Mechanical Analysis And Intelligent Control Strategy For Four-point Bending Incremental Forming Of Lsaw Pipes

In recent years,with the development of global economy,the demand for energy is increasing all over the world,which promotes the construction of oil and gas transportation pipelines.At the same time,the requirement for pipeline quality is increasing.Longitudinal Submerged Arc Welding(LSAW)pipe is an important part of long-distance oil and gas transportation pipelines.The main forming processes of LSAW pipe are JCO forming processe and UO forming processe.Due to the advantages of less investment and less equipments,JCO forming process has been greatly developed and widely used in China.At present,most companies adopt three-point bending incremental forming method to forming LSAW pipe.The forming process has many shortcomings,such as more forming passes,low production efficiency,long residual straight edge,low precision of product shape and large residual stress,which can not meet the increasing quality requirements of LSAW pipe in international and international markets.Therefore,the manufacturer urgently need to carry out technological reforms to meet the needs of the market,improve the quality of pipe and promot efficiency.Based on the analysis of the traditional three-point and four-point bending JCO forming process for LSAW pipe,the main factor friction is pointed out,which causes the uneven curvature distribution of sheet in four-point bending process.On this basis,the four-point bending process of symmetrical sheet is optimized,and the distribution of bending moment is recalculated.Combining with the classical elastic-plastic bending theory,the analytical formulas of the main factors,which affecting the forming process,such as bending moment,bending force,curvature radius,forming angle,deflection distribution and reduction,are obtained.The springback is analyzed and calculated by the plane elastic equation with small curvature,which provides theoretical guidance for the subsequent formulation of sheet symmetrical four-point bending JCO forming process parameters.In order to further reduce the residual straight edge and improve the forming quality of LSAW pipe,the sheet asymmetric four-point bending process is proposed based on the sheet symmetrical four-point bending process.In the forming process,one side of the sheet contact the die with straight section and the other side contacts die with the arc section.Due to the stress state during the bending process is complex,the bending process is divided into rigid rotation stage,asymmetric three-point bending stage and asymmetric four-point bending stage according to the force condition.The distribution of bending moments under different bending statge is analyzed and the calculation method of bending moments is given.The bending process and springback process of each stage are analyzed theoretically,and the analytical formulas of the main forming indexes before and after springback are obtained.It lays a foundation for establishing sheet asymmetric four-point bending JCO forming process.Based on the characteristics of LSAW pipe JCO forming process,the method for measuring single pass forming angle and ellipticity of formed part of pipe based on machine vision is proposed.The calculation method of forming angle based on contour feature and the calculation model of whole contour of formed part are given.In order to measure the forming quality of formed parts,the concept of part ellipticity is proposed and its definition formula is given.The comparison with the measurement results of three coordinate measuring instrument shows that the maximum error of single pass in y axis direction is 0.21 mm,and the maximum error of forming angle is 0.2°.The difference between the splicing result of the whole contour and the measurement result of three coordinates is 0.48% of the formed part.It can satisfy the engineering application and complete the on-line real-time monitoring of the pipe forming quality.Finally,an intelligent control strategy for LSAW pipe four-point bending JCO forming process is established,which includes basic parameters input module,process parameter formulation module,forming result recognition module and process parameter correction module.According to the basic parameters,the strategy can formulate the required parameters for JCO forming process according to the sheet symmetrical four-point forming process and the sheet asymmetrical four-point forming process.According to the forming results of real-time testing,the process parameters can be modified by the iterative compensation method to control the forming accuracy and improve the quality of the pipe.After the intelligent control strategy is programmed,an experimental system is established to verify the strategy by experiment and finite element simulation.The results show that compared with the three-point bending process,the four-point bending process can complete the JCO forming process with fewer passes and improve the production efficiency.The ellipticity of most of experimental pipes is less than 1.5% and that of simulation results is less than 1%.The experimental and simulated ellipticity of sheet asymmetric four-point bending process for the same size pipe are less than that of sheet symmetrical four-point bending process.The feasibility and reliability of the two new processes are proved,and the effect of the symmetrical four-point bending process on improving the quality of LSAW pipe is verified.It laid a foundation for the development of the LSAW pipe JCO forming process and the intellectualization of the forming process in the future.