Can an A356 T6 engine housing be run at 350 degrees? 3

[bludog]

My rotary engine housings were cast out of Aluminum A356 and a T6 heat treatment was performed on them (solution heat treated to 1000 deg (Fahrenheit) for 12 hours, quenched in water at 175 deg, precipitation harden at 310 for 2-4 hours). The part was supposed to have a T7 heat treatment (precipitation harden at 475 instead of 310).

Do you see any harm if I ran the engine and the parts reached 350 degrees (even though they were heat treated to only 310)?

Would we run into any problems if we age the parts at the elevated T7 temp (475 deg) even though they have already been precipitation harden at 310 deg?

 

[kenvlach]

Have the parts been machined yet?

The T7 (400^oF) and T71 (475^oF) heat treatments for A356 relieve quenching stresses to provide dimensional stability. The T6 treatment gives higher strength but only relieves 10-35% of the residual stresses. At 350^oF, the UTS of A356-T6 is only about 16 ksi (cf. 41 ksi at 75^oF), so an applied load plus the residual stress could conceivably cause warping. But, if uniformly bolted to something cool, strong and rigid, stresses could relieve in a desired pattern. How complex is the part, and are there points where mechanical binding might occur?

If you have precision clearances, get the extra stress relief. The usual T71 time is 2-4 hours. Since the part's already been heated some, do only 2 hours. If machining much metal, rough machine prior to the T71 treatment, and final machine afterwards.

Can the part actually reach 350^oF, or is that just a combustion surface temperature?

 

[swall]

I agree with kenvlach--go ahead and perform the T7 age, since you apparently had considered T7 mechanical properties to be satisfactory for the application.

 

[EdStainless]

Make sure that you do the re-aging before your final machining. You may get some stress relief related distortion, but doing htis now will save you pain later.

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[bludog]

Thanks for the help guys.

The parts have not been finished machined yet.

The parts are the housings of a small prototype 150cc rotory engine and are fairly complex is structure. They will be uniformly bolted and water will run through the cooling passages. There are no evident places that mechanical binding should occur. Areas of the part will reach 350 F and the combustion surface temp is much greater.

How do I find out the maximum operating temperatures of A356-T6 and A356-T71? No references seem to have this information.

Thanks again,
Justin Blumetti

 

[kenvlach]

"How do I find out the maximum operating temperatures of A356-T6 and A356-T71? No references seem to have this information."

Creepy subject. Maximum operating temperatures depend upon applied load and time at temperature. Plus, excess themal expansion and corrosion or oxidation may occur.

The thermal expansion coefficient (same as for 356.0) is given to 600^oF in a figure on page 3-505 of MIL-HDBK-5J. Section 3.9.5 gives a lot of room temperature data for A356.0-T6, but nothing at higher T (except heat capacity at 212^oF).

At elevated temperatures, T6 overages to T7, anyhow.
Some minimum 0.2% yield stresses for 10,000 hour testing:
17 ksi at 300^oF for both T6 & T7 initial tempers.
8.5 400
5 500
3 600
2 700

Cf. 30 ksi for T6 & 24 ksi for T71 at 75^oF.

For the elastic (Young's) modulus at temperature, multiply the 75^oF value by the following:
98% at 212^oF
95% 300
90% 400
80% 500
(for all Al alloys. from page 50, MIL-HDBK-694A).