Physical aluminum is white in color, with a decided bluish tint, which becomes very much more marked upon exposure, when the thin film of white oxide on its surface prevents further tarnishing from the air, but which seems to give it. by contrast to the metal as a background, an enhanced bluish tint. The addition of small percentages of silver. chromium. manganese. tungsten. or titanium changes the color of aluminum. render Mg it nearer that of silver. as well as considerably increasing the hardness and stiffness of the metal. Pure aluminum has no taste or odor. Under heat, the coefficient of linear expansion of I in. round aluminum rods of 118i per cent purity is .0000206 per degree C.. between the freezing and boiling points of water; that of iron being .00(10122; tin, .0000217; copper, .000017N (authorities, I hint, Langley. and Hall). Sound castings of aluminum can readily be made in dry sand molds, if the metal is not heated much beyond the melting-point, to prevent the absorption of gases. The metal does not need any Ilux. Its shrinkag,e in. to the foot. The mean specific heat of aluminum from 0° to the melting-point is 0-285, water being taken as one, and the latent heat of fusion is 28.5 heat-units (authority, Richards). The coefficient of thermal conductivity of aluminum. obtained by the method of Wiederman and Franz, sil ver being taken as 100 and copper as 73.0, is for unannealed aluminum 37.96, for annealed alu minum 38-S7. Aluminum stands fourth, being preceded only by silver, copper, and gold, as a conductor of both heat and electricity. One yard of annealed aluminum wire of 98i per cent parity, -0325 in. diameter, 14° C., has .05484 of an ohm resistance, a yard of pure copper wire having a resistance of .0315. The electrical conductivity of silver being taken at 100. copper as 90, pure annealed aluminum has an electrical conductivity of about 50. Pure nhuninum has no polarity, and indeed the commercial metal in the market is practically non-magnetic. Pure almninuni is very sonorous, and its tone seems to be improved by alloying with a few per cent of silver or titanium. Pure aluminum is, when properly treated. a very malleable and ductile metal. It can readily be rolled into sheets in. thick, or be beaten into leaf nearly as thin as gold-leaf, or be drawn into the finest wire. Pure aluminum stands third in the order of malleability, being exceeded only by gold and silver, and in the order of ductility seventh, being exceeded by gold, silver, platinum, iron, softest steel, and copper. Both its malleability and ductility are greatly impaired by the presence of the two common impurities, silicon and iron. Aluminum can be rolled or hammered cold, but the metal is most malleable at, and should be heated to, between 200° and 300° P., for rolling or breaking down from the ingot to the best advantage. Like silver and gold, aluminum has to be frequently annealed, as it hardens remarkably upon working. By reason of this phenomenon of hardening dur ing rolling, forging, stamping, or drawing, the metal may be turned out very rigid in fin ished shape, so that it will answer excellently well for purposes where the annealed metal would be entirely too soft or too weak or lacking in rigidity. Especially is this true with aluminum alloyed with a few per cent of titanium, copper. or silicon. The alloys do not show their increased hardness to anything like its maximum extent in at all in proportion to the increased brittleness. But when these castings are drop-forged, rolled, hammered or drawn down, with only sufficient annealings to prevent the metal from crack imz, the increased hardness appears in it remarkable degree. It can be safely stated, as a general rule, that the purer the idnininunn the softer and less rigid it is. The fracture of im pure alutninunt shows ordinarily hexagonal crystals. although the pure metal is very tough, and on breaking, by bending backward and forward, often appears distinctly fibrous and silky in fracture.
Ann paling anneal aluminum a low and even temperature should be main tained in the muffle—just such a temperature as will show an even red-heat in a pieee of iron or steel placed in the muffle, when viewed at twilight or on a dark day. The aluminum itself, however, should not appear at all red at this temperature. A ready test of this is that the metal has been heated enough to char the end of pine stick, which will leave a black mark on the plate as it is drawn across it. When the metal has acquired this temperature it should he taken from the. furnace and allowed to cool gradually. Very thin sections may be annealed by placing them in boiling, water, nd either allowing them to cool with t he water or taking them out to cool gradually. It is possible to anneal to any degree, by lowering the temperature to which the metal is heated below that specified by means of suitable appliances. Aluminnin wire alloyed wit It a few per cent of copper. titanium, or silver. can be drawn, having a tensile strength of 80.000 lbs. to the sq. in.. and which will have, weight for weight with copper wire. an electri cal conductivity of 170, that of colqter being 100. When it is taken into consideration that the collier has a tensile strength at a maximum of 30,000 lbs. to the sq. in., against 80,000 lbs. per sq. in. for aluminum titanium alloy, and that. iron and soft steel wire have each at con ductivity of 12 in the same scale, and at utmost a strength equal to that of the aluminum titanium alloy. a wide field for usefulness as electrical condnetors seems open fur alumi num. Aluminum can he easily welded electrically, and solders satisfactorily. The specific gravity of aluminum is one of its most striking properties, it being from 2.56 to 2-70; struct ural steel being 2.95. copper .:14111, ordinary high brass :1.45, niekel :1.50. silver 4, lend 4-8, gold 7.7. platinum 8.6 times heavier. A enb. m. of aluminum weighs -002 lbs., I II' oz. pois. Cast aluminum has about the ultimate strength of east-iron in tension. limit tinder
compression it is comparatively weak. The following is a table of average tensile and com pression strength of the metal, the average of many results of tests of the metal of US per cent purity : Under transverse tests pure aluminum is not very rigid. A 1 in. square bar of good east-iron supported on knife-edges 4 ft. 6 in. long and loaded in the center will readily stand 500 lbs. without a deflection of over 2 in. A similar bar of aluminum would deflect over '2 in. with a weight of 250 lbs., although the aluminum bar would bend nearly double before breaking, while the will ordinarily break before the deflection has gone very mulch beyond 2 in. Aluminum and copper form two series of valuable alloys, the alumni= bronzes ranging from '2 to 12 per cent of aluminum with copper, the copper-hardened aluminum series with from 2 to perhaps 20 per cent of copper with the aluminum. In the 5 to 12 per cent alumi num bronzes we oldain some of the densest, finest-grained, and strongest metals known— metals having remarkable ductility as compared with tensile strength. A 10-per-cent bronze can readily and uniformly he made in forged bars. with MAO Ms. per sq. in. tensile strength, with 60,000 lbs. elastic limit per sq. in., and with ;It least 10 per cent elongation in 8 in.: and aluminum bronzes can be made to fill a of even 130.0(10 lbs, per sq. in., and 5 per cent inS in. tineh bronzes have a specific gravity of about 7.50, and are of a light-yellow color. The 5 to is per cent aluminum bronzes of from 8.30 to 8 specific gravity, and a handsome yellow color, readily give 70,000 to 80,000 lbs. per sq. in. tensile strength, with over 30 per cent elongation in 8 in., and with an elastic limit of over 40.000 lbs. pr sq. in. It will probably he alloys of the latter eh:uaeleristics that will be most used—especially for marine work ; and the fact that 5 to 7 per cent bronzes can be rolled or hammered at a red-heat, proper precautions. which can readily be scenred, being taken, will greatly enlarge their use. Metal of this character can be worked in almost every way that steel can, and has the advantages of greater strength and ductility, and greater ability to withstand corrosion. The presence of silicon makes a harder bronze, but one of much less comparative ductility and a less malleable alloy. The presence of iron weakens, and very seriously interferes with the value of the bronze. The presence of zinc in aluminum bronze is not so deleterious—in fact, it makes the best aluminum brasses, much better than those having, tin in them. A leminnm in bronzes lowers the melting-point of the copper at least 101)' or 200°. The inciting-point of 10 per cent aluminum bronze is somewhere in the neighborhood of 1,700° P. Aluminum bronze is among the hardest of the bronzes, and hardens upon cold working considerably. This hardness, however, can be lowered by annealing at a red-heat and plunging into cold water. Aluminum bronze can readily be tooled in a lathe, and the chips being cut clean and smooth and long do not clog the tool. Aluminum bronze is a remarkably rigid metal under transverse strain, being much more rigid than ordinary brass or even gun bronze; and under compression strain, although rather low in elastic limit compared with its ultimate compressive strength, it is still much stronger than any of the other bronzes. It undergoes a long period of gradual compression before it finally gives way, malting it peculiarly a safe metal tinder compressive strain. Alinmiallin bronze has special anti-friction qualities, owing to its fine grain texture and peculiarly smooth and unetnous though hard surface, which resists abrasion remarkably. Attention has already been called to the anti-corrosive qualities of aluminium bronze, and, as its electrical conductivity is better than that of brass. it is especially well adapted for parts of electrical machinery. Aluminum bronze can be brazed and soldered nearly as well as brass. Sound, clean castings at ,Inuninum bronze can be safely and regularly made, either in sand molds or against chills, if the proper precautions are taken to avoid: 1. Oxidation. 2. Contamination from scum, or a cinder composed of oxide of aluminum with a little copper in it. 3. Contraction cracks, caused by strains due to shrinkage. 4. The shutting in of gas into the castings. The first trouble—oxidation—can be prevented by not heating the metal too hot in the plumbago crucibles. The second trouble—contamination from scum—can be avoided by pouring into a hot ladle or pouring-basin large enough to hold all the metal ne cessary to fill the mold, and permitting the metal to escape from the bottom of this receptacle, after giving sufficient time to allow the scum to come to the surface. Proper skim-gates should also be provided for each mold. The third difficulty—contraction—is overcome by giving plenty of allowand of metal to feed the casting in cooling. This can be done in several ways, each best adapted for varying conditions. The cores should be made of a yield ing character. using resin or other suitable substance. with coarse sand, that will yield under slight pressure. Unyielding iron metal cores should be dispensed with as far as possible. Castings shouhl have "risers" or "feeding-heads" with flaring openings large in section— even larger than the castings they are intended to feed. The feeding-heads should be refilled as often as they will take the metal. In this way the castings are solidified first, drawing the metal to supply their shrinkage from the still fluid ^ riser," having a level higher than the casting itself. The gates to the mold should be of sufficient and so arranged that they can be filled with metal as cold as it will pour and give full castings. The fourth difli culty—g,as in the castings—can be prevented by taking the ordinary precautions used by founders for this purpose.