装配间隙是影响国际热核聚变实验反应堆(ITER)校正场线圈盒激光封焊质量的重要因素之一。采用热丝填充激光焊接，在离焦量分别为+5、+10、+16 mm的情况下对5 mm厚核聚变用钢316LN在不同对接间隙条件下进行光斑直径与对接间隙裕度的研究。焊后进行了焊缝表面、截面形貌的分析，同时进行了金相组织、接头拉伸强度、显微硬度测试及断面电镜与成分分析。结果表明，在热丝填充激光焊接中，光斑直径大小不仅可以与对接间隙量相当，还可以在小于对接间隙量0.1～0.2 mm的情况下，得到表面成形较好且抗拉强度值较高的焊缝，当光斑直径小于对接间隙量达到0.3 mm时，焊缝表面成形变差且抗拉强度值下降。

The assembly clearance is one of the factors that affect the laser closure welding quality for international thermonuclear experimental reactor (ITER) correction coil case. An experimental study on relationship between the focused spot diameter and the butt joint gap margin in the defocusing amount of +5, +10, +16 mm respectively is performed in laser butt welding of 5 mm thick 316LN steel for nuclear fusion with hot wire filler under various butt gaps. The weld topography and cross section, metallographic phase, tensile strength, micro hardness test, electron microscope and composition of cross-section are analyzed after welding. The results show that in laser welding with hot wire filler the focused spot diameter can not be only equivalent to the butt joint gap, but also can be 0.1~0.2 mm less than the butt joint gap when the weld is of good shape and the higher tensile strength is obtained. When it is 0.3 mm less than the butt joint gap, bad weld beads are obtained and the tensile strength of the weld decreases.