INDUBTRIAL A N D ENGINEERING CHEMIXTRY
January, 1923
25
Die Castings By Sam Tour DOEHLERDIE CASTING CO.,BROOKLYN, N. Y.
IE CASTINGS, as referred to in this article, are castings made by forcing molten metal, under pressure, into a metallic mold or die. The casting of metal into metallic or permanent molds was practiced many centuries ago, but the application of force or pressure to this process is somewhat in the nature of a recent development, although machines for doing this were developed almost fifty years ago. The history and evolution of the process,' as well as the various types of casting machines,2 and the various types of alloys that are commercially die cast and their general properties, have all been ably described by Mr. Charles Packa3
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ALLOYSCAST Die casting, as it was first developed, was limited to tin and lead base alloys having melting points not exceeding 350' C. A short, time later, zinc base alloys having melting points up to approximately 450" C. were introduced. This was the status up until about 1914, when the die casting of aluminiurn base alloys, with melting points up to 650" C., was developed on a commercial scale. In other words, the tendency has been to go to alloys of higher and higher melting points. The proposed steps after aluminium alloys are the brasses, and &hen the bronzes, and so on to cast iron and then to steel. I n this progression, not only do the melting points increase, but also the total heat to be absorbed by the die increasers. I n Table I the figures have been largely taken from "Metallurgical Calculations" by Richards. To the left are the metals considered. With the exception of brass and cast iron, the elements have been chosen as they illustrate the point sufficiently, and also because there do not exist in technical literature many data of this nature concerning allPoys. It will be noticed that each step forward in types of alloys has been in the direction of higher temperature and greater heat to be absorbed by the die per casting made. It is this increase in temperature and heat that has for the last decade retarded the development of the art and industry of die casting. Die-casting machines and die-casting dies are complicated and expensive, and unless a considerable number of castings can be made from such equipment before it becomes unserviceable the whole process must fail. 1
Proc. Engineers' Society of Western Pennsylvania, 88 (1918).
2
"Die Castings and Their Application to the War Program," Am.
Ynst. Mining Met. Eng. Bull., 146. 'Paper presented a t the spring meeting of the American Society of Mechanical Engineers, in 1920.
LIFEOF DIES For tin and lead alloys having casting temperatures approximately 332' and 427" C. and heat-absorption values of 192 and 178 cal. per cc., respectively, dies made of plain carbon machinery steel without heat treatment will last almost indefinitely, so far as heat effect is concerned. For zinc alloys having casting temperatures up to 519" C. and heat absorption values of approximately 450 cal. averagesized dies made of plain carbon machinery steel not heattreated will develop heat cracks or thermal-fatigue cracks after having made from 20,000 to 30,000 castings. These same dies properly heat-treated will make 50,000 to 75,000 castings before developing these cracks. By using a high-grade alloy steel a life of 40,000 to 50,000 castings can be obtained in the non-heat-treated state, and by proper heattreatment of this steel dies can be made having a life of over 100,000 castings. For aluminium alloys having casting temperatures up to 757" C. and heat-absorption values approximately 562 cal., the life of a die is very much less. The type of steel a t present used for dies for die-casting aluminium alloys will, if not heat-treated, develop heat cracks before 1000 castings are made. This same steel properly heat-treated will give the die a life of from 10,000 to 15,000 castings. For brasses having casting temperatures around 1000° C. and heat-absorption values approximating 753 cal. no material has as yet been found that, even when heat-treated, will result in a die with a life of over 1000 castings.
THERMAL FATIGUE OF DIES The heat cracks referred to above are very clearly shown in Figs. 1 to 4, inclusive. Fig. 1 shows the initial casting made from the die in question; Fig. 2 shows the sixteenthousandth casting from a duplicate die. Fig. 3 shows the die block itself after the making of about 20,000 castings; and Fig. 4 shows an enlarged view of a portion of Fig. 3. As this was a casting much larger than the average, the die began to heat-check after only 5000or 6000 castings were made. It is, of course, seldom necessary to discard a die when it first develops heat cracks. As well illustrated in the photographs, the heat cracks in the die result in a network of fins on the casting. The die can be used until these fins develop to the point where they cannot be economically removed, or until they interfere with the operation of the die. The mechanism of the action which results in heat cracks in a die is very similar to that described by Tchernoff.4 While the die is filled with the molten alloy suddenly forced in under 4
"On the Erosion of Steel Guns by Powder Gases," Rev. me'tal, 1915.
TABLE I Melting Point C. SYMBOLM.P.
....
FORMULA Tin 232 Lead 327 Zinc 419 Aluminium 657 Brass 900 Copper 1083 Cast Iron 1200 Iron 1535
Specific Heat OOto M.P.
Sm
....
Heat in Solid a t M . P.
Cal./G. Qs
M.P.XSm
Latent Heatof Fusion L.H.
0.058 0.036 0.112 0.273 0.100 0.125
45.20 167.40
13.82 6.00 22.60 90.90
118.70
43.30
0.150
256.00
0.150
14.34
....
Heat in Liquid a t M.P. Cal./G. Ql as+-L.H.
11.60
.....
.....
.... .... 66.00
Specific Heat of Liquid
Casting Temp.
Superheat
oc.
o c .
SI
IC
....
....
26.16 17.60 67.80 258.30
0.060 0.040 0.179 0.308
162.00
0.156
332 427 519 757 (1000) 1233
..... ..... 322.00
....
.... 0.200
...
1735
tS
Tc-M. P. 100
100 100 100
... ... 200
150
Heat in Liquid a t Ti. Cal./G. Qc
Temp. of Removal C. tY
+ tsS1 ....
QJ
32.16 21.62 85.70 289.10 130.00 185.40 245 00 362.00
100 150 200 300 400 500 600 750
Heat Removed per Unit Weight Cal./G. Qwr Qc--kSm
26.36 16.22 63.30 208.20 90.00 122.90 155.00 236.40
Heat Removed per Unit Specific Volume Gravity Cal./Cc. d Qvr dQwr
....
7.29 11.00 7.10 2.70 8.40 8.30 7.50 7.70
192.1 178.4 449.5 562 753 1020 1162 1820
4
26
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FI< l
pressure, the extreme surface of the die is considerably heated but very quickly cooled by the conduction of the heat back into the underlying metal. If an elementary prism of metal (bfccfb, Fig. 5) free of surroundings is heated in the described manner, on the end surface, cc, it would expand to a shape bfeefb. If, however, it were held in sidewise constraint, as it would be in a die, the shape assumed during the transient heating would be bddb. It is apparent that if the heating has been high enough the surface layers will be overstrained in compression and the metal a t the surface will he plastically deformed. During cooling, therefore, the surface dd will contract more than enough to relieve the induced compressive stress, and a state of tension will exist in the cold surface now slightly higher than the original cc. If the range of temperature in the cycle is sufficiently high and wide, the stresses set up may develop cracks in only one cycle. Messrs. Guillet, Galibourg and Reurets found it possible to develop heat cracks at will by pouring a little molten copper on 5 ~ e u mcroi, . ism.
a flat hardened surface and then immediately drenching with a strong jet of water. If the range in temperature is not sufficiently high and wide t o develop cracks in only one.cycle but is sufficient to cause a slight amount of permanent deformation of the surface, then for each cycle the surface is alternately stressed in compression and tension. Under these conditions cracks will develop after a certain number of cycles. If the range in temperature is not sufficiently high and wide to develop permanent deformation, then the stresses in elements parallel to the surface --ill alternate between compression and zero and the stresses in elements perpendicular to the surface will alternate between tension and zero. As found by 13. F. Moore, of the University of Illinois, if this alternation of stresses is above the “endurance limit” of the material, it will cause fatigue failure (thennal-fatigue, heat-checking, heht cracks) after a certain definite number of cycles. The distance these stresses go beyond the “endurance limit” will determine the number of castings which ca be made in a die before it develops heat cracks.
Bio. 2
INDUSTRIAL AND E.VGI&-EERING CHEMiSTRY
January, 1923
27
Pro 3
The normal net result of the cycle described above and illustrated in Fig. 5 is shown in Fig. 0. But in die casting the cycle is not allowed to rake this normal course. Just as soon as a crack appears, molten metal is forced into it a t the start of the next cycle of operations and stays there m-hile that cycle is completed. This metal in the crack acts like a wedge and spreads the crack wider and drives it deeper. In Fig. 4 it will be noticed that some of the heat cracks are white while others are black. The white ones are those in which the vedge of aluminium completely fills the crack, while the black ones are those from which the fins on the casting shown in Fig. 2 were derived. Even the cracks which appear black are not free from aluminium. An investigation under the microscope of a properly polished and etched specimen shoivs aluminium in the bottom of every crack. ."I.UMINIUJ.I DIE CASTING
That this situation in regard to thermal-fatigue of dies is the most import,ant hindrancc to the development of the
made during the last few years in type of alloy steel used for dies and in methods of heat treatment have resulted in Curve 4 and have made possible the expansion of the aluminium die-casting industry to its present status. Upon further improvements along this line depends further expansion. With each few thousand average increased life of die the less expensive aluminium die castings will hecome and the more uses will be found for them. Die-casting concern throughout the country are continually experimenting. Hundreds of steels and other ferrous alloys and many nonferrous alloys are being tried under actual service conditions. It is possible that 8 better steel or material or type of treatment for aluminium diecasting dies is in its final development stage a t this moment. That such a development can be made and not become known for some time is quite possible, owing to the attitude of secrecy adopted hy practically all the die-casting concerns in regard t,o these matters. Efforts in the direction o i developing a new material are being made along the following lines: (a) steels which have greater normal strength; (b) steels which retain their strength a t high temperatures; (e) steels which retain their hardness a t higher tempering temperatures: ( d j steels which have the property of red hardness; (e) materials with lower thermal coefficients of expansion t,han steel; (f) materials which will give service equal to that d: - - ~ - - - - ~ : * now given by heat-treated alloy steels but which wig not give the usual trouble 9 ; ' ;y--::.: P in hardening; and (gj machinable ma>, ' ' terials not requiring heat treatment.
s-i;
BRASSDIECASTING
Fro.
*
die-casting industry is clearly shown by the curves in Fig. 7. In these CUNCS casting temperatures are shown as ahsoissas and thousands of casting until the appearance of heat cracks are shorn as ordinates. The improvements
In considering brass die castings another difficulty is encountered. Reattreated alloy steel dies have been found best for aluminium die casting at 750' C., but brass must be cast a t 10W' C. The extreme surface of the die does not b reach the same temperature as the molten alloy forced into it,, hut reaches FW. 6 a temperature variously estimated for aluminium die casting of 550' to 650" C., and for brass die casting at from 800" to 900' C. Between these two temperatures lies the transformation range for most steels. For brass die casting the simple thermal expansion and
28
INDUSTRIAL AND ENGINEERING CHEMISTRY
contraction illustrated in Fig. 5 is almost overshadowed by the changes in volume resulting from changes of state and by the neutralizing effects of these changes of state upon any previous heat treatment. This is all further complicated by the decrease in strength and hardness of the steel when at elevated temperatures and further by the alloying or solvent properties of molten brass on steel. Just as improvements have been made during the last few years in the alloy steels
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v
J
next few years will bring further im.-. provements, but it FICA6 is indeed doubtful if any simple alloy steel will be found that will make brass die casting a commercial possibility. The steel industry has for many years used cast iron for ingot molds, but this is a case of permanent mold casting and not die casting. Dimensions, surface, and life are not so important, no appreciable amount of machine work is done on the molds, the metal is not forced in under pressure nor instantaneously, the molds are not rapidly cooled, and the weight of the mold is small compared t o the weight of steel cast. I n die casting, the force and pressure behind the molten alloy cause it t o come into much closer contact with the mold or die, and much more suddenly, so that the heating conditions are many times more severe than in
.
ing approximately 200 castings. Brass die casting is further complicated by the shrinkage and high strength of brass, and the resultant gripping of cores before they can be removed. Cores must be hard enough so that they will not score, must be strong and tough enough so that sharp corners will not be eliminated, and must be strong enough to withstand the force necessary to extract them from the casting. The development of a lubricant or coating material which1 will not gasify and cause blowholes in the metal, but which will act as a protective film between the die and the hot metal and at the same time as a lubricant for cores, is certainly to be looked forward to. Such a lubricant or coating would materially cut down the rate of heat transmission t o the die, and in doing so would not only cut down the maximum temperature to which the die surface is heated but also. will lower the pressure it is necessary to apply to the molten metal in order to make it fill the cavity in the die.
A Good Beginning! In an editorial in our November, 1922, issue, we called attention to the need of supplying chemical facts to those who have the making of our country’s laws, with the suggestion that LocaZ Sections undertake the task of furnishing their congressmen with information which they should have in order to best serve the interests of the chemical industry. The following letter, sent to an Indiana representative, is the first response of which we are aware. May there be others! Hon. A. J. Hickey, Laporte, Ind.
11.34
Vol. 15, No. 1
November 23, 1922
DEARSIR: YOUmay not be aware that in this (South Bend) district,
Castwe Z m p e r a t u r c J
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FIQ. 7
permanent mold casting. I n die casting, the mass of the mold or dies is very great compared to the casting itself, and therefore the rate of cooling of the extreme surface of the die is many times greater than is the case in permanent mold casting. The foregoing explains why a piece of cast iron, such as regularly used for steel ingots molds, when used in a die for brass die casting developed excessive heat cracks before fifty castings were made. Under similar conditions heat-treated alloy steel such as has proved best for aluminium die casting developed heat cracks after mak-
which includes the counties of Elkhart, St. Joseph, Starke, Marshall, and Laporte, there is a Local Section of the American Chemical Society. Naturally there is little need to remind you of this organization in which all reputable chemists and chemical engineers are enrolled, but it might be worth while to tell you of the objects of the Society. The objects are the advancement of‘ chemistry and the promotion of chemical research. This, you can see, has nothing to do with selfish motives, or organization for profit. With such an aim in view, every American chemist desires the beneficial promotion of chemistry in America. The record of this branch of industry in the war will tell you its importance. As to your personal interest we are unaware, so we are writing to tell you we are quite ready to assist you should you wish to acquire information on any branch of chemistry that may not be clear to you, or on some particular phase on which you would like to be more thoroughly informed. Also, we wish to add that with the concentrated competition of foreign products, i t does not seem right that our own chemical industries should suffer extinction. We are enclosing a small digest, or “syllabus,” which will be worth while looking over, as it will give you an idea of the field of chemistry in America to-day. Please consider that twenty years ago there were hardly twenty chemists in the state of Indiana. We have nothing to ask except confidence and nothing to sell: except service; therefore, please accept our cooperation and counsel to the common end of a more unified and complete country. Our Section has expressed a desire to have you with us at one of our regular monthly meetings when convenient for you. It is anxious t o hear some of the chemical ,problems which confront our congressmen. We should, indeed, appreciate an opportunity of having you talk to us on this subject, perhaps to the better understanding of us both. Very truly yours, Secretary The South African Journal of Industries announced that the cover of the November, 1922, issue is of South African manufacture, having been made by the Premier Paper Mills, Klip River, Transvaal. This is the first occasion on which any South African journal has been issued with a cover of locally made paper.
