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Aviation History
1933
1933 - 1103.PDF
NOVEMBER 30, 1933 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT This series of uniform shot welds is by no means an isolated example. Testing by shearing of ten consecu tive shot welds is one of the standard methods adopted by the manufacturers in demonstrating this type of welding machine; and in no instance has there ever been a greater variation than 10 per cent, from the mean value obtained. For comparison Table B, which shows a similar series of test on D.T.D. 60A (20 per cent, chromium, 2 per cent, nickel) is included. Having arrived at a satisfactory state of affairs so far as uniformity of the individual weld is concerned, one may well consider the advantage offered by weld ing in the matter of strength. The strength of the riveted lap joint employing a single row of rivets is dependent upon the pitch of the rivets. By decreas ing the distance between the rivets the joint may be strengthened up to a certain point only. Beyond this point the strength is reduced, as failure will occur by fracture of the material between the rivet holes. With spot welding, however, there is no weakening of the material resulting from the removal of material by drilling. The strength of the joint may thus be in creased considerably by reducing the distance between the individual spots to a figure below the minimum per missible riveting pitch. As is well known, the chief problem hitherto experi enced in welding austenitic stainless steels has been the prevention of heating within the range 500°-900° C, which causes carbide precipitation with possible subse quent failure through corrosion. One advantage of shotwelding in this connection is that the extremely brief heating period can be so controlled that the zone of fusion is confined to the interior of the weld, a layer of material possessing a resistance to corrosion equal to that of the remainder of the sheet being left at the surface. Tests have been made to demonstrate the effect of beating, and to compare welds made by ordinary spot- welding machines with those produced on the Budd shot- welder. The duration of the spot welds ranged from 0.1 to 3 sec, and the shotwelds were timed to con siderably less than 0.1 sec. The pieces so welded were then subjected to a corrosion test, which consisted of immersing them for periods of 1 min. at 15-min. inter vals in a 20 per cent, sodium-chloride solution. The results showed that where the welding time ex ceeded 0.1 sec, rusting commenced very quickly, but on the faster welds signs of corrosion appeared only after several weeks' test; moreover, this corrosion was easily removed by wiping with a damp cloth. A further \ f . i ' ' ' I \ \ \ \ \ \ \\ \\V \\\\\\\v\\V AUTOGRAPHIC STRIP FROM THE WELD RECORDER. The limiting range for good welds is enclosed by the area " A." After the preliminary run on the left follows a run of good welds. Then a number of under-heated welds, and on the right a run of overheated welds. It should be pointed out that this strip was prepared to show how the quality of welds is indicated. The machine does not, of course, normally behave in such erratic fashion Tests have been carried out to demonstrate this point using 0.031-in. sheet steel to D.T.D. Specification No. 166. The material was cut into strips 3 in. long and 1J in. wide, the edges being ground to remove the effect of shearing. Four riveted samples were made up, each consisting of two of the above pieces overlapped along the l^-in. edge. Austenitic stainless-steel rivets, 5 in. diameter, were used, and the pitches used for the different samples were ^ in., § in., A| in., and \ in. respectively. On pulling the test-pieces to fracture in the tensile-testing machine, it was found that 4,300 lb. was the highest strength obtained, and this was the piece employing the f-in. pitch. With the smaller pitches the metal failed between the rivet holes. Simi lar shot-welded samples were prepared in which the number and pitch of the welds was varied from four welds with a pitch of f in. to 10 welds with a pitch of •& in. The maximum strength was obtained from the test-piece with eight welds and a pitch of -ft in., which broke at 5,825 lb. It is interesting to observe that no advantage was gained by decreasing the pitch beyond ft in. The frac ture in the caas of the test-pieces with 8, 9, and 10 welds was in the metal immediately adjacent to the welds. This is accounted for by the fact that the steel used for the tests has its high ultimate strength de veloped in it by cold working. The heat of welding re moves this cold work, and the physical properties of the material in the welds are those of the steel in the fully softened state. The ultimate strength of one of the 3-in. by l£-in. Strips used in the experiment was 7,250 lb. This strength could not be developed in either the riveted °r the shot-welded joint, but the latter was distinctly the stronger of the two. experiment in which the welds were treated with metal polish before testing for corrosion, was made, and it was found that even the slight tendency towards corro sion hitherto noted was eliminated. As a further investigation into the effects of heating time on weld characteristics, some strength tests were made, " Twoscore " steel (D.T.D.60A) being used for this purpose, 0.032 in. thick x 2 in. long x f in. wide. A weld with an extremely brief timing was made and pulled; it gave a strength of 550 lb. The timing period was then increased bv 0.02 sec. and it was found that ASSEMBLED BY SHOT WELDING : An aeroplane elevator 1198 c
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