Research On Effect Of Wire Preheatingon Heat Transfer At Electrode Tip And Metal Transfer In Gmaw

Gas metal arc welding (GMAW) is one of the most widely used welding method in industry. Development of new high quality high efficiency GMAW process is always one the hot spot in the welding community. A hot-wire GMAW process is proposed in this dissertation. By prehating the solid wire with a assistant TIG arc, the same deposition rate while lower heat input to the base metal could be achieved. On the other hand, with the same welding current of GMAW, the deposition rate could be significantly increased without additional heat to the base metal. Further, the heat transfer at wire tip and metal transfer are the two fundalmental issues of GMAW process, while the effect of wire temperature on them has not been reported yet. Define the point temperature of the wire at the end face of the contact tip as the wire preheating temperature, which can be adjusted y regulating the preheating TIG current. The heat and metal transfer under different wire preheating temperature in the proposed hot-wire GMAW are thus studied in this dissertation.A GMAW experimental system with TIG preheating circular has been established. A uniform GMAW torch with TIG preheating mechanic has been designed and optimized, which guarantees the preheat of the quick-feed wire with no compromise on the wire feed stability. Aiming at the GMAW process using constant current and the one with constant wire feed speed respectively, the effect of wire preheating temperature on the wire extension dynamic, wire temperature field, wire melting dynamic, and metal transfer behavior are both experimentally and theoretically studied.The TIG preheating effect on the feed solid wire is first studied. The wire preheating temperature under different wire feed speed and preheating current is measured. The wire feeding stability under different wire preheating temperature are evaluated. A heat input model of the TIG arc is established, which predicts the wire temperature at acceptable accuracy. The effects of wire preheating temperature on the temperature distribution and voltage on the wire extension are both experimentally tested and theoretically analyzed. The dynamic change of welding current and arc voltage between none-preheating GMAW and hot-wire GMAW is also studied.Heat conduction and convection are the primary heat transfer modes at the wire tip. The arc anode heat and resistance heat are the two heat sources for melting the wire. The assistant TIG arc provides additional heat to melt the wire. Through none-dimensional constant calculating of heat transfer, the effect of wire preheating on the wire melting dynamic and droplet temperature is qualitatively analyzed. The arc variables in real welding under different wire preheating temperatures are measured, then the toal heat input for wire melting is computed. The results indicate that the total heat consumption for melting wire of the same mass decreases when the wire temperature increases. A droplet temperature calculation model is established. The model computing results demonstrate that the wire preheating contributes to reduce the droplet temperature. The higher the wire preheating temperature is, the more the droplet temperature is reduced. The droplet temperatures with and without preheating are experimentally measured and the results agree with the model predictions.Then, the effects of the preheating temperature on the metal transfer are experimentally studied. In CV GMAW process, the stability of metal transfer is improved by preheating the wire. The metal transfer tends to change from irregular mixed transfer of large drop globular and streaming spray to regular small drop globular or drop spray transfer. In CC GMAW, the droplet detachment is obviously enhanced by increasing the preheating temperature, the droplet size is reduced and the transfer frequency is increased. The transition current of spray transfer is much lower than that in conventional GMAW. The mechanism of such enhancement on droplet detachment is explained through analysis on the changes of the arc shape and forces excreting on the droplet.Finally, the process feature of the hot-wire GMAW is studied. In CC GMAW, the deposition rate is significantly increased by preheating the wire. In CV GMAW, the deposition rate is the same after preheating the wire, while the average welding current is effectively reduced resulting in much less heat input to the base metal. This is beneficial to welds of thin sheets and heat-sensitive materials. High-speed welding experiments using both the hot-wire GMAW and conventional GMAW are conducted. The results clearly demonstrate that the undercut defect is almost eliminated or significantly weakened in the hot-wire GMAW process.