Laser Cutting Aluminum Parts 4

[SAITAETGrad]

Pardon my ignorance. How do you get to .605 and .613 compliance with laser cut holes on aluminum parts? Is there any commercial or military process specifications that others use?

My application is strictly static strength with 6000 and 7000 series aluminum. The capabilities of a local manufacturer are too good to ignore. One option would be to finish drill the holes conventionally but it seems a real shame. My mind's eye says that the HAZ can't be much more than .005. Of course not all laser cutters are created equal. We'd go to another process to avoid any testing.

Is there a dragon to worry about here? What references will cure my ignorance?

All material removal processes can result in HAZ if poorly controlled. Feel free to widen the discussion beyond laser cutting and beyond static strength structures.

 

[berkshire]

I have always used routing or waterjetting to avoid the HAZ . As you say you can use this for pilot holes and clean out upon assembly, but what about the perimeter of the sheet, you are going to need more than a deburring tool there.
B.E.

You are judged not by what you know, but by what you can do.

 

[SAITAETGrad]

Thanks Berkshire.

The production machine is one of these:

Link

 

[rb1957]

I guess you could characterise the laser cutting yourself, with a bunch of tests and a controlled procedure.

another day in paradise, or is paradise one day closer ?

 

[berkshire]

SAITAETGrad,
You are still going to have to deal with the HAZ no matter how small it is , or demonstrate that it will not affect the corrosion and strength properties of the metal you are using. I ran into a similar problem some years ago , when a company wanted to use a plasma torch to cut out aluminum aircraft parts . The only thing that is different here is that the HAZ from the lazer is much smaller.
We gave up on the idea, even though we were heat treating the metal after processing.
B.E.

You are judged not by what you know, but by what you can do.

 

[WKTaylor]

I have always treated laser cutting... and to a slightly lesser-degree water-jet cutting... of sheet metal parts similar to blanking/shearing shapes and punching-holes.

The residual edges and holes are guides: They must be trimmed-back or drill/reamer-enlarged ~0.020[+] to ensure rough deformed [and HAZ or rough-abraded] metal is 100% removed and is what remains is properly defect and residual-stress free [smooth/deburred] along every edge... or will be 'made-so' after final drilling/reaming/fitting.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
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[MikeHalloran]

The customer rejected our first attempt at making a large complicated '6Mo' (stainless with extra Nickel and Molybdenum) tube assembly because the welds looked like crap. They did. When asked, the welder said the laser cut edges 'welded funny'. We requested that he stop welding if that happened again, rather than wasting a couple thousand dollars worth of material and making an unsatisfactory ten thousand dollar product.

Under magnification, the laser kerf looked like a series of ~.005" diameter drilled holes, probably an artifact of pulsing the laser, and the blued HAZ extended maybe another .010" into the bulk material.

So we strongly suggested that our fabricators should snag grind the laser cut edges before fitting, which caused some irregularity in fitups because some of the fabricators leaned harder on the grinder.

So we went to waterjet cutting, which got us precise edges with no HAZ.
... but we had to wash the incoming parts, because the parts came in covered in fine white sand, as was the entire waterjet facility.

I'd expect similar adventures for aluminum.


Mike Halloran
Pembroke Pines, FL, USA

 

[Sparweb]

I much prefer to use router tables.
I have used water jet and laser cutting services in the past, but there were often problems.
It all just comes down to process control - what works on one thickness doesn't work on a thicker piece (tapered kerf). The effect on 6061 is not the same on 7075. Some shops will not clean up the table between a steel part and an aluminum part. Have they got the machine set up correctly? And so on...

This stuff is always a hassle with outside vendors, especially ones that aren't accustomed to complying with process controls of any sort.
You could send them 2 orders, 2 months apart, for the same part drawings, the same quantities, and supply stock material from your own stores, and still get back widely different parts.

Drilling on-size through the waterjet/laser cut pilot holes in aluminum isn't too bad. Re-drilling holes in steel is awful. You will throw away a lot of drill bits. Positional accuracy of the pilots is as good as a router table. I was satisfied that drilling a 1/16" pilot hole up to 1/8" for riveting was removing all evidence of HAZ. Unfortunately, re-drilling the pilot introduces some error in the final hole location unless you are very careful. As you say, this brings the process very close to the threshold of pointlessness.
There's often a faint spatter around all edges and holes, so that's something else needing removal.
Paint adhesion to the WJ/laser cut edges is usually terrible if they aren't cleaned up on a grinder/belt sander first. Yes, grinding the whole edge introduces its own heat problem.

Mike, go easy on your welders. You probably aren't the welder's direct supervisor (though I suspect you may be HIS boss).
Anyway it's that boss who sets the tone for whether the guys in the shop can speak up about quality or not.
The guy knew he had a problem but was either told to proceed, or expected it would look bad on him if he spoke up.
(Setting aside any possibility that anybody was covering their a** at the time, of course).

STF

 

[tbuelna]

The FAR 23.605 and 23.613 sections noted are requirements for controlled and validated manufacturing processes. This is to ensure the material properties in your finished product meet certain requirements. With the 6000 and 7000 series extruded aluminum tube used in your example that is initially trimmed by a multi-axis laser, you would need to machine off the HAZ surfaces to ensure the remaining material has properties conforming to those used in your certification analysis.

Probably easier to machine the part using conventional CNC milling rather than laser cutting it and then CNC milling to remove the HAZ areas.