| In the process of electrical contact induction brazing,in order to ensure the certain production efficiency,it is easy to appear incomplete penetrations,and the brazing ratio is unstable.In addition,there are still no effective on-line quality monitoring and on-line fast non-destructive quality assessment tools in the automated production of electrical contact induction brazing.Therefore,CJ400 electrical contact was taken as an experimental sample.The brazing ratio of pulse and continuous induction brazing was compared and analyzed.The on-line brazing ratio monitoring based on the integral value of process electrical parameters was studied.In view of low efficiency of brazing ratio measurement(about 30 minutes per workpiece),an on-line rapid nondestructive evaluation method for brazing ratio after welding was proposed based on the resistance of electrical contact.In order to improve the uniformity of radial temperature field,variable current pulse induction brazing method was proposed,and the radial temperature uniformity of continuous and pulse induction heating was compared and analyzed.Based on the numerical simulation,it is found that under the traditional continuous induction heating,the smaller the current density is,the more uniform the radial temperature field is.Under the condition of pulse induction heating,the temperature difference between the outer surface and the center is rising with the law of fluctuation because of the different heating rate of the base value and the peak value.Under the same heating time,the radial temperature uniformity of pulse induction brazing is better than that of continuous induction brazing.In order to meet the requirements of production efficiency,the total induction brazing time of CJ400 electrical contact is limited to about 20s(heating time 16s,holding time 4s).Based on the self-developed pulse induction heating power supply controller,the continuous and pulse induction brazing process was compared and analyzed.Under the condition of continuous induction brazing(Current of 45.0A),heating time is 17.0s,the brazing ratio reaches the maximum value of 87.62%.Under the condition of pulse induction brazing(base value 30.0A,peak value 60.0A),heating time is 16.8s,the brazing ratio reaches the maximum value of 96.61%,and the process window is 16.4s-17.2s.Under the same heating time,the brazing ratio of pulse induction brazing is higher,and the process window is larger,which means that the welding quality is more stable.The microstructure of brazed joints is composed of Ag based solid solution,Cu based solid solution and a small amount of Ag-Cu eutectic structure.With the increase of holding time and current,the Ag based solid solution increases,and the Ag-Cu eutectic structure decreases.In order to meet the needs of automatic production of electrical contact induction brazing,the on-line brazing ratio monitoring based on electrical parameters(the ultimate objective is monitoring and controlling)was studied.Based on the study of relationship between the process parameters(such as current,voltage and power)and brazing ratio,it is found that the relationship curve can be divided into four parts:filler metal unmelted area,increasing area,stable area(the brazing ratio is basically stable at above 80%)and decreasing area.This is because the integral value of electrical parameters are directly proportional to the output energy of induction heating power,and the output energy of induction heating power directly affects the melting process of filler metal and the changing rule of brazing ratio.The quality level and quality factor could be given based on the reasonable deviation extracted from the stable area.Finally,the fuzzy comprehensive evaluation of brazing ratio could be carried out by the weighting algorithm.By comparing the prediction results and the measuring results,it is found that the accuracy can reach 97%,99%respectively.In view of the low efficiency of brazing ratio measurement for electrical contacts,off-line random inspection was adopted in the quality system.An on-line fast nondestructive quality assessment scheme based on electrical contact resistance was proposed in this paper.Through the study of the relationship between electrical contact resistance and brazing ratio,it is found that the current will flow around the incomplete penetration,resulting in a weak current density region near the defect.The conduction channel of the current becomes narrow.Therefore,the resistance of electrical contact decreases with the increase of brazing ratio.Due to the weak current density region near the defect,the different height(within the range of 0.3mm)of incomplete penetration has little effect on the resistance.It is concluded that both of the resistance and brazing ratio reflect the area of incomplete penetration.High power intermediate frequency inverter resistance welding machine(maximum output current 60kA)was adopted in resistance measuring of electrical contact.The contact resistance could be eliminated to the maximum extent by means of the resistance heat and the electrode pressure,and the accuracy and stability of the resistance measurement were improved.After the optimization of measuring process,the electrical contact resistance was measured by multi pulse(6 pulses)instead of single pulse.The electrode force is 3000N,the time of each pulse is 100ms.The first pulse with large current(30000A)is mainly used to eliminate contact resistance.The rest measuring pulses with small current(20000A)is used to avoid the resistance measurement error caused by the increase of temperature in the measuring area.Furthermore,the temperature would be lower by the means of water cooling system during the interval time(1000ms)of pulses.The average value of R_t in the last four pulse is approximately regarded as R_w.And the relationship between resistance and brazing ratio is analyzed.The welding quality of electrical contact was predicted based on the resistance.By comparing the predicted results and measured results,the RMS error and maximum absolute error of brazing ratio is 3.05%and 5.47%.So it is concluded that the scheme of on-line fast nondestructive quality inspection based on the electrical contact resistance is practicable. |
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