Tài liệu CastingsJohn Campbell OBE FREngProfessor of Casting Technology pptx

[...]... the skin of the casting However, of course, the surface oxide is immediately and continuously re-forming, as though capable of infinite expansion This is a natural and protective mode of advancement of the liquid metal front It is to be encouraged by good design of filling systems As a fine point of logic, it is to be noted that the tearing and sliding process is driven by the friction of the casting. .. steel parts of the high pressure die casting machines and the die itself from rapid attack by zinc From the point of view of the casting quality, the film-formation problem does give some problems, assisting in the occlusion of air and films during the extreme surface turbulence of filling Nevertheless, these problems generally remain tolerable because the melting and casting temperatures of zinc pressure... rejection of stoichiometric amounts of the two gases to form porosity, the principal content of the pores will not be hydrogen and oxygen but their reaction product, water vapour An excess of hydrogen in solution will naturally result in an admixture of hydrogen in the gas in equilibrium with the melt An excess of oxygen in solution will result in the precipitation of copper oxide Much importance is often... onset of vigorous flaring at that point is sufficiently marked that in the years prior to the wider use of thermocouples foundrymen used it as an indication of casting temperature The accuracy of this piece of folklore can be appreciated from Figure 1.4 The flaring temperatures increase in step with the increasing copper contents (Le at greater dilutions of zinc), and thus with the increasing casting. .. the release of moisture, and the contamination of the charge with hydrogen and oxygen, will have time to be reversed In contrast, an addition of charcoal at a late stage of melting will flood the melt with fresh supplies of hydrogen and oxygen that will almost certainly not have time to evaporate out before casting Any late additions of anything, even alloying additions, introduce the risk of unwanted... lowered by deoxidizing additions of manganese, silicon or aluminium In modem practice a complex cocktail of deoxidizing elements is added as an alternative or in addition These often contain small percentages of rare earths to control the shape of the non-metallic inclusions in the steel It seems likely that this control of shape is the result of reducing the melting point of the inclusions so that they... the surface of the liquid The presence of a solid film on the surface will hold the surface stationary, and because of the effect of viscosity, this stationary zone will extend for some distance into the bulk liquid The thickness of the The melt boundary layer is reduced if the bulk of the liquid is violently stirred However, within the stagnant liquid of the boundary layer the movement of solutes can... solutes can occur only by the slow process of diffusion, Le the migration of populations of atoms by the process of each atom carrying out one random atomic jump at a time Another region where diffusion is important is in the partially solidified zone of a solidifying casting, where the bulk flow of the liquid is normally a slow drift In the solid state, of course, diffusion is the only mechanism by... the formation of oxide films from the decomposition of moisture, and the formation of graphitic films from the decomposition of hydrocarbons, both result in the increase of hydrogen in the metal The comparative rates of diffusion of hydrogen and other elements in solution in various metals are shown in Figures 1.6 to 1.8 These reactions will be dealt with in detail later In the case of liquid copper... addition of feeders to the casting Traditional users of high levels of hydrogen in this way are the permanent mould casters of automobile inlet manifolds and rainwater goods such as pipes and gutters Both cost and the practicalities of the great length to thickness ratio of these parts prevent any effective feeding Raising the temperature of the melt will increase the solubility of hydrogen in liquid aluminium . class="bi x0 y0 w0 h0" alt="" Castings Castings John Campbell OBE FREng Professor of Casting Technology, University of Birmingham, UK UTTERWORTH. understanding of this most complex of forming operations, the casting process. It is hoped thereby to improve the professionalism and status of casting technology,