Section 1:
Introduction
Direct metal laser sintering process involves the use of computer aided design to achieve the desired geometry of a part after which the geometry is transferred on a printing plate on a multiple axis where the component material is fed in grain sizes each layer at a time in varying thicknesses. The deposited material is heated via a high energy laser where it melts partially and sintering occurs to form a solid component (1 p. 85).
Metal components can be manufactured using direct metal laser sintering process, which is one of additive layer manufacturing technologies. However thermal stresses in these components can lead to cracks. However the manufacturing process can be optimized; using parameters including laser power, components size, material properties, building speed and layer thickness.
Laser sintering involves the process of producing parts by lying layer by layer of a working powder, which solidifies. While it’s notable that fatigue in metal part production is a common phenomenon, designers utilize annealing methods and heating processes to relief stress in metals.
Fatigue samples are always built in double conic and cylindrical shapes after which they are observed under electron micrograph to identify possible locations of initiation of cracks and fatigue points (2 p. 35) (3 p. 97).
Bártolo, Paulo. Virtual and rapid manufacturing : advanced research in virtual and rapid prototyping. New York: Taylor & Francis, 2008.
Huamin, Cao. Laser Technology. Boca Raton, Fla: CRC Press/ Taylor & Francis, 2010.
Additive manufacturing has been responsible in the production of complex niche parts as those used in medical and aero parts and industrial fields to impart fracture and wear resistance, reduce corrosion and to achieve biocompatibility as with use inside biological tissues. In selective laser melting, titanium alloys find wide range of uses following research in fractography that has proven to give good results (4 p. 134) (5 p. 153).
While additive manufacturing hold the key to complexity, its success can be attributed to its capabilities to utilize the processing powers of a computer, the graphics capability that ensure precision and versatility. The use of high capability graphics user interface has not only been of great importance in gaming but in design too. Computer aided designs are able to be designed and reproduced in a 3D form with ease enabling complexity to be achievable.
The use of computer numerical control machines does give precision to parts enabling high calibre machine control with the use of high frequency sensors for immediate feedback. Computation that is done real-time enables machine to use logic instantly by use of precision motors, lenses and mirrors to place laser in exact positions (5 p. 236) .
Lasers for heating requirement carry high energy in the ultraviolet range to cut through metal and cause metal fusion and melting in localized positions without heat build-up and thus the part is formed with minimal distortion due to thermal stresses. Regarding the setting of thermal stresses, key is the step by step procedure involved in selecting the metal powder to be used (6 p. 435). After this metal analysers record the particle sizes to establish size distribution between D10 and D90 values, usually the particles follow a normal distribution.
laboratories, sandia national, et al. Laser Engineered Net Shaping; a tool for direct fabrication of metal parts. washington DC: National Government Publication, 1998.
Laboratory, Knolls Power Atomic. Direct Laser Powder Deposition-'State of the Art'. Washington DC: US govt publishers, 1999.
Lefteri, Chris. Making it : manufacturing techniques for product design. London: Laurence King Pub, 2012.
Between testing phases most sintered materials will relieve stress through cracks in the fatigue relief regions that may occur on the surface or inside the core. By use of ultrasound inside core defects can be easily detectable (7 p. 324). These are sampled to get the mean defect characteristics as with relevance to shape and locations giving an easy data to analyse. The testing may usually involve compressive forces of 500 MPa to determine the repeatability of defects in the produced parts (8 p. 54).
The process of metal failure usually starts with the initiation of cracks within the matrix after which the crack propagates to other regions and what follows is ultimate failure. Gromov notes that it is a common phenomenon for some defects to arise due to the effectiveness of the metal powder sintering itself, thermal stresses are seen as a major propagator of defects as thermal stresses usually relieve in points of weakness where the sintering process may not have fully occurred (9 p. 86).Though internal and external pores can be observed to reduce defects in the method used to produce the part. As a result high wattage lasers are applied that lead to fast tooling and lesser defects (5 p. 187).
History of Additive Manufacturing
The history of additive layer manufacturing takes its roots from developments in 1980’s in computer aided design and laser technology to come up with a product that is complete having been produced a layer at a time. Additive manufacturing entails the creation of 3d objects by adding layer to top of another layer stepwise by use of computer digital designs.
Joshi, Shrikrishna N and Uday S Dixit. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi: Springer India, 2015.
Gromov, Alexander and Ulrich Teipel. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany: Wiley-VCH Verlag, 2014.
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
Kang, S -J L. Sintering : densification, grain growth, and microstructure. Amsterdam ; Boston ; London : Elsevier Butterworth-Heinemann, 2005.
In 1950s and 1960 show the revolutionize laser of computing technology and the rise of computers, lasers and logic controllers sparking research in the applicability of technology in fabrication processes. In the year 1984, there was the rise of patents in United States of America and japan elaborating the concepts of 3D systems in additive manufacturing and stereo lithography technologies. Further developments in the industry led to laminated object manufacturing (LOM), cubital and others like the selective sintering process by the DTM Company (8 p. 46).
In the contrary, additive manufacturing has not been without hitches, the LOM machine by Heilys failed with the solido process laminates polymer sheets failing in their manufacturability. It’s notable that 5-axis mechanisms have also failed most times in the past.
The process commonly known as stereo lithography or photo polymerization has been recently of great concern in fine work piece manufacturing owing to its versatility and able to optimize designs. Laser sintering started as a delicate process where thin sheets of thermoplastic were formed from their powders by use of carbon dioxide lasers to heat to a point below melting point, which cad forms the geometry. The technology having started with use of materials that were easily workable, today the technology applies the use of metal powders to undertake laser sintering, which results in durable parts, the new additive layer manufacturing was then born, the year 2000. As a result highly dense parts were now reproducible with this technology enabling the workability with ferrous and nonferrous materials and enabling engineers to work out alloys through wire extrusion and blown powders. Today, the use of laser sintering has enabled the forming of high temperature thermoplastics due to high forming temperatures achievable (5 p. 56) (10 p. 83).
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015
Kang, S -J L. Sintering : densification, grain growth, and microstructure. Amsterdam ; Boston ; London : Elsevier Butterworth-Heinemann, 2005.
Laboratory, Knolls Power Atomic. Direct Laser Powder Deposition-'State of the Art'. Washington DC: US govt publishers, 1999.
.
With laser technologies, processes like selective laser sintering have been produced enabling designers to come up with complex products through sintering localized portions of powder in cross sections to achieve solid parts. The sheet lamination process to is another method of producing complex shapes by use of cheap technology, invented by Michael Feygin, he proved that by placing sheets of plastic or metal on top of another sandwiched in a bond was a cheap and easy way of producing parts (8 p. 24).
Background/History of DMLS
Direct metal laser sintering has been developed to selectively melt powder in a localized location thus enabling parts to be produced by use of a high power laser beam that easily fuses the metal powders together. DMLS patented in 1995 by Fraunhofer institute in Germany saw the method tested successfully with results matching those of electron beam melting. The process starts with digitization of thin slices of the 3D object to be produced of thicknesses between 20 to 100 micrometres thick, an industrial standard. What follows is the deposition of these fine slices on the printing plate after which high energy laser is focused on an x and y axis parameters by use of precise high frequency scanning mirrors. Once each layer is focused by the high intense laser, usually with hundreds of watts of energy, the metal powder melts partially causing welding among the particles. The process is repeated resulting in a 3d part. Materials that can be produced include titanium, cobalt chrome, tool steel, stainless steel and aluminium. Since this process leads to rapid prototyping, it holds great potential in revolutionizing the manufacturing world (1 p. 36) (11 p. 256).
In DMLS applications carbon dioxide lasers have been used quite for many years. These lasers have been proved to seamlessly generate massive energy required for sintering processes. Today YAG lasers and other types that offer better beam quality, most common being the laser fibres and disc lasers have found great application.
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
Fang, Zhigang Zak. Sintering of advanced materials : fundamentals and processes. Oxford ; Philadelphia, PA: Woodhead Publications, 2010.
Boljanovic, Vukota. Metal Shaping Processes: casting and molding, particulate processing, deformation processes, and metal removal. New York: I ndustrial press, 2010.
While DMLS technologies produce high performance tool steel, it’s able to fine tune density details while achieving the required mechanical properties (3 p. 465). These technologies have played a key role in improving speed in production of components, since, components like dies and moulds are eliminated. This adds to cost reduction too, since, the design changes don’t require coming up with moulds that take time to fabricate hence giving the designer a freedom of choice. It thus enables on demand production of components thus relieving the consumer a time taking process to wait for fabrication and also relieves the producer the cost of producing items that may not be sold.
Types of Lasers and Metal Powders
While today DMLS technologies utilize a wide variety of lasers, the EOSINT M 270 laser commonly using the 200 watts ytterbium fibre laser has been commonly used. With alternatives like Trumaform LF 250 that use disc laser, they find it hard to compete with EOSINT 270 with beam qualities of magnitude 1.0 m^2 quality. Since this lasers have beam diameters as short as 100micrometers they are able to be focused on smaller build areas giving precision as opposed to carbon dioxide lasers. As a result this is attributed to shorter wavelengths thus enabling more power to be transferred to working area within short time intervals hence their 200 watt power usually corresponds to reliable power intensities in the magnitude of 25kW/mm^2. The shorter wavelength results in higher absorption rates in metal. It, therefore, enables effectiveness in sintering process and hastening build-up speeds (9 p. 234) (3 p. 26).
Yttrium aluminium garnet lasers commonly known as YAG lasers are made from doped neodymium and erbium. This laser developed in the 1960’s, is a solid state has been used in metal sintering processes due to its ability to deliver high energy quantities in the magnitude of kilowatts (3 p. 278).
Huamin, Cao. Laser Technology. Boca Raton, Fla: CRC Press/ Taylor & Francis, 2010.
Gromov, Alexander and Ulrich Teipel. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany: Wiley-VCH Verlag, 2014.
Initial lasers used 2D technologies with micro mirror devices being able to focus laser on a whole plane of the working plate. This process was known to fuse laminates, liquid polymer, molten material and mostly discrete particles of various building materials. It’s notable that most authors have argued that the future holds the possibilities to fabricate complete objects with this technology applying a single pass of laser by combining vector and raster-base methods of scanning the prototype (7 p. 54).
The use of discrete metal particles usually metal powders is a common method in direct metal laser sintering where the constituents may be graded according to specifications or in a normal distribution. For polymer fabrication to be successful they must process thermoplastic properties to enable melting to solidifying and the same process repeated again to enable the sheets to be placed on top of the new surface (3 p. 368) (10 p. 122).
The application of 3D printer technology in metal sintering was first developed by MIT le at the point. Advancements to molten material systems called the fused deposition modelling. This later led to the proliferation of direct metal systems. The presence of machines like the EOSINT-M20 have enabled laser sintering to use laser engineered lenses for precision, however alternatives by use of sheet laminates that can be sintered by laser has been an alternative in some component specific requirements optimized products. A company from Sweden however uses electron beam melting processes (EBM), a relatively similar operation to DMLS (4 p. 286).
Other existing methods include the LENS powder delivery methods where the lens is located in a tipped arm and the power is delivered within the lens to cause the sintering process. The powder happens to melt causing sintering at the point where the laser jet hits the grains of the metal enabling the repair of high end metal products such as blades of gas turbines (9 p. 401).
Joshi, Shrikrishna N and Uday S Dixit. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi: Springer India, 2015.
Huamin, Cao. Laser Technology. Boca Raton, Fla: CRC Press/ Taylor & Francis, 2010.
Laboratory, Knolls Power Atomic. Direct Laser Powder Deposition-'State of the Art'. Washington DC: US govt publishers, 1999
J.C. Heigel, P. Michaleris, E.W. Reutzel. Thermo-mechanical model development and validation of directed energy deposition additive manufacturing of Ti–6Al–4V. new york: new york, 2015.
With recent technologies aiming at precision and optimization of desired properties, manufacturers have come up with hybrid systems able to perform better. The common being the planar milling where each surface is milled before a new layer is applied for sintering. This is a process that is involved in the sanders and objects machines thus avoiding accumulation of errors due to errors that occur as a result of different heights of the particles deposited on each new surface, it thus achieves a smooth planar surface on every new deposition. This method has however been noted as not optimal despite the quality of the components produced due to wastage of material that may be hard to recycle with the same method (12 p. 63). However engineers have come up with methods that can merge subtractive and additive element manufacturing to come with optimized systems as is the case with Stratoconception approach. In this method the computer aided models are separated to thick layers that can be easily machined. Later after these parts are machined, they can be joined together to form a complete part, which could not be machined in the normal multi axis centres due to tool accessibility issues. Since the strength of bonding between the parts in this method is of great consideration, highly effective methods such as diffusion bonding are mostly used. Similarly lower cost solutions such as the subtractive RP methods can be used where Roland desktop milling machine produce slices configured to the required shape of each slice (13 p. 124).
The shape deposition method (SDM) is another method that has been used to undertake additive manufacturing. Although it has not been commercialized to date, it’s an easier version of its precursors by using east to manufacture parts, which are later combined together (8 p. 423). This has enabled the parts to be made from different materials and later combined together and as such materials like ceramics, plastics, metals and other materials may be modelled into one component. It’s notable however that the commercialization of this method is hindered by the fact that the material sheets are not necessarily planar hence making it hard to mass produce components of varying designs.
Lefteri, Chris. Making it : manufacturing techniques for product design. London: Laurence King Pub, 2012.
Xu, Dayun. Engineering solutions for intensification of production : selected, peer reviewed papers from the 2014 2nd International Conference on Manufacturing Engineering and Technology for Manufacturing Growth (METMG 2014), January 20-21, 2014, Miami, State of Fl. Zurich, Switzerland: Trans Trap Publications, 2014.
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
The use of titanium alloys especially the Ti 6Al-4v has offered lightweight and a high strength material that is highly formable (7 p. 76) (4 p. 83). It’s less corrosive and its lightweight properties have seen its application in aircraft industry in building turbine blades and other parts. With sintering processes the use of this titanium compound has enabled precise repairs in parts and offered efficiency. In today’s industry the manufacture of turbine blades that used to take 44 weeks to produce a blade takes 8 weeks to produce a better blade with direct metal laser sintering process (9 p. 375).
Literature Review
With powder generation methods varying across different laser type applications, precision is a great concern and as much as the production costs are involved. Despite powder spraying remaining relatively unpopular among manufacturers, EOS has patented the recoater blade system method that has proved to be effective and less complex. In this method a metal powder in excess is applied at the forming table with the rotating blade levelling across the table for a flatter sheet as the extrusion die sinks giving each layer chance to be sintered. With EOSINT M 270 lasers thin layers as thin as 20 micrometres are produced with little to no deviations with EOS’ 20 micrometre technologies. As a result some technologies with bigger errors have seemed unfit for high precision jobs as they add machining costs to fine tune dimensions to near precisions. Post machining is known to add more relative errors and making the producing hardware more complicated to the disadvantage of the manufacturer (1 p. 46) (5 p. 272).
Joshi, Shrikrishna N and Uday S Dixit. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi: Springer India, 2015.
J.C. Heigel, P. Michaleris, E.W. Reutzel. Thermo-mechanical model development and validation of directed energy deposition additive manufacturing of Ti–6Al–4V. new york: new york, 2015.
Gromov, Alexander and Ulrich Teipel. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany: Wiley-VCH Verlag, 2014.
Kang, S -J L. Sintering : densification, grain growth, and microstructure. Amsterdam ; Boston ; London : Elsevier Butterworth-Heinemann, 2005.
The use of direct metal laser sintering process is critical with relevance to the metal powders used, since, the optimality of the job produced is the main goal of a manufacturer, it’s critical for most parameters to be adapted for the specific metal element or alloy (14 p. 234).
When in practical applications, tooling by DLMS process is able to achieve high speeds, since, the inner core of tooling is of less importance relative to mechanical properties of the outer skin. This technology enables the product core to be produced 8 times faster than the outer core where finish properties such as smoothness, hardness, porosity and general skin parameters are of great importance. The use of EOSINT M laser technologies has enabled the working of gold, titanium alloys, stainless steels, copper and alloys, aluminium, metal composites and silver, which can also be easily produced. With EOSINT M 250 Xtended tooling higher efficiency is achievable with results showing considerable results both in porosity and tensile strength, tolerances of as low as 20micrometers, porosity of less than 5%, tensile strengths of greater than 1,100 MPa are achievable. It is also well noting that this technology can achieve hardness of up to 42 Rockwell C (7 p. 232).
The DMLS technology has been widely used in production of inserts used in injection moulds for their capabilities to resist high pressures and resisting wearing so as to maintain tight tolerances. However these attributes become more of a reality with each development, the sintered parts often fail to achieve polish finishes of greater quality as opposed to other parts produced by alternative methods. The remaining porosity after polishing has however been eradicated in various materials such as the Direct steel H20, which has shown the least surface defects with this method (3 p. 329).
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
Joshi, Shrikrishna N and Uday S Dixit. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi: Springer India, 2015.
Huamin, Cao. Laser Technology. Boca Raton, Fla: CRC Press/ Taylor & Francis, 2010.
In the thermal testing processes heat treated alloy or normal element pieces are immersed in 1038degrees Celsius for varying periods usually one hour or 843degrees Celsius for four hours to achieve various mechanical properties. It however common for direct metal laser sintered components not to be annealed as some of their properties near those of annealed metals a process called LH1150 or LH900 (12 p. 11).
The process will usually follow examination under optical microscope after the sample is etched or finely polished. The etching and polishing process is aimed at revealing the microstructure within the metal matrix. The polishing being carried by use of diamond paste on polishing pads is able to achieve polishing of 0.25 micrometres range for finer details, however Villella’s etching reagent consisting of hydrochloric acid, methanol and picric acid is usually used in exposing microstructure in steel metals. The prepared specimens then are coated with gold palladium, this serves to increase conductivity and so aiding in visibility under electron microscope. It’s however notable that backscatter electron imaging is an alternative to electron micrograph (9 p. 230) (12 p. 11).
Lattice distortions are then determined by use of x-ray diffraction to reveal the crystalline structure and packing of atoms within the metal structure. This is usually used to note alloy properties when heating occurs during sintering at different time lapses. By use of JADE software phase quantification method the volumetric percentages of austenite phase are determined and an x-ray diffraction curve plotted.
Lefteri, Chris. Making it : manufacturing techniques for product design. London: Laurence King Pub, 2012.
Gromov, Alexander and Ulrich Teipel. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany: Wiley-VCH Verlag, 2014.
Results from findings establish that between the grain matrixes, the combination of shorter and longer grains is responsible for affecting the response of a part to thermal stresses. The layering effect is found to occur at its best with a mix-up of varied metal grain sizes resulting in a part that usually has higher resistance to thermal stresses. Its notable that during rapid cooling, elongated metal grains will usually disintegrate to shorter grains, this effect is common owing to rapid cooling that occurs with direct metal laser sintering process, however a higher percentage of elongated grains will usually occur in the direction of heat flow growing as they exit the melt pool at the localized point (8 p. 366) (6 p. 12). It is this attribute that there is less considerable grain growth in a sintering process, this is due to lower temperatures resulting in better mechanical properties of the part.
Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin: Springer publishers, 2015.
Publishing(Bingley), Emerald Group. A focus on SLS and printingSLM methods in 3D. Wagon Lane: Emerald Publisher, 2015
Cross sections of parts produced by this method usually reveal gas pockets and voids that set as the metal cools instantly relieving through contraction processes. Moreover there is the entrapment of nitrogen gas during the gas atomization of the powder and since the process of cooling is usually rapid, the part solidifies in a defect. While most manufacturers advocate for a post heat treatment air cool of the parts to room temperatures, there has not been highly effective methods to avoid gassing that occurs rapidly during the heating process (5 p. 45). With most ferrite metals, an annealing process is considered as a last step towards achieving output samples that are similar to those produced with a cast process. While this process is known to increase the strength of the part, it however imparts negatively on a part that was designed for high ductility property due to retained austenite. Producers have replaced nitrogen environments to produce parts under DMLS in inert gases like argon to reduce gas atomization and absorption and arguably reducing austenite retention (2 p. 386).
Bártolo, Paulo. Virtual and rapid manufacturing : advanced research in virtual and rapid prototyping. New York: Taylor & Francis, 2008.
Figure 1 An image of a direct metal laser sintering process being done to produce a complex part
Figure 2 dental formula being produced by a DMLS
Figure 3 Illustrative picture showing the various components of a DMLS machine
Figure 4 layout of 3D model production
Section 2
Interaction of lasers on metal powder
The way material interact with laser to produce a component is a complex analogy usually involving energy transfer through a laser beam to impart the energy of the required magnitude. The analytical evaluation of the interaction between laser beam and metal powder takes into account the physical properties of the metal powder as it entails density, the specific heat and thermal conductivity involved the feed rate of the powder in the active region of sintering, the rate of power transfer through the laser and the scanning speed of the laser (15 p. 64). The active region of functional process is the melt pool which is in most cases determined by the rate of mass flow in the point of interest. Lasers provide the capacity to focus huge amounts of energy in a small point enabling the energy transferred to modify properties on impact or cutting a component (13 p. 246).
The sintering process is a temperature dependent process involving speed, accuracy and impact; this is a process involving rapid phase transformation of the metal powder to achieve a solidified high strength substance. It is worth noting that such rapid processes are affected in great magnitude with thermal capillary forces as the mass changes states partially.
Fischer notes that thermal stresses build up in entirely all direct metal laser sintering processes despite experimenting with different types of lasers. In this case, he notes that the thermal conductivity of the metal powder, the absorptivity index of the laser factoring its reflectance properties influence the time required to scan through any point for any specific metal powder (7 p. 86). He finds that owing to high scan frequencies of laser on the working metal powder, the grain particles partially melt on the outer cover instantly and sintering in the process, however the core was found to remain relatively at room temperatures constrained by metal thermo conductivity and as a result thermal stresses set in instantly (16 p. 155). This phenomenon can be observed on finished components that usually get defects without being to any service as observed under electron microscope.
15. Analysis and Modelling of Direct Selective Laser Sintering of Two-component Metal Powders. Chen, Tiebing. s.l. : Kanop, 2014, Direct Metal Manufacturing, p. 76.
13. Xu, Dayun. Engineering solutions for intensification of production : selected, peer reviewed papers from the 2014 2nd International Conference on Manufacturing Engineering and Technology for Manufacturing Growth (METMG 2014), January 20-21, 2014, Miami, State of Fl. Zurich, Switzerland : Trans Trap Publications, 2014.
7.Joshi, Shrikrishna N and Dixit, Uday S. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi : Springer India, 2015.
16. laboratories, sandia national, et al., et al. Laser Engineered Net Shaping; a tool for direct fabrication of metal parts. washington DC : National Government Publication, 1998.
Under the effect of laser in a rapid scan, metal power packed under a powder bed results in a balling effect under reducing scanning frequencies. This is due to the effect of the cohesive forces that set due to the partial melting of the metal powder, with increased laser impact time, balling increases leading to more defects and thermal stresses in the final product. The wetting phenomena observed in prolonged laser impact on metal powders is highly influenced by the density of the metal powder involved with higher density metal powders exhibiting higher wetting on the powder bed. Excessive balling wills results in recoil pressure hindering melt flow and as a result the formation of pores sets us on the surface of the metal powder and as results, some regions are imparted with metal scattering. Denudation zones sets in as a result of poorly sintered metal powders after laser effect, as a result the metal pool collapses causing the outer surface to suffer creep. In the event that such process occurs in the last process of finishing component, a final finish is always compromised with the last option being to polish the component. Its however notable that most product manufacturers avoid post manufacture processes such as milling and polishing to cut on costs and thus such an occurrence is to the disadvantage of the product maker. In single pass laser operations where only a single scan is performed with appreciable time between repeating intervals, a thin metal powder that melts on the surface usually results in roll forming or recoiling due to the high energy impact on the surface thus blowing the powder away. High frequency pulsation will result in energy homogenization within subsequent metal powder layers thus enabling a solid and compact component to be formed. This is due to the low energy supplied at that instant owing to a higher repetition rate of the beam supply. This will in most cases lead to higher component quality with bulk densities of 60% to 90%. Pulsed laser systems will usually have short periods of powder exposure to laser and as a results there is localised heating at the skin of the powders, this leads to lower average temperatures as opposed to the high single phased lasers or the so called continuous –wave laser sintering. The inner core of the metal powders will act as a heat sink minimising the rise of temperatures while the skin melts to sinter with adjacent powders, it is for this reason that pulsed systems result in sintered components whose grains retain the original crystalline structure within the cores of the metal powders. The pulsed laser sintering will in most cases result in residual stress in the produced component owing to high differences in forming temperatures between the core and the skin (9 p. 145) (3 p. 123) (13 p. 245).
The metal powder feed rate is always monitored for quality purposes on the produced product. This is a process highly dependent on the density of the metal owing to the ability of a metal to pack densely under sintering process. Higher density metals will usually suffer little quality issues under varying feed rates to the powder bed, however to achieve similar results for lower dense metals such as aluminium a more concentrated stream of powder is used to achieve a dense packing of the metal. Since most material is delivered under pressure in a gas matrix, the delivering gas flow rate is directly proportional to the velocity of the metal powder being delivered and so is the volume delivered too. Therefore a higher gas velocity will be coupled to lower metal powder feed rate and lower metal powder particle size for optimum results (6 p. 324). The analogy behind these practices being the ability of gas to transfer energy from velocity to momentum within metal powders at extrusion nozzles.
9. Gromov, Alexander and Teipel, Ulrich. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany : Wiley-VCH Verlag, 2014.
3. Huamin, Cao. Laser Technology. Boca Raton, Fla : CRC Press/ Taylor & Francis, 2010.
13. Xu, Dayun. Engineering solutions for intensification of production : selected, peer reviewed papers from the 2014 2nd International Conference on Manufacturing Engineering and Technology for Manufacturing Growth (METMG 2014), January 20-21, 2014, Miami, State of Fl. Zurich, Switzerland : Trans Trap Publications, 2014.
6. Publishing(Bingley), Emerald Group. A focus on SLS and printingSLM methods in 3D. Wagon Lane : Emerald Publisher, 2015.
In the use of coaxial powder feeder, it is observed that the rate of metal powder thermal conductivity imparts on the effect of the laser despite using high wattage laser beams. In practical applications over 95% of the energy dissipated by the laser is used to form the highly molten laser pool and compensate for energy losses due to high thermal conductivity in metal powders, the rest 5% is used to relatively heat the surrounding region from where the high energy is imparted on powder plate with partial melting and no appreciable sintering process. However despite the industrial application of direct metal laser sintering machines employing high wattage lasers, metal sintering can occur with low energy lasers such as 5watts (9 p. 65).
9. Gromov, Alexander and Teipel, Ulrich. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany : Wiley-VCH Verlag, 2014.
Stresses produced from direct metal laser sintering
During sintering, expansion and shrinkage of the metal sets up stresses within the matrix and as a result weakening the manufactured component. During sintering residual stresses set up as the powders become rounded and compact reducing porosity. Since most of DMLS processes do not involve metal annealing to relieve stresses, they retain such stresses and as a result the components are prone to failure in working environments. Immediately the melt pool solidifies, tensile stresses set up in the component, the stresses take the longitudinal direction to the trace of the laser thereby causing strain in the contracted solid metal (3 p. 127).
According to von misses, a ductile solid yields when the distortion energy density reaches a critical value for that material. Von misses stresses can set up in a normal component and build up to the magnitude of 60Mpa, as a result such a component is weakened and if its imparted with higher temperatures slightly below the annealing point for crystal growth, warping occurs. The solidifying mass loses elastic deformation as it cools with elastic deformation abilities up to 5ms occurring during the phase change to zero as it completely solidifies. Immediately after sintering process takes place, in the immediate cooling stage, von misses stress gains equals the yield stress of the component material at that point (9 p. 60) (7 p. 342).
Huamin, Cao. Laser Technology. Boca Raton, Fla : CRC Press/ Taylor & Francis, 2010
9. Gromov, Alexander and Teipel, Ulrich. Metal nanopowders : production, characterization, and energetic applications. Weinheim, Germany : Wiley-VCH Verlag, 2014
7. Joshi, Shrikrishna N and Dixit, Uday S. Lasers based manufacturing : 5th International and 26th All India Manufacturing Technology, Design and Research Conference, AIMTDR 2014. New Delhi : Springer India, 2015.
In the graph above it is observed that buckling of the yield stress curve occurs at roughly 400 degrees Celsius. This is as a result of yield limit curve attaining a change in direction at the tangent around 400 degrees Celsius. From the graph, a von misses stress loading near a yield point coupled with internal tensile stress at elevating temperatures of 400 degrees Celsius, the material becomes brittle. The result of such setting stresses during cooling may lead to defects such as delamination and warping of an entire or partial layer of metallic powder (17 p. 32) (18 p. 34).
In the graph above, the maximum temperature attained by the sintering powder is indipendent of the scanning speed.
The temperature homogenity is obsrrved at 0.4mm laser scanned way. From this graph it can be concluded that materials have different cooling behaviour which influences the high correlation of material properties to scanned vector length. The sintering process is usually a rapid process hence most of the heat developed at the melt pool is dissipated in the sintered body due to a high temperature gradient (17 p. 41).
17. applications, National Research council(US).Committe on a scientific assesment of free-electron laser technology for naval. Scientific Assesment of High Power free-electron laser technology. Washingtom DC : National Academies Press, 2009.
18. Gibson, I, Rosen, D W and Stucker, B. Additive manufacturing technologies : rapid prototyping to direct digital manufacturing. New York : Springer, 2010.
From this diagram it can be noted that high stress occurs mostly at the edges away from the melt pool. The high differences in stress loading between the melt pool and the surrounding metal pose a likelihood of delamination within the transitional layer.
A graph of temperature against time under a powder bed for stainless steel
From the graph it can be concluded that when the laser energy is increased the temperature of the bed increases too. Increasing laser lapse time reduces the temperature achieved at the laser bed indicating that temperature build up is dependent on laser exposure time. The magnitude of temperature from the melt pool spreads radially reducing as it moves from the laser application zone to the powder bed.
The diagram above indicates the processes that happen once laser hits the grain particles. Depending on its absorptivity, some particles absorb it while they reflect some of the energy back and to other adjacent particles. The arrangement and porosity of particles on the powder plate determines the percentage of particles that are hit by laser on first impact determining layer thicknesses for each forming process.
Figure 1 a graph indicating the porosity characteristics of the formed component depending on the laser power and their scan speeds
Figure 2 percentage of porosity obtained by two methods for Al/SiC composites by DLMS
This graph shows the effect of annealing a metal component that has been produced through DMLS method. This shows that crystal growth during annealing process relieves the thermal stresses inside the metal and consequently increasing stress.
Figure 3 a hardness evaluation of AlSiMg alloy and Si reinforced composites produced by DMLS
From this chart the effect of adding silicon to metal powders is observed to improve the hardness of the component produced while at the same time reducing the porosity in a component. Since DMLS faces a challenge of porosity issues it can be understood that research in elements like silicon additives can lead to better produced components that address the shortcomings of using DMLS in component production. The production of composites with DMLS is observed to minimize the deviations of material properties owing to the capability of composites to have different phases at any single moment during the sintering process. AlSiOMg composites exhibit anisotropy after being processed by DMLS and thus have excellent strength capabilities (17 p. 93)
17. applications, National Research council(US).Committe on a scientific assesment of free-electron laser technology for naval. Scientific Assesment of High Power free-electron laser technology. Washingtom DC : National Academies Press, 2009.
Figure 4 this figure shows the distribution of temperatures on the skin of iron powder being sintered by DMLS for different scan rates from a Gaussian source with a 0.4mm diameter. A coupling efficiency of 20% is assumed and temperatures calculated according to three dimensional heat flow equations
In liquid phase sintering processes where densification is a key property of concern, experiments have proven that DMLS processes to obey first order kinetics law in the way powders flow in semiliquid phase.
i.e.
Where represents the sintering rate
The temperature distribution with lasers using different scan rates can be defined with the following equation derived from the graph above;
i.e.
The experimental rates being that metal will exceed melting point with high laser scan speeds and exceed boiling pint with higher laser scan speeds,
In such a case the surface tension and shrinkage effects result in the formation of large pores. Therefore by varying the rates of laser scanning the densification rates can be easily manipulated. Since several parameters are critical to the densification rate, the following factors are factored in arriving at the first order kinetics law.
Laser power P
Scan line spacing h
Laser beam spot size d
Scan rate v
Powder thickness w
Therefore the energy density
However since the scanning process involves irradiating powder at successive times, the fraction overlap, O.
Therefore the delivered energy per unit volume of any individual laser trail;
This results in a rapid transfer of energy to the particles being sintered as they attain their melting point at sintering temperature Tѕ
Therefore, Tѕ= To =
Therefore since in the rapid sintering process, some metal evaporation takes place the specific energy (Ѱ) input must be put into consideration
From the equation it can be noted that Ѱ is directly proportional to Tѕ. Therefore
Conclusion
The use of direct metal laser sintering processes is a key milestone in the manufacturing industry since it has enabled rapid prototyping become a reality and more so the production of complex parts. With this technology developments such as the 3D printer have been created shining a ray of hope that manufacturing of custom made products will one day be at a household level. The application of this technology finds vast application across all fields and as such so are science hobbyists too. Cutting edge technologies to increasing the efficiency of DMLS will emerge owing to the much research undergoing in this field. Despite the fact that rapid prototyping has been used for polymers in the past, today various metals are being used enabling households to print aluminium shoes at their comfort in homes. Laser technologies improve the manufacturing of delicate parts and reducing production times thus enabling the commercial application of DMLS to not only finer parts but customized components. It is a cost cutting technology and with research and developments the process will be used in the production of many metal alloy components.
Limitations
The limitations of DMSL are some limitations due to some geometry which require support when being produced. Downward facing horizontal surfaces always require supports and as a result this may require time taking procedures like removing the support parts through machining. Such process increase the time taken as opposed to other geometries which are produced on one pass (8 p. 324).
The production of parts with these methods results in some stresses within the component matrix as the sintering occurs; this is due to a combination of different metal properties and laser scanning times and energy. The research to eliminate stresses across various components has not been satisfactory especially where high densities need to be achieved (19 p. 165).
Limitations in achieving high quality finishes such that high quality finish components require polishing before use, an undertaking that adds cost and production time. The grainy finish can be improved by using finer powders (19 p. 167).
Some components produced by DLMS are sometimes porous hence they can’t be used in some fluid systems such as gas tight and water environments, research needs to be done to achieve the best out of this technology and owing to the interest this field carries, there is potential for overcoming many of the limitations.
8. Gu, Dongdong. Laer Additive Manufacturing of High Performance Materials. Berlin : Springer publishers, 2015.
19. White, Lillian. Additive manufacturing materials : standards, testing and applicability. New York : Nova Science Publishers, 2015.
Recommendations
I recommend for engineers to research and come up with metal powders that are optimized for DMLS technologies. This entails with metal powders that are optimized for energy efficiency, porosity of the required percentage, higher density packing and the ones that achieve finer finishes to avoid post manufacture processes.
Engineers need to Come up with pneumatic systems that can be used to produce downward horizontal surfaces without the use of additional parts, these pneumatic systems should have the capacity to handle metal in isolation and suspension opening gates for manufacturing of even more complex components. Research is a prerequisite to invention and innovation and as such, means to avoid additional manufacturing processes can be achieved by placing such powders in complex geometries pneumatically.
Engineers need to establish finer metal powders to improve surface finish and establish fine powder application methods. They need to come up with laser polishing technologies that achieve near perfect polishing as that of mechanical means. However since laser polishing is a post DMSL the powders used and the sintering process can be controlled to optimize the surface finish qualities.
Research need to be done on the production of composites through DMSL process as better properties are exhibited with varying combination of different elements. DMLS can prove a way of making composites that would otherwise not be possible under conventional die casting processes.
Works Cited
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17. applications, National Research council(US).Committe on a scientific assesment of free-electron laser technology for naval. Scientific Assesment of High Power free-electron laser technology. Washingtom DC : National Academies Press, 2009.
18. Gibson, I, Rosen, D W and Stucker, B. Additive manufacturing technologies : rapid prototyping to direct digital manufacturing. New York : Springer, 2010.
19. White, Lillian. Additive manufacturing materials : standards, testing and applicability. New York : Nova Science Publishers, 2015.
20. Chua, Chee Kai and Leong, Kah Fai. 3D printing and additive manufacturing : principles and applications. Hackensack, New Jersey : World Scientific pub, 2015.
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