Geometric features of chill-cast surfaces
- PDF / 4,053,601 Bytes
- 10 Pages / 603.28 x 783.28 pts Page_size
- 39 Downloads / 210 Views
Geometric features such as grooves, wrinkles, and laps are commonly observed on chill-cast surfaces. With lead used as a representative material, the occurrence of these features over a range of casting speeds from 0.025 to 17 cm/s (0.6 to 400 in./min) has been examined. On the basis of their assumed origin, three types of surface features have been recognized. The first two are associated with the mechanical behavior of the thermally stressed solidifying shell. Fine Type I grooves are the remnants of periodic shell corrugations formed near the meniscus during the earliest stages of solidification, and their spacing decreases with increasing casting speeds. More widely spaced Type II wrinkles are concavities that develop as solidification progresses, and are associated with the temperature perturbations that cause uneven thickening of the shell. Type III surface features occur at slow casting speeds when freezing of the meniscus gives rise to periodic surface lapping. W H E N a liquid is cast against a mold wall the normal expectation is that the cast surface will merely assume the smoothness of the mold wall. In the case of rapidly cooled castings, however, the cast surface will often exhibit geometric features that are a consequence of the thermomechanical behavior of the solidifying shell and have little to do with the geometry of the mold wall. An example of a nonsmooth surface produced by rapidly casting against a polished metal wall is illustrated in Fig. I. This type of approximately equispaced horizontal grooving has been observed on the surface of rapidly cooled metal castings since at least 1919.14 Such a surface is generally considered to depreciate the quality of a casting as well as to reduce the heat transfer between the solidifying shell and the mold. Furthermore, in the continuous-casting process the surface roughness is expected to affect the friction between the continuously withdrawing shell and the mold. Detailed studies of features similar to those shown in Fig. 1 have been made by Siebel and co-workers for continuously cast aluminum, 5 by Waters for both statically and continuously cast leadr and by Collins for DC-cast aluminum. 8 However, the influence of the casting variables on the geometry of the cast surfaces has not been fully explored, and the origins of the different features have not been recognized.
against the mold. The speed of the ascending meniscus was measured and taken as the casting speed. A constant casting speed was achieved by constant displacement rate of the piston, the range of possible casting speeds being 0.01 to 20 cm/s (0.24 to 470 in./min). To minimize heating of the air above the rising liquid, an air reservoir was incorporated in the pump system. No attempt was made to maintain the ascending liquid inside the mold chamber at constant temperature. However, as an indication of the relative amount of superheat in the liquid, the temperature in the bath was measured just prior to casting. One type of mold chamber consisted of a vertically mounted stainless-steel fram
Data Loading...
