casting 101 C
Control Liquid Metal, Limit Oxide Inclusions KEVIN FLEISCHMANN, AFS INSTITUTE
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chosen method of manufacture for certain metallic shapes. However, the melting, holding, treating and transferring of liquid metal presents a number of specific challenges. The majority of metalcasting facilities operate in the open atmosphere; their melt surface is most often oxidized, depending on the alloy, with a film of varying thickness. This film will regenerate almost instantaneously whenever ruptured. The more cau- tion that is exercised when manipulating liquid metal, the higher the quality of the final castings. Oxides are non-metallic inclu-
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sions in castings. They are solid, stable, long-living ceramics that are insoluble in liquid. Splash- ing, turbulent flow of liquid metal entrains folded oxide films and rips them into tiny pieces, which often can entrap air bubbles and disperse them throughout the bulk fluid. Oxide inclusions are practically invisible; with only modern high pow- ered magnification methods allow us to study and measure sizes and shapes. These oxides have no strength, acting as tiny cracks in the casting, nucleating shrink and gas poros- ity sites, lowering mechanical properties and increasing hot tearing propensity. In steel casting produc- tion, oxide defects often are referred to as “ceroxides,” while gray and ductile iron
iquids flow much bet- ter than solids” is an obvious observation, but it is a primary reason casting is your
producers more commonly use the term ”slag” to describe iron-based oxide. Aluminum, magnesium, cop- per base and other nonferrous alloy producers refer to their oxides in castings as “dross.” A casting with fewer and smaller
oxide inclusions means better mechanical and physical properties, potentially approaching wrought al- loy strengths. To minimize oxidation production inside the mold, many
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ing suppliers can greatly improve the casting and meet your specifica- tions by paying special attention to the transfer of liquid metal from the furnace to the mold and its filling of the mold. A highly performing gating and
A casting with fewer oxide inclusions means better mechanical and physical properties.”
metalcasters see counter gravity fill- ing as an obvious solution. Because of economic considerations, however, many facilities opt instead to use gravity-filled horizontally or verti- cally parted green sand molds to limit cost. Castings with good prop- erties and surface finishes can be and are produced on a regular basis under these conditions. Metalcast-
risering system is not built by ac- cident. An understanding of how and where the casting will solidify will allow for proper feeding design. Metallostatic pressure from risers during solidifica- tion will help keep oxide inclusions compact, not allowing for an unfolding and stretching of these minute cracks. To minimize the presence of oxide inclusions be-
Limiting oxide inclusions, also known as ceroxides, slag or dross, can improve a metal casting’s fi nal properties.
fore solidification even takes place, the filling system should be kept relatively narrow, just wide enough to allow for the time needed to fill the mold. Maintaining positive liquid metal pressure on the mold during filling pushes mold gases out and traps oxide films against the mold’s wall surface. The cleaner mass of liquid should enter the mold as low as reasonably possible in the mold cavity, rising up to fill the void left by the pattern. Oxide inclusions have been the subject of many pieces of literature and presentations, if you’d like to learn more about this type of defect. Under- standing the casting process and how to troubleshoot specific problems are great approaches to see each and every cast- ing’s potential for improved quality.
Jan/Feb 2015 | METAL CASTING DESIGN & PURCHASING | 43
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