MEIGH
The casting tem perature was ab o u t 1,150 deg. C., the average time taken to fill the mould was 40 seconds, and the castings weighed 26 lb. each. A fter the m ould was full, the -re-in., i-in ., and |-in . steps solidified in 5, 14, and 32.5 seconds respectively, showing that solidi
fication had taken place during the pouring period to the extent o f 35/40 or 87 per cent, volume of the -re-in. step; 65 per cent, volume o f the f-in. step; and 20 per cent, volume of the f-in. step. Leaving the 1-j-in. and 3-in.
steps with the heart of the mass still m olten—
they solidified after periods of 78 and 210 seconds respectively. T he expression “ the heart of the mass ” is used because solidifica
tion had started in layers parallel to the walls o f the mould. W ith the object o f obtaining com parative figures for the solidification speeds of a non-m agnetic alloy containing 9.7 per cent.
Al, 3 per cent. M n, and the rest copper, and the complex alloy, previously m entioned, con
taining iron and nickel, other castings were run in, as far as possible, similar conditions.
In the non-m agnetic alloy, the -ré-in. step solidified 15 sec. after the mould was full, as com pared with 5 sec. for the nickel alloy. In term s of volume, 37.5 per cent, of the ré-in. step in the non-m agnetic alloy had solidified during the pouring, as com pared with 87 per cent, of the nickel alloy, and, the other steps in the n on
magnetic alloy were still molten at the heart, while the nickel alloy had solidified as previously mentioned.
The com parative behaviour of the two alloys is shown on Fig. 4. It is obvious that the pour
ing speed o f the nickel alloy can be increased, to a certain extent, to com pensate for its more rapid solidification, but there are limits of thick
ness and area, after which the nickel content must be cut down or eliminated, if moulds are to be filled under practical working conditions.
T hin sections solidify in their entire thickness, but the heavier sections solidify in layers approxim ately parallel to the m ould walls and, as the total period of solidification in the 6-in.
section is 750 sec. for the nickel alloy, and 1,300 sec. for the non-magnetic, -ré in. thickness o f metal is solidified after 7.5 sec. in the form er and 130 sec. in the latter; 1 in. thickness is
solidified after a period of 116 sec. for the nickel alloy, and 227 sec. for the non-m agnetic alloy. W hile, under average conditions, flow stops and piping starts after 395 sec. for the nickel alloy, and 645 sec. for the non-m agnetic alloy, leaving a final solidification o r feeding period o f 360 sec. for the nickel alloy, and 645 sec. for th non-m agnetic alloy.
R eferring to Fig. 2, it is obvious th a t the speed o f solidification, fo r any given thickness of section, can be influenced by a num ber of outside factors, and th a t the structure will vary accordingly, but a given structure has a definite ultim ate tensile strength value. M icro-exam ina
tion can therefore be used as a gauge of the tensile properties, rather than as an indication
Fi g. 1 .— In f l u e n c e o f Al u m i n i u m o n t h e Mi c r o- St r u c t u r e o f a n Al l o y CONTAINING C O P P E R AND M A N G A N ESE.
MFLU.Iff CE. OF ALU MI M U M
O/V f i l LOY C O ttT A IH /riG
■CC-t'&K ANJ>
K A O .
1 4 0
of thickness o r speed of solidification in seconds, which are only given here to com plete a picture.
T h e particle size of the constituents depends upon the com position o f the alloy, the speed of solidification, the subsequent heat-treatm ent and the m echanical or physical treatm ent which the m etal receives at different stages after solidifica
tion, b u t a given particle size represents a given tensile strength in a given alloy.
W hatever the com position o f the alloy, the particle size is the greatest in the annealed state o r in the “ as-cast ” state, in the case of heavy masses. T here are several ways of obtaining a given structure, but it is proposed to lim it this discussion to the modifications of structure brought about by different solidification speeds.
Speed of solidification, and heat-treatm ent after solidification, have as m uch influence as
F i o . 2 . — I n f l u e n c e o f S o l i d i f i c a t i o n S p e e d o n t h e M i c r o - S t r u c t u r e o f a C o p p e r - A l u m i n i u m - M a n g a n e s e - I r o n - N i c k e l A l l o y .
141
m echanical treatm ent on the m icro-structure, and hence on the physical and m echanical properties of alum inium bronze. Such technical in fo rm a
tion as has hitherto been published on the subject of solidification speed gives the im pression th a t there are only tw o possibilities, nam ely, those arising from sand and from chill casting, and th a t the latter m ust essentially be restricted to the relatively few castings th a t can be cast in a m etallic m ould. This impression is entirely erroneous, and the d ata given on the scales show how qualities m ay be varied in a sand casting, and how this variation depends upon mass, and has nothing to do w ith the use of chill moulds.
These data also show the fallacy of relying too m uch on results obtained from standard test-pieces no m atter w hether they are cast-on
Fi g. 3 .— St e p p e d Ca s t i n g f o r Te s t i n g t h e In f l u e n c e o f Th i c k n e s s o n Me c h a n i c a l Pr o p e r t i e s.
or separate. Such test-pieces give valuable indications as to the quality of the m etal from the furnace, bu t no t in the casting.
Design
T he characteristics given in Fig. 2 show, th at although exceptionally good properties are obtained in thick sections, they are im proved in every respect, as sections are reduced and solidi
fication is correspondingly accelerated.
Perfectly satisfactory alum inium bronze cast
ings can be m a ce from patterns intended for other metals. T he fullest advantage, however, can only be taken of the properties of these alloys by specialised design, reducing thicknesses w here it is desirable to do so, and thus effecting the m axim um economy, not only in the intrinsic value o f the m etal employed, bu t also in m ould
ing cost.
It is obvious th a t an appropriately lightened design, w hilst affording these economies will, at
the sam e tim e, offer a degree o f security equal to one in which thick and heavy sections are em ployed, m ore by force o f habit than from a stand p o int of practical utility.
A lum inium copper alloys pass almost instantaneously from the liquid to the solid state, and in doing so the m etal is reduced to 9 /1 0 th s o f its liquid volum e before the com
m encem ent o f ordinary lineal contraction.
These conditions of solidification offer very considerable advantage over those of alloys which pass through a “ p a s ty " phase prior to com plete solidification. A t the same time they necessitate special treatm ent during the casting operation, to ensure th a t an adequate supply of m etal is provided, to replenish what would otherwise be em pty space caused by reduction in volume. This reduction in volum e may be
1 g
WUOIflMTlON JPtID SECONDS
NICKELNO«
X 5 15 X It 15 X 32 39 r 49 142 IV n 227 2" IIS 337 3 ' 227 575 I,' 332 827 5 ' 555 1060 6' 730 1300
Fi g. 4 . — So l i d i f i c a t i o n Sp e e d s t o b e a s s o c i a t e d w i t h In c r e a s i n g Me t a l Se c t i o n.
term ed “ internal contraction ’ as opposed to shrinkage o r surface contraction.
Fig. 5 shows a casting consisting of a heavy boss 4J in. thick, and a heavy outer flange and ring I f in. thick, united by light webs, 1 in.
thick. It affords a good illustration of the advantages o f single-phase solidification. In the com m oner bronzes the webs solidify and begin to shrink while the boss, and outer flange, are still in a m ore o r less pasty condition.
F ractures are thus caused in the paste, which has neither strength n o r ductility. In aluminium copper alloys, the webs solidify and act as cool
ing fins, thereby accelerating solidification in those parts o f the m olten masses in their imme
diate vicinity. As soon as the m etal solidifies, it possesses both strength and ductility, and the lineal contraction, w hich is norm ally 1.8 per cent, and which w ould otherw ise set up internal stresses, is com pensated either by the sand being crushed or by the elongation o r deform ation of the m etal u p to a m axim um o f 1.8 per cent.,
whilst it is still at a tem perature within the annealing range.
If the m oulding m aterial has a uniform co
efficient o f conductivity, a casting of uniform thickness solidifies in layers parallel to the m ould face.
Fig. 7 represents a flywheel with a heavy rim and boss, and with spokes o f different design, and shows the order of solidification. W here a thin section joins on to a thick section, i.e., the rim and boss, it accelerates solidification a t a place near its point of contact I and J, whilst in section H , of m uch greater thickness, there is the reverse effect. If the rim and boss were cast separately, solidification w ould occur in
has fed the m etal previously frozen. Solidifica
tion will then progress, and the last places to freeze will be L, M and N respectively. These places in tu rn will be fed by suitable headers, and a perfectly sound casting will be produced w ithout the use of artificial chills. T he tensile properties of each p art of the casting will vary according to the speed of solidification, but neither weak places nor internal stresses will exist.
W hen radii are used for spoke F , the chilling effect on the masses is slightly reduced. This effect is still further reduced by the addition of necks and radii, as in spoke G. By increasing the section as at K , the mass of the rim is in
creased, and solidification is retarded, as shown by the black area.
It will be noticed that the use o f radii or necks for joining thin to thick section is o f no disadvantage. This is not the case, however, w hen two thin sections are joined together as is shown in Fig. 8, which illustrates suitable, and unsuitable methods. In this illustration P
Fi g s. 5 a n d 6 .— Pr im a r y Wh e e l f o r Hy d r a u l ic Cl u t c h w e i g h i n g a s Ca s t 2 ,3 6 6 l b s. ( Co u r t e s y, Ba r c l a y, Cu r l e & Co m p a n y, Li m i t e d.)
layers o f uniform thickness, A, C and D repre
senting the layer solidified after the first period, and B th a t after the second period. T o tal soli
dification m ight occupy 30 min. If the other sections, E, F , G and H , are cast with the rim and boss, solidification of both the latter is modified according to the respective heat- absorbing effects o f the arms. The section E is, say, | in. thick, and solidifies in, say, 170 secs., and, while p a rt o f its lineal contraction is taking place, it cools the m etal in the rim and boss, I and J. A t the same time, the general fall in tem perature is producing solidification layers, A, C and D.
D uring the second period of, say, 15 min., the area shown as grey becomes solidified, and
F i g . 7 ( L e f t ) — F l y w h e e l s h o w i n g Or d e r o f S o l i d i f i c a t i o n ; a n d F i g . 8 ( R i g h t ) s h o w i n g U s e o f R a d i i i n D e s i g n .
represents the wall of a cham ber; and the m etal in the walls o f even thickness solidify in, say, 30 seconds, for the junctions Q and R have no retarding effect on solidification. If, however, radii are used as at S and T. solidifi
cation takes m uch longer at the junctions, and as there is usually no possibility o f providing a supply of feeding m etal, cavities o r porous zones are form ed, w hich weaken the casting, at places where extra strength was expected by the designer. This effect m ay be overcom e by the use of artificial chills, but the operation is costly. M oreover, it is unnecessary, if the designer appreciates the difference between the single-phase solidification of alum inium copper alloys, and the dual-phase solidification o f those alloys of lower tensile strength, th at are in m ore general use.
143
U se of Chills
Chills are used in the foundry either to en sure sound castings or to produce local increase o f tensile strength in vital parts. T heir use is costly and should be avoided w henever pos
sible. M uch m ay be done in this direction by designing to ensure the m axim um uniform ity o f thickness throughout a casting. W hen a local increase o f thickness is required, such places should be situated w here they can be attached by feeder heads, or else they m ust be chilled.
It m ust be rem em bered that the m achining allow ances result in increases o f mass, an d that although a casting m ay be designed with un i
form thickness, the thickness “ as cast ” m ay vary considerably. These rem arks apply to such castings as valve seats, bu t the thickening up o f flanges, for m achining, presents no diffi
culty— the flanges being easily fed o r chilled, while the chilling o f the valve seats w ould be a t least a costly operation, an d in som e cases extrem ely difficult.
. cü llUrU-''"
_
— -/¡ y/
1/
THICKNESS OF SAND CAST.
Fi g. 9 .— Re l a t i o n s h i p i n Me c h a n ic a l Pr o p e r t i e s o f Ch i l l a n d Sa n d Ca s t i n g s.
T here is a lim it to the effect of chills, usually dependent on the pro p o rtio n o f the surface o f the m ass th a t can be covered by them. To illustrate this p o int Fig. 9 shows the com p arative effects, produced o n different thick
nesses, by full chill, semi-chill an d sand casting.
F o r exam ple, a cube o f the alloy w ith 8-in.
faces chilled on five sides will solidify a t the sam e speed as one with 7-in. faces chilled on one side, or as a 6-in. cube cast in sand w ith
o u t chills. In thinner sections for instance, a 3-in. thickness in the form o f a plate chilled on both sides, w ould solidify a t the sam e speed as a 2-in. thickness chilled on one side, or re-in. thickness w ithout chills, o r a 2-in. thick
ness chilled on both sides w ould solidify a t the sam e speed as l |- i n . thickness chilled on one side o r -re-in. thickness w ithout chill.
Such figures as these m ust be regarded as only approxim ate, since they are influenced by a
num ber of outside factors. T hey indicate, h ow ever, the limits w ithin w hich sound castings can be produced in spite of in appropriate design, or alternatively, the extent to w hich vital parts o f castings can be given increased tensile strength.
By reference to an ap p ropriate micro-scale, such as Fig. 2, and the corresponding graph, it is easy to determ ine, in advance, properties which m ay be given to the different parts of the casting, by taking into consideration the thickness o f the m etal and possibility o f using chills.
Chilling in the m ould, though beneficial when used in the m anner described, should be followed by a stress-relieving treatm ent, to eliminate internal stresses. Such stress-relieving occurs autom atically under ordinary conditions o f cool
ing in a sand mould.
Shrinkage
T he shrinkage o f alum inium bronze depends to a great extent on the m ethod of pouring, the
F i g . 10.— S h r i n k a g e i n C a s t i n g s o f D i f f e r e n t S e c t i o n s .
speed a t w hich the m ould is filled and the thickness o f the casting. T he m axim um shrinkage o f these alloys is 2.4 per cent., but the conditions under which such a high figure is reached are extrem ely rare. T hey occur, in fact, only in the case o f such castings as bars, w hich are cast very rapidly an d are free to contract w ithout hindrance.
In profiled castings o f all kinds there are various influences w hich have their effect upon shrinkages. These are as follow : —
(1) Slow pouring allows the m etal to solidify, and to start shrinking while the m ould is actually being filled, thus reducing the am o u n t of shrinkage in the finished casting. It has the greatest effect in the vertical direction in which a casting is being run, w here it reduces contrac
tion to ab o u t half th a t in o th er directions.
Take, fo r exam ple, the stepped casting already m entioned in which 87 per cent, volum e o f the
1% in. step : 65 per cent, of the f in. and 20 per cent, o f the 1 in. step solidified during the p o u r
ing period.
T a b l e I .— Solidification Conditions in Different Thicknesses
Percentage volume solidified
during pouring.
Thickness in inches.
Speed in seconds. Percentage volume solidified
during pouring.
Nickel alloy.
N on
magnetic
87 A 5 .0 15 37.5
65 f 14.2 45 Molten.
2 0 I 32.5 99
Molten 1 49.5 142 99
99 l i 78.0 227 99
99 2 116.0 337 99
99 3 227.0 575 99
99 4 392.0 827 99
99 5 555.0 1,060 99
99 6 750.0 1,300 99
Layers of Solidification in 6-in. Cube.
Alloy.
Percentage volume solidified.
Time in seconds.
Thickness in inches.
Nickel 73.0 555.0 Piping.
99 52.5 392.0
99 30.5 227.0 2.1
99 15.5 116.0 0.95
99 10.5 78.0 0.63
99 1 . 0 7.5 0.06
Non-magnetic 82.0 1,060.0 Piping.
99 64.0 827.0
49.5 645.0 99
99 44.0 576.0 2.55
99 £6 . 0 337.0 1.53
99 17.5 227.0 1.05
9 9 11.0 142.0 0.66
9 ) 1 . 0 130.0 0.06
(2) The extrem e plasticity of these alloys, just below their melting point, allows them to
“ give ” when resistance is encountered from a core, or projection on the m ould, which again results in reduced shrinkage in the casting. This plasticity produces its greatest effect in very thin sections, w hich are n o t strong enough to crush the sand. There is seldom any measure- able contraction in the walls of a casting A in.
thick.
(3) The high degree o f tensile strength which the m etal possesses at tem peratures o f from 700 deg. C. downwards enables it to crush the sand in the hardest mould. T he thicker the casting the m ore pronounced is this effect, as, although thick sections are norm ally cast m uch m ore slowly than thin, the layers o f m etal which solidify while the m ould is being filled attain sufficient strength to crush the sand before filling is completed. This tends to increase shrinkage of the finished casting.
W hilst these influences m ake it impossible . to form ulate definite rules for the estimation of shrinkage, the figures given in Fig. 10 may be taken as a general guide.
In cases w here accuracy of dimensions is of great im portance, it is advisable to allow for a lower rate of shrinkage in a bore, and a higher rate in overall outside lengths, and to leave an extra machining allowance to com pensate for any error which m ay occur. A fter the first casting has been m ade from any pattern, the latter may be rectified so that norm al machining allowances are left on it for future use.
The author wishes to express his indebtedness to the M cGraw -H ill Publishing Com pany, Limited, for permission to use some of the data appearing in his book, “ Practical Application of A lum inium Bronze.”
145 L