Fig 1. A schematic showing the AISF configuration
Titanium is widely used in the aerospace industry due to its relatively high yield strength combined with low density. To save machining cost and reduce metal waste, many titanium parts are manufactured by metal forming processes, which fabricate titanium components from sheet.
Currently, to shape complex titanium sheet components, the aircraft industry uses well established forming processes, such as deep drawing, hot forming, super plastic forming and hydro forming. In certain applications parts are even manufactured by hand working. Moreover, current industry practice requires significant heating in order to soften the titanium, and allow forming of high wall angles and the reduction of spring back.
Current titanium forming techniques, therefore, have many disadvantages, including high cost, long tooling lead time and high energy consumption. To address these drawbacks, the innovative Asymmetric Incremental Sheet Forming (AISF) technology developed within INMA will increase flexibility, reduce costs, minimise energy consumption and speed up the industrialisation phase. Project INMA is developing an intelligent, knowledge-based, flexible manufacturing technology for titanium shaping. This technology is based on AISF.
Funded by the European Commission, development of the proposed technology will transform the manufacture of many titanium sheet aerospace components, such as after pylon fairings, fan blades, exhaust ducts or air collectors. The innovative, cost-effective forming technology, with a low environmental impact, will provide a wide range of benefits for the European aircraft industry.
AISF is a non-conventional forming technology, where a localised and progressive plastic deformation of the blank is achieved by the action of a punch tool, which follows a continuous and controlled path. For localised heating, a heat source will travel with the forming tool in order to soften the metal and achieve high formabiity.
The project comprises technology development in five broad areas:
- development of the AISF process by which best practice, operational procedures and operating envelopes are identified
- development of numerical models for the cold and hot forming processes. Models are used to predict forces, strains and shape deviations during the forming process, in addition to the thermal field evolving during the hot AISF process
- material characterisation by which the formability limits, and the post forming properties of titanium sheets are identified
- development of a knowledge-based model that provides tool path correction and reduces geometric deviations from required tolerances
- developing a lean heating strategy to identify heating procedures to minimise energy consumption and achieve required wall angles
TWI's role in INMA addresses four aspects of the hot AISF process:
- identifying the most suitable and energy efficient heating techniques for hot AISF
- developing a localised heating model for the AISF. The localised heating model is developed to assist with selection of forming parameters in order to reduce cost
- developing the lean heating assessment strategy to reduce the energy consumption of the process
- developing hot AISF procedures that will minimise post-forming geometric distortion due to residual stress
Fig 2. Temperature contours for the titanium sheet during the 6th cycle of concentric circular path heating at 2kW nominal laser power.
A critical review of several potential heating methods has been delivered, in addition to a validated finite element model for sheet heating simulation for any tool path. It was found that laser heating is the recommended heating choice. TWI's approach combined experimental and theoretical investigation of localised heating of titanium sheets. Several physical experimental trials were carried out, where a 2mm thick titanium sheet was heated for different tool paths. The temperature field on the plate was measured by thermocouples and thermal imaging.
TWI is now leading work on the integrated assessment of heating methods, as we are currently comparing different heating processes, in order to optimise the hot AISF process, with regard to formability, technical feasibility, cost-efficiency and environmental impact.
The major impacts of the INMA project will be:
- Costs incurred by dedicated tooling will be reduced by 80%.
- Component lead times will decrease by 90%.
- Buy-to-fly ratios will be up to 20% lower.
Fig 3. Thermography image showing the temperature field in the titanium sheet due to linear laser heating.