Aluminium laser cutting - HAZ 1

[Lisa_247]

Hi all,

Does anyone have any information on heat affected zones for laser cutting of aluminium? Ive not been able to get any data off the company doing the cutting, therefore im going to have to make some conservative assumptions on the HAZ for the stress analysis. Ive never worked with laser cut parts before - is there any kind of conservative rule of thumb that can be assumed for the HAZ? Thanks.

 

[SWComposites]

Googling the topic brings up lots of info.

 

[rb1957]

who uses lasers to cut Al ? I thought these days it was all water jet ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

I think this question arises from the following very serious situation with Vans Aircraft ... designer/manufacture of homebuilt Acft ... via Kits.

https://www.vansaircraft.com/2023/10/business-announcement-from-vans-founder-dick-vangrunsven/

They outsourced kits to a vendor which LASER-cut holes to-size... and compounding the problem... the LASER starter-entry hole was placed at the edge of the nominal Dia hole... then the LASER 'drew' the correct size hole with the LASER... Soooooo each Nominal Dia hole has a small semicircular NOTCH at the edge.

It is unbelievable that the LASER starter hole did not get punched on the inside then disappear as the round hole was cut to Nom Dia. It is also unbelievable that these holes were not all 'pilot/undersized' so they could be line-drilled to size to clean/round-out the ragged holes for riveting. Problem with laser and water jet is the micro-ragged-edge from the cutting processes... VS a 'normal/nominal' round-smooth hole created by a drill-bit.

These holes are inserted into the sheet metal for 'fast-build match-hole tooled kits' that have nominal holes precisely placed for rapid Assy. Cleco-Cleco-Cleco-Cleco... etc to rapidly self-tool/build the sheet metal Assy.... then rivet-rivet-rivet-rivet... etc.

Also, VANS engineering/working specs NEVER covered this particular hole LASER-drilling practice to nominal size... nor appears to have intended the starter notch at the hole edge. LOTS of NON-STANDARD practices at play here. LOTS of unhappy kit builders.

SADLY I know too much about match-hole-tooling Assy method... John Thorp originated this in the 1960s for the Thorp T18... which I learned-about as I 'helped' my dad build his aircraft N455DT 1967-to-1972. AND I also knew about this issue with VANs [vendor] non-standard drilling-practices some months ago.

Apparently the shithashitthefan, now. SAD.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[SWComposites]

Wil - WOW, what a mess. 1800 customers affected. Is Vans going to survive?

 

[RoarkS]

SWC...Put it this way... I WAS building an RV8. I bought a Mooney. They don't even have a QA program...when I asked during the interview they were confused and handed me the build instruction book.

 

[Lisa_247]

Of course i have tried Googling the topic, i just cant find any quantatitive data and neither does the customer im working for. The Vans story is interesting, but doesnt answer my question. Thanks anyway!

 

[WKTaylor]

L247... theeeeeeen... can you expand on what you are looking for??????

Defects... tears, cracks, corrosion, scratches/gouges, work-hardening, etc... typically OCCUR/START at edges or 'vulnerable' surfaces. Focusing-on edges only: ragged edges are great crack starters and are prone to corrosion, regardless of how the edges are created, IE: saw-cut, machined, water-jet, LASER, etc.

My experience... and my company's fab processes... generally are in sync. All surfaces and wide edges shall be made smooth of irregularities... and all sharp corners shall be rounded-off... and all sharp edges shall made free of burrs [chamfering/rounding-off] for a host of reasons.

This 'mechanical process' removes the ragged/sharp/burred/tramp features on-edges [and surfaces, too] which are impossible to apply any 'CONTINUOUS/UNIFORM' corrosion protective treatments on. In doing this 'mechanical smoothing treatment', all-over, this simultaneously eliminates the vast/high % of crack starters... and make a high reliability part.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Lisa_247]

OK sorry if i wasnt clear enough! Im a stress engineer and my job is to ensure parts are strong enough and safe to fly. There are some components that i have done some analysis on, based on the assumption that the parts are mechanically cut, but the customer is now asking me if it is OK for them to laser cut the parts instead. SO what i need to do is look at the areas that could potentially lie in the HAZ (i.e. the edges) and assume that the strength of the material is reduced in this area. So for example, if there is a welded joint, we will typically reduce the strength allowables by 40% local to the weld to account for the reduced strength within the HAZ. So im looking for some kind of guidance on the width of the HAZ (0.1mm? 1mm? 10mm?)and the percentage reduction in the strength properties in this region. Thanks.

 

[SWComposites]

Ha, well the first thing that stress analysts need to learn is the word “No”.

So is it a part supplier asking or your aircraft OEM customer? If it is a part supplier, make them provide the data on the HAZ effect. If its an OEM, not sure why they are asking you if its ok.

Are they planning to just trim the edges of parts using a laser? Or also make fastener holes? Cutouts with high Kt’s? The laser cut edge finish and HAZ may have a larger effect of fatigue than static strength.

 

[rb1957]

yeah, "NO".

Don't you have M&P people to answer this type of question ? Maybe not, but then I suspect you're with a small outfit where stress fields all these analysis questions.

But rather than "no" I'd probably answer "I need to do fatigue tests on typical laser cut holes, oh, and have these tests approved by FAA (or whoever), and ..." making it really difficult to see the end of the tunnel. That should discourage most people. But then these guys would go to someone else (project ?) and say "I can save $X using laser cutting but your stress people aren't playing nice." (yeah well, what do expect from those stress guys !)

Is this primary structure or clearly secondary ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[RoarkS]

rb1957 agreed.

Don't screw around and get someone killed by not knowing. Aircraft structural aluminum alloys ARE effected by heat. The general rule is DO NOT add heat unless you know what you're doing (heat treat facility). Essentially a laser is completely uncontrolled heating and cooling in an uncontrolled atmosphere on parts where time to quench is literally seconds. Creates a grain structure change. That's a stress concntration... which is bad. Short answer does it fundamentally change the material properties expected of the component... YES.

I heard someone at Boeing onetime say it was okay because the high wattage lasers being used and gage of material was such that the HAZ was only a few thousanths. Well that's all it takes for a crack to grow as the part is being handled.

Even if you mecahnically remove the HAZ, Reaming/match drilling holes how do you know that a crack didn't already start and grow. You don't. AND I've never seen an at scale process that fully edge preps all parts... That's why we router cut "real" parts and DA sand them with a pad that rips burrs and sharp edges on the top and bottom. Even waterjet leaves a rough enough edge I never allowed it on sheet components. Usually that's a 80 grit garnet product which does okay most of the time, but what about a cracked ruby, bad nozzle, change in cut speed from a straight line... slows to line, then transitions into a near constant acceleration parabolic... significant change in edge quality... and guess what lasers usually run on the same type of cut routines. I have waterjet some parts out of thicker materials, but they were subject to pretty extensive post processes.

If you do the test, I suggest you add steps to handle the parts "severely" as they would be in the factory including time on shelf because of age hardening (at least 4 days for 2024)

 

[Tmoose]

I don't know about laser cutting, but as-cut EDM ruins many steels for welding and clobbers fatigue strength via bad surface geometry and micro cracks and probably a bunch more.

The operating stress would have to be mighty low for that not to matter.

This 1977 US Air Force Materiel Labs Technical Report might be worth a look.
2024 T3 and 7075 T6.

https://apps.dtic.mil/sti/tr/pdf/ADA051838.pdf

Here is link that almost seems to say it is OK on some materials.

https://www.army.mil/article/85097/laser_revolutionizes_sheet_metal_cutting_at_army_depot

MIL-STD-130N talks about laser engraving aluminum. No particular warnings or limitations from a "laser" word search , but I did not see actual materials mentioned either.

 

[rb1957]

I'm hearing ... "don't do it"

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[WKTaylor]

As far as welded aluminum goes... torch, arc, spot, LASER, etc... a smooth bead/nugget is relatively low stress and reliable strong [for weldable alloys] if 'done-by-the-book'. Spot-welded aluminum typically corrodes at the SW... which is the leading cause of failure. What is most damaging is burrs, pin-holes, oxides, slag, laps, cold-shuts, etc. These rough/ragged/oxidized surface/edge are prone to crack initiation and all-sorts of corrosion.

LASERS/water-jet/hole-punches/etc, create ragged axially-oriented rough holes and edges... which have built-in crack starters and are impossible to apply corrosion protection-onto with any reliability.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Sparweb]

Hello Lisa,
I'm a little torn, because I don't want to imply that the knowledgeable contributors so far don't know what they're talking about - they certainly do and every point they raise is an important consideration. However, it seems they haven't done the investigation and experimentation to overcome the HAZ in laser-cut parts and therefore take the conservative approach. This is wise and they are speaking purely from common wisdom.

On the other hand, there are ways to set up the process in such a way that laser-cutting can be used on "some" parts. The HAZ can be removed by subsequent cutting/drilling/filing/reaming/etc. processes, carefully selected to thoroughly remove it all. A few key concepts to bring to mind:
[ul]
[li]You need not be saving time by laser-cutting, if consistent dimensions, fast vendor turn-around, or other factors matter.[/li]
[li]Quality of your part is now driven by the quality of the laser cutting machine and its operator.[/li]
[li]Detailed process standards are needed to control many steps along the way.[/li]
[li]The question isn't "if" you inspect the parts, but "when" do you inspect them?[/li]
[li]Not every part you fabricate is the main spar splice of an A330. How critical are they in fact?[/li]
[li]You aren't the first person or organization to figure this out.[/li]
[/ul]
I'd like to be more specific but I'm trying not to reveal too much of my company's "secret sauce". We solved this problem, somewhat. There is a mitigation/correction/solution for each of the concerns that the experts point out above. I just want to let you know that if you and your organization are willing to invest the time, research, and diligence in adding this process to your capabilities, you can. In the end, you might show that the extra effort costs more than a common CNC router, like we did. But in my organization, we went ahead anyway, because it makes more vendors available when the preferred CNC vendors are busy with long lead-times.

It sounds like you are mostly concerned with stress-analysis, not quality control or manufacturing. That unfortunately puts you a step removed from the decisions that are being made about using laser-cutting. This could be awkward, if they are going ahead with it, but without the protections like the ones I've put in place.

 

[rb1957]

so you cut a hole with a laser, then drill/ream to final size ? and the saving is ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Sparweb]

Read it again RB;
"because it makes more vendors available when the preferred CNC vendors are busy with long lead-times."

 

[rb1957]

if you're going to spot the hole by hand, why create a (presumably) near sized hole in the first place ?

leave the part blank ?

if you must laser cut a pilot hole, and (presumably) clean out any HAZ going to full size.

I get it may make more vendors available, but I feel we shouldn't encourage "bad" processes.
I'd've thought there were more water jets around than lasers ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Lisa_247]

OK thanks everyone for your replies. We are a relatively small company. The parts they want to laser cut are tertiary structure, equipment shelves, small brackets to support lightweight items in the cabin, that kind of thing. I am the structures CVE on the project so ultimately its up to me to decide whether its acceptable from a structures point of view. If i cant obtain any quantitative data on how the parts are going to be impacted then i will have to just say 'no to all'. I was just trying to avoid that as in the DOA world its quite different to the OEM world!