According to the strategy of controlled pulse key-holing,a new sensing and control system was developed for monitoring and controlling the keyhole condition during plasma arc welding(PAW). Through sensing and processing the efflux plasma voltage signals,the quantitative relationship among the welding current,efflux plasma voltage and backside weld width of the weld was established. PAW experiments show that the efflux plasma voltage can reflect the state of keyhole and backside weld width accurately. The closed-loop control tests validate the stability and reliability of the developed keyhole PAW system.
During stable keyhole plasma arc welding,the pilot arc and the transferred arc exist at the meantime,and the arcs can be considered as a composition of two parts inside and outside the nozzle,respectively. Under the mechanical constriction and thermal contraction effects,the inside arc has certain arc length,electron density and arc profile etc. inducing constant tungsten-to-nozzle voltage. However,the arc outside the nozzle diverges at about 5 degrees and has certain characteristics similar to the free arcs. The nozzle-to-workpiece voltage (NTWV) depends mainly on the length of the arc,which gets bigger as increasing of the weld penetration and keyhole size. The NTWV sensor is developed for monitoring NTWV in real time. The welding experiments are designed to get different penetrations and keyhole sizes. It is found that as the weld penetration and the keyhole size increase,NTWV also increases linearly. The NTWV signals can be used as the feedback variable in automatic control of keyhole plasma arc welding.
