Aluminum is one of the youngest of the metals. It was discovered by Wcehler in 1827. At first its considerable expense prevented its being generally used. About 1889, however, the discovery of new processes for its reduction from bauxite, etc., cheapened aluminum, so that it has become a commercial metal. Since then the expiration of the patents covering many of the reduction processes has brought the price still lower.

Fairly pure aluminum is plastic at ordinary temperature, being sixth in ductility and second in malleability.1 It melts at 655 deg. Cent.; its plasticity increases with heat up to about 600 deg. Cent., when it becomes hot short, and will crumble under the hammer. It is easiest to work between 350 and 400 deg. Cent. Its tensile strength runs from about 14,000 pounds per square inch for cast bars up to 50,000 pounds per square inch for rolled metal and wire.

1 Aluminum Company of America.

It was at first thought that its lightness (sp. gr. 2.6) was its most valuable property. But the experimenters soon found that it formed valuable alloys. The aluminum bronzes, aluminum-iron (about 7 per cent.), and some of the three-and four-metal aluminum alloys were found to be good metals for castings for bearings, and in many instances will eventually displace brass, bronze, and even steel.

Aluminum has two natural disadvantages. It is electropositive and it is difficult to weld or solder. As late as 1903, one of the prominent periodicals1 said editorially: "Undoubtedly the man who discovers a good aluminum solder will make his fortune, for it is the want of this accessory that seriously hinders the development of aluminum manufactures."

Pure aluminum does not oxidize at ordinary temperature, but when heated becomes coated with a thin film. This film is presumably oxid, though it is so thin that not enough of it can be gathered for analysis. It adheres closely, is rapidly replaced when scraped off, and does not easily flux away. This film covers impure aluminum at ordinary temperature, and it is claimed that aluminum alloys of small percentage are troubled with the surface film. In pouring for aluminum-iron castings there must be only one flow: the molten metal is encased in a skin which retards it, and would hinder the union of two streams in the mold.

Being electropositive to all other metals used in the arts, aluminum soldered joints are troublesome. Electrolytic action sets in, especially when the joint is in contact with water, and the metal at the joint disintegrates. For this reason soldered joints are unsatisfactory.

Many solders for this metal have been recently patented. M. U. Schoop mentions his collection of 50 as being incomplete. Most of these inventions specify a flux that will remove the troublesome film. The solder is generally an alloy of aluminum with zinc, tin, lead, nickel, copper, or silver, or any two or more of these metals in varying proportion. The softer of these solders are fluxed on with zinc chlorid, mercuric chlorid, tallow, etc.; the harder solders are fluxed with fluorspar, borax, lithium chlorid, etc.1

1 Iron Age, Dec. 31, 1903.

Dr. Richards has invented a self-fluxing solder of a tin and phosphorus alloy. The phosphorus either reduces or dissolves the film that protects the aluminum surface, and the tin alloys with the clean surface. This solder is extensively used. The present composition of this solder is. 29 parts tin, 11 zinc, 1 aluminum, and 1 phosphor-tin.2

M. U. Schoop, who has also done considerable research work in soldering, has patented a flux. It is a mixture of fluorides of calcium, potassium, or boron and the chlorids of alkali metals, and is covered by British Patent No. 24283, Nov. 26, 1908.3 This flux may be used before soldering or the cleaned metal may be welded without soldering.

The softer solders are often too weak for good work. All of the solders seem liable to electrolysis. The flux, in a general way, may vary in constitution, should not contain water, and should have a moderately high melting point. It should attack the film, but not the metals. Apparently none of the solders can be guaranteed to last indefinitely on account of electrolysis.

Prof. O. P. Watts,4 of University of Wisconsin, says: "There has been considerable trouble with solders containing tin and possibly with some others. In some cases destruction may have been due to electrolytic action, but in others it appears to be due to a slow diffusion of the tin in the solid state, resulting in the formation of a layer of a very brittle alloy of aluminum and tin, so that the joint breaks. This is a slow action and may require a year or more for its completion."

Aluminum, as can be guessed by its properties, is a weldable metal, but the tenacious film prevents the natural flow. Accordingly, a flux such as suggested by Schoop is used to clean the surfaces. The pieces of metal to be welded cannot be heated above about 600 deg. Cent., because they will be hot-short.

1 "Die Gewinnung des Aluminiums," A. Minet.

2 The Metal Industry, 1906, p. 22.

3 Electrochemical and Metallurgical Industry, Jan., 1909.

4 Special Information.

In 1906, W. C. Heraeus,1 of Hanau a. M., exploited a method of welding aluminum that resembles the smith's treatment of malleable iron. It appears, however, that Heraeus' discovery was antedated by the work of Mrs. Emme of this country, who welded aluminum without melting it as early as 1897. Mrs. Emme sued Heraeus for infringement of patent in 1902, won her case, and was afterward bought out by him.2 The method as described by M. Minet3 is as follows: The two pieces of aluminum to be welded are polished carefully around the ends, and the surfaces to come in contact are polished. They are then heated with an oxy-hydrogen blow-pipe or Bunsen flame to the proper temperature, 400 deg. C. When this correct temperature is reached, the two pieces are pressed against each other and are hammered and worked as in ordinary welding, the temperature meanwhile being kept the same. The metal flows together at the weld. The success of the operation depends on heating in a complete reducing flame to keep the surfaces bright and on maintaining the proper temperature. It would take a skilled workman. Upon cooling it will be found that the joint will withstand concussion tests and sharp changes of temperature. Dick patented a somewhat similar process in 1900.

The limitation of this practice in welding is obvious. It is not easy to keep the metal ends in a reducing atmosphere, yet they will oxidize rapidly at welding heat in presence of oxygen. And, besides, aluminum is a rapid conductor of heat and, like copper, the heat will travel from the joint unless the flame is very hot.

Cowper-Coles has also done good work in welding aluminum. He has devised a machine in which the bars of aluminum, cleaned and faced off square, are placed and clamped. ■ The bars are heated with a benzene lamp, and when at the plastic point are squeezed together until the metal at the joint" forms a considerable blob and the oxid has been forced out of the junction. The weld is then quickly quenched with a jet of water and at the same time a screen shuts off the flame. This contrivance of the inventor's makes a weld that does not have to be worked.

1 Iron Age, Nov. 22, 1900.

2 Special information.

3 "Die Gewinnung des Aluminiums," A. Minet.

Schoop's method is not dissimilar to these three just described; only he cleans the oxid with a dissolving flux before welding. Thus his process does away with the difficulty of cleaning the metal and keeping it clean; the rapid conduction of heat is still a difficulty.

This latter property of aluminum, its high heat conductivity, is of least consequence in the oxy-acetylene welding process, where the estimated temperature of the flame is 3600 deg. Cent. This welding process is especially adapted to aluminum, takes less time, and is sure. The metal pieces need no preliminary cleaning, though if the body of the pieces is large, as with motor cases, it is best to heat the whole casting over a gas flame. This because of the expansion and the rapid conducion of heat from the fresh weld. The operator plays his flame directly on the fracture, using a small melt bar of aluminum to fill up the break and to reinforce the weld. Aluminum melts and behaves like solder under the flame. The operator gets a good melt at the break and works the soft metal in and out with the end of his melt bar. This prevents the solidification of any of the oxid film in the body of the piece.

This weld gives a cast aluminum reinforcement that is stronger than the body of the piece because it can be reinforced. There is no reason why this system of welding aluminum cannot be applied in all instances where aluminum is to be welded. There are two precautions necessary. Aluminum melts at 655 deg. Cent.; the temperature of the flame is at least 2000 deg. So the operator must take care that he does not get his metal too hot, or it will run away from the weld. (In the case of mending motor cases, the fracture is placed in a horizontal position and backed with asbestos paper.) Also, the operator should not use the customary high-oxygen flame. If he does his aluminum will scum.

As for strength of the welded joint, Cowper-Coles,1 who tested twelve consecutive welds of bars, claimed that the metal had not deteriorated. All of the bars broke outside of the weld, and also outside of the range of high heating. There is no doubt, however, that working the metal at the weld is advantageous, just as it is with iron, etc.

1 Electrochemist and Metallurgist, Nov., 1903.

Conclusion. - From the foregoing, it is plain that aluminum articles must be either welded or riveted - not soldered. It is likely that the manufacturers will soon begin to use this welding property of aluminum more extensively. Riveted ware is unsatisfactory, because the metal is too soft unless alloyed. From the fabrication of kitchen ware to the building up of light, strong metal frames, such as for automobiles, welds would be ideal joints.