Aluminium cast, Low Pressure Vs High Pressure

[Haz88]

Hi All,

Apologies if this has been a topic previously covered.

I have a product i am designing and of which a few parts will be Al Cast.
I have a few other engineers here who have backgrounds in the processes however two gentlemen who are both from credible backgrounds have conflicting opinions. This has unfortunately left me at a roadblock as i do not want to create the part in the wrong process.

Background:
The parts will be for "outdoor Use" and have the potential to be dropped on rocks etc. One part is a long slender piece which would be alot like a broom stick but not as long and the other a more solid shape close to that of a computer monitor base plate.

The information i have researched has left me thinking that they may be biased to whomever published the findings.

Engineer ONE believes High pressure die casting will be the better option due to the decrease in porosity and increase in production.
"High pressure die cast is merely a die casting process aimed at porosity minimization for applications such as pneumatics or hydraulics.
Note however that in my experience it is not infallible...only 'better than low pressure die casting'."


Engineer TWO has stated that low pressure die casting has a lower porosity allowance and has back this up with a paper. This paper however seems like it was sponsored by a LPDC company. Attached
"As we had discussed, the HPDC has the highest tool/die costs, but lowest piece price. The high pressure castings have the highest amount of internal porosity and the aluminium grades suitable for this process (should have higher silicon level) are not the strongest and as far as I remember, they cannot be heat treated to increase the casting strength.
Depending on the required strength of the part, the annual volume and the cost of the dies, you might need to consider the other options: permanent mold (gravity casting), low pressure die casting, investment casting, etc."

If anyone has any experience in this field i would greatly appreciate it, as its slowing my product up.

Thank you
Josh

 

[tbuelna]

Aluminum die castings are not typically heat treated due to potential surface blistering from internal porosity. So aluminum die casting alloys are optimized for use in the as-cast condition, the only exception being a stress relief to improve machinability. Regarding your concern over different internal porosity levels between the two die casting processes noted, are you sure the difference is an issue in your application? Porosity is primarily an issue where pressure tightness or fracture/fatigue are concerns.

 

[arunmrao]

Select an alloy suitable for ease of cast ability using any of the pressure decanting process. I am not sure about the competing processes influencing porosity.

You may also consider secondary process like HIP ( hot isostatic pressing) to improve structural density .

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[swall]

With traditional die casting (aka high pressure die casting) you are limited by alloy choice--typically high silicon hypereutectoid alloys like 380. As tbuelna stated, these are not heat treatable and you are stuck with "as-cast" properties. They also do not have great corrosion resistance. For an outdoor application, you probably want a corrosion resistant alloy like 535, which is castable by the permanent mold process. Low pressure die casting, also known as squeeze casting, is one of the processes in the same family as permanent mold casting. If you need higher strength and are willing to anodize the castings, something like permanent mold cast 356-T61 alloy would be a possible choice.

 

[Haz88]

Thanks tbuelna, arunmrao and swall.

The material our supplier has noted is ADC12, which i think is the 383. It is good to hear that the porosity might not be an issue for my application. However on another issue with the same parts i was intending on anodising them but a few test pieces i had anodised came back looking terrible, but that was my intention.

Again, Thank you for your help.

 

[SwinnyGG]

You're correct that ADC12 is approximately equivalent to 383 (SAE) or 384 (ASTM).

This is a modified alloy which flows better than 380.

Unmachined castings will almost never anodize well, and as a result will not gain a great deal of corrosion resistance from the coating process.

If you want good corrosion resistance you need a lower copper content (like 380 would provide) but the sacrifice will be more difficult mold fill on any intricate or thin features.

 

[Haz88]

Thanks jgKRI,
The old steal from peter to pay paul scenario.

Many thanks

 

[swall]

Haz88--yes, as you have discovered the anodize appearance of die cast alloys is poor--this is due to the high silicon content. My comment re anodize was directed at 356 alloy.
If surface treatment of die castings is needed, I would suggest an Alodine conversion coating with supplemental organic finish.

 

[WKTaylor]

Haz88... rhetorical question... sorta...

Are aluminum castings truly 'best' for 'outdoor parts subject to taking a beating'!? Casting quality combined with low strength might prove to shorten useable life of these parts... unless cheap/throw-away/recyclable is 'most important'.

The long slender object and case object vaguely sound like they could be made from a wrought alloy [6061-T6] in the form of a extrusion; and likewise the base-stand looking object. Sometimes, lightly trimming/cutting/forming an extrusion, in-lieu of a complexcasting, may help you design-out complicating features in-favor of simplifying features.

OR, perhaps a high-grade/density impact-toughened plastic molding would work better. I have plastic stakes for a pool cover that serve faithfully for ~6-months in the cold/and high winds... and take real pounding... and they were relatively dirt cheap and have served me for 5-years now. Also, what color do You want the plastic molded part?!

OH Well... just suggesting...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Haz88]

Thanks Swall and WKTaylor,

Your questions are valid WKTaylor, but due to external influences i have to maintain my current course. But I will use this as a reference if an improvement is needed.
Definitely not the way i would like to engineer, but sometimes its not my decision.
Cheers