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the terminating lamella a s elsewhere on the inter-face leads to Eq. [7].REFERENCES

'R . F. Mehl and W. C. Hagel: Prog. Metal Phys . , 1956, vol. 6, pp.74-134.

'W. H. Brandt: AlME Trans. , 1946, vol. 167, p. 405; J . A p p l . P h ys .,1945, vol. 16, p. 139.'J. Scheil: 2. Metallk. , 1946, vol. 37, p. 123.'M. Hillert: Jernkontoret s Ann. , 1960, vol. 144, p. 520.'C. Zener: AlME Trans. , 1946, vol. 167, p. 550.6K. . Jack son and B. C halmers: to be published.

Filtering and Fluxing Processes for Aluminum AlloysK. J. Brondyke and P. D. Hess

Two proce sses have been developed for imp rov -ing the quali ty of molten -alum inum allovs befor ecast ing . The Fi l t ra t ion Process . which involvespassing molten metal through a packed bed ofgranular f i l ter mate ria l , i s a rapid menns of r e -moving f inelv divided pnr ticle s. I t has the mo stpotential in those instances where removal ofinclusions i s o f primnry importance. The Com-bination Fil tration -Inert Gas Fluxing P roc ess in -uolzles introduction of an ine rt gas s o that i t wil ldif fuse co unterc urren t to meta l f low through thefi l ter bed of gvanular material . Dissolved hydrogenis re mov ed from the metal in addi tion to remova l off inelv dzvided particle s. The Combination Pr oce ssis mo st usefu l where both inclusion remova l andat ta inment o f cotu isten tly low -hydrogen -contentmetal are important . Metal t reated by the Com -bination Process is of higher and more uniformquality than heretofore attainable with prolmgedchlorin e f luxing. Co sts of the Combination Pr oc es scan, for the mo st part , be o f f se t bv sacings derivedfrom high rec ove ries and increased product ion ofsup erio r -quality products .IMPROVEMENTn the quali ty of molt en-al uminu malloy s is se cur ed generally through the u se ofsome fluxing practice in a crucible, holding furnace,o r ladle involving either gaseous o r solid fluxingmedia. Typical fluxing agent s may include thegas es nitrogen, argon, or chlorine used eithersingly or a s mixtures and the solids aluminumchlor ide or hexachloroethane. Reg ard les s of thefluxing means used, the pri mar y objectives ar e theadequate removal of both metallic and nonmetallicinclu sions and reducti on of hydrogen content to anacceptably low level. The ultimate goal i s to pro-duce ingots and ca st products of high quality fre efr om inclusions and porosity.

K. J. BRONDYKE, M ember AIME, and P. D. HESS areAssistant Chief and Research Enaineer, re s~ ec t iv el v. abr i -,c a t in g M e t a ll u rg y Di vi si on , ~ l c o a e ie a r i h abora to ri es ,Aluminu m Co. of America, Ne w Kensington, Pa.Manuscr ipt submit ted January 21, 1964. EMD

The development of ultr asoni cs fo r measu rem entof int ern al quality, tightening of qualit y control s,and the application of the aluminum a lloys t o newor different commerc ial fields have been responsi-ble for an e ver i ncreasing demand for improvementof quality of both cas t and fab ric ate d products. Th isdemand fo r high quality has not been res tri cte d toany single type of product. On the con tra ry, the de-mand encompass es about the enti re variety ofproduc ts including those for fabri cation, those to bemachined, buffed, o r finished for decora tive pur-poses, and those for critical applications where bothsurfac e and internal quality is of utmos t import ance.In many instanc es products acceptable in the 40'swould now be sc rap ped a s unacceptable by thes ehigher present-day standards.

With this eve r inc reasi ng demand for higherquality, it beca me appa rent at Alcoa that conven-tional means of melt treat ment w er e inadequate inmany instances and new approaches were neces sary.Consequently the res ear ch program was intensi-fied with attention focus ed on metal tr eatme nt dur -ing transf er in order to reduce the furnace pro-cessin g time to a minimum in an effor t to attain thedes i red resu l t s a t a minimu m of additional expense.As a res ult of thi s intensive resea rch programwhich covered nume rous var iation s and adaptationsof conventional m ethod s in addition to new methodsof melt treatment, se veral new process es weredeveloped, two of which will be described here.The firs t of these, a method of me lt filtratio n, waspatented in February, 1959.l The second proces sinvolving combination filtration-inert ga s fluxingwas patented in June, 1 9 6 ~ . ~ther melt-treatmentpr oc ess es developed during this period of i nvesti-gation were patented as l i ~ t e d . ~FILTRATION PROCESS

General Description. The Filtration Proces s,which involves passing molten metal through apacked bed of g ranula r filter ma teria l, i s a rapidmethod of effecti vely rem ovi ng finely divided pa r-ticles. It is an impingement-type filte r; hence thesize of the particle s removed fr om the metal a r econsiderably s ma ll er than the int ers tic es of the

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bed. The unit is normally placed in a transfersystem so that the metal is treated just prior tocasting.

The filt er unit cons ists e ssentially of a heatedref rac tor y container equipped with a verti cal baffleto dire ct the meta l through the bed of gra nul ar fi l-ter materi al. The baffle is offset s o that the inletside housing the filt er bed is larger than the out-let. The filter unit illustrated in Fig. 1 repre sentsa type commonly used in a transfer system fromfurna ce to casting station. This design, in whichthe filter bed is continually submerged by the metal,permits either continuous or intermittent operationwithout impa irin g eithe r efficiency of fil trat ion orflow capacity. Complete sub mer si on of the bed isesse ntial to the attainment of des ire d resu lts. Theunit is normally maintained at a temperat ureslightly higher than casting tem perat ure.

The granular bed of the filter may be made up ofany refr act ory of high melting point, ine rt tomolten aluminum with a specific gravity g re at erthan molten aluminum s o the granules will sink tothe bottom, and of sufficient hardn ess so a s to pre-vent powdering during use. Of the re fra cto rie spossessing these qualities, Tabular Alumina (syn-thetic corundum) is preferr ed. The refractory-par tic le s izing may be in the range of - 3 + 14 meshU.S. Standard.

The depth of the filter bed may vary from a mini-mum of 6 in. to any maximum practically conven-ient. Since filtration occ urs in depth, the flow ca-pacity of t his filtration proc ess is not affected tothe extent associated with refractory plate-typefil te rs o r scree ns where fi l trat ion occurs princi-pally on the leading surface. As shown in Fig. 1,the filte r bed is supported on a l ayer of TabularAlumina ball s. The latte r se rv es both as a plenumchamber to provide uniform metal flow through thebed and as a ba rr ie r against washing ref ract oryparticles into the trans fer system and ultimatelyinto the cast ings.

Metal flow through the unit is r elat ed to an ove r-flow principle in which flow rate is a function offil ter cr os s section and metal head (the difference

in metal level a s measur ed on the inlet and outletsid es of the baffle). With a 5-in. met al head, afilter bed of 8 in. depth is capable of pas sing 200lb of metal per h r per sq in. of filt er a r ea normalto metal flow. Capacities a t grea te r depths of fi lterbed a r e correspondingly les s. During prolongedoperation, the filter bed will become clogged pro-gre ssi vel y by inclusions removed; consequentlymetal flow ra te will decre ase if the metal head iskept constant. Installations a r e the refore designedwith exce ss capacity and with provision for am plevariation in metal head as required.

Results. No methods a r e available for auantita-tive direc t measureme nts of eithe r the number orsiz e of inc lusi ons in molten-aluminum alloys. Be-cau se of the problem of taking a repr ese nta tiv esample, there is no reliable method for oxide de-termination applicable to the low concentrationsnorma lly encountered in aluminum melting. Con-sequently, the effe ctiv enes s of the filtr ation pro-ce ss has been esta blished by plant and laboratoryevaluations of machining and tool-wear te st s. Ineach evaluation the filtration process was com-par ed to the standa rd method of me lt trea tmentnormally used.

In the castin g field, machining-tes t evaluationsincluded deter minat ion of numbe r of visible dr os sinclusions, de term inat ion of inc idence of ha rdspots, tool life, and tool wear based on radioactivemeasurements.

Since in most instances the castings a re ma-chined only by the customer, machining tests weredevised using spectrographic disk samples castfrom the molten-aluminum alloy to represent meltquality. Visual examination of machined surf ace sshowed a dec rea sed incidence of inclusions inmetal treated by the Filtration Pr oce ss. Typicalresul ts ar e shown in Table I. This beneficial effectof filtr atio n was als o obser ved in examinations fordr os s inclusions and hard spots of those ca stin gsroughly machined in our plants. Coincident withthese re sul ts was an improvement in the life of thelathe tool used in the se rough-machining operations.Radioactive tool-wear te st s on die castings showeda 50 pct de cr ea se in tool wear when machiningcast ings which had been poured fro m filte red

, l N i E T T I I O UG U / P U T L E T T R O U G H i;

Fig. 1 -Filtration -proce ss unit wi t h off-set baffle.

1554-VOLUME 230, DECEMBER 1964

metal.Coincident with the re su lt s of t hes e evaluations,

a substantial improvement by filtration was ob-serv ed in densi ties of vacuum ga s tes t sampl estaken fr om the molten metal. Regar dless of thedensity of the vacuum ga s te st s amp le of t he un-trea ted metal , densities of vacuum ga s test sa m-ples after filtration approached the theoreticaldensi ty of th e alloy sa mpl ed when the hydrogencontent was sufficiently low. In spi te of th es e in-cr ea se s in density, ther e was no evidence ofmeasu rable hydrogen remova l by the Filtrati onP ro ce ss using either the ~ e l e ~ a s ~nstrument o rthe solid-extract ion method5 for hydrogen deter -minations. Since hydrogen was not removed by f i l -tration, it is now concluded that the increased

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dens ities of the vacuum ga s tes t sam ple s were in-dica tive of efficient remo val of inclusi ons whichser ve a s nuclei for bubble formation during solidi-fication. This phase of the subject is d escri bed indetail in our companion paper entitled "Interpre-tation of Vacuum Gas Tes t Res ult s fo r Aluminum~ l l o ~ s " . ~COMBINATION FILTRATION-INERT GASFLUXING PROCESS

General Description. The Combination Filtration-Inert Gas Fluxing Pr oce ss provides a rapid methodboth for removing inclusions and for reducing hy-drogen content. Since hydrogen i s the only g as ap -preciably soluble in aluminum, other gas es a re notconsidered in the discussion in this paper. The im-port ance of attaining low concent rations of hydrogenin aluminum alloys lies in the drastic difference inhydrogen solubility between the liquid and the solidmeta l at the freezing tempe rat ure . Because of thisreduced solubility at the freezing point, ther e israpid precipit ation of the gas during solidificationwith the result that gas porosity often forms in thecas t metal. Voids thus forme d become discontinu-ities upon subsequent fabrication a s the gas p res -su re s developed preven t healing. In castings thevoids a r e undesirable fro m the standpoint of poten-tial leakers, reduced available cross section,lower strengths, and s o forth.

The bas ic unit of the Combination P ro ce ss i sidentical to that described for the Filtration Pro -cess . However, in the Combination Process, pro-vision i s also made to introduce the iner t gas sothat it will pass co unter curr ent to metal flow.Dissolved atomic hydrogen in the aluminum dif-fu se s into the ine rt gas and is thus removed.Tabular Alumina is pr efe rre d for the filter-bedmaterial a s in the Filtration Pro ces s.

In the Combination Pro ce ss , the ine rt ga spasses upwardly through the filter bed on the inletside. Argon i s preferre d a s the fluxing gas a s it issubstantially inert to aluminum. However, nitrogenmay al so be employed in the treat men t of thosealuminum-base alloys where nitride formation canbe tolerated. The efficiency of hydrogen removali s comparable to that when using argon. Chlorinei s unsuitable because of chloride formation with atendency toward rapid clogging of the fil te r bed.

The proced ure for charging the unit of the Com-bination Process is sim ilar to that for the Fi ltra-tion Process, except that the diffuser for introduc-ing the ine rt ga s should also be placed in position.Because of the count erc urr ent flow of the ine rtfluxing gas , metal flow capacity of the Combinationunit is not as grea t a s for a Filtration unit of thesame size.

Resu lts. The lack of quantitative methods fo rdetermination of filtration efficiencies discu ssedfor the Filtration P ro ce ss also applied to the Com-bination Filtration-Inert Gas Fluxing Pr oc es s.Efficiencies of hydrogen reduction , however, were

Table I. Inclusions in Spectrographic Dis c SamplesTes t Incidence ofNo. Sample Source Metal Charge Inclusio ns, PctI Gas-fired Ingot

crucible furnaceGas-fi red Molten metal

crucible furnaceGas-fired Filt ered molten metal 4. 1

crucible furnaceI1 Reverberatory Unfiltered molten metal 18.5

holding furnaceReverberatory Filtered molten metal 0.0

holding furnacedete rmine d by m easu remen ts using both the Telegasand solid -extr actio n method of determini ng absolutehydrogen contents. Vacuum gas test sa mple s wereals o taken to determine relative gas content and in-dicate melt cleanliness. These tests were supple-mented by dye-pen etrant examinations and ultra -sonic tes ts of both ingots and forgings. Material forforging stock and forgings was selected for theseevaluations, a s quality requirem ents fo r theseprod ucts a r e high and metal of low hydrogen con-tent i s desired.

The effectiveness of the Combination P ro ce ss onmelt quality was established by a comparison ofsamp les taken both before and aft er the unit in themetal-transfer system. Since there was intere st ineither reducing furnace fluxing time in some in-stances, or altogether eliminating furnace fluxingwith chlorine in others, numerous evaluations weremade to deter mine the effectiveness of the pr oce sseit her with o r without prio r chlorin e fluxing. Ineach instance re su lt s of the evaluations were com-par ed with those of the then existin g pr ac tic es in-volving only furnace fluxing with chlorine.

The improvement in densities of vacuum g as te stsamp les produced by the use of the CombinationPro ces s was, in all cases, equivalent to o r betterthan that attained by long-time chlorine fluxing.Because of the s imul taneous remo val of both hy-drogen and inclu sions, the den siti es of vacuum ga ste st sa mples were consistently high, always ap-proaching the theoreti cal density of the alloy undertest.Hydrogen remova l by the Combination P ro ce ssis dependent on metal flow rate, inert-gas flow rate,and the hydrogen content of the incoming me tal . Asin any gas-removal process, increased inert-gasflow rate s a re n ecessary either when operating withincreased metal flow rate s o r when treating metalof higher hydrogen contents. Under pro per operatingconditions, hydrogen conte nts af ter the CombinationPro ces s a re in the proximity of 0.1 ml per 100 g.Hydrogen reduc tions typical of the CombinationProcess under plant conditions are given in Table II.One will observe that the same low final hydrogencontent was attained reg ar dl es s of initi al hydrogencontent or metal flow rate. I t appea rs that the only

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- - - - ~ - - - - - - - - - - -~Table II. Typ ica l Hydrogen Reduction with Combination Filtra tion-In ert Gas Fluxing Process

.- -.

Hydrogen, ml per 10 0 g t HydrogenMetal Flow, Argon Flow , Reduct ion,Al loy lb per hr (CFH) After P ct- - --- Bef o re--2014 3,700 40 0.16 0.08 502014 4,200 40 0.18 0.09 506061 4,150 50 0.21 0.12 43606 1 8,300 50 0.24 0.12 506061 8,300* 130 0.41 0.12 7 16061 28,000* 280 0.33 0.08 767075 3,600* 40 0.29 0.10 667075 6,200* 40 0.28 0.14 507075 38,000 165 0.25 0.07 7 27079 6,200 50 0.34 0.13 65

~

'No prior C1, flux.+ ~ ~ d r o ~ e neterminat ions - y Telegas ins trument described in J. Inst. Metals, 1938, vol . 86, pp . 212-19.

8 E l O R C C O Y B l N l T O N P R O CE SS. ,, E R c o n s l N n r l o w P R O C E 5 5 samples and hydrogen determinations, was reflected

" 2 2 , [7 L R L . PO14 A L L O Y - 3 1 0 0 Le 5 < H e in examination of ingots and fabricated pro ductsNO C1? F U R N A C L FLUX\CII 20 C L ~ V ~ L A H DO R K 5 - X 2 2 1 4 ALL^^ produced therefrom. In each evaluation, these in-! P O 0 0 1 8 5 ,nP, N O C 1 2 F U R N A C L FLUXm 2 3 , [IID C L E V E L I N D W O R K 5 - 7 0 7 9 A L L ~ y gots exhibited increased soundness, with isolatedWL- 3 9 0 0 L BS I H R , P R l O R CI2 F U R N I C EF L U * inclusions virtually absent. This improved qualitymL resulted in fewer rejections by ultrasonic inspec-% 2 4 0 4

CII tion at the ingot stage, d uring fabrication, and atC 0W Q final inspection. This reduction in rejections r e-

2 5 ,m F O 3 sulting from th e improved ingot quality has beenu tz real ized in fabricated pr oducts in the fields ofW5 2 . 6 5 0 . 2 forging, rolling, and extrusion.30 07 zE 2 7 W 0 . 1

0SUMMARY

~3 LT0Cu 2 . 8 F Improved met al quality can be attained by us e of1 0

I 2 3 I 2 3 either the Filtration Pro ces s or the CombinationFig. 2-Average hydrogen and vacuum ga s tes t dens ity Filtration-Inert Gas Fluxing Proc ess. The Filt ra-improvement with Combination Filtration-Inert Gas tion P roc ess has the most potential in those in-Fluxing Process. stan ces where removal of inclus ions is of primary

restriction to the minimum hydrogen content attain- importance. The combination Process is mostable by the Combination Pro ce ss is one related to useful where both inclusion remova l and attainmentenvironmental conditions prevailing at the ti me of of consistentl y low-hydrogen-content meta l a r eoperation. importan t. Material trea ted by the Combination

The ability to furt her re duce hydrogen content P roces s is of a highe r and mor e uniform qualitywhen the Combination P ro ce ss supplemented chlo- than heretofore attainable with prolonged chlorinerine fur nace fluxing was duplicated in eve ry evalu- fluxing. Cos ts of th e Combination Pr oc es s can, fo ration reg ard les s of alloy. Data illustrat ing this the mos t part, b e offset by savings derived fromeffect ar e depicted in Fig. 2 , a ba r graph of av er - high recoveries and increased production ofage vacuum ga s t est densities and hydrogen con- superior- quality products. Use of the pro ces sestents of metal proc esse d at the Labor ator ies and ha s improved the internal integrity of Alcoa prod-at Alcoa's Cleveland Works. Fr om this graph and ucts. This i s especially significant by today'sTable I1 i t is apparent that the Combination Process standards with the demand for maximum reliabilityof equipment for both defens e and comm erci al a p-will produce:

1) high final densities of vacuum gas te st s am -plications.

ples re gar dle ss of whether the metal is previ-ously chlorine fluxed;

2 ) simi lar final hydrogen contents over a widerang e of met al flow ra te s;3 ) lower hydrogen contents than metal fluxed inthe furnace;

4 ) substanti al additional reduction i n the hydrogencontent of m etal fluxed in the furn ace.

The improvement in m elt quality by the Combina-t ion Process , as measured by vacuum gas tes t

REFERENCES'U.S. Pate nt No. 2,863,558.'U.S. Patent No. 3,039.864.'~Ju..a t e n t s N o s . 2,840,463 and 3,006,473.'C . E. R a ns l ev . D . E. 1. Talbot , and H. C. Barlow : .I . Inst. Metals,.1958, vol . 86, part 5.'1. L . Brandt and C. N. Cochran: .I. Metals, 1956, vol . 8, no . 12, pp .

16762-74.K. J . Brondyke and P. D. H es s : See co m pani on pa per Trons . Met. S o c .

AIME, 1964, vol . 230. pp. 1542-46.

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