is it necessary to replace axel shaft, when replacing cv joint boot cover?

IF you consider the additional labor cost to install a new cv-joint 'boot' only - as opposed to replacing the axle assembly .. You'd be better off money wise and have a more reliable repair replacing the assembly! It's actually a lot less labor that way - plus the axle shaft assemblies are very reasonably priced.

It is not necessary. They sell boot kits that come with grease too. If it has been leaking for a very long time or the joint is making noise, you should replace the entire axle assembly. Alex