The Alternative Methods of Improving Surface Finish during the Building Process Parts - Term Paper Example

 The Alternative Methods of Improving Surface Finish during the Building Process Parts Introduction Manufacturing technologies and product development all aim at improving the productivity by enhancing the product quality and efficiency, through the whole manufacturing process, from the detail design and theory to the packaging and quality control. Additive manufacturing (AM) technology have offered new insights into the development and design of products, as ergonomic models and visual aesthetic can be developed with minimal tooling and labor requirements (Bartolo, 2007). Since the inception of Rapid Prototyping (RP) processes, the technology has gone a notch higher and the increase of the accessibility of the manufacturing grade materials. The resultant effect has been the growth of interest in utilization of the technique to create end-use parts; the technique is referred to as the Rapid Manufacturing (RM). RM is an emerging manufacturing technique and it has lead to a number of research areas in the engineering industry and academia. Selective Laser Melting (SLM) is an additive manufacturing process that has been advanced from the Selective Laser Sintering (SLS) into a process that has the capability of manufacturing low volume, high value, end-use sections from the ever increasing inventory of alloys and metals (Daniel 2009). Additive Manufacturing AM is an overall term that is used to describe all the technologies that are used to manufacture parts by the addition of material in layers as opposed to the traditional subtractive methods or processes that involved the removal of material according to (Daniel 2009). In the past, AM technology was confined to the manufacturing of prototypes and models that led to the widely accepted term Rapid Manufacturing (RP) and it used to refer to the all layer additive manufacturing process. The advance of processes, machine hardware, and materials meant that the parts could be developed with enough mechanical properties that permit functional applications (Gibson, Rosen & Stucker, 2009). This had led to the adoption of Rapid Manufacturing (RM), to show the differences between the fully functional characteristics of the sections being manufactured from the prototypes and RP models. Additive Manufacturing is the overall term that is used to describe Rapid Manufacturing and Rapid Prototyping and their description in the application of the Additive Manufacturing technology (Daniel 2009). SLM Problems Selective Laser Melting is a recently created technology. The technology is used to manufacture full density metal part with properties that are comparable to or better than the wrought materials. In the SLM process, the laser beam completely melts down the powder particles. The SLM technology is capable of realizing rapid manufacturing of end-use complex metallic parts little treatment or any post-processing. The most significant variables controlled during selective laser melting include layer thickness, beam offset, laser energy, scan velocity and exposure strategies. From the mentioned characteristics, the surface energy and the required energy per unit length can be easily derived. The powder density and the laser energy are the most significant process variables because they increase the powder’s laser energy high temperatures. The mechanical strength, the density and the liquid phase proportion of the laser sintered part increases. The scan velocity and the beam offset play a significant role in the buildup of the speed, the part strength and in association with the particle sizes; it determines the surface roughness of horizontal surfaces. The vertical surfaces quality is affected by the special exposure strategy (Tolksdorf and Westkamper). The metallic powder particles are molten completely in order for the material to reach high density; the laser melting process accompanies the creation of the residual stresses that find their origin from the thermal gradients found within the materials. The stresses led to the part distortion, delaminations, and cracks. Vaporization is another phenomenon characteristic of this process and it happens when the powder bed is irradiated with energy of high intensity. In the laser melting process, the temperature exposed to the powder particles by the laser beam is high than the material melting temperature. An additional increase in the temperature (which is now twice the temperature of the material) leads to the powder evaporation; the evaporated particles expand very fast and generate an overpressure on the melted region and the material is removed from its bed. Spheroidization or balling is another problem that may happen during machining; balling is the creation of isolated areas that have an equal diameter with the focus of the laser beam and it inhibits the deposition and reduces the density of the created part. It happens when the molten material is not capable of fully wetting the substrate because of the surface tension. The phenomenon is as a result of an excessive amount of energy that gives the melted powder viscosity that is too low (Sabina, Nicola, Andrea and Antonio, nd). In order to minimize these problems, some technologies have been adopted in the SLM (Selective Laser Melting) process. To minimize residual stresses and distortions, the powder in the construct platform is sustained at an elevated or high temperature (temperature that is below the melting point of the material that has been powdered) (Yasa, Kempen, Kruth, Thijs, and Van-Humbeeck, 2009). Other technologies that have been adopted to offer solutions to these problems include the inclusion of the preheating phase and it is utilized in the minimization of porosity on the construct piece. The following are the most common solutions; (a) infrared heaters – they are placed near the building platform to sustain the elevated temperature that is around the part being created, (b) feed cartridges – they are used to preheat the powder before spreading to the build area, and (c) resistive heaters – they are about the building platform. The sustenance of a uniform and elevated temperature and the preheating of the powder are also used to reduce the power laser needs of the process, to enhance the absorption of the laser beam, prevent distortion of the part in the building due to the non-uniformity of the thermal contraction (curling) and expansion, and to minimize the temperature gradient and enhance the wetting properties (Sabina, Nicola, Andrea and Antonio, nd). Powder preheating is utilized in the enhancement of the laser absorption in order to minimize the gradient temperature and enhance the wetting properties. The preheating solution requires a cooling time. If the powders and the high temperature sections are exposed prematurely to the external atmosphere, there is a high probability of degradation because the powder and the present oxygen cannot be used for additional processing and recovery is not possible. The construct objects can distort due to the non-uniform thermal contraction. There is need to permit the parts to get to a sufficient low temperature that is uniform and can be taken care of and exposed to atmosphere and ambient temperature. The last step is the removal of the parts from the powder bed and the loose powder is removed from the parts and additional finishing is done if it is necessary (Sabina, Nicola, Andrea and Antonio, nd). Selective Laser Melting and Surface Improvement Surface improvement usually involves smoothing of the material used for making a product for a variety of reasons. Surface improvement may be done to reduce friction, improve the rate of wear, reduce resistance to air, for decorative purposes and as a preliminary activity before plating (Mumtaz, Hopkinson, & Erasenthiran, 2006). Surface improvement may be accomplished during the production of a component or alternatively once the component has already been produced. Surface improvement or finishing is usually done to engineering products in most cases as a final step before the component leaves the production line. Selective Laser Melting as an additive technique used in the manufacture of products has numerous advantages. However, one of its greatest limitation is with respect to its insufficiency in so far as surface quality is concerned. Other of its limitations regard balling, the staircase effect, poor accuracy in terms of dimension of finished products and the existence of residual stresses in products made by this method as noted by Mumtaz, Hopkinson, & Erasenthiran (2006). A number of methods can be used to enhance the surface quality of products that are manufactured through Selective Laser Melting. The alternative methods that can be used in this respect include mechanical, thermal and chemical processes. Yet another valuable alternative in this respect is laser remelting which notably is a clean and reproducible process given that the process has makes it possible to have better control of variables that impact surface finish. It is noted that laser remelting has the capacity to improve up to 90 percent of the total roughness values, averagely, of the surface produced by Selective Laser Melting (Khan and Dickens). Laser remelting may not only be applied in the modification of surfaces that have roughness values between five and eight but can also apply in improving other surface properties including corrosion resistance, wettability, microhardness, wear behavior and fricton. Mechanical, Thermal and Chemical Processes of Surface Improvement The chemical processes that are commonly applied in the surface improvement of SLM machined parts include oxidation and acid etching. Mechanical processes that applied in this respect include abrasive sandblasting and machining. Plasma spraying features as a thermal process that can alternatively be used in improving the qualities of parts or products manufactured by SLM (Mumtaz K., Hopkinson, & Erasenthiran, 2006). Etching involves the use of strong mordant or acid to chop of unprotected parts of a metal’s surface. Acid etching may be accomplished in two stages for the attainment of superior finish. First, the material may be etched to a matt finish. This is then followed by another type of etch so that a clean finish is brought back in some areas. On the other hand, during abrasive blasting, a stream of coarse material is channeled under high pressure to a surface that needs to be smoothened, roughened, shaped or cleansed of contaminants (Mumtaz K., Hopkinson, & Erasenthiran, 2006). Although various materials may be used as abrasive material, including soda and beads, during the blasting process, sand is most commonly applied possibly due to its easy availability in many locations. Materials or products may also be machined using smoothing machines so as to improve their surfaces before or after they have been machined by SLM. The surfaces can be ground or smoothed using smoothing machines. Thermal spraying is a coating process in which molten materials are, onto a surface, sprayed. The coating precursor is usually heated electrically or chemically, the material being used to form the spray always fed in wire or powder form. During plasma spraying, the feedstock is usually fed in various alternative forms as suspension, liquid, wire or powder. The feedstock is introduced into a plasma jet created by a plasma torch, and is sprayed at about 10,000 Kelvins, in molten form projected toward the substrate (Mumtaz K., Hopkinson, & Erasenthiran, 2006). The droplets of molten material fast deposit and form a layer of deposit. This method of surface improvement depends on a number of factors including substrate cooling, distance of torch offset, amount of energy used, flow rate, type of material used as feedstock and the composition of the plasma gas. Oxidation generally is the chemical reaction that involves addition of oxygen to an element of compound. Oxidation in real sense occurs when the oxidation number of atoms change in what is commonly known as a redox reaction. The chemical process may be applied in the removal of unwanted parts of a material. Conclusion Selective laser melting is a technology that is very promising; this is because of the nearly unlimited geometry it offers especially for the tooling applications. The technology offers the capability of producing complex geometries with inner cavities such as the conformal cooling channels. Conformal cooling channels are generally difficult to construct by the conventional machining techniques but they permit the main reduction of injection molding cycle period. The selective laser melting process provides a new opportunity in the tooling techniques by developing parts that have nearly full density and geometrical complexity with high freedom. The selective laser melting (SLM) machines can be used extensively in a number of applications such as the finishing of parts and the creation of complex parts. This can be attributed to the various characteristics this type of technology has and the ability of this technology to be integrated into the machining process. SLM is applied in both the ceramics and metals technological tools. SLM is a type of additive manufacturing technology rather than the traditional subtractive method. Its functioning is based on the laser technology; the technology permits the machine to add thickness from 20µm to 200µm. Although the machine is said to be efficient, there are a number of problems that can affect the proper functioning of the machine. These problems include balling, residual stress and other factors that affect the functioning negatively. When using these machines, one must take into consideration all these problems and factors and deal with them in the right way. When utilized fully, SLM can be a technology that is very useful. The limitations associated with the surface quality of products made using SLM technology can be corrected using various methods. The alternative methods that can be applied in this case include machining, plasma spraying, sand blasting, and acid etching. These processes can be carried out during or after the SLM have been done to the specific being made depending on how much applicable they are with respect with surrounding circumstances. References Bartolo, P. Virtual and Rapid Manufacturing: Advanced Research in Virtual and Rapid Prototyping. London, UK: Taylor & Francis, 2007. Gibson, I., Rosen, D. & Stucker, B. (2009) Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing. New York, NY: Springer. Khan, M. and Dickens, P. “Selective Laser Melting (SLM) of Pure Gold.” Gold Bulletin 43, no. 2 (2010): 114-121. Mumtaz, K. A., Hopkinson, N. & Erasenthiran, P. “High Density Selective Laser Melting of Waspaloy.” Loughborough University (2006): 220-232. Sabina L., Contuzzi, N.., Angelastro, A. and Ludovico, A. (nd) New Trends in Technologies: Devices, Computer, Communication and Industrial Systems. Italy: Polytechnic of Bari, n.d. Thomas, Daniel. “The Development of Design Rules for Selective Laser Melting” Ph.D. Thesis, University of Wales Institute, 2009. Tolksdorf E. and Westkamper N. (2006) “Developments in Precision Product Manufacturing for Laser-Sintering” (presentation, Third International Conference Multiscale Materials Modeling, Freiburg, Germany, September 18-22). Yasa, E., Kempen, K., Kruth, J., Thijs, L., and Van Humbeeck, J. (2009) “Microstructure and Mechanical Properties of Maraging Steel 300 after Selective Laser Melting.” Catholic University of Leuven (2009): 383-396. Read More