February. 1926
ILVDUSTRIALA S D E-VGIXEERING CHEXISTRY
125
Sand-cast Aluminium-Manganese Alloys' By Samuel Daniels WAR DEPARTMEYT, AIR SERVICE, MCCOOK FIELD,DAYTON, OHIO
at room temperature, which manganese than 2 per cent, ' is of importance when formthe workable materials beThe composition and characteristics of aluminium aling operations must follow ing less manloys to which manganese is added are outlined. The quenching. These strong, ganiferous than those for binary sand-cast aluminium alloys are hardly to be ductile alloys, with their low casting purposes. chosen for their mechanical properties, and heat treatspecific gravity, are of disThere are two binary ment does not improve them in any way. More than tinct importance from the positions employed comabout 2 per cent of manganese increases the shrinkage, engineering standpoint. mercially in this country, the unsoundness, and the difficulties in machining. It has been customary the 's a1uminium-2 manOn the other hand, this element does not, like most not to add to aluminiumganese, for castings others, detract from the normal resistance of combase alloys more than about required to resist corrosion mercially pure aluminium to salt-spray corrosion, and 1 per cent of manganese, by weak Organic in ternary and more complex alloys it probably-acts to because of the foundry and acids, and the 98'5 improve this resistance. The aluminium-manganese other difficulties which this a1uminium-1'25 manganese, alloys are less resistant to corrosion by distilled water element engenders. Manin the wToUght condition' salt spray. The occurrence of a new ganese has been known to for parts to be corroMnSi, is the metallographic feature sion-resistant. The former increase s h r i n k a g e . It has an ultimate strength of unites with aluminium to about 17,000 pounds per give the hard, brittle compound MnAl,, which, formsquare inch and an elongation in 5.08 em. (2 inches) of about 5 per cent, and the latter, ing a eutectic at about 3 per cent of manganese and a t apdepending upon the temper and gage may have an ultimate proximately 650" C. (1202" F.),2is of minor solubility in the strength of from 16,000 to 45,000 pounds per square inch aluminium-rich solid solution (Figure 1). The present paper is one of a number of investigations of and an elongation in 5.08 em. ( 2 inches) of from 25 to 1 per the properties of various aluminium-alloy systems undercent. Manganese is also a prominent constituent in a number taken by the Material Section, Engineering Division, Air of ternary and, more important, quaternary proprietary Service, U. S. A. I n this article are discussed the effects alloys. The 97 aluminium-:! copper-1 manganese alloy is of additions to aluminium of manganese in quantities less a soft, low-strength material for castings, possessing good than 10 per cent.1 resistance to corrosion, especially by salt water, but having Method of Alloying the disadvantage of a low proportional limit of less than 2000 pounds per square inch. It is not suitable for castings Five melts of aluminium-manganese alloys were sandwhich must be tapped for threaded parts and which are cast in the form of test specimens, to the calculatcd mansubjected to vibration. The 85 aluminium-14 copper-1 ganese contents of 0.5, 1.0, 3.0, 5.0, and 9.6 per cent, as manganese alloy received some prominence in England during Melts 4469, 4470, 4471, 4473, and 4474, respectively. The the war as a piston stock. The manganese was found to aluminium ingot was of "Special" grade under Air Service increase the strength a t moderately elevated temperatures, Specification 11,010-B and contained 99.55 per cent of and the loss in thermal conductivity it occasioned was re- aluminium, by difference, and no manganese. The manstored by annealing at about 454' C. (850" F.). The me- ganese was introduced through a commercial aluminiumchanical properties of this alloy as sand-cast are approx- manganese hardener bearing 9.66 per cent of manganese. imately 22,000-1.0-75. (These figures refer to ultimate The exact analyses of the aluminium ingot and the hardener strength-elongation in 5.08 em. (2 inches)-Brinell hardness, are given in Table I. 10 mm. ball, 500 kg. load, applied for 30 seconds.) The wrought, Duralumin-type of alloy, with about 4 per cent of Table I-Percentage Composition of Raw Materials Aluminium copper, 0.5 per cent each of manganese and magnesium, Melt MATBRIAL Copper Silicon Iron Manganese (diff.) and sometimes with a small quantity of chromium, is a better 3538 A1 ingot 0.04 0.16 0.26 None 99.55 3848 AI-Mn hardener 0.82 0.37 0 . 5 7 9.66 ... known material. The manganese therein contributes toward prevention of cracking during rolling, to a fibrous Four alloys were prepared in 15-pound lots by melting structure, and to enhancement of resistance to corrosion. the requisite-amounts of ingot and of hardener together in 1 Received September 3, 1925. Published by permission of Chief of Air Service, War Department.
2
Hindrichs, 2. anorg. Chcm., 69, 444 (1908).
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
126
a plumbago crucible. The alloy with 9.6 per cent of manganese was simply the remelted hardener. All the alloys were poured at 704' C. (1300' F.) except thislast, which was cast from 871' C. (1600' F.), in view of its much higher liquidus (about 820' C., or 1508' F.). It was found that
VOl. 18, No. 2
the analysis was made on 50 cc. Using the entire solution, the manganese content of the hardener was checked by the bismuthate method (0.25-gram sample) to within 0.06 per cent. The results of these determinations are indicated in Tables I and 111. Table 111-Chemical
Melt
3538 4469 4470
4469 4470 4471 4473 4474
Percentage of Manganese Diagram, Aluminium-Man$anese System (Hindrichs)
FI$me 1-Equilibrium
704' C. (1300' F.) was scarcely hot enough to pour the 5.0 per cent alloy properly. The complete schedule of the foundry practice is recorded in Table 11. Table 11-Foundry Practice (Furnace, Monarch; fuel, oil; weight of charge, 15 pounds)
Melt 4469 4470 4471 4473 4474
Time in furnace Minutes 20 20 15 25 20
TBYPBRATURB--
Max. furnace
-Pouring-
* C.
OF.
OC.
O F .
727 727 788 771 927
1340 1340 1450 1420 1700
702 702 704 707 871
1295 1295 1300 1305 1600
Method of Casting Test Specimens
From each melt two molds of tension test specimens were cast to size (1.28 cm. or 0.505 inch diameter), three to the mold, with individual risers and a common pouring sprue, in green sand composed of approximately equal parts of Sandusky and of Albany sand. A photograph of the Air Service standard TB-1 test bar mold has been shown in a previous a r t i ~ l e . ~Temperature measurements were made with a bare chromel-alumel couple and a potentiometer. Method of Testing
CHEMICAL ANALYsrs-The aluminium ingot and the hardener were analyzed for copper, iron, and silicon by standard methods. The manganese content was determined in all samples by the sodium arsenite method. The weights of samples for the aluminium ingot and for the 0.5, 1.0, 3.0, 5.0, and 9.6 per cent manganese alloys were, respectively, 0.1, 0.1, 0.1, 0.3, 0.2, and 0.2 gram. The last threesolutions were diluted to 500, 500, and 1000 cc., respectively, and Daniels, THISJOURNAL, 16, 1243 (1924).
Analysis and Mechanical Properties of AI-Mn Alloys Ultimate Elongation in Manganese strength 5.08cm. (2 in.) Brinell Specific Per cent hardness gravity Per cent Lh./sq. in. A s sand-cast None 11,750 38.5 20 2.68 0.54 25 13,930 27.2 2.691 1.02 30 17,030 22.6 2.696 2.88 36 14,900 2.6 2.713 4.56 39 2.720 16,860 3.0 48 12,560 9.60 None 2.765 A s wenched and aged (1025-96CW300-2) 0.54 13,960 35.8 24 2.690 1.02 15,430 23.3 27 2.697 2.88 15,300 3.3 35 2.715 16,930 2.3 40 2.723 4.56 9.60 11,300 None 48 2.769
MECHANICAL TESTING-Tensile, Brinell hardness (500 kg. load, 10 mm. ball, 30 seconds), and specific gravity tests were made according to standard methods and with the results embodied in Table 111 and in Figure 2. Each value is the average taken from three bars in similar condition of treatment, with the exception that only one specific gravity test was made to represent each mold of specimens. The as-cast bars were tested 11 days after casting, and the heattreated 3 days after the completion of heat treatment. CORROSION TESTING-The unmachined grip ends of broken specimens both as cast and as heat-treated were subjected to corrosion tests in distilled water and in salt spray. For the purpose of comparison there were included in the tests a sample of the aluminium (permanent-mold) ingot from which the aluminium-manganese series was prepared and unmachined grip ends from test bars in the 92 aluminium-8 copper alloy. I n the distilled-water test each manganese alloy was r e p resented in a 400-cc. beaker by two gripend specimens-one
Figure 3-Shrinkaee
(Pouring-Sprue) of Aiuminiurn-MenRanese hiloya, with Increasing Manganese CenteRt to Kieht
as cast, the other as heat-treated. These were then immersed in sufficient water to cover 5 em. (2 inches) of the specimen and to permit also a study of the water-line corrosion. The same treatment was simultaneously accorded unmachined specimens from the aluminium ingot and from the 8 per cent ropper alloy. The exposure consumed 30 days. The salt spray test involved placing a similiir set of specimens on glms supports in an inclosure~cont,inuallypermeated with the indirect spray from an atomized 20 per cent salt solution. The humidity was practically 100 per cent at all times. The spray period was 100 hours.
proscrrt, in all the alloys but that they are hardly to be di.g tinguished from the MnSi and MnAla. In several eases needles of FeAL were indentified on t,he bases of form and af e01nr. Among the etching reagents tried w'ere a 1per cent aqueous stdut.ion of sodium hydroxide, with (Lantsberry) and without 2 per cent of hydrogen peroxide, cold and boiling distilled wat,er, an acid ferric chloride reagent, 5 per cent aqueous sodium hydroxide, and 2 per cent aqueous hydrofluoric acid. The 1 per cent caustic solutions bring out latent scratches and stain the MnAI, and MnAl irregularly, without altering the color of the silicide, which is sharply outlined. Method of Heat Treatment In short etching periods MnAls turns light brown, while the cores are watery blue. The coloratiou becomes v a n s In order to ascertain whether quenching and aging would improve the mechanical properties of the alloys through gated with protracted immersion. The acid ferric chloride the possible solution and reprecipitat.ionof soluble compounds, solution blackens the silicide. Cold and boiling distilled one mold of each analysis was soaked for 96 hours at 580" C. water cause pitting and nonuniform staining of themanganese (1075" F.), about 52" C. (125" F.) below the eutectic tem- aluminides. A dip in 5 per cent aqueous sodium hydroxide perature, iii an elecbric furnace automatically controlled to until hydrogen is evolved turns MnAl, brown arid leaves within * 5.5' C. (10' I?.). The bars "ere then quenched the cores difficultly distinguishable in color, a t least, from into cold water and thereafter aged in an electric oven a t the silicide, which is not attacked. Longer immersions sharpen the boundaries of the compounds and make the MnAL 149" C. (300" F.) for 2 hours. blue to black and the cores brown. The shorter etch is Preparation and Examination of Metallographic Specimens preferable. Etching in 2 per cent aqueous hydrofluoric acid undil hydrogen is evolved sharply outlines all the conA 1.27-em. (0.5-inch) transverse metallographic specimen stituents, especially the MnSi, which, while the other comwas cut from the riser end of the middle bar in each cast pounds are not affected, is stained brown. This etch is preand in each heat-treated mold. The procedure for polishing ferred to longer immersions, which blacken the silicide. the specimens has been outlined in &previousarticle? They The 5 per cent sodium hydroxide and the 2 per cent hydrowere examined, unetched and etched, a t low, a t int.ermediat.e, fluoric acid etchants, used in conjunction with one another, a.nd at high (1.9 mm. objective nuder oil immersion) magni- are quite helpful. fication. A study of the constituents unetched revealed that silicon, Foundry, Machining, and Mechanical Properties of slaty tinge, is easily recognizable, but that the watery blue MnAL is difficult to distinguish from iron-bearing constituCAST A~~~ous-Theaddition of much more than about I ent,% from the bluegrey skeletons, filigree, and fringes of per cent of manganese results in foundry and other di5another constituent of intimate association, which are culties. This element increases the shrinkage (Figure 3) thought, at least in part, to be manganese silicide (MnSi, and promotes porosity when present in appreciable quantity. or perhaps MnAlSi), and, to less degree, from the cores of Such unsoundness waa especially observable in the metpinkish Mn&(?), occurring in the 9.6 per cent manganese allographic sections. In the 9.6 per cent manganese alloy alloy. The MnSi, however, appears to be harder than the internal stresses were of sufficient magnitude to cause the MnAlr on relief-polishing, which stains 4he former a an exudation of liquid eutectic. A lathe test to compare light brown. This type of polishing also lessens the natural the machinability of the 4.6 per cent manganese aUoy with color-contrast between MnSi and the cores; but the fact that of the 8 per cent copper alloy showed that the latter still remains that the silicide takes the characteristic forms gave a longer chip, was more free-cutting, and took a much of filigree, fringes, or detached skeletons, whereas the core smoother finish. The former material machined like cast are always sheathed with MnAla. A8 to the iron-bearing iron. The hard, brittle compound MnAla, which seems to constituents, it is believed that purple FeAb skeletons are drag and drop out, is probably responsible for this action.
MI,. X 100 i:3,!IiO-27.2 - a i . MnAIa
Figure 5-0.5
F i ~ u r e4-0.5
sad
iron-beating skcietoiis
mid
i 1
PiCurs 7-2.9 M n . X 100 Quenchedandaged. i5.?00-8.8.-3~. Needle lormation of prmary and fine e i i f c ~ f i careas 01 MnAh
Mn.
X 500
SnoB~cast. M s AI2 skeletoiis
Figure 6-2.9 M n . x IO0 Smd-cast. 14,YJO-2.6 -30. Disioiir?cd spines o bearing f MiiAla skeletons (iarger p$wlic!cr) and filmy 1r00-
Finure 9-9.6 M n . X 100 Sand-cart. 12 660-0-48. Duplex needles kontsiniag Miikli ilirathcs and hlniAi cures
F
Pisure 10-9.6 Mn. X 500 Sand cakt. DlnAlz llisht). MruAl cores Ihalftone). and MnSi I?) ftinseo
~ 12-9.6 o
Mn. ~
~ X IbU ~
~
As far as ultimate sireiigtli and percentage of elongaiion are concerned, there is not, murh to he gained for the sandcast, alloys by introducing much more than, perhaps, 2 per cent of mangmese ( T ~ h l e111 and Figure 2 ) . In fact, the binary alloys of ahiminiuni \vith manganese arc hardly to be rhoson for their strength, for 20,000 poiincls per square inch uvoiild prdidbly be a Inaxiinum. The decline in dmgation nliicli occurred for the alloys containing 1.0 and 2.9 per cent of maw ganese is so abrupt that the curvewould have been strengthened had a 2 per cent alloy been irtcliided ill tlic series. The Brinell hardness graph manifests a gradual increase n i t li Inn n ga iiese content froin 20 t.o 48. In thin respect its trendisdissimilar to that of ultimate strcngbh, which begins to fall away after ahout 5 per cent of inang:nltese is passed. I t is Figure 33-9.6 M n . X 500 thought, however, Quenched and aged. Curer "rsriy b) that too niiieh rehcal treafnient. Coinpare with Piguie 10 liaiice c a n n o t be placed U ~ O I the I strength and the elongation curve for the allovs higher in manganese,& they appeitr iensitive to pouring ternperature. The specific gravity rises regularly with tlie percentage of manganese, I-~E:XFTI%EATED ALLOYS.-Ifardly any difference WBS foulrd physiaally or mechanically betlveen the cast and tlie he& treated test specimens (Table 111and Figure 2).
Corrosion
In t.he distilled-water test the corrosion products of the manganrse alloys varied in color from a mixture of brown and white on the low-manganese irlloys to solid brown on the high-manganese alloys. This brown deposit is evidently manganic hydroxide.4 The extent of corrosion of the maoganese alloys was about the same as that of the pure aliiminium, brit slightly less than that of the 8 per cent copper alloy. Neither increase in manganese cont.ent over 1 per cent nor heat treatment altered the behavior of the several alloys. It seems peculiar that these tests showed that the RIUminium-manganese alloys are rather readily attacked by distilled water but are more corrosion-resistant to salt spray *Institute Mechanical Ewineers. Ninth Report to the Alloys Research Committee, 1910, p. 283.
('l'aljle IV), After 100 liours in t,lie a p aluminium ingot showed only slight cor white i n color a i d proliably aluiniiiiirni hydroxide. h d y s i s of the corrosion product oil tlie 9.6 pcr rent rnangancse alloy gave 1.5 per cent ol mangancse, rvhicit iodioates t.hat the solution of manganese from its hard compounds is qiiitc sluw. T!ie 1 per cent alloy, however, was practically as resist,ant as the alloys richer in manganese, and hest treatment did not affect the eorrodiliility. All these alloys were superior i n this respect to the 8 per cent copper alloy aiid conipnwd f:t\r(Jr:lbl) with t , h alumiuiiim-silioori al!oys. .%%'XdlIography
The :uirlition of 0.54 per eent of marigancsc causcd iii the as-cast :alloy tlic :Lpiwnranee of hfn& chieffy as skeletons (Figiims 4 m i l 5 ) , Init also :is needles and sinall rouuded particlrs. Iron-lienring ncedlcs aiid skektlctorrs were also evidently present, though not easily differcntinted, because ghter-colored MnA13. of meager &e, froin the ordiiia With this latter consbituent were assorialed tiny bluegray coionies of what may be MnSi. The heat treatinent oC this alloy prodoced no strixtural ehmgc except bhe appearmice of a slight intragranular preeipitnt,c of hfnAIa (1"igure 14). As tlie manganese rontent is increased t h e s k e l e t o n s of 11 n A I IJ e e a in e larger a n d more r a m i f i e d and the needles stouter until, with d i o i i t the ent e c t i c r a t i o , the rhomboidnl ha h i t hg:m to crop out and t h e criluinnar a r r a n g e m e n t was common (Figure 6). Primary MnAI, was found in the segregated arwis, close ijy more charactcristically eutectic Jlndb ( F i g u r e 7). The Pinure 14--9.6 M n . x I000
nature MuAb surrounded by MnSi (?I fringes. Note iiifragrnnulnr precipitate ot MnAla of the constituents other than and exccpt hiInAlr did not diange perceptibly. With 4.6 per cent of mangairese primary, rhomboidal MnM8 was abundant (Figure 8). After the heat treatment of these alloys the iiitragranular precipitate or eoagulate of PinL& was apparent-the only noticeable structural change. As ea&, the 9.6 per cent manganese alloy had cores of hIAl(?) sheathed with MnM3 (Figures 9 and IO). The evidence for the occurrence of the MnSi constituent is pre-
I X D VSTRIAL A S D ESGXNEERISG CHEMISTRY
130
sented in Figure 11. The wateiy blue particles of Alnhls have evidently reacted m-ith the liquid to form another bluegray enfringing constituent. Massive chunks of purple Fe-413 were found associated with M n d , but this type of Feilla was absent in the other alloys, in which the iron content was considerably lower. Heat treatment removed the cores nearly in entirety (compare Figures 12 and 13 with
VOl. 18, No. 2
Figures 9 and 10). Figure 14 is of hlnAls, the hlnSi(?) fringe, and the abundant intragranular precipitate of MnA13. Acknowledgment
Grateful acknowledgment is herewith made to A. C. Zimmerman, Clifford Mcllahon, and to John L. Hester for their assistance in this experimentation.
Factors Determining the Brightness and Opacity of White Paints’ By F. H. Rhodes and J. S. Fonda CORNELL UNIVERSITY, ITHACA, N. Y.
HE opacity of a paint may be defined as the ability absorbed, so that the brightness will be low. As the thickof the paint to obstruct the transmission of visible ness of the film of white paint is increased by increasing either light. This property is of very great importance the number of coats or the thickness of the individual coats, to the user, since upon the opacity depend the quantity and less and less light is transmitted and the brightness of the the number of coats of paint required to obscure the surface painted surface increases, until finally a point is reached a t over which the paint is applied. To obtain complete hiding which practically all of the incident light is either reflected with a paint of low opacity a large quantity of paint and a or absorbed by the paint film itself. When this point is large number of coats will be required, so that both the cost reached further increase in the thickness of the film produces of material and the cost of application will be high. By no measurable increase in brightness. The brightness of such an opaque film of any some authorities a distincp a r t i c u l a r paint may be tion has been drawn betermed the “ultimate brighttween opacity or degree of A formula is developed to express the relation between ness” of that paint. obstruction to visible light, the brightness of a film of white paint and the thickIt is obvious that the and hiding power or ability ness of the film, and experimental evidence in support thickness of the film of any to obscure the underlying of this formula is advanced. The well-known effect particular paint which is surface. This distinction is of the addition of a small amount of black pigment required to attain the ultinot a valid one; the two in increasing the hiding power of a white paint is exmate brightness for that terms are simply two plained. Attention is called to the possible effect of paint will decrease as the methods of expressing the the roughness of the surface of a paint film upon the opacity of the film increases. same property. brightness of the film. The actual brightness of All our so-called white any thin film of a paint is a paints absorb a t least a small functionof both theultimate amount of theincident light. so that the amount of lGht reflected by the painted surface is brightness and the opacity. The opacity of a film of paint is, always less than the amount of light incident to that surface. in turn, dependent on the thickness of the film, the ratio of I n other words, “white” paints are really gray instead of white. the refractive index of the pigment to the refractive index of The ratio between the amount of light reflected by a given film the vehicle, the volume percentage of pigment in the paint, the of paint and the amount of light incident to that film may average size (diameter) of the pigment particles, and the shape and structure of the individual particles of pigment. be taken as the measure of the brightness of the film. The substances that are used as paint pigments are, in The brightness of a white paint is of very great practical importance, particularly in the interior painting of resi- massive form, transparent. The whiteness of the white dences, office buildings, and factories. The intensity pigments in their usual finely divided condition is due to of illumination within a room depends not only upon the the multiple reflection and refraction of the incident light amount of light entering the room, but also upon the extent at the very large number of exposed surfaces. Since the to which this light is conserved by reflection from the walls amount of reflection that takes place in a light ray that passes and furnishings. Rooms painted with a white paint of a t a given angle from one transparent medium into another high brightness will show much greater average intensity increases with the difference between the refractive indices of illumination with the same input of light than will rooms of the two media, i t is apparent that the total amount of painted with a paint of low reflecting power. Moreover, reflection from a finely divided pigment suspended in a the use of a paint of high reflecting power facilitates uniform transparent vehicle will increase with the difference between illumination and minimizes the variation in the intensity the refractive index of the pigment and that of the vehicle. of light caused by the irregular distribution of light sources. Since the suspending medium in an ordinary paint is oxiIt is apparent that when a so-called white paint is applied dized linseed oil, the opacity of a paint should increase as over a black surface the brightness of the painted surface the difference between the refractive index of the pigment will depend to large extent upon the opacity and thickness and that of the oxidized oil. This fact has been recognized by most investigators in of the film. With very thin films or relatively transparent paints a considerable amount of the incident light will be this field. Several investigators have determined the brighttransmitted to the underlying dark surface and will there be ness and opacity of dry paint pigments or of suspensions of pigments in water and glycerol, and have proceeded to use I Received June 1. 1926.
T
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