| Magnetic-free drill pipe is an important part of directional drilling technology and equipment.For the engineering requirements of ultra-deep holes,the magnetic-free drill pipe composed of titanium alloy / magnetic-free steel is one of the solutions.The manufacturing method of non-magnetic drill pipe is friction welding.Through the preliminary engineering test,it is found that the friction welded joint of non-magnetic steel / titanium alloy has the phenomenon of virtual connection.This phenomenon leads to magnetic-free steel / titanium alloy non-magnetic drill pipe can not be well applied to engineering practice.Due to the different thermophysical properties of non-magnetic steel and titanium alloy,the formation mechanism of friction welding joint is different from the existing theory of friction welding of homogeneous materials,so the virtual welding mechanism of friction welding of non-magnetic steel and titanium alloy has not been solved.Due to the lack of basic theory,it can not provide strong theoretical guidance for friction welding process setting.Therefore,based on the above discussion,the temperature field,interface microstructure and mechanical properties are studied in this paper,which lays a theoretical foundation for solving the virtual connection essence and improving the strength of friction welded joints of magnetic-free steel/ titanium alloy.In this paper,the numerical simulation of rotating friction welding of titanium alloy and non-magnetic steel is carried out.The numerical simulation results show that the temperature field,plastic flow field and stress field of titanium steel friction welding joint are asymmetric and different.During the welding process,the maximum temperature on the titanium side is higher than that on the steel side,the plastic flow velocity on the steel side is higher than that on the titanium side,the stress on the titanium side of the friction interface is higher than that on the steel side,and the strain on the steel side is higher than that on the titanium side.When the pressure is small,the heat input is small,resulting in less heat generation at the interface.At the end of welding,most regions are in the low temperature zone,and good combination cannot be achieved.With the increase of friction pressure,the extreme value of interface temperature first increases and then decreases.The temperature of friction interface increases with the increase of welding.With the increase of welding time,the temperature diffuses,and the deformation of steel side is serious,resulting in bulging.With the change of different taper structures on the steel side,the temperature of the welding interface has little change.When the tapers are 5 and 10,the workpiece is well formed,which can effectively reduce the virtual connection phenomenon.When the taper increases by 15 and 20,a part of the material on the steel side is insufficient,and the workpiece is poorly formed.Through physical experiments,it is revealed that the virtual welding phenomenon in the friction welding system of magnetic-free steel / titanium alloy is due to insufficient deformation caused by insufficient friction heat generation of the material,and insufficient deformation makes insufficient softening of the steel side not conducive to interface bonding.In addition,sufficient friction heat makes the interface form a thicker brittle intermetallic compound,which is also not conducive to interface bonding.The process method of suppressing intermetallic compounds by reducing rotational speed,increasing torque,improving steel side deformation and promoting connection,and reducing interfacial heat generation was obtained,thereby eliminating the virtual connection phenomenon.The thickness criterion for eliminating the virtual welding phenomenon is constructed based on the thickness of the compound at the outer edge.When the thickness is greater than 12-15 ?m,the false junction is easy to occur.Prefabricating circular and triangular ends on the steel side can increase the torque more effectively,soften the steel side,improve the deformation of the steel side to promote connection,eliminate the virtual connection phenomenon and improve the performance.It is verified that the end face design method is one of the effective means to improve the mechanical properties. |
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