MIG welding dissimilar metals

[Wyo.welder]

Alright, so I had a little frankenmetal project come across my bench and I needed to join some mild steel to 304 stainless, some custom brackets for a friend who's apparently allergic to using the same metal twice. I went with MIG (solid wire, C25 gas), mostly for convenience, and... yeah, it worked, but I've got questions now.

Fusion looked good, but I'm not thrilled with how the bead behaved on the stainless side, the wetting felt weird, and the colour change was a bit unpredictable. Used ER70S-6 since it was already loaded, but I know that's not ideal for dissimilar metals. Didn't bother purging, obviously, since this wasn't TIG or pressure rated, but now I'm wondering if that corner I cut might bite later.
Has anyone else here welded mild to stainless with MIG and had long term success or regret? Do you bother with 309L wire, or just stick to mild steel filler and let it ride?

 

[PowerPo]

Did you notice any signs of early corrosion or cracking over time, especially at the transition point, or do you think the joint holds up fine as long as it's not structural or in a corrosive environment?

 

[brainiac1]

ER70S-6 is defiantly not ideal for these kinds of situations but it can work depending on the situation. If it's not a crucial bracket it will hold well enough and should be fine. If not, then you'll need to thing something different you'll need 309 or even 312

 

[Kenny C]

I wonder if silcon bronze would work?

 

[Lewislewis]

ER70S 6 will hold but it's not the best for longevity. If it's anything structural or exposed to weather, I'd switch to 309L wire next time

 

[StillAlive]

Did you notice any signs of early corrosion or cracking over time, especially at the transition point, or do you think the joint holds up fine as long as it's not structural or in a corrosive environment?

yep, that ER70S-6 is a bit of a gamble. The main problem is that mild steel and stainless steel have different compositions and expand at different rates when they get hot.
This can cause the weld to become brittle over time and possibly crack. This is where 309L filler wire comes in. It's a special blend of chromium and nickel designed to be a transition metal that can handle the stress of joining two very different types of steel. For a long term, reliable weld, using 309L is the right call