Campbell Complete Casting Handbook

Metal Casting Processes, Metallurgy, Techniques and Design
2. Auflage 2015
ISBN: 978-0-08-100120-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 1054 Seiten

Campbell's Complete Casting Handbook: Metal Casting Processes, Techniques and Design, Second Edition provides an update to the first single-volume guide to cover modern principles and processes in such breadth and depth, while also retaining a clear, practical focus. The work has a unique viewpoint, interpreting the behavior of castings, and metals as a whole, in terms of their biofilm content, the largely invisible casting defects which control much of the structure and behavior of metals. This new edition includes new findings, many from John Campbell's own research, on crack initiation, contact pouring, vortex gates, and the Cosworth Process. - Delivers the expert advice that engineers need to make successful and profitable casting decisions - Ideal reference for those interested in solidification, vortex gates, nucleation, biofilm, remelting, and molding - Follows a logical, two-part structure that covers both casting metallurgy and casting manufacture - Contains established, must-have information, such as Campbell's '10 Rules' for successful casting manufacture - Includes numerous updates and revisions based on recent breakthroughs in the industry

John Campbell OBE is a leading international figure in the castings industry, with over four decades of experience. He is the originator of the Cosworth Casting Process, the pre-eminent production process for automobile cylinder heads and blocks. He is also co-inventor of both the Baxi Casting Process (now owned by Alcoa) developed in the UK, and the newly emerging Alotech Casting Process in the USA. He is Professor Emeritus of Casting Technology at the University of Birmingham, UK.

Autoren/Hrsg.

Campbell, John

Leseproben

Chapter 2

Entrainment

Abstract

The entrainment process can be a folding action; alternatively, parts of the flow can impinge, as droplets falling back into the liquid. In both cases, the film necessarily comes together dry side to dry side. Entrainment does not necessarily occur only by the dramatic action of a breaking wave. The entrainment mechanism is a folding-in of the surface or impingement action between two liquid surfaces. It can occur simply by the contraction of a ‘free liquid’ surface. In the case of a liquid surface that contracts in area, its area of oxide, being a solid, is not able to contract. Thus the excess area is forced to fold in a concertina-like fashion.

Keywords

Bifilm; Entrainment; Spherodisation; Superalloy; Wetting

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 and mutually assimilate. The body of the liquid re-forms seamlessly. We do not normally even think to question such an apparently self-evident process.

However, the same is not true for many common liquids, the surface of which is not a liquid, but a solid, often invisible film of extreme thinness. Aqueous liquids often exhibit films of proteins or other large molecular compounds.

Liquid metals are a special case. The surface of most liquid metals comprises an oxide film. If the surface happens to fold, by the action of a breaking wave, or by droplets forming and falling back into the melt, the surface oxide becomes entrained in the bulk liquid (Figure 2.1).

The entrainment process can be a folding action, as seen in Figure 2.1. Alternatively, also shown in the figure, parts of the flow can impinge, as droplets falling back into the liquid. In both cases, the film necessarily comes together dry side to dry side. The submerged surface films are therefore necessarily always double.

Also, of course, because of the negligible bonding across the dry opposed interfaces, the defect now necessarily resembles and acts as a crack. Turbulent pouring of liquid metals can therefore quickly fill the liquid with cracks. The cracks have a relatively long life, and in many alloys can survive long enough to be frozen into the casting. We shall see how they have a key role in the creation of other defects during the process of freezing and how they degrade the properties of the final casting.

Figure 2.1 Sketch of a surface entrainment event.

Entrainment does not necessarily occur only by the dramatic action of a breaking wave, as seen in Figure 2.1. It can occur simply by the contraction of a ‘free liquid’ surface. In the case of a liquid surface that contracts in area, its area of oxide, being a solid, is not able to contract. Thus the excess area is forced to fold in a concertina-like fashion. Considerations of buoyancy (in all but the most rigid and thick films) confirm that the fold will be inward, and therefore entrained (Figure 2.2). Such loss of surface is common during rather gentle undulations of the surface, the slopping and surging that can occur during the filling of moulds. Such gentle folding might be available to unfold again during a subsequent expansion, so that the entrained surface might almost immediately detrain once again. This potential for reversible entrainment may not be important compared to the probability that much enfolded material will remain enfolded and entrained. Masses of entrained oxides will entangle and adhere to cores and moulds, but more severe bulk turbulence may tear it away and transport it elsewhere.

With regard to all film-forming alloys, accidental entrainment of the surface during pouring is, unfortunately, only to be expected. This phenomenon of the degradation of liquid metals by pouring is perfectly natural and fundamental to the quality and reliability issues for cast metals. Because these defects are inherited by wrought metals, nearly all of our engineering metals are degraded, too. It is amazing that such a simple mechanism could have arrived at the twenty-first century having escaped notice of thousands of workers, researchers and teachers for the past 6000years.

Figure 2.2 Modes of filling (a) a liquid metal advancing by the splitting of its surface oxide (this may occur via a transverse unzipping wave); (b) the retreat of a surface illustrating the consequent entrainment of the surface oxide. (Compare the flow behaviour of cast iron: Figure 6.34).

In any case, it is now clear that the entrained film has the potential to become one of the most severely damaging defects in cast products (and, as we shall see, in wrought products too). It is essential, therefore, to understand film formation and the way in which films can become incorporated into a casting so as to damage its properties. These are vitally important issues.

It is worth repeating that a surface film is not harmful while it continues to stay on the surface. In fact, in the case of the oxide on liquid aluminium in air, it is doing a valuable service in protecting the melt from catastrophic oxidation. This is clear when compared with liquid magnesium in air. Because magnesium oxide is not as protective, the liquid magnesium can burn, generating its characteristic brilliant flame until the whole melt is converted to oxide. In the meantime, so much heat is evolved that the liquid melts its way through the bottom of the crucible, through the base of the furnace, and will continue down through a concrete floor, taking oxygen from the concrete to sustain the oxidation process until all the metal is consumed. This is the incendiary bomb effect. Oxidation reactions can be impressively energetic!

A solid film grows from the surface of the liquid, atom by atom, as each metal atom combines with newly arriving atoms or molecules of the surrounding gas. Thus for an alumina film on the surface of liquid aluminium, the underside of the film is in perfect atomic contact with the melt, and can be considered to be ‘well wetted’ by the liquid. (Care is needed with the concept of wetting as used in this instance. Here it refers merely to the perfection of the atomic contact which is evidently automatic when the film is grown in this way. The concept contrasts with the use of the term wetting for the case in which a sessile drop is placed on an alumina substrate. Perfect atomic contact is now unlikely to exist where the liquid covers the substrate, so that at its edges the liquid will form a large contact angle with the substrate, indicating, in effect, that it does not wish to be in contact with such a surface. Technically, the creation of the liquid/solid interface raises the total energy of the system. The wetting in this case is said to be poor.)

The problem with the surface film only occurs when it becomes entrained and thus submerged in the bulk liquid.

When considering submerged oxide films, it is important to emphasise that the side of the film which was originally in contact with the melt will continue to be well wetted, i.e. it will enjoy its perfect atomic contact with the liquid. As such, it will adhere well and be an unfavourable nucleation site for volume defects such as cracks, gas bubbles or shrinkage cavities. When the metal solidifies, the metal-oxide bond will be expected to continue to be strong, as in the perfect example of the oxide on the surface of all solid aluminium products, especially noticeable in the case of anodised aluminium.

The upper surface of the solid oxide as grown on the liquid is of course dry. On a microscale, it is known to have some degree of roughness. In fact the upper surfaces of oxide films can be extremely rough. Some, like MgO, being microscopically akin to a concertina, others like a rucked carpet or ploughed field, or others, like the spinel Al2MgO4, are an irregular jumble of crystals.

The other key feature of surface films is the great speed at which they can grow. Thus in the fraction of a second (probably between 10 and 100ms) that it takes to cause a splash or to enfold the surface, the expanding surface, newly creating additional area of liquid, will react with its environment to cover itself in new film. The reaction is so fast to be effectively instantaneous for the formation of oxides.

Other types of surface films on liquid metals are of interest to casters. Liquid oxides such as silicates are sometimes beneficial because they can detrain by balling-up under the action of surface tension and then easily float out, leaving no harmful residue in the casting. Solid graphitic films seem to be common when liquid metals are cast in hydrocarbon-rich environments. In addition, there is some evidence that other films such as sulphides and oxychlorides are important in some conditions. Fredriksson (1996) describes TiN films on alloys of Fe containing Ti, Cr and C when melted in a nitrogen atmosphere. Oxide films are common, but nitride films are to be expected in circumstances were oxygen has been consumed in a submerged crack. Raiszadeh and Griffiths (2008) have done excellent work to illustrate this in aluminium alloys.

In passing, in the usual case of an alloy with a solid oxide film, it is of interest to examine whether the presence of oxide in a melt necessarily implies that the oxide is double. For instance, why cannot a single piece of oxide be simply taken and immersed in a melt to...