Shielded Metal Arc Welding (SMAW)
SMAW stands for Shielding Metal Arc Welding (In Canada/ America/ India). It is also called MMAW or Manual Metal arc welding (In UK). Other name is Stick electrode welding or commonly known as Electrode welding or Manual Welding. (Non-standards name which are common worldwide).
Heat for welding is generated by an electric arc established between flux-covered consumable metal rod (Electrode) and the workpiece as shown in figure 1. During electrode melting, the coating burns and produce gaseous shield which protect the electrode tip, welding puddle and highly heated workpiece from atmospheric contamination.
Watch this YouTube video for full best classroom training on Shielded metal arc welding process.
Figure 1 Process Principle
The basic components of SMAW equipment setup (Figure – 2) are the following;
- Power Source
- Electrode Holder
Please see the below schematic diagram (Figure – 2) of SMAW for a better understanding.
In SMAW, electric arc is used to melt the base metal, this arc is generated by striking the electrode with the work piece. Soon after generation of arc the electrode is withdrawn from the work piece, but it remains in close contact with the work piece. The gases present between the gap (between electrode and work piece) gets ionized producing a plasma path and smooth flow of electrons takes place hence, in spite of the gap between the electrode and the work piece the circuit remains closed (or energized) and hence arc doesn’t get extinguished.
A power source, electrode holder, welding handheld shield and wire brush for cleaning are shown in figure – 3.
If the electrode is connected with the positive terminal and the workpiece with the negative terminal, it is known as DCEP (i.e. direct current electrode positive or Reverse Polarity). Whereas if the electrode is connected with the negative terminal and the workpiece with positive, then it is known as direct current electrode negative (i.e. DCEN or Straight Polarity) as shown in figure 4.
In DCEP, the electricity flows into the tip of the electrode and concentrates about two-thirds of the heat, which gives good penetration. Hence it is usually used on thicker steels.
In DCEN, the electricity flows out of the rod, concentrating about one-third of the heat on the electrode. Hence the penetration is less, this a very good choice for thinner steels. Both types of polarity are shown in figure 4 for reader easy reference.
Purpose of Electrode Coating
- During welding, the flux material decomposes and produces fumes. These fumes shield the molten weld pool from the atmosphere. In the absence of those fumes, a molten weld pool will be exposed to the atmosphere and may react with oxygen present in the environment and thus oxidation may take place, which is very dangerous and may lead to failure. (i.e prevents oxidation of molten weld pool).
- The density of flux material is less than the weld material hence the flux comes upwards and floats on the weld pool and thus protects the weld pool. During solidification of the molten weld pool, flux also solidifies and forms a thin layer on the surface of the weld called as slag. This thin layer of slag helps in the slow cooling of weld material. In the absence of this thin layer of slag, rapid cooling of molten weld pool will take place which is very dangerous because rapid cooling results in microstructural changes of weld metal (martensite formation).
- Flux also helps in the initiation of arc. Especially when welding is done in AC (Alternating Current), polarity keeps on changing at a constant rate, i.e. it attains a positive value than a negative value and it happens within a fraction of a second. Polarity changes more than 100 times in a second. While going from positive to negative it attains a value ‘0’ and every time when it goes to zero, the arc has to be re-initiated and the flux plays its role. It happens very fast (i.e. within a fraction of seconds) hence we cannot see this with our eyes.
Welding Parameters for SMAW Welding
Typical current (Amperage) ranges for shielded metal arc welding (SMAW) electrodes is given below (Table 1);
|Electrode diameter mm (in)||E6010|
|Electrode diameter mm (in)||E7015|
Advantages of SMAW Welding
- This process is suitable for most of commercially available metals and alloys.
- The equipment is comparatively inexpensive and portable.
- Equipment is relatively simple.
- This process can be used in all welding positions.
- This welding process is flexible and can be applied to a variety of joint configurations and positions.
- No need of separate gas shielding.
- Less sensitive to wind and drafts if compared to gas shielded arc welding processes.
- It can be used in areas where access is limited (electrodes a be bent and even a mirror can be used at tight spaces).
Disadvantages of SMAW
- Low deposition rate than GMAW and FCAW
- More skilled welding operator required than many other welding processors.
- Not suitable for reactive metals such as Titanium, Zirconium, Tantalum, and Niobium.
- Not suitable for metals with low melting temperature such as Lead, Tin and Zinc and their alloys.
- This process cannot be automated.
- More slag due to flux shielded electrode.
- Repeated changing of consumed electrodes with a new one makes this process quite slow if compared to GMAW.