Cutting aluminium - has anyone been told not to laser cut if going to weld?

I was told by someone yesterday who should know what they are talking about that I shouldn't use a CNC laser to cut aluminium plate for boat building as the laser cutting creates a form of heat treatment for the edges and then you don't get good welds - has anyone else heard this or have any knowledge of it?

 

From Aluminum Temper Designations: How do They Work? - Gabrian https://www.gabrian.com/aluminum-temper/

The statement contained within this article : First, consider that certain aluminum alloy series can only be strain hardened or thermally treated, which may limit the number of choices you have in manufacturing. Wrought aluminum alloys 1XXX, 3XXX, and 5XXX non-heat-treatable, while alloys 2XXX, 6XXX, and 7XXX are heat treatable. Some 4XXX series wrought aluminum alloys are heat treatable, while others are not.

Assume that you are referring to sheet/plate profiles. Most boat builders use the 5XXX series for plating a boat. ( and more often than not, 6XXX for extrusions)
As 5XXX series are not heat treatable, I cannot see any reason why a laser cutter would impart characteristics that would ""create a form of heat treatment for the edges and then you don't get good welds""

Can you get your "someone who should know" to offer an explanation for this statement?

The fibre optic lasers that I have seen demo'd at Fabtech are faster than say a plasma table. So I suspect that the HAZ width may be less, but then as you weld up the joints, the aluminum will be exposed to more heat than the actual cutting process. The material has to reach a certain temperature to melt and it is quite low as compared to steel.

Was there any chance that he was referring to steel? Generally, high carbon steel is heat treatable but in its simplest process, the steel is heated, then quenched to increase strength. As there would not be a dedicated quench in the laser cutting system to "harden" the material, it is unlikely the high temp cut zone would harden.
I guess the question would be this: Would the cool surrounding material self quench the thin laser kerf line?

 

What Barry said...

It is fine. The process of welding is a far greater influence on the alloy and also, depends which alloy you are using.
Now since you're cnc cutting, it will be a 5000 series alloy, ergo, non-heat treatable. Therefore the temper will be the only influence.

 

.....as per above, BUT after more than forty years of alu-welding my experience is that plasma and laser cut edges demand a deeper oxide removal than edges that have been cut by router or carbide-tipped saw. My gut feeling (not scientifically validated though...) is that the increased surface temperatures from laser and/or plasma will accelerate the local oxide growth.

 

Im loathed to counter baeckmo, especially since it is loosely based on the basics of ally....again, so I have no hard evidence other than the basics.
But...
Most laser cutting temps are in the range of up to 1000c. Which easily metals ally.
But the oxide layer of ally requires a minimum of 2000c before it is removed....and the oxide layer forms almost instantaneously; it is measured in less than millisecond, for the oxide layer to grow, in the presence of oxygen.

I suspect it maybe the amount of time taken to "dress" the joint, post cutting, perhaps, which could be a function of the speed and width of the laser?

But, 40 years worth of empirical evidence is indeed hard to argue against!

 

I have read that some CNC laser cutters use nitrogen gas as a shielding gas during the cutting process - possibly to help overcome the problem that you have identified. I wonder if it is possible to run a CNC laser in an inert gas-filled enclosure?

 

I guess using a laser cleaner to remove the oxide is not going to work!

 

I suspect it maybe the amount of time taken to "dress" the joint, post cutting, perhaps, which could be a function of the speed and width of the laser?
Might very well be the case. My feeling is that I had to dress the corners of the edge slightly deeper to avoid oxide inclusions when welding immediately after a laser or plasma cut. Just as you have to go a little deeper into the corner on an edge cut by router and then stored for some time.

The problem area is just the edge corner, where the oxide layer becomes thicker due to oxygen access from both the plate surface and from the cut plane. I can not remember any difference between the CO2/N2 plasma and the newer air plasmas, so I don't think it is the laser or plasma gas per se, but the intensity of the oxidation after the burner passing.

 

You're not contradicting him at all. Laser and plasma melt the metal, the edge is bubbling. The oxides form instantly and sink into the liquid metal exposing new pure material. The process repeats until the metal cools enough to solidify. The net result is the inclusion of oxides into the edge, instead of beeing only on the surface, wich means more oxide to remove. It's the exact same phenomenon shielding gas is employed to prevent when welding, it gives the metal time to cool so the oxides form only on the surface instead of contaminating the weld.

As a side note, I have experienced plasma cut high strenght steel (special application not boatbuilding stuff) harden enough to dull and chip the beveling router in a few centimeters.

For RSD, before you purchase that awesome laser welder please have the manufacturer explain positioning tolerance for his machine. Even with laser cut parts the plating could easily exceed it and the framing might require some strong jigs.

 

Positioning tolerance and accuracy is something that I am going to be really interrogating them on as I have access to 30 foot long sheets.

CNC Laser cutting in an inert gas-filled room is sounding better and better all the time!

 

https://www.ijtre.com/images/scripts/2016030808.pdf

Laser cutting paper above containing this excerpt

1.4 Laser Fusion Cutting[14] In this process, the laser beam heats up the material to melt, using an inert gas such as nitrogen, to blow away the melted material. The inert gas also can help protect the heated material from the surrounding air as well as protecting the laser optics. The process is shown in Figure 1.2. The energy requirements are lower than in the vaporization cutting. Laser fusion cutting is mainly used for metal material this is also include highly alloyed steels like stainless steel, aluminium and titanium alloys. The advantage of this process is that the cut edge is free of oxides with high quality.

 

Welded Lazer cut parts last week, dressed the edge with a flop wheel same as I've done with hand cut, router cut, plasma cut, water jet and now laser cut parts. Admittedly not the most up to date on the intricacies of metallurgy. I know each iteration seems to be even better than the last. This is some sort of shielded laser, only know the guy that does it complains how much electricity it pulls compared to his older plasma table.

Makes a clean part, easy to adapt for a price I'd never hand cut for...

 

Many thanks for that - I haven't had a chance to read the full paper yet but from your excerpt it seems as though the nitrogen is the key part - and reading between the lines - the more nitrogen you can send at it the better - less potential for oxides. It might be worth investing in a nitrogen generator rather than using bottled nitrogen.

 

Many thanks for sharing that experience - very interesting that he is finding that it is using a lot more electricity than plasma - it should be the opposite.

 

OK I am starting to assemble a bit of a picture here -

@Barry - it seems that the naval architect was probably referring to aluminium - for the oxides reason that others have posted about - I am suspecting that he (or more probably one of his client) has had a bad experience with laser-cut sheet, and the cause would probably have been insufficient nitrogen shielding. BTW you mentioned Fabtech - are you going this year?

@baeckmo - it seems as though you have been experiencing aluminium that has been laser cut with insufficient nitrogen too - I may be wrong but I would have thought that oxides couldn't/shouldn't/wouldn't form if there was sufficient nitrogen shielding gas?

@Rumars - I think that you have nailed it with your explanation/comparison of why shielding gas is used for welding. It would be very interesting to compare aluminium samples from a laser cutter that gets its nitrogen from bottled nitrogen and one that uses a nitrogen generator where the process can be more readily "flooded" with nitrogen - I'm suspecting that the laser running bottled nitrogen will have more oxides as the boss would have dialled back the nitrogen flow to save money on gas costs - but at the expense of more oxides that cost money later to remove.

@comfisherman - if you get a chance it would be interesting to find out whether the laser uses bottled nitrogen or a nitrogen generator?