MARANGONI EFFECTS IN WELDING

The problem of variable weld penetration or 'cast-to-cast' variation i n GTA/TIG welding is discussed. It is shown that for normal GTA/TIG we lding conditions the Heiple-Roper theory is valid, i.e. that weld pene tration is controlled by the fluid flow in the weld pool which; in tur n: is controlled by the direction and magnitude of the thermocapillary forces. For most steels the direction and magnitude of these forces a re determined by the sulphur content, since the temperature coefficien t of surface tension (d gamma/dT) is negative when S < 30 ppm and this leads to a radially outward flow and poor penetration whereas a steel with S > 60 ppm has a positive (d gamma/dT) which produces a radially inward flow giving good weld penetration. Thermocapillary forces were shown to play a part in the problems of 'off-centre welding', 'porosi ty' and 'arc wander' in GTA/TIG welding and in the surface rippling of welds.