Blacksmiths have traditionally used collars
for joinery for thousands of years.
A suitable length of metal is heated in the forge,
then wrapped around the pieces being joined. As the collar cools
it shrinks, which causes it to strongly grip the pieces together.
These days electrical welding has largely replaced collars for mass produced work. However, many contemporary blacksmiths find collars more aesthetically pleasing than a welded joint and use them as a design feature. In particular, a collar can be used to great effect as a contrasting metal detail, as the above image shows.