Selective Laser Melting Process - Literature review Example

SELECTIVE LASER MELTING (SLM) PROCESS NAME COURSE DATE Selective Laser Melting (SLM) Process It is important to review the existing body of literature of SLM and its application in the production of Ti6Al14V alloys. It is because; the existing literature body will provide a deeper understanding of SLM processes, Ti6Al14V alloys and geometry formation parameters. Additionally, the literature will not only shape the objectives of the study topic by identifying the missing aspects in the research projects conducted on SLM, but also provide various models of solving the study topic. There are vast research findings on the performance or rather properties of SLM process in fabrication of various functional parts forged from various mechanical components such as metals, metallic alloys and other substances.1 In this context, SLM refers to a modern technique used to fabricate functional parts in layers, one after the other, using powders. It is common due to its cost-effectiveness in the construction of strong and complex structures. In this regard, the powders used in the process are bound strongly to the already treated underlying layer through continuous and point-directed heating coupled with melting by a scanning laser beam. However, the powders must form a thin layer over the treated surface before SLM process.2 Therefore, the initial heating and re-melting of the powder layer in the scanning region of the laser beam forces the powders to form a strong adhesive bond with the underlying layer of the substrate material thereby producing a finished product with desirable mechanical characteristics. However, to ensure that the SLM process yields desirable functional parts, several parameters must not only be put into consideration, but also optimized with appropriate standards.3 These parameters mainly include the thickness of the powder layer, the density of energy, the laser scanning speed and the laser beam diameter, among others. Therefore, the imbalances of the parameters might produce undesirable finished products that are weak structurally. For instance, a research experiment conducted to estimate the effect of parameter changes on temperature distribution at the point of interaction between the laser beam and the powder layers produced weak functional parts.4 In this regard, the experiment involved the study of a single-line scan on an unconsolidated thin powder layer. Table1: An overview of the process parameters, the resulting energy and relative densities for the different samples, three different scanning strategies were applied, namely identical layers with the zigzag scan vectors (zz) or unidirectional scanning vectors (uni) and the cross-hatching strategy (cross)(Grigoriev, S. et al.). The experiments revealed that scanning velocities’ interval are common where the re-melted powder trails are homogeneous. This means that the trails break whenever the velocity of scanning dropped outside the standard interval. It is negative effect termed as the ‘balling effect’ that produces undesirable properties. The experiment portrays that during SLM process the failure to consider some parameters may produce unstable functional parts.5 Hence, the researchers in this particular case employed a radiation-coupled numerical approach and transfer of heat to analyze the prospected instability effect of balling effect.6 Graph 1: Intensity for Al in EDX measurements along a vertical line in front view of sample E, starting at the top of the sample (Grigoriev, S. et al.). In the findings, they established that the melt pool Plateau-Rayleigh capillary instability model best explains the balling effect at relatively high laser scanning velocities (approximately higher than 20 cm/s with respect to the experiment’s condition in their particular research methodology). Whereby, only two parameters stabilized the process and eliminated the balling effect through the reduction of the laser scanning velocity. They include decreasing of the melt pool’s ratio of length-to-width and increasing the width contact of laser scanning beam and the immediate layer of the substrate. It is therefore clear that the parameters of laser-directed melting are important in the SLM process. Additionally, researchers show that SLM plays an important role in laser cladding methodologies. The cladding methodologies are attributed to the deposition of alloys or rather the coating non-metallic materials on metals.7 Graph 2: Vickers microhardness of samples A, B, C, E and F plotted against the respective energy densities applied (Grigoriev, S. et al.). The cladding methodologies allow the formation of a metallurgical bond that effectively binds two or more different materials together to yield products with finer grains and free of microspores. That is why SLM is superior over other coating techniques such as Atmospheric Plasma Spraying (APS) and High Velocity Oxy-Fuel (HVOF) among other methods in both tribological properties and strong adherent coatings. Additionally, research shows that SLM technique of cladding mechanical materials produces a reduced thermal load on the substrates’ layer which generates a very low regions affected by heat (Heat Affected Zones, HAZ ).8 Hence, HAZ causes very few distortions on the particular material. Table 3: Composition and properties for the materials employed. Extrapolated values are in italics (Gusarov, AV. et al.) These and other advantages have encouraged the engineering field to explore SLM as a sole method of coating metals. The coating is not limited on refurbishment of used materials, but also extends up to the direct industrial manufacturing processes. Apart from the effects noted earlier due to the variance of parameters, stresses generated during SLM also play a crucial role. The stresses inside the material originate from the increased temperatures of the laser scanning beam and the incompatibility of two different materials.9 Graph 3: Residual stress profile over the height of the sample near its centre, starting at the bottom of the plate, for the sample without preheating, measured by EDXRD, b) predicted by the 3D model (Suarez, A. et al). The stresses cause undesirable properties in the finished product by forming distortions and cracked openings. Therefore, some researchers proposed the use of the finite element method as an approach that shows potential in providing a better understanding of the SLM process. A better understanding of the SLM process will ensure that all parameters are optimized before conducting any coating technique using SLM so as to produce high quality and desirable mechanical products. However, the practical validation of the finite element method and other thermomechanical simulations proves futile. Therefore, in pursuit of a better understanding of SLM the researchers in this context employed a transient nonlinear thermomechanical FEM approach.10 In the validation of the experiment, the researchers used the energy dispersive X-ray diffraction method to take measurements in the equipped facility of European Synchroton Radiation Facility (ESRF). Graph 4: Residual stress profile over the height of the sample, near its centre, starting at the bottom of the plate, predicted by the 2D FEM model (solid lines) and measured by EDXRD (Suarez, A. et al). They compared the results of the tests performed on the samples of Stellite 6B alloy and the stainless steel ALSI 304. In this context, they employed both single track and multi-track analyses unlike in the initially discussed process. They proved that the energy dispersive X-ray diffraction method provides a better understanding and measurement of the SLM stress compared to other methods such as the 3D X-ray microscopy technique that only reveals hydrostatic stresses at only 500MPa.11 However, as the SLM becomes more viable in the mechanical industry and other engineering field, the optimization of its parameters remains elusive due to the complexity of the whole process and the lack of a better model to aid in understanding the process. Graph 5: (i) PH Hardening curves, (ii) PH Young modulus and expansion coefficient (Pyka, G. et al.) It is because; from the previously researched contents, the scientists have only used a particular model to study a single parameter at a given time before, during and after the cladding process. This means that there is still a big gap open for scientists to research and establish the most viable models of studying and understanding the SLM process so as to ensure that the modern SLM process are optimized. For instance, the recent advances in developing study models such as the energy dispersive X-ray diffraction method helped researchers in the study of hypereutectic Al-Si-alloys structure formation using SLM cladding technique.12 The Al-Si-alloys are important in the automotive and more particularly in the manufacturing industries for creation of tools. It is established that high content of Si in the alloy increase the wearing resistance properties of the material which demands the manufacturers to pay close attention or rather increase the quantity of Si used. However, the conventional coating methods used in the production of the alloy require the Si content to remain below 20 wt. % due to the formation of big Si grains (more than 30 micrometers) that directly influences the mechanical properties of alloy.13 This means that large grains prevent the formation of strong adhesive bonds between the distinct metals of the alloy. However, researchers showed that SLM technology circumnavigates the limits of Si content in the alloy. Whereby, it enables the manufactures to refine the structures of the alloy via brisk solidification process. In this experiment, the researchers used single track layering width of above 3mm and below 5mm. Graph 6: Thermal and cumulated plastic strain variation in the element at the part centre (Pyka, G. et al.) It called for an advanced research on single track layering with a reduced width and high Si content, that is, less that 500 micrometers and 30-60 wt. % respectively. Also, during the experimental tests the scientists studied closely the structural formation of the Al-Si alloys in terms of grain size distribution, microstructure of powders and surface morphology among other factors. The research findings portrayed that it is possible to produce the Al-Si alloys with single track layered Al widths of less than 500 micrometers and Si content of more than 30 wt. % and less than 60 wt. % using the SLM process.14 Therefore, the experiment in this case showed that SLM defies the conventional methods of plating; however, it failed to establish a viable model of understanding the parameters of the whole process. This shows that more result-driven research methodologies have to be pursued so as to demystify the SLM process. Table 4: Clad track geometry variation vs. powder feed rate (P = 200 W, v = 800 mm/min)(Thijs, L. et al.) Table 5: Clad track geometry variation vs. laser power (v = 800 mm/min, Pm = 10 g/min) (Thijs, L. et al.). Table 6: Clad track geometry variation vs. beam speed (P = 150 W, Pm= 10 g/min) (Thijs, L. et al.). That is why several mechanical engineers have undertaken the task of discovering different aspects of SLM based on its immediate applicability such as in production of Al-Si alloys so as to find appropriate knowledge that can help in the optimization of the parameters within that specific mechanization process.15 It is due to this specialty ventures that the research on SLM process has narrowed over time to deal with a specific parameter at a time. In this context, the most recent research entailed the observation of the surface morphology and roughness in the customization of the manufactured additive structures of open porous Ti6Al14V.16 The Additive Manufacturing (AM) enables the engineers to construct porous materials using a controlled system of geometry. However, the conventional AM processes does not allow surface-finishing the refurbished material. It forced researchers to look for alternative approaches to solve this limitation. They used Design of Experiments (DOE) to finish the surface of a post AM material using a 3D SLM. The DOE employed in this particular case involved a two-level and three-factor complete factorial model. The model assessed the distinctive features of surface treatment duration and chemical etching solution concentration on the resultant surface morphology and laser beam thickness of the refurbished porous materials.17 The engineers finally established that the amount of the surface treatment solution played a major role in influencing the reduction of roughness in the final Ti6Al14V product. On the other hand, when the engineers increased the width of the laser scanning beam, the rate of roughness reduction decreased. This means that an increase of the laser scanning beam width in the refurbishment of an already coated material decreases the activity of the surface treatment solution. This is an important insight or rather literature review information not only in the manufacture of Ti6Al14V, but also in the design of research models that aim at characterizing a given aspect of the Ti6Al14V alloys.18 Therefore, every milestone covered in the study of SLM does not only provide insight on a particular parameter or model, but also creates a route that allows other researchers to pursue and develop models of understanding and optimizing the SLM process. For instance, in the above discussed research experiment the engineers suggested that the post AM treatment of Ti6Al14V requires a standard width of a laser beam coupled with a per-determined concentration of the surface treatment solution.19 The engineers in this particular research experiment created a route for researching of the SLM single track parameters and geometry associated with the production of Ti6Al14V materials and other metal alloys. Graph 7: Diffraction pattern of AlSi30 powder (Pyka, G. et al.) Moreover, they showed that there is a lot of information that lacks on the complete production of Ti6Al14V. In this regard, they meant that SLM does not influence the outcome of the whole cladding process; there are other standardized factors that must accompany SLM for effectiveness and efficiency of the process. In another important research article of Ti6Al14V, the researchers investigated the method of improving the density of Ti6Al14V alloy via the use of SLM process.20 The conventional processes use ordinary inert gas that yield Ti6Al14V alloys with low density. Therefore, they used an inlet baffle to deliver a modified inert gas during the SLM process. Their findings revealed that the modified inert gas influenced the properties of the Ti6Al14V final products. This means that the modified inert gas reduced the porosity of the alloy up to less than 0.1 % by a given square area. Additionally, the SLM-forged Ti6Al14V alloy microstructure showed martenstic alpha-prime phase. This means that the alloy portrayed resultant tensile strength of 920 up to960 MPa coupled with an elongation of 3-5% at the structural fracture.21 In a nutshell, the fracture morphological surfaces of the tensile products demonstrated combined properties of fracture brittlineness and ductility. Therefore, the research experiment resolved part of the concerns that surround the use of SLM methods in production of alloys especially Ti6Al14V. Prior to the above discussed experiment studies had shown that high-density alloys of Ti6Al14V could not be created or achieved by the SLM method. The limit in this case was attributed to the additive properties of the SLM method. Whereby, the variation in the properties of the metals used to create the alloy discouraged the formation of strong bonds between layers that directly determine the density of the alloy. In this regard, tough bond increase the density of the alloy while weak bonds reduce its density. Hence, the experiment resolved the density issue that had limited the application of SLM in the production of high-density Ti6Al14V alloys.22 In other words, the experiment revealed a parameter which is necessary in single track layering of Ti6Al14V alloy components for the production of different densities of the material. Hence, this research experiment on the density parameter is important in any study topic that involves Ti6Al14V alloys and SLM processes.23 Table 7: Tensile test results of the Ti6Al4V alloy in different processing directions (Pyka, G. et al.). Alternatively, the research on the density variation of Ti6Al14V alloys created an understanding of the supporting parameters for effective SLM process. In this regard, there are several extended research on the properties of the Ti6Al14V alloys produced via the SLM method. The properties in this context are not limited on the parameters that influence SLM process rather they indulge the geometrical outcome of the Ti6Al14V alloys. For instance, scientists conducted a study on the evolution of the micro-structural morphology of Ti6Al14V alloy during the SLM process.24 They revealed unique characteristics that play an important role in the resultant alloy alongside the influence of the parameters. Whereby, they revealed that by standardizing the ordinary parameters, employing the martensitic phase and ensuring the continuous occurrence of the epitaxial growth, elongated particles emerged on the surface of the alloy during the SLM process. However, contrarily to the expectations of ordinary SLM procedures the direction of the elongated particles depended on the standardized SLM process parameters.25 Additionally, the scientists noted that high inputs of heat precipitate Ti3Al in the internetallic phase. In the above discussed experiment, the scientists resolved not only the issue of fast solidification in SLM processes, but also resolved the issue of internal heat build-up in the Ti6Al14V alloys which inhibits proper definition of the final product. Whereby, the thermal stress and fast solidification results into formation of non-equilibrium phases between the layers of the alloy.26 However, the experiment revealed that consistency between the phases can be achieved by optimizing the external parameters rather than the properties of SLM laser beam. Hence, as the information on SLM process of manufacturing Ti6Al14V alloys via single track layering increases due to multiple parameter research projects, the production of Ti6Al14V alloys increases due to the increase understanding of the parameters needed in the production of high quality and desirable alloys.27 Also, the research findings increase the number of models that are important in determining the optimal parameters of producing other alloys of mechanical importance other than Ti6Al14V. Hence, it is important to note that SLM cladding provides a wide scope of scientific study that will at the end increase the benefits of SLM process. Current statistics reveal that many industrial processes employ SLM coating procedures due to its advantages over the traditional procedures. They include reduced time of marketing, cost-effective production procedures, high utilization of the substrate materials, increased degree of flexibility and direct manufacturing procedures. Graph 8: Strain-stress curves of for samples prepared Figure 17. Fracture structure of samples built in Y and Z direction (Pyka, G. et al.). On a final note, the material specific research increases the availability of different models that can be easily studied to produce variance characteristics in Ti6Al14V alloys during SLM process. In this regard, the research recommendations reveal that there is still too little information on the process and geometric characteristics of single track on Ti6Al14V which forms the basic topic of research.28 This means that many studies according to the above discussed information are based on the parameters that surround the SLM process. The researchers have yet to narrow down to the geometric properties that are associated with single track layering of Ti6Al14V alloys. It is because; the scientific research is yet to establish constructive and reliable models of understanding and controlling the parameters of SLM to allow the comparison of the geometrics and the parameters. In this context, it is important to highlight that there exist few research findings that slightly compare the SLM produced Ti6Al14V alloys with its resultant geometry. Table 8: Shows an overview of the experimental conditions used for the three-factor two-level full factorial method (Pyka, G. et al.) For instance, in a recent experiment that involved analysis and modelling of laser direct metal deposition of SLM-processed Ti6Al14V alloy, the results showed that the laser direct metal deposition (LDMD) approach portrayed a major strength that can produce freeform geometries of Ti6Al14V alloys. Moreover, the LDMD created different and distinct surface coatings on the finished product. Also, the findings reported that LDMD converted porous graded metallic substrates into high density Ti6Al14V alloy samples. This SLM technique is important in the aerospace industry and biomedical field due to its adaptability, cost-effectiveness and flexibility. Therefore, LDMD presents an advanced stage in the evolution of SLM coating technique that has limited hindrances of both extrinsic and intrinsic parameters. However, it is important to note that LDMD was not possible in the initial launch of SLM processes due to the limitation in the models of understanding the parameters.29 Hence, as the research on Ti6Al14V production narrows down to the individual parameters, it important for the scientists to find the relationship between the geometry properties of single track layered Ti6Al14V alloy and the surrounding SLM parameters. That is why there is need to carry a research on the process and geometrics characteristics of single track on Ti6Al4V. Bibliography Dinda, GP. et al., "Fabrication of Ti-6Al-4V scaffolds by direct metal deposition", Metallurgical and Materials Transactions A, vol. 39, no. 12, 2008, pp. 2914-2922. Grigoriev, S. et al., "Structure formation of hypereutectic Al-Si-alloys produced by laser surface treatment", Strojniški vestnik-Journal of Mechanical Engineering, vol. 60, no. 6, 2014, pp. 389-394. Gusarov, AV. et al., "Heat transfer modelling and stability analysis of selective laser melting", Applied Surface Science, vol. 254, no. 4, 2007, pp. 975-979. Murr, LE. et al., "Microstructure and mechanical behavior of Ti–6Al–4V produced by rapid-layer manufacturing, for biomedical applications", Journal of the mechanical behavior of biomedical materials, vol. 2, no. 1, 2009, pp. 20-32. Pyka, G. et al., "Surface roughness and morphology customization of additive manufactured open porous Ti6Al4V structures", Materials, vol. 6, no. 10, 2013, pp. 4737-4757. Suarez, A. et al., "Study of residual stresses generated inside laser cladded plates using FEM and diffraction of synchrotron radiation", Surface and Coatings Technology, vol. 204, no. 12, 2010, pp. 1983-1988. Thijs, L. et al., "A study of the microstructural evolution during selective laser melting of Ti–6Al–4V," Acta Materialia, vol. 58, no. 9, 2010, pp. 3303-3312. Read More

Table1: An overview of the process parameters, the resulting energy and relative densities for the different samples, three different scanning strategies were applied, namely identical layers with the zigzag scan vectors (zz) or unidirectional scanning vectors (uni) and the cross-hatching strategy (cross)(Grigoriev, S. et al.). The experiments revealed that scanning velocities’ interval are common where the re-melted powder trails are homogeneous. This means that the trails break whenever the velocity of scanning dropped outside the standard interval.

It is negative effect termed as the ‘balling effect’ that produces undesirable properties. The experiment portrays that during SLM process the failure to consider some parameters may produce unstable functional parts.5 Hence, the researchers in this particular case employed a radiation-coupled numerical approach and transfer of heat to analyze the prospected instability effect of balling effect.6 Graph 1: Intensity for Al in EDX measurements along a vertical line in front view of sample E, starting at the top of the sample (Grigoriev, S. et al.).

In the findings, they established that the melt pool Plateau-Rayleigh capillary instability model best explains the balling effect at relatively high laser scanning velocities (approximately higher than 20 cm/s with respect to the experiment’s condition in their particular research methodology). Whereby, only two parameters stabilized the process and eliminated the balling effect through the reduction of the laser scanning velocity. They include decreasing of the melt pool’s ratio of length-to-width and increasing the width contact of laser scanning beam and the immediate layer of the substrate.

It is therefore clear that the parameters of laser-directed melting are important in the SLM process. Additionally, researchers show that SLM plays an important role in laser cladding methodologies. The cladding methodologies are attributed to the deposition of alloys or rather the coating non-metallic materials on metals.7 Graph 2: Vickers microhardness of samples A, B, C, E and F plotted against the respective energy densities applied (Grigoriev, S. et al.). The cladding methodologies allow the formation of a metallurgical bond that effectively binds two or more different materials together to yield products with finer grains and free of microspores.

That is why SLM is superior over other coating techniques such as Atmospheric Plasma Spraying (APS) and High Velocity Oxy-Fuel (HVOF) among other methods in both tribological properties and strong adherent coatings. Additionally, research shows that SLM technique of cladding mechanical materials produces a reduced thermal load on the substrates’ layer which generates a very low regions affected by heat (Heat Affected Zones, HAZ ).8 Hence, HAZ causes very few distortions on the particular material.

Table 3: Composition and properties for the materials employed. Extrapolated values are in italics (Gusarov, AV. et al.) These and other advantages have encouraged the engineering field to explore SLM as a sole method of coating metals. The coating is not limited on refurbishment of used materials, but also extends up to the direct industrial manufacturing processes. Apart from the effects noted earlier due to the variance of parameters, stresses generated during SLM also play a crucial role.

The stresses inside the material originate from the increased temperatures of the laser scanning beam and the incompatibility of two different materials.9 Graph 3: Residual stress profile over the height of the sample near its centre, starting at the bottom of the plate, for the sample without preheating, measured by EDXRD, b) predicted by the 3D model (Suarez, A. et al). The stresses cause undesirable properties in the finished product by forming distortions and cracked openings. Therefore, some researchers proposed the use of the finite element method as an approach that shows potential in providing a better understanding of the SLM process.

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