## 1 Introduction  
In order to improve fuel efficiency and crashworthiness in the automotive industry, the application of various materials rather than the conventional mild steel has been expanding [1]. Increased application of non-ferrous metals and dissimilar material combinations now requires non-conventional joining processes instead of resistance spot welding (RSW) and gas metal arc welding (GMAW), which are the main joining processes in car body manufacturing. Since non-ferrous metals and dissimilar materials have poor weldability due to poor solubility, cracking, porosity formation, and so on, mechanical joining processes have been applied instead of fusion welding processes. The representative mechanical joining processes are blind riveeting, clinching, self-piercing riveeting (SPR), and high speed bolt joining such as flow drilling screw [2; 3; 4]. Compared with other mechanical joining processes, SPR process has several advantages such as a wide range of base materials, no need for a pre-drilled hole, high load-bearing capacity, and fast cycle time. In the automotive industry, it has been successfully applied to high-volume vehicles such as the Ford F150, which sold over 1 million units. [5]. During the SPR process, a semi-tubular rivet is penetrated into two or more sheets, and its skirts flare into the bottom sheet with the guidance of a die. A high setting force and rigid C-frame are also essential requirements for the SPR process because it is a cold-forming process with piercing and flaring phases [5; 6]. As the need for SPR process increases, various kinds of studies have been published. [PERSON] et al. presented the failure load model for Al SPR joints under pull-out and shear loading conditions [7]. [PERSON] et al. also evaluated the joint strength in lap-shear and cross-tension loading conditions [8]. [PERSON] et al. [9] and [PERSON] et al. [10] defined the cross-sectional characteristics that can alter the mechanical properties of the joint. Previous studies have discussed the joint characteristics according to various combinations such as Al/Fe [11; 12], CFRP/metal [13], CFRP/steel/Al [14], clad steel/Al [15], and pre-coated steel sheets [16]. As the hardware elements of the SPR process, various combinations of rivet and die have been experimentally evaluated to investigate the effects of shape [17; 18; 19] and coating [20; 21; 22]. Numerical models have been developed to simulate the riveting process and optimize the rivet, die, and process parameters [23; 24]. Galvanic corrosion as an important issue in dissimilar metal joints has been discussed in the literature [25; 26]. The fatigue behavior of SPR joints has been investigated for Al alloys [27; 28; 29; 30] and dissimilar material combinations [28; 31]. In most studies, the rivet, die, and setting force are considered as the main process parameters for given material combinations. In a few publications, a rivet setting speed was additionally considered as a process parameter when a servo gun was used [32; 33]. From extensive experiences of RSW for sheet metal, it is known that there are diverse abnormal process conditions that are not usually considered as process control parameters but affect the quality of welding. [PERSON] et al. suggested four setup faults such as angular misalignment, fit-up faults, axial misalignment, and edge welds during RSW [34]. [PERSON] et al. added poor cooling and electrode wear as abnormal welding conditions regarding an electrode, and they investigated the relationship between abnormal conditions and RSW quality [35; 36; 37]. Recently, there have been attempts to apply optimization using machine learning algorithms to automotive spot welds [38; 39]. The distinction between control and noise parameters is a prerequisite for process parameter optimization and database construction. However, most studies for the SPR processes have only focused on the main process parameters, and far too little attention has been paid to the study of abnormal process conditions.