• ALIGNED POROSITY

    A localized array of porosity oriented in a line.

  • ALL-WELD-METAL TEST SPECIMEN

    A test specimen in which the portion being tested is composed wholly of weld metal.

  • ALLOTROPY

    The reversible phenomenon by which certain metals may exist with more than one crystal structure. For example, alpha, gamma and delta iron are three allotropic forms of iron with different crystal structures.

  • ALLOY

    A substance with metallic properties and composed of two or more chemical elements of which at least one is a metal. See STANDARD WELDING TERMS. The added element may be metallic or nonmetallic. See also STEEL, ALLOY; ALUMINUM ALLOYS; MONEL; COPPER ALLOY WELDING.

  • ALLOY POWDER

    Powder prepared from a homogeneous molten alloy or from the solidification product of such an alloy.

  • ALLOY STEEL

    See STEEL, ALLOY.

  • ALLOYING ELEMENTS

    The chemical elements comprising an alloy. In steel it is usually limited to the metallic elements added to steel to modify its properties. For example, the addition of copper, nickel, or chromium individually or in combination produces alloys or special steels.

  • ALNICO ALLOYS

    A series of alloys developed for use as permanent magnets. With the exception of Alnico 111, all of these iron-base alloys contain aluminum, nickel, and cobalt as the principle alloying elements (as the name Alnico indicates). Most also contain 3% or 6% copper. Because these alloys are available only in the cast or sintered condition, they are difficult to fabricate by welding.

  • ALPHA IRON

    The body-centered cubic form of pure iron.

  • ALPHABRASS

    A copper-zinc alloy with a copper content greater than approximately 64%. “Yellow brass” is the name used in metallurgical literature.

  • ALTERNATING CURRENT (ac)

    (Abbreviation: ac). A current which reverses directions at regularly recurring intervals. Unless otherwise distinctly specified, the term alternating current refers to a periodically varying current with successive half waves of the same shape and area.

  • ALTERNATING CURRENT ARC WELDING

    An arc welding process in which the power supply provides alternating current to the arc.

  • ALUMINOTHERMIC PROCESS

    A method of welding which makes use of the exothermic reaction which occurs when a mixture of aluminum and iron oxide powders is ignited. When ignited, this mixture produces superheated liquid steel and aluminum oxide slag at approximately 2760°C (5000°F). The liquid steel is sufficiently hot to melt and dissolve any metal with which it comes in contact and fuses with it to form a solid homogeneous mass when cooled. For this reason, this process is especially adapted to welding heavy steel and cast iron sections, such as those used in locomotive, marine, crankshaft and steel mill repairs, and is also used in pipe welding and rail welding. See THERMITE WELDING.

  • ALUMINUM

    (Chemical symbol: Al). Aluminum is a silver-white, malleable, ductile, light, metallic element with good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. Atomic weight, 26.97; melting point, 660°C (1220°F); specific gravity, 2.70 at 20°C (68°F). Aluminum is one of the most abundant constituents of the earth’s crust. It is found in most clays, soils and rocks, but the principal commercial source is the ore, bauxite, an impure hydrated oxide. The impurities are removed from bauxite by a chemical process leaving pure aluminum oxide, alumina. Pure metallic aluminum is obtained by electrolysis of the oxide.

    Aluminum is third on the scale of malleability and fifth in ductility. It is only slightly magnetic and is strongly electro-positive, so that when in contact with most metals it corrodes rapidly.

    Aluminum will take a high polish, but it is likely to become “frosted” in appearance due to the formation of an oxide coating. Its electrical conductivity is about 60% that of copper. Aluminum is used extensively as a deoxidizer in steel production, and as such it is an effective purifier. Aluminum lessens grain growth by forming dispersed oxides or nitrides.

  • ALUMINUM ALLOYS

    Commercial aluminum alloys are grouped into two classifications: wrought alloys and cast alloys.

    Wrought Alloys

    Wrought alloys are those alloys which are designed for mill products for which final physical forms are obtained by mechanical working, such as rolling, forging, extruding and drawing. Wrought aluminum mill products include sheet, plate, wire, rod, bar, tube, pipe, forgings, angles, structural items, channels, and rolled and extruded shapes.

    Cast Alloys

    Cast alloys are those alloys which are shaped into final form by filling a mold with molten metal and allowing it to solidify in the mold.

    Sand Casting

     Sand casting utilizes a mold in sand made around a previously formed pattern to the exact shape desired in the final casting, but slightly larger in size to allow for shrinkage of the cast metal as it cools.

    Permanent Mold Castings

    Permanent mold castings are made by pouring molten metal into steel or iron molds.

    Die Castings

    Die castings are also made in steel molds, but the molten metal is forced under pressure into the die or mold cavities. Die casting yields a denser casting with a better surface finish, closer dimensional tolerances, and thinner sections when desired.

    Clad Alloys

    Clad alloys, which may be up to 5% of the total thickness on each side, yield a composite product which provides the high strength of the core alloy protected by the cladding. Copper and zinc, when used as major alloying elements, reduce the overall resistance to corrosion of aluminum alloys. To gain the desired corrosion resistance in these alloys in sheet and plate form, they are clad with high purity aluminum, a low magnesium-silicon alloy, or an alloy of 1% zinc.

    Wrought Alloy Designations

    The Aluminum Association, an organization composed of manufacturers of aluminum and aluminum alloys, has devised a four-digit index system for designating wrought aluminum and wrought aluminum alloys. The first digit indicates the alloy group, ie; the major alloying element, as shown in Table A-2. The second digit indicates a modification of the original alloy, or the impurity limit of unalloyed aluminum. The third and fourth digits identify the alloy or indicate the aluminurn purity. See UNIFIED NUMBERING SYSTEM.

    Wrought Alloy Temper Designations

    In this index system, the letter following the alloy designation and separated from it by a hyphen indicates the basic temper designation. The addition of a subsequent digit, when applicable, refers to the specific treatment used to attain this temper condition.

    Alloys which are hardenable only by cold working are assigned "H' designations; alloys hardenable by heat treatment or by a combination of heat treatment and cold work are assigned "T" designations. Table A-3 shows the basic temper designations and resulting condition of the alloy.

    Casting Alloy Designations

    A system of four-digit numerical designations is used to identify aluminum casting alloys, as shown in Table A-4. The first digit indicates the alloy group, the second two digits identify the aluminum alloy within the group, and the last digit (which is separated from the first three by a period) indicates the product form. A modification of the original alloy or impurity limits is indicated by a letter before the numerical designation. The temper designation system for castings is the same as that for wrought product shown in Table A-3.

     

  • ALUMINUM BRAZING

    In brazing, specific fluxes and filler materials with melting points lower than that of the parent metal are used for making a joint without melting the pieces to be joined. Brazing can be used to advantage when sections are too thin for welding, and for those assemblies having many parts which must be joined in an intricate manner. Brazing is generally lower in cost than gas or arc welding and is adaptable to mass production. Brazed joints have a smoother appearance, with well rounded fillets which often require no finishing.

    Brazed joints should be carefully designed to provide for full penetration of filler metal, because its flow depends largely on capillary action and gravity. Joints should be self-jigging for easy assembly prior to brazing. Lock seams, lap fillet, and T-joints are preferred because they have greater strength than butt or scarf joints.

    Three commonly used aluminum brazing methods are furnace, molten flux dip, and torch.

    Furnace Brazing

    Furnace brazing consists of applying a flux and filter material to the workpieces, arranging them, then heating in a furnace to a temperature that causes the filler material to melt and flow into the joint without melting the parent metal. Filler material in various forms is added to the joint. In many cases, filler material in the form of a flat shim or wire ring can be fitted into the joint. Filler material is also supplied by using clad brazing sheet, shaped to fit the joint.

    Standard types of furnace heating systems include forced air circulation, direct combustion, electrical resistance, controlled atmosphere, and radiant tube. The selection of furnace type is determined by the application requirements, as furnace operation and results vary. For example, temperature is most easily controlled in electrical resistance furnaces. Although combustion furnaces are least expensive, some assemblies cannot be exposed to the gases which are always present in this type. Radiant heat furnaces are sometimes difficult to regulate, but the type of heat produced is excellent for most brazing requirements. Aluminum-coated steel or firebrick linings are preferred for all types of heating units.

    Rate of production is another consideration when selecting a heating unit. In batch furnaces, brazing is accomplished by placing a tray of assemblies inside, heating for the required time, then removing the batch. Though simpler, this furnace is slower than the furnace with a continuous conveying system in which the work moves through on a belt. The continuous furnace is more conservative of heat, and the gradual heating reduces danger of warping.

    Temperature for individual batches will necessarily depend on such factors as the design of the parts, size of fillets, and alloy to be brazed. However, furnaces should have operating temperature ranges from 540 to 650°C (1000 to 1200"F), with control capability within +/- 3°C (5°F). Since regulation of temperature is critical, automatic control is the rule in production jobs. If uniform rise of temperature does not occur naturally, forced circulation is essential.

     Assemblies are generally placed in the furnace immediately after fluxing. When large  areas have been fluxed, most of the moisture must be removed because the brazing process may be hindered if it is not removed. Preheating the parts for about 20 minutes at approximately 200°C (400°F) is usually sufficient.

    Brazing time depends on the thickness of the parts. For instance, material 0.15 mm (0.006 in.) thick reaches temperature in a few minutes, while 13 mm (0.5 in.) thick material may take up to 45 minutes. After the filler material begins to melt, it takes approximately five minutes for the material to fill the joints.

    Dip Brazing

    Parts are assembled and dipped into a molten flux in dip brazing. This method has been very successful for the manufacture of elaborate assemblies, such as heat exchanger units. The flux application does not require a separate operation and the bath transmits heat to the interior of thin walled parts without overheating outside surfaces. Contamination is also held to a minimum.

    Dip brazing is versatile. It is used in the manufacture of delicate specialty parts where tolerances up to k0.05 mm (0.002 in.) are maintained in production, or in making large parts approaching 450 kg (1000 lb).

    A separate furnace is necessary to preheat the assembly to prevent undue cooling of the flux bath. A furnace used for furnace brazing operated at 280 to 300°C (540 to 565°F) is satisfactory for preheating. It should be located near the dip pot so heat loss will be held to a minimum.

    Size of the dip pot will depend on the size of the assemblies to be brazed, but  should be large enough to prevent the parts from cooling the flux more than 5°C (10°F) below operating temperature when they are added.

    Dehydration of the flux bath is accomplished by dipping 1100 or 3003 alloy sheet into it. As the sheet is attacked, the hydrogen evolved is ignited on the surface. Residue that forms on the bottom of the pot must be removed on a regular basis.

    A modification of dip brazing is the application of a flux mixture to the assembly prior to immersion in a salt bath furnace. A typical example consists of making a paste of a mixture of a dry, powdered aluminum-silicon (548°C [1018"F] flow point) brazing alloy and flux, and water, and applying as much as required to fill the joints and make fillets. Next, the assembly is placed in an oven and heated to about 540°C (1000°F) to remove the water. This leaves the brazing alloy powder firmly cemented to the aluminum surfaces, the flux serving as the cement.  

    When the assembly is placed in the molten brazing salt, the alloy is held firmly in place by the flux cement while it is being heated and melted. The flux cement has a higher melting point than either the brazing alloy or the brazing salt, but it is soluble in  the salt bath, so the brazing alloy is held in place, even while melting, until the cement has been dissolved by the molten salt. As the flux cement is dissolved away from the molten filler metal, the alloy runs into the joint capillary spaces and also forms smooth fillets.

     

    Torch Brazing

    This method of brazing can be accomplished by using a standard torch as a heat source. Correct torch tip can best be determined through trial, and often depends on the thickness of the piece to be brazed. Filler alloys with suitable melting ranges and efficient fluxes are available for all brazeable aluminum alloys. Most work can be torch brazed with 3 mm (1/8 in.) diameter wire.

    A reducing flame with an inner cone about 25 mm (1 in.) in length and a larger exterior blue flame is preferred. Oxyhydrogen, oxyacetylene, oxynatural gas, or gasoline blow torches can be used. Ample clearance space must be allowed where the filler will flow, and a path for flux to escape must be allowed.

    After painting with flux paste, the entire area of the joint is heated until the filler melts when it is touched against the heated parent metal. Too hot a flame, or allowing the joint to cool repeatedly, will cause uneven results. Capillary flow tends to be toward the hottest spot, so it is important that the flow of the filler wire be controlled throughout. Heat should be applied just ahead of where flow is desired. Joints can be produced that have a final fillet that needs a minimum of finishing, if any. All flux should be removed after brazing. If joints are accessible, a fiber brush with boiling water bath can be used. Scrubbing with hot water and rinsing with cold, then drying is often effective, as is blasting with a steam jet. When possible, a chemical treatment should be used to clean the joint.

     

    Cleaning

    Clean surfaces are essential if strong brazed joints are to result. All grease should be removed. Solvent or vapor cleaning will probably be sufficient for the nonheat-treatable alloys, but the for the heat-treatable alloys, the oxide film must be removed with a chemical or by abrasion with steel wool, or stainless steel brushes. All burrs should be removed, as flux will not flow around them.

    In post-brazing cleaning, it is essential to remove all the flux. A solution of nitric acid (concentrated technical grade) in equal amounts of water is effective. When a large area is to be cleaned of residual flux, however, this method is not recommended because noxious fumes are generated. An exhaust system is advisable even for small production situations. To achieve a uniform etch and remove flux in one operation, the work can be immersed in a nitrichydrofluoric acid solution, using 2 L (0.5 gal) nitric acid, 1/8 L (1/4 pint) hydrofluoric acid, and 17 L (4.5 gal) of water. The major portion of flux should be removed first by immersing in boiling water, then immersing in the acid solution for 10 to 15 minutes, depending on the desired extent of etching. Parts are then drained and rinsed in cold running water, then in hot water. To avoid staining, the hot water bath should be limited to about 3 minutes. 

    Because of the reaction of a hydrofluoric acid solution with aluminum, in which hydrogen gas is generated, flux removal is efficiently accomplished by this method. The solution is compounded of 600 mL (1.25 pints) of acid, (technical concentrated grade) and 19 L (5 gal) of water. Though this solution is less contaminated by flux than those containing nitric acid, the hydrofluoric acid solution does dissolve aluminum. Therefore, immersion time should be limited to 10 minutes or less. Discoloration can be removed by a quick dip in nitric acid.

    When maximum corrosion resistance is important, or when parts are thin, parts can be dipped in a solution of 2 L (2.25 qts.) of nitric acid (technical concentrated grade), 1.8 kg (4 lb.) of sodium dichromate, and 17 L (4.5 gal) of water. The usual procedure is to immerse the parts in hot water, then in the dip solution at 65°C (150°F) for 7 to 10 minutes, followed with rinsing in hot water.

  • ALUMINUM BRONZE (9% Aluminum Bronze)

    A copper-aluminum alloy commonly used for the fabrication of corrosion resistant parts and marine hardware.

  • ALUMINUM CASTINGS, Welding

    Both the gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) processes are used for welding aluminum castings. In general, welding aluminum alloy castings requires a technique similar to that used on aluminum sheet and other wrought products. However, many castings are susceptible to thermal strains and cracks because of intricate design and varying section thicknesses. In highly stressed structures, castings depend on heat treatment for strength.  Welding tends to destroy the effect of the initial heat treatment. In these cases, welding is not recommended unless it is possible for the casting or assembly to be heat-treated again after welding, when the loss in strength can largely be restored.

    Preparation for Welding

    Before welding, castings should be cleaned carefully with a wire brush and an appropriate solvent to remove every trace of oil, grease and dirt. When welds are to be made in sections heavier than approximately 5 mm (3/16 in.), the edges should be beveled at an angle of about 45". When preparing defective areas for welding, any unsoundness or dross must be completely melted or cut away before proceeding with the weld. When two or more pieces are to be assembled, or if a broken piece is to be welded, the parts should be held by a fixture and clamped in the correct position for preheating and welding. The clamps should be attached in a way that will permit free expansion of the casting during heating, otherwise stresses may develop which will result in excessive distortion or cracks.

    Preheating

    Prior to welding a casting that is large or intricate in design, it should be preheated slowly and uniformly in a furnace to avoid thermal stresses and facilitate development of the required temperature for welding. A temperature of 370 to 425°C (700 to 800°F) is generally sufficient for preheating. If the casting is small, or if the weld is near the edge and in a thin walled section, an experienced welder can often do the necessary preheating with an oxyfuel gas torch applied in the region of the weld. After welding, the casting should be cooled slowly and uniformly to room temperature to reduce the danger of excessive stresses and possible cracks.

    Welding Precautions

    Surface defects and small holes in aluminum castings can be repaired by welding after the part is correctly prepared and preheated. However, when working with assemblies or broken castings, there are several points to consider during welding. The individual parts should first be tack-welded into place, and actual welding should begin at the center and proceed toward the end. When any difference exists in the thickness of the sections being joined, the GTAW welder must carefully distribute the heat from the torch in order to avoid melting the lighter section while bringing the heavier section up to welding temperature. A similar precaution must be taken with sheet and casting assemblies, and welders may require a little experience to develop the proper technique.

    Choice of Welding Rod

    When welding castings of the non-heat-treatable aluminum alloys or assemblies involving such castings, consisting of welding rod Al-S%Si or A1-4%Cu, 3%Si is generally used. However, in the case of castings requiring subsequent heat treatment, a welding rod of the same alloy as the casting should be used. The size of the rod best suited for the job will, of course, depend to some extent on the thickness of the metal being welded, but in general, a rod 1.6 to 2.4 mm (1/16 in. to 3/32 in.) diameter will be satisfactory.

  • ALUMINUM SOLDERING

    Soldering is an economical and practical means of joining aluminum on a production basis. With careful attention to such details as surface preparation, solder composition, temperature, and application of heat, a variety of joints can be soldered. Although less heat is required to raise the temperature of a piece of aluminum sheet of a given thickness than is  required for a sheet of copper or steel of the same thickness, aluminum must be heated from 55 to 110°C (100 to 200°F) higher than either of these metals when it is to be soldered. The higher  temperature is specified to produce joints with good resistance to corrosion, and is one of the key factors in producing successful soldered joints in aluminum.

    Preparing the Surface

    As a first step, it is necessary to remove the oxide film on aluminum so that the filler metal can contact and bond with the parent metal. This is accomplished by one of the following methods:

    (1) Mechanical abrasion

    (2) Application of ultrasonic energy

    (3) Electroplating

    (4) Use of either chemical or reaction-type fluxes

    Mechanical Abrasion

    Scraping is the simplest way to remove oxide. Due to the rapid rate at which the film re-forms on aluminum, scraping is impractical unless it is accomplished in the  presence of molten solder. The solder then wets and bonds with the parent metal and results in a precoated or “tinned” surface.

    Although there are many variations of the process, one example is as follows: Two sheets of aluminum are heated to the melting temperature of the solder. A small amount of solder is then melted on the sheets and rubbed with an abrasion tool until the solder wets the surface. The two precoated sheets are then placed together and held in contact until the solder solidifies. A strong joint results.

    A fibrous glass brush is one of the most satisfactory abrasion tools, since no corrosion hazard is created and the close-packed strands remove the oxide without damage to the parent metal.

    Some solder rods, called “abrasion solders,” have melting characteristics which permit them to perform the dual role of solder source and abrasion tool. However, only a precoated or “tinned” surface is produced, and a second operation is generally required to complete the joining.

    Ultrasonic Cleaning and Soldering

    Cleaning- Ultrasonic energy can be used to remove oxide film on aluminum. An electronic power oscillator is used to generate electrical impulses (currents) at frequencies from 15 to 50 kHz; these electrical impulses are converted to mechanical motion by a device known as a magnetostrictive transducer. Commercial transducers used in soldering tools consist of a nickel core and a coil around the core that is connected to the oscillator. When the nickel core (a laminated nickel core is generally used to reduce eddy currents) is subjected to an electromagnetic impulse resulting from electric current flowing through the coil, it constricts a maximum of 30/1 000 000 (30 x of its length. If the end of the vibrating core is brought into contact with molten solder, the vibrating core will produce numerous holes, or voids, within the liquid.When aluminum is immersed in the liquid solder, the collapse of the voids  reates an abrasive effect known as cavitation erosion on the surface of the metal. This erosive action removes the oxide film.

    Soldering- In ultrasonic aluminum soldering, the area to be precoated, or “tinned,” is cleaned, heated to soldering temperature, about 190°C (375”F), and the solder, usually a 90-10 tin-zinc combination, is applied. A quantity of solder is melted on the surface to form a molten puddle, and the end of the transducer is swept over this surface. The ultrasonic energy removes the oxide from the aluminum, allowing a firm solder bond.

    The ultrasonic method can also be applied in dip soldering, or, with modifications, in brazing and welding. The primary advantages of the ultrasonic process are that no flux is required, and joint quality is equal to that of joints soldered by any other process using the same solder and parent metal. The disadvantages are high cost of equipment, small capacity of the units, and the limitation that direct soldering of lap or crimp joints is not practical.

    Plated Surfaces for Soldering

    It is possible to prepare the aluminum surface to be soldered by electrolytically plating it with a metal, such as copper. Before deposition of the copper, the aluminum surface is treated by immersing the aluminum in a solution of alkaline sodium zincate. The zincated surface is then electrolytically plated with copper to produce a surface that can be easily soldered with the conventional solders and fluxes used to solder copper.

    Fluxes for Soldering Aluminum

    Chemical and reaction fluxes are the types generally used for soldering aluminum. Chemical fluxes are usually recommended when the joint temperature is less than 275°C (525°F). However, in some applications, the maximum temperature limit can be successfully raised to 325°C (620°F). At temperatures exceeding 275°C (525"F), the chemical fluxes decompose; at temperatures above 325°C (620"F), this decomposition becomes so rapid that it is impractical to use this type of flux.

    In general, chemical fluxes are used with the tinlead-cadmium-zinc solders. For best results, the magnesium content  if the aluminum alloy being soldered should not exceed 1%, and the silicon content  should not exceed 5%.

    All of the common commercial reaction fluxes deposit zinc or tin, or both, on the aluminum surfaces. These metals alloy with the aluminum, and a thin alloy layer is formed in the area near the original surface of the material.

    Solders for Aluminum

    There are four groups of commercial solders for aluminum: zinc base, zinc- admium base, tin-zinc base, and the tin-lead base. All these may contain appreciable quantities of other metals. Table A-6 shows the composition of typical solders for aluminum.

    The zinc-base solders produce joints with shear strengths of 103 MPa (15 000 psi) and higher, with good corrosion resistance. These solders require soldering temperatures ranging from 370 to 435°C (700 to 820°F).

    The zinc-cadmium base solders develop joints with shear strengths in excess of 70 MPa (10 000 psi), with intermediate corrosion resistance. They require soldering temperatures of 265 to 400°C (5 10 to 750°F).

    The tin-zinc base solders develop joints with shear strengths in excess of 48 MPa (7000 psi), with intermediate corrosion resistance. They require soldering temperatures of 290°C (550°F) or higher. The tin-lead solders containing cadmium or zinc produce joints with shear strength in excess of 34 MPa (5000 psi), with corrosion resistance adequate for interior applications only. These solders are applied at soldering temperatures of 230°C (450°F) or higher.

    Solders high in zinc content are applied to aluminum for a soldered system that is very resistant to corrosive attack. Hot dip tinned surfaces are used in special applications to produce readily solderable surfaces, since tin quickly wets an aluminum surface from which the oxide has been removed. Thus, pretinned aluminum soldering materials and techniques cannot be used. However, molten tin penetrates aluminum- magnesium alloys along the grain boundaries, and alloys containing more than 0.5% magnesium can be seriously damaged by this penetration. Cadmium is only slightly soluble in solid  aluminum and forms a very limited diffusion zone in aluminum soldered joints. Cadmium is not usually used as a solder by itself, but is used effectively to improve the  properties of zinc- and tin-base solders. Lead is practically insoluble in solid aluminum and is not normally used as a solder by itself. In combination with tin, zinc and cadmium, lead forms an important class of solders for aluminum.

    Joint Design

    The joint designs used for soldering aluminum are similar to those used with other metals. The most common designs are lap, crimped, and T joints. Capillary spacing varies with method, alloy, solder, joint, and flux. Generally, joint spacings from 0.25 to  0.60 mm (0.010 to 0.025 in.) are maintained when a chemical flux is used, and from 0.05 to 0.25 mm (0.002 to 0.010 in.) with reaction fluxes.

    Torch Soldering

    Air-fuel gas or oxyfuel gas torches are used effectively to solder aluminum assemblies. The flame temperature (gas mixtures) and heat output (torch size) can be independently adjusted to provide optimum conditions for specific applications. The flux is usually painted on the joint, and the solder is either pre-placed or manually fed into the joint using solder wire. The best torch soldering technique involves heating the assembly initially on both sides of the joint area until solder flow can be initiated in the joint area. The flame can then be moved to a position directly over the joint and slightly behind the front of the solder flow. In this way the flame does not come into direct contact with the flux before it has performed its function, and the speed and ease of soldering is at a maximum.

    Furnace Soldering

    Furnace soldering is a highly productive, efficient method for fabricating aluminum assemblies. In this process, the entire assembly is raised to temperature, thus minimizing distortion. The solder is usually preplaced in the joint, using wires, shims, or washers of filler material. Flux is applied by spraying, painting, or immersing the part in the flux by flowing a liquid flux over the assembly. The assembly is then placed in a furnace and brought to temperature. The flux must be carefully protected against charring or volatilization before it has performed its function. Joint design and furnace characteristics should be such that all sections of the joint are brought to temperature at the same time in order to prevent excessive alloying and penetration by liquid solder.

    Dip Soldering

    Dip soldering is an efficient process for joining assemblies at a high production rate. It is a versatile process because the same techniques used for other metals can often be utilized for soldering aluminum by merely changing solder and flux. Any of the solders listed in Table A-6 can be used for dip soldering. Solder selection should be based on service and operating characteristics required, and cost of the solder.

    In dip soldering, the flux tends to insulate the part to be soldered from the solder, thus a heavy coat of flux will reduce the rate at which the part is brought to soldering temperature. Since the rate of heating will be greatest if a small amount of flux is used, and because solder will prevent the surface from being reoxidized, a dilute liquid flux is recommended for dip soldering. Also, the flux should be selected to operate at the  optimum temperature of the solder to minimize drossing, dissolution, and liquid metal penetration, and to provide the best operating characteristics possible.

    Soldering Aluminum Alloys

    While aluminum and all the aluminum alloys can be satisfactorily joined by soldering, the alloying elements influence the ease with which they are soldered. Alloys commonly used in commercial applications are 1100, 1145,3003,5005, and 6061.

    Commercially pure aluminum (1100), aluminum of higher purity (1145), and aluminum-manganese (3003) alloys can be readily joined using all soldering techniques. Aside from ensuring that the surface is reasonably free of extraneous dirt or corrosive produced, no special surface preparation is needed for soldering these alloys. They are also resistant to intergranular penetration by liquid solder.

     Use of molten tin solders results in intergranular penetration in alloys containing 0.5% or more magnesium. Zinc solders will also cause intergranular penetration of aluminum-magnesium alloys, but the extent of penetration is usually not significant until the magnesium content of the parent alloy exceeds 0.7%.

    Aluminum alloys containing more than 5% silicon are not usually soldered by procedures requiring the use of a flux.

    The addition of zinc or copper to aluminum does not materially reduce the solderability. However, these metals are used in combination with other elements to form high-strength, heat-treatable alloys. Films formed on the surface during heat treatment reduce the solderability, so a chemical surface pre-treatment is usually recommended. In some instances, alloys such as 2024 and 7075 have been satisfactorily soldered using reaction fluxes without using chemical pretreatment. If chemical fluxes are used, a chemical pretreatment is usually required.

    Additions of small amounts of magnesium and silicon to aluminum produce an alloy system commonly referred to as the aluminum-magnesium-silicate alloys. These alloys, 6061 and 6063, are easily soldered and are not as susceptible to intergranular penetration by liquid solder as the binary aluminum-magnesium alloys of a similar magnesium content.

    Excellent Solderability

    Binary aluminum-magnesium alloys, in sheet and other forms, provide excellent solderability, and include:

    1030, 1050, 1060, 1070, 1075, 1080, 1085, 1090, 1095, 1099, 1100, 1130, 1145, 1160, 1171, 1180, 1187,1197, and 3003.
    Chemical or reaction fluxes may be used.
     

    Good Solderability

    Alloys considered “good” for soldering are:

    3004, 5005, 5357, 6053, 6061, 6062, 6063, 6151, 6253, 6951, 7072, and 8112.
    With the exception of the first two, reaction type flux is recommended.
     

    Fair Solderability

    Fair solderability is accorded alloys:

    2011, 2014, 2017, 2018, 2024, 2025, 2117, 2214, 2218, 2225, and 5050.
     

    Poor Solderability

    The alloys rated as poor for soldering are:

     5052, 5652, 7075, 7178, 7277, 4032, 4043, 4045, 4343, 5055, 5056, 5083, 5086, 5154, 5254, and 5356.

  • ALUMINUM WROUGHT ALLOYS, Welding

    Wrought aluminum alloys can be joined by most fusion and solid state processes, as well as by brazing and soldering (See ALUMINUM BRAZING and ALUMINUM SOLDERING).

    The relative weldability of the wrought non-heat treatable alloys is shown in Table A-7. Similar information for the wrought heat-treatable alloys is shown in Table A-8. In addition to the processes listed in the tables, wrought aluminum alloys are welded by electron beam and plasma arc welding, and such solid state processes as friction welding, diffusion welding, explosion welding, high frequency welding and cold welding. Submerged arc welding is one of the few processes not commercially used on wrought aluminum alloys.

    The selection of a process for welding wrought aluminum alloys depends on many factors, such as the application and service environment, the physical dimensions of the parts being welded, the number of parts involved, the joint design required for the application, and the welding equipment available to do the job.

    The selection of filler metals for welding wrought aluminum alloys depends on the particular alloy, but also may be influenced by the process selected and the service requirements of the product. Some additional considerations are joint design, dilution, cracking tendencies, strength and ductility requirements, corrosive environment, and appearance. Table A-9 shows a filler metal selection chart for welding aluminum alloys.

  • ALUMINUM, Gas-Shielded Arc Welding

    One of several advantages of gas shielded arc welding of aluminum alloys over other methods of fusion welding is that the need for flux is eliminated, thus removing a potential source of corrosion. Other advantages are that welding can be accomplished in all positions; there is better visibility and greater speed. Sound, pressure-tight joints with high strength and low distortion can be produced. Because of these advantages, the inert-gas-shielded processes are the predominant methods of fusion welding aluminum alloys.

    Relatively easy to perform, gas tungsten arc welding (GTAW) uses non-consumable tungsten electrodes, alternating current, and argon or helium shielding gas. When filler material is needed, it can be fed automatically or manually. Aluminum as thin as 0.6 mm (0.025 in.) can be welded, but production welding is more easily controlled when thickness is 1 .O mm (0.040 in.) or greater.

    Gas metal arc welding (GMAW), employs aluminum wire as both electrode and filler metal, uses direct current, and a shielding gas of argon or helium, or a mixture of these. The filler wire is fed automatically into the welding zone at a speed compatible with the arc length and welding current, resulting in higher welding speeds than possible with the gas tungsten arc method. Because the heat zone on each side of the weld is narrower, GMAW produces welds of superior strength. A further advantage is that metal of considerable thickness can often be welded without preheating because of high current densities and the concentrated heat of the arc.

  • ALUMINUM, Oxyfuel Gas Welding

    Satisfactory butt, lap, and fillet welds can be made with an oxyfuel gas torch on sections of aluminum ranging up to 25 mm (1 in.) in thickness. The oxyfuel gas welding process would only be used where a source of electric power is not available for arc welding. Oxyhydrogen or oxyacetylene flames produce the heat necessary to offset the high thermal conductivity of the aluminum. Generally, the other oxygen-gas combinations do not provide sufficient heat for welding, but may be used for preheating, which is often needed when joining thick sections.

    Overlap joints are not recommended for gas welding because there is danger of flux entrapment in the overlap. When possible, the joint should be designed as a butt weld. If an overlap joint is made, it should be completely welded around the edges to seal the overlapped area.

    Preheating is essential in gas welding to allow proper fusion. Sections thicker than 6 mm (1/4 in.) should be preheated to 310 to 370°C (600 to 700°F). Preheating above 425°C (800°F) is not recommended because there is danger of melting some of the alloying constituents. Heat should be applied uniformly to both parts being joined. See OXYFUEL GAS WELDING.

  • ALUMINUM, Pressure Welding

    Pressure welding or solid phase bonding of aluminum is accomplished by applying high pressure on the surfaces to be joined, either with or without heat, in the complete absence of melting.

     Pressure can be applied by aligning two punches or tapered rolls. Another method uses a shoulder punch on one side of the material and a flat plate or anvil on the other. A third method uses a single tapered roll and a flat surfaced roll. In some instances, punches with shoulders are employed to control the amount of punch penetration and flatten the deformation at the point of entry simultaneously.

    Wire brushing is the most satisfactory method of surface preparation.

     

    Pressure Gas Welding

    Metal flow between clean interfaces is essential to a cold pressure weld. Simple pressure is not enough.  Once started, metal flow must be vigorous and continuous,  although speed seems to have little bearing on quality of weld. Pressure must be applied over a comparatively narrow strip, so that the metal can flow away from the weld at both sides. When continuous welds are to be used, the indentor should be of waved design, rather than straight, for maximum strength. Strip and sheet can also be butt welded, but as the width increases, the gripping problem for the dies also increases.

    There are two basic methods of pressure gas welding: closed joint and open joint. Coalescence is produced simultaneously over the entire area of abutting surfaces by heating with oxyacetylene flames and then applying pressure. No filler metal is used.

    In closed joint welding (also called solid phase and closed butt welding), weld faces are in contact during the complete welding cycle. Ends are carefully cleaned, butted, and heated to a high temperature, but not to the melting stage. Pressure is applied, thereby upsetting the weld zone in a plastic deformation. Various refinements are used in this method, particularly in pressure. Often a low initial pressure is applied, and the pressure is increased as the metal attains its plastic state. Maximum pressure can be applied throughout the welding process, or different pressures may be applied at regular or varying intervals.

    In open joint welding, parts are spaced a short distance apart, and heated to the melting temperature. When melting temperature is reached, the parts are brought together rapidly, causing an upset, or partial fusion, weld. Most of the melted material is squeezed from the interface by the impact, and the resulting weld resembles a resistance flash weld.

  • ALUMINUM, Resistance Welding

    Resistance welding is a process in which the welding heat is generated in the parts to be: joined by resistance of the parts to the flow of an electric current. Spot welding, seam welding and flash welding are forms of resistance welding.

    All the aluminum alloys can be resistance welded. Because the physical characteristics of aluminum are different than those of steel, somewhat different equipment may be required, although modified equipment is often adapted with excellent results. More electrical capacity is usually required for aluminum than for steel.

    Advantages of resistance welding are low cost, high production speed, and automatic operation. The major disadvantage is the high initial cost of the equipment. Consequently, resistance welding is generally confined to mass production items where the low cost per weld will offset the high cost of the equipment.

    Spot and Seam Welding

    Three types of resistance welding equipment are used for spot and seam welding aluminum alloys. These are classified on the basis of the electrical system supplying welding current as follows: standard alternating current (ac), energy storage,  electromagnetic, and energy storage, electrostatic. Electrostatic welding may be either magnetic or condenser energy storage. The comparative current and pressure cycles for these systems are shown in Figure A-4.

    Alternating-Current Welding

    Since aluminum and its alloys have comparatively high thermal and electrical conductivities, high welding currents and relatively short welding times are required in spot welding.

    In the widely used alternating-current method for spot welding, the high welding current required is obtained out of the secondary coil of a welding transformer having a turns ratio in the range of 20:l to 100:l. The primary coil is usually connected to either 230 or 460volt, 60 Hz power supply. An electronic control is used to time the application of welding current ranging of 1 to 30 cycles.

    Current Regulation

    The secondary current required  varies with the thickness of the material to be welded, as shown in Table A-5. To obtain the correct current, an electronic control adjusts the current in steps of approximately 1000 amperes. Taps either on the primary of the welding transformer, or on a separate auto-transformer may be used. Where  necessary, a series-parallel switch is provided on the welding transformer primary to permit adjustment of the current down to 25% of the maximum, which is usually sufficient to cover the normal range of material thickness.

    Timing- The welding time is controlled by means of a switch in the supply line to  the welding transformer primary. Both mechanically operated and magnetically operated  welding contactors have been used for this purpose, but modern machines use solid state switches. Such devices should control the welding time to the values listed in Table A-5, with an accuracy of plus or minus one cycle. Improved welds result when the controls are adjusted to close the circuit at a uniform point in the voltage wave, and to open the circuit when the welding current passes through zero. However, some variations of this ideal condition is permissible for welding most of the aluminum alloys.

    Electronic equipment for controlling the duration of welding current is widely used with alternating-current welding machines. When these machines are provided with means to start the flow of current in synchronism with the supply voltage, the consistency of weld strength and the appearance of the welds are improved over that obtained when less precise timing equipment is used. Electronic timing equipment for controlling the magnitude as well as the duration of welding current provides a smooth adjustment of the welding heat.

    Current Demand- One of the chief objections to alternating-current spot-welding machines is that the high currents required for aluminum welding place a very high electrical demand on the system supplying the machines. This current demand is of intermittent nature, single-phase, and of very low power factor, and may cause disturbances in electric lights and other electric equipment. This condition can be  alleviated to a large extent by installing static condensers in series with the primary of the welding transformer. The manufacturer of the welding equipment should be consulted to  determine the size and number of condensers required.

    Magnetic-Energy Storage Welding

    The electrical current demand for spot welding aluminum can be reduced even further by using magnetic- energy storage equipment, which stores the welding energy in an inductor transformer by establishing a direct current of 100 to 400 amperes in the primary winding of this transformer. On interruption of the current by a contactor, a high value of current is established in the secondary circuit and through the work being welded. This current decays to a low value in 0.01 to 0.05 second.

    Equipment for this process also has an electrode pressure system which permits the welding pressure to be varied during the welding operation. The combination of a short duration welding current impulse and a varying pressure results in welds of very sound structure and good appearance.

    The maximum power demand for magnetic energy storage equipment is about one-tenth that required for alternating-current equipment, but this system can weld the same thickness of material because the energy is obtained by drawing a lower power for a longer time.

    Condenser-Energy Storage Welding

    The condenser-energy storage equipment utilizes static condensers to store the energy used for welding. Three-phase primary power is stepped up in voltage and rectified to charge the condensers to voltages of 1000 to 3000 volts. When this bank of charged condensers is connected to the primary of the welding transformer, an impulse of welding current rises rapidly to its maximum value and decays to zero at a somewhat slower rate. When welding with this equipment, a constant high value of welding  pressure is generally used. In some cases a higher pressure is used at the end of the weld to provide a forging action on the solidified weld metal.

    Welds produced on this type equipment are excellent in appearance and the structure is very sound. Another advantage is that the maximum demand on the power  system is about one-tenth of that required for a-c welding equipment to join the same thickness of material.

    Electrodes

    The correct selection of electrode shape and the maintenance of this shape in production is essential to achieving consistent spot welds on aluminum. Welding electrodes serve three functions:

    1) They conduct the welding current into the parts being welded.

    2) They exert sufficient pressure on the material to hold it in place.

    3) They conduct the heat out of the parts welded to aid in the prevention of outside materials being reached by the weld zone.

    At least one of the electrodes must be shaped so that current will be highly concentrated in the weld.  This electrode may be dome shaped with a 25 to 50 mm (1 to 2 in.) radius, or it may be conical with a 158" to 166" included cone angle.  Another tip shape often used with the energy storage welding processes consists of a truncated cone with a 160" to 130" cone angle and a glat spot with a diameter equal to twice the thickness of the weld materials, plus 3 mm (1/8 in.).

    The same sahpe electrode can be used on the other side of the work, or a flat electrode can be used on one side of the work to obtain a surface with the minimum of electrode marking.  These flat electrodes may be in the range of 16 mm to 30 mm (5/8 in to 1-1/4 ibn.) in diameter.  A further increase in diameter does not improve the appearance of the weld.

    The electrodes must be of sufficient diameter to carry the required welding currents without undue heating.  A 16 mm (5/8 in.) diameter electrode is suitable for currents up to 35,000 amp, and a welding time of 15 cycles when the rate of welding is not more than 40 welds per minute.  When higher welding currents or greater welding speeds are used, electrodes of 22 to 30 mm (7/8 in to 1-1/4 in.) diameter should be used.  For welding currents less than 20,000 amps and welding times less than 8 cycles, 12 mm (1/2in.) diameter electrodes are satisfactory.

    A coating of aluminum alloy gradually forms over the face of the electrode.  This alloy "pickup" is of low electrical conductivity, and eventually causes the electrodes to stick to the work and to melt the surface of the base material.  The pickup can be removed off the electrodes with No.160 or No.240 abrasive cloth, but in removing pickup off the dome-shaped electrodes, it is important to maintain the original electrode shape.

    On alternating-current welding machines, using dome or cone shaped electrodes, pickup must be removed off of the tips after 15-80 welds, depending on the material welded.  On energy-storage equipment using the truncated cone electrodes, less pickup is formed, and 60-300 welds may be made before the electrodes require cleaning.  The tip cleaning operation can require 2-3 seconds.

    Seam Welding

    Equipment for seam welding aluminum is similar to a-c spot-welding equipment except that the electrodes are replaced by roller electrodes in the ranges of 10 to 16 mm (3/8 to 5/8 in.) thick and 15 to 22 cm (6 to 9 in.) in diameter. One or both of these wheels are trimmed to an included "V" angle of 158" to 166", or a 25 to 50 mm (1 to 2 in.) radius to concentrate the current in the weld. The wheels and the work are cooled by a water flow of 8 to 12 L/min (2 to 3 gal/min), directed against the periphery of the wheel near the  weld. Usually one of the wheels is driven at an adjustable constant speed of 30 to 150 cm/min (12 to 60 in./min). It is essential in seam welding that the electronic timing control initiate and close off the weld current in synchronism with the supply voltage.

    Flash Welding

    Aluminum alloys in the form of sheet, tubing, extrusions, and rolled bar can be butt- or miter-flash welded to form joints of equal or greater strength than those produced by fusion welding. In flash welding, the parts to be joined are securely clamped in dies on the welding machine, and an electric arc is established between the ends of the parts to be welded. This arc is maintained by placing the parts together as the aluminum material is consumed in the arc. When the ends of the parts are sufficiently heated by this arcing process, the weld is made by rapidly driving the heated ends together with sufficient pressure to hold the material in intimate contact until the weld metal has cooled.

    Equipment- So that no arcing occurs when welding aluminum, the flash-welding machine must have sufficient transformer capacity to supply a current density of 15 500 amp/cm2 (100 000 amp/in2.) within the section welded, when the parts are in firm contact. The secondary voltage of the flash-welding transformer can be in the range of  2 to 20 volts. The machine must be equipped with appropriate dies and die-clamping devices to securely hold the parts being welded to prevent slipping during the upsetting action which takes place when the weld is formed. One of the clamping dies must be driven toward the other with an accelerated motion to establish and maintain the flashing, and to obtain a very rapid upset motion at the end of the flashing period. The mechanism for driving the movable die must be sufficiently rigid and strong to upset the largest area of section to be welded.

    Clamping Dies- Dies are made with hard-drawn copper or copper alloys. Water cooling is not required except on very high production machines. The clamping dies should securely contact at least 80% of the outside circumference of the part to be joined. The length of the dies is usually in the range of 25 to 50 mm (1 to 2 in.) and is limited only by the possibility of crushing the material if too small a die length is used. In addition to holding the parts, the die blocks serve as a means of conducting electric current into the parts being welded and of conducting heat out of the parts during the welding process. A secure electrical connection between one of the dies contacting at least 40% of the circumference of the part must be made.

    Flashing- The duration of the flashing motion must be sufficient to permit adequate coverage by the arc of the entire section welded. Considerable variation can be tolerated in both the amount of material flashed off and the time of flashing, providing a  uniform, steady flash is maintained. Total material flashed off both pieces varies, starting at 6 mm (1/4 in.) for small diameter wires, and up to 18 mm (3/4 in.) for large diameter rod. Flashing times in the range of one-half to one second are used, although the flashing time can be reduced to as low as 1/20 second, if sufficient current is available to maintain flashing.

    Welding Current- Welding current is adjusted by varying the secondary voltage applied to the dies. It is usually done with taps on the primary of the welding transformer. An adjustment which provides an upset current of about 15 500 amp/cm2 (100 000 amp. Per in.2) is used. The current obtained during flashing ranges between 1/5 to 1/3 of the current, which flows after the parts have come into good contact during the upset.

    Welding Time- The transformer is energized before the parts to be welded have come into contact and is de-energized by opening a contactor (or by other means) in the primary supply to the welding transformer. The time relation between the beginning of the upset motion and the cutoff of power to the welding transformer is the most critical adjustment in the flash welding of aluminum. The current is removed after 1 to 5 cycles following the initiation of the upset cycle. The time delay of mechanical current interruptions is critical. If the current is shut off too early, oxide inclusions occur in the welds; if it is shut off too late, overheating of the weld and low weld strength are the result.

    Costs- The economics of constructing special dies to hold the parts, and the time and material necessary to adjust the machine for production are such that 500 to 1000 joints can usually be required to justify the cost of setting up the flash-welding process.  Production rates starting at 60 and ranging to 200 welds per hour can be obtained, depending on methods used in clamping the parts. The actual welding operation lasts only one second.

    Finishing the Welds- Chipping or grinding methods are used to remove the excess upset material to finish the weld. Welds finished and treated by the anodizing process exhibit only a narrow line of slight discoloration at the weld.

     

  • ALUMINUM, Ultrasonic Welding

    Ultrasonic welding is a metal joining process in which high-intensity vibratory energy, usually at a frequency above audibility, or in excess of 15 kHz, is introduced into the area to be welded as the workpieces are held together under pressure. This process depends on the  conversion of high-frequency alternating current to mechanical vibration. Ultrasonic welding involves complex relationships between the static clamping force, the oscillating shear forces, and a moderate temperature rise in the weld zone, creating conditions which result in atomic diffusion across the interface. The metal recrystallizes to a very fine grained structure having the properties of moderately cold-worked metal. The magnitude of the factors required to produce a weld are functions of the thickness, surface condition, and the mechanical properties of the workpieces. See ULTRASONIC WELDING.

    Pieces to be joined are clamped at low pressure (4  to 160 kg [ l o to 350 Ib.]) between two welding members or sonotrodes, and the vibratory energy is introduced for a brief interval. The heart of the equipment is a magnetostrictive transducer, a rectangular stack built up of “A” nickel laminations wrapped with insulated wire. Nickel laminations are used for the transducer because of the transducer’s substantial change in length when magnetized. The equipment develops power at supersonic frequency to drive the transducer stack which, in turn, converts electrical current to mechanical vibrations, then transmits them to the upper sonotrode. The high frequency vibratory energy produced by the transducer passes from the welding head through the two pieces to be welded, where it disrupts the oxide film at the interface and eliminates the need for any further preparation.

    All combinations of aluminum alloys form a weldable pair. They may be joined in any available form: cast, extruded, rolled, forged, or heat-treated. Soft aluminum cladding on the surface of these alloys facilitates welding. Aluminum can be welded to most other metals, including germanium and silicon, the primary semiconductor materials.

    Applications include electronic components, electrical connections, foil and sheet splicing, encapsulation and packaging, and structural welding.