Basic Research On The Related Technologies Of Drilling Typical Difficult-to-machine Materials

The rate of drilling operation in machining is about 40%~50%. However, it is difficult to drill difficult-to-machine materials, which is mainly shown that: drilling deformation is complex, drilling forces (torque, thrust) are large, drilling temperature is high, drill bit wear is serious, and life of bit is short, and so on. With the development of aerospace, weapons, ships, power plants and other industries, the difficult-to-machine materials are used progressively, which brings serious challenges to machining. Although carbide drill can be used to drill some difficult-to-machine materials, its application is limit due to low flexural strength, poor impact strength, brittleness, easy to chipping or brittle fracture, high precision of drill press required, high cost, and difficultly grinding, so people still like to use the high-speed steel twist drill. It is necessary to system study the characteristics and laws of drilling deformation, drilling force, drilling temperature, drill bit wear, and the coolant and lubricant technology. It is not only has a theoretical significance, but also has a wide range of application.Firstly, based on the reviews of the status of drilling technology, drilling mechanism of typical difficult-to-machine materials was studied, that is, the drilling chip formation process. However, accessing drilling chip roots is the key, so drilling quick-stop device was developed for it. A series of tests were carried out for getting the drilling chip roots of AISI 1045, austenitic stainless steel 1Cr18Ni9Ti, titanium alloys Ti6Al4V and nickel-based superalloy GH4169, then chip roots were made to the chip roots specimen. Based on it, the chip deformation, laws and influence factors were studied, and shear angle empirical formula was also established. These are the theory foundation for studying the drilling process of difficult-to-machine materials.Secondly, researches on drilling force which is one of the difficulties in drilling difficult-to-machine materials were conducted. Working angles of drill bits were established according to drilling equivalent model, then a theoretical model of drilling force was established and the ratio of drilling force of the cutting edge drilling method was developed for theoretical and experimental studying the drilling force in drilling 3 kinds of typical difficult-to-machine materials. Besides, finite element method was also used for studying the drilling force. Results show that material mechanic properties has a great influence on the ratio of drilling force of the cutting edge, while it is the limit of drilling parameter; the thrust of GH4169 is the largest, that of Ti6Al4V is the second, that of 1Cr18Ni9Ti is the third; the torque of GH4169 is the largest, and that of 1Cr18Ni9Ti is the second, and that of Ti6Al4V is the third.Thirdly, a drilling temperature theoretical model was established for studying the drilling temperature which is one of the difficulties in drilling difficult-to-machine materials. Drilling temperature distribution model and average temperature model were developed according to the drilling equivalent model and two-dimensional cutting temperature theory. Then, the drilling temperature in drilling 3 kinds of typical difficult-to-machine materials was studied. A preliminary study on the distribution of drilling temperature was also carried out by using the finite element method. Results show that the calculated drilling temperature is in good agreement with the experimental value, so the model is effective; the drilling temperature of GH4169 is the highest, that of 1Cr18Ni9Ti is the second, and that of Ti6Al4V is the third.Finally, dry drilling tests of 3 kinds of typical difficult-to-machine materials, 1Cr18Ni9Ti, Ti6Al4V and GH4169 were carried out for finding the difficults and laws, then, empirical formulas of drilling forces and drilling temperature in drilling 1Cr18Ni9Ti and Ti6Al4V were established. In addition, drilling tests of difficult-to-machine materials were carried out under three coolant and lubricant conditions of dry drilling, oil emulsion and superheated water vapor, and a superheated water vapor supply system in drilling was developed for that. Results show that the index in empirical formula is related to the material mechanic properties; superheated water vapor can reduce the drilling force, lower the drilling temperature, and reduce drill bit wear. Although compared to oil emulsion the effects of superheated water vapor are not very obvious, it is the pure green coolant and lubricant instead of oil emulsion, which meets the development trend of green machining technology.