Friction stir welding apparatus for structural steel

A friction stir welding apparatus for structural steel, the welding apparatus comprising:

• a rotary tool that includes a shoulder and a pin disposed on the shoulder,
• the pin sharing a rotation axis with the shoulder,
• at least the shoulder and the pin being made of a material that is harder than steel sheets that are used as workpieces,
• a front heating device that is provided in front of the rotary tool and positioned higher than a top surface of the rotary tool, and
• a rear heating device provided behind the rotary tool and positioned lower than the front heating device,
• wherein:
• the front heating device is adapted to:
• form a heated region
• on the steel sheets such that:
• (a) a center of the heated region is located on the advancing side;
• and (b) a distance from the center of the heated region to the welding centerline is from 0.5 times to 1 time a maximum radius of the pin of the rotary tool,
• the heated region having a temperature TS (° C.) that satisfies TS≥0.8×TA1 (such that TA1 is described in Formula (1) below)
• TA1(° C.)=723−10.7[% Mn]−16.9[% Ni]+29.1[% Si]+16.9[% Cr]+290 [% As]+6.38[% W] Formula (1),
• where [% M] represents an amount (mass %) of element M in the steel sheets used as a workpiece,
• the rotary tool is adapted to penetrate into a non-welded part of the steel sheets and to move in a welding direction while the rotary tool rotates so that a portion of the steel sheets which has been preheated by the front heating device is:
• (i) softened by frictional heat between the rotary tool and the steel sheets and (ii) the softened portion of the steels sheet is stirred with the rotary tool to produce plastic flow, and
• the welding centerline is a straight line that passes through the rotation axis of the rotary tool and is parallel to the welding direction.