3218

casting cools down to room tomperature, the casting exhi­

is to be sound, with no cavities present, the riser must feed approximately % of the volume of the casting.

Unless the riser is considerably larger than the casting cavity it w ill freeze fir s t .

When shaped castings are produced the similar def-

ect can arise in their thicker parts. The change in metal

volume manifests itse lf as shrinkage during the transfor­ mation from liquid to solid . If insufficient attention

dification process, areas of liquid can become isolated. The shrinkage then appears as a cavity or porosity (Pig, 2,23). Voids due to shrinkage can be confused with blows, but usually have a rougher wall and an irregular shape.;.

Fig.2.2h, Volumetric contraction or shrinkage of metals and alloys on solidification (a),and tendency of binary alloys to show (1) dispersed shrinkage cavities

or (2)contral cavities (b) {£2j 0 •)

Fig* 2*25. Methods for eliminating hot spots in a

T - seetion [6].

the entrapped liquid volume* Therefore a central shrin­ kage cavity is formed easier*

At the volumetric solidification of an alloy cryst­ allization takes place simultaneously across the whole volume of the casting* In this case the shrinkage cavity ( pipe ) is not formed hut shrinkage porosity as small cavities occurs anywhere* These «avities can be so small that it is impossible to detect them with the unaided eye because they are located either between crystals and gra­ ins or even between dendritics*

Some alloys are able to give a center-line shrinkage tog­ ether with the porosity*

In general» the main reasons of the shrinkage defe­ cts appearance can be expressed as the following i -

1* Probable errors in an alloy preparation and a deviation from the optimum chemical composition of the alloy.

2* High overheating the alloy or metal*

3. Bad feeding heat centres of the casting as a

result of $

- imperfect gating system;

—either small dimensions of risers or their incorrect location on the casting;

charge so to obtain the needed chemical composition of

the alloy. It is inadmissible to overheat the alloy ra ther high without a special necessity.

In most cases the major cause of shrinkage defecti

is the failure to obtain directional solidification tow

ing of hot spots throughout the whole of solidification process.

Conditions of directional solidification of a ca&, ting are improved at the equalizing of the heat transmi ssion from heavy and light parts of the casting to the i uld. It can be approached by different ways :~

- by using external and internal ch ills (Fig.2,2^

-by setting a core in the place of casting gaud ions;

Variable methods are used to provide optimum cond­ itions of feeding the casting and to eliminate the shri­

nkage cavities and porosity formation, such as setting of risers with atmospheric or gas pressure, heating of risers and also different methods of physical coercion s ultrasonic, electromagnetic mixing, vibration, etc. Often proprietary methods involving exothermic heating in the mould cavity can be used to encourage directional solidi­

fication, When these are used together with judicious

location of refractory insert? and chills to control heat transfer to the mould ( Fig. 2,27), shrinkage defects can largely be overcome#

From shrinkage point of view the casting design is

the most important question. Temperature gradients in so­ lidifying castings must be favorably controlled i f sound castings are to be made.

It is obvious that heavy sections of castings cannot be fed through light sections, -The designer should attempt

fulfilment of the'directional solidification principle.

ted areas are potentially troublesome. Without special

casting are known as inclusions. Common inclusions inc­

Slag inclusions are reaction products /of the nets being cast or'other non^motallies from jjolting operatic