Test: Fabrication Processes (Welding) - 2 - Mechanical Engineering MCQ

The fabrication weldability test is used to determine the susceptibility of the welded joint for cracking. Weldability test may be classified as:

i) Theoretical test

ii) Simulated test

iii) Actual welding tests

a) Fabrication weldability tests that determine the susceptibility of the welded joint to crack. These tests are a hot cracking test, root cracking test, hydrogen-induced tests.

b) Service weldability test that measures mechanical properties which are considered essential for the satisfactory performance of the welded joint during service. These tests are: Tensile, hardness and bend tests, impact tests, fatigue tests, corrosion tests, creep tests etc.

Direct current reversed polarity (DCRP) : It is also known by the name of direct current electrode positive (DCEP) process. As name suggests, the electrode in the straight polarity process is on the positive terminal while the workpiece is connected with the negative terminal.
Reverse polarity is advantageous in overhead welding because in reverse polarity work piece is made negative (at positive electrode). So less amount of metal is melted.

Effective heat transfer per second (P) = η VI =0.85x25x300 J/s

Speed (c) = 8 mm/s

Net heat input 

Q = I²Rt = 5000² × (200 ×10⁻⁶ ) × 0.2J = 1000J

Correct Answer :- B

Explanation : EBW is carried out in vacuum to avoid dispersion or deviation from the focus.

Correct Answer :- b

Explanation : Residual stress depends on welding parameters such as current, voltage, speed, current.

Let's analyze the Assertion (A) and Reason (R):

Assertion (A):

"The electrodes of AC arc welding are coated with sodium silicate, whereas electrodes used for DC arc welding are coated with potassium silicate binders."

• This statement is true. In AC arc welding, electrodes are typically coated with sodium silicate because sodium helps stabilize the arc in alternating current. In DC arc welding, potassium silicate is preferred because potassium provides better arc stability with direct current due to its ionization properties.

Reason (R):

"Potassium has a lower ionization potential than sodium."

• This statement is false. In fact, potassium has a higher ionization potential than sodium. Sodium has a lower ionization potential, meaning it requires less energy to remove an electron from a sodium atom compared to potassium.

Conclusion:

• Assertion (A) is true, but Reason (R) is false.

Thus, the correct answer is:

Option 1: A is true but R is false.

Correct Answer :- a,b

Explanation : Although the classic flux for steel was borax, and the filler rod is dipped in a can of borax, while the rod is still very hot. The shielding gas is supplied by the torch itself as the work is done inside the area covered by the flame.

So the flux took care of minor oxide contamination, and the torch flame kept the weld puddle from seeing atmospheric oxygen.

Generally DCSP is used in GTAW with the exception of AC being used in aluminium and Magnesium.

Explanation:

Gas metal arc welding (GMAW) or Metal inert gas arc welding (MIG)

• In this process, the arc is formed between a continuous, automatically fed, metallic consumable electrode and welding job in an atmosphere of inert gas, and hence this is called metal inert gas arc welding (MIG) process.
• In this process small droplets of molten metal from the electrode are transferred to the weld area, at ratio of several hundred droplets per second, the transfer is spatter free and very stable, high dc current, high voltage, and large diameter electrode are used along with the shielding gas.
• The shielding gases for MIG welding are mixtures of argon, oxygen, and CO2, and special gas mixtures may contain helium. 

We need maximum metal deposition rate so choice SAW.
Note: You know maximum metal deposition rate of all welding is SAW and it may joint 40 mm thick plates easily.

H = I²Rt = 15000² × 0.0001× 0.25J = 5625 W − s

Electro slag welding is not a type of electric resistance method of welding.

• Percussion welding

  Uses a short electrical discharge to create a high temperature arc, then applies pressure to forge the materials together. This method is often used to join dissimilar metals or when flash isn't needed at the joint.

• Flash welding

  Doesn't use filler metals, but instead creates an arc by applying current to the metals while they're separated by a predetermined distance. Once the metals reach the right temperature, they're pressed together to forge them together.

• Seam welding

  Uses a series of continuous spot welds to create a leak-proof joint between two overlapping pieces of sheet metal. The workpiece is placed between two wheels that apply pressure and carry current to produce the welds.