Laser Welding Ferritic Stainless Steel with protective gas

[sampsonr]

Hi Guys,

I am laser welding thin (0.5mm) 430LNT (Ti, Nb stabilized for welding) using a 2kW Rofin 1070nm fiber laser using an automated production machine. We have been playing around with applying protective gases such as Nitrogen, Argon, and Helium via Ø4mm nozzle and found that the welds are oxidized unless the protective gas is applied in a very specific way. The direction of the nozzle and flow rate are critical to getting a shiny oxide free weld. A flow rate between 1.5 - 7.5LPM is required to get this condition and anywhere out of this flow rate will result in a weld with oxide. The other condition we get when applying the gas to get a shiny weld is a severe loss in penetration. For example normally with no protective gas, 1300W is adequate to get full penetration however when gas is added the power required could now be a high as 2000W.

Has anyone come across (1) how sensitive to gas application oxide free welds are? and (2) A severe reduction in penetration when an oxide free weld is achieved verses a weld with oxide?

Any help would be greatly appreciated.

Regards,
Roy

 

[btrueblood]

Sampson,

No direct experience with laser welding, but yes your results sound typical.

In GTA (TIG) welding stainless, having a diffuse, laminar flow of purge gas is important to keep oxides from forming, as is a backing purge when possible. Take a look at a "gas lens" Tig cup versus a standard cup (either visit a local weld shop, or use google images), and you can see what welders do to get the flow as laminar as possible. Turbulence in the flow (which will happen at higher flow rates with your narrow 4mm nozzle) will entrain (mix in) oxygen from the surrounding air, and spoil your weld. Too low a flow rate, and you won't keep the weld zone covered long enough before the nozzle/weld head moves away, and oxides will form. Trailing shields for Tig welders are used to keep this effect minimized. Not sure if that helps you at all, as making a wider, screened "gas lens" to replace your 4mm nozzle might not be feasible within the space available on the laser head, nor can I see you easily making a trailing shield that won't get all tangled up as the laser head moves around. Some laser welding is done in a sealed cabinet that can be evacuated and backfilled with shielding gas, but I don't know if that is feasible for you either.

Using nitrogen may give you some trouble with excessive weld hardness and possibly loss of corrosion resistance, but not sure how bad it will be for 430. And, yes, adding gas flow will tend to carry heat away, so you have to add heat to compensate.