Strength Reduction / Calculation of Cast Parts 1

[Derek57]

Good morning,

I am working on a cost reduction project, and the idea came up of casting a fairly complex component that is currently CnC'd. Of course, the strength and fatigue characteristics of the component if it is cast will be different, but I am not sure how to calculate them. Is there a generic "knock-down" factor / rule of thumb that can be used? The material is currently 416 SS, but another grade of Stainless Steel, Carbon Steel, or even Aluminum would be acceptable.

Thank you very much

 

[tbuelna]

Derek57-

Here is a link to FAR part 29.621 that gives an example of the casting factors required for various rotorcraft structural applications.

If fatigue life is a concern with your casting, things become quite a bit more complicated. The min mechanical properties you can expect with your castings is greatly influenced by the foundry process used, heat treatment, mechanical processing such as HIP, etc. FAR 29.621 gives guidance on standards for NDI/QA when qualifying the manufacturing process for a particular aircraft casting design. MIL-STD-2175 is also an excellent reference for casting quality standards.

Lastly, when switching from a 100% machined part to a part made from a casting, be sure to take into account the greater variation in cast feature size/location, cast wall thickness, draft, etc. existing in the casting when you perform your analysis. Your analysis should be based on the min material conditions (LMC) that may potentially exist in your casting from the combined effects of things like pattern/core shift.

Hope that helps.
Terry

 

[Tmoose]

Just like Tbuelna predics, we had ongoing issues with cast material properties and casting quality, apparently for decades.
Some quality problems were brought on by our required geometry that could not meet some basic casting design guidelines for transitions of thickness, radiuses and fillets, and many etc.
Some were normal variations in cast material properties, and some were variations resulting from sloppy practices by foundries. Be especially fearful of Foundry personnel changes since sometimes the only old lady that knows the recipe and technique moves on.

Presumably the existing part functions OK in terms of stiffness, deflection, fatigue life etc.
I think I'd need to know the stress levels in the existing part in some detail to know how much reduction in properties and risk of porosity etc I can stand. If the part is wildly overdesigned then there will be no problem as long as the geometry can successfully be cast, and not all can.

If there are areas that are too weak or flexible then it is often relatively easy to modify the pattern to add ( or sometime remove!!) material as needed.
Careful use of solid modeling and basic FEA would go a long way to getting the casting right structurally so the foundry can offer advice how to make it cast-able before the first pattern is made.

Lincoln electric has some bargain priced books.
This one has some details about converting castings to weldments considering among other things differences in material properties.

http://www.jflfoundation.com/ProductDetails.asp?ProductCode=DW