低圧ティグアークによるビード形成現象とアンダカットビード形成の防止法

抄録

Low pressure TIG arc has remarkable characteristics as a welding heat source such as low arc pressure and high temperature cathode zone. Bead formation by low pressure TIG arc in high speed condition is studied in comparison with those by atmospheric TIG arc. Mechanism of under-cut bead formation and a method of prevention of under-cut bead are also studied in the report. The results obtained are as follows;
(1) Bead shapes by low pressure TIG arc change with both arc current and welding speed similarly to those by atmospheric TIG arc. Welding condition ranges for bead mode are basically divided into three parts by Front Gouging Line, where gouging region occupies the front part of the molten pool, and Bottom Vacant Line, where vacant parts remain at the bottom part of the penetration.
(2) In low pressure condition, both Front Gouging Line and Bottom Vacant Line shift remarkably to higher current-higher speed side compared with those in atomospheric condition because of lower arc pressure. So, the suitable welding region, limited by the appearance of improper bead such as. under-cut bead, is enlarged much to high current-high speed side.
(3) Once gouging region appears in front of the molten pool under transition condition, a wide and shallow normal bead changes rapidly to a narrow and deep bead corresponding to the shape of cathode zone. Under-cut bead is formed when the gouging region occupies maximum bead width, because side part of the gouging region solidifies rapidly after detached from the cathode zone. (4) On the surface of the gouging region in under-cut bead, thin molten metal of several ten microns thickness flows uniformly toward backside, and it satisfies the condition of supercritical flow in fluid dynam-ics. Under such a condition, gravity head of molten pool is balanced with arc pressure, surface tension of molten metal and hydraulic jump effect of supercritical flow.
(5) For the prevention of under-cut bead formation in high speed condition, prevention of gouging formation by forward metal flow under forward torch angle in low arc pressure condition is an effective method. Uneven bead in the current region up to Bottom Vacant Line can be basically changed to the normal bead using the above method.