An investigation into the forming behaviour of metal composite hybrids

Abstract

In an effort to reduce gas emissions from fuel combustion generated by the transportation sector the use of lightweight materials such as composites are currently being intensively explored. Fuel-economy improvement for passenger vehicles is projected to increase by 6-7 % for every 10 % of weight reduction. Composite materials are stronger than steel and lighter than aluminium. Fibre Metal Laminates (FML) is a class of composite material comprised by alternating aluminium sheets and composites which has been successfully employed in the aerospace industry. The resulting synergy of material properties gives FMLs superior mechanical properties when compared with those of their individual constituent materials. FMLs based on thermoplastic composites have been shown to possess better fracture toughness, shorter processing times, good impact and localised blast resistance as well as being recyclable. For the automotive industry to take advantage of FML's superior mechanical properties, the forming behaviours of this class of material need to be understood. Stamp forming is a much cheaper way to produce FML parts in high-volume industries compared to autoclaving. FML systems can be immediately used as a replacement for metal in production lines with minimal change to stamp-forming tools. Two systems of FML with different fiber stiffness, FMIL-Curv and FML-Twintex, were benchmarked with monolithic aluminium of the same thickness to study their formability. Full factorial experimental work was conducted to study the influence of process parameters -binder force, feedrate and preheat temperature, using the Swift test. The Swift round-bottom test was chosen to study both stretch and draw behaviour during forming. A full field-strain measurement system was used to study the effects of varying forming conditions on strain evolution in real time during the stamp-forming process. A statistical test, ANOVA, shows that preheat temperature and binder force have significant effects on strain evolution for both FML systems. FML samples showed better strain distribution in stretch forming compared to monolithic aluminium. They also required less work to form and have a weight reduction of approximately 30% when compared to aluminium samples. High-temperature forming is preferred for FML-Twintex while low-temperature forming is better for FML-Curv. FML-Curv can also be formed to higher depths compared to monolithic aluminium. By proper selection of process parameters, contact conditions and composite fiber properties it is possible to produce FML parts that have superior formability characteristics compared to monolithic metals. A predictive finite element model was developed to model forming process. The results from the finite element solution showed a high degree of correlation with experimental results. Show more