Metal Casting Zone

hello,
We are casting plenty of aluminium alloy casting parts , iam attaching a a foto wherein we are faced with shrinkage porosity problem. in a ring type of component.

The gravity die is bottom gated and metal flows into a circular channel and ingates provided at segment location for metal flow into die cavity with heavy riser on top of the casting.

The casting is a ring of 360 dia od and 240 mm dia id and about 50 mm thick. Alloy is A356T6.

Your views and suggestions to eliminate this problem.

thanks
rags

To eliminate this as a suspect, are you using any type of flux or de-gassing the melt? That looks to be more of a gas defect than a shrinkage defect. I would say possibly sand or slag inclusions except you said you were using a die. Also, what are you using as a mold release? That may be gasing out as well.

we are nitrogen degassing the metal using a lance for purging, we use a RPT -reduced pr test to check for the gas and metal is free from gas.

We use a graphite based mold release agent, yes this is a mettalic mold and we have a ingate mesh filter to trap inclusions.

thanks again
rags

Looks like you have that covered then, you have a more professional setup than most of us. If you are nitrogen de-gassing then you are also probably checking the temp of the melt with a pyrometer and pouring at the same temp every time. Does the defect occur in the same location every time, or does it occur randomly? Have you tried varying the pour temp?

The only thing i can think of is there is some form of cantaminate. too much graphite?

raghuvir wrote:

We use a graphite based mold release agent, yes this is a mettalic mold and we have a ingate mesh filter to trap inclusions.
rags

how hot should be your mold before you pour?
what sort of release agent are you using?
in my foundry we are spraying spuncoat stuff (graphite based) but we are making sure that it is dry otherwise you might get porosity due to moisture

thanks all,
the mold is heated to around 250deg c before pouring and die temp will reach upro 400 deg c on continous opertion.

The graphite based coating is water based.

This defect occurs frequently only the percentage of scrap rate is inconsistent .
We have tried varying temp but results as well as location of defects are almost same, to be more precise below the riser .

thanks again
rags

If it is happening consistently at the riser, my guess would be that you need to have a look at your mold design. Perhaps the riser is too large and is allowing gas to re-enter the metal at that point, or some of the graphite is washing off and is landing there for the same reason. Have you cut through a casting (riser included)at that point and looked at the cross section? Is it a surface defect or does it happen in the cross section as well?

thanks odd duck,
i will try your suggestion, hopefully we should get some answers
thanks
rags