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Why is turbine Blade made by casting?

Why is turbine Blade made by casting?

(OP)
Hi All,
I was reading somewhere mentioned most turbine blade is made by casting? Made me thinking, why not forged or machined from block?
I won't think cost is the reason for such high end product. My limited understanding is that investment casting generally is not as good as forged product. Could some expert share some thoughts?
Cheers.

RE: Why is turbine Blade made by casting?

I imagine it gives you a better chance of retaining the single-crystal properties you worked so hard to create in the first place.

A.

RE: Why is turbine Blade made by casting?

I've seen two flavors:
Some larger blades or "buckets" (such as for steam turbines) will have internal cooling channels, casting is the preferred manufacturing process.
Smaller blades for high performance jet engines will be forged from solid blanks.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: Why is turbine Blade made by casting?

Having watched the time it takes to forge a turbine blade from nickel alloy at the Henry Wiggins plant in Hereford UK , I would think Casting if applicable would be the far quicker process.
B.E.

You are judged not by what you know, but by what you can do.

RE: Why is turbine Blade made by casting?

Spoonful-

The HP turbine blades and nozzle guide vanes of modern gas turbine engines use very sophisticated internal air cooling and external surface air film cooling to maintain safe operating temperatures. There are serpentine (multi-pass) air cooling passages inside the blades/guide vanes, and also internal passages for supplying air to the film cooling orifices. Currently, the most practical production method for making this type of structure is investment casting.



RE: Why is turbine Blade made by casting?

2
Disclaimer: I have no professional experience in this area. I'm just recalling a factoid I learned in a materials science class...a while ago.

One factor is the desire to have a blade end up as one single crystal. No grain boundaries.

To make this happen, some clever sacrificial geometry is added to the casting: a "pigtail". As I understand it, the material solidifies and multiple crystals begin to propagate through the pigtail. After some distance, though, only one crystal survives and propagates through the actual part.

RE: Why is turbine Blade made by casting?

While I agree that Investment Casting is the most preferred route. There are now several developments using 3D printing of these blades, and have undergone extensive trials by GE, Siemens etc. Exciting times for 3D printing .


https://www.siemens.com/innovation/en/home/picture...

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

Mahatma Gandhi.

RE: Why is turbine Blade made by casting?

There is a company repairing turbine blades by AM. They are rebuilding about 0.050-0.090" on the tips of the blades that was lost over time to seal wear.
They are controlling heat input and solidification rates so that hey can grow new tips with the original single crystal orientation.
The biggest problem with AM today is that you don't what microstructure/properties you will have until you make a part and then destructively test it. This is a big deal in alloys that are not cubic structure (such as Ti) because you different strength, fatigue, modulus, and even CTE in different orientations within the same part. That, and the fatigue and impact properties are very poor.
This isn't a big deal if you are taking a part that is over designed by 50x and reducing that to 20x. You still cut the weight by 60% and you still have good safety margins.
Once people develop closed loop control systems that allow prediction and control of structure and properties then they will have a breakthrough.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Why is turbine Blade made by casting?

Enjoy the read. Royce's is not far from where I live.

Link

A little side note: The car division is colloquially called Rolls and the aero division Royce's. They are actually completely separate companies.

RE: Why is turbine Blade made by casting?

As EdStainless noted, there are ALM processes (like SLE) suitable for repairing eroded surfaces of single-crystal superalloy HP turbine blades/nozzle vanes. The SLE process builds up a nickel superalloy surface layer with a micro-structure similar to the base material. However, while ALM processes are satisfactory for metal surface repairs, the creep properties are not yet good enough to replace single-crystal directionally solidified investment castings.

There has been quite a bit of development work done recently by GE and other engine OEMs using additive manufacturing with CMC materials for producing turbine engine components. CMC is light, strong, and can tolerate temperatures 300-400 degF higher than nickel superalloys. But the CMC materials produced by ALM are not quite ready for production manufacture of critical engine components like HP turbine blades/nozzle vanes. Some other less critical engine components like combustor panels and nozzle shrouds are now being made from CMC material, but they are manufactured using a conventional process.

RE: Why is turbine Blade made by casting?

Ed ,I agree with your observation, that little is known or discussed about structure/property correlation. Most of the discussions relate to accuracy surface finish, a few about structural integrity. I have attempted to draw attention but maybe they want to keep it a close secret.

My own work relates to development of patterns and core boxes in plastics, using 3D printing and now developing cores and moulds.

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

Mahatma Gandhi.

RE: Why is turbine Blade made by casting?

Another missing link to AM is that the alloys used are optimized for casting, at some point people will need to develop new alloys that are actually developed for one of the AM processes. However I am sure that different laser types (not to mention Ebeam) would each have different optimum alloys. Imagine trying qualify those parts. The total lack of standardization is great for the development phase but at some point AM will need standardization. Right now if you develop a part on one machine and then want to make on a different one (even just a larger one from the same supplier) you have to reformulate your process.
One more issue with AM is the lack of process tracking as it relates to internal part uniformity and NDT. If the local temp, weld pool size, brightness, or about 10 other parameters change you should know exactly where in the volume that occurred. NDT methods such as Xray will find voids, but what if the defect is a local area with a different grain size?
The CMCs are great for static parts in the hot gas path (nozzles and first stage stationary vanes).
Most people don't realize that current cast metallic blades have two or three layers of coating on them with cermets and ceramics used extensively.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

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