Effects of Friction Stir Welding Process Control and Tool Penetration on Mechanical Strength and Morphology of Dissimilar Aluminum-to-Polymer Joints

Abstract

An engineering grade polymer—glass fiber-reinforced polyphenylene ether blended with polystyrene—and an aluminum alloy—AA6082-T6—were joined by friction stir welding in an overlap configuration. A comprehensive analysis was conducted of the effects of the tool penetration by adjusting the pin length and the process control on the joints’ mechanical performance. To this end, a series of welds with a fixed 3° tilt angle, a travel speed of 120 mm/min, and 600 RPM of rotational speed was carried out. The analysis encompassed the mechanical strength of the fabricated joints and the mechanical energy input throughout the joining processes, the resulting cross-sectional interfaces, both on macro and micro scales, and the observed defects. The quasi-static shear tensile tests resulted in average tensile strengths varying between 5.5 and 26.1 MPa, representing joint efficiencies ranging from 10.1% to 47.4%, respectively. The joints that exhibited the lowest mechanical performance were fabricated with the highest level of tool penetration (higher pin length) with the process being position-controlled, while the best performance was recorded in joints welded with the lowest tool penetration and a force-controlled process. Nonetheless, the joint welded with a 2 mm long pin and position-controlled process exhibited a mechanical strength comparable with the highest one with a significantly lower standard deviation, a promising attribute for technological industrialization. In this way, it was found that the tool penetration, controlled by adjusting the pin length, played a significant role in the development of the joints’ morphology and, consequently, mechanical performance, whereas the process control exhibited a minor influence on the mechanical performance of the joints, but a considerable effect on process repeatability.

Thermo-Mechanical Characterization of Metal–Polymer Friction Stir Composite Joints—A Full Factorial Design of Experiments

“…The macrostructure of the joining region of the three joints was found to be remarkably similar, with the aluminum plate being clinched into the polymeric plate. The interface between the base materials evidenced a prominent double concavity shape, which is a geometrical feature that induces macro-mechanical interlocking between the base materials, as previously reported by Correia et al [ 19 , 40 ]. All three joints exhibited a small void located between the stirring zone (SZ) and the thermo-mechanically affected zone (TMAZ) toward the RS of the joints; this defect was also identified and explained by Derazkola et al [ 27 , 33 , 38 , 48 ] and Huang et al [ 28 , 49 ].…”

“…Since the DoE was designed with three factors (tilt angle, rotational speed, and travel speed) and two levels, a total of eight (2 3 = 8) composite joints with different parameter settings were fabricated. The range associated with each independent variable under analysis was based on previous research carried out by Correia et al [ 19 , 40 ]. The numerical expressions that relate the joining parameters to both mechanical strength and processing temperature are expected in the form of Equation (1).…”

“…Moreover, the energy released by the materials undergoing severe plastic strain is also converted into heat, and both mechanisms generate the necessary heat input to produce the desired joint; therefore, it is considered a clean and environmentally friendly technological process [ 15 ]. The FSW process is commonly divided into the following four stages: (i) the plunging stage, (ii) the dwelling stage, (iii) the welding stage, and (iv) the retracting stage [ 19 ]. The industrial application of such technology is not confined to the aeronautical/space industry, with the automotive industry also showing interest in applying FSW technology to the manufacturing processes associated with battery trays [ 20 , 21 ], battery packs [ 22 ], and the fuel cell system’s subframe [ 23 ].…”