Welding MCQ Quiz - Objective Question with Answer for Welding - Download Free PDF

Concept:

Carbon Arc Welding (CAW) is an older arc welding process that uses a non-consumable carbon or graphite electrode to create an arc between the electrode and the workpiece. This arc generates the heat required to melt the metals at the welding point.

Electrode Used:

The typical electrode used in CAW is a Graphite electrode or Carbon electrode, due to its high melting point and ability to conduct electricity without melting itself.

The correct answer is option 2.

In AC welding transformers, the secondary (welding) current is controlled by adjusting the amount of magnetic flux that links the primary and secondary windings. This is done through various methods:

Magnetic shunts or movable cores:

• Inserting or adjusting a magnetic shunt between the windings changes the leakage flux.
• By increasing the leakage flux, less magnetic flux links the secondary winding, resulting in lower induced voltage and current.

Tapping or variable core transformers:

• The tap change adjusts the turns ratio, which affects the induced voltage and load current.
• While the core flux stays relatively constant in no-load conditions, under loaded conditions, the load current and flux changes are influenced by the tap position.

Concept:

Resistance welding is a process in which heat is generated by passing a high current through the resistance caused by the contact between two or more metal surfaces, and then applying pressure to weld them together.

The two major elements required are:

• Current: Generates the necessary heat at the contact area.
• Pressure: Ensures proper contact between the metals and helps form a strong joint as the metals fuse.

Arc is not involved in resistance welding; it's typical in arc welding processes.

Concept:

Electric Arc Welding is a welding process that uses an electric arc to generate intense heat (around 6000°C) to melt the base metals. The molten metals fuse together upon cooling, forming a strong joint. No external pressure is applied in this process.

Calculation

Fusion welding involves melting of base metals, with or without filler material, and solidification upon cooling to form the joint. Since electric arc welding melts the metal at the joint, it falls under the category of fusion welding.

Other options like resistance welding and pressure welding involve mechanical pressure, while solid-state welding does not involve melting.

Resistance Welding

Resistance welding is a process in which heat is generated by passing an electric current through the workpieces, and pressure is applied to join them.

Some common types of resistance welding include:

• Spot Welding: Used for sheet metal joining.
• Seam Welding: A continuous form of spot welding.
• Projection Welding: Uses projections on workpieces to focus heat.
• Butt Welding: Joins two metal ends together.
• Percussion Welding: A type of flash welding where a high-voltage pulse is used.

Carbon Arc Welding is not a resistance welding process. Instead, it uses an electric arc between a carbon electrode and the workpiece to generate heat, melting the metal to form a joint. It falls under arc welding processes rather than resistance welding.

DC welding	AC welding
Motor generator set or rectifier is required in case of the availability of AC supply	Only the transformer is required
The cost of equipment is high	The cost of equipment is cheap
Arc stability is more	Arc stability is less
The heat produced is uniform	The heat produced is not uniform
Both bare and coated electrodes can be used	Only coated electrodes should be used
The operating power factor is high	The power factor is low, So, the capacitors are necessary to improve the power factor
It is safer since the no-load voltage is low	It dangerous since the no-load voltage is high
The electric energy consumption is 5-10 kWh/kg of deposited metal	The electrical energy consumption is 3-4 kWh/kg of deposited metal
Arc blow occurs due to the presence of a non-uniform magnetic field	Arc blow will not occur due to the uniform magnetic field
The efficiency is low due to the rotating parts	The efficiency is high due to the absence of rotating parts

The power factor of load using a welding transformer least depends on the material to be welded which is independent of the welding transformer circuit.

• Electric arc welding is the process of joining two metallic pieces by melting metal which is obtained due to the heat developed by an arc struck between an electrode and the metal to be welded or between the two electrodes.
• If the thickness of metal increase, the current requirement will increase as the heat requirement to melt the metal increases.
• To penetrate thick metal surface arc requires more power, for this purpose it requires more current and constant voltage.

Requirement of power supply for electric arc welding:

• ​Both AC and DC can be used in arc welding.
• Usually, 70-100 V on the AC supply and 50-100 V on the DC supply system is sufficient to strike the arc in the air gap between the electrodes.
• Once the arc is struck, 20-30 V is only required to maintain it.
• Arc welding transformer is also designed to have a high leakage reactance so as to stabilize the arc during the welding process.
• Due to the high leakage reactance power factor of the welding transformer is very low the order of 0.3 to 0.5 lagging.

Welding generator:

• A differential compound generator is used as a welding generator
• The current supplied by DC generator is alternating that can be converted to direct quantity by use of a commutator
• As the load increases, the net flux due to the series and shunt fields in opposition decreases hence the generated emf also decreases
• Welding generator can produce both DC and AC

Explanation:

Welding:

It is a metal joining process in which two or more parts are joined or coalesced at their contacting surfaces by suitable application of heat or/and pressure.

Welding processes can be broadly classified into

i. Fusion welding

ii. Pressure welding/solid-state welding

In fusion-welding processes, heat is applied to melt the base metals. In many fusion welding processes, a filler metal is added to the molten pool during welding to facilitate the process and provide strength to the welded joint.

• Types: Arc welding, Gas welding, Oxyfuel gas welding, electron beam welding, laser welding, Carbon arc welding

 

In solid-state welding, joining of materials is performed with the help of heat and pressure or pressure alone.

• Types: Forge welding, Resistance welding processes (spot, seam, projection, flash butt, arc stud welding), Ultrasonic welding, Explosive welding

Spot welding:

• It is used primarily for welding two or more metal sheets together by applying pressure and heat from an electric current to weld area.
• It works by contacting copper alloy electrodes to the sheet surfaces, whereby pressure and electric current are applied and heat is generated by the passage of current through resistive materials such as low carbon steels.

 

In Spot Welding:

Voltage = less than 8 V.

Current = 5000 Amp.

Power Factor = 0.3 – 0.5 lagging

Ex. Welding of a car body.

Welding:

• It is a metal joining process in which two or more parts are joined or coalesced at their contacting surfaces by suitable application of heat or/and pressure.
• Welding processes can be broadly classified into

1. Fusion welding
2. Pressure welding/solid-state welding
    

In fusion-welding processes, heat is applied to melt the base metals. In many fusion welding processes, a filler metal is added to the molten pool during welding to facilitate the process and provide strength to the welded joint.

• Types: Arc welding, Gas welding, Oxyfuel gas welding, electron beam welding, laser welding, Carbon arc welding

 

In solid-state welding (non-fusion type welding), joining of materials is performed with the help of heat and pressure or pressure alone.

• Types: Forge welding, Resistance welding processes (spot, seam, projection, flash butt, arc stud welding), Ultrasonic welding, Explosive welding

DC welding	AC welding
Motor generator set or rectifier is required in case of the availability of AC supply	Only the transformer is required
The cost of equipment is high	The cost of equipment is cheap
Arc stability is more	Arc stability is less
The heat produced is uniform	The heat produced is not uniform
Both bare and coated electrodes can be used	Only coated electrodes should be used
The operating power factor is high	The power factor is low, So, the capacitors are necessary to improve the power factor
It is safer since the no-load voltage is low	It dangerous since the no-load voltage is high
The electric energy consumption is 5-10 kWh/kg of deposited metal	The electrical energy consumption is 3-4 kWh/kg of deposited metal
Arc blow occurs due to the presence of non-uniform magnetic field	Arc blow will not occur due to the uniform magnetic field
The efficiency is low due to the rotating parts	The efficiency is high due to the absence of rotating parts

Polarity are of following two types:

1. Straight Polarity: In straight polarity, the electrode is having a negative terminal while the workpiece is connected to the positive terminal of the direct current power source.

2. Reverse Polarity: In reverse polarity, the electrode is connected to the positive terminal whereas workpiece is connected to the negative terminal of the direct current power source.

Application:

About 2/3 (67%) of total heat produced during welding is generated at the positive terminal while the rest of total heat is generated at the negative terminal.

So, if our job is thick, means we want more heat on the workpiece, so we connect workpiece to the positive terminal hence we adopt straight polarity.

Similarly, if the workpiece is thin, we connect workpiece to the negative terminal hence we adopt reverse polarity.

If our electrode is non-consumable, means we want less heat on electrode (generally) so connect electrode to the negative terminal i.e. use straight polarity for doing welding.

Reverse polarity (i.e. electrode positive) is used with basic low-hydrogen electrodes and for most non-ferrous metals. It produces maximum penetration for a given set of conditions. This characteristic makes reverse polarity the better choice for vertical and overhead welding.