Metal Shaping Processes. Vukota Boljanovic
the sprue filled and reducing pouring height.
•Increasing static pressure by lengthening the runner height.
c) Discontinuities
Discontinuities include cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Although many factors are involved in tearing, coarse grain size and the presence of low-melting segregates along the grain boundaries (inter-granular areas) increase the tendency for hot tearing. Incomplete castings result from the molten metal being at too low a temperature or from the metal being poured too slowly. Cold shut is an interface in a casting that lacks complete fusion because of the meeting of two streams of liquid metal from different gates.
Example: Hot cracking.
Hot cracking is a crack that is often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.
Possible causes:
•Damage to the casting while hot, due to rough handling or excessive temperature at shakeout.
Remedies:
•Care in shakeout and in handling the casting while it is still hot.
•Sufficient cooling of the casting in the mold.
•For metallic molds; delay of knockout, assuring mold alignment, using ejector pins.
d) Defective Surface
Defective surfaces are ones that have folds, laps, scars, adhering sand layers, or oxide scale.
Example: Flow marks.
On the surfaces of otherwise sound castings, the defect appears as lines that trace the flow of the streams of liquid metal.
Possible causes:
•Oxide films that lodge at the surface, partially marking the paths of metal flow through the mold.
•Metal, flask or both being too hot.
Remedies:
•Increasing mold temperature.
•Lowering the pouring temperature.
•Modifying gate size and location (for permanent molding by gravity or low pressure).
•Tilting the mold during pouring.
•In die casting, vaporblast or sand blast mold surfaces that are perpendicular, or nearly perpendicular, to the mold parting line.
e) Incomplete Casting
Incomplete casting, such as misruns (due to premature solidification), insufficient volume of metal poured, and runout (due to loss of metal from mold after pouring).
Example: Poured shot.
The upper portion of the casting is missing. The edges adjacent to the missing section are slightly rounded, but all other contours conform to the pattern. The sprue, risers, and lateral vents are filled only to the same height above the parting line as is the casting (contrary to what is observed in the case of defect).
Possible causes:
•Insufficient quantity of liquid metal in the ladle.
•Premature interruption of pouring due to workman’s error.
•Metal too cold when cast.
•Mold too cold when cast.
Remedies:
•Having sufficient metal in the ladle to fill the mold.
•Checking the gating system.
•Instructing pouring crew and supervising pouring practice.
f) Incorrect Dimensions or Shape
Owing to factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting.
Example: Distorted casting.
Inadequate thickness, extending over large areas of the cope or drag surfaces at the time the mold is rammed.
Possible causes:
•Rigidity of the pattern or pattern plate is not sufficient to withstand the ramming pressure applied to the sand.
Remedies:
•Assuring adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased.
g) Inclusions
Inclusions form during melting, solidification, and molding. Generally nonmetallic, they are regarded as harmful because they act as stress raisers and reduce the strength of the casting. They can be filtered out during processing of the molten metal. Inclusions may form during melting because of reaction of the molten metal with the environment (usually oxygen) or the crucible material. Chemical reactions among components in the molten metal may produce inclusions; slags and other foreign material entrapped in the molten metal also become inclusions. Reactions between the metal and the mold material may produce inclusions. Spalling of the mold and core surfaces also produces inclusions, indicating the importance of the quality and maintenance of molds.
Example: Metallic inclusions.
Metallic or intermetallic inclusions can be of various sizes; they are distinctly different in structure and color from the base material and most especially different in properties. These defects most often appear after machining.
Possible causes:
•Combinations formed as intermetallics between the melt and metallic impurities (foreign impurities).
•Charge materials or alloy additions that have not completely dissolved in the melt.
•Molten metal containing excess flux or foreign oxides.
•During solidification, the formation and segregation of insoluble intermetallic compounds concentrating in the residual liquid.
•Contaminants in wax pattern.
Remedies:
•Assuring that charge materials are clean and eliminating foreign metals.
•Using small pieces of alloying material and master alloys in making up the charge.
•Being sure that the bath is hot enough when making the additions.
•Not making additions too near to the time of pouring.
•For nonferrous alloys, protecting cast iron crucibles with a suitable wash coating.
Examples of seven basic categories of defects are shown in Fig. 1.8.
h) Porosity
Porosity is the presence of holes, spaces, or gaps inside a solid. Two main sources of porosity during casting are shrinkage and gas porosity. The first of these occurs due to the volume contraction between solid and liquid during solidification; if additional liquid is not supplied to compensate, then porosity will appear in the casting.
Fig. 1.8 Schematic illustration of seven basic categories of casting defects: a) joint flash, b) blowholes, c) hot-cracks, d) flow marks, e) poured shot, f) distorted casting, g) metallic inclusion.
The most obvious porosity defects