MMAW

MMAW (Manual Metal Arc Welding)
or
SMAW(Shield Metal Arc Welding)

Shield or Manual metal arc welding is a process, in which the heat for welding is generated by an electric arc, established between a flux covered consumable electrode and the work piece.The electrode rod is made of a material that is compatible with the base material being welded and covered with a flux that gives off vapours that serve as a shielding gas, and provides a layer of slag both of which the protect the weld area by atmospheric contamination.

Types of welding:-

• Autogenous Welding- It is a kind of welding in which only similar metal parts can be welded. e.g. mild steel to mild steel, brass to brass, copper to copper.  
• Heterogeneous Welding- It is a kind of welding in which only dissimilar parts can be welded. e.g. mild steel to copper, cast iron to mild steel, stainless steel to mild steel etc.

Advantage of Shielded Metal Arc welding-

1. This is the simplest among all arc welding process.
2. It is portable.
3. This process is benchmark for all arc welding process.
4. Cost economical.
5. This can be use of outdoor and indoor.
6. Welding can be carried in any position.
7. This process finds countless application because of availability of wide range electrode. 
8. A big range of metal and their alloys can be welded.
9. The power cable of SMAW can be extended, thats why it can be used for long distance and height process.

Application:-

SMAW is used both in fabrication and maintenance repair job. This process find application in 

• Tank, boilers, and pressure vessel fabrication.
• Ship building
• Pipes and penstock joining
• Building and bridge fabrication
• automotive and aircraft industry etc.

Equipment required for shielded metal arc welding:-

• Arc welding power source 
• Welding cables 
• Electrode holder
• Earth clamp
• Welding electrode
• Welding helmet and hand shield
• Protective coating including hand gloves
• Chipping hammer, wire brush etc.
• PPE (Personal protective equipment)

Power sources for SMAW:
In SMAW both AC and DC power sources can be use. All of them supply the high current used for welding at a sufficient safe lower voltage. The output requires is either AC or DC. Welding current plays a significant role in deposit quality and deposition rate.
In manual metal arc welding, short circuit voltage can fluctuate due to arc length chances. Hence if a constant voltage power source is used, current will fluctuate with arc gap in manual arc welding.
To overcome this problem a constant current power source is used in manual arc welding. In direct current, the current flows continuously in one direction and hence welding arc is relatively steady and smooth. DC is superior to AC especially while welding with lower currents.

Polarity in DC welding:- 

Polarity indicates the direction of current flow in welding circuit.

Flow of electrons is always from negative to positive.

Kinds of polarity

1.  Straight polarity
2. Reverse polarity

Straight polarity:- In straight polarity, the electrode is connected to the negative and the work to the positive terminal of the power source.

Reverse polarity:- In reverse polarity the electrode is connected to the positive and work to the negative terminal of the power source.

Why polarity is important in welding:

In DC welding, 2/3 of the heat is liberated from the positive end and 1/3 from negative end. To have this advantage of unequal heat distribution in the electrode and base metal, the polarity as an important factor for successful welding.

Straight polarity is used for:

• Welding with bare light coated and medium coated electrodes.
• Welding the thicker sections to obtain more fusion and penetration.

Reverse polarity is used for:

• Welding of non-ferrous metals.
• Welding with heavy and super-heavy coated electrodes.
• Position welding 
• Sheet metal welding

ARC Lenght:-
Arc length is known as the gap or distance between tip of electrode and work
piece during welding.

Types of arc length:

1. Standard arc length
2. Short arc length
3. Long arc length

Advantage of standard arc length:

1. Good weld joint
2. Good penetration
3. Maximum strength
4. Less chance of porosity
5. Metal deposition as per standard
6. Maximum ductility
7. Less or no oxidation