Today, aluminum and its alloys are used in different areas of manufacturing and technology (e.g., automotive and aerospace) [1
]. Modern industries have been aggressively pushing the limits of aluminum alloys in strength, damage tolerance and corrosion resistance fronts to develop strong and tough aluminum alloys for various parts to increase the overall efficiency [2
]. The aluminum alloy 5xxx series is widely utilized in marine applications, such as ship hulls. AA5083 is considered one of the well-known representatives of the 5××× series, with many applications in aircraft, marine structural parts and automobiles [4
]. Welding of aluminum alloys is important for fabricating structural constructions and mechanical fabrications such as aircraft and marine vessels. However, welding exhibits problems and can be challenging in many cases. Indicative welding defects common to aluminum include incomplete fusion, hot cracking and porosity [5
Friction stir welding (FSW) is a solid-phase welding process giving good quality butt and lap joints and is being widely considered by the modern aerospace and automotive industries for high-performance, structurally-demanding applications. The peak welding temperature can be limited to 80% of the melting temperature of the base metal [6
]. Therefore, this process can be considered as a hot working process. Thus, the FSW process has proved to be ideal for creating high quality welds in several materials, including those which are extremely difficult to weld by conventional fusion processes [7
]. Furthermore, FSW is proven to avoid severe distortions, and the generated residual stresses are found to be particularly low, compared to traditional welding processes [9
Surface modifications of engineering components that comprise surface interactions are of major importance in the industries nowadays. The surfaces of engineering components can be traditionally protected by different methods (e.g., protective coatings and induction hardening). During the last decade, surface composites have been emerging as an effective way to improve the surface hardness and protect the components against wear and to reduce friction. One of the most important surface composite manufacturing processes is the friction stir process (FSP) [10
]. FSP is a surface modifying technique which involves the generation of friction heat and intense plastic flow. FSP was developed from the basic principles of FSW. Owing to its high mixing ability, methods based on FSP have been developed for the incorporation of reinforcing particles into the modified surface (stir-zone) to further improve the hardness of the base materials [11
]. The particles used as reinforcing fillers are mostly either ceramic (e.g., Al2
) or carbide (e.g., TiC, SiC or WC) powders. Kurt et al. [12
] incorporated SiC particles into cold-rolled plates of AA1050 using FSP; the hardness values were increased from 67 HV (plain specimen) to 80 HV, and the bending strength was increased from 60 MPa to 84 MPa. Navazani et al. [13
] used FSP for the introduction and fine dispersion of nano-ZrO2
particles in AZ31 alloy; the obtained specimens introduced enhanced hardness values from 55 HV for the plain material to 87 HV for the two FSP pass reinforced one. The yield stress and the ultimate tensile strength were found increased in the FSPed specimens; on the other hand, the toughness was decreased. The change in the mechanical properties was found attributed mainly to the refinement of the microstructure. Mirjavadi et al. [14
] manufactured AA5083/ZrO2 nanocomposites through FSP process. The hardness values were gradually increased by increasing the FSP passes due to constant refinement of the microstructure; 130 HV hardness were achieved for eight FSP passes. Another approach in surface reinforcing using FSP is in-situ reinforcing. In-situ approach of surface reinforcing involves synthesizing desirable reinforcements during processing itself. In-situ composites present many benefits, such as reinforcement-matrix interfaces free of defects, thermodynamically stable reinforcements, improved compatibility and higher bonding strength between the reinforcements and the matrix [15
Based on the above, a key limitation is identified and concerns the fact that aluminum-based and aluminum-alloy-based metal matrix composites, formed during friction stir process by an in-situ reaction, are of high interest and have not been well researched. Thus, in the current study, a new approach in applying friction stir process for fabricating in-situ composites is presented. The method starts with the machining of a shallow groove in the surface of the base material. The groove is then filled with a thin sheet of the reinforcing metal, and then multiple FSP passes are carried out for the integration of the reinforcing metal into the FSPed stir-zone. The logic of the process is that: (a) the intense stirring action of the process will integrate the thin soft sheet of the reinforcing metal in the form of small particles inside the stir zone; (b) due to the high heat input of the FSP process, the integrated reinforcing particles are expected to form in situ intermetallic compounds, and these intermetallics will act as secondary phase reinforcements to the matrix; (c) at the same time, diffusion of the reinforcing metal in the matrix could lead to a solid solution matrix that is mechanically tougher. Thus, in the present study, plates of the non-heat treatable aluminum alloy AA5083-H111 were used as the base material. As reinforcing material, a thin sheet of pure copper was used; the reinforcements in the aluminum matrix were synthesized by in-situ chemical reactions between copper and AA5083 during the FSP process. Copper was used because it is one of the few elements that has relatively high solubility in aluminum. Furthermore, Al–Cu systems are being extensively studied due to their potential applications at ambient to elevated temperatures [17
]. Specimens with different numbers of FSP passes were successfully created and analyzed by the means of optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques. Finally, the macrohardness distribution was calculated, evaluated and correlated to the microstructural outcomes.
2. Materials and Methods
As the base material, 6 mm thick plates of aluminum alloy AA5083-H111 (in work hardened condition) were used. Aluminum alloy AA5083 is a non-heat treatable aluminum alloy widely used in the transportation, marine and chemical industries. It presents excellent weldability and formability, mild strength and mild corrosion resistance [18
]. A spectrometer was used to verify the chemical constituents of the AA5083 plates. The analytical chemical composition is given in Table 1
. The given values are the mean values of three independent measurements. A thin sheet of pure copper with cross-section 4 mm width and 0.8 mm thickness was used as the reinforcing material. Furthermore, a thin sheet of pure aluminum with cross-section 4 mm width and 0.6 mm thickness was used to cover the copper sheet (the usage of this layer is analytically explained in Section 2.2
The implementation of a copper strip instead of powder minimizes the volume of reinforcing material needed (and the groove’s dimensions), reduces the air inserted into the stir zone and introduces a more economical process of surface composite fabrication.
2.2. Friction Stir Process
illustrates the methodology followed in the manufacturing process. Initially, a groove with cross-section of 4 mm width and 1.4 mm deep was machined in the aluminum plate. The copper thin sheet was placed in the bottom of the groove and the pure aluminum sheet was placed above (Figure 1
a). The groove was covered/closed, using a pinless tool, by a single FSP pass (Figure 1
b). This stage was necessary in order to create a surface aluminum layer that keeps the thin copper sheet stable inside the groove during the next FSP stage. The parameters used for the pinless FSP pass were 1000 rpm rotational speed combined with 120 mm/min transverse speed. Preliminary experiments necessitated the use of the upper thin aluminum sheet because of the direct contact of the FSW tools with the copper led to aluminum/copper intermetallic material getting bonded on the steel FSW tool due to high heat input and intense plastic deformation. The adhesion of the intermetallic compounds to the FSP tool threated pin resulted in wormhole type defects and the adhesion in the shoulder resulted in surface morphology defects. The addition of a thin aluminum sheet that covered the copper eliminated the adhesion of compounds to the FSP tool.
Initially, experiments were conducted with the intention of identifying the optimum parameters that result in a FSPed stir zone consisting of good material mixing and no defects. The optimum operational parameters were obtained for 1000 rpm rotational speed (maximum of the FSW machine) combined with 13 mm/min transverse speed (lowest of the used FSW machine). Those parameters, due to the high heat input (high weld pitch) and the intense plastic deformation, were projected according to literature to present the optimum material mixing for AA5083 friction stir processing [19
Thus, after the stage of closing the groove with the pinless tool (Figure 1
b), FSP passes were carried out in the same direction, successively and without allowing the samples between the FSP passes to cool down to room temperature (Figure 1
c). Furthermore, in order to study the influence of the number of FSP passes on the integration of copper in the stir-zone; one, two and three passes were performed respectively.
The FSP experiments were carried out by the use of a modified milling machine. The friction stir welding/processing tool was made from heat-treated tool steel. The shoulder of the tool was flat with a diameter of 22 mm. The pin of the tool was cylindrical with a right-handed thread, 4 mm diameter and 4 mm height.
2.3. Microstructural Characterization
Initially, a metallographic specimen preparation was performed. The FSPed specimens were cut perpendicularly to the direction of the FSP. Then the specimens were mounted in epoxy resin, and their surfaces were grinded and polished, reaching a final polishing stage of 0.05 μm colloidal silica suspension. The samples were then treated for a short period of time in an ultrasonic bath in order to remove the remaining particles from the polishing stage. Finally, the samples were etched by a soft Keller's reagent for ten seconds.
After the metallographic specimen preparation, an analytical microstructural characterization was performed on the stir zone of the friction stir processed specimens. Firstly, the specimens were observed using optical microscopy (OM) methods. An optical stereoscope Leica MZ6 (Wetzlar, Hesse, Germany) was used for the macroscopic observation of the FSPed stir zones, and an optical microscope Leica DMILM was used for investigation of the material flow inside the stir zone. The embedded copper-based intermetallic particles were further characterized using a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) detector at an accelerated voltage of 20 kV. Furthermore, in order to observe the surface topography characteristics, atomic force microscopy (AFM) experiments were carried out. Finally, an X-ray diffraction (XRD) study was performed in order to investigate the phases formed in the friction-stir-processed composite stir-zone.
2.4. Macrohardness Distribution Evaluation
The microstructural observations were correlated to macrohardness distribution analysis. The macro-Vickers was preferred over micro-Vickers due to existence of large (up to 80 μm) copper-based particles in the stir-zone. Under these conditions, the stir zone was not homogeneous and the large indent size of the macro-Vickers averaged out the inhomogeneities to obtain a bulk hardness value. The calculation of macrohardness distribution profile was carried out by an Instron Wolpert 4021 V-Testor (Norwood, MA, USA) (3000 gf for 15 s). Furthermore, the macrohardness values were correlated with a FSPed specimen without copper addition (AA5083 plate with three FSP passes without any Al–Cu intermetallic compound reinforcements).
Based on the present work, a very promising method for surface modification and surface hardening is presented. The main novelty of the suggested method lies in the fact that a copper strip can be easily integrated in the aluminum stir-zone, with the use of FSP, in te form of micron-sized particles, and thus modify the mechanical and physical properties of the weld. The copper particles, during the FSP process, due to the intense heat input and plastic deformation, tend to get enriched with aluminum atoms through interplanar diffusion–interfacial migration effects, and layers of different hard and complex phases are created in-situ. The implementation of a metal strip instead of powder minimizes the volume of reinforcing material needed, reduces the air inserted into the stir zone and introduces a more economical process of surface composite fabrication. The proposed process could have potential applications for manufacturing hybrid alloys. For example, materials that on one side are pure AA5083, and have very good corrosion resistance and moderate mechanical properties, on the other side (the cu reinforced side) have better mechanical properties with increased hardness and wear resistance. The possible applications of this method are of high interest in modern industries, and research is needed for the application of the method for different combinations of reinforcing and base metals.