Oxy-Acetylene Welding of Aluminum
While aluminum has a melting point less than half that of steel, its conductivity is over three times as great, so we use a tip about the same size for this metal as we do for steel, and because of this conductivity, we should realize that the effects of expansion and contraction must be particularly guarded against, since there is a large area which is heated and expanded and consequently a large area which must cool and contract. The oxy-acetylene welding of aluminum is a viable and important process. The preparation requirements for which is practically the same as for iron or steel sheets, with the addition of a proper flux. Aluminum castings vary in their composition, and success in welding them will depend somewhat upon the alloy used. Copper increases the strength, but machine work is made difficult; many times there is a large percentage of zinc, which makes machine work easy, but the casting is more or less brittle. For example, an aluminum case, welded without attention paid to the effects of expansion and contraction, may distort or it may break depending upon the alloy used. The first consideration in aluminum repair work is expansion and contraction. We realize its importance, remembering that with aluminum the shrinkage is a great deal more than with other metals, and warpage or breakage are certain unless we understand it. Plan, then, to heat and cool slowly and evenly.
Oxidation takes place very easily when welding aluminum, more so than with other metals, and this oxide has a very high melting point. As the metal comes to the melting point, this oxide forms a film which prevents the edges from flowing together and it must be destroyed before a bond can be effected. The method generally practiced by experienced welders when oxy-acetylene welding aluminum is to destroy this oxide by means of a small iron rod or paddle, and the edges joined by puddling the metal with this rod. This method is the least expensive, but it calls for the exercise of considerable skill and is not so effective as the destruction of the oxide by a flux, since it introduces the oxide into the weld. Beveling aluminum is not so important as with other metals, since the action of the oxy-acetylene welding flame causes the edges to slightly draw apart from each other. At this moment, the welding rod coated with flux, the same as with cast iron, is added, the flux chemically removes the oxide and the edges are bonded and material added from the rod at the same time.
With aluminum, the metal is especially likely to collapse because the heated area, due to the high conductivity, is large and the metal is without “strength” when it is very hot. How hot should we preheat aluminum castings? It depends upon the alloy used. Ordinarily, a safe rule to follow is to stop heating when the casting gives off a dull sound when tapped lightly with a hammer. We can tell little by its color, as we can with iron and steel and even with the welding flame playing upon the break, it gives practically no warning that it is at melting temperature, other than when it is ready to add the flux and welding rod, it has a wrinkled appearance, dull gray in color.
Special care must be given to even heating, since most castings have bosses thinner or thicker than adjoining sections and we are likely to melt the thin part unless we particularly guard against it, or, on the other hand, not give the heavy section enough heat and so as to cause an unequal expansion. Remember that aluminum must be carefully supported when preheating, since it is very brittle when hot, and as well keep it protected from drafts, as with other castings.
Aluminum is not a difficult metal to weld. The difficulty lies in properly taking care of expansion and contraction, and if we take this into consideration we will be successful when welding aluminum.
When finishing the welded section of an aluminum casting, the ordinary wheel quickly fills up with chips. If this wheel is kept well oiled with a heavy lubricant, to a large extent this will be overcome. A Vixen file should be used, as ordinary files are useless.