Entertaining result apparently from peening action on aluminum surface

[Guest]

I have been cleaning up metal parts in preparation to recover my Aeronca.

I glass blasted the painted side of some inspection covers and marveled at their condition. They were a wavy as a potato chip!

Soon I noticed that they were quite flat before I blasted them and the waves appeared afterward.

This may seem odd that it took a while to notice but working in a blast cabinet with dust flying and light being obscured make it more tenuous.

So I then began to blast the unpainted side and sure enough they snapped back into shape.

Pictures are attached.

Apparently the glass peens the surface and adds a compression stress that makes them become wavy until you blast the other side and "equalize" it.

I also think that I noticed that if I blasted the painted perimeter first the waviness was less.

[Jim Hartley]

Re: Entertaining result apparently from peening action on aluminum surface

Thanks Dave! I learn something new every day.

[N96337]

Re: Entertaining result apparently from peening action on aluminum surface

The guy that taught me to "knock metal" always told me the only thing you can do to metal is stretch it...even the "shrinking" is done by stretching.... it took me a long time to wrap my head around that, but he's right.
Did you experiment with lower pressures, Dave?
John

[astjp2]

Re: Entertaining result apparently from peening action on aluminum surface

Chemical strip is the only way to go, blasting will only cause the metal to crack because of the work hardening it causes. Tim

[Robert Lees]

Re: Entertaining result apparently from peening action on aluminum surface

Soda blasting works well on uber-thin ali as well, but introduces it's own problems of too much alkalinity which needs addressing before painting.

Like Tim, I chemically stripped mine, and also de-riveted the spring steel clips to get rid of those pesky rust-forming steel rivets.

Rob

[Guest]

Re: Entertaining result apparently from peening action on aluminum surface

Chemical strip is the only way to go, blasting will only cause the metal to crack because of the work hardening it causes. Tim

Tim,

Do you think an inspection cover is going to crack?

Dave

[Guest]

Re: Entertaining result apparently from peening action on aluminum surface

The guy that taught me to "knock metal" always told me the only thing you can do to metal is stretch it...even the "shrinking" is done by stretching.... it took me a long time to wrap my head around that, but he's right.
Did you experiment with lower pressures, Dave?
John

No I did not. I think I have it set for about 65 psi.

I may try lower psi later. I think that I have some of those kind of covers to test the lower pressure.

[PA1195]

Re: Entertaining result apparently from peening action on aluminum surface

Somewhat off topic, but we can add an additional rivet to each spring element leg (like original Piper/Univair does and then charge ~$35/cover) so the cover's two added rivets fit just inside the inspection hole. That way they won't slide and blow off in the prop slipstream as easily.

Example: http://www.univair.com/piper/piper-j...on-hole-plate/

gary

[astjp2]

Re: Entertaining result apparently from peening action on aluminum surface

They might around the rivet hole if its in the prop wash area. Blasting causes them to also lose their springback. The aerobatic ones use a different stamping, some also have 2 pairs of clips instead of one. The extra rivets also were used. I don't think any inspector will ever know the difference if they are modified though. Tim

Tim,

Do you think an inspection cover is going to crack?

Dave

[3Dreaming]

Re: Entertaining result apparently from peening action on aluminum surface

The old style vented covers are the best for not moving around. There was a fellow making them for the Waco folks, but he was using a thicker piece of aluminum. (.020 I think)

[Robert Lees]

Re: Entertaining result apparently from peening action on aluminum surface

I secure them with PK screws through the inspection ring and through the spring steel clip (like some aerobatic planes do).

Incidentally, the spring clip should be installed transverse to the airflow (and not in line with it) so that if slippage occurs, you won't lose the cover.

[Mark Bowden]

Re: Entertaining result apparently from peening action on aluminum surface

Hi, what you have observed is a commercial process, "Peen Forming", that is some 50-60 years old. It is used to form the wing skins of a really, really fast business jet after they have been machined to final thickness which tapers from wing root to the tip. The process uses a combination of metal shot size and intensity of impact on the surface to-be-formed. Think CNC sand blaster gun (like you demonstrated) or pellet-type lawn fertilizer spreader.

It can be used to form shapes, improve material properties, apply decorative finishes and tailor the insides of tubes for better flow and material movement. It is generally a CNC process but is some cases done by hand. The compressive stresses on the material surfaces improve fatigue performance and reduce crack initiation and propagation.

The more recent developments are the conversion to lasers in lieu of metal shot. The shock wave of a laser pulse impacts the surface creating the same effect, but it is more controllable and has deeper effects into the thickness of the part. However it is less exciting without the 10's of thousands of tiny shot balls flying all over the place. Metal or laser shot can be applied to any kind of metal part including gears and other machined components to improve wear and corrosion performance. It can also rework damaged parts.

Metal Improvement is one of the more prominent shops providing this service.

[Guest]

Re: Entertaining result apparently from peening action on aluminum surface

Good post Mark, thanks.

You made me I recall that peening is stress relief method for engine parts.

Dave