Advanced Solid-State Dissimilar Material Joining and Additive Manufacturing for Enabling Multi-Material Lightweight Structures

Abstract

Multi-material lightweight structures by “using the right material at the right place” have become increasingly important for ensuring environmental sustainability. Conventional mechanical fastening and/or adhesive bonding processes can be inefficient, unreliable, and time-consuming, therefore unable to take full advantage of multi-material structures and emerging advanced manufacturing processes, such as additive manufacturing (AM). Due to material incompatibility between some of the dissimilar material combinations, e.g., between aluminum alloy and steel or between polymer composite and metal, there have been major challenges in achieving direct welding or joining. For instance, fusion-based dissimilar metal joining or AM, e.g., between aluminum alloy and steel, introduces brittle intermetallic compounds (IMCs) between the aluminum-steel interface, which can cause severe cracking and galvanic corrosion or stress corrosion cracking. However, there have been some promising developments over the recent years. Some solid-state dissimilar material joining and AM processes, e.g., friction-based processes for producing direct bonding between dissimilar metals, e.g., between aluminum alloy and steel and between polar polymer (PA66) and aluminum, and friction-based solid-state additive manufacturing (additive friction stir deposition or AFSD). However, the direct joining between non-polar polymer (such as polypropylene or PP) and metal and suppression of IMCs for improving corrosion resistance in bimetallic joints is still not achieved. The solid-state additive manufacturing method available to date (e.g., AFSD) imparts severe compression force and overheats the substrate, making the process challenging for relatively thin substrates and safety-critical applications. In this research, solid-state friction-based dissimilar material direct joining and additive manufacturing methods are investigated. Using the friction-based solid-state method, it first achieved a novel metal to hard-to-weld polypropylene (PP) composite direct joining using a functionally active interface modulator insert layer under localized temperature and pressure conditions. Utilizing carbon-oxygen-aluminum (C-O-Al) type covalent bonding between aluminum alloy and polypropylene composite, it eliminated fasteners and adhesives for a robust joint stronger than the base composite material. Subsequently, it utilized previously discovered, nanoscale shear localization-induced amorphization (NSLIAA) to join thin gauge aluminum to steel materials directly under controlled friction spot joining configuration for the suppression of IMCs and increased corrosion resistance. It has then demonstrated a novel friction-based solid-state metal additive manufacturing method to enable melt-less dissimilar material additive manufacturing of metals and alloys directly at a large scale without solidification defects and minimal substrate compression force-induced instabilities. It was termed SoftTouch AMTM due to its low temperature and low compression force features and defined as an additive friction extrusion deposition (AFED) process due to its friction-based extrusion and additive processing features. SoftTouch AMTM utilizes super-plasticity to soften the feedstock in two steps, eliminating the need for direct contact between substrate and feedstock and providing grain refinement and severe cold working (severe plastic deformation) below the melting temperatures of metallic materials. It avoids using pricy powders and is fully capable of utilizing off-the-shelf metallic bars as feedstock. It has proven crucial for future demands of large-area additive manufacturing, energy sectors, and sustainability.