Metal transfer in MIG welding

The metal transfer process in MIG welding has been investigated. The heat balance of the melting process, forces acting on the wire tip and droplets, and droplet movement were examined quantitatively both under steady current and pulsed current conditions. A novel transistorised power source was employed for precision current adjustment which with the use of high speed cinephotography and careful metallographic techniques has allowed a re-assessment of current theories to be made. A new metal transfer mode designated as ’Drop Spray* has been discovered. This transfer mode is located between the well known globular and spray transfer modes and only occurs in a very narrow current range of 20A, but it has several important features. The relationship between metal transfer mode and the welding variables has been established quantitatively for the first time. It was found that the extension resistance and heat content of droplets are determined by current and hence metal transfer modes. The amount of spatter and fume was also found to be determined by transfer mode. Metal transfer under pulsed current was also investigated. It was found that the metal transfer modes under pulsed current are the same as that of steady current. It was also found that the first drop of every pulse is of drop spray mode and the subsequent droplets will be stream spray. Careful observations and measurements have been made at various stages of the current pulse to enable greater understanding of the influence of the pulse parameters. Based on the results mentioned above, a concept of controlled MIG welding was proposed, based on the control of metal transfer mode. By this concept any predetermined feature of conventional MIG welding can be achieved consistently and repeatedly. As an application of the proposed concept, drop spray transfer was reproduced over a wide current range by means of metal transfer control. A new controlled MIG process designated as ’controlled drop spray MIG' has been developed which features high efficiency, all positional ability, freedom from spatter, low fume generation and good bead appearance. The preliminary bead on plate trial shows that all the expected results have been achieved although many more trials are necessary to fully prove the process.