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Aviation History
1937
1937 - 1106.PDF
SUPPLEMENT TO FLIGHT 4206 29 APRIL 29, I937 THE AIRCRAFT ENGINEER FIG.l BLOWPIPE LEFTWARD METHOD may be described briefly as follows: The edges are first saw-cut inward to a depth about equal to the thickness of the material, the saw cuts being about twice the thick ness of the material apart. This type of preparation, of course, applies particularly when unbevelled edges are butted close together, and undoubtedly assists in the pene tration of the weld and activity of the flux. Notching is also useful when it is necessary to join on bevelled edge and unbevelled edge, the notching being applied to the unbevelled edge. Also, if the bottom edges of bevels are close butted for some reason or other, or the edges are not bevelled for their full thickness, notching is again use ful in securing adequate penetration. To get the best results when welding aluminium it is necessary to use a suitable flux. The purpose of the flux, of course, is to remove the refractory oxide of aluminium, alumina, and to enable a good clean joint to be made free from inclusions. The flux must fulfil a number of require ments ; it must, in the first place, be capable of dissolving the aluminium oxide and melt at about ioo°-i5o° C. below the m.p. of the alloy being welded. In the fused state it must have a specific gravity less than the molten alloy so that it may float on the surface of the metal, and it must have the correct degree of viscosity. Not only must the the flux contain the correct constituents necessary to give it the properties outlined, but they must be in the correct proportions and properly mixed, hence it is advisable to use only fluxes of guaranteed efficiency, manufactured by experts. The flux may be applied either as a powder or a paste, but it should always be used sparingly both for reasons of economy and efficiency. Method of Welding As far as single-operator underhand welding on sheet up to tVin. or £in. is concerned, the best technique is the left ward or forward method. The arrangement of the rod and the blowpipe is that shown in the sketch (Fig. 1). When the sheet thickness exceeds Jin. the two-operator vertical method should be adopted wherever possible, and the corresponding arrange ment of blowpipes and welding rods is that shown (Fig. 2). A neutral flame should be used, and care should be taken to see that the acetylene is quite free from impurities. Dissolved acetylene will be found the most suitable fuel gas. When welding aluminium an acceleration of welding speed takes place during welding, due to the rapid rise in temperature of the metal surrounding the edges con sequent on the high thermal conductivity of aluminium, and this is apt to give a certain amount of trouble to the beginner. Once the essentials have been mastered, how ever, it is possible to make excellent welds in aluminium, and to render them quite invisible after suitable dressing and polishing. In the case of aluminium welding, finishing off the weld Fl G.2. TWO-OPERATOR, UPWARD METHOD is just as important as the welding itself. The first matter to which attention should be paid is the removal of flux. The fluxes used for aluminium welding are strongly corro sive in action, and every trace of flux must be removed on completion of the weld. A good method is to wash in warm water and then to brush vigorously with a metal brush. Wherever possible the welded article should be dipped in a warm 5 per cent, solution of nitric acid and immediately rinsed in warm water. It is sometimes recommended that a second washing be carried out after a few days in order to make certain that the last trace of flux has been removed. The second important point connected with finishing off the weld is the improvement in properties brought about by heat treatment and mechanical work on the metal. As deposited the aluminium has a cast structure and is rather coarse grained. This may be seen in the micrograph (Fig. 3). On blowpipe annealing and hammering, how ever, the metal undergoes considerable refinement of grain * I h ' ) ¥• V <V t%*\; 0 f f • *•-*> •V". •iJk-iX'* K r • \ it*** i ;,-•'"*•.' 5 • v** •• *••-*• i i I SK W ' "!4* > . * jk .)y V'' >T *\\ \ *' • . - - a ' \ "**' i 4 • \*> < \ t V'» -"»' 1 "5 * \ k ' •t- ''U ' 1 f, 'J /V L- /'• x •YV'«|! < /La, V...» \ ¥ • • t . V ,*? * *>„•; *,<* *-",,-' >** SR ' ' V ^*JL ** '9t ?'* / b * Fig. 3.—Aluminium weld, as welded.
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