Along with the edge preparation, cleaning of parent plates prior to welding is indispensably necessary in order to get defect free sound welded joint. Such unwanted deposits can be removed from work surface in various ways, including air blowing, wire brushing, etching, ultrasonic cleaning, etc. Sand blasting or grit blasting can also be used if work surface contains thin layer of coating. Surface of base plates may contain one or more of the following unwanted items.
- Oxide layer
- Color layer
- Coating layer
- Oil or grease
- Loose foreign particles
Oxide formation on metals
Most metals (except noble metals) are sensitive to oxidation, for example, aluminum and magnesium. When bare aluminum comes in contact with atmospheric air (oxygen), it quickly forms a tarnishing layer of alumina (aluminum oxide—Al2O3). This alumina is hard, brittle, insulate and refractory material. Its melting point is also very high as compared to parent metal (melting point of aluminum is 660ºC and that for alumina is 2072ºC). While arc welding of aluminum components, this thin alumina layer will restrict the free flow of electrons from or to the base plate, thus arc will not stabilize. It can also be observed that the base aluminum has melt down but a layer of alumina still remains on it in solid hard condition, which will not allow molten filler metal to mix with the molten base metal and thus proper coalescence will not be formed. Similar situation occurs in case of magnesium also.
Oxide cleaning by electric arc in arc welding
Since oxide forms on few metals quickly, so removal of oxide layers prior to welding is practically difficult task, especially when work room environment is humid and hot. Fortunately, oxide cleaning occurs inherently while arc welding by specific polarity. In arc welding, electrode is given one polarity, while base plates are given other polarity. Electrons flow from negative polarity to positive polarity in the external circuit due to the application of sufficient potential difference. Flow of avalanche of electrons basically constitutes the arc (main source of heat for melting).
Now if base plates are connected with negative terminal of the power source and electrode is connected with the positive terminal then electrons will flow from surface of the base plate toward electrode. During this outward flow, electrons rupture the insulate oxide layer present on work surface to make a free less resistive path. This inherent oxide layer breaking is termed as arc cleaning phenomenon. It is also called oxide cleaning or ionic cleaning. Such a connection where electrode is made positive and base plates are made negative is called Direct Current Reverse Polarity (DCRP). So DCRP is the best polarity in terms of oxide cleaning action.
However, if the connection is opposite (electrode is made negative and base plates are made positive) then electrons will flow towards base plate after liberating from the electrode. These electrons will break the oxide layer but downward force will mix such oxide particles with molten work metal. Presence of oxide particles within the weld bead will result in defective welding. Such a connection where electrode is made negative and base plates are made positive is called Direct Current Straight Polarity (DCSP). Thus DCSP gives poor arc cleaning action.
In case of AC power supply, both the above situations occur simultaneously one after another in each cycle. So when the base plates become negative, a good arc cleaning action is obtained; however, in next half of the cycle, base plates become positive and arc cleaning action changes to poor. On the basis of arc cleaning action, welding polarity can be arranged as follows.
- Direct Current Reverse Polarity (DCRP)—Best oxide cleaning.
- Alternating Current (AC) polarity—Moderate oxide cleaning.
- Direct Current Straight Polarity (DCSP)—Poor oxide cleaning.