Aluminium Pressure Die Casting

In pressure

die casting, die temperature, molten metal pouring temperature, injection pressure and speed

are optimised for a special casting.

Metal for a single shot is loaded into a cylindrical chamber through a pouring aperture. A

piston then forces the metal into the die, the entire operation being completed in a few

seconds, so that iron contamination is virtually eliminated. Using this technique much higher

injection pressure in the range of 70140 Mpa is feasible, enabling lower metal to be

employed and greater intricacy achieved. The castings are less prone to entrapped air and a

higher standard of soundness ensures from the smaller amount of liquid and solidification

shrinkage occurring within the die.

In cold chamber operations the molten metals is usually maintained at constant temperature

in an adjacent holding furnace, where transfer of successive shots to the machine chambers

can be accomplished manually. Holding furnaces may be electrically heated types or the one

using immersion heating types or the one using immersion heating device, which has a close

control over the molten metal.

The molten metal is degassed by chlorine or hexachloroethane followed by modification

with suitable modifier. For thinner sections the working temperature of the molten metal

should be 680 deg. C to 690 deg C and for thicker sections this should be between 650 deg. C

to 680 deg C.

The die temperature should be maintained so that castings of good quality are produced.

Structure of the EN AC-AlSi9Cu3(Fe) aluminium alloy, high pressure cast alloy, light microscope

Oxide cross-sections of anodic layer generated on theENAC-AlSi9Cu3(Fe) alloy, high pressure cast alloy, light microscope

structure of theEN AC-AlSi12(b) alloy, sand cast alloy, light microscopeFig

Structure of theEN AC-AlSi9Cu3(Fe) alloy, sand cast alloy, light microscope

Oxide cross-sections of anodic layer generated on theENAC-AlSi12(b) alloy, sand cast alloy, light microscopeFig

Oxide cross-sections of anodic layer generated on theENAC-AlSi9Cu3(Fe) alloy, sand cast alloy

Structure of theEN AC-AlSi12(b) alloy, high pressure cast alloy, light microscope

Oxide cross-sections of anodic layer generated on the EN AC-AlSi12(b) alloy, high pressure cast alloy, light microscope

furnace

atmosphere should be slightly

reducing, and maximum temperature

should be controlled.

Zinc

chloride / Ammonium Chloride is useful

for cleaning. It separates the metal

from the dross by changing the

interfacial energy.

Melting crucible

Graphite, SiC crucibles or Cast

Iron pots (coated with a thin layer of

refractory powder with sodium silicate

mixed with water as a binder) are used

for melting. Fuel economy is good with

Cast Iron pots, but efficient lining is

needed to avoid excessive

contamination of the melt with Fe.

Reworking of coating at regular interval

and prior drying is a must, as loose

oxide pieces or moisture may affect

the melt.

Temperature control is

essential in aluminium alloy melting.

If the dross formed by adding flux is

allowed to come in contact with air, at

temperature above 9500 C, a strong

crystalline aggregate of corundum is

formed. Its specific gravity of 3.9 gm/

cc is higher than that of Al, hence it

settle down, exposing new melt

surface to further oxidation losses and

hydrogen pick-up. Presence of Alkali

(Na, Ca, Sr, Li) of rare earth elements

like Ce or Be reduces oxidation

tendency even at higher temperature

(if sufficient content in the melt). This

is because high temperature leads to

loss of these elements.

Dross formation (combination

of Al2O3 &other oxides is beneficial to

some extent as it prevents diffusion

of hydrogen into the melt during

melting. On the other hand, it indicates

loss of alloying elements, which

necessitates remelting with flux to

recover the same.

Deoxidation & degassing

Fluxes are used to combine

with oxygen and oxides on melt

surface, enabling recovery of base

metal with alloying elements, making

viscous slag and protecting the melt

from further oxidation attack.

Therefore to homogenize the melt

stirring from bottom upwards, without

excessively disturbing the surface,

should be followed. Fluxes are a must

when excessive proportion of foundry

scrap is used in the charge. Foundries

operating fast-melting crucible

furnaces rarely use fluxes to minimize

the danger from contamination.

Fluxing at low temperature is

desirable to form oxide layer. As once

corundum (crystalline aggregate) is

formed at high temperature, it would

be very difficult to decompose it and

recover base elements.

Proper degassing will

eliminate the chances for occurrence

of defects due to gaseous elements.

In melting Aluminium alloys C2 Cl6

(Hexachloroethane) tablets were

preferred over chlorine gas. At present

use of SF6 (Sulfur hexafluoride) with

carrier gas has become popular. It is

better than earlier practice as it does

not cause damage to environment.

C2Cl6 not only degasses the melt, but

also refines the grains (heterogeneous

nucleation). Gas flushing is more

effective than salt addition for

degassing since chloride salts need

time to decompose, liberate Chlorine

and then remove hydrogen. It may

introduce gas rather than removing it,

if it is not properly dried.

Inclusions (oxides, carbides,

graphite flakes or borides introduced

by grain refiners) are dangerous in

aluminium castings, especially under

fatigue load. Crack initiation,

propagation and failure are steps

involved in fatigue failure which is

favoured by surface irregularities or

cracks. These inclusions affect surface

appearance and also act as nucleating

site for cracks. Proper skimming action

before pouring, use of tea-spout ladle

for pouring, use of strainer core, skim

bob gating or filter usuage are

followed.