SOLIDIFICATION OF CASTING

SOLIDIFICATION OF CASTING

The function of a riser is to feed the casting during solidification to avoid shrinkage cavities. The requirement of casting depends on type of metal powered and complexity of the casting. Different metals may have different volumetric shrinkages. Grey cast-iron sometimes may have negative shrinkage.

As time progresses, the metal starts loosing heat through all sides and as a result starts fruzing from all sides equally trapping the liquid metal inside. By further solidification and subsequent volumetric shrinkage and the metal contraction due to change in temperature. Causes formation of a void. On completion of solidification finally results in the shrinkage cavity.

Liquid shrinkage occurred ends up as a void. Such isolated spots which remain hot till the end are called „hot spots‟.

The turn directional solidification normally used in the casting terminology. Since cooling is achieved by the removal of heat from all surfaces which are exposed to the atmosphere or sand, cooling normally starts from the point which is the thinnest or is exposed over a larger surface area.

DEFECTS IN CASTING:

a. Gas defects b. Shrinkage cavities c. Moulding material defects

d. Powering metal defects e. Metallurgical defects.

(a)Gas defects:-

(i) Blow holes and open blows:-

These are spherical, flattened or elongated cavities present inside the costing or on the surface. On the surface they are called open blows and inside they are called blow holes. These are caused by the moisture left in the mould and in the core. Due to the heat in molten metal, the moisture is converted into steam, part of it is untrapped in the casting ends up as blow hole or open blow. This defect is also caused due to the lower permeability or lower venting of mould.

(ii) Air inclusions:

the atmospheric and other gases absorbed by the molten metal in the furnace, in the laddle and during the flow in the mould when not allowed to escape would be trapped inside costing and weaken it.

(iii) Pin hole porosity:

This is caused hydrogen in the molten metal. The formation of hydrogen is due to dissociation of water inside mould cavity. As the molten metal gets solidified it looses the temperature. Which decreases the solubility of gases and there by expelling the dissolved gases. The hydrogen while having the molten metal would cause very small diameter. And long pin holes showing the path of escape. These pin holes causes the leakage of fluids under high pressure. The reason for this is high powering temperature. Which increases the gas pickup.

shrinkage cavities:

These are caused by liquid shrinkage occurring during solidification.

Moulding material defects:

Cuts and washes – metal penetration – fusion – run out – rattails and buckles, swell and drop. The above defects occur due to the lack of required properties to moulding materials and improper ramming.

SPECIAL CASTING PROCESS

a. Centrifugal casting b. Die castings c.Investment casting

a. Centrifugal casting:

This is a process of producing hallow cylindrical costing without core. The molten metal is fed into the revolving mould. The axis of rotation is usually horizontal but can be vertical for short pieces. Moulds are made of steel or graphite may be cooled with a refractory material to increase mould life. The metal is forced against the mould surface by the centrifugal force until its solidifies. After solidification mould rotation is stopped and the casting is pulled out. This is used to make cylindrical parts such as pipes, gun barrels and street lamp post.

Advantages:-

1. The mechanical properties of centrifugally east jokes are better compare to other processes since in elusion such as slag and oxides gets segregated towards the centre and can be easily removed by machining pressure acting on metal throughout the solidification, causes the porosity to be eliminated giving rise to dense metal.

2. Up to certain thickness of objects, proper directional solidification can be obtained starting from the mould surface to the centre.

Limitations:-

1. Only certain shapes which axi symmetric and having concentric holes are suitable for true centrifugal casting.

2. The equipment is expensive and thus is suitable only for large quantity production.

Semi centrifugal casting:-

This is used for more complicated jobs which are not possible to cost by true centrifugal casting, that are axi symmetric in nature. No need of central hole which is to be obtained with the help of core. The mould made of sand or metal is rotated about a vertical axis and the molten metal enters the mould through a central powering basin. For larger production rates, the moulds can be stacked one over the other, all feeding from the same central pouring basin. The rotating speeds are not high as in true centrifugal costing.

CENTRIFUSING:

This metal is used to get higher metal pressure during solidification, when casting shapes are not axi symmetrical. Used for small jobs of any shape. A number of such small jobs are joined together by means of radial runners with a central sphere on a revolving table. The jobs are uniformly placed on the table around the periphery so that here masses are properly balanced. This process is similar to semi centrifugal casting.