Study on Heat Input Control and Weld Microstructure Performance of Double Plus Metal Inert-gas Welding

Author:Jin Li

Supervisor:xue jia xiang

Database:Doctor

Degree Year:2019

Download:43

Pages:162

Size:14558K

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Lighter weight,stronger toughness and higher precision are the development trend of manufacturing industry.In this situation.Many traditional steel materials are gradually replaced by new lightweight materials with a wide range of excellent comprehensive performance,among which the representative is aluminum alloy.In terms of aluminum alloy welding,Double plus metal inert-gas welding(DP-MIG)is able to modulate the high frequency by selecting the appropriate low frequency,and has the advantage of refining grain,reducing the heat input,large range of welding parameters adjustment,wide joint assembly clearance,etc.Based on the control of input welding heat in the aluminum alloy DP-MIG,the paper mainly studies fuzzy PID parameters self-tuning control algorithm of the pulse current and double closed loop control algorithm of the arc voltage and current,as well as DP-MIG welding self-learning expert database based on the algorithm of distance calibration and Taylor interpolation.Moreover,as well as the impact of the current waveform parameters and low frequency on the DP-MIG welding heat input and welding seam organization performance.The main achievements of this paper are as follows:(1)The fuzzy PID parameter self-tuning control algorithm of current waveform was studied to realize the adaptive control of DP-MIG current waveform.The algorithm of double closed-loop current and voltage control for arc length of aluminum alloy pulse welding arc was explored,improving the anti-interference ability and control accuracy of current waveform,which built up a solid foundation for DP-MIG current waveform control and welding heat input control.Simulink modeling and Simulation of the power supply system for aluminum alloy arc welding are carried out in this paper.In the micro-control of current waveform,the self-tuning control algorithm of fuzzy PID parameters is adopted to realize the self-adaptive control of current waveform for DP-MIG welding.The double closed-loop control algorithm of current and voltage for the arc length of aluminum alloy pulse welding is explored.The simulation and verification results show that the control algorithm has good robustness.With self-adaptability,the arc length can be controlled accurately,and satisfactory weld quality can be obtained.(2)In more than a drop of arteries and veins and arteries of molten drops transition based on interval,based on the algorithm of distance calibration and taylor interpolation,a kind efficient rapid build method of aluminum alloy DP-MIG welding self learning expert database was put forward,and made the digital welding machine with parameter"self learning"and"self-adjusting"function,reducing the aluminum alloy DP-MIG welding heat input and welding quality control difficulty.For fixed frequency welding,the parameters of DP-MIG welding with a full 10 ampere current between 50A and 220A are calibrated,and the parameters of DP-MIG welding at other points in 50A to 220A are calculated by Taylor interpolation algorithm.After verification test,the parameters are fine-tuned and saved to save the optimal parameters in expert database.For changing frequency welding,a variable frequency welding method for DP-MIG welding of aluminum alloy is proposed.The process test results show that the frequency conversion DP-MIG welding method is stable and reliable,which provides a new basis and idea for establishing the self-learning expert database of frequency conversion DP-MIG welding of aluminum alloy efficiently and quickly.(3)The influence of current waveform parameters on the heat input and weld microstructure of surfacing welding seam was analyzed,and the effect of the ratio of base current to the number of strong and weak current pulses on the microstructure and mechanical properties of aluminum alloy DP-MIG welding joints is studied.The influence of heat input on the arc initiation and closing of double pulse MIG welding is studied by digital arc initiation and closing technology.By changing welding current or welding speed,the influence of heat input on welding seam forming of AA6061-T6 aluminum alloy with different plate thickness is studied.It is concluded that the optimal heat input of AA6061-T6aluminum alloy with 2mm,3mm and 6mm thickness is 123 J·mm-1,189J·mm-1 and600J·mm-1,respectively.The influence of the base current on the microstructure and mechanical properties of the weld is studied.The experimental results show that the width of the heat-affected zone and the tensile properties of the joint increase with the increase of heat input.The effect of the number of strong and weak current pulses on the microstructure and mechanical properties of the welded joint is investigated.The experimental results show that the width of the heat-affected zone and the grain size of the fused zone increase with the increase of the ratio of strong and weak current pulses,and the tensile properties of the joint improve.(4)The effect of low frequency on thermal input and microstructure of different welded joints is analyzed,and the microstructure evolution,microhardness,mechanical properties and corrosion behavior of the same aluminum alloy joint,different aluminum steel joint and aluminum alloy multi-pass welding are studied.DP-MIG butt welding of the same AA6061-T6 aluminum alloy is carried out with multiple low-frequency frequencies.When5Hz is applied,finer weld microstructure,better mechanical properties and corrosion resistance are obtained.DP-MIG plate butt joint of AA6061-T6 aluminum alloy and galvanized Q235B steel plate is carried out.When the frequency is 3Hz,4Hz and 5Hz,the results showed that the weld forming and mechanical properties of the joints are almost the same.DP-MIG multi-pass welding of AA6061-T6 aluminum alloy is carried out.The test results show that good microstructure and mechanical properties can be obtained by adjusting the low-frequency frequency,and the optimal low-frequency frequency is 5Hz.