Question

5. Identify any THREE hazards associated with the welding process.
6. What is difference between consumable and non-consumable electrodes? Identify one welding process that employs a consumable electrode and one that employs a non-consumable electrode?
7. Identify any THREE functions of electrode coatings used in SMAW process?
8. What information the electrode designation such as AWS E6018 provides?
9. What are advantages of GMAW compared to the SMAW process?
10. Identify any three metal transfer modes in relation to the GMAW process?.
11. Why shielding gases are used in the welding process?
12. List equipment and consumables used for the GMAW process, GTAW and SMAW processes.
13. Which of the arc processes cannot be employed for overhead welding position?
14. How the GTAW process is different from the GMAW process, and how it is similar to the GMAW process?
15. While using DC; which polarities can be employed for the GTAW process?
16. Which polarity results in high deposition rate? DEN or DCEP
17. Which polarity is better for deeper welds: DCEN or DCEP?
18. Compare heat affected zone for the laser beam welding process and the oxyfuel welding process, and comment on the difference.
19. Explain the mechanism of resistance spot welding, and steps involved in the process.
20. Which welding process would you recommend to fabricate the food cans: resistance seam welding or laser beam welding, and why?

Answer 1

(5)Hazards associated with the welding process:

• Electric shock.
• Ultraviolet radiation.
• Toxic fumes and gases.

(6) Consumable electrode:If the electrode is used for closing the electric circuit hence melting and also for supplying additional molten metal.It should be of the same material as that of the filler rod.MIG (metal inert gas welding) uses consumable electrode.

Non consumable electrode:If the electrode is used only for closing the electric circuit hence meltingTIG(tungsten inert gas welding) uses Non consumable electrode.

(7)Shielded metal arc welding, or SMAW uses consumable electrode.

functions of electrode:

• Arc stability.
• A primary source of alloying and additional filler metal.
• shielding from contamination.

(8)AWS E6018

AWS -american welding society.

E- elctrode.

60-First two digits designates the minimum tensile strength in Ksi.

1-Second last digit designates position for which the electrode can be used.

• 1 – Flat, Vertical, Overhead and horizontal (All Position)
• 2 – Horizontal-Fillet & Flat
• 3 – Flat, Horizontal, Overhead and Vertical-downwards.

18-Last two digits together shows two things :Polarity in which electrode can be used and Flux composition.

18=AC OR DCEP ,Low-hydrogen potassium, iron powder.

(9)Advantages of GMAW compared to the SMAW.

GMAW(Gas metal arc welding).

• In SMAW, welders must periodically stop to change out for a fresh electrode. In GMAW, by contrast, the wire electrode feeds automatically into the spool gun. This boosts productivity by eliminating the need for constant stopping and starting.
• GMAW can be performed even by less skilled welders when compared to SMAW.
• GMAW is Capable of welding Dissimilar metals.SMAW isn’t suitable for metals that are reactive.
• GMAW have high travel speeds and deposition rates compared to Shielded Metal Arc Welding.

(10)The three transfer modes in GMAW are globular, short-circuiting, and spray.

• Globular:GMAW with globular metal transfer is considered the least desirable of the three major GMAW variations, because of its tendency to produce high heat, a poor weld surface, and spatter.
• Short-circuiting: the current is lower than for the globular method,as a result of the lower current, the heat input for the short-arc variation is considerably reduced, making it possible to weld thinner materials while decreasing the amount of distortion and residual stress in the weld area.
• Spray: Spray transfer GMAW was the first metal transfer method used in GMAW, and well-suited to welding aluminium and stainless steel while employing an inert shielding gas.

(11)Shielding are mainly used to protect the molten metal from the contamination and oxidation caused by these harmful gases in the atmosphere,to prevent exposure of the molten weld pool to oxygen, nitrogen and hydrogen contained in the air atmosphere. The reaction of these elements with the weld pool can create a variety of problems, including porosity (holes within the weld bead) and excessive spatter.

Different shielding gases also play an important role in determining weld penetration profiles, arc stability, mechanical properties of the finished weld, the transfer process etc.

(12)GMAW

equipments:Welding Power Source,Wire Feeders,Supply of Electrode Wire,Welding Gun,Shielding Gas Cylinder.

consumables:welding rods and wires, as well as front-end parts of welding guns, such as contact tips, nozzles, and diffusers.

GTAW

equipments:welding torch,filler rod,tungsten electrode,power supply, shielding gas.

consumables:Cups, collets, back caps, tungsten, and gas lenses.

SMAW

equipments:power supply,onnecting cables or leads,electrode holder, cable connectors, ground clamp,electrode.

consumables:Stick electrode, flux-cored wires, solid wires, SAW wires and fluxes.

(13)SMAW cannot be employed for overhead welding position.

(14)

GTAW and GMAW.

difference.

• Non consumable tungsten electrode in GTAW.
• GTAW require cooling for tungsten electrode as it is not consumed.
• GTAW upto 5mm thickness, GMAW upto 30mm.

similarity.

• same inert gases are used.
• same welding technique.
• low and high melting point reactive metals can be welded.

(15)straight polatity and reverse polarity :

DCEN- deep penetration, narrow melted area, approximate 30% heat in electrode and 70% in base metal.

DCEP-shallow penetration, wide melted area, approximate 70% heat in electrode and 30% in base metal.

(16)DCEN has a higher deposition rate because there is more electrode heating.One-third of the welding heat on the electrode and two-thirds on the metal being welded. DCEN welding current produces a high electrode melting rate.

(17)DCEN- deep penetration, narrow melted area, approximate 30% heat in electrode and 70% in base metal,

hence it is better for deep welds.

(18)Laser beam machining

Welding processes result in a narrow heat affected zone due to Key hole phenomenon.The Keyhole model of welding is what leads to the narrow heat affected zone. The high intensity radiation produced by the welding process vapourizes the metal and penetrates deep into the metal. This melted pool has a high aspect ratio compared to the Conduction model in which the width of the melted pool is almost equal to the depth.

laser beam welding

Oxyfuel welding uses high heat input and increases the size of the HAZ.High HAZ is due to the large exposing area.

(19)resistance spot welding.

Eventhough resistance welding is classified as a non fusion welding operation but it is a fusion pressue welding.The heat required for melting and joining is obtained electrical resistance of the circuit.

The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint.The welding heat is generated by the electric current, which is transferred to the workpiece through copper alloy electrodes. Copper is used for the electrodes as it has a high thermal conductivity and low electrical resistance compared to most other metals, ensuring that the heat is generated preferentially in the work pieces rather than the electrodes.

(20)welding process to fabricate the food cans : small metal sheets are rolled into cylinders then cylinder edges are welded by squeezing them together whilst passing an electric current through them, the resistance provides heat for melting.So the process is resistance seam welding.