Aluminium Die Casting Porosity

Probably too far in the future to help you immediately, but I read this article earlier this morning.

Maybe you need to experiment. Add more time in slow cooling and vacuum degassing.

As you say, mold and product design are, for the moment, beyond your control, you must consider the die cast process and the various parameters that you can control. A few things come to mind. (assuming that you are using a typical cold chamber die casting machine)

The first is the cold chamber. The diameter and length from the pour hole should ideally be such that as the piston closes the pour hole at he end if the slow shot, the shot completely fills the volume of the chamber. Too large a diameter will trap air in the cold chamber which will be injected in the mold with the aluminum.

The second is timing of the intensifier. The intensifier should be initiated just as the mold cavity is completely filled, that is, just before impact. If started too early it will run out of stroke before the maximum compaction pressure is achieved.

Finally, vacuum, which you should be using. The vacuum should be initiated at the start of the fast shot. If started too early it will just pull air into the mold cavity through the still opened pour hole.

Is the quality of the aluminium you are using poor (to many impurities)?

Have you considered sealing the aluminium component afterwards to cope with the porosity issue? We do this with our cast brass boxes (not sure if this is suitable for aluminium or your application).

Search for answers from users of the material.

Worst case, vacuum impregnate the finished castings with a sealer.

casting-porosity-solutions - Ultraseal America Inc.

finding a solution to the eternal problem of porosity in casting

Understanding Porosity | AFS

It is very difficult to answer this question with the information you bring on.

Die-Casting is the worst process for making Al parts with respect to uniformity of the part. But it is the cheapest, by far, thus choosen by all purchasers and accountants.

There are several mechanisms that induce porosities to occur.

* Dissolved gas (you can not take it all out, unless you have your furnace and crucibles all kept in the vacuum, hydrogen is removed as it causes embritlement)

* shrink voids

* surface oxides that mix in due turbulence

....

As a general rule it is assumed that the skin is the ultimate layer of protection and if inclusions can be avoided to damage this skin the leak tightness will be OK. Unless you remove the skin. (thus reduce the machining really to the minimum required)

Quite some of the porosities I have found are due bad design, resulting in local hot spots in the tool, that damage the surface when opening (not sufficiently solidified)

Add/move overflows to move all inclusions out of the part

Apply casting simulation with porosity prediction and adapt the injection point and part design to improve, not forgetting all the other requirements.

Good post Gwen.

If I might add to it, I would say to the OP, (as he has given us VERY limited information) that:-

1) secondary sprues/air holes may need to be increased in thickness and number (possibly no sprues have been used up to now?) Bring them up to the level of the funnel when full. This is very important.

2) Temperature of the aluminium may need to be increased

3) Mould may need to be heated to a higher temperature before pouring

4) The size of the "funnel" part and the hole leading into the mould may need to be increased quite dramatically. Thus allowing more weight of liquid aluminium to force the molten aluminium into the mould. This also means that a far greater amount of aluminium must be melted for each pour and more sprue material must be cut off and made good...

5) Cleanliness of mould is suspect. Blow though with air before usage.

6) aluminium scrap is being reused without proper cleaning of the molten metal?

7) The crucible is not being properly cleaned.

I am sure that I have missed a lot of other important points that could be part of the problem....sorry!

Why not join CR4 and stop posting anonymously?

You make some very good points. Both product and mold design are critical to quality. However the original post implied that these were outside his control. That leaves us with the die-cast process itself and the die-cast machine parameters which he may have control of. In my experience as both a die maker and die-cast process engineer, These parameters, timing, temperature, and pressure make all the difference in the final product quality.

Unfortunately, we have not had any feedback from the OP to help us to drill down to a solution.

You are right: in most cases the design is out of control of the die caster

This business has a problem: customers push for the cheapest solutions, casters don't always understand exactly what they start with.

My experience is that a bad design or tool cannot be saved by tweaking the process conditions.

Leaktight parts in die casting is an art, thixomolding would be a much better solution but I can't even convince purchasing to even evaluate it. They don't understand the issue: just force the die caster to supply good parts.

Recently I had a caster refusing a design, after the first evaluations they found a zone that was going to leak and they found no way to solve it.

I was happy they did this, mitigating and solving issues with leaking parts consume a lot of energy in our business. We discussed the solutions and found a new design that solved the issues.

If you are following best practices and still getting excessive porosity, perhaps consider ultrasonic argon degassing.

Usually some of the porosity comes from trapped air in the mold. On a couple of parts I have done, we used a sacrificial chamber just beyond the far end of the part so that the initial metal into the mold flushes out into the chamber. the slug formed by the chamber gets clipped off and fed back to the melting pot. If you also observe wall thickness uniformity in the part and do some mold flow modeling to confirm initial metal flow through, your casting uniformity and quality will improve.

I am not a metallurgist, but I don't think casting is the best process. Forging would be much better. Think SCUBA tanks.

I wonder is a cheaper solution might be to accept the problems as they are and add a simple process to seal any possible holes with say epoxy resin?

I am assuming of course that in usage,the temperatures do not exceed what the resin can handle.

If once it has been finally finished (turned on a lathe maybe?), the unit is well cleaned (alcohol comes to mind), then immersed inside and out in a resin, then mechanically cleaned off, leaving resin in all the tiny holes, then allowed to harden or speeded up with heat, then pressure tested before polishing! - or something similar!!

There are methods of pressure handling that may work even faster.....

Naturally, many units could be handled together, to speed up the process dramatically....

Just a thought!!

That's a great idea, I wish I had thought of it. Oh, wait, I did. #5

Batch vacuum impregnation is a fairly normal way to solve porosity issues in cheap problem castings.

I doubt we'll ever hear from the OP to hear if anything suggested here was even considered.

My Bad, I missed it.

By the way I think is a VERY clever idea of OURS!!!

Impregnation is indeed a common solution, it works fine in a certain range of porosities.

Too small the surface tension of the "glue" is to high to get it pushed in.

Fuel has a very high wetting capacity, it can go in and through those holes.

That is why fuel and hydraulic parts are sweating.

There are epoxies that are very "thin". With some added pressure, surely it would work....worth trying I feel.....

Porosity sealing - Wikipedia

Looks very practical and effective.