Joining of Dissimilar Materials

Material manufacturers and engineering structure designers are currently focusing new ways to exploit the benefits of light-weight, hybrid materials with improved properties at a low cost. The ability to join dissimilar materials is enabling the design engineers to develop light-weight and efficient automobiles, aircraft and space vehicles. The objective of this PhD research study was to produce alternative and efficient joining solutions for automotive and aerospace applications. The joining of dissimilar material was experimented to obtain light-weight Fibre Reinforced Polymer (FRP) sandwich composites, Al-foam sandwich (AFS) composites, hybrid dynamic FRP epoxy/polyurethane composites and the joining of Ti6Al4V alloy with and without surface modification to Ceramic Matrix Composite (CMC) and itself. The joining of Al-foam and Al-honeycomb to FRP skins was performed. The experimental results show that higher flexural properties can be achieved by replacing Al-honeycomb with low-cost Al-foam as a core material in the sandwich structures. Compared to FRP-honeycomb sandwich panels, FRP-Al foam sandwich panels display ~25 % and ~65 % higher flexural strength in a long and short span three-point bending tests respectively. AFS composites with complete metallic character, to withstand high-temperature application conditions, were produced by soldering/brazing techniques using Zn-based and Al-based joining alloys. A post-brazing thermal treatment was designed to recover the mechanical properties of AFS composites, lost during the soldering/brazing process. The microstructural analysis of the Al-skin/Al-foam interface revealed that the diffusion of joining materials into the joining substrates (Al-sheet and Al-foam) was achieved. Around 80% higher bending load before failure was observed when the AFS specimens produced with Zn-based joining alloys were subjected to flexural load compared to those produced with Al-based joining alloys. Hybrid dynamic Carbon Fibre Reinforced Polymer (CFRP) composites with enhanced impact properties were produced by exploiting the reversible cross-linking functionalities of dynamic epoxy and dynamic PU resin systems. By joining dynamic CFRP-epoxy and dynamic CFR-PU laminates, hybrid dynamic composite in three different configurations and a non-hybrid composite were obtained. The four dynamic composites were characterised for structural, thermal, flexural and impact properties. The damage initiation upon impact was observed at around 95% higher energy level in the hybrid configuration (CFRP-4), compared to the non-hybrid configuration. The hybrid configuration CFRP-3 responded with around 55% higher perforation threshold energy compared the non-hybrid configuration. Preliminary work on Adhesive joining of the Ti6Al4V alloy to itself was performed to analyse the effect micro-machining on adhesion and the effect of shape/design of micro-slots on an adhesive joint strength. Three types of micro-slots: V, semi-circle and U-shaped micro-slots were produced on Ti6Al4V sheet surface by using an in-house developed Micro-Electro-Discharge Machining (Micro-EDM) setup. Ti6Al4V alloy specimens with and without micro-machined surfaces were bonded together using a commercial epoxy adhesive. The Single Lap Offset (SLO) shear test results revealed that the micro-slot oriented perpendicular to the applied load displayed ~23 % higher joining strength compared to when the micro-slots were oriented parallel to the applied load. U-shaped micro-slots configuration displayed ~30 % improvement in the joint shear strength compared to the specimens with un-modified surfaces. The fractured surfaces analysis revealed mix (adhesive-cohesive) with cohesive dominated failure in bonded specimens with micro-machined surfaces compared to the as-received where pure adhesive failure was observed. The joining of CMCs (C/SiC and SiC/SiC) to Ti6Al4V alloy was experimented using active brazing alloy (Cusil-ABA) and Zr-based brazing alloy (TiB590) in a pressure-less argon atmosphere. The CMC-Ti6Al4V joint strength was further improved by modifying the surface of Ti6Al4V alloy using an in-house built Micro-EDM setup. Around 40% higher joining strength was recorded when the Zr-based brazing alloy was used as a joining material compared to the conventional active brazing alloy, Cusil-ABA. Improvement in the joining strength was noticed when the Ti6Al4V surface was modified prior to joining.