Check how to set Weblogic take automatically back up of you configuration (config.xml file) in Hasim's post Weblogic : Back Up Config.xml file.
Reposting some part of his post:
Login to weblogic console
Click Domain –> Configuration –> General (Advanced)
Check Configuration Archive Enabled and Count.
true - backups of the configuration will be made during server boot.
Change will take place after you redeploy module or restart the server
it will create following files in domain.