If you're a hardy do-it-yourself-er, you might think you can avoid having to call a repairman to replace a broken garage door spring, or even get out of having to replace it at all. After call, you're clever, right? Why can't you just weld it back together?
There are two main reasons why this would be risky and foolish. One, the welding job may fail because it's not strong enough or has accidentally compromised the structural integrity of nearby parts of the spring. And secondly, if you think about it logically for a moment, the fact that the spring broke in the first place means that it is old enough and worn out enough to just keep on breaking.
There are other improvisations with spring replacement that might seem like good ideas but are very foolhardy. If the spring has broken in a location that is close to a winding cone, it may seem like good sense to simple discard the broken piece of the spring, clean of the end of the long spring that is remaining, and insert it into the cone. This, again, is a bad improvisation, because the part you replaced will not be correctly calibrated to bear the weight in the garage door, and the shorter spring will, in comparison, be over-wound, increasing the amount of stress to a potential breaking point, bringing your garage door crashing down. And again, if the spring broke once, it's age is probably advanced enough for another breakage to be likely.
If your garage door opener spring breaks, always replace BOTH with new ones. The extra cost is worth the security.