Hydroforming Material Help Needed

[nickjk]

I am currently working on a project that involves hydro forming an aluminum tube and need help on the recommended grade of aluminum.

The tube I.D. is approx 1" with a wall thickness of .020".
The minimum hydro forming radius is .078".
Calculated elongation during forming is approx. 4.8%
Ratio of min. radius to wall thickness is 3.85:1

The max. hoop stress during cycling is approx. 22,000 psi.

My first attempt used 6061 T651 which hydro formed wonderfully but fractured after approx. 30,000 cycles. I believe it to be caused by a low fatigue strength 14,000 psi. Note: I have lowered the max hoop stress since then. I believe hoop stress was around 30,000 psi.

My second attempt used 7075 T651 which would not hydro form properly. I needed a pressure of approx. 20,000 psi to form the tube, but the tube would fracture at 5,000 psi.
I thought because I was only elongating the material 4.8% and because 7075 T651 spec's call for elongation of 11% that I would be able to use the material. I was so wrong.
7075 T651 is not a very formable material in the hardened state and I do not believe I could heat treat later because of the wall thickness, it would distort.

I wanted to try 5086 H32, but am scared because of the Magnesium content 3.5% that the material could stress corrode if held above 150F. Since used in a hydraulic application I am very concerned with the temp.

I am looking at 5052 H34. I will be giving up a little in yield and ultimate strength compared to the 6065 T6 but will gain in the fatigue strength. Elongate at break spec is 14%.

I have no idea how well 5052 H34 will hydro form or if there is a better aluminum suggested.

I have been searching for quite awhile.

Thanks in advance for any support.

Nickjk

 

[MikeHalloran]

Excuse me, an aluminum tube in a hydraulic system?

Sounds like a bad idea to me, because hydraulic systems normally have a lot of pressure cycles associated with the pump pistons or vanes. Not to mention the consequences of a tube failure.

Car guys use aluminum tubes in the A/C system, but the pressures are lower than in a typical hydraulic system, and they use relatively thicker walls than you are proposing. I have no idea what alloy they use.





Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

nickjk,

5052 aluminum forms very nicely in the annealed (O) condition. And your hydroforming operation would likely work harden it to a higher strength level.

However, if you start with .020" 5052 in an H34 condition, an elongation rate of 4.8% during forming is probably a bit too high. MIL-HDBK-5 gives a min. elongation rate of 4% for .020" thick 5052 H34 sheet.

Assuming your part's working stress is 13ksi or less (including any knockdowns for Kt or R), then 5052 H34 is probably OK for 3x10^4 fatigue cycles.

Hope that helps.
Terry

 

[nickjk]

Terry your response has been very helpful.

I am looking for 1,000,000 fatigue cycles and I believe my maximum working stress is closer to 20ksi.

I have been using data from Matweb and noticed that the elongation values are different from MIL-HDBK-5H.
I like that MIL-HDBK-5H specifies elongation in closer material thickness ranges.

Matweb is also showing a Fatigue Strength of 18ksi 5x10^8 cycles for 5052-H34. I am not sure and may be all wet but was wondering if there could be an increase in Fatigue Strength from the cold work of hydroforming?

Do you think this material may be worth a try?

Again, Thanks for your help.

Nick

 

[tbuelna]

nickjk,

Establishing accurate fatigue data is difficult, since it is a statistical value. There are many variables involved to boot.

MIL-HDBK-5 does not give any fatigue (S-N) data for 5052 aluminum. It does give an Ftu/Fty of 34/26 ksi in the longitudinal for cond. H34 sheet (AMS 4017). But 77% of yield (20ksi)is probably too high for 10^6 fatigue cycles, even without load reversals.

The hydroforming operation will naturally increase the tensile strength of 5052, since it is a strain hardening alloy. But this effect may not be uniform over the entire part. And also remember that the hydroforming may also cause some local thinning.

Good luck.
Terry