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Aviation History
1936
1936 - 0828.PDF
APRIL 2, 1936. FLIGHT. 353 WELDING in AIRCRAFT CONSTRUCTION Possibilities of Electrical Resistance Welding in Stressed-Skin Structures : Timing Welds by Mercury Valves With Grid Control INTERESTING references to the possibility of using spot welding in the construction of stressed-skin structures were made in a paper by Mr. R. H. Dobson and Mr. R. F. Taylor, read before the R.Ae.S. the other day. Mr. Dobson, as most Flight readers will probably know, is works manager of A. V. Roe & Co., Ltd., Manchester, and the use of welded steel tube construction based on Fokker practice has been a familiar feature of Avro fuselages for several years, so that the authors of the paper have had long practical experience of welding. Two forms of electrical resistance welding were dealt with in the paper: Seam welding and spot welding. The former has been used in the manufacture of main plane spars of nickel chromium steel strip, comprising two booms and a web, pro duced by the usual draw-rolling operation. It was explained that the two booms were roughly assembled on the web, and the whole spar then passed through guide rolls into the seam welder and between two pairs of electrode discs. The current was continually flowing between upper and lower electrodes, so welding the booms to the web. The finished weld consisted of a series of spots or stitches, spaced about nine per inch. The spacing was obtained by using a motor-driven variable induct ance motor which varied the peak welding current and caused intermittent or stitch welding. By varying the speed of this motor the pitch could be altered at will. The process was very satisfactory, and a 15ft. spar could be assembled in three minutes, coming out of the machine perfectly straight. The spars had to be heat treated subsequently to give the desired physical properties~by means of the tension electrical process. Comparative vibration tests carried out on seam welded and riveted spars for the Avro Tutor showed that the welded spar developed a slightly higher stress. Spot Welding light alloys. Table I gives the mean results obtained, using a spot welder with automatic control, in the welding of the ordinary steels : — Material. Gauge. 22 20 18 17 16 14 12 10 Material. 18 16 14 12 JO M.S. Sheet. Load Single. Shear. 1,100 1,600 1,90!) 2,400 2,500 3,400 3,000 5,800 Carbon Steel. 2,700 4,400 5,600 6,700 7,600 B.S.I. Spec. 2 S 3. per Spot. Double Shear. 1,700 2,300 2,700 8,400 4,200 5,600 6,600 8,400 D.T.D. Spec. 124. 4,400 6,700 8,700 11,000 13,000 In the welding of the non-corrodible steels and light alloys contactor operated welders are not suitable. With these materials it is essential that the structure of the material on the outsides of the spot is unaffected by the welding heat, and this necessitates extremely short welding times ranging from 0.01 to 0.08 sec, together with fairly high currents, otherwise the corrosion resisting properties would be seriously impaired. Using contactor controlled equipment on a 50-cycle supply, the variation in operation of the contactor determined by oscillographic tests may be as much as two cycles, and as under certain conditions of welding this variation may be 70 per cent, of the total welding time, consistent results are almost impossible of attainment. Numerous tests have been carried out on Alclad to specifica tion D.T.D. 118 welded on an ordinarv welder with contactor control which have given very satisfactory physical properties, although the variation has at times been rather large due to insufficient control of the time of passing current and also owing to insufficient amperage being available. Table II gives the average figures obtained for different gauges: — TABLE II. Properties of Light Alloy Spot Welds. Load per Spot in Pounds. Gauge. Single Shear. Double Shear. 20 168 308 24 243 392 22 336 530 20 425 952 18 458 951 It will .be noted that as the 18s G. material is approached that there is a falling off in the strength; this is entirely due to the capacity of the machine being approached. A large number of tests carried out on similar material, welded by means of remote electrodes and mercury valve grid- controlled ^equipment, have shown an improvement on these figures of 15 to 20 per cent. It should be pointed out that this does not represent finality as numerous tests remain to be carried out before it can be decided what is the correct relationship between current pres sure and time for optimum results in a given thickness. There is, however, little doubt that with controlled condi tions welds may be consistently produced which are strong, sound, and which do not penetrate to the outside surfaces of the material. . As a result of these investigations I feel that the day is not very far distant when electrical spot welding will find extended use in aircraft construction and the possibility of stressed-skin structure being manufactured by this method should not be overlooked. Modern methods of stressed-skin construction, whether for fuselages or wings, are of necessity expensive owing to the enormous amount of riveting entailed, and if such structuies can, in future, be fabricated by spot welding, using remote elec trode, a considerable saving in manufacturing costs may be effected. There are no grounds for assuming that a structure fabricated in such manner will be any less efficient than a riveted structure. In view of the part spot welding may play in stressed-skin construction in the future, the section of the paper which dealt with this subject may be quoted in full: The electrical resistance process of spot welding has not received the attention which so useful a production process merits. Main plane spars have been produced by this method, hut the seam welding process is more satisfactory for such components. Spot welding shows up to good advantage when used in the manufacture of ribs, fuselage and wing fittings, cowlings and general sheet metal parts, whether in steel, aluminium or aluminium alloys. There are several aircraft in existence in this country having seam welded spars, spot welded steel wing fittings and spot welded aluminium ribs. Electrical resistance spot welding consists essentially of clamping two or more pieces of metal to be joined between upper electrodes under pressure and passing a heavy current of low voltage from one tip to the other, through the.metalto be welded. The electrical resistance of the two contacting faces and the column of metal between the electrodes is greater than that of toe electrodes and the column is raised to welding heat "mist under mechanical pressure and fusion occurs. Ihe factors governing the amount of heat in a weld are: (i) Tv,CUrr&IVt' ^ **me °* Pass*ng current; (3) electrode pressure. these and other variables materially affect the resulting Physical an<f metallurgical properties of the spot welds, and it is the accuracy with which these are controlled that deter mines to a large degree the success or failure of the process. th e mechanical pressure is, of course, readily controllable and bv m^lutude of the welding current is in most cases governed • tselectlng the correct transformer tapping to suit the rent" g weIded- The time of passing the welding cur- Wean1S rfather more difficult to control, but may be done by switch re'ay~°perated contactors, synchronous motor-driven Thef' °r by mercury valves, having grid control, trouble \ mention<rd type is preferable as it is free from the with erf t t0 ar"nS and wear which are always associated of cont Yrctors and synchronous motor switches. It is capable ^curate"8 the weidin8 time from °01 sec- upward. Such really n COntro* °*- welding conditions does not appear to be kcornes^8^1^ WheiX considering the ordinary steels, but it 'Xntial in the welding of non-corrodible steels and
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