Machinery's Handbook, 31st Edition
1496 DEFECTS IN CASTING regions that have not yet solidified, and porous regions may develop at their centers due to contraction as the surfaces of the thicker region begin to solidify. Micro-porosity can develop when the liquid metal solidifies and shrinks between dendrites or between den - drite branches. Porosity caused by shrinkage can be reduced or eliminated by various means. Adequate liquid metal should be provided to avoid cavities caused by shrinkage. Internal or external chills used in sand casting also are an effective means of reducing shrinkage porosity by increasing the rate of solidification in critical regions. Internal chills usually are made of the same material as the casting and are left in the casting; external chills may be made of the same material or of iron, copper, or graphite. With alloys, porosity also may be re- duced or eliminated by making the temperature gradient steep. For example, mold materi- als with higher thermal conductivity may be used. Subjecting the casting to hot isostatic pressing is another method of reducing porosity. Most obvious porosity defects are caused by entrapment of gases within the molten solution. Because liquid metals have much greater solubility for gases than solid metals, when a metal begins to solidify, dissolved gases are expelled from the solution. Typi- cally, hydrogen precipitates into melt because of contact with the atmosphere or excessive moisture in the flux. Since hydrogen is highly soluble in molten metal, it is best to avoid superheating metals beyond their melting temperature and to avoid holding the material in a molten state any longer than necessary. To reduce absorption of gases from the atmosphere, which may leave slag or dross, cover molten metal until just prior to pouring it into the mold. Gases also may result from re- actions of the molten metal with the mold materials, either accumulating in regions of existing porosity or causing micro-porosity in the casting, particularly in cast iron, alu- minum, and copper. Dissolved gases may be removed from the molten metal by flushing or purging with an inert gas or by melting and pouring metal in a vacuum. If the dissolved gas is oxygen, the molten metal can be deoxidized. Steel is usually deoxidized with alu- minum, copper-based alloys with phosphorus, silicon, titanium, and zirconium-bearing materials. If the porosity is spherical and the walls are smooth, porosity usually is the result of gases. If the walls are rough and angular, porosity is likely the result of shrinkage between dendrites. Gross porosity is caused by shrinkage and usually is called a shrinkage cavity . The loss in casting properties measured by a tensile test may reflect the amount of poros - ity in a casting; because imperfections become areas of higher stress concentration, the percentage of property loss becomes greater when the strength requirement is higher. Porosity also is detrimental to the ductility of a casting and its surface finish, making it permeable and thus affecting the pressure tightness of a cast pressure vessel. A metallo- graphic examination can determine whether porosity exists in a casting. X-ray techniques also are used for nondestructive evaluations of porosity. METAL CASTING AND MOLDING PROCESSES Metal casting processes may be classified in four different ways: According to the Mold Type: (1) expendable mold (destroyed after each casting); or (2) permanent mold (reused many times). According to the Type of Pattern Used for Making a Sand Mold: (1) expendable pattern (melted for each mold), using wax as the pattern material; or (2) permanent pattern (reused for many molds), using wood or metal as the pattern material. According to the Type of Core Used for Producing a Hole in Casting: (1) expendable core (used in both sand and metal molds), using sand as the core material; or (2) permanent core (used with a permanent mold only), using metal as the core material. According to the Method by Which the Mold is Filled: (1) gravity (sand casting, gravity die casting); (2) pressure (low- and high-pressure die casting); or (3) vacuum (vacuum investment casting).
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