The pilot reported that after taking off, dense white smoke began to fill the cockpit and seemed to come from under the instrument panel. He stated that the instruments were in the normal operating range, but the smoke became so dense that the visibility in the cockpit was reduced to a few inches. The passenger in the right front seat was also a pilot and assisted in navigating back to the airport. The aircraft was landed downwind on runway 20, and after touchdown, the nose gear collapsed. An examination revealed that a turbocharger exhaust coupling had failed and allowed hot exhaust gases to expel into the engine compartment. The oil pressure hose line melted and oil was sprayed onto the turbocharger and mixed with the hot gases and burned. The smoke entered the cockpit through the vent beneath the instrument panel. An examination of the nose gear revealed that the gear down lock springs were damaged by fire, thus allowing the nose gear to collapse.