Development of an experimentally supported Friction Stir Welding model for predicting the strength of dissimilar joints based on the example of aluminum-steel joints

搅拌摩擦焊是摩擦焊接的一种特例，在这种焊接中，连接伙伴之间不会发生相对运动。旋转焊接工具插入连接伙伴，并沿连接线移动。工具的旋转会导致材料受热、塑化并移动到工具后面，从而形成焊缝。在搅拌摩擦焊接过程中，焊缝的强度特别取决于其几何形状和材料的热机械处理。对于相似材料之间的接头，有许多数值过程模型，但对于异种接头的搅拌摩擦焊接过程没有工艺模型，因此，异种接头的强度特性只能通过实验来确定。本项目的目的是开发一种仿真模型，以预测不同金属搅拌摩擦焊接接头的强度特性随焊接参数的变化。该模型是以技术相关材料铝-钢组合为例建立的。为了实现这一目标，采用了三分程序。为了验证单步焊接的结果，采用了搅拌摩擦焊接过程模型。第一步，采用应变、应变率和温度相关材料模型对搅拌摩擦焊接过程进行了连续介质力学模拟。为了处理非常大的变形，并将不同材料之间的相互作用视为多相问题，必须使用复杂的计算方法。工艺模型的结果-焊缝几何形状以及两种材料的随时间变化的温度和应变场-被用作以下两个步骤的输入。采用损伤力学模型描述了搅拌摩擦焊热处理过程中材料的形变、组织变化、屈服强度与损伤力学参数之间的关系。根据工艺模型的结果，在参考焊缝中识别出类似形变处理的区域。最后，将上述两个步骤的结果合并到焊缝强度模型中。由过程模型产生的焊缝的破坏行为由损伤力学模型决定。通过实验拉伸试验验证了数值确定的接头强度特性，通过这三个步骤的组合，可以从搅拌摩擦焊接过程的工艺输入变量预测不同类型接头的几何形状和强度特性。

Friction stir welding is a special case of friction welding where no relative movement between the joining partners is used. A rotating welding tool plunges into the joining partners and is moved along the joint line. The rotation of the tool causes the material to heat, plastify and move behind the tool creating the weld seam. The strength of the weld seam is particularly dependent on its geometry and the thermomechanical treatment of the materials during the friction stir welding process. For joints between similar materials there are a number of numerical process models.However, there is no process model for the friction stir welding process of dissimilar joints.Therefore, the strength properties of dissimilar joints can merely be determined by experiments.The aim of this project is the development of a simulation model to predict the strength properties of friction stir welded joints of different metals depending on the welding parameters. The model is developed at the example of the technically relevant material combination aluminum-steel. To accomplish this aim, a three divided procedure is applied. To validate the results of the single steps reference welds are used.In the first step, the so called process model, the friction stir welding process is continuum mechanically simulated using strain, strain-rate and temperature dependent material models. In order to deal with the very large deformations and to treat the interaction of different materials as a multiphase problem, complex computation methods have to be used. The results of the process model - weld seam geometry as well as the time dependent temperature and strain fields of both materials - are used as input for the following two steps. The results of the process model are validated by micrographs and temperature measurements.In the subsequent step a damage mechanics model is adapted, to describe the relation between thermomechanical treatment of the materials during friction stir welding, the resulting changes in microstructure, the yield strength and the damage mechanical parameters. Based on the results of the process model areas of similar thermomechanical treatment are identified in the reference weld seam. From these areas samples are taken and tested for calibrating the damage mechanics model.In the last step, the results of both preceding steps are merged into a strength model of the weld seam. The failure behavior of the weld seam resulting from the process model is determined by the damage mechanics model. The numerically determined strength properties of the joint are validated by experimental tensile tests.With the assembly of the three steps, it is possible to predict the geometry and the strength properties of dissimilar joints starting from the process input variables of the friction stir welding process.