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A concept of combining friction stir spot welding (FSSW) and clinching, denoted as friction stir clinching (FSC), was proposed to join alclad 2024-T3 aluminum sheets. A tool, having a smooth probe and a flat shoulder, and a die, having a circular cavity and a round groove, were used to make FSC joints. The failure loads and fatigue lives of FSC joints made by various punching depths, rotational speeds, and dwelling times, were evaluated to obtain the admissible processing parameters. Optical micrographs of the FSC joints, before and after failure, were examined to understand the effects of processing parameters on the mechanical interlock and alclad layer distribution, which strongly correlate to the failure load, failure mode, and fatigue life of FSC joints. Finally, the static and fatigue performance of FSC joints made by the admissible processing parameters was obtained. The feasibility of the FSC process for alclad 2024-T3 aluminum sheets was confirmed.

In this paper, failure modes of dissimilar laser welds in lap-shear specimens of low carbon steel and high strength low alloy (HSLA) steel sheets are investigated based on experimental observations. Micro-hardness tests across the weld zones of dissimilar laser welds were conducted. The hardness values of the fusion zones and heat affected zones are significantly higher than those of the base metals. The fatigue lives and the corresponding failure modes of laser welds as functions of the load ranges are then examined. Optical micrographs of the laser welds before and after failure under quasi-static and cyclic loading conditions are then examined. The failure modes and fatigue behaviors of the laser welds under different loading conditions are different. Under quasi-static loading conditions, a necking failure occurred in the upper low carbon steel sheet far away from the laser weld.

Fatigue analysis of swept friction stir clinch (Swept-FSC) joints between 6061-T6 aluminum (Al) and S45C steel (Fe) sheets was conducted through experimental approaches. Before fatigue tests, a parametric study for the probe geometry of FSC tools was conducted in order to eliminate the hook structure inside the joint and improve the mechanical performance of the joint. Then a series of quasi-static and fatigue tests for Al/Fe Swept-FSC joints in lap-shear (LP) and cross-tension (CT) specimens were conducted. The fatigue data were recorded. The fatigue behavior of Al/Fe Swept-FSC joints in LP and CT specimens were examined through optical and scanning electron microscopes. Experimental results indicated that LP specimens have two failure modes, while CT specimens have only one failure mode. The dominant fatigue crack of each failure mode was identified.

Fatigue behavior of self-piercing rivets (SPRs) and clinch joints in lap-shear specimens of 6111-T4 aluminum sheets is investigated based on experimental observations and a fatigue life estimation model. Lap-shear specimens with SRPs and clinch joints were tested under cyclic loading conditions. Under cyclic loading conditions, fatigue cracks start from the curved interfacial surface of the upper sheet and then grow into the upper sheet thickness for both self-piercing rivets and clinch joints. The self-piercing rivets and clinch joints fail finally through the circumferential/transverse crack growth in the upper sheets and inner button crack growth, respectively. The structural stress solution and the experimental stress-life data for aluminum 6111-T4 sheets are adopted to estimate the fatigue lives of both types of joints. The fatigue life estimations based on the structural stress model show good agreement with the experimental results.

Failure modes of spot friction welds in cross-tension specimens of aluminum 6061-T6 sheets are first investigated based on experimental observations. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. Experimental results show that the failure modes of the welds under quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, the failure mainly starts from the necking of the upper sheet outside the weld. Under low-cycle loading conditions, the dominant fatigue cracks are the kinked cracks growing into the upper sheet from the crack tips; hence, the upper nugget pullout failure mode can be seen. Under high-cycle loading conditions, the dominant fatigue cracks are kinked cracks growing into the lower sheet from the crack tips; subsequently, the lower nugget pullout failure mode can be seen.

This paper studied the process development of dissimilar clinch joints in cross-tension specimens of aluminum alloy 5052-H32 (AA5052-H32)/thermoplastic carbon fiber reinforced plastic (TP-CFRP) sheets. The AA5052-H32 and TP-CFRP sheets with a thickness of 1.6 mm were used. The important processing parameters for AA5052/TP-CFRP clinch joints, such as the punching load, heating mode, heating temperature, and die depth, were considered. The failure loads, failure modes, and metallographic micrographs of AA5052/TP-CFRP clinch joints were analyzed to determine an available processing parameter set for fatigue tests. Finally, the fatigue performance and failure mode of AA5052/TP-CFRP clinch joints were obtained.

Local stress intensity factors (LSIFs) for kinked cracks of spot welds in four specimens, lap-shear, cross-tension, U-shaped, and coach-peel, were studied by three-dimensional finite element analyses. Finite element models for spot welds without and with kinked cracks were developed. Semi-elliptical cracks with various kinked crack lengths were assumed. Two dominant cracking modes for each specimen were considered. The global stress intensity factor (GSIF) solutions for spot welds without kinked cracks were first obtained to determine the analytical LSIF solutions for spot welds with infinitesimal kinked cracks. The LSIF solutions for spot welds with finite kinked cracks were then obtained. The LSIF solutions of the four specimens show similar general trends. As kinked crack length increases, the mode I LSIF solutions gradually increase and then decrease, while the mode II LSIF solutions show inverse trends.