【作者】 贾传宝；

【导师】 武传松；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 穿孔等离子弧焊接作为一种高能量密度焊接方法,能够对中厚板不开坡口一次焊透,具有很大的应用潜力。但由于小孔状态对焊接工艺参数的变化敏感,获得良好接头质量的合理工艺参数范围窄、可调裕度小,制约着该工艺在制造业中的大量应用。对传统的穿孔等离子弧焊接工艺做出新颖的改进,使其自动适应焊接工艺条件的变化,提高焊接过程和焊接质量的稳定性,拓宽其适用的工艺参数区间,对于推动穿孔等离子弧焊接工艺在制造业中的大量应用,从而大幅度提高焊接生产率,具有重要的意义和工程实用价值。本研究基于“受控脉冲穿孔”的控制策略,研制出等离子弧焊接实验系统,设计和调试出主控计算机与等离子弧焊机之间的接口电路与控制软件,实现了焊接过程的信息采集、处理、运行等各项控制功能。研制出尾焰电压信号的检测装置,进行了大量的焊接试验,建立了尾焰电压、焊缝背面熔宽和焊接电流三者之间的定量关系,能够可靠地表征熔池的穿孔状态以及小孔的尺寸。提出了一种新的小孔检测技术-NTWV传感法,进行了初步的试验验证。设计出适用于受控脉冲穿孔控制策略的特殊焊接电流波形,在电流峰值下降沿增加了两个缓降坡度,以进一步降低焊接热输入,改进电流波形的可控性和灵活性,并保证焊接过程中小孔“开-闭”过渡更加平稳,避免普通方波电流作用时的小孔特征信号倒“V”型剧烈变化。焊接试验证明,受控脉冲电流波形作用于焊接过程时,在每一个脉冲循环过程中,小孔特征信号均呈现倒“U”型变化,证明了小孔从“开”到“闭”转变过程的平稳,有利于获得更好的焊缝成形质量。从熔池穿孔过程各时间段和穿孔反馈信号幅值两个角度对焊接过程进行分析,确定了两种受控脉冲穿孔控制方法,分别以小孔建立时间和小孔平均尺寸为系统被控制量,脉冲峰值电流及其下降斜率为控制量。基于经典控制理论,针对两种控制方法,设计适当的焊接试验获得系统的输入输出数据对,进行系统辨识,经整定,获得比例积分（PI）控制器的参数。集成上述传感检测技术与PI控制技术,研制出受控脉冲穿孔等离子弧焊接控制系统。设计了平板-恒定目标值、平板-变化目标值和变厚度工件-恒定目标值焊接试验,对控制系统和控制方法的实际效果进行验证。试验证明,系统运行平稳,控制算法可靠;被控制量目标值变化时,控制量迅速随之调整,以保证焊接过程动态稳定。当工件厚度连续变化时,系统根据反馈信号与目标值的偏差实时调节脉冲峰值电流及其下降斜率,使被控制量稳定在目标值附近,从而保证小孔尺寸和熔宽的均匀、稳定。通过比较两种控制方法的控制效果,发现以小孔平均尺寸(脉冲尾焰电压平均值V_EP作为表征)为被控制量的控制方法更优。更多还原

【Abstract】 Keyhole plasma arc welding (PAW), as one of the high energy density welding processes, can fully penetrate the workpieces of middle thickness with single pass, so that it has great potential of application in industry. However, since key-holing condition during PAW process is sensitive to the variation of welding process parameters, the ranges of appropriate welding parameters are narrow, and the tolerance is less, which limits its wide applications in manufacturing. In this study, the traditional keyhole PAW was novelly improved to make it be adaptive to any variation of welding conditions. The ranges of appropriate welding parameters were expanded with higher process stability and weld quality. It is of significant value for promoting wider applications of keyhole PAW and increasing welding productivity in modern manufacturing industry.Based on the ’controlled pulse key-holing’ strategy, a keyhole PAW experiment system was developed with the functions such as signal acquisition, data processing, process control etc. The interface circuits and the system software were designed to integrate the main control computer and the PAW machine. Efflux plasma voltage signal acquisition device was constructed, and large amount of sensing experiments were conducted. The quantitative correlation between the efflux plasma voltage, backside weld width and welding current were established through data analysis. It was found that efflux plasma voltage can reflect and characterize the keyhole status and size reliably. In addition, a new method for sensing keyhole- NTWV was proposed with preliminary experimental verification.The welding current waveform for controlled pulse key-holing strategy was developed and applied in the keyhole PAW system. Two slow-decreasing slopes were added at the dropping point from peak current to base current to further reduce welding heat input and improve the controllability. Such current waveform assured more stable and smoother transition of the keyhole from ’open’ to ’close’ status. The sudden change of keyhole signals like an inverted ’V’ shape under square-waveform current was avoided. The test results showed that when the welding current waveform for controlled pulse key-holing strategy was applied, the corresponding keyhole signal changed with a smoother mode, like an inverted ’U’ shape. It proved that the transition of keyhole from ’open’ to ’close’ status is more stable, which results in higher weld quality.Keyhole PAW process was analyzed according to the different time intervals of keyhole establishing and sustaining, and the keyhole signal describing the averaged keyhole size during a pulse cycle. Two control methods were determined, i.e., taking the keyhole establishing time and the averaged efflux plasma voltage as the controlled variables, respectively, while taking the peak current and its slow-dropping slopes as the controlling variables. Based on the classical control theory, system identification and model construction were conducted, and PI controller parameters were acquired.Through integrating the keyhole sensing device and the PI controller, a system of controlled pulse key-holing plasma arc welding was developed. To verify the effectiveness of the control system, various kinds of welding experiments (bead-on-plate welding with constant setpoint and varied setpoint, varied-thickness plate with constant setpoint) were conducted. It has proved that the developed system works stably and the control algorithms are reliable. When the setpoint of controlled variable varies, the controlling variable can be adjusted rapidly to maintain the dynamic stability of the welding process. When the workpiece thickness changes continuously, the system adjusts the peak current and its decreasing slopes according to the deviation of the feedback signal from the setpoint. Therefore, the controlled variable fluctuates around the setpoint within the permitted range, and keyhole size and backside weld width can be guaranteed to be uniform although the plate thickness changes. Through comparing the welding results under two control methods, it was found that the controller taking the averaged keyhole size as the controlled variable is better.更多还原