Titanium Rear Subframe Bonded titanium structures Lotus Engineering has recently completed an exciting new project which pushed the boundaries of materials, joining and manufacturing technologies, resulting in a lightweight titanium rear subframe for the Lotus Exige S. The adhesively bonded titanium frame is 36% (18 kg) lighter than the current high strength steel assembly. The mass reduction has been achieved whilst increasing first torsion mode by 19%, with comparable torsional stiffness, point stiffness and crash performance. This is a major achievement given the subframesâ€™ tight package and the fact the Exige S is already a lightweight sports car. The six month collaborative project between Lotus and its partners Caged Laser Engineering and S&D Speciality Metals was funded by the Niche Vehicle Network (NVN). The objective of the project was to design and develop a rear subframe for the Lotus Exige S with a 30% mass reduction whilst attaining the same structural performance as the current steel frame. Key to the success of the project was the assessment and selection of a suitable structural adhesive and associated pre-treatment to achieve strong and durable bonded titanium
joints. This included testing and evaluation of the laser ablation pre-treatment process. Titanium is a lightweight metal which is as strong as steel, but with almost half the weight. Itâ€™s also highly resistant to corrosion due to the formation of a passive oxide layer which protects the metal and requires no further corrosion protection. Titanium is also a poor conductor of heat, which can be advantageous if a structure is required to minimise the transfer of heat, such as a subframe surrounded by an engine and exhaust system. These factors make titanium an attractive material for certain lightweight automotive parts, especially in combination with structural adhesives which are known to improve joint and overall assembly stiffness. Project Activity Summary The research project incorporated five main phases of work. The first phase focused on the testing and selection of a suitable adhesive and associated pre-treatment, in order to enable prototype frame build. The second phase covered the design and computer aided engineering (CAE) simulation of the frame, which included two iterative loops.