxed crystal formation) in the solidified button. SuchThe yield strength of alloys developed for the manufactureheat transfer effects are of considerable interest in relationof turbine discs for advanced aerogas turbines has dramati-to a range of secondary melting processes (such as vacuumcally increased over the past 60 years or so. This increasearc remelting, electroslag refining, and electron beam coldin yield strength has been parallelled by a decrease in thehearth refining).critical defect size for fracture. For example, McLean1 and

An investigation has been made of the effects of variousShamblem et al.2 estimate that the critical defect size forflux additions on the cleanness and microstructure producedthe most advanced current disc alloys, like Udimet 720by EBBM of Udimet 720 which is a particularly difficultis about 50 mm which is well below the resolution of avail-alloy to solidify in ingot form. Udimet 720, an advancedable non-destructive inspection techniques. The process ofturbine disc alloy, is commonly processed by powderelectron beam button melting (EBBM) has been widelymetallurgy routes to avoid segregation. However, there isused for assessing the inclusion content (or cleanness) ofconsiderable interest in developing an ingot/forging processhigh strength alloys, notably of nickel base superalloysfor economic reasons and to avoid the inherent qualityintended for turbine disc applications.3 The techniquecontrol problems of powder metallurgy. The alloy isdepends on the removal of non-metallic inclusions fromprone to high levels of macrosegregation which can onlythe liquid by controlled solidification that concentratesbe avoided through effective control of the solidificationthem in the final molten fraction that solidifies as anprocess.inclusion rich raft on the button surface. In addition to its

normal analytical function, the technique also constitutes aconvenient small scale procedure to investigate fundamental

Experimentalphenomena that could influence electron beam remeltingprocessing for alloy purification. The present study utilisesthe EBBM process to investigate the potential for control The EBBM procedure used in the present study was thatof inclusion removal through the use of fluxes. developed by Quested et al.3 at the National Physical

Various investigations have been made of the melting Laboratory. Indeed the Laboratory’s facilities were used inand solidification process occurring during EBBM. These the present investigation. The technique consists of driphave concentrated on the mechanisms by which the melting an ingot of alloy (~1 kg) into a water cooledinclusions are rejected into the melt on solidification to copper crucible and solidifying the molten alloy in aeventually form this raft.2,4–8 It has been proposed7–9 that controlled manner by controlling the electron beam heatingnon-wetting inclusions such as Al2O3 rapidly segregate to source. Full details of the technique are given by Questedthe surfaces of the molten droplets while they fall from the et al.3 The water cooled copper crucibles used in theelectrode on to the melt surface, where thermocapillary present study had profiles of hemispherical segments. The(Marangoni) effects are involved in moving them to the buttons produced were in the range 50–70 mm in diametercentre of the button surface. The work reported here and 20–30 mm in depth. The electron beam was manipu-investigates the potential for influencing the efficiency of lated in a cycle that alternated between being focusedinclusion removal through control of the capillarity and directly onto the electrode (which consists of the alloy tothermocapillarity forces with molten fluxes which influence be melted) and onto the surface of the liquid alloy in the

crucible. A typical process cycle included a period of dripthe surface chemistry. It also considers the effect of a

Materials Science and Technology April 2000 Vol. 16 457ISSN 0267–0836

458 Halali et al. Effects of flux additions on electron beam button melting of Udimet 720

Table 1 Compositions of fluxes, wt-% alumina, silica, lime, and calcium fluoride (the smallamounts added lowered the liquidus temperatures but were

Flux Al2O

3CaO SiO

2CaF

2 too low to corrode the equipment. The compositions of thefluxes reported here are given in Table 1.S13 44 45 6 5

S14 46 47 7 ... The fluxes were prepared in batches of 30–50 g byS15 47 48 ... 5 thoroughly drying, weighing, and mixing the constituent

powders before melting the mixture in an argon atmospherein a graphite crucible using an induction furnace. The meltmelting followed by a period of controlled solidification,was held for 1 h at 1300°C. (The highest melting point ofachieved by appropriate adjustment of factors such asthe fluxes was close to 1270°C.) Following solidification thepower and time.flux was heated at 800°C for 24 h to oxidise any tracesFor comparison purposes two different procedures wereof graphite.adopted when adding the flux during the button melting

A total of nearly 40 buttons was prepared in the courseprocess. In the first procedure the flux was added after dripof the work, including a number which were required tomelting had started so that the flux entered a molten poolestablish the appropriate process cycles of melting andof the alloy, and then the drip melting step was resumed.solidification associated with flux additions. The resultsThis practice allowed investigation of the extent to whichfrom seven buttons which are reported here, illustratethe molten flux could flow into the shrinkage gap betweenthe systematic trends observed. Specimens were cut fromthe metal and the surface of the water cooled hemisphericaldifferent parts of the electron beam melted buttons, ground,mould and change the heat transfer from the sides. In thepolished, and examined by SEM (using Jeol JSM35, T200,second procedure the solid flux was placed in the bottomand T220 instruments) to characterise the nature andof the water cooled copper crucible and melted by thedistribution of inclusions. Also specimens were analysedelectron beam before the drip melting started. This practicechemically to determine whether flux additions had led toleads to the total heat flow into the crucible occurringcompositional changes in the alloy. To study the largerthrough a layer of flux. Buttons were also prepared withoutscale microstructural features, such as grain morphologyany flux addition but with the same power and time cyclesand size, half sections of the buttons were ground, polished,as those with the flux addition for purposes of comparison.and etched for 60 s with a mixture of Marble’s reagent andThe fluxes used were designed to consist mainly of2–5% of a molar chromate solution. Light microscopy wasoxides, with the objectives of:used to produce ‘composite’ photographs to represent half(i) forming a molten layer at a temperature below thesections of the buttons. Measurements of secondary den-melting point of the alloy being processed and ofdritic arm spacing were taken using these photographs.sufficiently low viscosity for flow to occur into theThe macrostructures of the partial or complete buttonshrinkage gap between crucible and metal duringsections were also photographed.solidification

(ii) influencing the microchemistry of the inclusions inorder to alter the inclusion wettability and assist in

Results and discussiontheir removal(iii ) achieving thermodynamic stability in order to

prevent reduction of components of the flux by There are two main aspects of the present work which arereactive metals in the alloy discussed separately below. First, the chemical effects

(iv) being sufficiently inert in order to avoid damage to associated with the interaction of the metal and thethe melting equipment inclusions with the flux additions, and second, the thermal

(v) avoiding the formation of high melting point oxide effects dependant on the process cycle and the effect of fluxphases such as spinels (e.g. Al2MgO4 ) which could additions on heat transfer.generate additional inclusions.

Taking into account points (i) to (iii), a variety of oxidesCHEMICAL EFFECTS – BUTTON COMPOSITIONSwere considered, particularly alumina, silica, lime, magnesia,AND INCLUSION CONTENTSand reactive metal oxides and halides. Point (iv) eliminatesChemical compositionproprietary mould mixtures containing substantial amounts

of reactive metals and oxide powders, although fluxes A comparison of the compositions of Udimet720 in the assupplied condition and after various EBBM treatments,containing small amounts of calcium fluoride were consid-

ered acceptable. Point (v) eliminates magnesia, rutile, both with and without the presence of fluxes, are detailedin Table 2 together with information on the processzirconia, and other refractory oxides. The choice of

constituents for the fluxes was accordingly limited to conditions used.

Table 2 Comparison of compositions of Udimet 720 as supplied and after EBBM*

C, S, N, O, Si, Ca, Al, Ti,Specimen Flux wt-% ppm ppm ppm wt-% wt-% wt-% wt-% Ni

As supplied None 0·021 14 14 21 0·01 <0·001 2·9 4·7 Bal.Button 9† None 0·027 11 14 19 0·02 <0·001 2·6 5·2 Bal.Button 19‡ S13Button 23‡ S14 0·008 11 13 15 0·10 <0·001 2·5 5·1 Bal.Button 24‡ S14 0·003 4 14 25 0·08 <0·001 2·5 5·0 Bal.Button 29§ S13 0·003 8 16 17 0·05 <0·001 2·4 4·7 Bal.Button 36† NoneButton 37† None

* As supplied Udimet 720 also nominally contains 17·9 wt-%Cr, 14·7 wt-%Co, 3·0 wt-%Mo, and 1·3 wt-%W.† Buttons 9, 36, and 37 were melted without flux additions. The processing conditions gave a lower energy input to button 9 relative to button 36. Inthe case of button 37 the energy input was similar to that of button 36, but at the end of the drip melting stage the power was cut off, instead ofadopting a controlled power reduction during solidification.‡ For buttons 19, 23, and 24 the flux was added at approximately the same time after the beginning of drip melting; the process cycle was similar,but not identical to that of button 36.§ For button 29 flux was added directly to the crucible and melted by the electron beam before drip melting was started, using a cycle similar to thatof button 36.

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Halali et al. Effects of flux additions on electron beam button melting of Udimet 720 459

a

b

1 cm

500 µm

a button 37, vertical section; b button 9, chill zone at the base of button

1 Buttons melted without flux: for process conditionssee Table 2

When flux additions were made, the button melted alloysshowed no major changes in aluminium and titaniumcontents compared with the as received cast material,although there was a small reduction in aluminiumconcentration. Of the trace elements, use of the fluxcontaining no calcium fluoride produced a substantialreduction in the sulphur concentration (button 24).However, when the flux contained calcium fluoride therewas only a small (button 29) or no (button 23) reductionin sulphur content. The reason for this difference inbehaviour is not understood, but it is suggested thatsulphur removal is mainly the result of reaction with CaOin the flux to form CaS. In the three flux melted alloys thecarbon content of the as received alloy was substantiallylowered from ~0·02 wt-% (in one case by nearly an orderof magnitude). Carbon can exist in solution in the liquidalloy and also as carbide particles. The removal of carbonfrom the liquid is attributed to its action as a reducingagent

C+MO(in flux)�CO+M(alloy)The formation of CO is likely to be enhanced by thevacuum in which melting is carried out. The observationthat the silicon content of the flux melted buttons is higherthan in the as received material suggests that silica in theflux is reduced by carbon.

Non-metallic inclusion contentA comparison is reported here of inclusions observed in

1 mm

a b c

buttons processed under similar conditions with and a button 36 (melted without flux); b button 23 (flux added afterwithout use of a flux (buttons 29 and 29 respectively). The beginning of drip melting); c button 29 (flux added before start offlux was added before the start of drip melting in processing drip melting)button 29. The data, shown in Table 3, were obtained using 2 Composite parts of vertical section of buttonSEM. Specimen 1 was taken from the middle of the buttonsection and specimen 2 from near the surface. On each

THERMAL EFFECTS – BUTTON SHAPE ANDspecimen 10 different areas (1·75 mm2 ) were observed. TheGRAIN/DENDRITE STRUCTUREdata indicate that the button prepared using a flux additionButton shapehad significantly fewer inclusions than that without the

flux. There is also a tendency to a smaller inclusion size. The use of fluxes completely changed the nature of thesolidified buttons. Without the fluxes the buttons had aThe present work confirms and extends the conclusions of

previous studies3,4,7,8 that electron beam button melting, central inclusion rich raft and were humped towards thecentre (Figs. 1a and 2a). With the addition of the fluxes,with or without the use of fluxes, reduces the content of

inclusions in the as supplied material. the inclusions were dispersed to the outside of the button

Materials Science and Technology April 2000 Vol. 16

460 Halali et al. Effects of flux additions on electron beam button melting of Udimet 720

1 cm

500 µm

a b

d

c

1 cm

1 cm

a button 23, vertical section; b button 19, concave upper surface showing non-metallic inclusions at rim, there is no central raft of inclusions;c button 19, lower surface of button, mottled and rough from contact with crucible, prior to flux addition, upper part of button surface in contactwith molten flux is smooth; d button 23, transition from chill to dendritic crystals

3 Buttons with flux added after beginning of drip melting: for process conditions see Table 2

and the button surface had a central hollow (Figs. 3a, b, reduction in sulphur content, of the magnitude shownand 4a). This is compatible with surface fluid flow, in the above to result from the use of fluxes, will reverse theabsence of the flux, being towards the centre of the button, direction of Marangoni convection in agreement with thesweeping the inclusions with it and directing the molten present observations.metal away from the chilled crucible surface, reducing theheat transfer coefficient there and thus retarding solidifi-

Macro- and microstructural effectscation. With the addition of flux, the flow is reversedThe macrostructures of the buttons typically consistedtowards the button perimeter dispersing the inclusions andof an extensive zone of columnar crystals, extending fromdelivering the molten metal to the chilled crucible surfacethe bottom and side walls of the crucible, and a centralwhere it solidifies to form a shell of solid alloy that is largeupper zone of equiaxed crystals. The relative extents ofenough to contain the volume of liquid metal. As the mainthe columnar and equiaxed crystal zones depended on thevolume of liquid solidifies, the volume of metal is reducedprocess conditions. Figure 1a illustrates the structure ofcausing the hollow in the surface of the button. Thesebutton 37, melted without flux addition, in which theobservations are consistent with fluid flow being controlledelectron beam was switched off at the end of the dripby surface tension gradients. Lee et al.9 have modelled the

electron beam button melting process showing that a melting stage, instead of adopting a controlled power

Table 3 Inclusion counts on selected specimens

Inclusion count

Specimen 1 2 3 4 5 6 7 8 9 10 Mean Comments

Button 9, specimen 1 9 4 16 23 17 11 8 1 12 12 12·3 Largest inclusion 70 mm, most 10–20 mmButton 9, specimen 2 9 12 20 22 17 16 8 4 18 18 14·4 Largest inclusion >100 mm, most 10–20 mmButton 29, specimen 1 3 9 2 8 5 4 9 10 4 5 5·9 Largest inclusion 40 mm, most <10 mmButton 29, specimen 2 19 9 14 6 7 9 11 5 8 5 9·3 Largest inclusion 80 mm, most <10 mm

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Halali et al. Effects of flux additions on electron beam button melting of Udimet 720 461

Sp

acin

g, µ

m

100

90

80

70

60

50

40

300 10 20 30 40

Distance from bottom of button, mm

2 9; & 23; + 29; × 36

5 Secondary dendrite arm spacings as function ofdistance from base of crucible for given specimen: forprocess conditions see Table 2: button 29 had fluxadded before start of drip melting

added after the start of drip melting, the lower region thecrystal boundaries were not revealed by the etching,consistent with the rapid quenching and small grain sizecharacteristic of flux free buttons. The figure illustrates theonset of relatively coarse grains and dendritic growth

1 cm

500 µm

a

bemanating from the part of the hemispherical surface thata vertical section; b dendritic growth at the base of the buttonis smooth, which appears to coincide with the height of the

4 Button 29: flux added before start of drip melting metal in the mould when the flux was added.The secondary dendrite arms spacings are shown in

reduction during solidification. In contrast, in button 9, Fig. 5 as a function of distance from the base of themelted with a lower energy input the columnar zone crucible. For the buttons prepared without flux the spacingsextended almost to the top of the button. lie in the approximate range 40–50 mm. There is a slight

A feature common to the buttons melted without flux increase with distance consistent with a decrease in coolingadditions was a mottled, rough surface and the presence of rate as solidification proceeds. Near the top surface aa chill zone consisting of very small equiaxed grains at the decrease in spacing occurs as the controlled solidficationhemispherical surfaces of the buttons (e.g. Figs. 1b and 2a). stage proceeds. A similar variation with distance is observedNo cellular or dendritic structure was revealed in these when the flux is added during the melting process, withgrains. The absence of substructure suggests a combination slightly larger dendrite spacings indicating a decrease inof a high temperature gradient and high solidification rate cooling rate due to the flux layer. When the flux was addedas the molten droplets impact the water cooled copper before the start of drip melting, a significant effect issurface of the crucible. There may be planar growth observed in that the spacings rise to a peak of aroundassociated with a high G/R ratio in this stage of solidifi- 70 mm at a distance of 15 mm from the base of the button.cation. It is likely that the initial stage is splatting of the There followed a decrease in the spacing as was found inalloy droplets. Solidification proceeds upwards constrained the other buttons. The results indicate clearly that theby conduction heat loss through the solidified metal, heat presence of a molten flux layer between the crucible andtransfer across the button/mould interface and the energy the button does not provide more efficient heat transfer.input from the electron beam. This leads to a dramatic The flux prevents rapid cooling of molten drops of alloydecrease in G, R, and the G/R ratio resulting in dendritic on impact with the cooled crucible surface. However, thesolidification over most of the button (Fig. 2a). Equiaxed flux layer does not have better initial heat transfercrystals are formed in the upper, central regions of the characteristics than the button/crucible shrinkage gap. Inbuttons and in the later stages of processing heat loss from fact, the higher dendrite spacings in the main part of buttonthe upper surface, by radiation and convection, has a 29, where the flux was added before the start of dripsignificant influence. melting, imply a considerable reduction in heat transfer

In contrast to the mottled, rough surface of buttons through the flux layer as compared with the button/cruciblemelted without flux addition, the button surface was shrinkage gap.smooth when solidification occurred in contact with a layerof flux. Figures 2b and 3c show cases (buttons 23 and 19respectively) where the flux was added during the drip

Summary of experimental observationsmelting stage. The lower surface of these buttons showedthe typical mottled, rough surface associated with initial

The addition of fluxes during the electron beam buttoncontact of molten metal with the crucible wall (see Fig. 3c).melting of Udimet 720 produced a number of significantThe upper part of the button surface showed a smootheffects:surface resulting from the presence of flux. A further notable

structural feature was observed when solidification begins (i) there is a reduction in the non-metallic inclusioncontent in the main volume of the button relativein the presence of flux. Figure 2c and 4b show the structure

of button 29 in which flux was added before the start of to both the original barstock and buttons producedwithout the use of flux. There is also evidence of adrip melting. No chill zone was formed, but instead

dendritic growth occurred in large grains that originated reduction in the sulphur content. These featuresmay be mainly the result of the presence of CaO inat the hemispherical surface (i.e. from the onset of

solidification). In button 23 (Fig. 3d) where the flux was the flux

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462 Halali et al. Effects of flux additions on electron beam button melting of Udimet 720

(ii ) the central raft of inclusions that forms on the top on fluid flow in the molten metal. This in turn changes theshape of the button and the the distribution of inclusionssurface of buttons melted without flux is absent

when fluxes are added. Inclusions are dispersed in collected at the top surface.2. It reduces the cooling rate at the initial stage ofa ring around the perimeter of the buttons prepared

using fluxes solidification. Without the use of flux the drip melted alloyrapidly solidifies on impacting the water cooled crucible.(iii ) a concave top surface develops in the buttons

produced with fluxes, in contrast to the convex The flux creates a thermal barrier to this initial rapidsolidification. The effect of the flux in filling the gapprofiles normally associated with buttons produced

without flux. This is thought to be a consequence between solid button and crucible has a relatively minoreffect on cooling rates.of the lower sulphur contents altering the surface

energies and causing a reversal of thermocapillary(Marangoni) flow

(iv) the formation of smooth hemispherical button Acknowledgementssurfaces where a molten flux layer was interposedbetween the button and the crucible wall, contrasts

Acknowledgements are made to EPSRC (GR/J 33654) forwith the mottled, rougher surfaces when flux is

support of the work, to the National Physical Laboratorynot present

for providing facilities for button melting, to Dr P. Quested(v) the elimination of the fine grained, equiaxed chill

and Mr D. Hayes for their cooperation and assist-zone which is formed on the button hemispherical

ance, Professor K. C. Mills for helpful discussions, Mr D.surface in the absence of a molten flux layer at the

Phan for carrying out dendrite arm spacing measurements,crucible wall

and to Rolls-Royce for supply of the alloy.(vi) the formation of large columnar crystals growing

from the bottom of the crucible when the flux isadded before the start of the drip melting cycle, Referencescompared with the small rapidly solidified grains inthe absence of flux. This indicates a reduced

1. . : Philos. T rans. R. Soc. (L ondon) A, 1995, 351, 419–nucleation rate associated with lower solidification433.rates when fluxes are used

2. . . , . . , and . . : Proc. Conf.(vii) an increase in secondary dendrite arm spacing when‘Electron beam melting and refining – state of the art 1983’,the flux is added at the start of the drip meltingReno, NV, USA, November 1983, Bakish Materials, 61–94.

cycle, indicating a considerable reduction in the3. . . and . : Proc. Conf. ‘Electron beam

heat transfer through the flux layer at the wall and melting and refining – state of the art 1990’, Reno, NV, USA,base of the crucible. October 1990, Bakish Materials, 10.

4. . . : PhD thesis, ‘Quality assurance by electron beambutton melting’, University of London, 1992.

5. . et al. : Proc. Conf. ‘High temperature materials forConclusions power engineering’, Liege, Belgium, September 1990, Centre des

Recherches Metallurgiques, 1653.6. . . , . . , and . . : Mater. Sci. Eng. A,The use of flux during electron beam button melting has

1993, A173, 371–377.two major effects.7. . , . , and . . . : Mater. Sci. Forum,

1. It modifies the chemistry. The primary effect is to1995, 189–190, 423–428.

reduce the sulphur content and consequently change the 8. . , . . . , and . : in ‘Superalloys 1996’,surface and interfacial energies of the system, and their (ed. R. Kissinger et al.), 457–465; 1996, Warrendale, PA, TMS.dependence on temperature. This has a dramatic effect on 9. . . , . . , and . : Philos. T rans. R. Soc.

(L ondon) A, 1998, 356, 1027–1043.efficiency of capture of inclusions at the melt surface and

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