Is there a difference between deadweight loss and welfare loss?
I would say all deadweight loss is welfare loss but not all welfare loss is deadweight loss. 

For example an unregulated polluter causing a negative externaly results in a welfare loss compared to the social optimum. There is no 'missing production' and hence no deadweight loss in this case.

Edit: According to Wikipedia, the loss created by externalities is also classified as deadweight loss.