Friction spot joining (FSpJ) of aluminum-CFRP hybrid structures

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.

Durability of Metal-Composite Friction Spot Joints under Environmental Conditions

“…In contrast to the SB and CC specimens, the PAA-P pre-treated joints showed a reduction in the lap shear strength in the first six months, while retaining their residual strength on the same level afterwards. PAA-P pre-treatment led to strong chemical carbon-carbon bonds between CF-PPS and aluminum [6,30]. It seems that moisture does not have a great influence on such chemical bonds, and the joint retained more than 80% of its initial strength.…”

“…FSpJ was used in this work to join the parts together. The principles of the process have been explained in our previous publications [1][2][3][4]6]. Briefly, the process uses a non-consumable tool, plunging into the aluminum sheet, which was placed on top of the composite in an overlap configuration to a pre-defined position while rotating at high speed.…”

“…Four surface pre-treatments were selected; these were sandblasting (SB), stand-alone conversion coating (CC), phosphoric acid anodizing (PAA), and PAA with a subsequent application of primer (PAA-P). Although SB + CC gave a slightly higher lap shear strength than stand-alone CC in dry conditions [6,30], the CC specimen was selected for the aging experiments, in order to understand the behavior of chemical pre-treatment under environmental conditions. For a detailed explanation of each surface pre-treatment, refer to [6,30].…”

“…It can be argued that the moisture and humidity could penetrate slightly into the overlap area, deteriorating the bonding between the aluminum and composite, especially in the area of the consolidated molten PPS (known as AZ). Because AZ is the weakest part of the bonding area in a FSp joint [4,6,52], the penetration of the moisture inside this layer should be easier. As the fracture surface of the joints shows in Figure 16a,b, no visual changes were observed within the bonding area of SB and CC specimens after six months.…”

“…In our previous publications, we investigated the influence of various process parameters on the mechanical performance of the FSp joints [3,5]. Moreover, the effect of different aluminums' surface pre-treatments on the lap shear strength of the aluminum alloy 2024-T3/carbon-fiber-reinforced poly (phenylene sulfide) (CF-PPS) FSp joints was recently reported [6], as well as its impact resistance [7] and corrosion properties [8] for Al-sand-blasted treatment.…”