【作者】 徐磊；

【导师】 张忠典；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 硕士

【摘要】 采用外加磁场的方法改善点焊熔核组织、提高点焊质量已为以往的研究所证实。但是,以往均采用永磁体作为磁场源,其可控性和实用性很差。本课题探讨了用电磁线圈产生可控磁场改善点焊接头组织的新方法。为了了解外加可控磁场对点焊接头组织的影响,本文以普通低碳钢和马氏体不锈钢为研究对象,详细地分析了外加径向和横向磁场对不同钢种的电阻点焊接头结晶组织的影响。本文首先根据电阻点焊机的实际情况设计了电磁线圈,并通过解析法和ANSYS数值模拟的方法详细计算了电磁线圈所产生的磁感应强度;为了验证计算结果的正确性,利用霍尔传感器设计了磁场测量装置,并对其进行标定,以保证所得到的磁感强度的准确性。实验测量结果与两种计算方法结果吻合良好。其次,探讨了外加径向和横向可控磁场对点焊熔核一次结晶组织的影响,发现在正常点焊时,钢点焊熔核的凝固组织为单一的粗大柱状晶组织;在外加径向磁场作用下,与无外加磁场相比,点焊熔核组织没有明显变化。主要原因是励磁线圈所产生的径向磁感强度过小。在外加足够强度的横向磁场作用下,点焊熔核为柱状晶和等轴晶组织,晶粒更加均匀,有明显细化;并详细分析了外加横向磁场作用下熔核的结晶特点和导致组织形态变化、晶粒细化的原因。外加磁场作用使熔核中的液态金属的流动速度增大,导致合金元素的分布更加均匀。实验结果证明,选择合适的磁场参数,外加可控磁场可改变晶粒的结晶方向,细化一次组织,提高焊缝的机械性能,是一种很有发展前景的点焊质量控制方法。更多还原

【Abstract】 The crystallization and the quality in the nugget are improved by external controllable magnetic field in this paper. The permanent magnets was adopted as the magnetic field source in the study before, which had poor practicability and controllability. This paper discusses a new method that the crystallization be improved by controllable magnetic field produced by the electromagnetic coils.This paper proposes a new method that controls the crystallization of spot welded joints by external controllable constant magnetic field which produced by electromagnetic coil. Taking spot weld of low carbon steel and stainless steel for example, this paper systematically analyzes the effects of external radial and transverse constant magnetic field on spot welding crystallization of low carbon steel and stainless steel.Firstly, the electromagnetic coils are designed and produced according to practice case of the welding machine. The magnetic induction intensity is calculated by analytical method and ANSYS numerical simulation. Hall sensor is designed to measure the magnetic induction intensity. The results of three methods conform to each other very well.Secondly, the effects of external radial and constant magnetic field on crystallization in nugget are studied. The nugget of both of steels is made up of coarse columnar crystals under general spot weld process. Comparing with the case which without external magnetic field, the microstructure has no evident change under the effect of external radial magnetic field. It is because the radial magnetic induction intensity is not strong enough. However, with the effects of external transverse constant magnetic field, the nugget is made up of refined“columnar + equiaxial”crystals, and the microstructure is more uniformly. So the reason is analyzed for which change take place. And the effects of external constant magnetic field on phase composition and microstructure are studied too. Because of the effects of external constant magnetic field, the distribution of elements is homogeneous in nugget controlled by magnetic field. The crack in the spot welded joint of stainless steel could be eliminated by external transverse constant magnetic field. So the mechanical properties of spot welded joints can improve.It is obvious that external constant magnetic field can modify the direction of crystallization, refine grain, and improve the mechanical properties of spot welded joints. So this method has unexceptionable prospect in spot welding quality control.更多还原