Shielded Metal Arc Welding (SMAW), or, Manual Metal Arc Welding, or Stick welding, is the uncrowned champion of all welding methods capable of providing a great variety of welds. This type of welding is the one that jumps to mind when we think of welding. A person is sitting haunched, wearing overalls and a visored helmet with a brilliant flurry of sparks at his feet.
How Is Shielded Metal Arc Welding Done
The process of shielded metal arc welding, in brief, involves a coated metal electrode of suitable composition. The molten metal fills the weld gap and joins the workpieces when the ignition of the arc takes place between the electrode and the parent workpiece. An appropriate structure of shielding flux coats the electrode. The flux and the metallic electrode metal melt together. Gas and slag, the products of the flux, shield the arc and the weld pool. The electrode flux also cleans the metal surface, adds alloying elements to the weld, stabilizes the arc and protects the molten metal from oxidation. The removal of the slag follows solidification.
Process Fundamentals Of Shielded Metal Arc Welding
The distinctive feature of shielded metal arc welding is the coating surrounding the core wire of the consumable electrode. It is termed as an electrode because it functions as the terminal from which the electric flow changes from the conducting solid to the conducting plasma of the welding arc. The electrode must meet the following three criteria for any given application.
- Should shield the weld metal and the arc
- Must add metal to the weld
- It must sustain a welding arc.
The ingredients of the coating, namely the flux, accomplish these functions. When the flux is heated sufficiently, the coverings constituents do the following namely;
- Decompose into gases and displace the air at the weld site thus shielding the arc and the weld metal
- Support also the arc plasma by ionization
- Flux the metal, and on cooling, form a protective slag over the weld bead. The coverings may also contain metal powders that enhance the metal contribution of the electrode to the weld pool.
Electrodes And Electrode Coverings
Based on the type of electrode in use, the covering achieves the following:
- Protection of the molten metal to begin with from excessive atmospheric contamination is by gas-shielding.
- Dissolved gaseous elements can cause porosity. To eliminate these, adding deoxidizers to react with them
- Fluxing agents to accelerate chemical reactions and also weld pool cleansing.
- A slag blanket has several roles; exclude air from the hot weld metal; augment the mechanical properties, in addition to maintaining bead shape and surface cleanliness of the weld metal.
- To achieve the desired microstructure by adding alloying materials.
- Compounds and elements to control the growth of grains.
- In brief, improve the mechanical properties of the weld metal by the addition of alloying metals.
- Constituents to affect the shape of the weld pool.
- Elements that affect the wetting of the workpiece in addition to the liquid weld metal viscosity
- Stabilizers to establish desired electrical characteristics of the electrode and minimize spattering.
Advantages Of SMAW
- Inexpensive equipment, portable, and straightforward.
- Wide range of metals, welding positions and electrodes possible
- Suitable for outdoor applications also
Disadvantages Of SMAW
- Discontinuous process due to the limited length of the electrodes.
- Slag inclusions may, however, make their way into the weld
- Fumes usually make the process control difficult
In spite of its ease of everyday use and simple operation, the verdict is out-shielded metal arc welding is on the decline and expectedly so given the enormous strides taken in the development of more futuristic, advance and efficient welding technology.
The old shall fall to give way for the new. So be it.