The thin sheet metal was a gasket. If it is in good shape, feel free to reuse it. I just throw them away. With how often we remove axles on this fleet, it is just easier to use some silicone sealant. Either way, a thin coat will do since the machine surfaces will push most of it out.
Typically, the bolts are in a blind hole. If this is not the case on your axle, then a bit of sealant on the threads will seal any leaks. Again, no need to gob it on.
