Formation of thin bands of interference colours or “tramlines” adjacent to arc welds in titanium and its alloys

Abstract

Arc welds in titanium typically exhibit thin strips of blue interference colours parallel to the welds. These features can cause difficulty in meeting conventional colour acceptance criteria, even though the fused zone may be silver. They have been called “tramlines” or “rail tracks” and develop within the shielded area, largely irrespective of the integrity of the shielding. In keyhole plasma welds, the “tramlines” are far broader than for tungsten inert gas welds, presenting a better opportunity to study their formation. Experimental evidence showed that the tramlines consisted of an oxide scale, significantly thicker than surface oxides on either the as-received parent metal surface or the silver metal between the tramlines and the weld metal. The formation of tramlines parallel to titanium weldments, within a properly gas-shielded area, is a consequence of surface condition, rather than an effect of the local environment during the welding operation. Since the heat-affected zone tramlines are very thin, colour criteria for weld acceptance should allow for the formation of these features in many applications.