Thermische Analyse des Zerspanens metallischer Werkstoffe bei hohen Schnittgeschwindigkeiten

The temperatures occuring in a machining operation affect the whole process, since parameters as tool wear, material behaviour and friction are influenced. A turning process with cutting speeds up to 100 m/s is investigated in this work for the workpiece materials AISI 1045 steel, AA 7075 aluminium, and Ti6Al4V titanium. The results show a strong temperature increase with rising cutting speed for the chip bottom surface approaching asymptotically a limiting value, which corresponds to the melting temperature in case of AA 7075 aluminium alloy. The workpiece surface temperatures also show a strong increase with cutting speed and are strongly affected by tool wear. For temperature measurements of chip and workpiece surfaces a fast fibre-optic two-colour pyrometer has been developed. The two-colour principle allows temperature measurements with high absolute accuracy without knowledge of the surface emissivity. Quartz fibres with small diameters enable measurements at locations with limited optical access. Additionally a high-speed infrared camera has been applied to measure temperature distributions during the chip formation in an orthogonal turning process. The temperature distributions in the primary shear zone have been evaluated, which show that material behaviour is depending on the cutting speed. The time dependent temperature distribution of the tool has been measured in an alternative experimental set-up. Temperature measurements are hardly possible at each location of interest, e. g. in the friction zone between tool and chip or in the finished workpiece subsurface layer, due to the small scales and high temperature gradients. To determine the complete temperature distribution of tool, workpiece, and chip the two-dimensional energy equation is solved numerically. A finite-volumemethod with structured grids is applied for the solution, which allows the calculation of steady-state and time dependent temperature fields. The theoretically determined distribution of the heat source terms at different locations has been corrected by a comparison of the calculated temperatures with measured values. The results show that the melting temperature of the workpiece material can be reached in the friction zones for AISI 1045 steel. Extremely high temperature gradients occur in the workpiece subsurface layer and the temperatures exceed the structural transformation temperature at which Ferrite/Pearlite changes into Austenite. The influence of the transformation enthalpy on the temperatures has also been investigated. Heat fluxes have been evaluated from the temperature fields. The heat flux into the tool decreases strongly with increasing time and is negligible for the steady state. Even for conventional speeds the heat flux into the workpiece is significantly higher than assumed in the past.