Hot stamping allows producing complex geometries from high performing materials. The use of this technique is increasing among automotive manufacturers because of the possibility to reduce weight without compromising with performance.
The aim of this study was to tailor the press hardening technique to fit the materials and demands of the turbofan engine, hoping to reduce weight, waste, machining and cost. Aerospace steel grades had not previously been used in press hardening. The possibilities and limitations of these new materials were thoroughly examined as well as the benefits using this manufacturing technique compared to conventional methods.
A selection of aerospace grade steels was characterised to find the material best suited for press hardening. For this material a thermo-viscoplastic material model was created. Using this model geometries from the turbofan engine, suited for press hardening, were optimised and the feasibility of forming these details investigated. The first of the geometries was a sheet metal disc. In this proposal it was suggested to also investigate one of the flow surfaces and a guide vane. To validate the simulation process and the prediction of detail quality from the forming analysis physical parts of the disc and the guide vane were produced.
For the disc and the guide vane benefits using near-net-shape blanks were investigated. A method to achieve non uniform blanks is rolling. Flat rolling is majorly done to produce a large amount of standard strip/plate. Flat rolling into a shape is not widely used but it has a large potential to save material and machining by rolling a Near-net-shape product. The guide vane was manufactured using this technique.
Also, a technical study of serial production was performed to compare the new production process to conventional design and manufacturing methods with regards to detail quality, weight, cost, material properties and manufacturing robustness.
The ViMaQ project explored the applicability of press hardening technology in aerospace applications. The aims of project were:
- To find, characterize and model a suitable steel grade;
- To evaluate the accuracy of the model for simulation of design feasibility and production;
- To find a process to produce near net shape blanks to be used in press hardening;
- To investigate the possibilities for serial production.
Three different precipitation hardening aerospace grade steels were investigated; baseline material 17-4PH, PH13-8Mo and Custom465. Gleeble investigations showed that C465 needs to be cooled below room temperature in order to get a complete martensite transformation and was therefore not a suitable choice.
Tests of the two remaining candidates, 17-4PH and PH13-8Mo showed that the formability and maximum forming force are more or less equal for in the tested temperature region but compared to the conventional hot stamping boron steel, Boron02, the formability is significantly worse. Due to the significantly higher cost for the PH13-8Mo investigations on this material was not taken any further and the physical parts were produced in 17-4PH.
The investigations showed that it is possible to use 17-4PH in press hardening process. A process window that results in stable, acceptable material properties has been defined. However the material showed an increased friction compared to the conventional hot stamping boron steel and also a completely different behavior with sticking and adhesive wear instead of abrasive wear. Friction tests were made using three different die surface modifications but it was, unfortunately, not possible to determine the friction coefficient for any of the settings and hence not possible to create a friction model to use in forming simulations.
Both an isothermal and a thermo mechanical material model were created for 17-4PH. When comparing simulation results to produced physical parts these models show positive results. The models are accurate enough to be used to simulate the production process and to use for product feasibility studies during product development.
From the produced parts it can be concluded that the sheet metal disc is a product that can be industrialised with the techniques covered in this project. The present production equipment at Gestamp HardTech can be used but some investments need to be made with regards to higher furnace temperature and increased tool wear as well as equipment that enable compliance with aerospace standards on monitoring of the production process of the individual parts. It would also be necessary to train personnel and add processes to comply with these standards. A rough estimate of the cost for this had been made but it has not been possible to compare to a conventional design since this part is not in production.
The production of the part with non-uniform thickness is further from industrialisation. The rolling trials show that it was possible to produce the desired profile, however the process generates a thick oxide-scale which results in a deterioration of the surface quality. Use of induction heating could reduce the oxide layer and also improve the productivity.
The main problem though is that the final parts do not reach the requirements with respect to hardness. The theory is that the hot rolling process does not deform the material enough to create a sufficient number of recrystallization points in the second heating processes, resulting in a coarser, softer structure. An increased reduction could possibly improve this but as the intention is that the parts will be welded together and after that heat treated to achieve homogeneous material properties this problem is probably not easy to solve for the finished part.