Laser Cutting Flashcards
What are laser cut quality characteristics?
- Cut is narrower.
- Cut edges are square.
- Cut edge can be smooth and clean.
- Narrow heat affected zone (HAZ).
What are laser cut process characteristics?
- Fast.
- Tool wear is zero.
- Cut in any direction.
- Low noise.
- Easily automated.
- Can cut all materials.
What are laser cut features?
Kerf
HAZ )Heat affected zone)
Striations
Bevel edge
Top spatter
Top edge rounding
What are the 5 main methods of cutting?
Vaporisation
Fusion Cutting - melt and blow
Reactive fusion cutting - melt, burn and blow
Controlled fracture cutting
Cold cutting
What is vaporisation cutting?
Not used unless essential due to high energy requirements and low efficiency=higher costs.
* The focused beam first heats up the surface to boiling point and thus generates a keyhole.
* Keyhole causes a sudden increase in the absorptivity due to black body multiple reflections and consequently deepens quickly.
* As deepening progresses, vapour is generated which escapes, blowing ejecta out of the hole/kerf.
* Better for cutting non-conductive materials which do not melt – wood, concrete, carbon.
* Inert assist gas used only to provide a shroud (protect nozzle and remove vapour)
What is a vaporisation cutting issues spatter?
Spatter formation normally associated with drilling (although similar conditions created in cutting)
* Keyhole in top layer of material caused melt pool and increase in absorptivity
* As hole deepens, vapour is generated and escapes
* This vapour exerts force on melt pool, effectively pulling molten material out of top of hole
* Melt falls back as spatter around the lip of the hole (or cut) or as a recast layer within the hole (or cut)
What is fusion cutting ?
Fusion Cutting or “Melt and Blow”
* Once a material has been melted and a hole is made, then with a
sufficiently strong gas jet it is possible to blow the molten material out of
the cut – melt and blow.
* Because boiling does not occur, the HAZ in fusion cutting is very small
and the cuts themselves are clean.
* Fusion cutting is ideal for flammable materials or oxidation sensitive
materials; e.g., stainless steel.
* The gasses used are inert (N2, He, Ar) or non-reactive (to the material)
and are supplied coaxially at high pressure (10 Bar).
* The sole role of the gas in fusion cutting is to drag the melt away.
* Consequently, the nozzle design and its alignment with the kerf, as well
as the alignment of the laser beam are important insofar as they affect
the drag of the gas on the melt.
What is the fusion cutting method?
1.Energy absorbed at top edge (typically where focal point is) and where coupling is more efficient (normal to beam propagation)
2.Molten material forced downwards by gas flow
3.The ‘rear’ side of the beam is absorbed nearer the top (as workpiece moves)
4.The ‘front’ side of the beam absorbed at top and towards bottom of cut (as it travels in a favourable direction)
5.Molten material flows down cut edge (sloped)
What are the three main parts on thick materials from striations?
- Direct penetration
- The shelf zone
- Washout region
Why do striations happen?
potentially pulsating droplets of melt or sideways burning
What is dross in fusion cutting
Dross (sometimes slag or burr) is the resolidified metal that adheres to the top and bottom
edge of the material being cut.
* Bottom dross occurs at slow cutting speeds but also present at high speed
* Normal range of speeds called the Dross-Free-Zone (DFZ) at which no dross is formed
* Certain materials and surface conditions make the DFZ very narrow or non-existent (e.g.
sandblasted surfaces provide a rough texture for the dross to adhere.
* Ensure higher speeds are maintained at corners (where effective speeds can be reduced)
* Close proximity of cuts can cause dross if the bulk material gradually heats up. Plan cut
paths carefully.
What is the method for reactive fusion cutting?
- If the inert gas in fusion cutting is replaced with one capable of reacting
exothermically (i.e. oxygen or a mixture containing oxygen) then another
source of heat is added to the process. - Gas now not only drags the melt away, but also reacts with the melt
- The amount of energy supplied by the burning reaction varies depending
upon the material being cut (and obviously the gas mixture) - The reactive (burning) gas (oxygen) reacts exothermically with the
workpiece (iron) adding a second heat source (laser = 60% of heat &
oxygen 40% of heat ) to speed up the process. - With titanium, as much as 90% of energy comes from assist gas
What is reactive fusion cutting advantages?
- Cutting speeds are usually roughly doubled compared to fusion melting
- Generally faster cuts = less heat penetration = better cut quality
- Must appreciate the formation of a oxide layer
- In titanium the cut edge becomes harder and more liable to cracking
- Dross is no longer a metal, but usually an oxide
- For mild steel – does not adhere well to surface
- For Stainless and Aluminium, dross is made up of high melting point components and
freezes quickly adhering to surface - Due to ignition, a further cause of striations is introduced – sideways burning
What are reactive fusion cutting limitations?
Reactive fusion cutting is only useful for inflammable materials.
* The reactive gas used is usually O2, or some mixture containing O2,
supplied at high pressure (10 Bar).
* As with fusion cutting, this means that the nozzle design and its
alignment with the kerf, as well as the alignment of the laser beam are
important insofar as they affect the drag of the gas on the melt.
What is controlled fracture cutting method?
Brittle materials which are vulnerable to thermal fracture can be quickly and neatly severed by guiding a
crack using a using a fine laser beam.
* Localised heating causes localised expansion leading to tensile stresses.
* Upon rapid cooling (as the beam moves away) a crack is initiated which acts as a stress raiser and cracking continues in the directions of the hot spot (laser beam).
* Speeds for this type of ‘cutting’ are of the order of 1
m s-1
