US3369591A - Process for die casting and heat treating aluminum alloy and resulting products - Google Patents

Description

Unite States Patent O 3,369,591 PRGCESS FOR DIE (IASTENG AND HEAT TREATING ALUMINUM ALLOY AND RE- SULTDQG PRODUCTS Paul R. Welch and Harrison G. Brogden, Gardena, Califl;

said Welch assignor, by mesne assignments, to International Telephone and Telegraph Corporation, New

York, N.Y.

No Drawing. Continuation of application Ser. No. 278,758, May 7, 1963. This application June 20, 1966, Ser. No. 559,001

1 Elaim. ((Jl. 164-113) ABSTRAiIT OF THE DISCLOSURE The sluminum alloy used is restricted in iron content to less than 0.15 percent by weight, and the alloy is protected against iron contamination during melting by handling in a non-ferrous receptacle. Other aspects are: maintaining the melt in the temperature range of 1340- 1400 degrees F., injecting at an initial low pressure and finishing at a higher pressure, modifying the alloy composition to include specified amounts of magnesium, and maintaining the mold at a particular elevated temperature.

This application is a continuation of application Ser. No. 278,758, filed May 7, 1963, now abandoned.

The present invention relates to a novel method for die casting and heat treating an aluminum alloy wherein the resulting products have superior physical properties of yield point, elongation and tensile strength.

Die casting is a particularly economical process for producing aluminum parts, and lends itself well to high production. However, prior to the present invention, die cast aluminum parts had relatively inferior physical properties, not having an adequate yield point, elongation or ultimate tensile strength to satisfy many needs. Thus, more sophisticated processes which are more expensive than die casting and not as well adapted for high production, such as permanent molding, forging, cold rolling, cold drawing, or cold finishing impact extruding, have been required in order to achieve adequate physical characteristics for many types of aluminum parts, as for example military hardware, internal combustion engines and components, auxiliary power drives, household appliances, hardware and structural components, outboard motor components, and many others. As a more particular example, alurninum alloy electrical connector shells produced under MIL-C 26500 and 26518 of 6061-T6 aluminum heretofore had to be impact extruded in order to meet the military standards.

The difiiculty with prior art die cast aluminum parts was not only that had relatively inferior physical properties in the as cast condition, but also because of the fact that those skilled in the art of die casting aluminum have heretofore found it impossible to provide any useful improvement in the physical properties of die cast aluminum parts by heat treating the parts after they were cast, so that in order to obtain the benefits of heat treatment, the parts were required to be produced by other methods.

Even where other, more sophisticated processes have been employed in the prior art for producing aluminum alloy parts which could be heat treated, it was difficult to control the ultimate physical properties of the parts by the heat treatment, and controlled relative variations in the yield point, elongation and ultimate or tensile strength usually could not be provided.

In view of these and other problems in the art, it is an object of the present invention to provide a novel 3,369,591 Patented Feb. 20, 1968 method of die casting aluminum alloy which results in as cast parts having substantially better physical properties than aluminum alloy parts produced by prior art die casting methods.

Another object of the invention is to provide a novel method for die casting an aluminum alloy which results in parts which are capable of having their physical characteristics of yield point, elongation and tensile strength substantially improved by subsequent heat treating and aging.

A further object of the present invention is to provide a method of the character described for die casting and heat treating an aluminum alloy wherein relative adjustment of the individual physical properties of yield point, elongation and tensile strength may be accurately controlled by varying the heat treating and aging cycles which are applied to the die cast parts.

The foregoing and other objects and advantages of the invention will become more fully apparent from a consideration of the detailed description which follows.

Hertofore, in the art of die casting aluminum alloys, it has been considered necessary to include in the alloy at least about 1% by weight of iron. One of the reasons why this iron content was considered necessary was that whenever less iron was used, the aluminum alloy became stuck to the mold by a soldering or welding action, so that the molded parts were destroyed in attempting to remove from the mold, and also the mold frequently became damaged, sometimes beyond repair. This iron content of at least about 1% was, at least in part, obtained from iron in the walls of the receptacles employed in melting and handling the molten aluminum, because of the great affinity of aluminum for iron. Thus, in typical prior art aluminum die casting practices, some iron was picked up by the molten aluminum from both the oven or melting pot and the ladle or other receptacle employed for transferring the molten aluminum from the melting pot to the dies. Whenever it appeared that the die cast parts were tending to adhere to the mold, the usual prior art procedure was to add iron to the melt. This was generally done in an uncontrolled manner by the operator merely tossing a few nails into the melting pot.

According to the present invention, the iron content of the molten aluminum that is injected into the dies during the die casting operation is greatly reduced from the amount which was employed in prior art die casting, to an amount ranging from none to not over 0.5% by weight, and preferably from 0 to about 0.15%.

In order to achieve this low iron content of the molten aluminum alloy which is die cast, it is necessary to protect the aluminum alloy from contact with any ferrous material while the aluminum is in the molten state. Thus, the furnace or melting pot is lined with silicon carbide, or an equivalent liner, to prevent iron pick-up by the molten aluminum. In other words, the liner in the furnace or melting pot is composed of a non-ferrous, refractory type of material. Ladles, if used to transport the molten alloy from the pot to the die casting machine, should be composed of a ceramic material or coated with a refractory material which contains a minimum amount of iron. Caution against iron contamination is desirable to the point of providing silicon carbide or equivalent protector tubes or covers for the thermocouple tubes and all supporting equipment which comes into contact with the molten metal.

The low iron content employed in the present invention appears to be important in a number of different respects which will be briefly outlined at this point, some of which will be developed in more detail hereinafter.

Thus, with the reduced iron content of the present invention, the as cast parts have better yield, elongation die castings, these improved physical properties being attributable at least in part to a substantial reduction in the amount of crystallization within the castparts, and to nesium content, accompaniedby a reduction in the copper content, provides a greater as cast ductility. Such varia' tions in'the physical properties of die cast aluminum alloy parts could not be achieved with the higher prior art iron content.

a substantial reduction in theamount of gas inclusions 5 and hence a more solid product. Further, the as cast Regarding the magnesium, with the relatively high iron parts have sufficient formability or malleability so that content of prior art die casting alloys, only a relatively they can be swaged, formed, coined or otherwise worked, small amount of magnesium could be included in the which was not heretofore possible with die cast aluminum alloy, because in combination with the iron, any'substanalloy parts, and such working can be applied in the as tial amount of magnesium tended to make the resulting cast condition of the parts, or after heat treating but cast parts too brittle. However, with the greatly reduced before aging, or even after aging, if desired, although at iron content of the present inventiomit has been found the final stage the strength of the parts is so high that that a substantial increase in the amount of magnesium they are more difiicult to work. contained in the alloy is not only permitted but is benefi- Another advantage of the low iron content is that it cial to the physical properties of the resulting cast parts. has a cooperation with other ingredients of the. alloy. For example, where the standard prior art alloy used for For example, the reduced iron content amplifies the most die castings considered to be of high quality had importance of both magnesium and copper in improving a magnesium content of 0.1% by weight, we have obthe physical characteristics of the cast parts, and also tained particularly good resultswith a magnesium con. permits variations not previously possible in the relative 2Q tent ranging from about 0.35% to about 0.65% by weight, proportions of magnesium and copper in the alloy, to pr0- and preferably about 0.45%. If the magnesium content vide variousproperties of thecast parts as desired. Furgoes much below about 0.35%, the as cast ultimate ther, with the low iron content, a much larger amount tensile strength and elongation become materially reof magnesium can be included in the alloy, benefiting the duced, and the susceptibility of the cast parts to effective as cast" product and making the heat treating and heat treatment and aging suffers a marked decrease. On aging more effective for furtherimprovement of the cast the other hand, if the magnesium content goes much over parts. about 0.65%, ductility or elongation is adversely affected.

Finally, the reduced iron content of the aluminum alloy This increased magnesium content of the alloy ememployed in the present invention permits, for the first ployed in the present invention, combined with the retime, die cast aluminum alloy parts to be heat treated duced iron content, appears to be an important factor in and aged with resulting substantial increases in the physiobtaining improved physical characteristics by the heat calproperties. In fact, the die cast aluminum alloy parts treating and aging of the die cast parts. of the present invention are so well adapted and sus- Table I below gives a comparison of ingredients beceptible to both heat treating and aging that the yield, tween an alloy employed in the present invention and a, tensile strength and elongation factors of the finished prior art alloy employed for die casting. The alloy ofthe products can be varied with respect to each other, and can present invention is designated by the numeral 1, and has be predetermined with a high degree of accuracy, such been found to provide excellent as cast physical properrelative variations of the different physical characteristics ties, and also to be heat treatable with substantial improveand such accurate predetermination or preselection of the. ment in the physical properties. It is to be understood, physical characteristics not having been possible prior however, that the present invention is not limited to the to the present invention, even with the more sophisticated particular alloy set forth in the table below. The alloy methods of providing aluminum alloy parts, such as perdesignated by the numeral 2 is the standard alloy used manent molding, forging, cold rolling, cold drawing, or for most high quality die castings, per Federal Specificold finishing impact extruding. cation QQ-A-591B.

TABLE 1 Cu Fe Si Mn Mg Z Ti Ni Tin Al 1 3.7 .14 8.2 .02 .45 .02 .05 0 0 87.42 Bal. 2 3.0-4.0 1.3 7.5-9.5 .5 .1 1.0 0 .5 .3 Bal.

Both copper and magnesium are also important ingre- Referring to the above chart, it will be noted tliat'in clients of the alloy, although it is not certain that copper addition to the large differences in the iron and magnesium is a necessary ingredient. Copper and magnesium con contents of the two alloys, there is also a considerable tribute to a snowflake or interleaving type or crystalline difference in the manganese content. The much larger structure in the aluminum alloy, which is a modification manganese content of the prior alloy is required to keep of the alloy grain structure making it more ductile and the larger iron content of the prior alloy in solution. capable of being made harder, with the net result that The other ingredients which are listed in the above the presence of copper and magnesium improves the as chart, including silicon, zinc, titanium, nickel and tin do cast physical properties of tensilev strength, yield and not pp to adversely affect thfi die casiillgopefafion, in elongation f the aluminum alloy the quantities present. The silicon appears to be useful as Whil copper d magnesium were h f d in some having lubricating qualities which reduce the likelihood of aluminum alloy materials employed in prior art die castthe Cast Partsstickingin the diesing, we have found that the importance of both of these found it extremely diflicult and entirely elements in improving the physical properties of the die merclany lmpracncal.to cast the i iron aluminum cast parts is greatly amplified by the reduced iron conaupy of mvenilon accordmg 9 conventional tent of the present invention. g gf castltrig i f and g i f a Pafrt of Another factor of importance with respect to the copper 6 en i Ion mvo a same? 9 c anges P and magnesium content of the alloy of the Present 531. 21? aiiifiliifii iiii y tfiii ifjeiiifiii lli 1 2.235.1135; 3 12 223 i zi gmaffly reducd d content 13 Ihat die cast without serious problems of soldering or welding q n 1 185 O w P' an PPF can be of the parts to the dies. Additionally, in the phases of varied to affect the resulting physical p pe cs f the the casting operation which may generally be considered die cast parts. For example, an increase in the magconventional, particularly good mold practices must be employed to secure the beneficial results of the low iron content and relatively high magnesium content.

According to conventional die casting practices, the melt temperature for the aluminum alloy is usually within a range .of from about 1120 F. to about 1250 F. While such temperatures are satisfactory for die casting aluminum alloys having about 1% or more iron content by weight, with the reduced iron content of the present invention of not over 0.5% by weight and preferably from to .15% it is extremely difficult if not impossible to cast the parts at these temperatures. Accordingly, in the present invention the melt temperature is maintained above 1300 F., and preferably within a range of from about 1340 F. to about 1400 F., at which temperatures the low iron aluminum alloy casts satisfactorily, and the parts are readily separated from the die without damage from soldering or the like.

An additional advantage of the higher melt temperature is that it tends to provide better solubility of the alloy materials, avoiding spotty areas and providing uniform alloying throughout the cast parts, which is an important factor in obtaining uniform physical characteristics of the parts. If the melt temperatures go much above 1400 F., some the ingredients, such as the magnesium, start to burn out or deteriorate.

In adidtion to the increased melt temperature, we have also found it highly advantageous to increase the die temperature substantially above that which has conventionally been employed in the prior art. Thus, where the temperature of the die was normally below 300 F. in prior art die casting, we have found that with the reduced iron content it is desirable to have the die within a temperature range of from about 325 F. to about 600 F. Below this temperature range there is a strong tendency of the injected molten alloy to solder at the gate, while above this temperature range the excessive heat tends to weaken the mold.

As stated above, the equipment for melting and handling the molten aluminum alloy is suitably lined so as to protect the molten alloy from contact with any ferrous material during the melting cycle, in order to accurately control the low iron content of the alloy.

It is necessary to prevent surface oxidation .on the melt, and this is accomplished by providing a covering agent of one or more fluxes or inert gases, or by handling the melt in a vacuum area, or by other conventional means. The melt should be de-gassed by an addition of chlorine, chlorine-nitrogen, or other similar fluxing agent to the melt prior to casting, and this process should be repeated either manually or mechanically throughout the melting operation to maintain the gases at the lowest possible level. Provision is made at the time of fiuxing to permit the impurities to fall out of solution before the casting process is continued.

It is desirable to provide the densest possible casting, for if a gas pocket is inadvertently formed in the casting, the subsequent heat treatment will result in a bubble and consequently a worthless part. Accordingly, the aforesaid degassing process is an important one. Another important factor in the provision of dense cast parts is the greatly reduced iron content, since the higher iron content of prior art aluminum alloy die castings appears to have a strong tendency to promote the formation of gas pockets in the parts.

Another procedure which cooperates in providing the densest possible castings is to move the mass of molten metal into the die cavity at relatively low pressure and as slowly as possible without the metal setting up or tending to harden, so as not to agitate the metal as it flows into the die cavity, which permits the gases in the mold area to escape, and then when the die cavity is partially filled, to suddenly increase the rate and pressure of injection. The final injection speed and pressure may be as much as four times as great as the initial speed and pressure of injection. The high casting pressure will usually be on the 6 order of about 600 to 800 psi. It should be noted, however, that the individual configurations .of the parts will require suitable variations in the rate and pressure of injection.

It is also helpful to employ a large gate orifice in the die so that soldering or atomizing will not be so likely to occur during the relatively slow initial phase of the injection. Further, it is desirable to employ a relatively large overflow in order to channel off most of the gases from the mold area.

Additionally, it is desirable to employ a minimum amount of lubricant in the mold in order to reduce the likelihood of the lubricant forming foreign inclusions in the cast parts.

Despite the precautions set forth above to decrease the likelihood of gas inclusions in the parts, the cast parts are so vulnerable to bubbling during the subsequent heat treating that we prefer to employ a further step when setting up a new die for operation. At the tryout stage for a new die, normal gating, venting and runner practices are used to prepare for an initial proof run on the die. Upon completion of the proof run, the entire shot including the runners and the cast part itself are placed in a heat treat oven and elevated to a sufiicient temperature to cause a combination of reduced strength in the aluminum and expansion of any gases in the part so as to force any bubble or blister which may occur to the surface of the part in order to determine the exact area of the die which should be corrected. Correction of the various factors such as injection rate, temperature, venting, amount of overflow and the like can then be made until such proof parts can be so treated without bubbles or blisters appearing in the actual part itself. In some cases it may be necessary to provide overflows in excess of those considered normal by the designer of the die. In practice we have found that a temperature on the order of about 1,000 P. for a relatively short period of time, as for example about one hour, will normally be sufficient to cause such blistering to occur.

The revision of the die which is made as a result of the aforesaid blister test does not actually remove the gas from the aluminum alloy, but moves the gas to some portion of the shot which is discarded in the following trimming operations so that the gas is not present in the finished piece part to the extent that it is normally encountered.

Upon removal of the shot from the die after solidification no additional or unusual practice such as quenching is required, and the parts may be permitted to cool to room temperature and the heat treatment applied immediately thereafter, or the parts may be stored for future heat treating, without any appreciable variation in the ultimate results of the heat treating.

Any of the known standard heat treating and aging practices which have heretofore been used in connection with aluminum alloy parts made by procedures other than die casting may be employed effectively in connection with the die cast parts of the present invention. Although such heat treating and aging was ineffective to improve the physical properties of prior art die cast aluminum alloy parts, it results in a considerable improvement of the parts which are prepared in accordance with the present invention.

In addition to the considerable improvement in the physical properties which results from heat treating and aging, by employing suitable variations in the temperature and time of the heat treatment and in the temperature and time of the aging, a wide range of values for the yield point, tensile strength and elongation is available to completely meet varying requirements of the designer.

The heat treatment is a solution heat treatment wherein the part is heated to a temperature which is below but relatively close to the melting point of the alloy, so as to cause proper dispersion of the alloy ingredients throughout the part. This is desirable because the ingredients of the alloy tend to stratify when the part cools from its outside after being removed from the die. Such solution heat treating can be to within 10-15". F. of the melting point, if desired. The solution heat treatment can be within the range of from about 890 F. to about 960 F., but the preferred and most effective range appears to be from about 920 F. to about 940 F. Although the time for the heat treatment can vary over relatively wide limits, a preferred time range is from about 4 hours to about 6 hours.

If desired, the solution heattreatment may be employed without a subsequent artificial aging step, in which case a natural aging will occur at room temperature and the parts will pick up added physical properties within about 2 to 3 days. Part which have thus been naturally aged are commonly referredto as being in the T5 condition. However, further improvement of the physical characteristics of the parts can be achieved by an artificial aging step, which results in parts that are in what is commonly referred to as the T6 condition.

It is desirable but not necessary to cold water quench the parts between the solution heat treating and artificial aging steps.

The artificial aging, like the solution heat treatment, involves relatively flexible range limits. Although a wider range of temperatures can beemployed for the aging, it has been found that a temperature range of from about 300 F. to about350 F. appears to be the most satisfactory. Similarly, although wide limits of aging time are available, aging times varying from about 3 hours to about hours appear to be particularly satisfactory.

With the alloy of the present invention designated by numeral 1 in Table I above, variations in the temperature and time of the solution heat treatment and in the temperature and time of the aging have resulted in an elongation factor varying from about 3.2% to about 11%, a tensile strength varying from about 41,660 p.s.i. to about 67,000 p.s.i., and a yield point of from about 24,440 p.s.i. to about 57,040 p.s.i.

A specific example of the effect of variations in the aging time on parts composed of the material identified by numeral 1 in Table I above, all of which have been solution heat treated at the same temperature and for the same time, is as follows: A single lot of parts were all solution heat treated at a temperature of 920 F. for 5 hours, and then cold water quenched. At this point, the parts were divided into three lots to establish the effect of age time on the physical properties. The aging temperature in all three lots was 320 F. Table II below gives the results.

TABLE 11 Age Yield Tensile, Elongation, Lot No. Time, hrs. Point, p.s.i. p.s.i. Percent;

Not only can the elongation, yield and tensile strength of the present die cast aluminum parts be varied according to the specific requirements .of the designer, but additionally these factors can be very accurately controlled by careful control of the temperature and time in both the solution heat treating and the aging steps of the process. For example, the yield point and tensile strength can be assured to within 150 p.s.i.

Such variations in the elongation, yield and tensile strength cannot be achieved by varying the heat treatment in parts which are produced by such other processes. as rolling, impact extruding, permanent molding or the like. Furthermore, it is impossible to assure the yield point and tensile strength of parts made by such other processes to an accuracy which even approaches 150 p.s.i., as can be done with the present invention.

8 It is important to note that most United States Military Specifications require that aluminum alloy parts have a minimum of at least 5% elongation and of at least 42,000.

p.s.i. yield. All three examples given in Table II are well above these requirements. However, the best physical properties which could be obtained in prior art die cast aluminum parts were an elongation of about 1 /2 to about 3%, a yield point of about 26,000 p.s.i., and a tensile strength of about 43,000 p.s.i. These characteristics could not be materially improved by any heat treating proced-- ures.

Even without the heat treating and aging of the present invention, the die cast parts of the present invention in their as cast condition have much better physical properties than those of prior art die cast aluminum parts. Thus, in the as cast condition, parts made of the composition designated by numeral 1 in Table 1 above will have an elongation of from about 5 /2% to about7 /2%,

a yield of from about 30,000p.s.i. to about 38,000 p.s.i.,

and a tensile strength of from about 48,000 p.s.i. to about 51,000 p.s.i.

As an example of the economic advantage of the present invention, an aluminum alloy electrical connector shell produced under MILC 26500 and 265 18 of 6061- T6 aluminum which heretofore had to be impact extruded in order to meet the Military Standards, had a factory cost of about $1.06 when made by impact extrusion. The equivalent part made in accordance with the present invention has a factory costof about 23 and additionally has approximately 20% better properties of yield, elongation and tensile strength than the impact extruded part.

One reason why many types of parts heretofore had to be made of steel, even though lightness in weight and other characteristics of aluminum would have been preferred, is the almost infinite range of such factors as yield,

which is therefore not to be limited to the details disclosed herein, but is to be accorded the full scope of the appended claim.

What we claim is:

1. The method of fabricating shaped metal bodies, said method comprising the step of: injecting a molten aluminum alloy at a temperature of from 1340 to 1400 degrees Fahrenheit into a mold having a temperature of from 325 to 600 degrees Fahrenheit, said alloy containing the following metals in the proportions indicated:

Metal Proportion in percent by weight Copper 3.7 Iron .14 Silicon 8.2 Manganese .02 Magnesium .45 Zinc .02 Titanium .05 Aluminum, :bal 87.42

References Cited Aluminum Alloy Castings, Carrington, 1946, Charles Griffin and Co., London, relied on pages 59-66, 1962, 163, 167 and 200404.

Metals Handbook, 1948 Edition, published by ASM, relied on pages 803, 804 and 816.

CHARLES N. LOVELL, Primary Examiner. DAVID L. RECK, Examiner.