Advanced Processes and Technology 1
Laser Materials Processing
Contents 1. Describe the interaction mechanisms of IR and UV laser beams with the surface of an opaque material. .................................................................................................................................................. 2 2.
What role does processing gas play in: ........................................................................................... 3 2a. Laser cutting (all types)? ............................................................................................................... 3 Vaporisation .................................................................................................................................... 3 Fusion Cutting (Melt and Blow) ...................................................................................................... 3 Reactive Fusion Cutting (Melt Burn and Blow) ............................................................................... 3 Controlled Fracture Cutting (Thermal Stress Cracking) .................................................................. 4 2b. Laser welding? .............................................................................................................................. 4 2c. Laser transformation hardening? .................................................................................................. 4 2d. Laser shock hardening?................................................................................................................. 4 2e. Laser cladding and laser drilling? .................................................................................................. 5
3.
For what types of materials processing tasks is: ............................................................................ 5 3a. The excimer suitable? ................................................................................................................... 5 3b. The CO2 laser suitable? ................................................................................................................ 6 3c. The HPDL laser suitable? ............................................................................................................... 6 3d. The fibre laser suitable? ................................................................................................................ 6 3e. The Nd:YAG laser suitable? ........................................................................................................... 6
4.
Fully describe the process of: ......................................................................................................... 7 4a. Laser percussion drilling; ............................................................................................................... 7 4b. Laser keyhole welding; and........................................................................................................... 7 4c. Laser cladding (pre-placed powder, blown powder and wire fed) ............................................... 8
Appendices.............................................................................................................................................. 9 References .............................................................................................................................................. 9
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1. Describe the interaction mechanisms of IR and UV laser beams with the surface of an opaque material. There is a large difference between the interaction mechanisms of that of an infra-red (IR) and ultraviolet (UV) when they hit an opaque material. The differences in properties will affect the different results seen when they come in contact with an opaque material. The IR laser light comes from the opposite end of the spectrum than the UV laser light, giving them completely different properties. Appendix 1 gives a visual representation of the light spectrum and of the visibility of the light as you work along the spectrum. It shows that at either end of the scale, where lasers sit, the light is not visible to the human eye. The colour of the light is dependent on the wavelength produced by the photon of that light. The IR laser has a relatively long wavelength, where the UV laser at the opposite end of the spectrum has a short wavelength. IR lasers usually produce wavelengths of between 10.6µm (CO2 laser) and 1.6µm (Nd:YAG). UV lasers have a much shorter wavelength of 193nm and 353nm (Excimer). With the differences in wavelength comes a difference in energy of the light. The equation for working out light energy (Appendix 2) has wavelength as a denominator, this means that the shorter the wavelength the higher the energy. With this in mind, UV light has a significantly shorter wavelength, giving it a higher energy. The opposite to what is found with IR lasers. With there being a lot of differences with the two types of lasers, there is a large difference in the interaction mechanisms when the beam hits an opaque surface. Interaction methods come in two forms, reflection or absorption. These are governed by the surface itself and the wavelength of the light hitting the surface. The difference wavelength is the main reason for the difference in interaction mechanisms when looking at IR and UV lasers. Reflectivity for opaque material can be calculated as 1 – Absorptivity, with Absorptivity being a “measure of an objects ability to absorb incident energy. [1]” “At shorter wavelengths, more photons can be absorbed by a greater number of bound electrons. Thus the absorption increases as the wavelength reduces. [2]” With UV lasers having a shorter wavelength than IR lasers, UV lasers absorption is considerably better than that of an IR laser. The surface of the material also dictates the absorption of the material. If the surface is perfectly flat any photons that do get reflected will reflect normally and away from the surface. This means that those photons will never be absorbed by the surface. If the surface is rough then the light is not reflected normally and the reflected photons also have the potential to be absorbed afterwards. This extra chance at absorption for the reflected photons means that a rougher surface has a higher absorption rate. The different lasers, with different wavelengths and energies, are absorbed in different ways. IR lasers are absorbed using a method called “Fresnel Absorption”. UV absorption is called “Ablation”. With the beam produced by IR lasers having longer wavelength, the photons interact with the free electrons. This interaction causes them to vibrate and, in turn, produce heat. This absorption of photons and converting it to heat is known as a “Photo-thermal process.” With UV lasers having a shorter wavelength and higher energy the UV laser is able to directly break the bonds within the material. With their being a similarity between UV laser wavelength and the excitation energy of most bonds holding the material together, UV lasers can directly break these bonds. This only removes small layers at any one time. This process is known as “Ablation” and is a “photochemically” induced process. 2 David Coombes A575990
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2. What role does processing gas play in: 2a. Laser cutting (all types)? There are many different ways to cut a material using lasers. Each of these processes are very different from one and other and the role of the gas within the process differs from method to method. There are four methods to cut materials using lasers, these are; Vaporisation, fusion cutting, reactive fusion cutting and controlled fracture cutting. Every method is different in principle but has similar features. One of the similar features is the way the gas is fed into the process. This happens through a small aperture nozzle around the laser beam and surrounds the laser beam. This gas can sometimes be jetted at high pressures depending on the process. Vaporisation This method uses a very high power density and essentially burns through the material. The beam heats up the surface to boiling point and produces a keyhole, “this keyhole increases the absorptivity, due to black body multiple reflections, thus increasing the depth of the keyhole quicker. As the deepening progresses, vapour is produced blowing material out of the hole [3]”. The initial part of the process happens very quickly; once the hole is formed the laser can traverse across the material cutting as it passes. During the whole process the beam is encased by gas which is been fed in through a nozzle form above. In this method the gas has two main roles. The first being to protect the optics, the surrounding gas prevents any material debris from returning up the nozzle and damaging the optics. The second role of the gas is to prevent oxidation. If the material being cut is prone to oxidation surrounding the cutting area by an inert gas can prevent the oxidation from happening. Fusion Cutting (Melt and Blow) The method of fusion cutting uses a lower power density than the method of vaporisation. The initial step is the same; by creating a hole by heating the spot where the laser beam and the material meet, to past the materials melting temperature. Then with “sufficiently strong jet it is possible to blow the molten material out of the cut [4]”. This method has three uses for the gas. The first being to blow the molten material out of the cut, once the material has been sufficiently melted the gas will blow the molten material away. With the gas removing the material, there is no need to vaporise it and will save on power. This process uses 1/10th of the power in comparison to vaporisation and thus uses half the energy. With the gas being inert the other two methods are the same as before; to protect the optics and to prevent oxidation, if needed. Reactive Fusion Cutting (Melt Burn and Blow) The process of reactive fusion cutting is very similar to fusion cutting and the only difference arriving due to the different roles of the gas. Instead of using an inert gas as in fusion cutting, a gas is used to react exothermically with the material being cut producing an additional heat source to the laser beam present. This additional heat source means that a lower powered laser can be used to cut the material. There are three more roles; these are the same as before. The gas is forced in at high pressure to blow away the molten material, also to protect the optics and finally prevents oxidation if needed.
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Controlled Fracture Cutting (Thermal Stress Cracking) This final cutting process is undertaken on brittle material such as glass and ceramics. The laser beam can cause localised heating on the surface and causing tensile stress. As the material heats up, the unit cells expand causing stress between the bonds of the atoms. As the beam moves away, rapid cooling occurs and a crack is initiated. The crack acts as a stress raiser and then continues in the direction of the laser. This process is not as accurate as the pervious methods due to unpredictable manner of the cracking. The nature of cracking leads to limitations in the profiles which can be cut, straight lines and simple curves are about the limitations of this method. The advantages to this are the lower use of power, with the laser only heating the surface and not melting the material. Also it is very fast and can cut up to one metre per second. The gas in this method has three functions. Two of which are the same as the previous methods; to protect the optics and prevent oxidation. Also the gas can help the rapid cooling of the material once the laser moves.ac
2b. Laser welding? The gas within laser welding has a number of roles. The main roles is to protect the optics, the gas prevents anything from returning up the nozzle and damaging the optics. Using an inert gas will also prevent the material from oxidising. With the gases main roles are to protect the material and the laser itself, the nozzle in which the gas comes out if a relatively large nozzle and comes out at low pressure and gives a low flow rate. Finally the gas used in Laser welding can become plasma. Plasma is "A low-density gas in which individual atoms are ionized.�[5] The steep temperature gradients, produced by the laser, cause the plasma to ionize. This has two detrimental effects on the process. The first effect is that the plasma can refract the laser beam; this will defocus the beam and cause a drop in energy density. This will cool the weld and make it very inconsistent. Secondly the plasma can directly absorb the laser beam; this will reduce the heat considerably and will disrupt the weld or potentially stop it. To prevent the plasma, a gas with a high ionization potential, but also heavy enough to hold around the weld is needed. Helium has a very high ionisation potential and would be ideal of laser welding if it was heavier. Helium can be mixed with a heavier gas such as Nitrogen, which has a low ionization potential, to produce a gas with properties between the both.
2c. Laser transformation hardening? Laser transformation hardening is when the microstructure of the material is altered using a combination of heating and rapid cooling, the change in the microstructure of the material will also change the properties of the effect area. This change takes the metal from Body Centred Cubic (BCC) to Face Centred Cubic (FCC). The later of the two structures can be considerably stronger and in the case of a ferrous alloy “the BCC crystal structure in ferrous alloy can only take into solution 0.02% carbon. However, the FCC crystal structure can take into solution over 0.8% carbon.�[6] The use of gas is for two main reasons. Firstly it is to protect the optics; secondly it prevents oxidation of the material.
2d. Laser shock hardening? In the past shock hardening had been achieved through the process of shot peeing. This is when a metallic shot impacts the surface at high speeds; this force produces compressive stresses in the surface and increases the fatigue strength of the material. Using a very short pulsed laser, with a low order beam similar results can be achieved. By directing the laser and setting it to a short pulse (10-
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100ns) the compressive strength of the material can be improved. Gas is used again in this to protect the optics by preventing anything from returning up the nozzle, and also to prevent oxidation.
2e. Laser cladding and laser drilling? The common role of the gas, that is every present through all the processes mentioned, is still present in these to processes. Initially the gas is used to prevent anything from returning up the nozzle and damaging the optics. In blown powder cladding the gas is used to blow the powder onto the melt pool created by the laser. This can cause spattering, to prevent this; powder is pre-placed onto the material making this blowing use of the gas redundant. Instead a skirt is used to provide a large shroud gas rich region. In laser drilling that gas protects the optics, as with all processes, but also aids the removal of material. It is very similar to the “melt and blow” process, the difference comes when this is not the main form of removal of material. The main removal is achieved by vaporization-induced recoil pressure and nucleation mechanisms. Plasma formation is not a problem like it is in cutting processes. This is due to cutting using pulsed lasers; this gives the plasma time to dissipate between pulses.
3. For what types of materials processing tasks is: For any of the lasers below, the properties of the individual laser will determine its ability to undertake certain tasks. The properties that affect its use are;
Type of light produced (Infra-Red or Ultra-Violet) If the beam produces heat Continuous or pulse wave High or low order beam Flexibility.
Continuous and pulsed waves relate to the relationship between intensity and time. Continuous wave lasers produce a constant level of intensity over a set period of time; this intensity is both the maximum and the minimum that can be produced. Pulsed waves consist of series of intensity peaks and troughs over a set time. High order and low order beams relate to the focal point of the laser. High order beams are sometime called doughnut beams. The main focus is in a ring shape, with the centre of the ring having a lower intensity, and the further away from the focal ring the intensity decreases. A low order beam is a Gaussian beam; this beams focal point is directly in the centre and the further from the centre the lower the intensity. (Appendix 3) Flexibility is mainly to do with the wavelength and whether it can be sent down an optical wire making the laser easier to use. The different properties for each laser will dictate which processes each excels at.
3a. The excimer suitable? The Excimer laser is a gas based laser, using a combination of an inert gas and a reactive gas. The beam produced is in the ultra-violet region of the light spectrum and produces different wavelengths within the ultra-violet boundaries depending on the mix of gases used. It produces a pulsed wave, which can be shaped to prolong or shorten the pulse and increase and decrease intensity. The UV wavelength means that it is unable to pass through an optical cable, reducing the flexibility. It produces a good stable beam of low order. This UV beam produces a photochemical reaction rather than photo thermal. The photochemical reaction also means it is unsuitable for laser welding, laser 5 David Coombes A575990
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transformation hardening and laser cladding and the fact that it low power means it is unsuitable for laser shock hardening. However the photochemical reaction as opposed to photothermal means that excimer lasers can be used to process heat sensitive materials and small fragile components.
3b. The CO2 laser suitable? CO2 lasers use a combination of Helium, Nitrogen and Carbon Dioxide to produce the IR beam. CO2 laser can produce powers from one watt to greater than thirty kilowatts. These lasers produce IR beams, around the 10.6µm wavelength. CO2 lasers can produce both continuous and pulsed waves. The output is in the form of a stable low order beam. The large wavelength makes it unsuitable to be delivered by fibres, making it less flexible than other lasers. Even with this restriction in its flexibility, it can be used in; laser cutting, laser welding, laser transformation hardening (of simple shapes) and laser cladding. With the not being peak pulsed lasers, they cannot be used in laser drilling and laser shock hardening.
3c. The HPDL laser suitable? HPDL laser is also known as a diode laser, it used doped p-n junctions such as Aluminium, Gallium and Arsenide. It produces continuous wave lasers with wavelengths between 810nm and 940nm, the degree of doping determines the wavelength. A single diode will produce only a few milliwatts but stacks of diodes can produce powers of kilowatts. The beam can be delivered down an optical fibre and is high order. HDPL lasers can be used for laser welding, laser transformation hardening and laser cladding. The beam that is delivered is always multimode never Gaussian meaning this type of laser is no good for cutting operations, also the laser cannot be peak pulsed meaning it cannot be used in laser shock hardening or drilling.
3d. The fibre laser suitable? Fibre laser is a solid state laser, using Ytterbium, Erbium and Thulium to produce their beam. It produces both continuous and pulsed wave at powers less than two hundred watts. The pulsed waves can be shaped to alter peak height and to prolong the pulse time. The wave length is between 0.9µm and 1µm. This shorter wavelength means that it can be passed down an optical cable. It uses a low order beam that can be focused into a very small spot. With fibre lasers being able to focus the spot size a lot smaller than most other lasers, they are beginning to dominate the laser cutting industry. Fibre lasers are also used for laser welding, laser cladding and laser transformation hardening. They cannot be used for laser shock hardening or drilling because even though the laser can be peak pulsed the power output is too low.
3e. The Nd:YAG laser suitable? Nd:YAG laser is a solid state laser, producing an IR beam with a wavelength of 1.06µm. This short wavelength allows is to travel through an optical cable, adding to its flexibility. This laser can only produce pulsed waves; these waves can be shaped to alter the length and intensity. This laser can also alter the frequency of the beam produced, by doubling and tripling the frequency the laser can produce green or UV light. The output beam is of low order. CO2 and Fibre lasers are usually preferred to the Nd:YAG when it comes to laser cutting, but the Nd:YAG laser can still perform laser cutting operations. Nd:YAG lasers are also used for; welding, drilling, cladding and shock hardening. They cannot be used for transformation hardening as this requires a constant wave beam as opposed to a pulsed beam.
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4. Fully describe the process of: 4a. Laser percussion drilling; This is a very fast process where a series of pulsed waves are used to create a hole in a given material. This process is used to create holes between 200Âľm and 800Âľm in diameter and up to a depth of 6mm. A number of laser pulses performed in series, all with a given energy and duration, are directed onto the same spot on a substrate surface. These pulsed are usually split into three different types, depending on their energy and duration. The initial pulse is different to the rest, this is due to its job of beginning the hole. Usually has a lower energy and longer duration in comparison to the other pulses. This is because the first pulse only needs to melt the surface and this only requires a lower temperature. Once the material is melted the second through the penultimate pulse are all of high power, with a very short duration. With high power and short duration, these middle pulses cause vaporisation of the material. The final pulse then is the finishing pulse. This is similar to the first pulse, with lower power and longer duration. This gives a better quality finish to the hole. The main way melt is ejected from the hole in this process is through vaporisation. Two other mechanisms aid the removal of melt material. The first is vaporisation-induced recoil. This is where the reaction force on the liquid surface which pushes the melt layer out of the beam path, this mechanism is more apparent in deeper holes. The second is through nucleation mechanisms, this causes violent boiling in the liquid. The liquid below the surface becomes superheated, this leads to a rapid change in phase (liquid to vapour) and the resulting expansion causes neighbouring liquid to be ejected. This additional melt ejection happens in a ratio of about 4:1, with the vaporisationinduced recoil being the most prevalent. Plasma is formed during this process, like with most other process, but with the laser being pulsed it gives the plasma time to dissipate between pulses. For this reason alone it is critical to get the time spacing between pulses correct. Not enough time and the plasma will not dissipate, resulting in poor quality and damage equipment. Too much time between pulses leads to cooling of the material and usually an unsuccessful operation. Nd:YAG lasers are ideal for this process due to their pulsed wave properties. With this laser peak pulsing can be achieved giving you more flexibility in the process. Excimer lasers can be used but only on very thin materials. This is because of it photo-chemical process and only removes one atomic layer in any one pulse. Any thicker material would result in a very lengthy process.
4b. Laser keyhole welding; and The process relies of the melting and flowing together of material to produce a weld, almost all materials can be welded together using laser welding processes. Laser welding is a high precision process for high value or high production rate applications. Laser welding uses both high powered pulsed and continuous wave lasers. All Nd:YAG, CO2, HPDL, and fibre lasers can be used, Excimer lasers cannot be used due to lack of heat produced and the ablation interaction mechanism used by the laser. Weld thicknesses between 0.001mm up to 50mm achieved depending on type of laser and power used.
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Keyhole welding uses a vaporised channel, called a keyhole. This vaporised channel is immediately below the interaction region. A deep melt pool is created as energy is absorbed at the surface and by the walls of the keyhole through a phenomenon called “wave guiding�. The void that is created by vaporisation is refilled by molten material as the laser moves and therefore key hole along the weld track. A high flow of low pressure gas is used to shield to weld from oxidation and to maintain a low temperature to keep the weld stable. A balance needs to be achieved in this gas, it need to have ionization positional to prevent the formation of plasma but also needs to heavy enough for it to stay in position. Plasma formation can result in the laser beam being refracted and defocused which leads to a drop in energy density cooling the weld making it inconsistent, the plasma creation can even lead to the weld being disrupted or stopped completely.
4c. Laser cladding (pre-placed powder, blown powder and wire fed) There are three types of laser cladding, all of which differ in the way the clad material is added. The aim of any cladding operation is to overlay one material with another material to form a sound interfacial bond without contaminating the cladding metal with the substrate. Clad and substrate material can be either metal or ceramics. This process is used in the hardfacing of; poppet valve faces, bearing races and turbine blade tips. It is also used in the repair of turbine blades and engine shafts. CO2 lasers are usually used but Nd:YAG, HDPL and fibre lasers can also be used. In pre-placed power the powder layer laid onto the substrate surface prior to laser irradiation. With the powder being placed prior to laser irradiation, it can only be completed on flat substrate geometries. Alcohol is usually used as a binder, to ensure the powder sticks to the substrate material. An inert gas shroud is provided and the laser beam is then traversed over the powder initiating a melt front that proceeds in a downwards direction, the substrate is melted after the powder therefore limiting adhesion. This method is not good for anything experiencing corrosion or erosion and the track tends to be non uniform and slightly porous. Blown-powder laser cladding offers the most versatility due to its well defined heated region, a fusion bond with low dilution can be automated. The powder is blown by an inert gas into the melt pool either off axis from the side or into the laser beam itself coaxially, thus cladding can proceed in any direction and on any surface geometry. In either case the melt front proceeds in an upwards direction and results in tracks with low dilution; good surface strength and are usually pore free. The cost of this process is very high, with the nozzles being very complex; prices can be up to ÂŁ750,000. The powders used are those developed for plasma spray there is a large selection, but the nature of the process results in a lot of waste powder. The strength of this process lies in its ability to clad small areas with precision, usually for use on high end applications. Finally wire-fed cladding, this is a similar process to arc welding. This is where a wire follows the laser, taking heat energy from the melt pool created by the laser, a large melt pool is required for this to occur making the method more inefficient than other methods. The large melt pool and the mixing of material result in a dense clad tracks. The need for the wire to follow the laser means that the process can only complete straight line cladding and not complex geometries.
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Appendices
Appendix 1 - http://www.johnsonlevel.com/ap/images/visible_light_spectrum(1).png
Appendix 2 - http://csep10.phys.utk.edu/astr162/lect/light/waves.html
Appendix 3 - http://www.thefabricator.com/Images/1510/TEM-diagram.jpg
References [1] – Google Definitions, 2008. Definitions of absorptivity on the Web. [Online] Available at: http://www.google.com/search?hl=en&safe=off&rls=com.microsoft%3A*%3AIESearchBox&rlz=1I7HPEA&q=define%3A+absorptivity [Accessed 13 December 2008] 9 David Coombes A575990
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[2] – Lawrence, Jonathan., Advanced Processes and Technology 1, Laser, Course Literature, Received in week 7 [3] – Lawrence, Jonathan., Advanced Processes and Technology 1, Laser, Course Literature, Received in week 7 [4] – Lawrence, Jonathan., Advanced Processes and Technology 1, Laser, Course Literature, Received in week 7 [5] –The Naked Singularity, 1998. Glossary. [Online] Available at: http://www.rdrop.com/users/green/school/glossary.htm [Accessed 15 December 2008] [6] – Metal Tech, 2005. Processes Introduction. [Online] Available at: http://www.metaltech.co.uk/processes.htm [Accessed 3 January 2009]
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