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CAST TIP Control Liquid Metal, Limit Oxide Inclusions KEVIN FLEISCHMANN, AFS INSTITUTE “ L

iquids flow much better than solids” is an obvious observa- tion, but it is a primary reason

casting is your chosen method of manufacture for certain metallic shapes. However, the melt- ing, holding, treating and transferring of liquid metal presents a number of specific challenges. Te majority of metalcasting facilities oper- ate in the open atmosphere; their melt surface is most often oxidized, depending on the alloy, with a film of vary- ing thickness. Tis film will regenerate almost instanta- neously whenever ruptured. Te more caution that is exercised when manipulating liquid metal, the higher the quality of the final castings. Oxides are non-metallic inclu-

counter gravity filling as an obvious solution. Because of economic consid- erations, however, many facilities opt instead to use gravity-filled horizontal- ly or vertically parted green sand molds

A well performing gating and risering system is not built by accident. An understanding of how and where the

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

casting will solidify will allow for proper feeding design.

sions in castings. Tey are solid, stable, long-living ceramics that are insoluble in liquid. Splashing, 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; only modern high powered magnification methods allow us to study and measure sizes and shapes. Tese oxides have no strength, acting as tiny cracks in the casting, nucleating shrink and gas porosity sites, lowering mechanical properties and increasing hot tearing propensity. In steel casting production,

oxide defects often are referred to as “ceroxides,” while gray and ductile iron producers more commonly use the term “slag” to describe iron-based oxide. Aluminum, magnesium, copper base and other nonferrous alloy pro- ducers refer to their oxides in castings as “dross.” A casting with fewer and smaller

oxide inclusions means better mechani- cal and physical properties, potentially approaching wrought alloy strengths. To minimize oxidation production inside the mold, many metalcasters see

44 | MODERN CASTING April 2015

to limit cost. Castings with good prop- erties and surface finishes can be and are produced on a regular basis under these conditions. Metalcasting suppli- ers can greatly improve the casting and meet your customer’s specifications by paying special attention to the transfer of liquid metal from the furnace to the mold and its filling of the mold. A well performing gating and

risering system is not built by accident.

of oxide inclusions before 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. Te 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 litera- ture and presentations, if you’d like to learn more about this type of defect. Understanding the casting process and how to troubleshoot specific problems are great approaches to see each and every casting’s potential for improved quality.

Oxides have no strength, lower mechanical properties and increase hot tearing propensity.

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