Effect of cutting media and cutting conditions on temperature and residual stress of machined surface at milling of difficult-to-cut alloy Ti6Al4V.

Provedený experimentální výzkum byl zaměřen na určení vlivu řezných podmínek a řezného prostředí na velikost teplot a velikost, směr a charakter zbytkového napětí v obrobeném povrchu při frézování těžkoobrobitelné slitiny titanu Ti6Al4V. Pro experiment byla použita kombinace tří řezných rychlostí vc: 50, 100, 150 m.min-1, tří posuvů na zub fz: 0,05, 0,085, 0,12 mm a tří řezných prostředí: obrábění za sucha, olejová mlha (aerosol vzduchu a oleje) a povodňové chlazení olejovou emulzí. Znalost teplot při obrábění je v daném případě nezbytná vzhledem ke skutečnosti, že při teplotách větších než 600 °C má Ti a jeho slitiny vysokou afinitu k intersticiálním prvkům obsažených ve vzduchu (a řezném prostředí), které vedou k tvorbě tvrdé vrstvy kysličníků a pod ní ležící zpevněné vrstvy. V kombinaci s nízkou tepelnou vodivostí slitiny Ti6Al4V dochází k lokalizaci vysokých teplot v těsné blízkosti břitu a tedy i pod obrobeným povrchem. Tato skutečnost posléze výrazně ovlivňuje velikost a charakter zbytkových napětí.This research deals with a determination of effect of cutting conditions and cutting environment on temperature and behaviour of residual stress in machined surface. The experiment was based on milling of difficult-to-cut titanium alloy Ti6Al4V. A combination of three cutting speeds: 50; 100; 150 m.min-1, three feed per tooth: 0,05; 0,085; 0,12 mm and three different cutting media: dry cutting, oil mist and flood cooling, has been applied in the experiment. Understanding to behaviour of cutting temperature is necessary. Titanium alloys have very high affinity to interstitial elements contained in air (and cutting medium). When the cutting temperature is higher then 600°C, these chemical elements create hard oxidic cover and hardened underlays. The highest temperature is localized very close to cutting edge and also under machined surface. The reason is low thermal conductivity of Ti6Al4V. Phenomena mentioned above influence magnitude and character of residual stress in machined surface. Measurement of temperatures has been splited on two parts – measurement of cutting temperature and measurement of temperature of machined surface. A natural thermocouple has been used for measuring of cutting temperature. For surface temperature measurement has been used a semi synthetic thermocouple. The magnitude of residual stress in surface layer was analysed using a destructive method of measurement. The method is based on electrolytic removal of material from inspected surface. Removing of surface layer causes a deformation, which is possible to determine a stress, which is proportional to the deformation. The accuracy of the measurement is dependent on thickness of removed layer, which has to be equal all over the surface. In accordance to the results of this research it is possible to recommend for finish milling of Ti6Al4V alloy by sintered carbide cutting tool following cutting conditions - cutting speed between 50 m.min-1 and 100 m.min-1, feed per tooth fz approximately 0,05 mm. These cutting conditions have been determined with reference to cutting temperature and residual stress in machined surface. The most suitable type of cutting environment is the flood cooling (5,5% Ecocool 2520 + water). Usage of flood cooling lowers surface temperature and cutting temperature. That causes decrease of residual stress in machined surface. A disadvantage of the flood cooling is, that sudden destruction of tool tip can occurs. The titanium alloy has a very low heat conductivity, therefore most of heat goes through the cutting tool. The reason of sudden destruction is a thermal shock, which causes chipping of tool tip