This set of Manufacturing Processes Multiple Choice Questions & Answers (MCQs) focuses on “Laser Welding – 7”.
1. The difference between welding at long and short wavelengths is due to _____
a) high absorptivity
b) high reflectivity
d) shroud design
Explanation: Due to the high absorptivity within the “keyhole” there is little operational difference when welding with long or short wavelengths. Shorter wavelengths do not get absorbed quickly by plasma therefore they are more useful in welding.
2. The pressure at the centre is a function of the _____
a) surface tension
b) joint geometries
c) material composition
d) gap tolerance
Explanation: The centre has a pressure which is a function of the surface tension and the curvature. This leads to pressure instability causing the “pinch” effect. The regions of high curvature flow to regions of lower curvature resulting in large humps.
3. The gap between the components should be smaller than _____
a) half of the beam diameter
b) beam diameter
c) square of the beam diameter
d) one quarter of the beam diameter
Explanation: In butt joints, the gap must be small enough that the beam cannot pass straight through the joint. The gap should be smaller than half the beam diameter (<200 mm).
4. For welds having larger gaps,_____
a) the beam is rotated
b) an extra lens is added
c) high energy density beam is used
d) zif-zag welds are made
Explanation: For welds where there is a large gap the beam is sometimes rotated by rotating the lens off axis from the beam. Also, there is a chance of some drop out or an underfill in the weld.
5. The welds which do not require filler material are called _____ materials.
Explanation: The welds which do not require filler material are called “autogenous” materials. If there are larger gaps, they can be corrected by adding filler material as a wire or powder.
6. Which of the following is not affected by the gas shroud?
a) Formation of plasma
b) Hindrance to the beam
c) Absorption of the beam
d) Pool width
Explanation: The gas shroud can affect the formation of plasma which may block the beam and thus the absorption of the beam into the workpiece. The formation of plasma is thought to occur through the reaction of the hot metal vapours from the keyhole with the shroud gas.
7. The shroud gas enters the keyhole because of _____
a) the fast emission of vapour from the keyhole
b) the narrower shape of the keyhole
c) higher wavelength of the beam
d) molten material
Explanation: The shroud gas enters the keyhole because of the fast emission of vapour from the keyhole. The plasma blocking effect will be less for those gases having a high ionization potential.
8. Which of the following gases is most favourable as shrouding gases in laser welding?
Explanation: Helium is favoured, in spite of its price, as the top shroud gas in laser welding. The shroud underneath the weld would be of a cheaper gas e.g. argon, N2 or CO2.
9. How many issues are there concerning laser welding?
Explanation: The main issues laser welding are:
– crack sensitivity,
– HAZ embrittlement
– poor absorption of the radiation.
10. In case of laser welding of dissimilar metals, there are chances of formation of _____
a) porous welds
b) brittle welds
c) thin welds
Explanation: For welds of dissimilar metals there is the additional problem of the possible formation of brittle intermetallics. Crack sensitivity of the material refers to centreline cracking, hot cracking or liquation.
11. Cracking happens due to _____
a) shrinkage stress
b) improper gas shielding
c) high weld speeds
d) high wavelength
Explanation: It is due to the shrinkage stress building up before the weld is fully solidified and strong enough to take the stress. It usually happens in metal alloys having a wide temperature range over which solidification occurs, e.g. those with high C, S, and P contents.
12. The temperature required for using arc augmented welding phenomenon is around _____
a) 100 °C
b) 200 °C
c) 300 °C
d) 400 °C
Explanation: In arc augmented laser welding the arc from a TIG torch mounted close to the laser beam interaction point will automatically lock onto the laser generated hot spot. The temperature required for this phenomenon is around 300 °C above the surrounding temperature.
13. Arc Augmented Laser Welding is used for _____
a) higher penetration
b) reducing power loss
c) stabilizing the arc
d) improve wed quality
Explanation: Arc Augmented Laser Welding is used for either to stabilize an arc which is unstable due to its traverse speed or to reduce the resistance of an arc which is stable.
14. The arc is on the same side of the workpiece as the laser.
Explanation: The arc is on the same side of the workpiece as the laser which allows doubling of the welding speed for a modest increase in the capital cost. The locking only happens for arcs with a low current and therefore slow cathode jet.
15. Weld pool geometry can be controlled by using multiple laser beams.
Explanation: If two laser beams are used simultaneously then there is the possibility of controlling the weld pool geometry and the weld bead shape. It is known as twin beam laser welding.
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