I suppose they're heat-welding the two tank halves...and judging by the pictures, they're welding top and bottom mold...
On September 14, 2020, a non-US field report indicated that a 2019 BMW Z4 experienced a fuel leak. An
engineering review was initiated, and the fuel tank supplier was contacted.
A review of supplier production records indicated that a deviation in the welding process had occurred when joining (welding) the two halves of the fuel tank. The review indicated that a specific process parameter involving the heating time of the tank halves may not have met and, therefore, could lead to an out of specification weld condition between the two tank halves.
Vehicle assembly information and supplier production records were reviewed to determine the number and production dates of potentially affected vehicles.
On November 5, 2020, BMW Group decided to conduct a voluntary recall for potentially affected vehicles.
BMW Group has not received any reports, nor is otherwise aware, of any accidents or injuries related to this issue.