Alloy Macro Shrinkage

Macro shrinkage is a phenomenon associated with metal casting process. Macro shrinkage occurs, when the liquid metal is surrounded by significant amount of solid material, which is strong enough to resist the depression of the contracting liquid.

The phenomenon of macro shrinkage occurs as a concentrated zone of shrinkage holes or single shrinkage cavity in cast products that can be detected through non-destructive tests, such as radiography, ultrasound, and magnetic particle method. The non-destructive tests and techniques help in eliminating the problem of macro shrinkage in cast products and improve their quality.

Macro shrinkage results, due to the interaction of several complex influences in iron. The grating, risering, and the iron chemistry needs to revised, when the shrinkage is constantly present in cast products on a regular basis. It has been observed that the problem of macro shrinkage is within the control and timing of the graphitizing process, when it occurs irregularly and the iron chemistry is consistent during the episodes of shrinkage.

Some of the significant factors that influence episodes of shrinkage in cast products include: - the thermal cooling conditions, casting temperature, alloy content and the amount of graphite expansion taking place during solidification.

Macro shrinkage can occur at three different levels in thermal analysis cups in cast products. Shrinkage occurs at the point of the last metal to solidify and so is located around the thermal couple for easy detection. Graphite, which is a hexagonal-close pack form of carbon that can grow in both liquid and solid form of iron, has emerged as a significant material to overcome the problem of macro-shrinkage in cast products.

It is necessary to maximize the formation of late graphite, without having to reduce the actual amount of graphite, in an effort to minimize the problem of macro shrinkage in cast products.

The size of the pores resulting from macro shrinkage is around 3 mm. Thee defects are internal, which can be prevented by optimal pouring temperature, riser positioning, and size. Several researchers continue to conduct studies and develop models, to study the phenomenon of macro shrinkage. Researchers throughout the world are involved in conducting studies and models, such as:-

  • Simple thermal models and criteria functions, to channel,
  • Porous medium models based on hydrogen diffusion, and the latest
  • Model based on oxide entrapment.

Researchers are focused on studying the phenomenon, due to its potential contribution towards improving the quality of cast products. Several computer simulation software packages have also been developed that are being used by foundries to predict casting defects and the phenomenon of macro shrinkage.

It is essential for researchers and materials professionals to study the phenomenon of macro shrinkage in cast products, as it causes decrease in number and lengths of most planar pores and a decrease in size and number of all intra-aggregate pores. The loss in pore space and continuity affects resin impregnation in most of the cast products.

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