A downhole well pump assembly includes a pump and a motor. The motor has a motor housing with a motor head adapter on one end, the motor head adapter having a threaded connector. A seal section seals around a shaft of the motor, the seal section having a seal section adapter connected to the motor head adapter with the threaded connector. The motor head adapter and the motor housing have cylindrical exterior surfaces and end faces that face each other, defining a joint between the end faces. A first strip of metal foil is welded around the exterior surfaces of the motor head adapter and the motor housing at the joint, sealing the entry of well fluids into the joint.