February 2020 Volume 2

FORGING RESEARCH

3. Several forging conditions have been examined at both room and elevated temperatures (100°C and 200°C), including lubricated and un-lubricated, un-coated, nitrided, and a number of PVD coatings (i-Kote, Super MoS 2 , SiC, diamond-like carbon [DLC], TiCN and AlCrTiN). 4. In the unlubricated condition, the lowest friction factors were obtained with two PVD coatings, i-Kote and Super MoS 2 . These are nanocomposite, thin-film coatings containing lubricious particles such as graphite and/or molybdenum disulfide (MoS 2 ). 5. Based on the results of this study, the best coatings to provide low levels of friction during forging, and thus may allow a reduction (or even possibly elimination) in the use of conventional lubricants, are hard thin-film PVD coatings containing lubricious particles. These should be examined in more detail in future research. FutureWork Future work in this project should therefore focus on the testing of PVD hard coatings that condition lubricious particles, and the development of coatings with lower friction factors than the i-Kote and Super MoS 2 coatings examined in this study. Future work could include the following: • Optimize the structure of the nanocomposite film-film coating • Identify which lubricious particles provide the lowest friction factors • Identify the optimum concentration of lubricious particles • Determine the optimum distribution of the lubricious particles in the thin-film coating • Examine the thermal stability of these types of coatings • Perform extended forging series at elevated temperatures Once the optimum coating has been identified based on the laboratory ring compression test, plant trials using actual forging dies should be performed to evaluate the long-term stability of the coatings. Acknowledgements Funding for this project was provided by the Forging Industry Educational Research Foundation (FIERF). Additional funding was also provided by the National Science Foundation, through their Research for Undergraduate (REU) program provided through the Center for Advanced Non-Ferrous Structural Alloys (CANFSA) at Colorado School of Mines. The authors of this report would like to acknowledge the various companies that provided coatings for testing and Finkl Steel, Bohler-Uddeholm and Hitachi who provided the die steels used in this research.

Table 12: Friction factor values for TiCN coated steel inserts measured at 100 ° C with lubrication.

Discussion This study has Identified and validated a laboratory test, the ring compression test, that simulates the forging process and can discriminate between coatings that reduce friction and coatings that have little-to-no effect. This laboratory test, therefore, can be used in future research to identify coating compositions that minimize friction during the forging operation, and so should allow a reduced amount of conventional lubricants to be used in the forging process. Based on testing performed using the ring compression test, the lowest friction factors were obtained with when testing with the conventional lubricants, Molykote and graphite. However, similar low friction factor values were obtained with two PVD coatings when tested in the unlubricated condition, i-Kote and Super MoS 2 . As summarized in Appendix 1, both these coatings contain lubricious particles such as graphite and molybdenum disulfide, and so focusing on PVD coatings that contain such lubricious particles would appear to be the best approach for future research. Other PVD coatings examined in this study were simply hard coatings without lubricious particles (such as TiCN, AlCrTiN, Sic and DLC). Although previous research had shown that these coatings had exhibited lower coefficients of friction in alternate testing (pin-on-disk, for example), testing performed in this study using the ring compression test indicated that they did not reduce the friction factor during simulated forging trials, and so do not appear useful for reducing conventional lubricant during forging. Therefore, based on the results in this study, future research should focus on PVD coatings that contain lubricious particles. To further improve performance, future research should focus on identification of the best lubricious particles (graphite versus molybdenum disulfide versus boron nitride, for example, the distribution of the particles in the coatings, and the optimum structure of the nanocomposite thin-film. Summary and Conclusions 1. A laboratory test has been identified and validated that provides a quantitative measurement of the impact of a range of substrate conditions on the level of friction between a forged aluminum workpiece and an H13 steel forging die. The ring compression test is easy to perform, and it simulates metal deformation conditions present in commercial forging applications better than other test procedures (such as pinon- disk). 2. A modified ring compression test apparatus has been designed and built that allows the examination of a range of substrate conditions, including varying temperature, on friction.

FIA MAGAZINE | FEBRUARY 2020 60