Arc welding is a liquid state welding process in which electrical energy produced by an arc between an electrode and a work piece is converted into heat and used for fusion welding. Shielded metal-arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes. The electrodes are in the shape of thin, long sticks, so this process is also known as stick welding. The heat generated melts a portion of the tip of the electrode, its coating, and the base metal in the immediate area of the arc. A weld forms after the molten metal-a mixture of the base metal (work piece), electrode metal, and substances from the coating on the electrode solidifies in the weld area.
The process has several variations depending on a number of process variables including: welding electrode, shielding method, welding equipment, current density, polarity etc.
This laboratory session is intended to give the student an overview of the Shielded Metal Arc Welding Process. The students will be introduced to:
- Energy sources
- Arc shielding
- Current density
- Arc blow
- Welding machine sizes
- Electrode types
- Practicing arc welding
The magnitude of the current used directly effects the melting rate, therefore for effective welding a constant current type of machine is required. It should minimize the sudden surges in current.
A bare section at the end of the electrode is clamped to one terminal of the power source, while the other is connected to the workpiece being welded. The current range between 50A and 300A, with power requirements generally less than .1kw.Too low a current causes incomplete fusion, and too high a current can damage the electrode coating and reduce its effectiveness. The current may be DC or AC.The power supplied from an electrical source can be calculated by a formula P=I^2R: Where I= current, t= time, R= resistance.
The electrode tip, weld puddle, arc and adjacent areas of the workpiece are protected from atmospheric contamination by gaseous shield obtained from combustion and decomposition of the flux covering. And to start welding, an arc is struck by touching the workpiece with the tip of the electrode, much like striking a match.
- It takes less time to weld by electric arc
- It can be used for joining almost all metals
- It is less expensive
- Portable equipment is also possible
- Joints in blind and restricted areas can be welded
- It is difficult to join thin sheets and alloys
- It is not continuous
Polarity indicates the direction of flow of current in a circuit. DC generator or AV-DC rectifier supplies DC current that flow in one direction only.
- Straight polarity, the work piece is positive and the electrode negative; this type of polarity is used for large deposit on heavy base metal.
- Reverse polarity, where the electrode is positive and current flows from base metal to the electrode as in figure 4.in this type electrode melts fast give good penetration.
Current density is a measure of the density of flow of a conserved charge.
- When the current flow through the electrode, base metal and ground clamp generates a magnetic field which may cause the arc to deviate from its path, this deviation of arc is known as ARC BLOW. The bending of the arc is due to the effect of an unbalanced magnetic field.
- It may often be minimized by shifting the earth clamp to another location of the workpiece or by simply twisting the electrode lead and the earth lead loosely together.
- AC Transformer
- Supplies AC where power is taken directly from a power supply
- Freedom from arc blow
WELDING MACHINE SIZES
- The size of welding machine is designated according to their output rating of current (ranges 150 –1000 amperes).
- The 60 % duty cycle means the machine can be operated for 6 minutes out of every 10 minutes.
In arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding. For a direct current system the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes. For an alternating current arc welder the welding electrode would not be considered an anode or cathode.
PRACTICING ARC WELDING
Arc welding technique
- Welding position :
Horizontal, Flat, Vertical, and Overhead
- Types of Joints :
Butt, Corner, Edge, Lap, and Tee
- Electrode Diameter :
Equal to thickness of the base metal
- Current Setting :
Depends on the thickness of the base metal and the actual welding position
- Arc Length :
Arc is generated by scratching the end of the electrode on the base metal. As soon as the arc is started, the electrode is immediately raised a distance equal to the diameter of the electrode. A shorter arc is usually better for vertical and overhead welding.
- Travel Speed:
It is the rate at which the electrode travels along the weld Seam. It is influenced by the skill of the welder, position of weld, type of electrode, and the required joint penetration
- Single and Multiple Pass:
A single pass involves depositing one layer of Pass Welds weld metal. On heavier plates two or more layers are deposited for strength
- Weaving the Electrode :
Weaving is a technique used to increase the width and Volume of the weld.
- SAFETY TOOLS:
In this experiment, we used many things like the following: Figure 1
|Safety gloves||Laboratory Face Shields||Safety apron|
|Safety boots||Lab coat|
- TOOLS FOR SMAW:
- power supply :
- DC generator: the direct current is driven by an electric motor or gasoline engine.
- power supply :
- AC transformer, provide alternating current where power is taken directly from a power supply.
- Rectifier (change Ac into DC).
- Electrode holder
- Ground clamp
- Electrical cables
- Chipping hammer
- Wire brush
|Power Supply||Electrical Holder||Ground Clamp|
|Electrical Cables||Chipping Hammer||Wire Brush|
- Put on safety vest, gloves, goggles
2. Observe the correct method of starting the electric arc
- Demonstrate the Arc welding process for laying a bead and emphasis on the effect of process variables.
- Get the sheet being welded
- Turn the power on
- Practice holding the electrode holder
- Start the arc by touching the tip of a coated electrode against the work piece and as soon as the arc is started raise the electrode distance equal to the diameter of the electrode and withdrawing it to the distance you want.
Put the remaining electrode in a safety place.
Because it is the first time I did like this welding, our joint has many defects like this:
- There was a small hole in the welding area
- There were some gaps and no welding in one edge
- The width of welding was not uniform and the welding wasn’t straight line.
- Some spatter on the sheet
- We didn’t clean perfectly the joint from shield.
I learnt that the energy source should be between the 50 and 300A and it DC. The arc is struck by touching the workpiece with the tip of the electrode. There are two types for polarity which are DC generator, flow in one direction only, or AV-DC generator. The Current density is found . Arc blow is the arc to deviate from its path. The size machine is between 150 and 1000 amperes. The electrode is either consumable, gas metal arc welding or shielded metal arc welding, or non-consumable, gas tungsten arc welding. The defects happening to my welding were because:
- I hold the electrode more time at that area
- The speed of my hand was not constant.
- Weaving and speed was not good