Aluminum Casting Alloys

The mechanical properties of alumi- num casting alloys are obtainable only if the chemical and heat treating specifications are followed carefully. It should be noted that the properties obtained from one particular combination of casting alloy, foundry practice and thermal treatment may not necessarily be identical to those achieved with the same alloy in a different foundry or with a different thermal treating source. In all aluminum casting alloys, the percentages of alloying elements and impurities must be controlled carefully. If they are not, characteristics such as soundness, machinability, corrosion resistance and conductivity are affected adversely.

Elements such as calcium and sodium, in ranges of a few thousandths of a percent, can have either a beneficial or a detrimental effect upon both soundness and machinability of alloys containing a high percentage of silicon. Similarly, the use of phosphorus in controlled amounts has a beneficial effect on the hypereutectic aluminum Casting alloys containing silicon in excess of 12%.

Special mechanical properties are obtainable in many alloys through accurate control of chemistry, manufacturing process and heat-treating processes. Reliable ingot producers can supply foundries with special ingot and with heat treating specifications designed to obtain special mechanical properties. Within practical limits, certain mechanical properties can be improved at the expense of others. For example, tensile and yield strengths can be increased, but this results in lower elongation and higher hardness. Higher elongation and lower hardness usually mean slight losses in tensile and yield strengths.

Thermal Treatment

Thermal Treatment Sand and permanent mold castings may be heat treated to improve mechanical and physical properties. Die castings can be stress relieved only (and not solution heat treated) because they generally have a porous internal structure containing some included gas, and they may blister at high temperatures. Some alloys, such as 443.0 (which is cast in sand or permanent molds but contains little or no copper, zinc or magnesium), do not respond to heat treatment to increase mechanical properties.

Thermal treatment designations (tempers) and what they specify are listed below. For aluminum castings, &qout-T” designates thermal treatment and is always followed by one or more digits that indicate specific sequences of basic treatments. A second digit indicates a modification of the heat treatment to obtain specific properties.

F-As Cast
O-Stress Relieve or Anneal
-T4-Solution Heat Treat and Quench: This is an unstable treatment. While it improves mechanical properties (such properties increase to some degree through aging at room temperature over a period of weeks), it is usual practice to artificially age to attain maximum mechanical values.

-T5, -T51-Artificially Age: This type of heat treatment is done at a comparatively low temperature and serves to eliminate growth on thermal cycle and to stabilize castings dimensionally (improving mechanical properties somewhat), to improve machinability and to relieve stress.

-T6, -T61-Solution Heat Treat, Quench and Artificially Age: Such heat treatment usually results in maximum tensile and yield strengths with adequate elongation. Aging stabilizes the properties.

-T7, -T71-Solution Heat Treat, Quench and Artificially Overage: This heat treatment improves mechanical properties to a large degree, completely degrowths and stabilizes the castings, and usually results in a slightly lower tensile and yield strength but an increased elongation value compared to the -T6 series of heat treatments.

Selecting Alloys and Casting Methods

Choosing the alloy, casting process and thermal treatment requires a complete knowledge of the service conditions of the part under consideration. A good knowledge of the alloys, heat treatments and various casting methods in common use is necessary.

Over 60 casting alloys are in use today, with as many as five different thermal treatment possibilities for some. This results in a large number of alternatives to choose from to satisfy individual requirements. Because of these many alloy and thermal treatment combinations, the possible range of typical mechanical properties varies widely.

Often, commercial castings do not have critical service requirements. In such cases, the foundry should be consulted to ascertain the most economical alloy and production method for the job.

In most cases, aluminum die castings are designed for maximum efficiency. In such cases, the alloy and heat treatment must be selected carefully from those available. In doing this, designers, with their knowledge of the service requirements for the casting, must confine the choice of alloys to those that provide the necessary properties. They must then be guided by the foundry in the final choice.

Sometimes, the alloy that shows the best properties on paper may have some production characteristics that would make it less desirable on an overall basis than other eligible alloys. The foundry is in the best position to advise on such factors as availability, relative ingot costs, production costs and reproducibility of results. When this is coordinated with the designer’s knowledge of service requirements, such as strength, hardness, corrosion resistance, impact strength and machinability, the best possible selection will result.

Because of this coordination, changes from initial design may be indicated to improve design efficiency, to lower production costs, or both. For instance, a casting with sound design from other standpoints may have a size or shape conducive to distortion in heat treating, which could be minimized through design changes.

Production and service requirements have a large bearing on the casting method, as do the size and shape of the part. For example, castings required in fairly large numbers should be made either by the permanent mold, diecasting or automated sand casting processes, provided the size and design features of the casting and available alloys are suitable.

Sandcasting is often used to produce parts with hollow cavities and a complex arrangement of ribs, pockets, etc., and to sizes that make parts unsuitable for casting in metal molds. In some cases, it is advantageous to redesign a casting to make it suitable for either permanent mold or diecasting methods. Sand casting usually requires minimum tooling charge, but the unit price of the castings and the finished part can be high. Permanent molding involves higher tooling charge, but the unit price is lower, particularly for longer runs. Diecasting usually involves the highest tooling charge and lowest piece price on large quantities.

Once the casting method is determined, the alloy choice is narrowed down appreciably. Next considered are the service requirements. If high strength is required, heat treatable alloys should be used. Alloy choice can be narrowed down further when remaining requirements, such as pressure tightness, corrosion resistance and machinability, are considered.

In some instances, it may be required to maximize one certain property-for example, highest possible yield strength. This immediately limits the alloy and heat treatment choices, as well as the casting method, to a possible one or two. Greater compromises will have to be made on the other requirements, particularly ductility.

Because die castings are usually used in the as-cast condition, thermal treatment is generally of little

significance, and choice of a diecasting alloy is based primarily on properties and castability of the alloy. Sandcasting, permanent mold casting and diecasting are the more widely used production methods. However, in special cases, a part may be produced more efficiently by other methods, such as shell molding, plaster molding, investment casting, lost foam, centrifugal or squeeze casting.

Aluminum Alloy Designations

Aluminum casting alloys in the U.S. are numbered according to a three-digit (plus decimal) system adopted by the Aluminum Association (AA) in 1954 and approved by the American National Standards Institute in 1957 (ANSI H35.1). The American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and the Federal and Military specifications for aluminum castings conform to the AA designation system.

In this system, major alloying elements and certain combinations of elements are indicated by the following number series:

 

Series Alloy Type
1XX.X 99.0% minimum aluminum
2XX.X Al + Cu
3XX.X Al + (Si-Mg), (Si-Cu) or (Si-Mg-Cu)
4XX.X Al + Si
5XX.X Al + Mg
7XX.X Al + Zn
8XX.X Al + Sn

 

The 6XX.X and 9XX.X series are not currently in use, but they are being held open for possible use in the future. The digit following the decimal in each alloy number indicates the form of product:

  • a &quot0″ (zero) following the decimal indicates the chemistry limits applied to an alloy Die casting;
  • a &quot1″ (one) following the decimal indicates the chemistry limits for ingot used to make the alloy casting;

  • a &quot2″ (two) following the decimal also indicates ingot but with somewhat different chemical limits (typically tighter, but still within the limits for ingot).

Generally, the XXX.1 ingot version can be supplied as a secondary product (remelted from scrap, etc.), whereas the XXX.2 ingot version is made from primary aluminum (reduction cell).

Some alloy names include a letter. Such letters, which precede an alloy number, distinguish between alloys that differ only slightly in percentages of impurities or relatively minor alloying elements (for example, 356.0, A356.0, B356.0 and F356.0).

Former alloy designations (if any) are shown in parentheses in the headings of the alloy tables. The properties shown for the alloys were obtained from ASTM, SAE and federal specifications as applied (sand, permanent mold and diecasting). Chemical composition limits are for standard foundry alloy ingot, percent maximum, unless shown as a range; the remainder is aluminum.

Properties and Characteristics of Some Common Casting Alloys

Typical Mechanical Properties Alloy 308.0
 

Casting Process and Temper

P.M.-F

Tension Shearing Strength(ksi) Comprehensive Yield Strength(Set 0.2%)(ksi) Brinell Hardness (500 kg load on 10 mm ball) Endurance Limit (ksi)
Ultimate Strength (ksi) Yield Strength
(Set 0.2%) (ksi)
Elongation (% in 2 in.)
28 16 2.0 22 17 70 13

Alloys 242.0 & A242.0

Alloys 242.0 and A242.0 are used extensively for applications where excellent strength and hardness at high temperatures are required. Heavy-duty pistons; motorcycle, diesel and aircraft pistons; aircraft generator housings; and air-cooled cylinder heads are typical applications.

Castability-These alloys have good fluidity, are fair for pressure tightness, and show fair resistance to hot cracking and solidification shrinkage.
Machinability-Machining characteristics of are very good. The usual precautions for machining aluminum will give the best results.
Weldability-Arc and resistance methods are very good for welding these alloys. Gas welding is satisfactory, but brazing is not recommended.
Finishing-Electroplated finishes are excellent on both alloys. Mechanical finishes are very good, and anodized appearance is good.
Corrosion Resistance-The resistance of these alloys to most forms of common corrosion is fair. Some additional protection can be gained by using chemical conversion coatings.

Typical Mechanical Properties Alloy 242.0
Casting Process and Temper Tension Shearing Strength(ksi) Comprehensive Yield Strength(Set 0.2%)(ksi) Brinell Hardness (500 kg load on 10 mm ball) Endurance Limit (ksi)
Ultimate Strength (ksi) Yield Strength
(Set 0.2%) (ksi)
Elongation (% in 2 in.)

Sand 242.0-F

31

30

<0.5

Sand 242.0-O

27

18

1.0

21

18

70

8.0

Sand 242.0-T571

32

30

0.5

26

34

85

11.0

Sand 242.0-T77

30

23

2.0

24

24

75

10.5

P.M. 242.0-T571

40

34

1.0

30

34

105

10.5

P.M. 242.0-T61

47

42

0.5

35

44

110

9.5

P.M. 242.0-T75

31

2.0

75