Thermal Effects Flashcards
What are the four basic phases of laser generated heat effects?
Heating, melting, boiling and plasma formation.
What is reflection?
When the laser beam is incident on the workpiece and energy is reflected.
Approx 90% for an infrared laser to be reflected
What is transmission of a laser beam?
None of the energy is absorbed and it propagates through the workpiece with no change in amplitude.
What is absorption of a laser beam?
As the laser propagates through the workpiece the energy is absorbed. Causing the amplitude of the laser to reduce exponentially and the temperature of the workpiece to increase from the energy absorbed.
What two elements of the work piece and the amplitude of the laser are affected by absorption?
Temperature of the workpiece and amplitude of the laser beam.
what are the 4 steps of heating a surface? What material process does this achieve?
1.Initially the substrate absorbs small amounts of laser beam energy (» 5 % for a CO2 laser), the remainder is reflected.
2.The small amount of energy absorbed raises the temperature of the substrate.
3.As the temperature rises, so the reflectivity reduces and the fraction of incident energy absorbed increases.
4.This is the regime for laser surface hardening. The surface temperature must be high enough (for most steels this is about 720 C), but the surface must not melt.
What are the 2 steps of melting the surface? What material process does this achieve?
1.When the substrate melts, the reflectivity drops considerably and the molten region grows rapidly.
2. This is the regime for surface alloying or cladding. The additional material is fed into the molten pool, often in a
powdered form.
What are the 4 steps from melting to vaporising?
1.Once the metal starts to vaporise, a keyhole and a partially ionized plasma forms.
2.The keyhole is filled with partially ionized metal vapour and penetrates through the workpiece. For cutting applications, the molten material is blown away by a high pressure gas jet.
3. During keyholing, the absorption of the laser beam is high, the beam is absorbed through multiple reflections on the keyhole walls and excitation of the metal vapour.
4.The excited electrons reemit the energy as they return to the ground state. The light emitted is over a
range of wavelengths, giving the plasma region a white/blue colour for most substrates.
What are the 2 methods of laser energy absorption in a laser keyhole?
- multiple reflections at the
keyhole walls (FRESNEL ABSORPTION)
* electron excitation in the substrate vapour (INVERSE BREMSSTRAHLUNG)
What is plasma formation?
The plasma region starts to limit the process. It absorbs the laser radiation and prevents the energy being transferred to the cavity
What is laser drilling-2 steps? And what happens if the material melts?
1.Drilling requires the removal of material, either through ejection or
vaporisation.
If the material melts it must be ejected. A high pressure gas jet, similar to that employed during cutting is used.
2.Resolidification of the molten, ejected material forms a recast layer and
leaves splatter on the surface of the workpiece.
The recast layer often includes faults e.g. delamination, cracks, inclusions.
What is laser cutting?
MELTING and BLOWING
1. With the use of a high pressure gas jet, to make a cut the molten material from the meltpool must be removed.
2.The front face directly below the laser beam heats up and is
removed, leaving the next level exposed to the laser beam. The jet
of molten metal blown away may cause the typical striated effect on the side wall of the cut.
What is the kerf width?
The width of the cut
What is Dross?
the melt that has adhered to the cut
What happens when the cutting reacts with air?
There is oxygen in the air that can react exothermically(give heat out) with oxygen.
If oxygen is used as the cutting assist gas, less laser power is required to progress the cut. For cutting iron based alloys, the effective input power may be equal to the incident laser power.
The burn front the the cause of striation during oxygen assisted cutting.
The formation of viscous oxides will make the melt adhere to the workpiece and form dross.
