Characterization and machining performance of a chipping resistant ultra-soft coating used for the machining of Inconel 718

Abstract

Inconel 718 is widely used in the aerospace industry due to its excellent mechanical properties, such as high corrosion and heat resistance and great strength at elevated temperatures. Unfortunately, the poor machinability of this alloy still presents a major challenge for manufacturers. Developing different types of tool coatings is a common method for enhancing the machinability performance of this class of materials. Almost all of the coatings selected to improve tool productivity are super-hard coatings. Authors have shown in their previous study that the deposition of a soft monolayer Al-Si coating on a tool, which has never been used before for machining applications, significantly improves tool performance and outperforms most of the recommended hard coatings during the machining of Inconel 718. Since deposition parameters in the PVD process play a significant effect on the properties of coatings and since no data is present in the literature for this tool coating, the effect of three main deposition parameters including bias voltage, deposition time, and gas pressure were investigated around their proposed extreme ranges. The Al-Si coatings deposited under different parameters were characterized and discussed in terms of hardness, roughness, coverage, adhesion, residual stress, coating microstructure, and coating material distribution. Based on the results of thorough coating characterizations, the Al-Si coating with full coverage and uniform material distribution was selected for further machining investigations. The results showed that most defects and complications in commercial hard coatings used for the machining of Inconel, including hardness, roughness, and residual stress formation, are not of any concern for Al-Si coatings, and the limits of the parameters should be defined mainly based on the microstructure and composition uniformity of the coating. The results also showed that a soft Al-Si PVD coating improves tool life by around 500%, which is much higher than the commercially available coatings, and decreases the initial cutting force by 150% during the machining of Inconel 718. The in-depth material characterizations of the coating, including XPS analysis and machinability analyses, showed that the considerable improvements were mainly attributed to the superior lubricating properties of the Al-Si coating deposited on the cutting tool.