Substitute material for al alloy 771 in T7 condition

[Nadimuthu]

Dear Friends,
We have an order for sand casting in 771 alloy in T71 condition(.Aluminium).But the mechanical properties could not be attained.
WE have been asked by our customer to suggest an alloy that will have the same mechanical properties.
THe castings will undergo cyclic loading .WE thought of alloy 204 T6 but the ASM handbook of Aluminum does not recommend this alloy in T6 condition for stress concentration areas.
Do cyclic loading and stress concentration mean the same thing?
Is it possible to go for 204 alloy and any other.
I look forward to your valuable suggestions and expert advice.
Nadimuthu.

 

[TVP]

First, there is a significant difference between cyclic loading and stress concentration. Cyclic loading means that the application of forces varies with time. This is the opposite of static loading. Most structural components in the transportation industries (automotive, aerospace, etc.) involve some amount of cyclic loading.

Stress concentration means that areas of a component have higher stresses than the nominal stress on the rest of the component. For example, if a solid cylindrical bar is stretched, a uniform stress is produced. If the solid cylindrical bar now has a circumferential groove machined into it, the nominal stress remains in the center of the bar, but the area around the groove has a higher stress (stress concentration).

There are several high strength alloys suitable for sand casting that can be considered. Alloys 201, 204, and 206 are all capable of being heat-treated to have similar properties to 771-T71. ASTM A 26 lists 201.0 T7 with minimum yield strength = 345 MPa, minimum tensile strength = 415 MPa, and minimum elongation of 3 %. Alloy 201 is also covered by SAE AMS 4228 and SAE AMS 4229, which are aerospace materials specifications. Alloy 206 is covered by SAE AMS 4235, SAE AMS 4236, and SAE AMS 4237. I could not find any ASTM or SAE AMS standards for 204, which appears to be a proprietary alloy from Pechiney.

I would investigate A T7 temper for alloys 201 and 206, which makes them less susceptible to stress corrosion cracking (SCC). This is the reason listed in the ASM reference for avoiding stress concentration with the T6 temper. You should investigate if alloy 204 responds to a T7 treatment in the same manner as 201 and 206. T7 consists of the following:

Solution treatment - 510 to 515 C for 2 hours, followed by 525 to 530 C for 14 to 20 hours, and then water quench (temperature to be 65 to 100 C).

Aging treatment - 185 to 190 C for 5 hours.

 

[Nadimuthu]

Dear TVP,
Thank you for your excellent and precise info (that I am always happy to read).
I shall look into customer approval for trials with 201 and keep you informed of the developments.
Nadimuthu.