No abstract

Friction spot joining is an alternative technique to produce metal-composite overlap joints. The main process parameters are tool rotational speed, plunge depth, joining time and joining force. In this study, the individual effect of the process parameters on the microstructure and mechanical strength of hybrid AA6181-T4/CF-PPS double lap joints was investigated using Taguchi method and analysis of variance (ANOVA). Produced joints presented mechanical performance from 2107 N to 3523 N. Joints failed by brittle fracture at the interface between aluminum alloy and composite, with displacement-at-peak load values from 0.7 mm to 0.9 mm. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by the joining time, plunge depth and joining force. Higher strength was correlated to the extension of the bonding area and macro-mechanical interlocking related to the formation of a metallic indentation (metallic nub) slightly inserted into the composite. Larger bonding areas were shown to be related to higher heat input (as a result of longer joining times and intermediate rotational speeds) leading to larger consolidate polymeric layers at the metalcomposite interface. Higher macro-mechanical interlocking was obtained at larger plunge depths. Joining force was shown to be related to crevice and pore filling of the metal surface by supporting spreading of the molten polymer. Higher joining forces led to better wetting of the interface increasing adhesive forces and joint mechanical performance. Nevertheless excessive joining forces caused squeezing flow of the molten layer reducing joint strength, since a large adhesive area was lost.

a b s t r a c tThe effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370-474°C.

No abstract

No abstract

The employment of various materials (such as lightweight metal alloys and composites) with distinct physicochemical properties in the automotive and aerospace industries has opened a new field of research into the joining of dissimilar materials. Several alternative methods have recently developed for joining metalcomposite multi-material structures. Friction spot joining (FSpJ) is an innovative technique within welding-based joining technologies suitable for metal-composite structures. This work aims to address and overview different aspects of FSpJ. Case-study overlap joints using aluminum alloy AA2024-T3 and carbon-fiber reinforced poly(phenylene sulfide) (CF-PPS) were produced. Peak temperatures of up to 474°C were recorded during the process. Such temperatures are well below thermal decomposition of PPS and extensive thermal degradation of PPS was not detected by thermal analysis in this work. Microstructure analysis was performed showing usual metallurgical phenomena (recovery and dynamic recrystallization) taking place with friction-based aluminum joining. Microstructural changes caused an alteration to the local mechanical properties as confirmed by microhardness and nanohardness measurements. Moreover, microstructural analysis of the composite part revealed the formation of a small number of volumetric defects such as pores and fiber-matrix debonding. Bonding mechanisms at the interface were studied into details by microscopy analysis and X-ray photoelectron spectroscopy. The influence of various aluminum surface pre-treatments on the bonding mechanisms and mechanical performance of single-lap shear joints was studied. In addition, fatigue life of the joints was investigated using an exponential model to obtain S-N curves. Finally, the quasi-static strength of the friction spot joints was compared with the state-of-the-art adhesive bonding. Friction spot joints showed 50% stronger joints than adhesively bonded joints, indicating the potential of the technique to be used for joining lightweight metals to composite materials.

Welding-based joining technologies have been recently developed for metal-composite lightweight structures. In this work, the welding-based joining technology, friction spot joining, was selected to study the failure and fracture micro-mechanisms of an aluminum-composite single lap joint. Failure analysis suggested that the radial cracks nucleate at the periphery of the bonding area and propagate rapidly until failure of the so-called adhesion zone. Upon further loading the cracks propagate into the transition and plastically deformed zones leading to a reduction of the stiffness of the joint. The final stage of failure occurs rapidly in a catastrophic manner. The findings of the fractography using scanning electron microscopy demonstrated a mixed brittle-ductile fracture. Three zones were identified on the fracture surfaces: a smooth and featureless area known as the mirror zone demonstrating brittle fracture, a quasi-smooth area representing a mixture of ductile and brittle fractures and finally a zone with a highly rough surface implying ductile fracture of the composite part. Further, fiber pull-out and breakage were identified as additional fracture micro-mechanisms. All in all, metal-2 composite joints produced by welding-based techniques demonstrate a mixed brittleductile fracture.

No abstract