The addition of second phase particles such as MnS is a common method to improve the mechanical processing properties of powder metallurgy products. Some studies even believe that adding MnS powder to prealloyed powder is the best way to improve the mechanical processing properties of powder metallurgy parts.
There are two main theories explaining the addition of MnS to improve the machinability of materials. The currently accepted MnS explains that the machinability of materials is explained by the fact that MnS improves the machinability of the material. It is believed that MnS plays a role between cutting tools and debris. Lubrication; another theory holds that the energy lost on the surface of the cutting tool and debris is only about 20% of the total energy consumption of the cutting process, and most of the energy is lost in the main shear zone of the cutting tool tip.
In order to reconcile these two opposing theories, this experiment used a transmission electron microscope to observe the formation of debris and the role of MnS particles during cutting.
Test method: Add MnS powder with 0.5% by mass to FC-0205 alloy powder to prepare pre-alloy powder, and press pre-alloy powder into a cylindrical test with a density of 6.8g/cm3 and a diameter of 38.1mm and a height of 50.8mm. Then, the green compact sample is sintered in a mixed gas of 10% by volume of hydrogen and 90% of nitrogen, the sintering temperature is 1120, the sintering holding time is 25 min, and the sintered sample is placed on a lathe for cutting, after cutting. The debris was observed by a transmission electron microscope, and a sample for observation by transmission electron microscopy was prepared by a focused ion beam method, and the sample was formed into a film having a thickness of 50 mm to 80 mm. The film samples were observed by transmission electron microscopy and found to have two kinds of MnS particles. The first particle has a nearly spherical shape. The particles are usually composed of a mixture of two phases of MnS and (MnSiO3). The shape of the second particle is Rod shape, composed of MnS single phase.
Discussion: (MnSiO3) is harder than MnS, and the mixture of MnS and (MnSiO3) is more favorable for stress concentration than MnS single phase. The presence of MnS and (MnSi)3) two-phase mixture reduces the plasticity of MnS and is favorable for formation. Microcracks and debris. Although the effect of stress concentration in the pure MnS phase is not as good as the mixture of two phases of MnS and (MnSiq), the MnS phase can function as a thermoelastic shear band, and it is advantageous to improve the machinability in the second shear zone. The deformation inside the material during chip formation is uneven, although most of the deformation occurs in the main shear zone, and considerable deformation occurs in the second shear zone. The mixture of MnS and (MnSiq) is in the first The effect of the second shear zone is not as great as that of the main shear zone because the contribution of the mixture of MnS and (MnSiq) phases to stress concentration is mainly due to its relative motion from the main shear zone to the second shear zone. At the same time, the movement of the cutting tool also changes the stress distribution in the material. The MnS phase has a greater effect in the second shear zone. The MnS phase can reduce the width of the strain zone. Reducing the width of the strain zone can reduce the resistance of the cutting tool and increase the cutting speed of the tool. Therefore, the MnS is used to cut the tool. The lubrication of the debris may not be as important as originally thought.
Conclusion: The main shear zone at the tip of the cutting tool contributes a great deal to the stress concentration by a mixture of two phases, MnS and (MnSiq), rather than single-phase MnS. In the second shear zone, MnS reduces rheological zones and cutting. Tools and debris play an important role in the friction, the role of MnS lubrication during the cutting process may not be as important as originally thought.
This paper was ranked first in the research and development category of the Powder Metallurgy Metallurgical Paper Competition of the American Metal Powder Industry Federation in 2003.
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