CASTING DEFECTS -Occurs because some steps in manufacture cycle do not get properly controlled and somewhere goes wrong 1.Blow Holes - appear as cavities (holes) in a casting - visible on the surface of a casting are called open blows whereas those occur below surface of castings and not visible from outside are termed as blow holes/

2. Shrinkage - occur during solidification - Shrinkage or contraction stages: 1.Contraction in volume of the liquid metal as it cools to solidification temperature. 2.Shrinkage as the liquid metal changes state and becomes a solid. 3.Contraction of the solid metal as it cools to RT. - cavity in the surface or an internal void in casting - due to lack of feed metal to compensate for volumetric contraction during solidification - subdivided into macroshrinkage, filamentary shrinkage and microshrinkage.

Macroshrinkage -‘pipe’ formation -Formation of macroshrinkage in ingot shown in Fig. 1 -Formation of macroshrinkage in isolated area is shown in Fig. 2. Cause: Faulty gating and risering. Improper chilling - as the casting shrinks, the riser supplies additional melting. The risers must freeze last, otherwise, shrink holes are created. If risers are properly located they an reduce the number and size of shrink holes. Remedies: Ensure proper directional solidification by modifying gating, risering and chilling

Fig 1 Shrinkage of a cylindrical casting during solidification and cooling: (0) Starting level of molten metal immediately after pouring, (1) Reduction in level caused by liquid contraction during cooling, (2) Reduction in level caused by solidification shrinkage, (3) Further reduction in height and diameter due to thermal contraction during cooling of the solid metal.

- Cause: Faulty gating and risering. Improper chilling - as the casting shrinks, the riser supplies additional melting. The risers must freeze last, otherwise, shrink holes are created. If risers are properly located they an reduce the number and size of shrink holes. - Remedies: Ensure proper directional solidification by modifying gating, risering and chilling

Fig 2 Creation of shrink holes due to improper location of a riser.

Control of Solidification to obtain Sound Castings -Solidification starts from the mould wall toward the centre line off the cavity (Lateral or Progressive solidification). -The longitudinal or directional solidification occur at right angles to lateral solidification at the centre line. - Directional solidification must be progressive toward the riser from the point/s, most distant from the riser. i.e solidification of molten metal from the sprue to the molten cavity and then to the riser to produce a casting which is free from voids and internal cavities.

Filamentary shrinkage -Flaws maybe a continuous structure of connected lines or branches of variable length, width and density, or occasionally as a network -Occurs in the centre of a section where the gradually thickening walls of solidified metal from two surfaces meet. -Occurs most frequently in alloys having a short solidification range and low thermal conductivity. Fig 3 Centre-line shrinkage

Microshrinkage -As tiny voids scattered through an area of metal. -Caused by inability to feed metal into the spaces between the arms of the individual crystals or grains of metal. -Most often found in metals having a long solidification temperature range Fig 4 High magnification of shrink area in an aluminium casting showing dendrites

3. Hot tears - unwanted cooling stress may cause the rupture (tear) because metal has low strength at higher temperature - Characteritics: The form is ragged, branching crack Generally intergranular Dendritic morphology on failure surface and heavily oxidised prior to any subsequent heat treatment Often locate at hot spots Occur randomly and their extent is also variable under identical casting conditions - Causes: Lack of collapsibility of core and mould, faulty design, hard ramming. - Remedies: improve collapsibility, modify design, provide soft ramming