59 Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2015R1A5A1037627). This work was also supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the Encourage- ment Program (R&D, P0002115) for The Industries of Economic Cooperation Region. The authors appreciate the help in the preparation of the first AMed specimen from Dinh Son Nguyen and professor Hong-Seok Park (University of Ulsan, South Korea).
60 References
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62 CHAPTER 4
MANUFACTURING OF MAGNESIUM/ALUMINUM AND COPPER/ALUMINUM BIMETALLIC RING COMPONENTS BY FRICTION STIR ASSISTED
SIMULTANEOUS FORGING AND SOLID‑STATE JOINING
ABSTRACT
Bimetallic ring components are forged with the assistance of friction stir forging (FS- forging). The cylindrical bimetallic blank made up of magnesium AZ31 cylinder (Mg core) tightly fitted inside an aluminum 6061-T6 tube (Al skin), is employed. During the course of FS- forging, the frictional heat and stirring of the rotating tool forge the bimetallic blank into the desired shape, while simultaneously generating a solid-state joint between the Mg core and the Al skin, without additional external heating, as confirmed by the microstructural analysis and mechanical tests. The microstructural analysis also shows that the characteristics of the solid- state joint can be different depending on the process parameters.
The concept of simultaneous joining and die forging with friction stir assisted forging (FS- Forging) is confirmed by manufacturing a bimetallic ring structure consisting of copper and aluminum. Cylindrical pure copper Cu 1100 (Cu core) tightly fitted inside a tubular aluminum 5052 (Al skin) is used as a bimetallic blank for conducting the process. The frictional heat generated from the stirring action of the rotating tool during FS-forging forges the bimetallic blank into the desired shape inside the die, while a solid-state joint between Cu core and Al skin is formed without external heating at the same time. Joining between Cu and Al inside the forged structure generates a nonuniform intermetallic layer of CuAl2. The joining between the Al-Cu final finished product offers an electrical resistivity of 35 μΩ cm, measured using a precise nano
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voltmeter. In detailed EBSD maps reveal that the core copper undergoes CDRX generating fine microstructure and enhanced hardness. In contrast, the skin Al undergoes DDRX suffering grain coarsening and reduction in hardness
KEYWORDS: Forging, Solid-state joining, Intermetallic, Friction stir welding
64 4.1 Introduction
The use of bimetallic structural components to minimize issues from using a single material has drawn interest from industries. However, with conventional processes, researchers have reported complications in the manufacture of such bimetallic components, generally due to the dissimilar mechanical/material properties of the selected metal alloys [Fan S et al.,2017].
In the manufacture of bimetallic materials or components, a simultaneous forming and joining is frequently considered. Bae et al. [Bae JH et al., 2011] cladded magnesium (Mg) and aluminum (Al) alloys using twin roll casting and reported the formation of Mg17Al12
intermetallic layer resulting strong bonding. Golovko et al. [Golovko O et al.,2015] performed hot extrusion of an Al–Mg bimetallic rod for simultaneous forming and joining with the help of furnace heating. Ng et al. successfully performed electrically assisted roll bonding of ultra-thin Al alloy sheets with the same alloy or Cu alloy sheets at a relatively low bonding load and achieved higher bonding strength as compared to the conventional process. Recently, Napierala et al. [Napierala O et al.,2019] proposed draw forging for simultaneous cold forming and joining of Al alloy and steel to manufacture multi-material component.
Friction stir welding (FSW) is an effective solid-state joining technique for nonferrous dissimilar materials. During FSW, frictional heat and severe straining by a rotating tool are applied to the workpiece material, which induce dynamic recrystallization, and consequently fine microstructures inside the material. The unique characteristics of FSW can be effectively used to manufacture bimetallic structural components, as suggested in friction stir extrusion of bimetallic Al/Fe blanks [Evans WT et al.,2015]. Compared with other forming processes, such as roll bonding, cladding, and extrusion, used for fabricating bimetallic components, approaches based on friction stirring remove or minimize the need for external heating, thus reducing both the
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energy requirement and the production time [Buffa G et al.,2020; Das H et al.,2018].
Ring components are finding increasing applications in many industries as rotary machine components or connecting components due to recent developments in ring forming processes, such as incremental ring rolling [Allwood JM et al.,2005], orbital forging [Shivpuri R et al.,1988], and spinning [Luo W et al.,2018]. Cleaver et al. [Cleaver CJ et al.,2016]
successfully demonstrated that ring components with variable wall thickness can be formed by a ring rolling technique. Traditionally, most metallic ring components are manufactured using a single material.
The high specific strength and low density of Mg alloys can make them candidates for structural components, including ring components. However, the limitations of the material properties, including poor formability, brittleness at room temperature, and low corrosion resistance, hinder the use of those alloys in practical applications. To counter the limits of Mg alloys for structural components, the concept of bimetallic components can be considered.
With their low density and good chemical compatibility with Mg alloys, Al alloys can be candidates to compose light-weight bimetallic combinations with Mg alloys. By properly combining Al alloys with Mg alloys, it is expected that the relatively high ductility and good corrosion resistance of Al alloys can minimize the issues from the material properties of Mg alloys [Whalen S et al.,2019; Tokunaga T et al.,2015]. However, a simultaneous forming and joining of such bimetallic combinations generally requires external heating or preheating of the materials, mostly due to the limited formability of Mg alloys at room temperature. Naturally, the simultaneous forming and joining with external heating or preheating increases the production cost and consumes longer time.
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In the present study, a friction stirring based approach to simultaneously forge and join a bimetallic ring component [Hong S-T et al.,2019] is suggested. The friction stir assisted forging (simply, FS-forging) can eliminate the need for external heating or preheating for materials with a limited formability at room temperature (such as Mg alloys). The FS-forging is also expected to generate a dynamically recrystallized fine microstructure inside the forged component by friction stirring. Reported here is the demonstration of the feasibility of FS-forging by fabricating Mg/Al bimetallic ring components.
Nowadays, manufacturing lightweight components by incorporating energy-efficient techniques is of great interest to academia and industries. For fulfilling the economic and ecological benefits simultaneously, various industries are looking forward to upgrading energy- efficient manufacturing techniques. However, during manufacturing, the energy efficiency factor solely depends on the weight of the components that make up the system and the number of steps involved in manufacturing. [Zhu Z et al.,2020; Watanabe T et al.,2006; Fan S et al.,2017]
Nonferrous metals with an excellent strength-to-weight ratio like aluminum, magnesium, and titanium can effectively reduce the overall weight of any component. But most lightweight materials might lack a specific property like strength, conductivity, or corrosion resistance useful for a distinct component or system [Mondal M et al.,2019; Haddadi F et al.,2015].
To meet the desired property of a particular component while maintaining lightweight sincerely involves the application of bimetallic components [Shang C et al.,2019; Bae JH et al.,2011]. However, manufacturing bimetallic components, processes involving simultaneous joining and forming is most preferable. Ng et al. achieved higher bonding strength than the conventional process by performing electrically assisted roll bonding of ultra-thin Al alloy sheets with Cu at a relatively lower force. Guo et al. manufactured Al/Cu multi-layered laminates by
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diffusion bonding with the formation of intermetallic compounds such as Al4Cu9, AlCu, and Al2Cu. Hot pressure welding and forge welding were applied to form and join simultaneously to manufacture Al/Fe bimetallic components by Kong et al., Zhang et al. manufactured a three- layered bimetallic composite Al/Mg/Al using roll bonding, inducing an equiaxed grain structure and forming a diffusion layer. Tokunaga et al. coated corrosion-prone magnesium alloy with aluminum by applying a hot extrusion process. They reported enhancement of corrosion resistance of magnesium alloy near like the aluminum but reduction of formability due to the formation of brittle intermetallic compounds.
The joining of dissimilar metals by conventional joining process with the enormous difference in thermal, chemical, and mechanical properties involves the formation of thick intermetallic compounds (IMC), which deteriorates the joining quality [Carvalho GH et al.,2018].
However, friction stir welding (FSW) is an efficacious solid-state joining process for similar and dissimilar nonferrous material combinations. The frictional heat and high straining generated by the rotating tool joins the material below its melting temperature by inducing dynamically recrystallized fine grain structure and avoiding the formation of thick IMC [Hou W et al.,2020;
Bhattacharya TK et al.,2015; Bhattacharya TK et al.,2017]. The rotating tool plunges into the overlapping sheets for friction stir spot welding (FSSW) to accomplish the welding. By modifying the workpiece geometry and its interaction type with the rotating tool, FSSW can be used effectively for simultaneous joining and forming of the materials. Buffa Et al. successfully recycled aluminum chips by friction stir assisted simultaneous joining and forming to manufacture consolidated billets. Mondal et al. simultaneously joined and forged Al/Mg bimetallic blank into a ring structure, applying friction stirring and incorporating dynamically recrystallized grain structure inside the magnesium. They also report a solid-state joining
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between Al and Mg inside the forged ring by producing a relatively more rigid IMC layer, whose thickness variation solely depends on the process parameters. Evans et al. also friction stir back extruded Al/Fe to make a successful joining.
In many industries, ring structured products are being used widely for rotary machine components or connecting components because of the recent advancement in ring-forming processes like incremental ring rolling [Allwood JM et al.,2005], orbital forging [Shivpuri R et al.,1988], and spinning. Luo et al. successfully spinned large thin-walled parts by integrating counter-roller spinning, multi-neck spinning, and hot spinning to manufacture the ring structure.
For most cases, the researchers report creating a ring structure of a single material only.
Copper is the most common material used for several applications like electrical connectors, heat-exchanger tubes, power generation, refrigeration tubes in the electronics, transportation, and aerospace industries due to its excellent electrical and thermal properties. But due to its excessive weight, sometimes it can be disadvantageous for the applications for which energy efficiency is the most critical, especially for the transportation and aerospace industry.
Aluminum with low specific weight and good conducting properties can be joined with copper to reduce weight-related to a single piece of Cu [Galvao I et al.,2016; Saeid T et al.,2010; Zhang C et al.,2018; Abdollah-Zadeh A et al.,2008].
Thus Al/Cu both being suitable conductive materials, a bimetallic component made of these two materials can be used in similar applications as conventionally used for a single Cu.
The pre-existing bonding process by executing forming like roll bonding, cladding, and extrusion for manufacturing bimetallic components uses external heating or is mainly divided into two steps. In comparison, the friction stir assisted simultaneous joining and forming process avoids any external heating as it uses the frictional heat to join and form the material combination and
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incorporates dynamically recrystallized grain structure inside the formed product [Hou W et al.,2020; Kuang B et al.,2015].
In this work, a bimetallic ring of Al/Cu is manufactured using friction stir assisted simultaneous joining and forming inside a closed die. The joining mechanism and evolution of the IMCS during the process are also studied using electron microscopy. The effect of the type and thickness of IMC on the electrical conductivity are briefly analyzed. An in-depth microstructural investigation has been made to evaluate dynamic recrystallization phenomena inside a forged part using electron backscattered microscopy. Later the microstructural results were correlated with hardness variation and resistivity inside the forged part