【作者】 彭乃康；

【导师】 李冬青；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 作为新一代高性能钢铁结构材料代表的超细晶粒钢,因其晶粒超细化从而实现了性能的强韧化。但在焊接热循环作用下,焊接接头会出现晶粒长大,软化及强度下降等问题,从而降低了产品质量、可靠性和使用寿命。针对这个问题,本文探讨了外加交变磁场改善超细晶粒钢点焊接头质量的新方法。文章以400MPa超细晶粒钢为研究对象,系统分析了外加交变磁场对超细晶粒钢点焊接头质量的影响。本文首先对点焊接头宏观形貌和晶粒形态进行分析。在选定焊接参数下,正常点焊接头截面接近矩形,外加磁场后,接头截面为椭圆形,并且在外加磁场作用下,熔核直径增大,焊点高度和焊透率略有降低。正常点焊时,熔核的凝固组织为单一粗大的柱状晶,方向性很强。外加磁场后,熔核内柱状晶方向性减弱,熔核中心及边缘出现不等量的等轴晶。研究分析表明,同一励磁电流下,外加反向磁场比同向磁场作用效果更强;同一磁场方向下,施加150A励磁电流较其他大小电流作用效果好。其次对接头的显微组织和力学性能进行研究。超细晶粒钢点焊熔核的显微组织为珠光体、马氏体和极少量铁素体。外加磁场后,熔核内组织分布更为细小均匀,并且熔核内平均硬度有所提高。拉剪试验结果表明,外加磁场后,点焊接头的抗剪切能力提高,采用向内磁场,施加150A励磁电流,接头的最大拉剪载荷较无磁场时提高了1.9%。最后对点焊接头内部缺陷进行分析。当电极压力不足时,熔核内部易出现裂纹、缩孔和疏松缺陷。在外加磁场作用下,熔核内缺陷得到不同程度的改善,同样,采用向内磁场,施加150A励磁电流时,磁场对缺陷的改善效果较好。实验结果证明,选择合适的点焊工艺和磁场参数,外加交变磁场可以改变晶粒的结晶方向,改善一次、二次结晶组织,提高焊缝的力学性能,是一种很有发展前景的点焊质量控制方法。更多还原

【Abstract】 As representative of high-performance steel material of new generation, ultra-fine grained steel realize, for its super-fine grain, realize the performance strengthening and toughening. But in welding processes, the welded joints of ultra-fine grained steel will be grain growth, softening and strength decline, which will reduce the product quality, reliability and service life. For this issue, the method, which is the quality improving of spot welding of ultra-fine grained steel in the external alternating magnetic field, will be discussed by this article. Taking 400MPa ultra-fine grained steel for an example, this paper systematically analyzes the effect of external alternating magnetic field on the quality of spot welded joint.This paper analyzes the macro-features and crystal habit of spot welded joint first. Under the selection of welding parameter, the section of normal spot welded joint is close to rectangular, but which is ellipse in external magnetic field. And it has a greater nugget diameter, a smaller joint height and penetration rate in alternating magnetic field. The nugget of ultra-fine grained steel is made up of single and coarse columnar crystals under general spot weld process, which has strong orientation. When it is applied external magnetic field, the orientation of columnar in nugget is weakened and there are variable amounts of equiaxed crystals in the center and fringe of the nugget. The research indicates that, as the same magnetic current, the effect of reverse magnetic field is superior to that of the same magnetic field; as the same magnetic direction, the effect of 150A magnetic current is better than others. Second, the microstructure and mechanical properties are researched. The microstructure of spot welded joint of ultra-fine grained steel of made up of pearlite, martensite and little of ferrite. When it is applied external magnetic field, the structure distribution is more fine and uniform and the average hardness in the nugget increases. The result of tensile shear experiment shows that, compared to the normal spot welding, the load against tensile shear improved the 1.90% when applied reverse magnetic field and 150A magnetic current. Finally, the inner defect of spot welded joint is researched. When the electrode force is below, there are cracks and shrinkage in the nugget. When it is applied magnetic field, the defects in the nugget are improved to same degree, similarly, the improvement effect is the best for150A reverse magnetic.It is obvious that external alternating magnetic field can modify the direction of crystallization, improve primary and secondary solidification structure, and increase the mechanical properties of spot welded joints. So this method has unexceptionable prospect in spot welding quality control.更多还原