On August 9, 2001, about 1530 eastern daylight time, an experimental, Avid Mark IV, N567SM, registered to a private individual, operating as a Title 14 CFR Part 91 personal flight, crashed in the vicinity of Salisbury, North Carolina, Visual meteorological conditions prevailed and no flight plan was filed. The private-rated pilot, the sole occupant, received serious injuries, and the aircraft sustained substantial damage. The flight originated about 10 minutes before the accident. According to the pilot, the flight was the first for the newly completed homebuilt. After suitable taxi runs and an air-taxi at about 30 to 50 feet agl, down runway 20, using the controls in all axes, the pilot deemed the aircraft ready for its first flight. After takeoff, as the pilot attempted to level out his left crosswind turn in the landing pattern, the aircraft did not respond to aileron inputs, although the pilot did have rudder response. Using only pitch, rudder, and power control, the pilot managed to enter a left shallow spiral that resulted in a controlled collision with the terrain about 1/2 mile from the runway. Unfortunately, the slide after terrain collision encountered a railroad berm and contributed to additional aircraft damage. According to an FAA inspector, examination of the wreckage revealed a broken weld that should have secured a threaded rod-end stud to the flight control rod mixer unit. The mixer unit is designed to coordinate inputs from the pilot's and copilot's flight control columns, (sticks) to the ailerons and flapperons of each wing, and is located under and behind the cockpit seats. The threaded stud was designed to be permanently welded to the aileron rod-ends and to afix the rod-ends of the aileron/flapperon control rods to the mixer unit. A photograph of the separation is an attachment to this report.