SEGREGATION AND COMPOSITIONAL DEFECTS IN

CASTING

SUBMITTED BY

WHAT IS SEGRAGATION AND COMPOSITIONAL DEFECT?

• This type of defects arise due to compositional difference arising during solidification and persisting in some cases as a permanent feature of the cast structure.

• Since equilibrium and homogeneity require prolonged time at temperatures high enough for diffusion , segregation persists in the solidified structure.

• The inherent segregation tendency in an alloy can be represented by the equilibrium distribution coefficient K where

K=Cṣ/Cʟ

• For K>1 or , K<1 , segregation of second phase would result.

CONSTITUTIONAL FACTORS WHICH PRODUCE A STRONG SEGREGATION

TENDENCY ARE:

1. Long freezing range

2. Gentle liquidus slope

3. Low solid solubility

BESIDE K, OTHER SOLIDIFICATION CONDITIONS DETERMINING THE DEGREE

AND PATTERN OF SEGREGATION ARE:

• Freezing rate• The mode of development of grain structure• Motion of crystal and residual liquid

CLASSIFICATION OF SEGREGATION

Micro segregation• Extending over dimensions of the order of a

single grain or less (~ 10 – 100 micron).• Mechanical properties of castings are

sensitive to micro segregation.• Can usually be removed by

homogenisation treatment.• Example: dendritic segregation.

Macro segregation• Occurs over a distance ranging from 1 cm to 1 m,

and,

therefore, can not be removed.• Example: V-segregation, inverse V-segregation,

negative segregation, surface segregation, tin sweat.

• Normal segregation This type of segregation occurs , when solidification

moves away from the mould walls as a plane front(no formation of dendrites) segregation freezing point constituents of the alloy are driven to the centre of the resulting casting.

• Gravity segregation This type of segregation takes place due to gravity.

Higher density elements will sink to the bottom of the mould and lighter elements will float to the surface.

• in tool steels (containing W, Mo) and nickel-based alloys (containing Ni , W, Hf , Mo, etc), the highly densed segregated liquid sinks downward , creating channel segregates which flow in the opposite direction to those in conventional steels. This type of segregations is called “freckel deffect”.

• Different types of channel or gravity segregation can be controlled by

1. Decreasing time available for their formation by increasing rate of solidification.

2. adjusting chemical composition of the alloy to give a solute-rich liquid which has a more nearly neutral buoyancy at the temperature with the freezing zone.

INVERSE SEGREGATION/DENDRITIC SEGREGATION

• Crystals having a narrow necked shape roots are med before the formation of a stable solid skin. When crystals come in contact with the adjacent crystal, the liquid of high concentration is trapped at the crystal root , and form inverse or , dendritic segregation.

• Inverse segregation can be reduced by: 1. forming fine grained equiaxed crystals (no large dendrite are formed) . 2. rapid cooling ( a stable solid skin is formed immediately) .

• The root causes of these defects, as well as methods for their control, are described below.

• Oxide inclusions result from the failure to maintain clean melt-handling and melt- transfer systems. To

minimize the likelihood of introducing metallic inclusions, filters should be included within the melt-

transfer system, or molten metal turbulence should be minimized when filling the die cavity. Preventing

foreign objects from entering open dies is also helpful.

• Porosity and voids can occur when insufficient pressure is applied during squeeze casting operations.

Porosity and/or voids are usually eliminated by increasing the casting pressure when the other variables

are optimized.

• Extrusion Segregation. The relative microsegregation that occurs in squeeze cast components is much

less than that in other cast components. Such defects can be avoided by designing dies properly, by

using a multiple gate system, by increasing die temperature, or by decreasing delay time before die

closure.

• Centerline segregation is a defect that is normally encountered with high-alloy wrought aluminum alloys at

lower solute temperatures. As solidification begins on the die walls, the liquid phase becomes more

concentrated with the lower-melting solute, which is trapped within the center areas of the extruded

projections or more massive areas of the casting. Such defects are avoided by increasing die temperature, by

minimizing die closure time, or by selecting an alternative alloy.

• Blistering. Air or gas from the melt that is trapped below the surface during turbulent die filling forms blisters

on the cast surface upon the release of pressure or during sub- sequent solution heat treatments. Methods of

avoiding such defects include degassing the melt and preheating the handling transfer equipment, using a

slower die closing speed, increasing the die and punch venting, and reducing the pouring temperature.

• Cold laps are caused by molten metal overlapping previously solidified layers, with incomplete bonding

between the two. To alleviate cold laps, it is necessary to increase the pouring temperature or the die

temperature. Reducing the die closure time has also been found to be beneficial.

• Hot tearing takes place in alloys that have an extended freezing range. When solid and liquid coexist over a wide range

of temperatures, contraction of the solid around the rigid mold surface can initiate rupture in partially solidified

regions. The methods used to avoid hot tearing in squeeze cast products include reducing the pouring temperature,

reducing the die temperature, increasing the pressurization time, and increasing the draft angles on the casting.

• Sticking. A thin layer of casting skin adheres to the die surface because of rapid cycling of the process without sufficient

die/punch cooling and lubrication. To avoid sticking, it is recommended to decrease die temperature or pouring

temperature.

• Extrusion debonding takes place when the casting has deeply extruded details and the metal remains in the open die

for a long period of time before it is extruded to fill the die cavity. The oxide present on the partially solidified crust in

the die remains there after the melt has been extruded around it, resulting in the absence of a metal-to-metal bond at

oxide stringer locations. Extrusion debonding can be prevented by increasing the tooling temperature or the pouring

temperature. Decreasing the die closure time can reduce the oxide formation on the semi- liquid metal present in the

die.