Development of fed friction-stir (FFS) process for dissimilar nanocomposite welding between AA2024 aluminum alloy and polycarbonate (PC)

Highlights

• A novel fed friction-stir (FFS) technology was introduced for nanocomposite joining.

• FFS process implemented for dissimilar welding of AA2024 alloy and PC.

• An attractive lap-joint design was produced as the weld nugget strengthened by Al₂O₃ nanoparticles.

• Dissimilar joining mechanisms were controlled in terms of processing parameters optimization and reinforcing powder feeding.

• The highest tensile and bending joining efficiencies up to ∼53 % and 93 % ratios attained.

Abstract

In this research, a new fed friction-stir (FFS) technology was implemented for dissimilar nanocomposite joining between AA2024 aluminum alloy and polycarbonate (PC) polymer with a T-joint design. Alumina (Al₂O₃) nanoparticles in the form of paste were injected through the stir zone (SZ) during dissimilar mixing of aluminum and polymer using the FFS system to strengthen the polymer side. The effects of processing parameters on the soundness and characteristics of dissimilar weldments were examined in terms of rotational tool speed (w = 1400−2100 r/min), traverse velocity (v = 25−55 mm/min), and reinforcement powder feeding. Stiff competition was found between w and v on controlling the weld nugget formation and its property. Increasing w and heat input ratio showed a beneficial influence on the materials flow and intermixing. Meanwhile, increasing v revealed detrimental impact by affecting the alumina powder feeding rate into the SZ. According to experiments, to attain a sound and defect-free dissimilar nanocomposite joint with superior mechanical property, the central processing parameters were optimized in the moderate range as w = 1750 r/min and v = 35 mm/min. A maximum tensile strength of ∼36 MPa and maximum flexural strength of ∼74 MPa were noted. Hence, the maximum joining efficiency was determined by ∼53 % and 93 % ratios of PC as the weakest part of dissimilar weldment during tensile and bending tests, respectively, with a failure behavior across the SZ.

Introduction

Recently, the dissimilar joining of metals and polymers has considered significant attention in automotive applications for weight reduction and a combination of different properties [[1], [2], [3], [4], [5], [6], [7]]. In this regard, linear friction-stir welding (FSW) was considered as an advanced solid-state joining technology for the production of such metal-polymer hybrid structures [1,[8], [9], [10], [11], [12]]. By promoting the materials intermixing and interaction in the solid-state, FSW showed an admirable capability on the formation of dissimilar bonding, as reported in the literature for different systems from various metals and polymers [8,9,[13], [14], [15], [16], [17], [18]]. In the first research by Moshwan et al. [19], the feasibility of dissimilar friction-stir joining between AA7075 aluminum alloy and polycarbonate (PC) polymer was assessed. A butt-joint design was used, and the formation of several defects as shrinkage micro-voids and inferior materials intermixing was noted through the weld nugget. Rahmat et al. [20] worked on the same system of AA7075-PC in a butt-joint design, however, the experimental results showed a feeble intermixing of dissimilar materials in the weld nugget with a rather low strength ratio of <10 %. Then, Liu et al. [21] employed friction lap welding for dissimilar joining between metal and plastic. In this context, AA6061 aluminum alloy and MC Nylon-6 were used as the raw material. Produced different weldments were failed at the interface during transverse tensile testing with a joint strength ratio of ∼30 % concerning the polymer side as the weakest counterpart. Higher mutual satisfaction between metal and polymer was attained using friction lap welding in the research of Okada et al. [22]. The joint strength ratio for such dissimilar metal-polymer weldments by the FSW process was enhanced in the study of Khodabakhshi et al. [6] by the promotion of materials intermixing in a butt-joint design and activation of physical and chemical materials interaction. For dissimilar weldment between AA5059 aluminum alloy and high-density polyethylene (HDPE), a maximum joint strength ratio of ∼50 % was measured during transverse tensile testing with failure location from the weld nugget and aluminum alloy interface. After that, in another study by Khodabakhshi et al. [5], the possible bonding mechanisms between metal and polymer during FSW joining were examined by the implementation of high resolution-transmission electron microscopy (HR-TEM) analysis from the interaction area at the interface. In that research, the formation of chemical bonding during solid-state chemical mixing was noted for the first time further than the occurrence of mechanical interlocking and Van-der Waals physical interaction. Accordingly, a thin semi-crystalline alumina layer at the interface was observed.

After these preliminary results, state of the art on this field was extended in the following years with a focus on studying the other metallic-polymeric systems and promoting the mechanical performance of dissimilar weldments. In one of the subsequent studies by Ratanathavorn et al. [23], the feasibility of dissimilar joining between AA6111 aluminum alloy and polyphenylene sulfide thermoplastic using friction-stir lap welding was assessed. In the different joint designs, the aluminum part was placed on the top, and joining mechanism controlled by fragmentation of aluminum and its dispersion through the polymer matrix led to the formation of a mixed weld nugget with a joint strength ratio of ∼55 %. Dissimilar joining of AA6061 aluminum alloy and polycarbonate (PC) sheets using the FSW process in a butt joint design was examined in the work of Patel et al. [10]. The formation of micro-void shrinkages across the weld nugget deteriorated the strength ratio of dissimilar weldment lower than 35 % with fracture location across the stir zone (SZ). In the research by Aghajani Derazkola et al. [4], temperature profile, materials flow pattern, and intermixing between AA5058 aluminum alloy and poly-methyl-methacrylate (PMMA) polymer during dissimilar friction-stir joining were simulated based on finite element (FE) analysis. In the experimental aspect, the formation of some surface tunnels noted using a lap-joint design and considering the polymer part on the top-side. In another research by Aghajani Derazkola et al. [3] in following of the previous study, the effects of processing parameters in terms of rotational tool speed, traverse velocity, tool tilting angle, and plunge depth on the soundness and mechanical performance of dissimilar friction-stir weldment between AA5058 alloy and PMMA polymer were examined. By optimizing the main processing parameters, an enhanced dissimilar joint strength ratio of ∼60 % was reported during transverse tensile testing. In new research by Huang et al. [9], dissimilar friction-stir lap welding between AA6061-T6 aluminum alloy and polyether ether ketone (PEEK) polymer was assessed. It was tried out to promote materials mixing and mechanical interlocking between metal and polymer by placing the metal plate on the top side and utilizing a threaded conic tool. The maximum tensile-shear bonding strength of ∼20 MPa was reported, which estimated a less than 20 % ratio of the polymer as the weakest base material. Effects of processing parameters on the dissimilar friction-stir weldments between AA5058 aluminum alloy and polycarbonate (PC) polymer with a lap-joint design and placing the polymer part on the top side were studied in a new work by Aghajani Derazkola et al. [24]. Bubbling mechanism controlled the formation of micro-void shrinkages found as a deteriorative phenomenon against the mechanical property of dissimilar weldments. Meanwhile, by optimization of processing parameters to minimize the welding defects, a maximum tensile-shear strength ratio of ∼68 % was reported with failure location from the SZ toward the polymer side.

Despite these reports, there is still a big challenge on the occurrence of mechanical and chemical bonding between metals and polymers during the FSW process needing more development by some modifications on the design and technology. As mentioned, the maximum report for joining efficiency was ∼70 % of the polymer side. Such a report is not a satisfactory criterion for practical applications under severe mechanical loading conditions such as the automotive industry. Newly, a novel process was developed by Aghajani Derazkola et al. [25] based on the traditional FSW route called as fed friction-stir (FFS) by feeding of reinforcing material through the rotating tool inside the weld nugget to form a dissimilar nanocomposite weldment between metal and polymer. According to the presented results in the first report on AA6082-PMMA system by feeding of alumina nanoparticles, the impact of this innovative method was on improving the joint quality by strengthening the polymer side and reducing the physical and mechanical differences between metal and polymer parts. In this research, the application of the FFS process was developed for dissimilar T-joint weldments between AA2024 aluminum alloy and polycarbonate (PC) polymer by the implementation of Al₂O₃ nanoparticles feeding and reinforcing the weld nugget. The main focus of this study was on elaboration the influence of main processing parameters. They were assessed in terms of rotational tool speed and traverse velocity to control the reinforcing powder feeding rate as well as the tool plunging depth, offset distance, and tilting angle on the microstructural features and mechanical characteristics of dissimilar weldments.