Past 4 exams Flashcards
(32 cards)
Which of the following does NOT affect the productivity of a laser stripping process?
o Beam waist diameter
o Process efficiency
o Laser emission power
o Scan speed
o Hatch distance
o None of these parameters
o Beam waist diameter (determines spot size -> precision)
o Process efficiency (Efficient use of energy -> Faster)
o Laser emission power (High power -> faster process)
o Scan speed (Directly affects the laser moves & speed on the surf)
o Hatch distance (distance between lines -> affect coverage time)
+ None of these parameters (all contribute)
Which of the following statements is FALSE regarding the keyhole welding process?
o Keyhole welding is always preferrable to conduction mode welding
o Keyhole welding allows to achieve higher penetration welds in comparison to arc based welding processes
o Plasma formation may hinder the keyhole stability during the welding process
o Processing of a surface from an inclined angle may impede keyhole formation
o None of these options
+ Keyhole welding is always preferrable to conduction mode welding (is NOT ALWAYS, because is deep prenetation, but, risk of porosity, lack of fusion and instability)
o Keyhole welding allows to achieve higher penetration welds in comparison to arc based welding processes
o Plasma formation may hinder the keyhole stability during the welding process
o Processing of a surface from an inclined angle may impede keyhole formation (impede forming the deep cavity “keyhole”)
o None of these options
*Conduction mode is better for shallow welds or when lower heat input is needed
With regards to the laser cutting process, which of the following statements is TRUE:
o Very high levels of emission power (P>30 kW) always correspond to an increase in productivity
o Spatial beam shaping cannot be employed due to limitations in the optical chain
o Pulsed wave emission is often preferred for the processing of high thickness materials
o The technology cannot be employed for the processing of non-metallic materials
o 3 axis gantry systems are the preferred solution for the processing of metallic sheet
With regards to the laser cutting process, which of the following statements is TRUE:
o Very high levels of emission power (P>30 kW) always correspond to an increase in productivity (possible thermal damages)
o Spatial beam shaping cannot be employed due to limitations in the optical chain (it’s used)
o Pulsed wave emission is often preferred for the processing of high thickness materials (Not ideal, Continuous wave better)
o The technology cannot be employed for the processing of non-metallic materials
+ 3 axis gantry systems are the preferred solution for the processing of metallic sheet (precise control and effective ideal)
Which of the following is FALSE regarding the Plasma Arc Cutting process?
o Active gases can be employed to improve the process productivity
o Higher levels of current allow to increase the cutting speed
o Thermal models may be employed to estimate the heat affected zone
o A larger nozzle allows to redistribute the plasma jet and improve the cutting capability
o Active gases can be employed to improve the process productivity (like O2 via exothermic reactions)
o Higher levels of current allow to increase the cutting speed (More thermal energy)
o Thermal models may be employed to estimate the heat affected zone
+ A larger nozzle allows to redistribute the plasma jet and improve the cutting capability (a large nozzle actually disperses the plasma arc -> lowering energy, density, cutting perf)
In AWJ cutting which of the following cannot reduce the presence of the uncut triangle defect?
o Increase the pressure
o Increase the abrasive feed rate
o Reduced the feed rate
o Modifications to the cut path trajectory
o None of these options
o Increase the pressure (higher Kinetic energy in the jet)
o Increase the abrasive feed rate (more erosive power)
o Reduced the feed rate (more time -> better penetration)
o Modifications to the cut path trajectory (avoid stress concentration or cutting resistance zones)
+ None of these options (all help reduce the defect)
*Uncut triangle defect is a common imperfection at the exit of the cut
Which amongst the following material-process combinations is NOT feasible?
o Submerged Arc Welding of high thickness pipes
o Laser welding of plastics
o Micro-machining of a ceramic mould by means of EDM
o Abrasive Water Jet cutting of marble plates
o All are feasible
o Submerged Arc Welding of high thickness pipes
o Laser welding of plastics
+ Micro-machining of a ceramic mould by means of EDM (EDM only works on conductive materials)
o Abrasive Water Jet cutting of marble plates (great for hard, brittle materials)
o All are feasible
*EDM (electrical discharge machining)
Thermal models can aid us in the comprehension of the physical phenomena underlying a technological process. Which of the following is NOT a solution to reduce the heat affected zone generated during the EDM process?
o Reduce the pulse duration
o Reduce the current
o Reduce the thermal diffusivity of the workpiece
o Reduce the initial temperature of the workpiece
o Reduce the voltage
o None of these options
o All of these options
o Reduce the pulse duration
o Reduce the current
+ Reduce the thermal diffusivity of the workpiece (CAN’T control the thermal diffusivity of a material during the process - it’s a material property)
o Reduce the initial temperature of the workpiece (limits total thermal damage)
o Reduce the voltage
o None of these options
o All of these option
*all the option respect with the pulse, reduce the heat input per pulse
Which of the following is NOT a useful information to calibrate the process efficiency of a thermal model?
o Measure the depth of a GTAW joint from a metallographic cross-section
o Evaluate the heat affected zone generated by a single EDM spark
o Assess the crater size during an EDM process
o Measure the temperature variation by means of thermocouples on a workpiece surface during laser cutting
o All of these options
o None of these options
o Measure the depth of a GTAW joint from a metallographic cross-section
o Evaluate the heat affected zone generated by a single EDM spark (localized thermal effects)
o Assess the crater size during an EDM process (energy transfer and material removal)
o Measure the temperature variation by means of thermocouples on a workpiece surface during laser cutting (direct thermal data)
o All of these options
+ None of these options
Which of the following statements is WRONG regarding the melt and blow laser cutting process?
o High pressure inert gas is required to expel the molten material from the kerf
o Reactive fusion cutting allows to cut higher thickness materials
o The surface quality obtainable by means of a fiber laser is typically higher than the one obtainable with a CO2 laser source
o When processing high thickness material the cut velocity must be reduced
o Lenses with a shorter focal length facilitate the cutting of low thickness materials due to the increased irradiance of the beam
o All of these options
o None of these options
o High pressure inert gas is required to expel the molten material from the kerf
o Reactive fusion cutting allows to cut higher thickness materials
+ The surface quality obtainable by means of a fiber laser is typically higher than the one obtainable with a CO2 laser source (CO2 lasers have longer wavelength -> better surfaces finish)
o When processing high thickness material the cut velocity must be reduced
o Lenses with a shorter focal length facilitate the cutting of low thickness materials due to the increased irradiance of the beam
o All of these options
o None of these options
*Fiber laser are fast, efficient, and thin metal cutting, but not surface quality
With regards to laser beam propagation, which of the following statement is WRONG:
o Diode laser systems typically require larger lenses with respect to active fiber laser systems
o Mirrors cannot be employed to deflect light emitted by an active fiber laser source
o CO2 laser systems generally provide larger beams with respect to disk solid-state laser sources due to the higher emission wavelength
o A transport fiber is typically coupled to deliver the laser radiation emitted by an active fiber source
o All of these options
o None of these options
o Diode laser systems typically require larger lenses with respect to active fiber laser systems (Diode lasers have larger divergence, require larger lenses)
+ Mirrors cannot be employed to deflect light emitted by an active fiber laser source (mirrors CAN deflect fiber laser beams, with coating)
o CO2 laser systems generally provide larger beams with respect to disk solid-state laser sources due to the higher emission wavelength (Longer wavelength -> larger beams)
o A transport fiber is typically coupled to deliver the laser radiation emitted by an active fiber source (for beam delivery)
o All of these options
o None of these options
According to Liu’s ablation model for laser micro-machining, which of the following statements is TRUE?
o Even if the threshold fluence is not achieved, some material will be ablated
o The use of a frequency duplicated laser source will reduce the threshold fluence
o A top hat beam distribution could make the ablation process more energy efficient
o The threshold fluence is only dependent on the material characteristics
o Whilst maintaining all other process parameters fixed, the use of a beam expander allows to increase the ablation diameter
o Even if the threshold fluence is not achieved, some material will be ablated (below-threshold -> No ablation)
o The use of a frequency duplicated laser source will reduce the threshold fluence
+ A top hat beam distribution could make the ablation process more energy efficient (distributes laser energy evenly -> uniform ablation -> efficient)
o The threshold fluence is only dependent on the material characteristics (many other factors)
o Whilst maintaining all other process parameters fixed, the use of a beam expander allows to increase the ablation diameter (beam expander spreads energy -> lower fluence)
Regarding arc welding processes, which of the following statements is TRUE?
o Submerged Arc Welding can be employed for welding in overhead configuration due to the presence of the protective granular flow
o Gas Tungsten Arc Welding provides higher penetration than laser welding due to the higher heat input in the workpiece
o In butt joints configuration Gas Metal Arc Welding allows to bridge larger gaps with respect to laser welding
o The molten pool is not protected from the atmosphere in Manual Metal Arc Welding thus leading to oxide inclusions
o None of these options
o All of these options
o Submerged Arc Welding can be employed for welding in overhead configuration due to the presence of the protective granular flow (not suitable due to gravity-fed flux)
o Gas Tungsten Arc Welding provides higher penetration than laser welding due to the higher heat input in the workpiece (High-quality, not necessarily deep)
+ In butt joints configuration Gas Metal Arc Welding allows to bridge larger gaps with respect to laser welding (LW doesn’t use a filler material, which can lead to problems)
o The molten pool is not protected from the atmosphere in Manual Metal Arc Welding thus leading to oxide inclusions (MMAW uses coated electrode, so the pool is protected)
o None of these options
o All of these options
Regarding AWJ cutting which of the following statements is WRONG?
o The material removal rate is strictly correlated to abrasive capability of the jet
o AWJ cutting is the preferred technology for cutting non-metallic materials for the food industry
o AWJ can cut high thickness metal sheets, comparable to Plasma Arc Cutting but with higher quality
o Laser cutting is typically preferred to AWJ cutting of thin metal sheets due to the higher process productivity
o None of these options
o All of these options
o The material removal rate is strictly correlated to abrasive capability of the jet (correlated)
+ AWJ cutting is the preferred technology for cutting non-metallic materials for the food industry (uses abrasive particles -> contaminates the food -> NOT USED)
o AWJ can cut high thickness metal sheets, comparable to Plasma Arc Cutting but with higher quality
o Laser cutting is typically preferred to AWJ cutting of thin metal sheets due to the higher process productivity (Laser is preferred for thin metals due to higher speed)
o None of these options
o All of these options
In Electric Discharge Machining, which of the following statements is WRONG?
o Closed-loop control of the electrode-workpiece gap is required to maintain high quality machining
o Reducing the pulse duration limits the heat affected zone generated by the material removal process
o The formation of the plasma channel is fundamental in enabling current flow between anode and cathode
o Plunge EDM is typically employed for the realisation of moulds
o It is possible to cut high thickness metal sheets (20 mm) by means of Wire EDM
o None of these options
o All of these options
o EDM require closed-loop gap control
o Shorter pulses reduce HAZ
o Plasma channel is essential
o Plunge EDM is standard for Molds
o Wire EDM can cut thick parts
+ None of these options
In PAC modeling, which phenomenon does NOT explain kerf width mismatch?
o Thermal energy was removed by means of radiative and convective heat transfer
o Exothermic contribution due to reactive fusion cutting of mild steel
o Stand-off distance closed-loop control to account for sheet deformations
o Electric arc transformation efficiency into heat due to the Joule effect
o None of these options
o All of these options
+ Stand-off distance closed-loop control to account for sheet deformations
** Stand-off control is a mechanical adjustment for distance, and does not directly affect kerf width in the heat transfer model.
*** Other phenomena (radiative losses, exothermic reactions, arc efficiency) all impact the heat input and thus kerf shape/width.
Laser surface hardening with diode laser didn’t change hardness. What should you do?
+ Increase the process duration (long duration -> high thermal input to austenitize the surface layer)
o Reduce the laser power (doesn’t help)
o Apply forced convection with Ar gas at room temperature (convection would cool down the area)
o Reduce the spot size
You are commissioned to carry out laser cutting of 2 mm - thick AISI 301 stainless steel parts employing a 4 kW Yb:glass laser system. You plan to cut out 10 pieces out of sheet materials with 1 m × 0.5 m size. You start the operation, and you observe that the quality appears to get inferior after you cut the first 2 parts, as you observe excessive dross formation, loss of cut and higher roughness (LOSS OF QUALITY). Which IS NOT a plausible cause?
o The gas pressure is dropping in time due to clogging issues in the gas line
o The focusing optics are dirty causing heating up and thermal lensing in the head
o The optical absorptivity of the material is too low
o The capacitive height sensor is not working
o The gas pressure is dropping in time due to clogging issues in the gas line (gas pressure drop reduces molten material ejection)
o The focusing optics are dirty causing heating up and thermal lensing in the head (Dirty lens cause thermal lensing and beam distortion)
+ The optical absorptivity of the material is too low (AISI absorptivity does not change significantly during cutting)
o The capacitive height sensor is not working (sensor failure leads to incorrect focus height, affecting cut)
Considering the design of a laser welding head, which of the following statements are
correct?
o If a CO2 laser is used a collimating and a focusing lens are required
o If a Yb:glass laser is used a collimating and a focusing lens are required
o The plume blower helps to cool down the molten material
o High pressure inert gas (16 bar) is employed to push the molten metal to flatten its
surface
o If a CO2 laser is used a collimating and a focusing lens are required (use mirros, not just lenses)
+ If a Yb:glass laser is used a collimating and a focusing lens are required (due to the high divergent nature, they require collimation to straight and focusing to concentrate)
o The plume blower helps to cool down the molten material (plume blower is for clearing vapor, not cooling)
o High pressure inert gas (16 bar) is employed to push the molten metal to flatten its
surface (HPIG is for cutting)
Regarding laser welding, which of the following is NOT an issue for the process?
o 3 mm Cu-alloy – 1 kW Nd:YAG laser – vaporization cutting
o 10 mm Al-alloy – 6 kW CO2 laser – melt and blow cutting with N2
o 20 mm steel – 6 kW fiber laser – reactive fusion cutting
o 5 mm pure Ti -6 kW CO2 laser – reactive fusion cutting
o 3 mm Cu-alloy – 1 kW Nd:YAG laser – vaporization cutting (Cu is highly reflective, thus, poor coupling)
o 10 mm Al-alloy – 6 kW CO2 laser – melt and blow cutting with N2 (poor absorption and reflection losses)
+ 20 mm steel – 6 kW fiber laser – reactive fusion cutting (Suitable for deep steel welding)
o 5 mm pure Ti -6 kW CO2 laser – reactive fusion cutting (risk of oxide formation)
Concerning the properties of the laser beam, which statement is wrong?
o Monochromaticity refers to a single wavelength, which is easier to focus
o Collimations refer to high directionality, which allows smaller beams
o Spatial modes are related to the energy distribution within the beam, where only
TEM00 is useful for industrial applications
o Beam coherence is referred to spatial and temporal coherence
+ Spatial modes are related to the energy distribution within the beam, where only TEM00 is useful for industrial applications (is ideal for precision, but other modes can be used for other ind applications)
*The other ones where true
**Beam coherence is referend to spatian and temporal coherence
Amongst the following, which laser has the highest peak power under the hypothesis of a
square wave pulse shape?
o Flash pumped Nd:Yag laser source with 100 W average power, 40 ns pulse duration
and 1% duty cycle
o Single mode laser source with 1000 W average power, 200 µs pulse duration and 50
% duty cycle
o Nano-second pulsed fiber laser source with 12 mJ pulse energy, 55 ns pulse duration
and 10 W average power
o Pico-second pulsed fiber laser source with 10 µJ pulse energy, 35 ps pulse duration
and 5 W average power
o Flash pumped Nd:Yag laser source with 100 W average power, 40 ns pulse duration
and 1% duty cycle
o Single mode laser source with 1000 W average power, 200 µs pulse duration and 50
% duty cycle
o Nano-second pulsed fiber laser source with 12 mJ pulse energy, 55 ns pulse duration
and 10 W average power
+ Pico-second pulsed fiber laser source with 10 µJ pulse energy, 35 ps pulse duration
and 5 W average power
- Peak power = energy/pulse duration
What of the following sentences are true regarding the Lazarenko’s circuit in EDM?
o It is suitable for roughing
o It is characterized by high current values
o It allows the pulse shape control
o It is not used anymore
o It is suitable for roughing (not the best choice)
+ It is characterized by high current values (simple and robust -> high discharge current)
o It allows the pulse shape control (limited control due to use of passive components)
o It is not used anymore (used as low-cost EDM setup)
In EDM, why a pulsed material removal is needed?
o It is not needed
o It is needed for obtaining the extraction of the molten material thanks to the
implosion of the plasma bubble created by the spark
o Because only the Lazarenko and power transistor circuits are available
o It is needed to always keep the dielectric conductivity higher than a threshold
o It is not needed (if not, no cooling or debris removal)
+ It is needed for obtaining the extraction of the molten material thanks to the
implosion of the plasma bubble created by the spark (the plasma creates a shockwave that removes molten material)
o Because only the Lazarenko and power transistor circuits are available
o It is needed to always keep the dielectric conductivity higher than a threshold (Pulsing helps mantain the dielectric insulation properties)
CO2 laser sources are not employed in Laser Metal Deposition processes because (indicate
the right answer:
o Reactive fusion cutting of carbon steel
+ Fiber transportation mechanisms is more convenient in robotised sysems
o Laser keyhole welding of 10 mm thick stainless steel
o Reactive fusion cutting of carbon steel (True but irrelevant)
+ Fiber transportation mechanisms is more convenient in robotised sysems
o Laser keyhole welding of 10 mm thick stainless steel (True but irrelevant)
*CO2 lasers use mirrors, not fibers, making them bulky and hard to integrate into robotic systems.
** Fiber lasers are compact and flexible
