Responce from Marbles reagent etching of weld profile 2

[AbyssUK]

Hi everybody,

I am no metallurgist but I have a basic understanding of metals.

We have been laser spot welding 304SS and to check out the welds I have been creating profiles of the weld. i.e. I grind down into the centre of the spot weld, polish down and then etch with marbles reagent so I can see the penetration of the weld.

Now, we are matching a previously done weld by a third party, we have our laser welder set to make welds just like the third party. When I create a profile I can see the penetration is spot on... however our welds are still much weaker! Now the only difference I can see on the etched profiles is that the third party weld appears to give a stronger clearer response (i.e. the weld area gives a darker colour after the same immersion time in the marbles reagent)

I understand that the marbles reagent lets me see grain boundaries, so I guess this means the third party weld has more closely packed grains to give the darker colour... how can we replicate this ? do we perhaps need to cool the weld pool down quicker ? heat it faster ? keep it hot for longer ?

Any help would be great, thanks in advance!

 

[Maui]

Posting images of both microstructures would go a long way in helping us to help you.

Are you and the third party using the same machines with the same set-up for spot welding, or are there significant differences in the machines and the set-ups?

Maui

http://www.EngineeringMetallurgy.com

 

[AbyssUK]

Here is the 3rd party weld

 

[AbyssUK]

Here is our attempt.

You can see that ours is much lighter, however a darker shade can be seen within the weld.

Welding machines are very different, this is obviously the major problem, if we can figure out why the weld is darker we can tweak the settings to try and give the same strength.

Thanks for looking.

 

[Maui]

AbyssUK, those images are thumbnails, so I can't really see as much detail as I would prefer. Could you load the full pictures so that we could get a more detailed look at the microstructures?

Maui

http://www.EngineeringMetallurgy.com

 

[AbyssUK]

Sorry this is the best we have, our digital microscope is pretty rubbish.... 1 megapixel

I've also had to cut out the weld image as the full image contains propriety information.

 

[metengr]

You’re going to need much more information to determine how to replicate a laser weld made by a 3rd party other than viewing cross sections of laser welds.
I presume you don't have any access to the 3rd party laser welding procedure otherwise duplication would be straight forward. So, you are attempting to reverse engineer a quality laser weld using your own equipment.

What are your current welding parameters for the laser spot welds, preparation, cleaning, etc.,? Do you have a welding procedure to follow or developed your own? The appearance of the weld does not necessarily equate to strength. Have you tested the weld coupons to determine and compare strength levels?

 

[AbyssUK]

We have access to the 3rd party welding procedure and I am told we are following it. I am told the only differences are machine maker and that we are welding horizontally while the other was welded vertically (but same angle to the object). Same laser power, same duration, same beam profile, same wavelengths, same shielding gas and the same prep and pre clean. We are confused!

I have tested the welds on a tensile machine and the 3rd party welds are at least 50% stronger..

This is a guess, but perhaps the 3rd party welder cooled quicker? some sort of heat sink?

 

[Maui]

AbyssUK, are you using a protective atmosphere as you perform this welding operation? According to the following article, which I recommend that you read,

http://www.uslasercorp.com/envoy/welding.html

Chemical reactions, such as oxidation or nitriding, with atmospheric gases at high temperatures can pose problems, particularly when the oxides or other elements formed have disassociation temperatures far above the melting point of the metal. The result is brittle, porous welds. Covering the welding area with an inert gas such as argon or helium minimizes these reactions in most cases. For some materials, it may be necessary to weld within a sealed chamber to prevent outside contamination.

Maui

http://www.EngineeringMetallurgy.com

 

[AbyssUK]

Thanks for the input Maui, but yes we are using helium as a shielding gas. We tried without and it did make much weaker welds.

 

[metengr]

I would recommend for the laser weld microstructural evaluation to use the following etchant.

Waterless Kalling's reagent

 

[Maui]

I'd also pay close attention to your mechanical fit-up prior to performing the welding. If the parts don't match up properly, that could create issues as well.

mAUI

http://www.EngineeringMetallurgy.com