| Plasma transferred arc welding(PTAW),as a kind of surface engineering technology in which alloy powder materials with special properties such as wear resistance and corrosion resistance are deposited on the surface of substrate by welding method,enjoys the advantages of low cost,high material utilization and high production efficiency.Therefore,it often appears in the fields of surface strengthening and remanufacturing of key parts in many industries such as machinery,metallurgy,energy,aerospace,equipment manufacturing and so on.Because the molten pool temperature changes in real time in the process of plasma transferred arc welding,the temperature of alloy powder material and substrate will be easy to overheat or under cooling if the input is not adjusted in time,which brings about obvious differences in the performance of spray welding layer,such as deformation,cracking,incomplete fusion,coarse structure and other problems;In the current plasma transferred arc welding process,the commonly adopted method is to judge the change of molten pool temperature according to the manual observation of the fluidity of molten liquid,and then adjust the input current of plasma power supply to control the molten pool temperature;However,there are disadvantages such as large errors,over-adjustment and adjustment lag in this operation method.In this thesis,the research on the influence of molten pool temperature on microstructural properties of Ni-based welding layer and the development of real-time measurement-control system is carried out to address the practical problems in plasma transferred arc welding technology development and production application.Based on the real-time measurement of the molten pool temperature in the plasma transferred arc welding process and the simulation of the temperature field and stress field in ANSYS,the influence of the molten pool temperature on the microstructure and properties of the Ni-based welding layer is analyzed;Provide theoretical support for the development of real-time automatic control of molten pool temperature.The closed-loop control of automatic adjustment of molten pool temperature in plasma transferred arc welding process is realized through PLC and analog expansion module.By using the built colorimetric infrared thermometer,the molten pool temperature in the spray welding process is monitored real-time in this thesis.After adjusting the process parameters such as plasma transferred arc welding current,walking speed and powder feeding amount,the Ni-based spray welding layer corresponding to the molten pool temperature in different spray welding processes are obtained,and the microstructure and composition of the Ni-based spray welding layer are detected by scanning electron microscope and EDS.The results show that the cooling rate of Ni-based spray welding layer is faster in the solidification process;Due to the different solid phase composition of crystallization at different times during non-equilibrium solidification of solid solution,the high melting point element Cr content of dendrite axis formed preferentially is higher,while the low melting point element Ni content between dendrites is higher;The process parameters such as plasma transferred arc welding current,spray gun travel speed and spray powder feeding speed have a great influence on the microstructure and performance of the Ni-based spray welding layer.When the spray welding current is large,the speed of the spray gun is low,and the powder feeding speed of the spray welding is low,the temperature of the plasma transferred arc welding pool increases,among which the spray welding current has the greatest influence.If the process parameters are not properly controlled,it may cause the molten pool to overheat,resulting in serious burning of Cr and Ni elements in the Ni-based spray welding layer,increase in dilution rate,decrease in hardness,and decrease in corrosion resistance.Under this test condition,when the plasma transferred arc welding process parameters are controlled as spray welding current 110 A,spray gun travel speed 100 mm/min,spray welding powder feeding speed 18.8 g/min,the spray welding molten pool temperature is between 1300 ℃-1400 ℃.During this time,a more ideal Ni-based spray welding layer can be obtained.The above studies demonstrate the importance of real-time temperature adjustment from the changes in the microstructure and properties of the Ni-based spray welding layer.In this thesis,ANSYS numerical simulation analysis is carried out to analyze the effects of spray welding current,walking speed of the spray gun and feeding speed of the spray welding powder on the temperature field of molten pool in plasma transferred arc welding process.According to the results,it can be concluded that the increase of spray welding current,the maximum surface temperature of spray welding weld pool increases gradually;With the acceleration of the gun walking speed,the maximum temperature of the surface of the melt pool gradually decreases.Due to the influence of the powder melting rate,the maximum temperature of the melt pool surface fluctuates insignificantly with the increase of the powder feeding speed.The accuracy of the model is verified by the fact that the variation trend of the peak temperature of the measured average surface temperature and the simulated peak temperature of the molten pool surface is consistent.The fitting equation of the measured average surface temperature of the molten pool and the simulated peak temperature of the molten pool surface is established,and the actual molten pool temperature under different spray welding currents can be predicted by the fitting equation,which provides a new approach and method for the optimization of process parameters.In order to solve the problems of relying on manual intervention and low degree of automation in the traditional plasma transferred arc welding process,this paper developed a real-time automatic control set for the melt pool temperature embedded in the plasma transferred arc welding system.The closed-loop control system for the molten pool temperature in the plasma transferred arc welding process is mainly composed of two parts: The real-time measurement of the molten pool temperature and the PLC operation processing and feedback of the temperature analog signal.The closed-loop control of the automatic adjustment of the melt pool temperature in the spray welding process is realized by using colorimetric infrared thermometer and PLC analog expansion module.The molten pool temperature of the samples processed by the complete set of real-time automatic control of molten pool temperature fluctuates at 1300 ℃-1400 ℃.Under this test condition,the feedback program molten pool temperature control accuracy is 15 ℃-20 ℃,and the purpose of improving the performance of the plasma transferred arc welding process and the tissue properties of the sprayed layer is achieved. |
PDF Full Text Request