Common defects of aluminum die castings

Features

Causes

Solutions

Oxidation slag

Oxidation slag distributes on the casting surface,in the stuffy corner parts of the mould.Fracture ashen white or yellow, It could be found by X-ray or during machining,can also be found in the alkaline cleaning, acid pickling or anodized.

a.the furnace material is not clean and used material is overmch.
b.Bad design of gating system.
c.The slag is not cleared away in alloy liquid.
d.Improper pouring operation and slag is involved.
e.Standing time is not enough after refining.

a.Furnace material should be through blowing sand and appropriate reduce used material.
b.Improve gating system and strength the ability of prevent of salg.
c.Using appropriate slag flux
d.Smooth poring and pay attention to prevent slag.
e.Stand for a certain time after refining.

Air hole

Air hole inside casting wall  is normally circular or elliptic with smooth surface,sometimes a yellow oil.Surface air hole could be found by sand blasting.Internal air hole have to be checked by X-RAY or machining.

a.Casting alloy is not smooth, involved in gas
b.Core sand is mixed with organic impurities (such as coal, grassroots horse manure, etc.).
c. Adverse ventilation  of mold and sand core.
d. Shrinkage cavity is exsited on the surface of cold iron.
e. Bad gating system design of aluminum die castings.

a.Well control of casting speed and avoid gas.
b.Core sand shall not be mixed with organic impurities in order to reduce the gas evolution of molding materials.
c.Improve exhaust ability of core sand.
d.Correct selection and processing of cold iron 
e.Improving pouring system design.

Shrinkage porosity

Shrinkage of aluminum die castings is usually produced near thick parts of riser boots,large scale thin wall parts.Could be tested by X-ray,fluorescence macroscopic fracture,ect.

a.Poor feeding function 
b.Air content in the furnace material is too much.
c.Parts near the sprue is overheated.
d.Too much moisturein in sand and sand core is not drying.
c.Big and coarse grains of alloy.
e.Inappropriate position of casting in the mould.
f.Pouring temperature is too high and the casting speed is too fast.

a.Add pouring metal liquid from the riser and improve the riser design. 
b.The furnace material should be clean and no corrosion.
c.set the riser casting shrinkage, put cold iron or chill and riser combination.
d.Control of molding sand moisture,dry sand core.
e.Refined product grain measures.
f.Improve the casting position in mould,reduce the pouring temperature and casting speed.

Crack

a.Crack produced in casting process.Developed along the grain boundary, often accompanied by segregation, formed in a high temperature 

b.Crack produced in treat treatment process.Caused by heat treatment burnt or overheating.

a.Casting structure design is unreasonable, there are sharp corners.
b.Deformability of sand core is bad.
c.Local overheating of aluminum die casting
d.High pouring temperature
e.Early taking aluminum die casting from the mold.
f. Excessive heat or burn of heat treatment and aggressive cooling speed

a.Improve the design of casting,avoid sharp corners,try to keep wall thickness uniform when design.
b.Increasing the deformability of sand mold. 
c.Ensure coagulation of aluminum casting at the same time and improve the design of gating system.
d.Appropriate reduce the pouring temperature. 
e.Control cooling time of casting from mould.
f. Using correction method if there is deformation of aluminum die castings.
g.Correct control of heat treatment temperature and reduce the quenching cooling rate.