Multi-material Joining of an Aluminum Alloy to Copper, Steel, and Titanium by Hybrid Metal Extrusion & Bonding

ORIGINAL RESEARCH ARTICLE Multi-material Joining of an Aluminum Alloy to Copper, Steel, and Titanium by Hybrid Metal Extrusion & Bonding TINA BERGH, HURSANAY FYHN, LISE SANDNES, JØRGEN BLINDHEIM, ØYSTEIN GRONG, RANDI HOLMESTAD, FILIPPO BERTO, and PER ERIK VULLUM Hybrid metal extrusion & bonding (HYB) is a solid-state welding method where an aluminum (Al) filler wire is continuously extruded into the weld groove between the metal parts to be joined by the use of a rotating steel tool that provides friction and plastic deformation. Although the HYB method was originally invented for Al joining, the process has shown great potential also for multi-material joining. This potential is explored through characterization of a unique Al–copper–steel–titanium (Al–Cu–steel–Ti) butt joint made in one pass. Each of the three dissimilar metal interface regions are characterized in terms of microstructure and tensile properties. Scanning and transmission electron microscopy reveals that bonding is achieved through a combination of nanoscale intermetallic phase formation and microscale mechanical interlocking. Electron diffraction is used to identify the main intermetallic phases present in the interfacial layers. Machining of miniature specimens enables tensile testing of each interface region. Overall, the presented characterization demonstrates the great potential for multi-material joining by HYB and provides fundamental insight into solid-state welding involving bonding of Al to Ti, steel, and Cu. https://doi.org/10.1007/s11661-023-07047-3  The Author(s) 2023 I. INTRODUCTION MULTI-MATERIAL or hybrid structures consist of two or more dissimilar materials that are joined together, which allow the properties of the parent materials to be jointly exploited.[1] Multi-material joints enable optimization of the material selection in each individual structural component and can be used to improve functionality or performance and/or to reduce weight or cost.[2] With such joints, lighter structures that retain high load-bearing capacities can be achieved,[3] which are crucial in reducing the environmental TINA BERGH is with the Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway and also with the Department of Physics, NTNU, 7491 Trondheim, Norway. Contact e-mail: HURSANAY FYHN and RANDI HOLMESTAD are with the Department of Physics, NTNU. LISE SANDNES, JØRGEN BLINDHEIM, ØYSTEIN GRONG, and FILIPPO BERTO are with the Department of Mechanical and Industrial Engineering, NTNU, 7491 Trondheim, Norway. PER ERIK VULLUM is with the Department of Physics, NTNU and also with the SINTEF Industry, 7034 Trondheim, Norway. Manuscript submitted October 5, 2022; accepted March 31, 2023. METALLURGICAL AND MATERIALS TRANSACTIONS A footprint in transportation industries. A prime example is joints between aluminum (Al) alloys and steels. They combine the light weight of Al alloys with the high strength of steels and enable improvement of the strength to weight distribution in, e.g., automotive components.[4] Furthermore, titanium (Ti) and its alloys have excellent corrosion resistances and high specific strengths that can be retained at high temperatures. They are, therefore, commonly used in the aerospace industry, often together with lightweight materials such as Al alloys.[5,6] Moreover, both Al and copper (Cu) have high electrical conductivity, and substituting Al with Cu by the use of Al–Cu joints may offer great weight and cost saving potential for electrical devices.[7] To realize use of such multi-material components, cost-effective, robust, and flexible welding methods capable of joining dissimilar materials without significantly deteriorating their properties are crucial. Dissimilar metal welding is challenging due to the differences in thermo-physical properties between the materials to be joined. Also, brittle intermetallic phases (IMPs) may form along the bonded interfaces.[8,9] In particular, the phases h-Fe4 Al13 and g-Fe2 Al5 ,[10,11], h-CuAl2 and c1 -Cu9 Al4 ,[12] and TiAl3 ,[13,14] often form during welding of Al–steel, Al–Cu, and Al–Ti, respectively. Studies have showed that as the IMP layer thickness increases, the joint strength decreases in Al–steel,[15,16] Al–Cu,[9,17] and Al–Ti[18,19] joints. Conversely, a thin IMP layer contributes to low interfacial electrical resistivity for Al–Cu joints.[17,20] The growth of IMP layers is often diffusion controlled, both for Al–Fe,[21] Al–Cu,[9,22,23] and Al–Ti layers.[13,24] Thus, the IMP layer growth typically accelerates at elevated temperatures, so that welding with low heat input is necessary. Limiting the heat input is also advantageous for welding of Al alloys in general. This is because both age-hardened and work-hardened Al alloys are sensitive to reheating and often develop a soft heat-affected zone following welding which may reduce the overall joint strength. Important solid-state welding methods include, e.g., cold-pressure welding and friction stir welding (FSW) techniques.[25,26] The solid-state welding method hybrid metal extrusion & bonding (HYB) was developed more recently[27,28] and was originally designed for butt joining of Al plates and profiles.[29–31] The HYB method relies on filling the weld groove between the base materials (BMs) to be joined with a solid filler metal (FM), based on the principles of continuous extrusion. A specially designed extruder tool is used that comprises a non-consumable rotating steel pin equipped with a set of moving extrusion dies at the bottom end.[29] During HYB, the extruder tool travels along the weld line, and a filler wire is fed into the tool and subsequently becomes forced to flow out of the extrusion dies and into the groove behind the pin. At the same time as freshly extruded FM is deposited, the rotating steel pin typically deforms the edge of at least one of the BMs and drags it into the weld groove. This combination of continuous extrusion, friction, and plastic deformation is the fundamental working principle of the HYB method. The method is flexible and allows various joint configurations.[28,32] It can also be used as a basis for additive manufacturing of small parts by depositing the Al FM in a layer-wise manner.[33] Due to the low process temperature and flexibility, the HYB method has shown great potential also for dissimilar metal welding. Three generations of Al–steel HYB butt joints have been characterized, and they showed progressively improved tensile properties.[34–36] Furthermore, it was recently reported that HYB offers the rare capability of producing multi-material Al–steel–Ti and Al–Cu–steel–Ti joints in one pass.[28,32] Previous characterization of HYB joints has included Al–Al, Al–steel, and Al–Cu HYB butt joints, but similar types of exploratory studies are also needed for other types of HYB joints with different geometries and/or other BM combinations. This article focuses on exploring the feasibility of multi (...truncated)