Magnetic Pulse Crimping Process,Performance And Quality Inspection Method For Cable Joints

With the rapid development of electric power,energy,automobile,aviation,electronics and other industries,lightweighting has become one of the main ways to energy saving and emission reduction,cost reduction and efficiency improvement in all professions and trades.Copper and aluminum are both good conductors and have been widely used in the wire and cable industry.Compared to copper,aluminum is lighter and less expensive.Especially in recent years,aluminum cables have been widely used in power systems due to the soaring copper price,as well as the increasing demand and lightweight development trend for high-voltage cable harnesses.The number of aluminum cable joints used is also gradually increasing.However,traditional joining methods（hydraulic clamp,press,tin soldering）are limited by manufacturing processes,technician experience,and other factors that make it difficult to ensure a reliable joining of cable joints.The short-circuit fire faults in cable joints occur frequently,causing a lot of economic losses and posing a great threat to the safe operation of the power system.The crimping quality of cable joint is directly related to the safe operation of the entire circuit.The use of stable and reliable joining method for cable joints can effectively reduce economic losses and dangerous accidents,thus safeguarding the lives and property of users.Therefore,an advanced manufacturing technology is urgently needed to guarantee the safety and reliability of cable joints.Aiming at the industry pain points of traditional cable joints,such as easy heating,great difficulty of quality inspection in the production process,and lack of quality assurance,etc.,this paper applies the magnetic pulse crimping（MPC）technology to the manufacturing of cable joints.For the crimping process for cable joints of high-voltage aluminum harnesses,matching MPC coil and tooling are designed and developed to realize the crimping between DT70 copper terminals and YJLV aluminum harnesses.The mechanical properties,electrical properties,microstructural properties,and service performance under extreme environments（fatigue and corrosion）of cable joints at different process parameters are explored.Nondestructive testing of MPC cable joints is achieved based on the characteristics of the crimping process.Main research contents and conclusions are as follows:（1）Relation between crimping parameters and copper terminal deformation,microstructure as well as mechanical properties for the cable joint was revealed.Crimping tests of aluminum harnesses（50,70 and 95 mm²）and copper terminals（DT70）cable joints were carried out to explore the effect of process parameters such as discharge energy,crimping gap,crimping times and crimping direction on the performance of the joints.The influence of crimping process parameters on the evolution of microstructure and interfacial morphology was elucidated.The optimal combination of process parameters for MPC was a discharge energy of 35 k J and a crimping gap of 1.1 mm.The electrical connection performance and tensile strength of MPC joints were at least 35.5%and 62.7%higher than those of hydraulic clamp crimping（HCC）joints.The compaction rate of cable joints should be controlled within 10%～25%.Welding was formed between Cu/Al interfaces.（2）The finite element method and boundary element method（FEM-BEM）coupling model of MPC process for cable joints was established.The coupling change law of electromagnetic and structural field during the whole deformation joining process of the numerical model was elucidated.The reliability of the numerical simulation was verified by extracting the collision velocity and deformation degree.The mechanism of MPC for cable joints was revealed.A three-dimensional process window was constructed for the MPC of high-voltage aluminum harnesses and copper terminals.The simulation parameters varied unevenly in circumferential,radial and axial directions.The collision point moved from the starting point to the ending point,and the Cu/Al interface underwent a transition from un-welded to local welded to un-welded in sequence.The local diffusion welding phenomenon at the Cu/Al interface was confirmed.（3）Fatigue failure models for aluminum harnesses and copper terminals cable joints were developed.The reasons for the transformation of the failure mode of cable joints under different stress levels were elucidated.The S-N curves of the two cable joints at different reliability were obtained by the two-parameter Weibull distribution model.Fatigue life of the cable joints at different stress levels was predicted.Fatigue failure characteristics and fracture mechanisms of the two joints under constant cyclic loading were revealed.The fatigue performance of both MPC and HCC joints decreased with increasing maximum cyclic stress.There were three failure modes of cable joints at different stress levels:aluminum harnesses fracture,copper terminals fracture,and mixed fracture failure of both.As the fatigue loading level decreased,the fatigue life gradually increased and the failure mode gradually changed from aluminum harnesses fracture to copper terminals fracture.In the mixed failure mode,both the aluminum harnesses and the copper terminals reached or approached their own fatigue limits,and eventually the parent metal,which reached its fatigue life limit first,fractured.（4）The corrosion characteristics of copper terminals-aluminum harnesses MPC and HCC cable joints in neutral salt spray environment and the degradation law of joint performance after corrosion were elucidated.Corrosion life prediction relationships for cable crimping joints were established.The corrosion area morphology of the cable joints was reconstructed by 3D scanning,and combined with corrosion fracture（SEM and EDS）analyses to reveal the corrosion failure mechanism of the cable joints,which provides ideas for the design of corrosion protection of the MPC cable joint structure.The main products after corrosion of aluminum were Al（OH）₃ and Al₂O₃.The corrosion products generated by pitting on the surface of copper terminal were Cu₂O、Cu O、Cu₂（OH）₂CO₂ and Cu₂（OH）₃Cl.The corrosion resistance of MPC joints was much better than that of HCC joints.（5）A nondestructive inspection method based on 3D image processing and machine learning for MPC cable joints was proposed.The external surface contour of the joint（3D point cloud）was collected using 3D reconstruction technology,and the corresponding efficient and accurate feature extraction algorithm was designed.The performance of different machine learning models in the regression task was explored,and the model was further optimized by fine-tuning to successfully solve the regression task from the deformation zone characteristics to the mechanical properties of the MPC cable joints.A high-precision performance prediction model based on extracting feature and bearing capacity was established.The cylindrical surfaces of copper terminals and aluminum harnesses were fitted by the RANSAC algorithm and the corresponding axes were obtained,where the radii of the terminals and cables were measured with an error of 0.5%and 1.1%.The calculation process of the average radius was simplified by coordinate transformation,and the running speed was increased by 18.6%.A high-precision prediction model with an average absolute error of 0.348 k N（Stacking）and an average absolute percentage error of 5%was developed.