【作者】 刘祖明；

【导师】 武传松； YuMing Zhang；

【作者基本信息】 山东大学 ， 材料加工工程， 2013， 博士

【摘要】 受控脉冲穿孔等离子弧焊工艺对传统工艺做出了新颖改进,具备获得优质接头并拓宽焊接工艺参数裕度的潜力。但是,原有系统通过检测等离子弧尾焰电压信号来间接反映小孔状态,不足以充分描述小孔动态变化过程的细节。本研究采用视觉传感方法获取工件背面的小孔图像,能够直观清晰地表现小孔特征参数随脉冲电流的瞬时演变过程。本文研究结果为深入理解小孔与熔池热状态的变化规律奠定基础,具有重要的理论意义和实际应用价值。构建了以普通工业CCD摄像机为核心的视觉传感系统,不仅可以测量背面小孔出口的长度和宽度等尺寸参数,而且可以测量背面小孔中心与正面焊枪中心轴线之间的偏移距离(即小孔中心偏移量)。在恒定参数穿孔焊接过程中,通过使用不同的焊接电流、焊接速度和离子气流量调节电弧穿透能力,观察背面小孔参数的变化。发现背面小孔中心偏移量对电弧穿透能力的变化更为敏感。电弧热输入变化时,小孔周围热状态发生变化；这不仅可以影响焊缝熔宽,而且可以显著地改变小孔前壁的倾斜程度。小孔前壁的倾斜程度决定了背面小孔中心偏移量的大小。因此,小孔中心偏移量可以很好地反映小孔前壁熔化状态。小孔热状态发生变化时,背面小孔中心偏移量可以比小孔尺寸更好地反映小孔的动态特性。背面小孔中心偏移量的大小,代表了小孔孔道的弯曲程度,是影响焊缝缺陷形成的关键因素之一。实验结果表明,如果小孔中心偏移量过大,焊缝中容易形成气孔,且焊缝表面成形质量差；如果小孔中心偏移量过小,熔池体积过度长大后,容易出现焊缝塌陷甚至烧穿。根据背面小孔中心偏移量可以预测焊缝成形不佳、焊缝气孔、焊缝塌陷和烧穿四种缺陷。研究发现,受控脉冲穿孔焊接过程中,每一个脉冲周期内,在穿透小孔形成瞬间,背面小孔中心偏移量最大；随后,小孔中心偏移量迅速降低。在闭合小孔阶段终了或者穿透小孔建立之初,小孔前壁倾斜程度最大。穿透小孔维持阶段越长,小孔前壁曲面倾斜程度越小,则背面小孔中心偏移量越小。通过调节脉冲电流后沿的下降斜率,可以控制穿透小孔维持阶段的时长,从而控制背面小孔中心偏移量。基于合适的预测控制算法,建立了以实时检测的背面小孔中心偏移量为被控制量、脉冲电流下降时间(表征脉冲电流后沿的下降斜率)为控制量的受控脉冲穿孔等离子弧焊控制系统。9.5mm厚度不锈钢板穿孔等离子弧焊接控制实验表明,控制过程稳定,每个脉冲内小孔可靠地穿透和闭合,焊缝成形和接头质量良好。更多还原

【Abstract】 The controlled-pulse keyhole plasma arc welding (PAW) is a novel improvement on the traditional pulse PAW. It has potential to produce high quality joints under wider range of process parameters. However, the developed system used the efflux plasma voltage to reflect the keyhole state indirectly, such a voltage signal can only describe whether the keyhole is fully-penetrated or not, but can not give more comprehensive information of the keyhole dynamic behaviors. In this study, vision sensing technology is employed to capture the keyhole image from backside of the test pieces, and the keyhole image sequence directly shows the keyhole evolution process at different instants on the welding current pulse waveform. The research results lay foundation for deeply understanding the thermal behaviors of both the keyhole and weld pool during the keyhole PAW process, and hence have important theoretical significance and will promote the practical applications of the controlled-pulse keyhole PAW process.The vision sensing system is constructed based on a common industrial CCD camera. It can measure not only the length and width of the backside keyhole exit, but also the deviation distance from backside keyhole exit center point to the welding torch axis. Different levels of welding current, welding speed and plasma gas flow rate have been selected in series of constant-parameter keyhole PAW processes. From these tests, it is observed that keyhole parameters, including the keyhole size and keyhole deviation distance, vary as the plasma arc penetration ability changes, but the keyhole deviation distance experiences much larger variation than the keyhole size does when a welding parameter has the same varying amplitude. Changing of the heat input alters the thermal state in the keyhole and weld pool and will induce the variation of the melting condition around the keyhole, so that the weld width and the front keyhole wall inclination will be both changed. The inclined degree of the front keyhole wall directly determines how far is the backside keyhole exit deviated away from the torch axis (deviation distance). Thus, the keyhole deviation distance is a very useful parameter to reflect the keyhole front wall melting state and its thermal condition. As the thermal state changes in the fully-penetrated keyhole process, the backside keyhole deviation distance varies with much larger scope and faster speed than the keyhole size parameters do. The deviation distance hence is a better variable to describe the keyhole dynamic characteristics.The quantity of the backside keyhole deviation distance, i.e. the inclined degree of the keyhole channel, is one of the critical factors to affect the weld defects formation. If the keyhole deviation distance is too large, porosity will easily occur with a bad front weld surface. If the deviation value reduces to nearly zero, the high level heat energy in the keyhole will speed up the melting process of the solid metal around the keyhole, and the weld pool will over-grow so that it is easy to collapse or even burn-through. Hence, the backside keyhole deviation distance can be used to predict the formation of four kinds of weld defects:low quality front weld surface, porosity, weld pool collapse and burn-through.In each pulse cycle of the controlled-pulse keyhole PAW process, backside keyhole deviation distance reaches its peak value when the fully-penetrated keyhole firstly forms; then, it decreases fast during the keyhole open period. It indicates that the keyhole front wall is inclined most severely at the instant of the fully-penetrated keyhole just forms or the closed keyhole period ends. Increasing of the keyhole open period decreases the keyhole front wall inclined degree and hence decreases the backside keyhole deviation distance. Adjusting of the current falling rate at the trailing edge of the welding current pulse will control the duration of the keyhole open period, hence control the level of the backside keyhole deviation distance.Taking the backside keyhole deviation distance as the controlled variable and current falling rate at trailing edge of the welding current pulse as the controlling variable, a modified controlled-pulse keyhole PAW system has been developed based on the predictive control algorithm. The welding tests have been carried out on stainless steel plates of thickness up to9.5mm. It demonstrates that the control process is stable, keyhole opens and closes smoothly in every pulse, and high quality welds have been obtained.更多还原