Research on Double Pulse MIG Welding Power Supply System and Welding Heat Input Control of Aluminum Alloy
Supervisor:xue jia xiang
The trend of development in the fields of aerospace,rail transportation and automobile manufacturing is to realize lightweight manufacturing.Aluminum alloy is the best engineering material to meet such demand because of its high strength and corrosion resistance.Because of the thermal conductivity,easy oxidation and high sensitivity to the input energy of the aluminum alloy,if the input energy is not properly controlled during the actual welding process,the defects of the initial fusion,later collapse and welding and so on are easily caused.Therefore,the dual pulse MIG welding has many adjustable parameters,and it can match the process parameters to control the input energy reasonably,and provide reliable support for the stable welding of aluminum alloy and the acquisition of high quality welds.This paper focuses on the simulation and optimization of dual pulse MIG welding power supply hardware circuit,DSP digital control,adaptive neural network intelligence and expert database system.On this basis,the heat input control of double pulse welding and the internal quality of the weld are combined with the mechanical properties,and the effect of heat input on the performance of the aluminum alloy welded joint is explored by the quantitative evaluation of the current sample entropy,which provides a new idea and method for the heat input control of the aluminum alloy double pulse MIG welding.The important meaning and the main work of the paper are as follows:(1)An adaptive PID control method based on feedforward neural network for double pulse MIG welding is proposed,which combine the adaptive feedforward neural network control and PID control.Based on the e correction method,the anti-interference of the system waveform is processed to improve the accuracy of the waveform,which establishes a solid foundation for the control of double pulse waveform and the stability of the welding process.The hardware circuit of arc welding power supply is designed by modularization method.The circuit of the power supply and control system of the dual pulse MIG welding power supply is optimized.The welding data transmission is realized by welding the WIFI communication network.A complete matching of welding current and wire feeding speed is achieved by curve fitting.Based on Newton interpolation and large step calibration technology,a unifiedexpert database is established,based on the existing process parameter database system,the system can improve expert database by interpolating and fine-tuning new process parameters.Based on the research of PID,feedforward control and artificial neural network control algorithm,an adaptive PID control algorithm based on feedforward neural network for the current waveform of double pulse MIG welding is proposed.The results show that this method can effectively match process parameters and achieve satisfactory weld quality.(2)Aiming at the problem of energy mutation in the conventional rectangular modulated aluminum alloy double pulse MIG welding,it is proposed that the trapezoid wave is used to realize the flexibility control of the heat input energy in the welding process,and the heat input precision control of the double pulse welding is realized by controlling and adjusting the number and ratio of the pulse number of the strong and weak pulse group.Because the trapezoid wave modulated double pulse strong weak pulse has a smooth transition of the group current and the heat input,the matching range of the wire feeding speed and the welding current parameters is enlarged under the condition of ensuring the heat input change;At the same time,when the low frequency modulation frequency of strong and weak pulse group changes,no need for dynamic response capability of wire feeder is required,which ensures the stability of welding process.(3)The evolution process of pulse modulation method for double pulse welding is studied.,the stability of double pulse welding process based on the rectangular wave,trapezoidal wave,and sine wave modulation is compared and analyzed.By adjusting the frequency of the weak pulse,the number of pulses and the speed of welding,the influence of heat input on the stability of the aluminum alloy plate surfacing and the internal quality of the weld is explored.The quantitative evaluation method of welding quality based on the entropy of current sample is put forward.The comparison of several modulation methods shows that:In the rectangular wave modulated double pulse MIG welding,because of the sudden change of the strong weak pulse,the welding process is sharp,and the probability of the backfiring and the top wire in the longtime welding process is great;The trapezoid wave and the sine wave modulated double pulse MIG welding has the transition pulse in the process of thestrong and weak pulse transformation.The welding process is smooth,the sound is soft,and the appearance quality of the weld is good.For AA606 aluminum alloy,we must ensure that the process parameters of strong pulse group remain unchanged and achieve stable droplet transfer,the average current of welding can be effectively reduced by adjusting the number of pulse number of the weak pulse group,and the input energy is reduced by 14%,while the trapezoid wave modulation is more uniform than the scale pattern of the surface of the rectangular wave modulated double pulse welding;By changing the base value time of the weak pulse group current,the modulation frequency of the strong and weak pulse group is improved,and the heat input is reduced.The appearance quality of the rectangular and trapezoid wave modulated double pulse welds is compared and analyzed,and the metallographic structure in the welding street is analyzed;By adjusting the welding speed and changing the heat input during the double pulse welding process,the hardness distribution of the welded joint of aluminum alloy is studied;A quantitative evaluation system for current sampling entropy of double pulse MIG welding is established based on embedding dimension,similarity tolerance,and spline length,the reliability of the analysis of welding stability is verified by the sample entropy,this method is helpful to study the mechanism of arc welding,improve the quality of welding and evaluate the performance of the process objectively.(4)The influence of different input line energy of rectangular wave modulation and different modulation frequency of the trapezoid wave on the appearance,microstructure,grain refinement and mechanical properties of the butt welded joint in double pulse MIG welding of aluminum alloy are studied.Using 6061-T6 aluminum alloy,the double pulse MIG welding process test of rectangular wave modulation with different input line energy and trapezoidal wave modulation in different frequency is carried out.The results show that with the increase of line energy,the width of the heat affected zone and the grain size of the fusion zone of the rectangular wave double pulse MIG welding head increase correspondingly,and the welded tensile specimen is broken in the heat affected zone,which is a plastic and toughening mixed fracture.When the modulation frequency is 3Hz,4Hz and 5Hz,the amplitude and amplitude of the current and voltage waveform of trapezoidal wave double pulse MIG welding are small,consistent and repeatable,and the transition pulse is clear.There is no arc breaking phenomenon in the whole welding process.The welding process is stable,and the tensile strength of the welded joint gradually increases.The tensile specimen is broken to the base metal and the joint is plastic fracture.