Aluminum Types and Aluminum Welding

Characteristics

Aluminum alloys are used in engineering design chiefly for their light weight, high strength to weight ratio, corrosion resistance, and relatively low cost. Aluminum is the most widely distributed of the elements, except for oxygen, nitrogen, and silicon. Aluminum alloys weigh about 0.1 pound per cubic inch. This is about one third the weight of iron at 0.28 pounds and copper at 0.32, is slightly heavier than magnesium at 0.066, and somewhat lighter than titanium at 0.163.

In its commercially pure state, aluminum is a relatively weak metal, having a tensile strength of approximately 13,000 PSI. However, with the addition of small amounts of alloying elements such as manganese, silicon, copper, magnesium, or zinc, and with the proper heat treatment or cold working, the tensile strength of aluminum can be made to approach 100,000 PSI.

Corrosion resistance of aluminum may be attributed to its self-healing nature, in which a thin, invisible skin of aluminum oxide forms when the metal is exposed to the atmosphere. Pure aluminum will form a continuous protective oxide film - i.e., corrode uniformly - while high-strength alloyed aluminum will sometimes become pitted as a result of localized galvanic corrosion at sites of alloying-constituent concentration.

Aluminum is easily fabricated - one of its most important assets. It can be cast by any method known to the foundryman; it can be rolled’ to any thickness, stamped, hammered, forged, or extruded. Aluminum is readily turned, milled, bored, or machined at the maximum speeds of which most machines are capable, and is adaptable to automatic screw machine processing. Aluminum can be joined by almost any method - riveting, gas, arc, or resistance welding; brazing; and adhesive bonding.

Many aluminum alloys have wide property ranges as a result of tempers attainable through treatment, both thermal and mechanical. With these wide ranges, much overlapping of properties exists among the various alloys thus making available a large number of compositions from which to choose. This increased selection provides for a greater latitude in the choice of fabricating techniques, and permits the selection of the most economical method.

CLASSES OF ALUMINUM AND ALUMINUM ALLOY

Types Available Aluminum is available in various compositions, including “pure” metal, alloys for casting, and alloys for the manufacture of wrought products. (Alloys for casting are normally different from those used for rolling, forging, and other working.) All types are produced in a wide variety of industrial shapes and forms.

"Pure" Aluminum "Pure" Aluminum. Pure aluminum is available both as a high-purity metal and as a commercially pure metal. Both have relatively low strength, and thus have limited utility in engineering design, except for applications where good electrical conductivity, ease of fabrication, or high resistance to corrosion are important. Pure aluminum is not heat treatable. However, its mechanical properties may be varied by strain hardening (cold work). Pure aluminum exhibits poor casting qualities; it is employed chiefly in wrought form. Commercially pure aluminum is available as foil, sheet and plate, wire, bar, rod, tube, and as extrusions and forgings.

Casting Alloys The aluminum alloys specified for casting purposes contain one or more alloying elements, the maximum of any one element not exceeding 12 percent. Some alloys are designed for use in the as-cast condition; others are designed to be heat treated to improve their mechanical properties and dimensional stability. High strength, together with good ductility, can be obtained by selection of suitable composition and heat treatment. Aluminum casting alloys are usually identified by arbitrarily selected, commercial designations of two- and three-digit numbers. These designations are sometimes preceded by a letter to indicate that the original alloy of the same number has been modified.

Wrought Alloys Most aluminum alloys used for wrought products contain less than 7 percent of alloying elements. By the regulation of the amount and type of elements added, the properties of the aluminum can be enhanced and its working characteristics improved. Special compositions have been developed for particular fabrication processes such as forging and extrusion. As with casting alloys, wrought alloys are produced in both heat-treatable and non-heat-treatable types. The mechanical properties of the "non-heat-treatable" type may be varied by strain-hardening, or by strain-hardening followed by partial annealing. The mechanical properties of the heat-treatable types may be improved by quenching from a suitable temperature and then aging. With the heat-treatable alloys, especially desirable properties may be obtained by a combination of heat treatment and strain-hardening.

Major Alloying Elements

The principal wrought forms of aluminum alloys are plate and sheet, foil, extruded shapes, tube, bar, rod, wire and forgings. Wrought aluminum alloys are designated by four-digit numbers assigned by the Aluminum Association. The first digit indicates the alloy group; the second digit indicates modifications of the original alloy (or impurity limits); the last two digits identify the aluminum alloy or indicate the aluminum purity. The system of designating alloy groups is shown in figure 2. Experimental alloys are also designated in accordance with this system, but their numbers are prefixed by the letter X. This prefix is dropped when the alloy becomes standard.

Welding Aluminum

The welding of aluminum is common practice in industry because it is fast, easy, and relatively inexpensive. It is especially useful in making leakproof joints in thick or thin metal, and can be employed with either wrought or cast aluminum, or a combination of both. Not all compositions of aluminum alloy are suitable for welding, and not all methods of welding can be used with them. The suitability for welding and the relative weldability of some aluminum alloys are given in tables 1 and 2.

Characteristics and Uses of Wrought Aluminum Alloys

Characteristics and Uses of Wrought Aluminum Alloys

Weldability Ratings for Cast and Wrought Aluminum

The welding of aluminum consists of fusing the molten parent metal together (with or without the use of filler metal), or of upsetting by pressure (with or without heat generated by the electrical resistance of the metal). A wide variety of welding methods are employed in the welding of aluminum. These include torch (gas), metal-arc, carbon-arc, tungsten-arc, atomic hydrogen, and electric-resistance welding. The equipment used is the same, except that it must be modified in some instances to permit slight changes in welding practices.

The corrosion-resistant oxide film that protects aluminum, deters the “wetting” action required for coalescence of the metals during welding. To effect a successful weld, this tough coating must be removed (and prevented from reforming) either mechanically, chemically, or electrically. Mechanical removal consists of abrading with a sander, stainless-steel wool, or some such means. Such a method is fast, but it is a manual operation, and should be reserved for comparatively small amounts of work.

Chemical removal is accomplished with fluxes that dissolve and float the oxides away. It is the most practical means of penetrating the glass-like oxide coating, and is well suited to the production of larger amounts of work. Its drawbacks include the danger of leaving voids or blow holes as a result of entrapment of slag, and the need for cleaning operations to remove any remaining corrosive flux

Electrical removal, used in some forms of arc welding, consists of the application of a reverse polarity (work negative) of welding current which loosens the oxide by electron emission. The reforming of oxides is prevented during welding and cooling of the weld by the cover of flux or by the use of inert gases to blanket the weld area.

The good thermal conductivity of aluminum allows the heat of welding to spread rapidly from the weld zone; this can result in a loss in strength in work-hardened or heat-treated alloys through annealing. It can also cause buckling or total collapse of the parent metal if the metal is not supported properly during welding. The good electrical conductivity necessitates the use of higher currents in resistance welding.

The low melting point of aluminum, in the range of 900°F (482°C) to 1216°F (658°C), increases the need for care in preventing the melting away of the metal parts that are to be welded. Since aluminum gives no visual indication of having attained welding temperature (that is, it does not become red, as does steel), the temperature has to be measured by the physical condition of the aluminum instead of its appearance. Aluminum tends to simply run like water when it reaches its melting point.

In welding applications where a considerable amount of general heating can be tolerated and where an easily finished bead is desired, gas welding is preferred. However, where minimum general heating, absence of flux, and very good properties are requirements, one of the types of inert-gas-shielded arc-welding method should be selected.

Gas welding is commonly done with oxypropane or oxyacetylene mixtures. The oxyacetylene flame is used most widely because of its availability for welding other metals. Butt, lap, and fillet welds are made in thickness of metal from 0.040 up to 1 inch.

Carbon-arc welding is an alternative method for joining material about 1/16 to 1/2 inch thick. The carbon arc affords a more concentrated heat source than a gas torch flame. Hence, it permits faster welding with less distortion. Soundness of welds is excellent and is comparable to that of good gas welding.

Gas metal-arc welding (GMAW) is especially suitable for heavy material. Welds in plate 2 1/2 inches thick are made satisfactorily by this method. Unsound joints are likely to appear in gas-metal-arc-welded material which is less than 5/64 inch thick. Weld soundness and smoothness of the surface are nearly as good as other arc-welding methods. This method has gained popularity as it uses no flux, but does employ a gas shielding of argon or helium, or a combination of both. This method also has a high production rate and reduces labor costs.

Shielding-metal-arc-welding (SMAW) is another electric arc method of welding aluminum. This method is difficult to master and often leaves slag inclusions in the deposited weld. This method of welding aluminum is more suitable for small repairs.

Gas tungsten-arc welding (GTAW) has two distinct advantages over some other forms of fusion welding; no flux is needed, and welds can be made with almost equal facility in the flat, vertical, or overhead positions. The advantages are the result of the ability to concentrate the heat, and the blanketing of the area with inert gas (argon or helium). The process can be used for either manual or automatic welding on metals 0.05 inch thick or thicker.

Resistance welding is especially useful for joining high-strength aluminum alloy sheet with practically no loss of strength. It includes three main types of processes; spot welding, seam or line welding, and butt or flash welding. The type adopted for assembly operations depends mainly on the form of material to be joined. Spot welding is widely used to replace riveting; it joins sheet structures at intervals as required. Seam welding is merely spot welding with the spots spaced so closely that they overlap to produce a gas-tight joint. Flash welding, sometimes classified as a resistance welding process, differs from spot welding in that it is used only for butt joints; the metal is heated for welding by establishing an arc between the ends of the two pieces to be joined.

Brazing Aluminum

Brazing differs from welding, in that filler metal is melted and flowed into the joint with little or no melting of the parent metal. (The brazing alloy melts at about 100°F (38°C) below that of the parent metal.) As a result, brazing is ideally suited to the joining of thinner material. It is also lower in cost than welding, has neater appearance, requires little finishing, and is suited to mass production methods. In addition, the corrosion resistance of brazed aluminum joints compares favorably, in general, to welded joints in the same alloy because, unlike solder, the filler metal is an aluminum alloy.

The strength of a brazed joint is equivalent to that of the metal in the annealed condition. However, in some instances where an age-hardening alloy is used, the mechanical properties of the metal can be enhanced by treatment. For example, alloy 6061 (61S), when quenched from the brazing operation and then artificially aged, will exhibit a tensile strength of approximately 45,000 psi, a yield strength of 40,000 psi, and an elongation in two inches of 9 percent. Brazeable alloys are available in plate, sheet, tube, rod, bar, wire, and shapes. They are generally confined to alloys 1100, 3003, and 6061.

Soldering Aluminum

Aluminum can be joined to aluminum and to other solderable metals by means of a soldering iron or torch, and an alloy of approximately 60 percent tin and 40 percent zinc. This method of joining is satisfactory for such applications as indoor electrical joints; it is not recommended for joining structural members or for use in moist or corrosive atmospheres because of the low mechanical properties of the solder and the difference in electrical potential between the solder and the aluminum.

The soldering of aluminum is similar to other forms of soldering, but it is somewhat more difficult to perform because of the high thermal conductivity of the aluminum and the presence of a tough oxide film. The thermal conductivity increases the problem of maintaining sufficient heat at the working area to melt the solder. (Aluminum solder melts at 550°F (288°C) to 700°F ( 371°C) as compared with 375°F (190°C) to 400°F (204°C) for most other solders.) Thus only small parts (20 square inches or less) which can be preheated, are suitable for soldering with an iron; larger parts require the use of a torch to concentrate sufficient heat.

The tough oxide film may be removed by dissolving it with a flux or by abrading it with a soldering iron or other mechanical means. In each instance, the working area must be kept covered with fluid flux or molten solder to exclude oxygen from the surface and to prevent the formation of a new oxide coating. However, after the surfaces are tinned, they may be joined in the usual manner.