Aluminum 6061-t6 metal shaping help

[trstek]

For an experimental aircraft application, is there an issue with using a plasma cutter on 6061-T6 Aluminum sheet?

The application would be structural parts. The parts are riveted together after shaping.

Looking at the option of scratch building vs buying the kit. (kit has laser cut parts)

Specifically, will the heat of the cut effect the metal in a bad way?

One practice recommended is stacking blanks together, then using a band saw to cut out parts. I was thinking plasma cutter with a pattern guide would be faster. Both processes would have edge clean up, not sure about the heat zone.

Any thoughts?

Thanks,
Tom

[Graham08]

Ordinarily I would say go for it, but being that this is a structural application on an airplane, I would use the bandsaw. I can't intelligently speak to how the HAZ compares between the plasma and laser, but I would assume the plasma to be larger, which could mean riveting through soft material.

[FM117]

It's going to soften the heat effected zone.
The question is how far from the edge?
If I was going to do this, I rig a "stack" of scrap
make the plasma cut then take them out to a shop
that has a hardness tester and have them check
how far into the sheet the soft area extends.
I've never been real happy cutting aluminum
with a plasma, maybe I'm just not very good
at it.......
Dave P.

[Aerometalworker]

For an experimental aircraft application, is there an issue with using a plasma cutter on 6061-T6 Aluminum sheet?

The application would be structural parts. The parts are riveted together after shaping.

Looking at the option of scratch building vs buying the kit. (kit has laser cut parts)

Specifically, will the heat of the cut effect the metal in a bad way?

One practice recommended is stacking blanks together, then using a band saw to cut out parts. I was thinking plasma cutter with a pattern guide would be faster. Both processes would have edge clean up, not sure about the heat zone.

Any thoughts?

Thanks,
Tom

Tom,
YES THERE IS AN ISSUE!!!! DONT DO IT!!!
The edge left by the plasma cutter is actually full of micro cracks that WILL propogate in. This is not a "might happen" this is a "WILL HAPPEN". Normal practice is to cut the edge back with snips 1/4" from the plasma cut.

*Note this is for aluminum

-Aaron

[Graham08]

Tom,
YES THERE IS AN ISSUE!!!! DONT DO IT!!!
The edge left by the plasma cutter is actually full of micro cracks that WILL propogate in. This is not a "might happen" this is a "WILL HAPPEN". Normal practice is to cut the edge back with snips 1/4" from the plasma cut.

-Aaron

Would laser cut parts have the same issue?

Is this a problem only with heat treatable aluminum, or is this an issue with steel parts as well?

I have witnessed cracking during the quench cycle while heat treating lasered steel parts which were prototypes made prior to having stamping dies. Every part in the batch of 50 or so had cracks visible in magnaflux inspection around holes that were lasered into the blanks.

My assumption was the rough edge from the laser made for stress risers which were crack initiation points during the quench. From what you're saying, the parts could have had micro cracks to begin with, and the quench process caused them to propogate quickly.

[Aerometalworker]

Would laser cut parts have the same issue?

Is this a problem only with heat treatable aluminum, or is this an issue with steel parts as well?

I have witnessed cracking during the quench cycle while heat treating lasered steel parts which were prototypes made prior to having stamping dies. Every part in the batch of 50 or so had cracks visible in magnaflux inspection around holes that were lasered into the blanks.

My assumption was the rough edge from the laser made for stress risers which were crack initiation points during the quench. From what you're saying, the parts could have had micro cracks to begin with, and the quench process caused them to propogate quickly.

The aluminum aerospace parts I have seen laser cut, have no issues when cut correctly. Plasma cuts are a problem with all aluminum, has nothing to do with heat treat. The cracks you saw in the steel may have been due to the rapid cooling rate of a laser cut causing embrittlement and/or shrinkage cracks depending on the steel alloy.
-Aaron

[Graham08]

Just to clarify, the steel parts were not cracked prior to heat treatment. Sounds like my original theory is probably right (stress risers). The problem went away when the holes were punched and coined during the blanking process.

Interesting. I will keep using the jigsaw, snips, etc. on all aluminum.

[trstek]

Thanks Guys,

Thats why I asked.
Tom

[bjw4076]

Why 6061-t6

are you working with t6 condition? Is this sheet stock? Are u bending it? If so, remember min bend rad of t6 is 3X thickness.

Yes there is a haz from plasma cutting(PAC).

"The appearance of the aluminum cut edge also varies with PAC process and gas selection. The surface condition of aluminum edges frequently is characterized by relative roughness, thin oxide layers, intergranular cracks, and porosity near the cut edge surface.

The HAZ of aluminum alloy cut specimens analyzed has both a resolidified layer, as well as a solid-state phase transformation. However, the phase structure of the aluminum alloy often is only faintly visible in the metallographic micrographs.



The extent of the HAZ (including both the resolidified and phase-transformed layers) generally is indicated by the presence of grain boundary silicide precipitates. The microstructure of cut specimens prepared with CDG air plasma and CDG argon/hydrogen plasma is qualitatively similar.

Because of the high thermal conductivity of aluminum alloys, the overall thickness of the HAZ may be significant. The resolidified layer often is as thick as the region defined by the phase transformation. The thickness of the HAZ is related to process conditions, such as cut speed and process gas, as well as material thickness. The width of the HAZ is greater for thicker material that is cut at lower speed.

Microhardness measurements of aluminum alloy 6061 near the cut edge and in the cut sample core region indicate that hardness is decreased significantly in the HAZ. The thermal cycle of heating and cooling that occurs during the PAC process essentially is an annealing process. This thermal cycle degrades the heat treat of the metal (usually T6) and returns it to an annealed state."

[FM117]

Tom,
Are these going to be blanks for ribs?
What are you building?
You know, depending on thickness a
regular wood router with a carbide bit
running around a pattern will just ZIP
through aluminum.
Dave P.

[trstek]

At the front end of this process, Purchased the plans this last weekend. It is a Sonex.

link to the web page - http://www.sonexaircraft.com/

Doing the homework to decide if I want to buy the "kit" or do a scratch build. The "parts" would be everything except the main spars if I go scratch build. It is an all aluminum airplane. About 10k rivets. Lots of drilling and riveting.

Meeting with a guy on Monday who has a metal fab shop to review the plans and see what he thinks. That will weigh heavy into my decision. If I can gain access at night to brakes and other time saving machines I am leaning towards scratch. If not, kit all the way. I just don't have the facilities to make the parts in a timely manor and the kit is a known quantity, just more money.

I would like to do much of the work here at the house, and have a plasma cutter, ie the question.

All the parts are sheet aluminum with some channel and plate. Essentially all light metal.

The plane has been built by others with snips and a band saw and a few other low cost tools. Just looking for ways to speed up the process, (and use what I already own).

Dave, A router is not something I had thought of, thanks for the tip. What bits do you use? Run it full speed, any lube?

I was also told turn the blade backwards in a table saw, and be careful of the chips.

If I am understanding this process, the quality in the parts is in the deburing and taking out the stress risers.

Thanks for the help, (sorry for the book)
Tom

[FusionKing]

Don't do the blade backwards in a tablesaw....or in any saw. Just get the correct blade. People who say that just don't know
And for that matter don't use the tablesaw to cut metal. That is just a little to far out there unless you really know your stuff and then you should know better

[trstek]

I was wondering about the backwards blade advice...

There is a program for members of the EAA, "Tech counselors" , before I start fabricating will get involved in that. Basicly, someone looks over my shoulder at every step to make sure I am doing it right. Someone local so they can stop by and inspect. (if I understand how that works)

This by all accounts will be a 2 - 3 year adventure for me, if life cooperates.

Appreciate all the knowledge, and help put forth here.
Tom

[Laiky]

I am envious! I have always wanted to build my own airplane. When i was in high school (Aviation High School!!) they had a BD5 (if memory serves) and i looked at it every day wondering what it would be like to have one. It's like the ultimate form of Hot Rodding, you build it as an experimental and then fly it any way you want. Someday!

Please keep us updated with planty of pics!

[FM117]

Tom,
I just use regular carbide bits, smallest dia. I can find. My router
only has one speed, not sure of it's rpm. I run it dry with a scrap
piece of plywood under the aluminum sheet. You need to be careful
of scratching the aluminum. Some people report using some of
the wax type "stick lubes", I've never tried then with the
router but they seem to help on carbide burs in the mini-grinder.
I have not really kept up on homebuilts in the last 10 years or so,
but I think there's something to be said for "kits" unless you really
really.....really enjoy making all the parts.
Never heard of mounting sawblades backwards.
Hope your neighbors like the sound of rivet bucking!
Dave P