Cyclic Deformation and Damage Mechanisms in additive manufactured Ti-6Al-4V with Graded Microstructures

Additive manufacturing (AM), also called 3D printing, has been widely recognised to be capable of offering a range of logistical, economic and technical advantages when compared with conventional manufacturing processes. For example, AM has opened up new business opportunities for the aerospace sector because of its reduced time-to-market and higher buy-to-fly ratio, in particular for titanium alloys. However, the widespread adoption of AM by industries for the production of metallic engineering parts/components remains a challenge. Lack of knowledge about the reliability of AM-built materials and associated concerns about engineering structural integrity has been identified as one of the hurdles to successfully commercialise this promising manufacturing process.

Ti-6Al-4V is the most prevalent titanium alloy and is widely used in aerospace applications. Rolls-Royce PLC at Derby has applied AM technologies to repair aero-engine BLISKS (Bladed-Disks) and GE Aviation in the USA has developed AM-built fuel nozzles for the LEAP aero-engines. With AM moving into the production of more heavily loaded end-use components, particularly within the aerospace sector, improved understanding of the mechanical behaviour of AM parts/components under cyclic (fatigue) loading is essential. This requires the development of a mechanistic based lifetime assessment procedure for safety-critical engineering systems that contain AM-built parts/components.

By engaging actively with both the UK's world-renowned AM manufacturing technology centre (MTC) and an end-user (Rolls-Royce PLC), this project will develop an experimentally-validated microstructure-based model that can be adopted when undertaking a lifetime prediction against fatigue crack initiation in AM-built Ti-6Al-4V.

The aim of this project is to enhance the confidence of both AM manufacturers and end-users that AM-built materials (in particular Ti-6Al-4V for the aerospace sector) are at least as safe and reliable as those produced by the traditional machine from solid processes. This will be achieved by developing a microstructure-based self-consistent model, validated by multi-scale experiments, to predict mechanical behaviour of the AM-built Ti-6Al-4V under cyclic loads at high temperature. Since the presence of graded microstructures for AM-built metallic materials, including AM steels, nickel and titanium alloys, is directly related to the essence of AM processes, i.e. the layer-by-layer building philosophy, this project will have broad benefits to both current and potential AM end-users ranging from aerospace, nuclear, automotive as well as biomedical industries. For example, the nuclear industry will use AM to make safety-critical lugs on steam generators (current forging capacity in the world isn't big enough to make these lugs).

This project is of strong relevance and interest to industry, with significant potential future benefits for the UK economy. Rolls-Royce PLC, a global leader in aerospace manufacturing, have applied AM processes to repair damaged aero-engine BLISKS for more than 5 years and produced a prototype front bearing aero-engine by AM in 2015. They will be one of the project beneficiaries by gaining a mechanistic understanding of fatigue behaviour of AM-built Ti-6Al-4V with graded microstructures. The project will provide Rolls-Royce PLC and other AM end-users with much needed assurance to the use of AM-built components. The widespread adoption of AM technologies by industry will improve manufacturing energy efficiency (an economic impact) by increasing material utilisation and minimising scrap material associated with component fabrication. In this sense, this project will help reduce the CO2 footprint and thus generate an environmental benefit.

One of the goals for the UK's stakeholders in aerospace, and other industries who are considering the utilisation of AM technologies, is to keep fully informed of advances in manufacturing technologies. This also aligns with UK's strategic decision of keeping and developing the core technology in advanced manufacturing. The proposed work will represent an outstanding example of world-leading AM research carried out in the UK. By interacting with the UK's High Value Manufacturing Catapult (the MTC), I will be able to meet and discuss the research outcomes with policy-makers, legislators and a wide range of professionals including business people. This will improve information flow and accelerate innovation and development in the AM sector.

The postdoctoral researcher as well as myself (an early career academic at Coventry University) will gain skills in both in situ neutron and synchrotron X-ray diffraction and tomography and developing self-consistent models for materials with graded microstructures. This project will expose me to wider networks of academic and industrial partners both nationally and internationally.