Dissimilar Material Joining Technology

Definition and Technical Background

Dissimilar material joining technology refers to an umbrella term for specialised manufacturing and joining processes engineered to reliably bond two or more materials with distinctly contrasting physical and chemical properties (such as steel and aluminium, aluminium and carbon fibre composites, or metals and plastics). It serves as the foundational technical prerequisite for enabling multi-material hybrid vehicle bodies (BIW). With multi-material lightweight construction becoming the mainstream industry trend, traditional single-method joining techniques like resistance spot welding (RSW) are no longer sufficient to meet dissimilar material joining demands.

Technical Challenges

Joining dissimilar materials presents several inherent technical hurdles. Drastic disparities in melting points and thermal expansion coefficients make interfacial reactions notoriously difficult to control during welding, often precipitating brittle intermetallic compounds (IMCs) that severely compromise the joint's mechanical integrity and long-term service reliability. In steel-to-aluminium joining, for instance, conventional welding processes suffer from substantial thermophysical mismatches and poor interfacial wettability, forming thick, continuous, and brittle IMC layers that drastically undermine joint strength. Additionally, galvanic corrosion readily occurs in humid environments due to the differing electrochemical potentials of dissimilar metals, necessitating mitigation strategies such as structural adhesive barriers or protective coatings.

Main Technical Routes

To date, several parallel technological pathways have been established for dissimilar material joining. Self-Piercing Riveting (SPR) stands as the most prevalent mechanical joining method in steel-aluminium hybrid body-in-white structures, creating a solid joint as the rivet pierces the top sheet and expands to form a mechanical interlock within the lower substrate. Adhesive-Riveting Hybrid Joining integrates structural adhesives with SPR; the adhesive bead serves as an electrical insulator to suppress galvanic corrosion whilst capitalising on superior shear strength, more than doubling the joint's shear load capacity compared to standalone riveting. Laser Braze-Welding tackles steel-aluminium fusion challenges through low heat-input control and targeted interfacial reaction management, restricting the brittle IMC layer to just 2 to 3 microns to achieve joint strengths exceeding 90% of the base material—a method already commercialised in components like side doors and crash boxes. Resistance Rivet Welding (RRW) merges resistance spot welding with mechanical riveting interlocks, allowing dissimilar metal joining on conventional spot-welding assembly lines with high compatibility and low tooling conversion costs. Emerging techniques such as Friction Self-Piercing Riveting (F-SPR) are also under active development, creating an interfacial oxide layer over the rivet to provide galvanic insulation and curb corrosion in dissimilar joints. For carbon fibre reinforced polymer (CFRP)-to-metal assemblies, structural bonding, bolting, and induction welding each serve distinct applications, with bonded joint mechanics remaining a core research focus. When joining carbon fibre reinforced thermoplastics (CFRTP) with aluminium alloys, pre-treating the aluminium surface with laser texturing to create micro-structures is currently explored to enhance interfacial bonding strength. Meanwhile, the cutting-edge NCW (Nugget Controlled Welding) resistance welding technology claims to achieve direct steel-to-aluminium fusion welding, currently undergoing industrial-scale validation on structural parts such as EV battery trays.

Industrial Significance

Dissimilar material joining technology represents a pivotal enabler for transitioning multi-material hybrid vehicle bodies from design concept to high-volume series production. It fundamentally dictates whether lightweighting initiatives can achieve stringent weight-reduction targets without compromising structural crash safety and long-term durability. Whether for aluminium-steel hybrid bodies, carbon fibre-aluminium composite structures, or multi-metal configurations incorporating magnesium alloys, overcoming joining bottlenecks remains vital to unleashing the full potential of automotive lightweight engineering and manufacturing.