Study On The Effect Of Extrusion And Turning Forming Method On The Loosening Behaviour Of Threaded Joints

The threaded connection structure is widely used in various mechanical structures due to its simple structure,easy assembly and disassembly and reliable performance.With the development of internal threads in the direction of low processing cost,low energy consumption and high performance,the application prospect of this structure in the fields of aviation,aerospace and rail transportation has broadened.At present,the common methods of internal thread processing are turning and extrusion,and scholars at home and abroad have conducted a considerable amount of research on the performance of turning/bolting structures,but the research on the reliability of extruded internal threads/bolting structures is limited.In this thesis,the damage morphology and mechanical properties of turned and extruded internal threads were analyzed,and the loosening behaviour of turned and extruded internal threads/bolts under axial excitation was systematically investigated based on the self-designed loosening and random vibration test fixtures for threaded connection structures.Based on the loosening behaviour of the threaded connection structure,the damage morphology of the threaded surface was analyzed to reveal the loosening failure mechanism of turned and extruded internal threads and bolts and the main wear mechanism of the threaded surface.In addition,the stress–strain distribution of the internal threads was analyzed by using the finite element method to study the stresses on the internal thread/bolt connection structure in both steel and aluminium alloy.The main conclusions obtained are as follows.(1)The plastic flow of the material occurs in the internal thread during extrusion forming,and the surface layer of the thread has a streamlined distribution.The cold work hardening when plastic deformation occurs results in the high hardness and good quality of the thread surface.The low extrusion speed and the GUEHRING M16-6HX PM HSS-E taps provide a good surface quality and resistance to loosening.(2)The degree of loosening and damage to the threaded joint structure decreases as the preload increases.because the frictional stress at the threaded contact interface increases slightly as the preload increases,but the relative slip and the frictional dissipation energy per unit area of the surface decrease,and the wear of the threaded surface gradually decreases.During the test,the axial force of the bolt did not decrease monotonically but first increased and then decreased during the local cycle.(3)The damage to the thread surface is characterized by spalling,plastic deformation and ploughing.The wear mechanisms on the thread surface are mainly fatigue wear,abrasive wear and adhesive wear.(4)Aluminium alloys have a lower modulus of elasticity than carbon steel,and the deformation of aluminium threads under the same load is greater than that of steel threads.