Castings John Campbell docx

[...]... oxygen in aluminium is extreme1 small; less than one atom in about or IO4 atoms This corresponds to less than x 100 I I I - - f E n, 5 6 Castings water vapour, causing the concentration of gas in solution to rise to nearer 10 ml.kg-I This spells disaster for most normal castings Such metal has been preferred, however, for the production of many non-critical parts, where the precipitation of hydrogen pores... cobalt-based high temperature alloys for turbine blades contain aluminium and titanium as the principal hardening elements Because such castings are produced by investment (lost wax) techniques, the running systems have been traditionally poor It is usual for such castings to be top poured, introducing severe surface turbulence, and creating high scrap levels In an effort to reduce the scrap, the alloys... entrainment during the production of cast material is an engineering problem too large to be covered in this book It has to await the arrival of a second volume planned for this series Castings I1- Practice listing my ten rules for good castings Chapter 2 ~~ Entrainment If perfectly clean water is poured, or is subject to a breaking wave, the newly created liquid surfaces fall back together again, and so impinge... (b) high magnification of the double film shown above, revealing its canyon-like appearance (Green and Campbell 1994) Figure 2.6 Polished section of Al7Si-O.4Mg alloy breaking into a bifilm, showing the upper part of the double film removed, revealing the inside of the lower part (Divandari 2000) 22 Castings author’s laboratory by Divandari (2000) It shows the double nature of the bifilm, since by chance,... the surface turbulence event is usually chaotic (Waves in a storm rarely resemble sine waves.) The resultant scattering of porosity in castings seems nearly always to originate from the pockets of entrained air This appears to be the most common source of porosity in castings (socalled ‘shrinkage’, and so-called ‘gas’ precipitating from solution are only additive effects that may or may not contribute... Entrainment 23 Figure 2.7 Schematic illustration of bi$lms with their trapped microbubbles, and actively buoyant macrobubbles castings, accounting for perhaps 80 per cent of all casting defects It is no wonder that the current computer simulations cannot predict the problems in many castings In fact, it seems that relatively few important defects are attributable to the commonly blamed ‘gas’ or ‘shrinkage’... will be explained in detail in Chapter 2) so that they can be seen The radiography of the cast test pieces reveals the size, shape and numbers of such important inclusions, as has been shown by Fox and Campbell (2000) The small cylindrical test pieces can be sectioned to yield a parallel form that gives optimum radiographic results Alternatively, it is more convenient to cast the test pieces with parallel... with the gas in solution in a liquid We shall digress to present a few examples to illustrate the concept Consider a liquid containing a certain amount of hydrogen atoms in solution If we place this 4 Castings This concept can be grasped by considering bubbles of gas which have been introduced into the liquid by stirring or turbulence, or which are adhering to fragments of surface films or other inclusions... demonstrates that on a normal day with 30 per cent relative humidity, the melt at 750°C should approach about 1 ml.kg-' (0.1 ml.lOO g-I) of dissolved hydrogen This is respectably low for most commercial castings (although perhaps just uncomfortably high for aerospace standards) Even at 100 per cent humidity the hydrogen level will continue to be tolerable for most applications This is the rationale for...Introduction I hope the reader will find inspiration from the new concepts described in this work What is presented is a new approach to the metallurgy of castings Not everything in the book can claim to be proved at this stage Ultimately, science proves itself by underpinning good technology Thus, not only must it be credible but, in addition, it must really .