LASER MATERIAL PROCESSING PDF
PDF | Light amplification by stimulated emission of radiation (laser) is a matter interaction and classification of laser material processing has been provided. W. M. Steen, J. Mazumder, Laser Material Processing. . advantage for the laser over electron beam processes, which require  Lambda Research Optics ( ) Radial polarizer for CO2 laser systems. myavr.info . pdf. Materials processing experts face a wide range of choices in determining the right laser we work with you to determine the right laser for your process needs.
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to the work piece. • Happens only if the material has high absorption at the wavelength corresponding to the laser beam. • Once the surface of the materials . INTRODUCTION TO LASER. MATERIALS PROCESSING. COURSE NOTES. RONALD SCHAEFFER, PhD. CEO, Photomachining, Inc. Pelham, NH. Printed in India. Laser processing of materials. J DUTTA MAJUMDAR and I MANNA. ∗. Metallurgical and Materials Engineering Department, Indian Institute of.
This tight lateral focusing comes at a price since it narrows the depth of field DOF along the axial direction, causing a concomitant drop in machining efficiency outside the reduced range. Therefore, the surface of the workpiece has to be carefully maintained within the focal position of the laser beam to ensure a high efficiency; as material is removed, it is important to continue to adjust the location of the laser focus to follow the change in topography of the workpiece One common method used to mitigate a narrow vertical machining range consists of extending the DOF of the system by using, for example, a low focusing power lens or structured light.
However, such attempts lead to a significant loss in lateral resolution Alternatively, the focused beam can be adjusted with respect to the surface location throughout the material removal process, but this requires real-time knowledge of the surface location as well as a fast focusing method.
While some studies have attempted to acquire real-time surface location information to increase the machining efficiency, the complexity and inflexibility of the resulting machining system drastically reduces its potential for practical use In addition to the difficulty of real-time surface monitoring, most controlled focusing methods also suffer from low response rates compared to the repetition rate of the laser 16 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , Here, we demonstrate a new method for increasing the micro-machining efficiency by combining a high repetition rate laser with an ultrafast z-scanner.
This creates an axial distribution for the focused beam that allows the pulses to be focused onto different surface locations of the material without real-time monitoring.
More importantly, the extended range for focal positions also relaxes the constraints on surface flatness and positioning.
The laser scanning speed also influenced the pitting potential and was attributed to the effect of power density. In summary a range of stainless steels show an increase in pitting potential due to LSM and can be attribute to enhanced surface chemistry and an increase in the homogeneity of the microstructure.
Laser Material Processing
Aluminium alloys have shown a mixed response in pitting potential due to LSM. While has shown an improvement in pitting potential due to LSM it was found that the most significant result was a change in the morphology of the pits to be shallower due to both the distribution of copper and the refinement of the microstructure [ 4 ].
This was attributed to an increase in the silicon aluminium boundaries caused by the refinement of the microstructure by LSM because these boundaries are more susceptible to the corrosion action.
Aluminium alloy shows improved pitting resistance due to LSM brought about by a more homogeneous microstructure which increased the initial resistance to pitting [ 7 ].
The poor corrosion properties of magnesium alloys have resulted in many investigations using LSM to improve the pitting potential. ACM has also been investigated [ 11 ]and it was found that the rapid solidification of LSM supressed of the formation of Al2Ca phase which is present in the cast alloy in the grain boundaries.
This change coupled with the extended solubility of aluminium in the microstructure increased the pitting potential. Rare earth containing alloy MEZ showed improved corrosion resistance [ 3 ]. It was suggested that the decrease in the anode cathode area due to the refinement of the microstructure coupled with the extended solubility of the rare earths increase the pitting corrosion resistance.
Alloy also responded well to LSM and due to its high chromium content and forms an excellent passivation film during testing [ 14 ].
This increase was attributed to the dissociating and refinement of large carbides and the increase of the passivation alloying elements in the ultrafine solid solution of austenite and martensite.
Metal matrix composites have also responded well to LSM [ 15 - 17 ]. The corrosion behaviour of metal matrix composites is due to heterogeneity of their structure.
Table of contents
LSM increase the homogeneity by dissolving the reinforcement phase. Cavitation erosion and corrosion Cavitation erosion corrosion occurs due to the formation and collapse of bubbles in fluid near the metallic surface. When a metal surface is subjected to high speed fluid flow sudden changes of liquid speed cause changes in the local vapour pressure which induces the formation and collapse of bubbles. The collapse of the bubbles generates shockwaves, which impact on the metal surface and leads to fatigue damage.
LSM has been used on several types of stainless steels to improve the resistance to this form of corrosion [ 18 - 20 ]. The response of different alloys varies for example S shows improved performance while S and S showed a decrease in the cavitation erosion corrosion performance. The decrease in performance for the two alloys was attributed to increased levels of twining in the two alloys. For martensitic stainless steel S the cavitation erosion corrosion performance was improved through increased levels of retained austenite, which for this alloy is highly transformable.
The stresses of the cavitation process are absorbed more readily due to the stress inducing a transformation of the retained austenite, hence increasing the resistance of the material to this form of corrosion. Stress corrosion cracking Stress corrosion cracking SCC is a form of cracking where an applied stress or residual stress in a component in combination with a corrosive environment acts to accelerate crack growth of the component.
LSM has been conducted on AA in two separate studies [ 21 , 22 ] to counteract this effect. Both studies found that the homogeneous refined microstructure produced by LSM delayed the onset of SCC when compared to the untreated material.
Electrochemical impedance measurements showed that the film resistance was higher than that of the untreated material and provided an effective barrier to corrosion attack. It was also found that a nitrogen atmosphere during processing forms AlN phase which improves the corrosion resistance of the film further. Laser surface alloying Laser surface alloying LSA is where a preplaced powder is melted with the substrate to form a layer with a combined composition.
The high cooling rates associated with the process novel microstructures are formed, which can exhibit improved corrosion performance. Like for LSM, process parameters are dependent on the thermo-physical properties of the substrate and alloying material as well as the processing speed.
The level of dilution plays a big role in the composition of the melted layer, which in turn can have a dramatic effect on the microstructure produced. There are several techniques for preplacing the alloying element s , the powder can be combined with a polymer to form a paste, which is then smeared onto the surface to be alloyed. Flame spray is another method to preplace the powder as is hot dipping.
One avenue to improve the corrosion performance using LSA is to alloy with an element, which forms a strong passivation film.
For example, alloying AA with chromium to improve its crevice corrosion performance [ 23 ], the alloying of ductile iron with copper [ 24 ] and the alloying of copper with titanium[ 25 ].
Advances in Laser Materials Processing
Another method is to alloy to form an intermetallic with alloying elements that exhibit good corrosion performance such as the alloying of steels with aluminium to form iron aluminides [ 26 ]. The area that has received the most attention is alloying to improve the cavitation corrosion properties.
LSA is ideal for this application because not only can the corrosion resistance of the surface be improved by alloying but the hardness of the surface can be improved as well, which are both properties required of a cavitation corrosion resistant material.
Aluminized steel prepared by LSA showed a 17 fold increase in the cavitation corrosion resistance [ 27 ]. The high hardness of the intermetallic layer and the shift in the pitting potential to a more noble level was used to explain the dramatic increase.
Variations in the power density during processing influence the cavitation corrosion behaviour and it was found that it was a compromise between cavitation erosion performance and corrosion, the harder layers exhibited improve cavitation corrosion and while the corrosion is improved by a more homogeneous microstructure.
A substantial investigation into LSA of stainless steel has been conducted by Kwok et al [ 29 ]. As would be expected a vast variety of microstructures were produced and were interpreted with reference to the appropriate phase diagrams. Essentially the layers alloyed with Co, Ni, Mn, C contained austenite as the main phase while alloying with Cr and Mo resulted in ferrite.
While the corrosion was improved in some cases, there was no underlying trend between the different alloying elements. This highlights the complexity of the microstructural formation in LSA and its dependence on process parameters. A traversing laser is used to heat the substrate and form a molten pool.
Into the pool either a wire or a powder is blown of a desired composition, which then melts and then quickly solidifies when past the laser beam, forming a new layer.
The spot size of the laser, the traversing speed, the powder feed rate and power of the laser influences the resulting clad layer.
By cladding materials that exhibit good corrosion performance the corrosion performance of a component can be increased.
This field of research has received limited attention. The cladding of stainless steels on mild steel has been investigated and like for laser surface melting the high cooling rates produce a fine microstructure, which coupled with the excellent corrosion properties of stainless steel the corrosion performance of the surface of the mild steel was improved [ 30 ].
The effect of La2O3 on ferritic steel was investigated [ 31 ] an proved to increase the corrosion resistance of the substrate by increasing the passivation effects of the film during testing. Nickel silicide as a clad layer have also been investigated [ 32 ]. This particular type of intermetallic displayed excellent corrosion response, which was attributed to the large amount of silicon which promotes the formation of highly dense and tightly adherent passive thin film when contacting with corrosive media.
Laser heat treatment Laser heat treatment LHT is where the laser heats the substrate without melting the top surface layer.
Diagrams for laser materials processing
The heating then allows solid-state phase transformation s to occur which when coupled with the high cooling rates of laser materials processing, new microstructures can be formed. This is a new field of laser material processing for improved corrosion performance and has the advantage over LSM in that the residual stress after processing is lower. Figure 4. Figure 5. Linear polarisation curves for as cast substrate and laser processed nickel-aluminium bronze. An example of this is laser transformation hardening of steels.
The heat from the laser causes the ferrite and pearlite to transform to austenite and then the rapid cooling provided by conduction to the substrate produces hard martensite. Typically this technique is uses to produce a wear resistant layer, however it has been applied stainless steel for improved cavitation corrosion resistance [ 20 ]. It was found that the ratio of austenite to martensite was better than for LSM.
Another example of this is the laser heat treatment of nickel-aluminium bronze NAB. LC of NAB produces the Widmanstatten morphology microstructure, which has poor corrosion properties [ 33 ]. Whereas the LHT of NAB produces a microstructure that is free form precipitates, Figure 4 , which is responsible for the reduced corrosion performance and is free from the Widmanstatten morphology microstructure [ 34 ].
This new type of microstructure exhibited improved corrosion properties as shown by the linear polarisation results, Figure 5. Hybrid coatings Laser melting or heating can also be used in conjunction with other coating techniques.
Thermal spray which covers a range of processing techniques applies coatings to metallic substrates by heating a powder and propelling into a substrate. Each individual particle forms a splat on the substrate and the accumulation of many splats result in a coating. While these coating have found many applications including thermal barrier coatings and wear resistant coatings they have flaws. They are that the coatings have poor bond strength between the coating and the substrate, and high porosity.
Laser melting offer the opportunity removes these two issues and so hybrid coating applications have been investigated. In relation to developing coatings for improved corrosion resistance the hybrid thermal spray laser melting have found applications with Ni coated WC coatings [ 35 , 36 ], thermal barrier coatings [ 37 , 38 ] super alloy WC coatings [ 39 ] Al-SI alloy coatings on AZ91D magnesium alloy [ 40 ] and NiCrBSi alloys [ 41 , 42 ].
For all these applications the laser melting consolidated and homogenised the coating which improved the corrosion performance.
In some cases if the wrong laser parameters were used cracking of the coating occurred mitigating the effect of reducing the porosity and should be considered when developing this type of coating. The second group of hybrid coatings and one that has received less attention is electroplating followed by laser melting.
Steen W.M., Mazumder J. Laser Material Processing
Electroplated coatings like thermal spray coatings have porosity and therefore their corrosion performance is reduced. Laser melting of the coatings consolidates the porosity and improves their performance. Examples of this are Ni-W-P coatings [ 43 ] and gold coatings [ 44 ].
Superhydrophobic surfaces produced by femtosecond lasers Superhydrophobic surface are a biomimetic surface structure where nano scale features on their surface cause a hydrophobic response [ 45 ]. These surface can be produced on metallic materials using femtosecond laser technology pulsed laser ablation [ 46 ].Shinmi, Proe. Laser Treatment of Materials edited by B.
Arata and I. Easterling, Acta metall. This can be applied to the welds of stainless steels to restore corrosion performance. Practising engineers and technicians in these areas will also find the book an authoritative source of information on the ever- expanding use of industrial lasers in material processing.
Eager and N.
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