**1. Introduction**

Additive manufacturing (AM) is a promising technology in numerous engineering applications. It involves the fabrication of various 3D objects by adding layer by layer material (alloy, plastic, concrete, human tissue, etc.) regardless of any size and shape [1,2]. Fusionbased additive manufacturing (F-BAM) techniques are used for different alloys [3]. Still, they are not suitable for aluminum-based alloys, especially the heat-treatable alloys (2xxx

**Citation:** Ahmed, M.M.Z.; El-Sayed Seleman, M.M.; Elfishawy, E.; Alzahrani, B.; Touileb, K.; Habba, M.I.A. The Effect of Temper Condition and Feeding Speed on the Additive Manufacturing of AA2011 Parts Using Friction Stir Deposition. *Materials* **2021**, *14*, 6396. https:// doi.org/10.3390/ma14216396

Academic Editor: Józef Iwaszko

Received: 9 September 2021 Accepted: 19 October 2021 Published: 25 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and 7xxx), due to their sensitivity to porosity formation, liquation cracking, segregation, solidification cracking, and anisotropic microstructure [2,4]. In contrast, friction stir deposition (FSD) is solid-state additive manufacturing (S-SAM) technique that can be used to deposit metals and composites [5,6]. The main advantage of the FSD as a solid-state process is that it can eliminate all problems of melting and solidification, and also, the feed material is mainly rods or wire without a need for special specifications of the used feed material. Most of the current works today are carried out on AM of aluminum-based alloys using a S-SAM technique [1,7,8]. In recent years, there has been increasing interest in utilizing FSD in many applications. This technique can be used for many purposes, including additive manufacturing [1,9–11], surface protection [11–13], and repair of defective components [6]. Thus, it can be said that the FSD-based AM is considered a new innovative approach to AM for building 3D parts ultimately in a solid state. The main processing parameters are the rotation rate, feeding speed, downward force, consumable rod material, and substrate material. These parameters govern the heat input and material flow processes. In the FSD process, the final build part's height depends on the layer thickness and the total number of assembly layers. Moreover, the modifications in the geometry of the building design can obtain manufacturing parts with different geometries [1,5]. Thus, the final FSD product is a near-net-shape with enhanced microstructure and isotropic mechanical properties [2,4,14]. Low porosity and low residual stress are the main privileges of the as-deposited part; this will make post-processing heat treatment unnecessary in many cases. However, surface finishing will usually be required [15–17]. Boeing and Airbus companies are considered the first use of additive manufacturing (AM) based on FSW principles [10,18,19]. Meanwhile, Airbus [20] presented the capability of achieving lightweight/low-cost structure parts by manufacturing 2050 Al-Li wing ribs by the FSAM process. Boeing assessed this process as a pre-form fabricating tool for manufacturing energy-efficient structures [10,21,22]. In addition to the capability of the FSD technique to generate material builds of high-performance structures, it is considered an energy and cost-saving process [10,23]. Elfishawy et al. [24] studied the possibility of multi-layers formation of die-cast Al–Si via FSD at the spindle rotation rate of 1200 rpm and different feeding speeds from 3 to 5 mm/min. The results showed sound structure with recrystallized refined grains. Therefore, from a scientific and technological point of view, it is of great importance to study how FSD works for additive manufacturing parts (AMPs) production in heat-treatable aluminum alloys. Although AA2011 is used extensively in aerospace and automotive components, there is a lack of publications discussing the applicability of AA2011 fabrication using FSD. Thus, the current work intends to explore the effect of the initial material conditions of AA2011 alloys on the properties and microstructures of the final produced AMPs. Three levels of feeding speeds of 3, 6, and 9 mm/min were associated with a high rotation rate of 1200 rpm/min to friction stir deposit AA2011-T6, and three other feeding speeds of 1, 2, and 3 mm/min were chosen with a low rotation rate of 200 rpm/min to deposit AA2011-O.

This study aims to study the effect of the consumable rod alloy temper condition on the behavior of the FSD process in terms of the parameters suitable for each temper condition as well as the properties of the AMPs.

#### **2. Materials and Methods**

To study the effect of the temper condition of AA2011 alloy on the properties of the produced AMPs, two groups of specimens, AA2011-T6 and AA2011-O, were used as consumable bars against a substrate of AA5083 alloy. The nominal chemical composition of AA2011 is given in Table 1. The annealing process for the as-received was carried out at 415 ◦C for 2.5 h followed by slow furnace cooling to the room temperature. Figure 1 illustrates the Cu-rich portion of the Al–Cu binary phase diagram with the annealing temperature range indicated [25,26].

**Table 1.** Chemical composition of AA2011 aluminum alloy (in wt.%).

**Figure 1.** A sketch for the Cu-rich portion of the Al–Cu binary phase diagram with the annealing temperature range indicated.

For comparison, three deposited materials were manufactured from each group of the AA2011-T6 and AA2011-O rods. The FSAM was carried out using the friction stir welding/processing machine (EG-FSW-M1) (Suez University, Suez, Egypt) [27,28]. Table 2 summarizes the deposition process parameters of both Al alloys.


**Table 2.** Consumable rod dimensions and FSD processing parameters.

The consumable aluminum rods are fixed using the machine shank to ensure the complete fixation of the rods throughout the process; Figure 2 shows a photograph of the actual AM process applied to AA2011. The additive manufacturing (AM) process involves three steps: fixing the consumable Al rod in the spindle shank (Figure 2a) and rotating it at a constant rotation rate while moving downward to reach the substrate material (Figure 2b). Finally, under a continuous feeding speed, the rod plastically deformed due to the high friction and the generated heat between the rod and the substrate that causes the material to transfer from the consumable bar to the substrate to build a material upwards. This process may continue until all the rod length is consumed and became insufficient for more deposition. The shape of the consumed tool tends to form a conical shape, as

shown in Figure 2c. For AA2011-T6 group specimens, careful processing parameters were selected based on our experience in the field and the published data [24,29] to produce additive manufacturing parts. The required heat input to friction stir deposit such a hard material limits the process parameters to be 1200 rpm as a spindle rotation rate with 3, 6, and 9 mm/min feeding speeds. For the AA2011-O group specimens, experiments start with shortening each of the three specimens to 110 mm in length. Of this length, 70 mm of the total length was consumed as a fixing base of the rod inside the shank to ensure tight gripping and prevent rod deflection during the deposition process, and 40 mm was functional during the process of friction deposition. Less heat input is needed to deposit this soft material; that is why after many trials, the optimum process parameters obtained were a 200 rpm spindle rotation rate and feeding speeds of 1, 2, and 3 mm/min. Figure 2d,e show schematic drawings of the AMP sections showing hardness measurement points, and the second half of AMP shows the specimens cut for OM and SEM examinations, respectively.

**Figure 2.** Photographs for the stages of the FSD process: (**a**) Fixing the AA2011 consumable rod and substrate AA5083 on the FSW/FSP machine, (**b**) feeding process during the FSD showing the building up of the part, and (**c**) the end of the deposition process for the additive manufacturing part (AMP). (**d**) and (**e**) are schematic drawings of the AMP sections showing hardness measurement points, and the second half of AMP shows the specimens cut for OM and SEM examinations, respectively.

Additive manufacturing parts (AMPs) have been sectioned vertically along the building direction (z-direction). The deposited layers were oriented perpendicular to the specimen axis/loading direction. The longitudinal sections were prepared according to the standard metallographic procedures by grinding up to 0.05 µm alumina polishing surface finish. The polished sections were investigated using an optical microscope (Olympus, BX41M-LED, Tokyo, Japan) after etching according to ATSM standard E407 using Keller's etchant of the chemical composition of 100 mL distilled water and 3 mL hydrofluoric acid. Microstructural examinations of the AMPs were also carried out using a scanning electron microscope (SEM, FEI, Hillisboro, OR, USA). SEM examination was carried out on the long-transverse sections of the cylindrical friction deposits using secondary electron (SE) imaging modes. Moreover, the grain size of all AM specimens and the base metal have

been analyzed by the grain interception method using Olympus Stream Motion Software. A Vickers Hardness Tester (Qness Q10, GmbH, Golling, Austria) with 0.2 kg load and 15 s dwell time was used to evaluate the average hardness of the starting and the AMPs. This test was carried out according to ASTM E92 by measuring twelve readings at least for each AM specimen on the longitudinal sections of the cylindrical friction deposits. The hardness maps were also drawn by collecting four horizontal (perpendicular to building direction) lines and five vertical lines measurements across the AMPs. The free space between any two indentations was 2 mm.

## **3. Results and Discussions**

#### *3.1. Fabrication of AMPs*

For conducting the friction stir deposition and forming the AMPs, the axis of the consumable rod is positioned exactly in the center of the square-shaped substrate to ensure the symmetry and homogeneity of heat dissipation through the substrate. Preliminary tests have been carried out to view the behavior of the rod to avoid buckling, physical discontinuities, or other defects of the rod and ensure the build of the part. Based on these preliminary tests, the rotation speeds, feed rate, and length of the consumable rod have been chosen. In addition, the length of the consumable rod out of the shank holder is varied with the temper condition, as the soft alloy tends to buckle easier than the hard alloy that allows more length to be used.

The rubbing between the two surfaces during the rotation and feeding speed of the consumable rod generates frictional heat, which softens the rod's rubbing end, causing plasticized material at the abutting ends. As the process continues, more plasticized material is built up [14,15]. As the required plasticized material thickness is gained, the rotating consumable rod is stopped and withdrawn; this process promotes a deposited layer on the substrate due to torsional shear. Figure 3 illustrates the remains of AA2011-T6 and AA2011-O consumable rods and their AMPs. Figure 3a–c shows the produced AA2011- T6 AMPs fabricated at a constant rotation rate of 1200 rpm at different feeding speeds of 3, 6, and 9 mm/min, respectively. For the AA2011-O specimens, the consumable rods of AA2011-O are softer than the AA2011-T6 rods. Therefore, the energy required to soften the AA2011-O consumable rod is lower than that needed for softening the AA2011-T6 one [30]. Thus, the AM process was conducted after many trials at a constant rotation rate of 200 rpm and various feeding speeds of 1, 2, and 3 mm/min, as shown in Figure 3d–f, respectively. It should be remarked that the higher feeding speeds of 9 mm/min and 3 mm/min at the rotational rates of 1200 and 200 rpm, respectively, are not recommended to fabricated AMPs of AA2011 alloys, where it is not easy to build continuous multi-layers upward to specific height and diameter. The increase in heat input due to an increase in feeding speed over the optimum condition also produces excessive flash around the AMPs, as given in Figure 3c for AA2011-T6 AMP and Figure 3f for AA2011-O AMP. Thus, it was noted that the conical shape at the end of the consumable rods after finishing the FSD process is flattened in a thin thickness, where the other materials are transferred to flash around the fabricated AMPs.

**Figure 3.** Optical images for the AMPs using FSD and their rods counterparts that remain after obtaining the required part length. AA2011-T6 AMPs processed at 1200 rpm and feeding speeds: (**a**) 3 mm/min, (**b**) 6 mm/min, and (**c**) 9 mm/min. The AA2011-O AMPs processed at 200 rpm and feeding speeds: (**d**) 1 mm/min, (**e**) 2 mm/min, and (**f**) 3 mm/min.
