Electrode size and type and thickness of coating on it determine the arc voltage requirement (overall range 16—40 V) and current requirement (within an overall range of 20—550 A). The current could be direct of alternating depending upon the electrode being used. Almost all electrodes work well on d. c. but only a few flux compositions give stable arc operation with a. c.
• Transformers, on the other hand, are easier to maintain and are more robust as compared to d. c. generators or rectifiers. During d. c. welding, direct current flows
between the electrode and the opposite terminal clamped to the workpiece. This current flow leads to the formation of a magnetic field which deflects the arc from the joint causing problems. This phenomenon is called arc-blow. It does not occur with a. c. as no stable magnetic fields are produced with a. c. (Fig. 4.1).
Fig. 4.1 Arc blow in SMA welding with direct current
• A. c. has another problem. The arc is extinguished each time the current pulse is reversed (i. e., for 50Hz power supply, every one-hundredth of a second) To maintain a stable arc, the arc must be instantaneously re-ignited. A voltage in excess of 80 V must be supplied each time the current falls to zero. These high voltages are safety hazard and d. c. with an o. c.v. of about 60 V is preferred from this point of view.
Fig. 4.2 Current and voltage waveforms in a. c. welding
4.2 COVERED ELECTRODES
In addition to establishing the arc and delivering filler metal to the weld, the electrode introduces other materials into and around the arc and weld pool through its covering. The main purpose of using a flux covering is to protect the molten metal from atmospheric contamination, the flux performs the following functions leading to the formation of a successful weld.
— weld-metal protection
— provides scavengers, de-oxidants, and fluxing agents to cleanse the weld and prevent excessive grain growth in the weld deposit.
— provides a slag blanket to protect hot metal from air, enhance mechanical strength, bead profile and surface cleanliness of weld bead.
— coating melts slower than the core wire, forming a cup the electrode end which additionally protects droplets of molten metal and makes touch welding possible and spatter loss is reduced.
— provides a means of adding alloying elements to enhance weld metal properties or adding iron powder to increase deposition efficiency.
In the following paragraph these factors will be briefly discussed.