Commercial Forms of Aluminum Alloys: Part One

Mill products of aluminum vary from foil thinner than tissue paper to plate 6 in. thick, and from wrist-thick stranded electrical power transmission conductor to magnet wire finer than human hair.

Ingot

Ingot, from which all other aluminum products are made, is generally supplied in one of seven commercial forms.

Unalloyed Ingot. Unalloyed aluminum ingot is furnished in sizes ranging from small-notched bars weighing a pound or less to large ingots weighing a ton or more. Unalloyed ingot may vary from about 98 to 99.999% Al; 99.5% Al is the most common grade. Electrical conductor (EC) ingot and rotor ingot for motors are special grades in which impurities objectionable for these applications are controlled.

Rich Alloy Ingot. Although unalloyed ingot is often used as produced, it is more often necessary to alloy it with other elements such as chromium, copper, iron, magnesium, manganese, nickel, silicon, titanium, and zinc. To accomplish this easily in production, rich alloy ingot, or "hardener", is employed; hardeners contain from less than 1% to as much as 50% of alloying elements.

Casting Alloy Ingot. Large quantities of scrap are consumed in the production of casting alloy ingot. When the impurity limitations on such alloys permit the introduction of large amounts of scrap, the resulting product is referred to as secondary casting alloy ingot. When the composition limits restrict the use of scrap, the product is designated as primary or virgin casting alloy ingot. There are many areas of overlap between primary and secondary.

Extrusion ingot. Extrusion ingot is usually furnished in cylindrical form, both solid and hollow. It varies in outside diameter from 75 to 800 mm. This product is most often made by the direct chill (DC) casting process or by some modification thereof. It is sawed to lengths varying, conforming to the requirements of the press in which it will be extruded. With certain alloys, extrusion ingots are often soaked or preheated before shipment to the extruder. This treatment, sometimes called homogenizing, permits higher extrusion speeds, improved surface finish on the extrusions, and longer die life. When extrusions of the highest quality are required, as in strong alloy aircraft parts, extrusion billets may be scalped before shipment to remove surface liquation. Hollow ingots are used to extrude tube and other hollow shapes. These ingots are normally cast to the required dimensions in the direct chill casting machine, but manufacturing limitations often require machining of the inside diameter.

Forging Ingot. Although most small forgings are produced from rolled or extruded stock, cast ingots are often used for large forgings. To prevent irregularities in the cast surface from affecting the quality of the forgings, these ingots are always scalped before shipment. Forging ingot is generally supplied in cylindrical form.

Hot Metal. Molten metal is sometimes transferred directly in insulated ladles from the smelter to the customer’s plant, occasionally at distances up to several hundred miles.

Castings

In order of commercial importance, the three major forms of aluminum castings are: die, permanent mold, and sand.

Die casting. Die-casting is inherently suited to large-quantity production of both ornamental and structural aluminum parts. If the quantity permits investment in a die, if wall thickness can be reduced as much as the casting process permits, and if objectionable undercuts are not present in the design, aluminum castings can usually be produced by this process at a lower cost than by any other method.

Permanent Mold Castings. In the permanent mold process, cast iron molds and cores are generally used; less frequently, steel and inlays of other metals are employed. Molten aluminum is poured into the mold cavity under a normal gravity head. In special cases, a small amount of pressure is applied to the mold through the application of vacuum, by pumping the molten metal, or by centrifugal force. In the semi permanent mold process, cores of dry sand or other expendable material are employed, overcoming many of the design limitations imposed by metal cores. Permanent mold castings are metallurgically superior to die or sand castings, having greater soundness, pressure tightness, higher strength, greater speed of production, and thinner walls.

Sand Castings. The sand casting process is the most versatile method of producing a cast aluminum shape and is characterized by universal adaptability. It is employed to produce small quantities of identical castings, parts requiring intricate coring, and very large castings. Modern high-speed molding equipment and methods produce sand castings relatively cheaply.

Other Cast Forms. Direct chill castings -- both solid and hollow, having round, rectangular or odd-shaped sections -- are used because of their outstanding mechanical properties after heat treatment and their sound structure. Mechanical properties approach those of wrought products except that the elongation is lower. Cast tool and jig plate and large sizes of cast bus bar are commonly made by this process. When thin walls and close dimensional tolerances are required, and if the quantity does not warrant die-casting, shell mold casting, plaster mold casting, or investment casting are often employed. The last two processes are also used where difficult undercuts and intricate coring are required. The centrifugal casting process is used for the production of large aluminum alloy tubes and rings.

Forgings and Pressings

Aluminum alloy forgings and pressings are produced commercially in conventional hammer equipment, on hydraulic and mechanical presses, in ring rollers, and on upsetters. They may be classified as die forgings, hand forgings, and rolled rings.

Die Forgings. Depending upon the amount of machining necessary to obtain a finished part, aluminum die forgings and pressings are categorized as (a) blocker, requiring the most machining; (b) conventional, providing a good balance between die cost and machining cost; or (c) precision forgings and pressings. Precision forgings are of advantage where it is desirable to obtain thin web sections and thin, accurate ribs with a minimum of machining. Draft angles are controlled to less than 10 if desired; corner fillets can be held to a minimum.

Hand forgings are produced by working aluminum stock between flat dies or other simple tools that shape the piece roughly to the required contour. Prototypes are frequently made as hand forgings to reduce delivery time. Slabs as large as 7 in. thick by 120 in. wide and 450 in. long, weighing over 35,000 lb, are available in the form of hand forgings.

Rolled Ring Forgings. Precision ring rolling equipment is available to produce rolled rings in a wide range of diameters up to 150 in., in any wall thickness and alloy. Larger rings can be fabricated by forging over a mandrel.