Hot forging of nickel-base superalloys is a key manufacturing operation in terms of the aerospace, power, and nuclear industries. Due to high, cyclical mechanical, thermal and tribological loads, however, this process is particularly prone to deterioration of die surface condition over the course of a run. This ultimately necessitates production stoppage for changeover, and resources are required to repair the worn die set and rework out of tolerance components. Financial ramifications in terms of yield and expenditure can be significant, i.e., as high as 30% of production costs. Consequently, a surface treatment that improves damage resistance beyond that of the state of the art potentially has considerable benefits.
Since the early 2000s, the combination of a nitrided layer with a physical vapour deposition (PVD) coating (a so-called duplex treatment) has been considered one of the most effective such treatments. The nitrided layer increases substrate resistance to plastic deformation and cracking and protects the coating from loss of cohesion and adhesion. The coating protects the nitrided substrate surface from abrasion and thermal effects and can reduce the coefficient of friction. In recent years, high power impulse magnetron sputtering (HIPIMS) technology has enabled PVD coatings with enhanced adhesion, superior density, and higher H/E ratios. However, HIPIMS coatings are as yet unexplored in this capacity.
Ultimately this project will evaluate and derisk a candidate HIPIMS coating in terms of hot forging a nickel-base superalloy. Performance will be benchmarked against conventional die surface treatments. To achieve this: (1) a hot forging wear test within a full-scale press will be developed, and (2) a modified version of the upsetting sliding friction test will be constructed and retrofitted to a universal testing machine. Such capability development is necessary because the extreme tribo-conditions and tendency for interplay of wear modes that occur in hot forging render commercially available tribometers inadequate. Furthermore, there is presently no accepted best way to accurately reproduce these aspects in a controlled environment. When coupled with the risk averse nature of the forging industry, this is a principal reason for the low uptake of technological innovation in this area. Within the UK research sector, the AFRC has unique access to industrial-scale forging presses and is thus well-positioned to develop tests to rectify this situation.
The methodology behind evaluating wear resistance will be for the student to employ material testing and FE-methods to design a die geometry that will promote wear without yield and to determine optimal billet dimensions. Analyses will be conducted to characterise and quantify abrasive wear, adhesive wear, surface fatigue, thermal fatigue, mechanical fatigue, thermal-mechanical fatigue, thermal softening, and plastic deformation. For the friction tests, the student will research mechanical and electrical components and employ FE-methods to construct an upsetting sliding test configuration that reproduces critical hot forging operational parameters. Analyses will be conducted to obtain quantitative and qualitative insight into process friction.
This PhD project will be supported by The National HIPIMS Technology Centre, Sheffield Hallam University. They will apply all HIPIMS coatings necessary for testing and collaborate with analysis, e.g., Raman spectroscopy, scanning electron microscopy, micohardness. It is important to highlight that the candidate HIPIMS coating has previously achieved a 10x increase in tool life for an industrial hot rolling operation at 900 °C.
Subsequent to the necessary supporting data being obtained, the AFRC supervisor, Dr Christopher Fleming, and Professor Hovsepian at The National HIPIMS Technology Centre plan to apply for Innovate UK funding to facilitate industrial production trials and implementation.