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It would be a waste of time. Fan would be out of balance and near impossible to rebalance, results would be vibration and motor bearing failure. Invest in a new blade, would be less expensive than replacing the blade and motor afterwards.
Are you nuts? Why take the risk? if the solder fails, you've got a sharp blade and sharper arm flying through the air at high speed. And you've voided the warranty. But hey, that $20 bucks you saved can go towards your new life as a blind person.
Rather than go blind...
1. If the blade is still intact, replace the arm. Since new arms are usually lighter than older arms, hardware stores sell inexpensive weights that can be clipped or "stuck" to the top of the fan blade. But they come off easily. More blindness risk...
2. In fact, if the arm broke due to anything other than accidental impact, I'd replace all of the arms and I'd ask the manufacturer to replace them for free due to defect. I'd even check for recalls.
3. Replacement blades are fairly inexpensive but I'd replace all of them at the same time, again due to balance.
But hey, if you are just looking for an excuse to solder something, sever your brake line on your car and solder that back together.