Numerical and experimental investigation of clinching joints of dissimilar metals

Abstract

The manufacturing and assembly of thin metal sheet structures and products involves the unavoidable process of selecting the appropriate joining techniques. The continuing search for novel, sustainable, and advanced lightweight products creates a high demand on the technical aspect of the joining. To integrate these new lightweight materials in different structures, such as vehicles or other products, suitable joining techniques have to be presented. Spot welding is one of the most traditional methods used for joining sheet metal in automotive and aircraft structures. Therefore, the new technological clinching method used for mechanically joining sheet metal of different thickness and properties can play a significant role as an alternative method for joining dissimilar materials. In this study, the clinching process for joining dissimilar materials is investigated in order to achieve the highest quality and strength of the clinching joints. The study is divided into four major parts. In the first part, finite element analysis (FEA) for a fixed and extendable die of clinching joining techniques is proposed. LS-DYNA explicit and implicit software is employed to simulate the process in order to evaluate the joint strength. FE results were validated using the experimental data from the open literature for the fixed die, while the clinching process using the extendable die was validated by experiments. In the second part of the study, an optimization for the process parameters, such as the shape of punch and die, was conducted. The optimization of the process parameters is developed based on Taguchi and the Taguchi-based Grey method. Taguchi's L27 orthogonal array design and the notion of the signal to noise (S/N) ratio is utilized to obtain the objective function. The results demonstrate that the optimal combination of the process parameters, which leads to a high-strength connection, is dictated by multiple characteristics, namely, the bottom thickness, the interlock length, and the neck thickness of the joint. In the third part of the study, experimental investigations with the aim of validating the proposed method for finding the optimal combination of process parameters are conducted. In this context, clinch joints created from the same or two different materials with varied plastic and strength properties are experimentally tested. Two types of samples are then investigated using the tensileshear and pull-out tests, one with a single clinch bulge and the other with two or more clinched bulges. The experimental results show that there are different factors controlling the mechanical connection strength such as the tools' geometry and the final shape of the joint. Additionally, the experimental results show that the clinching joint quality and functionality may have undesirable features because of the choice of the forming process parameters. Moreover, the testing results of the hybrid clinchbonded joints compared to the conventional clinched joints show that there is a significant increase of the clinched joint quality and strength. The fourth part of this work focuses on the applications of the Digital Image Correlation (DIC) technique as a nondestructive testing (NDT) tool for the inspection of mechanical clinching joints. In this approach, the joint is loaded with the maximum load that it is expected to encounter during actual operation (i.e., the design load) and two images are recorded, one before and one after applying the load. Due to the thin sheets, the artificially synthesized defects are very difficult to implement during the metal forming. The results of this study show that this approach can be used for detecting the initiation of the defects, in other words, the starting failure point of the clinched joint.