Dendrites’ Clues for Castability
How dendrites form in a casting during solidification informs plant metallurgists of its final properties. GEOFFREY SIGWORTH, GKS ENGINEERING SERVICES, DUNEDIN, FLORIDA
Tis article is the second in a series
on solidification in aluminum castings. While the series focuses on solidification principles in aluminum alloys, many can be applied to other metals, as well.
photographs of individual snowflakes. In the introduction, the author claims each snowflake is unique, and no two crystals are alike. Tis claim may be true, in spite of the incredibly large number of snowflakes that form each winter. Te variety of snowflakes shown in the book is mind boggling. Something similar happens every time metal solidifies in the mold. Te liquid-to-solid transformation involves the formation of many small, individual crystals of solid aluminum. Tis is a fascinating area, one which has received a great deal of study. A brief overview will be given in this article, touching on the aspects of solidification most impor- tant to the casting industry. Te first article in this series
A
described the use of phase diagrams to see the sequence of phases forming during solidi- fication, which influences final casting properties and can provide
book on snowflakes at the Carnegie Library in Pittsburgh depicts many
insight into castability issues. Even more about alloy properties can be gleaned from delving deeper into how the structure of a metal changes as it freezes. Tis knowledge is an impor- tant tool in choosing alloying combi- nations for your desired result. Te solid aluminum crystals forming
during solidification are like snow- flakes. Te metallurgists first observing these crystals thought they resembled trees and called them dendrites, after the Greek word for tree (δένδρον or déndron). Dendrites were first observed by polishing metal samples or by etch- ing the polished surface. More recently, real-time X-ray studies have observed the in situ formation of dendrites in
Al-Cu alloys. Because the aluminum crystal contains much less copper than the surrounding liquid, they appear lighter in X-ray images. Examples are shown in Fig. 1. Te formation of dendritic crystals is
a curious phenomenon, and many scien- tists have studied them. Te technical lit- erature in this area is extensive; however, a relatively simple explanation will suffice to understand what is happening. One important clue is that pure metals do not form dendrites. But when silicon or other elements are alloyed to aluminum, dendrites appear. From the Al-Si phase diagram, only 13% of the silicon in the liquid metal remains in the first solid. Tis means that the silicon atoms pile up in front of the growing solid crystals. Te situation is shown schemati- cally in Fig. 2. In keeping with the snowflake analogy, the growing aluminum grain is represented by a snow plow. Consider the act of shoveling
Fig. 1. These are dendrites found in Al-20% Cu liquid. (Pic- tures were taken (a) 110 seconds, (b) 139 seconds and (c) 360 seconds after the first grains appeared.)
snow. When a shovel is pushed, the snow quickly piles up in front, so one can go no farther. Years ago, sidewalks in some cities were cleared of snow by a horse-drawn plow. Te plow would use a “V”-shaped blade the width of the sidewalk which easily cut through the snow, pushing it to the sides of the walkway. Tis is shown schematically in Fig. 3.
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