Aluminum fan blade failures 1

[ruble3]

Just looking for general comments on a problem we are having with aluminum cast fan blades. Without getting into a lot of fan details, we've had 4 recent failures ( catastrophic) of the blades. Visual inspection of the blades looks like there are large cavities or pockets in the casting ( predominently where the blade meets the stub end where it is held on to the hub). Should the casting, irregardless of the type (grade) of aluminum be 'bubble free? or are bubbles inherent in casting aluminum? Would different grades be more prone to 'cavities' or bubbles? (The seller is only a supplier, who gets the blades from a foundry )

Thanks

 

[CoryPad]

Castings inherently contain porosity, which lead to diminished mechanical properties (as you have learned). Alloy type and part design/geometry have large influences on porosity level. Porosity can be minimized/eliminated by using vacuum casting techniques, or by using Hot Isostatic Pressing (HIP) after casting.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EngineerDave]

I have worked with a company on this very issue (perhaps it's even yours, but I won't name names of course). There is a whole testing plan that is recommended. If you are interested in details on it, please give me your e-mail address.

As Cory stated, porosity is to be expected. It can be rated by radiographic analysis and metallurgical cross section.

 

[arunmrao]

Normally bubble term is used for gas entrapment. Is this a case of gas porosity or shrinkage porosity. There are methods to detect and avoid such defects in a casting. are these sand cast or die cast?

 

[stanweld]

Assuming that you are the end user of these fans, you may wish to specify a radiographic quality level for the involved castings or specify forgings to be used in the fans' fabrication from now-on. For fans in use that you suspect may also have this problem, you should periodically liquid penetrant inspect based on service life to date. Repair or replace as warranted.

You should also involve the fan manufacturer in determining the root cause of failure. Is it materials related, design related or operating related? Are you operating in a harmonic regime - in a corrosive atmosphere? Did the manufacturer receive substandard material?

 

[ruble3]

Thanks for the replies- the vendor is working with the manufacturer to rectify the problem - just for additional info, one airfoil ( blade) is 6-1/4" long at base ;to attach it to the fan hub, there is an inverted bell shape cone approx 3-1/4" wide at the base of blade, then 1 3/4" fine thread stud to attach the blade to the hub- where the 'bell' meets the blade there is an internal cavity about 1-3/4" long ( along the 3-1/4" dimension noted above)x1/4" wide x 1/4" deep- seems like this is an awfully large 'cavity'? - previous failures show similar cavities in castings - I would think any gas entrapped would show as very tiny 'bubbles'? -I'll try & get a picture posted so its clearer

 

[arunmrao]

From what you describe it is a case of air entrapment. this can be due to poor gating, insufficient vents or slow pouring speeds. Is the cavity wall smooth and shining if so it indicates air entrapment. In thin wall castings this is a common defect observed and can be avoided .

 

[ruble3]

I took another look at the blade- there is actually a pinhole through the side of the cavity right through the casting! the cavity is fairly rough inside ( not smooth and shiny)& bright

 

[foundrymaninchina]

I think the casting's chemical composition is not fit to the specification:mil-21180a.and the heat treat is not do ,or is not good.The porosity in the casting means that the casting process is also not good.
You can tell your supplier about strength,enlogation,hardness.I think a experience foundry engineer can help you.And can tell you proper casting method.

 

[ruble3]

Apparently we are the only ones having problems- we will be getting all blades tested - is best way ultrasonically? I was going to post a picture but not sure how its done?

 

[unclesyd]

Radiography, actually x-rays, is the best method to look for porosity. It very quick and relatively cheap and you have a picture for show and tell. We went through a similar experience with large Cast Aluminum fan blades failing catastrophically. These blades were mounted to a hub with a cast-in 1 ½" fine thread bolt(grade 5). The bolt was about 5" into the blade (approximately 6' long). The failure had initiated at the bolt head, there was considerable porosity in the inboard end of the failed blades. We radiographed and fluorescent PT the remaining blades on the failed fan and four were removed for a to total of 6. NDT was performed on 2 other identical fans 3 blades were removed from one fan and none from the oldest which had no unacceptable porosity. On the oldest fan some loose blades were found. MT inspection of the embedded studs revealed 2 bolts were beginning to fail from fatigue, they were replaced. Nothing had been noted by anyone about vibration while running. At the time the fans had slipped trough the cracks, they were not on a reliability check sheet. The replacement hub and blades for the one that failed had through bolting on the flange to blade connection.