Goblin Welder + Dreadnought Shenanigans

[Fallen_Empire]

I have a Phyrexian Dreadnought in hand and in the graveyard.
I also have a Goblin Welder and Lotus Petal in play.

If I play my Dreadnought from my hand, can I use Goblin Welder's ability to swap the in play Petal with the Dreadnought in my graveyard, in order to sac it for the trigger cost of the Dreadnought entering play from my hand?

[Dice_Box]

Yes. The trigger goes on the stack, but the card is still in play until you finish resolving the trigger. While that's happening you have a window to interact.

PD is a etb effect, it's still on the Battlefield so you can do what you like.

[Fallen_Empire]

Thanks for the clarification Dice! Bout to brew up some spice!

[KobeBryan]

Too many steps for a weak combo bro.

[Fox]

Mechanics questions really aren't about top tier competitiveness @KobeBryan; very few decks which surpass whatever threshold you're setting as not being "weak" and still pose interesting mechanics questions. The point here is that if someone wants to play a deck that will have sophisticated interactions, that they need to know how they work before going to a paper event.

The initial question needs to be clarified:
If I play my Dreadnought from my hand, can I use Goblin Welder's ability to swap the in play Petal with the Dreadnought in my graveyard, in order to sac it for the trigger cost of the Dreadnought entering play from my hand?

Yes you can make a Dreadnought, but that Dreadnought will never be the one you played from hand:
1) Dreadnought from hand cast [at sorcery speed] and resolves, trigger on the stack
2) Activate Welder bringing Dreadnought #2 from yard into play [at instant speed], trigger on the stack
3) Sacrifice Dreadnought #1 to Dreadnought #2's trigger.
4) Dreadnought #1 trigger will be the next to resolve; the default here is nothing happens as Dreadnought #1 is in the graveyard.

Technically you have two other options for step 4:
-sac Dreadnought #2 to #1's trigger for probably no reason.
-sac Dreadnought #2 and Goblin Welder to #1's trigger for probably no reason.

[Dice_Box]

The real fun here is if you play Fling. Welding in the one from the Grave and throwing it at someone with the trigger still live. You can do all kinds of Jank while that trigger is live.

[FourDogsinaHorseSuit]

I have a Phyrexian Dreadnought in hand and in the graveyard.
I also have a Goblin Welder and Lotus Petal in play.

If I play my Dreadnought from my hand, can I use Goblin Welder's ability to swap the in play Petal with the Dreadnought in my graveyard, in order to sac it for the trigger cost of the Dreadnought entering play from my hand?

No, because of timing. When you resolve the hand dreadnought you can't activate welder until you put dreadnought's trigger on the stack. Then when you return the graveyard dreadnought it's going to put its trigger on the stack on top of the hand dreadnought's. You then resolve graveyard dreadnought's trigger first.
But wait! It turns out that it doesn't matter: you can simply feed hand dreadnought to graveyard dreadnought's trigger, since I believe the goal here is to have a dreadnought, and you don't really care where it came from.

[FourDogsinaHorseSuit]

The real fun here is if you play Fling. Welding in the one from the Grave and throwing it at someone with the trigger still live. You can do all kinds of Jank while that trigger is live.

Why throw away a perfectly good dreadnought? Torpor Orb, Illusionary mask, and sundial of the infinite are all great ways to keep your dreadnought alive. And that last one has some great built-in gotcha!

[Fallen_Empire]

Why throw away a perfectly good dreadnought? Torpor Orb, Illusionary mask, and sundial of the infinite are all great ways to keep your dreadnought alive. And that last one has some great built-in gotcha!

It's not plan A or even plan B. It is just another tool in the tool box. You don't put simian spirit guide in your deck to attack, but sometimes you need to etc.

[sco0ter]

I've played this interaction casually, it's quite fun, although it was only a nice side interaction.

Add some Torbor Orb (which plays nicely with Goblin Welder, too), Lightning Greaves (same), Sol lands, lock pieces and swing with Dreadnought on turn 2.