Characteristics and Uses of Aluminum Alloys

Its Increasing Importance

I THE increasing importance of aluminum and aluminum alloys as mate- • rials of construction makes desirable the promulgation of certain information in regard to general use, installation, and preservation of these relatively new materials. Aluminum and aluminum alloys for ship use have long been regarded with skepticism, largely due to improper uses, no doubt. Recent improvement in quality and a broader knowledge of application have permitted designers to accept the known limitations of aluminum* in order to take advantage of its special merit, namely, maximum strength for minimum weight. In commercial fields aluminum alloys are finding wider and wider applications; large quantities are used for automotive work, chemical equipment, aviation, textile machinery, radio, foil, collapsible tubes, conductor cable, etc. At least one railroad has built and is operating a train of coaches in which practically all the structure above the trucks is made of aluminum. Street car companies are employing aluminum alloys for practically all car parts except the wheels and axles. Aluminum furniture is being extensively developed. A 3,800-pound cored casting has recently been poured for a Diesel engine foundation aboard a yacht. Aluminum paint is being widely used, even as a priming coat for steel. The uses for aluminum will be discussed in this paper as applied primarily to naval vessels and will not cover the specialized uses in the field of aeronautics.

* The term “aluminum” is used in this paper generically and refers to either the commercially pure aluminum or to the several aluminum alloys.

2. On naval vessels aluminum has heretofore been used but little because of definite limitations of this metal with respect to its physical properties as compared to those of the metals commonly used in ship construction. The Treaty for Limitation of Naval Armaments has, however, given an impetus to the study of the utilization of aluminum on shipboard. As a result, aluminum alloys have been authorized for certain shipboard uses by the various technical bureaus of the Navy Department. Such uses of aluminum are restricted to those in which a failure of the material would not endanger the safety of the ship or cause injury to personnel. Additional uses will of course follow if the service installations already authorized are sufficiently satisfactory. Satisfactory usage of aluminum will depend to a large extent upon the care and preservation accorded by the forces afloat.

Limitations

3. In addition to its disadvantage of limited strength, aluminum has always been regarded as highly susceptible to salt water corrosion. In the case of the older aluminum alloys especially, salt water usually caused rapid deterioration and practically precluded their use on naval vessels. As a result of intensive metallurgical research, however, there has recently been great improvement in the properties of aluminum. These improvements have not entirely removed the former weaknesses, but have brought about an understanding of the properties of aluminum such that its limitations and restrictions can be fairly well met. One of the noteworthy limitations in the physical properties of aluminum, often overlooked, is the fact that the modulus of elasticity is about 10,000,000 as against 30,000,000 for steel. Because of this fact an aluminum beam, for example, would suffer about three times as much displacement in bending as would a similar steel beam subjected to the same load. This modulus is constant for all aluminum alloys and must be compensated for in design and application. Great effort is being made by engineers to develop designs which will permit a weight saving of about sixty per cent by substituting aluminum for steel and yet give the same factors of safety as the original steel design. This is often accomplished, in spite of the low modulus of elasticity, by efficient distribution of cross sectional areas and by suitable bracing. Though aluminum is being extensively used in very important commercial applications it cannot be used safely in naval design where the structural or watertight integrity of the ship might be affected. Further improvements in aluminum will of course eliminate some of the present necessary restrictions as to its use.

General Physical Properties

4. For ready reference some of the physical properties of aluminum are noted:

Specific gravity—2.65 to 2.90.

 Approximate weight per cubic inch—0.10 pounds.

Modulus of elasticity—10,000,000 pounds per square inch.

Melting point—approximately 1220° F.

Shrinkage allowance for pattern making—0.156 inch (5/32 inch) per foot.

Ratio of Weights of Equal Volumes

Commercial aluminum ...............  1.00

Zinc ............................................  2.63

Cast iron ....................................  2.63

Tin ............................................    2.70

Soft steel ...................................  2.89

Cast brass (Cu 70-Zn 30)............ 3.12

Cast bronze (Cu 90-Sn 10) .........  3.26

Nickel .........................................  3.26

Copper .......................................  3.30

Lead ...........................................  4.18

Forms Commercially Available

5. Aluminum is furnished in cast, rolled, forged, drawn, or extruded form as required. Pure aluminum must be alloyed with other elements such as copper, manganese, silicon, magnesium, etc., to provide satisfactory physical properties. These alloys are commonly referred to as aluminum-manganese, aluminum-silicon, “Duralumin,” etc. Navy Department leaflet specifications covering the alloys considered most desirable for Navy use have been prepared. In Navy specifications the trade mark or copyrighted names are not used. For instance the alloy marketed by the Bausch Machine Tool Company under the name “Duralumin” and by the Aluminum Company of America as “No. 17S” is described in the Navy leaflet specifications 47A3 as “Aluminum Alloy, Sheet (Aluminum-copper-magnesium-manganese).” For the sake of brevity the commonly known term “Duralumin” is used in this paper when referring to this alloy.
6. For the benefit of repair and foundry personnel on shipboard and elsewhere the uses and effects of the various alloying elements are outlined below:

Copper.—Copper is the most commonly used hardener and is present to some extent in the majority of the prominent casting alloys. The most widely used casting alloy in this country is that used in the automotive industry and generally known as “No. 12,” or “S.A.E. No. 30.” This alloy contains about eight per cent copper. It is not suitable for naval work on account of its notable lack of resistance to salt water corrosion and its low ductility. The elongation in two inches is from one per cent to two per cent.

Zinc.—Zinc has been used as a hardener for aluminum to a greater extent in Europe than in this country. Zinc is cheaper than copper and when properly used the castings conform sharply to the patterns and are sound and clean. There are numerous difficulties, however, in the casting of the aluminum-zinc alloys due to the volatility of the zinc. These alloys are heavier than many of the other aluminum alloys and lose their strength rapidly with increased temperatures.

Manganese.—Manganese when added in small percentages increases the physical properties and the corrosion resistance of aluminum. Castings containing manganese are more or less difficult to make owing to the high solidification shrinkage. An example of the successful use of manganese is Class 1 alloy in Navy Department leaflet specifications 46A1. This alloy was developed by the naval gun factory.

Silicon.—Silicon as an alloying element with aluminum is one of the newer developments which has come to the front rapidly in the last two years. Silicon greatly increases the fluidity of aluminum and enables the casting of intricate shapes and thin sections contiguous to heavy sections. The extremely low solidification shrinkage of the silicon alloys increases the ease of casting. The use of silicon produces an aluminum alloy with maximum resistance to salt water corrosion which makes it of particular value for naval work. Class 2 alloy in Navy Department leaflet specifications 46A1 is probably the most reliable and useful of the silicon alloys.

Iron, Magnesium, and Nickel.—Iron is nearly always present in aluminum as an impurity. Magnesium and nickel are used to a less extent with aluminum than the previously mentioned elements. An alloy of considerable prominence in England and frequently referred to in technical literature is Rosenhain’s “Y” alloy. This alloy contains about four per cent copper, two per cent nickel and one and one-half per cent magnesium, and is a good example of the nickel-magnesium series. Castings of this alloy are susceptible to heat treatment and develop high tensile strength but have low ductility. One of the advantages of this alloy is its ability to retain its strength at high temperatures. “Y” alloy is used but little in this country owing to the development of heat treated casting alloys in American foundries which are better suited for general uses.

Navy Casting Alloys

7. The bureaus of the Navy Department have a record of some seventy casting alloys, together with their chemical characteristics and physical properties. Of the available alloys three were selected for inclusion in Navy Department leaflet specifications 46A1 as being best suited for naval work. Strength, ductility, shock resistance, and resistance to salt water corrosion were the main considerations in this selection. Foundry personnel are reminded that satisfactory physical properties cannot result from mongrel alloys; the chemical requirements of the specifications should be rigidly adhered to. The chemical and physical properties of the three Navy alloys are outlined in Table 1.

Class No. 1.—This alloy has been developed by the naval gun factory and is seldom used by commercial foundries, largely because of the high shrinkage. The naval gun factory, however, obtains uniformly consistent results with this alloy. It has the highest degree of resistance of the four alloys to impact as shown by Izod tests, and is the most ductile.

Class No. 2.—This alloy is commonly termed the five per cent silicon alloy. It is the easiest of the four alloys to cast, its fluidity being due to the silicon content. It has low shrinkage and is adaptable to castings of thin section or intricate design. It has a dense structure making it suitable for work under moderate pressure and is one of the best alloys as regards resistance to salt water corrosion. Class 2 alloy is regularly produced by several of the navy yards and is readily obtainable from many commercial foundries. No heat treatment or special process is involved in its production.

Class No. 4.—Castings of this alloy require heat treatment to develop the required physical properties. This is the strongest, stiffest, and hardest of the four alloys and has good resistance to impact as shown by Izod tests. Of the four alloys, Class No. 4 has the greatest resistance to deformation by suddenly applied loads. It is not believed to be as resistant to salt water corrosion as the other alloys. This alloy is not particularly difficult to cast nor is its heat treatment involved.

Table 1.

Class	Chemical Requirements								Physical Requirements
	A1 Min.	Cu Max.	Si Max.		Zn Max.	Fe Max.	Mg Max.	Mn Max.	Min. Tensile lbs. per sq.in.		Min. Elong. in 2 in.
No. 1....	96.5	1-1.5	0	50		0.7		0.7-1.2	19,000		10.0
No. 2....	92.5	0.6	4	5 -6.0	0.20	1.0		0.2	17,000		3.0
No.4....	94.5	4.5	1	20	0.25	1.20	0.35		28,000		6.0

No. 4 is a heat treated alloy.

Table 2.

Class	Resistance to Corrosion	Stiffness	Hardness	Shock Resistance Izod Values	Ease of Casting	Machin- ability	Proportional Limit lbs. per sq. in.	Brinell Hardness Number
No. 1....	1	2	2	1	3	2	3500	40
No. 2....	1	3	3	3	1	3	3000	40
No. 4....	2	1	1	2	2	1	12000	70

Previously an alloy (No. 3) known as the thirteen per cent silicon was included in Navy specifications but has been eliminated due to difficulty in procuring consistent properties, particularly elongation.

8. Table 2 lists the approximate comparative characteristics of the three alloys. Research to date has not been extensive enough to establish definitely some of the factors listed. Comparative resistance to corrosion, in particular, has not been definitely established. This table, however, represents the best knowledge at present available and may serve as an aid to designers and users.

Rolled, Drawn or Extruded Alloys

9. Of the many commercial alloys available in the above forms, the Navy has standardized on three: “Duralumin,” aluminum-manganese, and commercially pure aluminum.
10. “Duralumin."—This alloy is the strongest and probably the most prominent of the rolled alloys. It is marketed by the Bausch Machine Tool Company under that trade name. The Aluminum Company of America markets this alloy under the trade name of “No. 17S.” Navy Department leaflet specifications described this alloy as “Aluminum Alloy (Aluminum-copper-magnesium-manganese).” The following leaflet specifications have been prepared for this metal.

For sheets ..................................  47A3

For rods, bars and shapes ..........  46A4

For seamless tubes   44A2

The chemical composition of this alloy is as follows:

Copper ............................  3.5% to 4.5%

Magnesium...................... 0.2% to 0.75%

Manganese....................... 0.4% to 1.0%

Aluminum ..........................  92.0% Min.

This alloy must be heat treated to obtain the maximum strength. The physical properties vary according to the cross section and shape of the material.

“Duralumin” sheets show a range in physical properties (depending upon the thickness) approximately as follows:

Tensile strength—50,000 to 65,000 lbs. per sq. in.

Yield point—25,000 to 38,000 lbs. per sq. in.

Elongation in 2 inches—25% to 12%.

In the annealed condition the strength is cut down about one-half and the structural value of the material is seriously reduced as well as is its resistance to corrosion.

For “Duralumin” shapes the following minimum requirements are specified:

Tensile strength—50,000 lbs. per sq. in.

Yield point—25,000 lbs. per sq. in.

Elongation in 2 inches—16%.

The physical properties of heat treated rods and bars vary with the cross section. The following are the approximate ranges:

Tensile strength—45,000 to 60,000 lbs. per sq. in.

Yield point—20,000 to 33,000 lbs. per sq. in.

Elongation in 2 inches—25% to 12%.

“Duralumin” should never be welded where strength is an important factor. If welding is resorted to, however, the complete part must be heat treated subsequent to welding. Drop forgings having the strength of mild steel may be satisfactorily made from this alloy.

11. Aluminum-Manganese Alloy. — The aluminum-manganese alloy has come rapidly to the front in the past year as being one of the most useful of the rolled alloys for purposes where the higher strength of “Duralumin” is not required. The aluminum-manganese alloy is more resistant to salt water corrosion and is considerably cheaper than “Duralumin.” It may be rolled in various tempers from soft to hard as desired, the rigidity, hardness and tensile strength increasing as the temper or amount of cold working, is increased. The chemical composition is as follows:

Manganese .........  1.0% to 1.5%

Copper ............................... 0.2% Max.

Aluminum ..........................  97.0% Min.

The range of physical properties depending on temper and thickness of sheet is as follows:

Min. tensile strength—16,000 to 33,000 lbs. per sq. in.

Min. elongation in 2 inches—25% to 1%.

Sheets of this alloy are described in Navy Department leaflet specifications 47A4. Pipe, tubing, rods, bars and rivets are also produced from this alloy.

This alloy has superseded “Duralumin” as a material for motor boat canopies, largely on account of its superior workability, lower price, rigidity in the temper used (}i hard), and availability in larger sizes. This alloy is marketed by the Aluminum Company of America under the trade name “3S” and is not proprietary.

12. Commercially Pure Aluminum.— Commercially pure aluminum (ninety-nine per cent al.) is furnished in rolled, extruded or drawn forms and is described in the following Navy Department leaflet specifications :

For sheet ...................................  47A2

For rods, bars and shapes.......... 46A3

For tubes ...................................  44A1

This material is no cheaper than the aluminum-manganese alloy and has considerably lower physical properties. It is softer and easier to deep draw, spin and weld than the aluminum-manganese alloy and is largely used for the manufacture of utensils, tanks and fabricated articles where especial strength is not required.

Application

13. In the paragraphs immediately following are listed some of the general shipboard applications of aluminum alloys which may be considered as typical.

Casting Alloys.—

Class 1.—Electrical equipment for fire control system, junction boxes, etc.

Class 2.—Watertight covers for ventilation systems; airport frames and lens frames for airports well above the waterline; cast portions of guards for gears; base castings for individual waterclosets; brackets for standing lights; metal hose racks; bedplates for motors and pumps; furniture fittings, etc.

Class 4.—Blast covers for airports, fittings for ammunition hoists, ladder treads for companion ladders, bed plates, hand wheels, etc.

Duralumin.—For minor structural purposes as shapes for stowage, airplanes, rigid airships, furniture frames, cabinets, file cases, shelving, bins, etc.

Aluminum-Manganese Alloy.—For motor boat canopies, divisional bulkheads, joiner doors,cabinets, file cases, shelving, bins, etc.

Commercially Pure Aluminum.—This material has very little application for structural purposes, but is used to some extent in the manufacture of cooking and other utensils.

Installation

14. The determination of proper methods of installation and preservation of aluminum alloys has been the aim of thousands of tests and experiments. Corrosion has been the bugaboo of aluminum on board ship, and very careful steps must be taken at the time of assembly and thereafter to prevent corrosion. While some test results have been differently interpreted by interested engineers, the broad results have been in general agreement regarding the corrodibility of aluminum. As regards method of installation on shipboard, there are three important conclusions or cautions:

1. Do not install aluminum where it will be in continuous contact with salt water or salt spray.
2. Do not install aluminum in metal-to- metal contact with copper alloys; suitable insulating materials should be used.
3. Do not install aluminum adjacent to wood which may be damp most of the time from the intermittent presence of sea water.

15. The fabrication and assembly of aluminum is not especially difficult. Due to its greater ductility aluminum is more easily worked than steel, but also more easily abraded and abused. Tools for working aluminum should be very sharp and highly polished. Cutting tools should have considerable side and top rake and should be used with soluble oil cutting compounds.
16. Working Fits.—In fitting working parts, considerable looseness in fit must be provided to prevent seizing. This is particularly true of aluminum against aluminum. Contact surfaces of working parts should be periodically oiled or greased. Where a threaded connection is used between two parts of aluminum a suitable “anti-seize” compound should be used, such as a mixture of zinc dust and vaseline or an emulsified asphalt. Rust preventive compounds should be satisfactory. Tight fitting threads in aluminum may cause “freezing” ; “shakey” fits are essential. Brass threads should never be used in contact with aluminum. Brass pins should never be used with aluminum hinges. Steel pins are best, preferably of corrosion resisting steel.
17. Riveting and Bolting.—Fittings may be secured by steel bolts, screws, or rivets. Aluminum alloy rivets (“Duralumin” or aluminum-manganese alloy depending on the service) may be used in sizes up to and including one-half inch diameter. “Duralumin” has approximately twice the strength of the aluminum-manganese alloy, and rivets made of this alloy should be heat treated before using and driven cold as soon as possible (within one hour) after heat treating. On aluminum rivets a compression riveter should be used where practicable. Aluminum rivets should be driven with a few heavy blows rather than by numerous light blows.
18. Welding.—Autogenous gas welding is, in general, satisfactory for welding aluminum to aluminum. Due to its lower temperature the hydrogen flame is more suitable than the acetylene flame for most work. The average efficiency of a weld properly made in commercially pure aluminum or in the aluminum-manganese alloy is (on a percentage basis) comparable to a similar weld in steel. In the high strength alloys, however, the elongation and tensile strength of the material (cast) in the weld are low and therefore the ductility and strength of such a weld are not comparable in efficiency to a similar weld in steel. "Duralumin” should never be welded where strength is important, but when it is welded the complete part must be heat treated subsequent to welding. It should be especially noted that aluminum fittings cannot be satisfactorily welded to steel. Soldering is practicable under certain conditions, but is not very satisfactory. In welding it is important that the proper technique be used by personnel trained in aluminum welding. A suitable flux must be used. In order to prevent alkaline corrosion great care must be taken to scrub off all traces of the flux with hot water. A better method is to dip in a hot two per cent solution of nitric acid, followed by a hot water rinse. This is most essential as such fluxes contain chlorides and fluorides which will absorb moisture from the atmosphere and cause rapid corrosion of the metal even though painted. Welding should not be done on board ship where the opposite side of the material is inaccessible for the removal of the flux.
19. Electric spot welding is entirely satisfactory and is in general use except for parts subjected to vibrational stresses. The welded spot consists of metal in the cast state and therefore has low ductility. The copper electrodes of the welding machine tend to become fouled with oxides, but this is obviated if the tips of the electrodes are chromium plated.
20. Experience is the best teacher, and is essential for satisfactory fabrication and installation. The points mentioned above are only the high lights; complete information on any of the operations such as casting, heat treating, or machining is readily procurable from the leading manufacturers of aluminum.

Preservation

21. The chief danger in the use of aluminum in the Navy lies in its susceptibility to corrosion. Though aluminum in the open air may be somewhat less corrodible than steel, yet corrosion of aluminum is of an insidious nature. When corrosion once begins it is as fatal as galloping consumption unless arrested in the early stages. The first stages of corrosion are difficult to detect, at least for the unpracticed eye. The small gray powder deposits which first appear, quite soon under certain conditions grow into sizable flakes and later into general disintegration of the metal. In the case of “Duralumin” at least, “intercrystalline corrosion” seems to occur with disastrous effects on the physical properties, especially the elongation. This internal corrosion is not always readily observable on the surface.
22. In order for the Navy to benefit from the weight saving permitted by the use of aluminum, the chief effort to insure satisfactory utilization must be centered on preservation. Many preservatives have been tested and some have been found fairly good; however, all have a limited life thus necessitating periodic refinishing. Suitable protection against corrosion therefore depends upon continual vigilance to detect corrosion and to arrest it. Repainting of aluminum is handled along the same lines as is the repainting of steel.
23. All surfaces of aluminum should be painted with several coats of approved preservative such as aluminum varnish or bituminous varnish. This should be done as soon after fabrication as practicable.
24. It is good practice to apply one coat of preservative prior to assembly, especially so if the parts are to remain in storage very long. Faying surfaces should be painted during assembly. A satisfactory formula for the manufacture of aluminum varnish is as follows:

Aluminum powder ..................  100 lbs.

Water resisting spar varnish ... 50 gals.

The naval aircraft factory has developed a formula for satisfactory bituminous varnish which is as follows:

Aluminum powder ..................  100 lbs.

Bituminous primer .................  50 gals.

This formula is used for airplane hulls or other parts exposed to salt water or salt spray.

25. Although the above named varnishes appear most satisfactory as priming coats, ordinary red lead is permissible. The theory that lead paints might cause local corrosion due to galvanic action between the aluminum and the paint ingredients has been fairly well disproved. Where it is necessary to make the color of the paint applied to the aluminum conform to the color of the surrounding paintwork, standard oil paints should be applied over either one or two coats of aluminum varnish.
26. In case aluminum is necessarily installed in contact with a dissimilar metal, the faying surfaces must be properly insulated from each other by the use of bituminous solution or bituminous varnish. Tarred felt is good but should not be used where it might soak up water.
27. Before applying any kind of paint to aluminum, it is essential that the surfaces be properly prepared. Castings should be sand blasted, rumbled, or wire brushed. Wrought material should first be thoroughly cleaned of any oil by a solvent such as gasoline, benzine or carbon tetrachloride. Then the material should be sand blasted or wire brushed in order to roughen the surface sufficiently to permit adhesion of the paint. When conditions permit, more thorough methods such as a caustic dip followed by a hot water rinse, and then by a dip in hot concentrated nitric acid and final water rinse, should be used in lieu of the above. Some alloys of aluminum are delivered with a fairly rough surface known as “gray finish.” For such materials sand blasting or wire brushing is not generally necessary. Before painting over welds, extreme care must be taken to completely remove all trace of the welding flux by thorough scrubbing with hot water. A better way is by dipping in a hot two per cent nitric acid solution followed by a hot water rinse. After thorough cleansing a priming coat of paint should be applied without delay.
28. Contact with Copper.—In fabricating and installing any parts made of aluminum, it is essential that care be used to avoid intimate contact between aluminum metals electro-positive to aluminum such as copper, brass or bronze, monel metal, etc. Such contact causes severe electrolytic action resulting in the rapid corrosion of the aluminum. When contact between aluminum and brass or bronze is unavoidable, the faying surfaces should be coated with marine glue or bituminous solution. Where practicable, bushings or spacers of corrosion resisting steel are recommended.
29. Contact with Steel.—Electrolytic action between aluminum and steel is not severe; especially when the steel is galvanized. Where aluminum is secured to steel the adjacent surfaces should be coated with marine glue or bituminous varnish before assembly. Where steel bolts, black or galvanized, are used in conjunction with aluminum, both the bolt holes and bolts should be painted prior to assembly. The most efficient method is to coat the bolt holes and bolts with sufficient bitumastic or red lead putty to fill all crevices when the bolts are tightened. In all cases, openings or crevices should be avoided as these collect moisture and provide a starting point for attack by corrosion.
30. Contact with Wood.—Damp wood in contact with aluminum is almost certain to cause corrosion. When aluminum is installed in contact with wood, a generous quantity of marine glue or bitumastic should be applied to faying surfaces before assembly. Unless this is done the aluminum will corrode badly under salt water exposure, as exemplified in the past by canopy hood installations.
31. Preservation of aluminum by the anodic process should be mentioned. This consists of anodic oxidation of the aluminum in an electrolyte of three per cent aqueous solution of chromic acid. Anodic oxidation furnishes an excellent surface for painting or for other coatings to resist corrosion; lanolin is sometimes used. The anodic process with the necessary appropriate coatings is used extensively in England especially for airplane parts. While this means of protection might be satisfactory in the case of small parts it is entirely too expensive for general use.
32. A very important development in corrosion resistance of “Duralumin” has recently been made by the Aluminum Company of America. By a secret process they are able to clad “Duralumin” products with a thin but homogeneous surface of pure aluminum, the latter being very resistant to corrosion. The name of this product is “Alclad.”

Conclusion

33. The improvements made in the production of aluminum and the Navy’s use of the newer alloys are of such recent development that there is yet much to be learned on

the subject. The information contained in this paper is perhaps the best available in concentrated form at the present time. It is realized that this paper is somewhat too technical to be interesting reading matter for the average person. The primary purpose is to invite attention to the difficulties which must be met to permit important weight saving on naval vessels through the use of aluminum. The precautions outlined herein are essential to the use of aluminum and must be followed closely to obtain reasonably satisfactory results. It is expected that some failures will be noted in service. Such failures should be studied and steps taken to prevent repetitions. The information gained from the installations now authorized will control the extension of the use of aluminum for more important work.

34. The writers are indebted to the representatives of the aluminum industry, particularly the Aluminum Company of America, for much of the information contained herein, and have freely drawn from their papers and discussions.