MECH0055_MAIN Answers_2018 19_MOODLE UPLOAD.

Answers to Q1

 

Q2 Parts b&c (aka a&b)

 

 

 

 

 

Question 2 continued. d)

General: [9 marks] Laser Beam Welding

  • Thin column of vaporized metal when used in keyhole mode (focused)
  • Narrow weld pool, thin HAZ
  • Usually performed autogenously (without filler) but filler can be used on shallower welds
  • Usually used with inert shielding gas (shroud or box) or vacuum
  • Some materials reflect light so photon absorption and thus transfer efficiency varies on the material:
  • highly reflective materials (Al) ~ 10%
  • non-reflective materials (graphite) up to 90% • Special coatings can be used to increase efficiency…
  • no physical contact required – weld through window!
  • Sharp focus allows v. small welds, low total heat (electronics)
  • Laser beam can be shaped, directed, and focused with both transmission and reflective optics (lenses and mirrors) and can be transmitted through fiber optic cables
  • No direct contact is necessary to produce a weld, only optical accessibility
  • Welds can be made on materials encapsulated within transparent containers,

e.g. components in a vacuum tube…

 

Electron Beam Welding

  • The electron beam is focused (ø 0.8 – 3.2 mm) + can produce high temperatures but requires vacuum (10-3 –10-5 atm) to prevent electrons from interacting with atoms/molecules in atmosphere
  • Imposes size restrictions (but vacuum cleans surfaces) + slow changeover – hence expensive.
  • Some allow exterior sample welds but high losses, shallower weld depths & x-ray hazard; some machines operate with sample in “soft” vacuum (0.1-0.01 atm).
  • Good for difficult-to-weld materials; Zr, Be, W
  • Very narrow HAZ, deep penetration
  • high power + heat, deep, narrow welds, high speeds
  • no filler, gas, flux, etc.
  • Expensive equipment, joint preparation has to be good, alignment, etc. …
  • Normally done autogenously (no filler metal is introduced into joint)

–     joints must fit together very well before welding and tend to be simple straight or square butt joints

  • Filler metal can be added as wire for shallow welds or to correct underfill in deep penetration welds
  • Usually used in keyhole mode
  • Electron absorption in materials is high so transfer efficiency is high (>90%)
  • Routinely used for specific applications in the aerospace and automotive industries…

 

Specific topics

  1. i) Differences in mode of operation [4 marks]

Laser and electron beams produce the highest heat source intensities used in welding. Although intensities of 109 W/cm2 are possible, only levels of 106 or 107 W/cm2 are useful for welding. Above this level vaporisation of the metal is so intense that holes are drilled rather than welds being formed. These intensities are possible because laser and electron beams can be very finely focused, due to their monochromatic nature.

CO2 lasers emit at 10.6 μm wavelength, which is not readily absorbed by metals.

Requires KCl optics. Powers up to 20 kW available.

Electrons have sufficient energy to penetrate a fraction of a millimetre beneath the surface creating the greatest heating there, whereas laser heat is all produced on the surface of a metal.

 

  1. ii) Equipment requirements and costs. [4 marks]

The workpiece in electron beam welding must be an electrical conductor, whereas lasers can heat insulators with equal or even greater effectiveness compared with metals. This is due to the fact that the free electrons in metals tend to reflect the laser light. Indeed the best electrical conductors such as silver, copper, gold and aluminium are highly reflective and may absorb less than 5% of the incident radiation

Electron beam generally requires a vacuum for consistent results. The laser can easily be operated in the ambient atmosphere.

Electron beams are easily distorted by magnetic fields. In some cases thermoelectric currents are produced during electron beam welding of dissimilar materials. If the materials are very thick the currents will produce magnetic fields which will distort the beam, causing it to deviate from the weld seam. Lasers on the other hand are not affected by stray magnetic fields.

Electron beams produce X-rays. Low voltage machines of less than 30kV produce soft X-rays which are relatively easy to shield, while high voltage machines of 100kV or greater require elaborate shielding. Laser radiation is relatively easy to shield although the beam is invisible and thus the equipment requires many safety interlocks. The laser radiation is also easily reflected by metal surfaces and most work must therefore be performed inside protective enclosures.

Laser and electron beam processes have many similarities and many differences. They provide some unique advantages such as low distortion and extremely rapid processing but they can be very costly except in high-volume applications Question 3

 

a Q3i)  [8 Marks]

  • Identification of the correct cooling rate which just avoids martensite formation.

Read from the TTT Diagram in the question, that transformation starts in 0.8s at 550C and use this to determine cooling rate.

237.5  /

 

Rearrange the equation for              .

2

 

Or give value and plug in below…

 

Substitute in equation for            .

2

 

 

Rearrange the equation for .

 

2

 

Insert data and compute answer.

237.5    /       0.9                                             25     50

5.55                                                                    /

2             0.028 /         . .                    550             273          300

 

 

Q3ii  [8 Marks]

  1. Cooling takes 2.5 times as long. Read from the TTT Diagram in the question, that the cooling curve intersects with the TTT at 550 ,           2 . The microstructure at 550      will be 50% Pearlite+bainite and 50% austenite.

Upon further cooling this remaining austenite will transform to Martensite. Crystal Structures:

 

 

 

Microstructures:

 

  1. Martensite: A high percentage of martensite, which is a hard and brittle phase with low ductility. This phase and may even crack while cooling.

 

Pearlite+Bainite: will have improved ductility, low hardness, and less chance of cracking during cooling. [Q1iib: 5 Marks]

 

  1. Student should list at least 2 benefits and 2 drawbacks for each process. E.g. SAW provides better cover but slower cooling under flux, automated but only needs to be horizontal, etc as per notes.

[Q1iib: 5 Marks]

 

  1. Detail 4 of the Possible defects listed below, adapted for AM [8 marks, 2 marks each]

 

[Q3iic: 5 Marks]

 

 

Answers 4:

 

 

 

 

 

END OF Q4

 

Answers for Q’s 5 (aka 1) and 6 (aka 2)

 

 

 

 

 

END OF PAPER