Not sure on the metal either. But, I doubt that it's bronze, and I also doubt that it's cast iron, so that sort of narrows it down eh? There is a possibility that it's plastic or a composite, but providing that it's metal: To answer your question, yes it is possible to weld the crack. But, this would a) take a lot of time and effort, b) create an aerodynamic distortion within the manifold resulting in reduced power and efficiency, and c) not result in a piece as solid as the original casting. In the case of manifolds it is recommended that the entire part be replaced. The labour to remove and replace the unit is expensive, so if you're already going that far, you mineswell put in a new part. If you were to weld it, to avoid problem b), you would need to grind off the weld from the inside which is a labour intensive and therefore expensive task. You may be able to source an old, used part from a wrecker given that there's so many civics on this continent. One is bound to have the part you need. Call around, find a used part, and get that installed. Or, even better, just buy a new part and get it over with. Your choice. Hope this helps, Good luck

unless you are a good (as in professional)welder,leave it alone ,call the local junkyard and swap it out...

It's alumnium