Some hydraulic pump manufacturers (the minority) opt for making barrels entirely of copper alloy (bronze), in contrast to a more common bimetallic design.
Why? Any advantages besides skipping thermal bonding operation? So far I haven't been able to find a definite final answer. Just small talk, like the less noticeable thermal expansion deformation of single metal parts in comparison with bimetallic parts and such... Is it really that important? Practice shows that both designs work fine, and, as you can see here, the more material-expensive full-bronze barrel can fail exactly as its wider-spread bimetallic brother.
The pics are courtesy of a Liebherr LPVD open circuit pump.
The full-bronze barrels do look a lot neater though!..