The role of cold nitrogen gas and oil mist on tool wear and surface roughness is investigated in turning the K424 nickel-base super alloy with Sialon and SiC whisker-reinforced alumina ceramic tools. A new cooling system is developed and used to lower the temperature of the compressed nitrogen gas. Experiments are performed in three different cooling/lubrication modes, i.e. the dry cutting, the cold nitrogen gas （CNG）, and the cold nitrogen gas and oil mist （CNGOM）. Experimental results show that the depth-of-cut notching severely limits the tool life in all the cooling/lubrication modes. Compared with the dry cutting, the use of CNG and CNGOMcan yield higher wear rate of depth-of-cut notching and worse surface finish.
High speed milling experiments using nitrogen-oil-mist as the cutting medium were carried out to investigate the characteristics of chip formation for Ti-6Al-4V alloy.Within the range of conditions employed(cutting speed,vc=190-300 m/min;cutting depth of axial,ap=5,7 mm),saw-tooth chips were produced in these experiments.During the macro and micro analysis of the Ti-6Al-4V chips,an optical microscope and a scanning electron microscope(SEM)were used to study the microstructure and the morphology of the chips,and the X-ray photoelectron spectroscopy(XPS)was employed for chemical analysis.Comparisons were made to study the influence of different cutting media(nitrogen-oil-mist,air-oil-mist and dry cutting condition)on chip formation.Results indicate that cutting media have significant effects on chip formation.Nitrogen-oil-mist is more suitable for improving the contact condition at chip-tool interface and increasing the tool life in high speed milling of Ti-6Al-4V alloy than air-oil-mist and dry cutting.
Milling Ti alloy is a very difficult technology. The primary problem is that the miller wear is very rapid and makes the miller break or rapture. Although cutting fluid is mainly used to reduce friction and temperature in cutting area to enhance tool life, it is the largest source of environmental pollution. To develop a technology for the clean and efficient milling Ti alloys, nitrogen gas is used as a cutting media in this paper. Based on lots of experiments and researches, the tool life and wear mechanism of high speed steel miller is analyzed. A conclusion is drawn that, milling with nitrogen gas media yields much longer tool life than dry milling. Tool life equations (Taylor′s equations) are derived for both milling types.
