Principles of the process

The process allows multiple types of control, such as force control, time control, and multiple joining phases. For instance, a process with force control regulates the joining force by maintaining a pre-set level while the time is a response. Alternatively, the process can use time control, obtaining the force as a response. Moreover, the duration of each joining phase can be limited by time and displacement. For a time-limited process, the transition to the next joining phase happens as soon as a pre-set time of the previous joining phase is reached. When such a switch of joining phases depends on a pre-set axial position of the spindle, and consequently of the rivet, the limitation happens by displacement. Each combination of control and limitation is a possible process variant. The variant can be selected according to the requirements of joining cycle and knowledge maturity, regarding the material’s response to heat generation during the process and geometric restrictions of the joining parts (e.g. accuracy of rivet penetration depth into thin polymeric plates).

The most consolidated and studied process variant is a force controlled and time-limited process comprising three phases: friction, forging and consolidation phases. The main steps of this process variant are depicted in Figure 3.6 for an overlap joint configuration. The polymeric parts are pre-assembled and fixed onto a backing plate, while the metal rivet, attached inside the chuck in the welding head, is aligned with the center of the polymeric parts (Figure 3.6-a). After the positioning step, the rivet is rotated at a pre-set rotational speed and pressed against the surface of the polymeric plate (Figure 3.6-b). The high rotation and axial force heat up the polymer part leading to softening and melting of a thin polymeric layer around the metal rivet. Due to the continuous plunging of the rivet into the polymeric part, the softened or molten polymeric layer is expelled from the joining area, forming a flash. Despite the unavoidable flash formation, such material can be removed during the joining process by using additional cutters on the welding head or be entrapped into features such as reentrances in the rivet head. Additionally, due to the low thermal conductivity of the polymer, the local temperature at the rivet tip rises and approaches the plasticizing temperature of the metal (60 % to 90 % of the melting temperature of metal alloys, such as aluminum and titanium). Thereafter, the rivet’s rotation is stopped and the axial force is increased (Figure 3.6-c). The plasticized rivet tip is pushed against the cold polymeric layer beneath it, which creates the resistance required to deform the metal, anchoring it in the polymeric part. Finally, the joint is consolidated by cooling under pressure (Figure 3.6-d), the spindle retracts and the friction riveted joint is created.

Figure 3.6 Schematic representation of Direct Friction Riveting process steps: a) positioning of the joining parts, b) friction phase (plunging of the rotating rivet through the upper part), c) forging phase (plunging of the rivet through the lower part and rivet plastic deformation), and d) joint consolidation. The flash formed

during the process was not illustrated for simplification.

The plastic deformation of the rivet tip is a function of frictional heat and axial forces.

However, when the heat supplied during the friction phase is sufficient to plasticize a large volume of metal and partially deform the rivet tip, the forging phase can be omitted, as reported by Proença et al. [145]. The authors showed that, for the combination of glass fiber reinforced polyamide 6 and aluminum alloy 6056-T6, an increase of axial force during the process did not have any significant influence on deformation of the rivet tip and likewise the mechanical performance of the joints produced. However, this behavior was only observed for samples

produced with high heat input conditions, which yielded high plasticizing levels in the metal part during the friction phase. Achievement of this can increase the energy efficiency of the process while also contribute greatly to the overall understanding of friction riveting.

Figure 3.7 illustrates the monitoring diagram of a force controlled and time-limited process with (Figure 3.7-a) and without (Figure 3.7-b) the forging phase. As shown in both cases, during the frictional phase, both rotational speed (black line) and joining force (dark gray line) increase to the defined level while the spindle moves downward (bright gray line). For the process without a forging phase, the axial displacement of the spindle decelerates whereas the force and rotational speed remain constant. In the case of a process with forging phase, the axial displacement accelerates as a response to increasing axial force while the rotational speed decreases. The final consolidation phase is similar in both cases, with no axial movement of the spindle and no rotational speed, but under some pressure, either additionally imposed or simply due to the weight of the spindle.

Figure 3.7 Schematic force-controlled FricRiveting diagram for a) a process with forging phase, and b) without forging phase.

The primary bonding mechanisms of friction riveted joints are mechanical interlocking, by the rivet tip widening into the polymeric part, and adhesion forces believed to be present where the molten polymer is in intimate contact with the metal. The efficiency of the rivet’s mechanical anchoring has been shown to be the main contribution to the joint’s mechanical performance under quasi-static loading [25,27,28]. According to Pina et al. [146], not only the widening of the rivet tip affects the joint strength, but also its shape, noting that overdeformation has a significant negative influence on the joint’s strength. Although reported, this effect has not had any deep scientific investigation, and the topic will be covered in Section 6.3.2.