Oxy-acetylene welding manual

by Lorn Campbell

Part Two.—ALUMINUM WELDING

(123) It will be noticed, when welding aluminum, that bright surfaces will oxidize immediately when exposed to the air. This action occurs perhaps faster on aluminum than on any other metal. With this oxide or scale present the metal will not run together nor fuse, no matter how much heat is applied. The metal may be molten on each side of an oxidized crack and at times will cause the line of fracture to even float, but if the oxide is not destroyed the metal will not fuse. As has been noted previously, two methods are used to destroy this oxide, namely, the flux method and the puddle system. On account of this exceedingly rapid oxidation, it will be found to the operator’s advantage to complete his aluminum welds as quickly as possible in order that he will have less of this oxide to combat. It will be found in using the puddle system that greater haste can be made by using the torch in the left hand, leaving the right free to do the puddling and to add whatever metal is necessary. In this method most of the success depends upon the operator’s skill in handling his puddle stick and puddling in additional metal. Generally the right hand can do this more rapidly than the left.

(124) It is well to learn how to make a successful weld from one side of the metal only, and while this will apply to all metals, it is especially advantageous in working aluminum. Where a small layer of metal has been added to one side of an aluminum job, such as a crank case, and it does not penetrate the entire thickness of the metal, when the other side is turned, and the flame applied to it, a difference in temperature and the loss of strength in this metal when heated will cause the first side welded to crack unless the operator is extremely cautious. Therefore it is always well to learn how to penetrate the entire thickness of the metal from one side and make a satisfactory weld in this manner.

(125) As previously stated, aluminum when melted loses most of its strength, and if not supported by some means or other the metal will collapse. On account of this it is advisable to back up aluminum work, when possible, whether the job is to be done cold or in preheated condition. The most successful manner of backing up is shown in [Fig. 78], wherein A represents a thin sheet of copper which has been fitted to the work, and daubs of asbestos cement shown at B will aid to some extent in holding the plate in position, but this alone should not be depended upon. A prop or fire brick, upon the top of which has been placed a cushion of cement, will serve as a good backing, but where this cannot be accomplished filler-rods may be bent in the manner shown in [Fig. 78]. These filler-rods are not of the springy type, but are of soft wire and the loop as shown is not for a spring effect, but merely to take care of the contraction and expansion of the wire. Copper is given a preference over most of the other sheet metals, because it can be peened with a hammer to any shape desired, and many odd shaped additions can be formed by its use.

Fig. 78.—One Method Used to “Back Up” Aluminum Work, when Welding. A Represents a Sheet of Copper; B, Asbestos Cement.

(126) The use of clamps, when working on aluminum, is not recommended on account of the great conductivity of heat and the weakening of the metal as it approaches the melting-point. Pressure of any kind is not desired and the operators who attempt to use clamps will regret it sooner or later.

(127) In aluminum work contraction and expansion take place the same as in other metals, only to a much greater extent, and greater allowances must be made. However, the same rules can be applied when determining whether work should be preheated or not, for if the ends are free to move, the work can usually be accomplished without preheating, whereas if confined, it will be necessary. When preheating is necessary the whole piece must be treated in the same way, regardless of the size. If only part of the work were preheated and the balance left exposed, it would be almost impossible to avoid warpage and shrinkage strains, which would render the work useless. Always preheat the entire piece if any portion requires it.

(128) Great care must be exercised when setting up aluminum work for preheating. Its weight should be distributed equally on whatever support is used, so that there will be no danger of any one part sagging, thereby throwing the whole piece out of alignment. A good way of accomplishing this is to lay fire bricks on their flat side, in such a manner that the weight of the work will be fairly well distributed. Then put a daub of clay or asbestos cement on each brick and press the aluminum piece down on this cushion. This will overcome the use of shims and other methods used for jacking up the work, which are unreliable.

(129) If charcoal is to be used as a preheating fuel, an oven of fire brick should be built up with draft holes in the bottom layer of brick, as described in the chapter on [Preheating]. A layer or two of charcoal is then ignited. The oven is then covered with asbestos paper or a piece of sheet metal. Asbestos paper is preferable as the metal becomes very hot and is apt to burn the operator. After the fire has received a good start, additional charcoal is added until sufficient heat is obtained. This can be determined by sprinkling a little sawdust on the surface of the aluminum, and if it chars readily, the work is ready to weld. Other methods have been outlined previously, any or all of which may be used in learning this heat. In executing the weld as little of the work is exposed to the air as possible, in order to hold a uniform heat and not permit any part to become chilled. At the completion of the weld the oven is covered over, the openings in the bottom row of bricks are stopped up, and the piece allowed to cool with the dying fire. The charcoal process is the slow but sure method of handling preheated aluminum work, and is always recommended.

(130) When preheating aluminum with torches burning kerosene or gas a different kind of oven is built, as previously described in the lecture on [Preheating]. No openings are left in the lower row of bricks and the oven is built very much closer to the work being preheated. As the object is to confine as much heat as possible and have a uniform temperature throughout, it is not desirable to have such ovens loosely constructed. If the bricks are irregular, a double wall can be built with a layer of asbestos between them. Such a procedure is always recommended if time and bricks permit. A hole is left in one end of the oven for the preheating torch flame to enter. On aluminum work the flame is never played directly upon the metal. A baffling plate of metal or fire brick is used to distribute the flame around the sides of the piece and very satisfactory results may be obtained by preheating in this manner.