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Aviation History
1941
1941 - 2439.PDF
OCTOBER I6TH, 1941. FLIGHT PREPARING ALUMINIUM ALLOYS mits, lengthy treatment is always tobe advocated. The latest types of polishing powders appear to be madeby an electrolytic process which en- sures the finest possible particles, andare regarded as an advance over the usual prepared alumina powders. Thespeed at which the disc revolves is not of great importance, but manyworkers prefer some 1,000 revolutions per minute, and the whole art dependson the light pressure of the specimen against the felt. After the foregoing treatment, thesurface is cleaned with spirit, and finally given a fine polish by hand. Too Great Relief With some of the die-casting alloysthe use of petroleum introduces an almost photographic effect, as the sur-face is produced in too great relief. In a similar manner moEt of the soft clu-minium alloys, vhich are not too tough, tend again to show up thisrelief when given a too lengthy spell of polishing. Should the disc at anytime become dry dus to the alcohol evaporating, the surface would becomeseriously oxidised. When the disc is too wet, the operator is liable to besplashed with particles, but, as a matter of fact, working under theseconditions gives the most perfect speci- mens. More care has to be accordedwhere photo-micrographs have to be taken than for a direct examination. Etching Regarding etching, this should fol low immediately upon polishing, in order to avoid slight oxidation, and any samples which have to be left for even an hour will generally display signs of tarnish. The etching is never so complete when a skin of tarnish has to be removed, and where time has inadvertently intervened it is advis- able to etch, polish gently, and repeat the etching to bring up the best fresh structure. Although sodium hydrate is widely used, the outlines with some alloys are not brought sharply into relief, and further, the sodium salts appear to introduce a tint of colour to certain of the constituents. Sodium bifluoride reacted in much the same manner, and after much research it was decided to, abandon the use of etching agents containing sodium. Tha softer alloys are best etched in dilute sulphuric acid (about 1 per cent, solu- tion), and the etching or:ly occupies a few seconds. Some investigators prefer to rub the surface gently with felt dur- ing the etching, so as to prevent any possibility of air bubbles being present. For most of the harder cast alloys, an 0.5 per cent, solution of hydrofluoric acid is used, which shows practically no tendency to tinge the components with colour, and displays the crystal- line separations very distinctly. This etching solution is satisfactory for a wide range of aluminium-magnesium, aluminium copper, and aluminium- manganese alloys, and with the mag- nesium-silicon alloys, but not for rich magnesium alloys. The latter, of softer disposition, rre better dealt with by the very dilute sulphuric acid solu-tion More care is devoted to etching where photo micrographs are used asevidence in a matter of dispute, and the long-continued use of sodiumhydrate is largely because so many ex- aminations are simply made on thespot to keep a check on anneal.g nnd heat-treatment operations Wheremany photomicrographs hav< to be taken on short notice, it is a good planto Mow air from a fan, or compressed- air jet, ever 'he surface of the speci-men, so that all interstices are per- fectly dry. H this is not done thereis always the risk that the reflecte.l light may be affected and prevent aperfectly true rendering being ob- tained on the photographic plate. Thefiling of specimens is greatly facilitated by using sets of small pill boxes, as inthis way all alloys of the same com- position and physical disposition maybe kept together, or allocated to he one section ~ Instead of comparing afresh sample -vitb simply a photo; micrograph, the previous specimen isthus at hand, and the whole field is available for comparison Easy Comparison With a regular "library" of actualspecimens t.> examin5 at a moment's notice, all copper-aluminium eutectics,dendritic structures, irregularly dis tributed aluminium-rilicon eutectics,and mixed aluminium magnesium crystals formed by compressed airquenching, etc., etc., can be compared and a sound training is thus availableto almost any metallurgist, which otherwise would take long to learn bysimply perusing written descriptions. Dutch Empire Air TrainingT HE Dutch East Indies Ait Force,which is steadily expanding, will be given increased impetus shortly as theresult ot increased training facilities by their Volunteer Flying Corps at Batavia.The V.V.C. (to give it its Dutch title) has just announced the purchase of fortytraining aircraft (type not specified) and when these are delivered they will gointo service immediately. Ordnance for the Orient ""THE Burma Road i.s expected soon to•^ be handling something like 50,000 tons of Lease-Lend material every month,and certain problems involved have been investigated by Sir Kobert Brooke-Popham, C.-in-C, Far East. Among this war material for China,incidentally, is the latest version of the Vultee Vanguard, the P-60, several ofwhich are said to be already on their way. • Airacobra Speed-up I HE introduction of new technicalL methods in the production of the Airacobra has, according to the NewYork correspondent of The Times, le- duccd from 16,000 to less than 8,000the number of man-hours required to turn out these machines. Many operations which formerly re-quired skilled mechanics can now be performed by semi-skilled labour; thecompany's factories at Buffalo and New York have increased their personnel ten-fold during the past 21 months. AT REYKJAVIK: A Curtiss Toma-hawk being unloaded on the quayside. It is interesting to note the Americanmarkings which mean that the U.S. occupation army will have its ownair force.
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