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Powder Bed Based Laser Additive Manufacturing Process of Stainless Steel: A Review
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ScienceDirect Materials Today: Proceedings 5 (2018) 18510–18517
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ICMPC_2018
Powder Bed Based Laser Additive Manufacturing Process of Stainless Steel: A Review Adebola Adeyemi, Esther T. Akinlabi, Rasheedat M. Mahamood,* Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park, Kingsway Campus, Johannesburg, 2006, South Africa
Abstract This study presents a review on powder bed laser additive manufacturing of stainless steel. The powder bed laser additive manufacturing processes that are presented in this paper are the selective laser sintering and selective laser melting. The powder bed laser additive manufacturing process of stainless steel are reviewed in this paper. The process parameters was found to plat an important role in the evolving properties of the powder bed based laser additive manufacturing process. The paper ends with the perspective of future research directions. . © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Additive manufacturing;, laser additive manufacturing;
selective
laser melting; selective laser sintering ; Materials
characterization
1. INTRODUCTION Steels are important engineering material invented by mankind because of their extreme multiplicity in properties. Stainless steel originated from steel as a result of the addition of chromium. The percentage composition of the chromium is sufficient enough to prevent rusting in corrosive environment. Different grades of stainless steel includes, Martensitic stainless steel, Ferritic stainless steel, Austenitic stainless steel, superferritic stainless steel, duplex stainless steel, precipitation hardening stainless steel and super austenitic stainless steel [1]. *Corresponding author. E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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Asides from its high corrosion resistance property, stainless steel are malleable enough to be bent, folded, welded, machined and deep drawn, they also have a high heat conductivity and high strength. Stainless steels find extensive applications that include: chemical equipments, food processing equipments, cryogenic vessels, X-ray tube bases, heat exchangers, cutleries, jet-engine parts, automotive fasteners, valves, brewing equipments, and aircraft fittings. Methods of fabrication of stainless steel include hot forming processes and cold forming processes. Complex parts are broken down into smaller parts when these conventional manufacturing processes are used. These does not only make the process to be cumbersome but also heavier because of extra materials that are used in joining the several parts together. Additive manufacturing processes is an advanced manufacturing process that can produce complex parts no matter the complexity as a single unit part [2]. Additive manufacturing is a modern method of fabrication process which is used in producing a functional engineering metallic components one layer at a time from computer aided design (CAD) model data [1]. There are various types of additive manufacturing technology, which include: vat photopolymerization (stereolithography), fused deposition modelling, selective laser sintering/melting, laminated object manufacturing and Laser metal deposition that is also referred to as (Laser engineered net shaping) [3] . Vat photopolymerization (stereolithography) is the first commercial additive manufacturing method that is used to create a layer of solidified material using ultraviolet radiation to selectively polymerize a curable resin until a complete part is formed (Cooper et al, 2001). Its advantages includes high building speed and flexibility. Major disadvantages are high cost of materials and process errors due to over curing [2, 3]. Fused deposition modelling additive manufacturing is also referred to as material extrusion process, this process is used for fabricating 3D parts by deposition of laser heated thermoplastic filaments in a layer wise manner [4]. With this method, complex durable parts can be easily manufactured with high accuracy. Draw-backs of this additive manufacturing technology include poor surface finish, time consumption and high porosity of manufactured parts. This method has been applied in automotive , aerospace, medical and plastic industries. Selective laser sintering/malting is a powder bed additive manufacturing method that involves atomic fusion/melting of metallic powder deposited in form of layers [1]. Parts can be easily processed within a short time frame, it is flexible and accurate. The main challenge of this method is that the process is difficult to control due to large number of parameters involved. This additive manufacturing method has been mostly used with metals such as stainless steel 316L austenitic grade, precipitated hardened stainless steel [5-6]. Selective laser sintering has an extensive application in the field of aerospace, medical and automotive engineering due to the ability to control the stiffness of components in a desired model. Selective laser melting is a powder based bed fusion that is used to produce metallic components by deposition of a thin metallic powder on a substrate and using a high intensity laser beam to melt and fuse selective region of metallic powder according to the computer aided design data (CAD) in a layer-wise fashion [7]. This method has an advantage of less porosity of built parts with better mechanical property, manufacturability of complex shapes and excellent scanning efficiency. The disadvantage is similar to that of the process discuss earlier in terms of process control challenge due to too many parameters and also there is a material wastage. Selective laser melting has been extensively used with the employment of stainless steel [9]. Selective laser melting has an extensive application in the field of aerospace, medical, automotive engineering and medical health care sectors [1]. Benefits of this methods include good efficiency of material usage, parts with complicated shapes can be built, high strength material can be achieved, materials can be customized, it takes less time and it eliminated oxide impurities due to vacuum environment . Disadvantages of this methods are high price of set-up due to integration of vacuum with the machine for good thermal and impurity free environment, X-rays are formed during the process which is detrimental to human health. Laminated object additive manufacturing is an additive-subtractive rapid prototyping manufacturing process where 3D objects are manufactured by metal sheet material that are bonded together by thermally activated adhesive coating layer by layer. Each layer is formed from a sheet of paper coated with a thermoplastic adhesive and sheet is bonded together by using a heated stainless steel roller after which a CO2 laser cuts cross-section into a layer of paper according to the information from the CAD model repeatedly until the required object is formed and lamination is actualized [10]. The process is simple and faster since the laser doesn’t have to scan the entire area of the cross section. Merits of this method includes that fact that large size parts can be built, it is cheap, no microstructural alteration during the process, and it is flexible in the sense that component does not need support structures. Setbacks of this methods includes wastage of material during the subtractive (actual forming) process,
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complex internal cavities and hollow parts are difficult to build, and it has poor surface finishing [11]. Laminated object manufacturing has been demonstrated in aerospace and tool design industries . Laser Metal Deposition, an additive manufacturing process, is used in building parts by melting a metal powder (either in composite or single metal) that is injected into a specific location by mean of a high power laser beam [12-14]. The process of solidification and cooling occur in a closed chamber in an argon atmosphere so as to prevent oxidation of the melt pool. This process permits the use of high variety of metals and composites such as stainless steel mostly austenitic grades, which are mainly 316, SS 316L, SS 304L and other exotic metals such as titanium and its alloys, composites and functionally graded materials [15-18]. A major benefit of laser additive manufacturing technology is that, it provide new chances for customization of metallic components in terms of material composition manipulation and properties, improvements in product performance, and lower overall manufacturing costs due to its unique capabilities. Lots of research and discoveries has been achieved with laser metal deposition as an additive manufacturing method with the employment of stainless steel and stainless steel composite with much effort in enhancing wear resistance and strength property. This paper presents an overview of selective laser sintering and selective laser melting process of stainless steel. Some recent research works on stainless steel using these powder bed additive manufacturing techniques are presented and future research need are also proposed. 2. Selective Laser Sintering of Stainless Steel This additive manufacturing process is a powder based layer-additive manufacturing process where metallic components are built section by section. A moderately low laser power is used in this process as metallic material never reach a liquid phase during the heating process. The process occurs at a faster rate at high temperature which is why it involves heating a powder [10]. The process is achieved when the laser scan powder material deposited on the substrate on the 2D cross-section of the part created in 3D geometrical shape [20]. The process is repeated in which after the first laser scan, the powder bed is lowered by the amount of thickness of the layer produced initially, and then a new layer of powder material is spread on the powder bed again on top of the initial scanned layer until a fully dense built part is produced. Selective laser sintering (SLS) process has the potential to become one of the most valuable additive manufacturing techniques, because it has potential to easily produced complex shapes. The Figure 1 shows the schematic diagram of the SLS process.
Fig. 1. Schematic diagram of selective laser sintering process [20]
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Few research investigations and studies have been done in the application of selective laser sintering with stainless steels. Stainless steel grades that has been involved in this process are the austenitic type mainly SS 316L and the precipitated hardened stainless steel grade PH . Ibrahim et al, [16] studied the fabrication of a novel porous electrode scaffold made from stainless steel 316L powder using selective laser sintering by careful selection of process parameters and also how the property such as porosity, electrical conductivity and optical microscopy measurements were used to investigate the properties of the fabricated sample. In this investigation stainless steel, SS316L with particle size of 25 to 50 micro-meter were built with 30 W laser power and 1500 mm/s scanning speed. Density and porosity properties were investigated and it was discovered that high porosity metal parts can be produced by using a low laser power and high scan speed. This study also revealed the feasibility of producing porous metal sintered parts for electrochemical devices using the right processing parameters. Xie et al, [6] studied the mechanical and structural characteristics of porous 316L stainless steel fabricated by indirect laser sintering. In this investigation, a simple encapsulated method was developed to coat 316L SS powder fabricated by indirect SLS process, with ethylene-vinyl acetate copolymer (EVA) resin. In this experiment, a water atomized 316L stainless steel powder with particle size of 45 micro-meter and ethylene-vinyl acetate copolymer (EVA) resin were used to encapsulate the stainless steel metallic particle together . The selective laser sintering method was performed in a pure argon environment on a WYS600 SLS equipment with powder bed temperature 5 0 C below the melting temperature of ethylene-vinyl acetate copolymer (EVA) resin. The processing parameters employed were scan spacing of 0.10, 0.15 and 0.20 mm, laser power ranges between 10 and 35 W at interval of 5 W, scanning speed was set between 1000 and 2000 mm/s at difference of 200 mm/s and layer thickness was set at 0.15 mm. Before characterization of the EVA resin, post-processing was carried out in pure hydrogen contained furnace. The EVA resin was then characterized in terms of its thermal behaviour, density, porosity, average pore size. Mechanical test was performed on a CMT4305 electronic testing universal testing machine with a purpose of investigating the yield strength and young modulus. It was discovered that the laser power and the sintering temperature are determining factor on the reduction in the porosity of the material. It was concluded that the characteristics of the sintered porous stainless steel 316 L produced can be use as a substitute for bones in biomedical applications. Pal et al , [7] investigated the effect of post-processing and machining process parameters on the mechanical properties of stainless steel (precipitation hardened, PH1) product produced by direct laser sintering. A sample of stainless steel was fabricated in an argon atmosphere in a 40 degree centigrade pre-heating machining chamber with a fibre laser system (EOSINT-M270) having beam diameter of 0.1 mm. Processing parameters of tensile specimens were 195 W laser power, scan speed of 900 mm/s with 40 micro-meter thickness layer. Tensile test was then performed with specimen dimensions 80 mm in total length, 40 mm gauge length, 5 mm gauge diameter, length of holding part was set at 20 mm and diameter 6 mm. It was discovered that the tensile strength of the DLMS part increased after heat treatment. Residual stresses remain in the het treated part with increased tensile strength due to rapid cooling without undergoing any post-processing stage. It was discovered after the analysis that the energy density will determine the mechanical property which implies that tensile strength of the stainless steel (precipitation hardened, PH 1) can be controlled by the combination of the machining parameters and energy density. Laser power and scanning speed will also determine the extent of surface roughness of the stainless steel. AlMangour et al, [9]. studied the deformation mechanism of 17-4 precipitated hardened stainless steel fabricated by direct metal laser sintering using micro pillar compression testing and transmission electron microscopy. 17- 4 stainless steel were first produced using direct metal laser sintering system ( EOSINT M 280, EOS GmbH) in an argon atmosphere with spherical size of approximately 15 – 45 micro-meter in diameter. Scanning speed was 750 mm/s with scanning direction made 67 degrees between successive building layers with hatch spacing was 0.11 mm. micro compression properties of the 17-4 precipitation hardened stainless steel was evaluated. Outcome revealed that the microstructure and properties of the 17-4 stainless steel stainless steel specimens vary significantly from those produced by conventional manufacturing methods because of fine grain evolution that emerged. .
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3. Selective Laser Melting of Stainless Steel Selective Laser Melting process is an additive manufacturing technology that can be used to produce solid metallic components from metallic powder by using a high intensity laser to melt and fuse selective region of the metallic powder layer by layer according to the computer aided design data (CAD). A new layer of metal powder is applied and the build platform is being lowered by the amount of thickness of one layer. The process involves building of component layer by layer by depositing a thin metallic powder on a substrate. A high intensity power laser is then used to melt and fuse together a specific area of the metallic powder according to the data from the 3D CAD. Once the laser scanning is completed, succeeding layer of metallic powder is deposited on top and laser scans another new layer until the required component is completely built after repeated successive layer of metallic powder is deposited [3]. Once the laser scanning processes completed, loose powders are removed from the building chamber and the component can be separated from the substrate plate manually or by electrical discharge machining (EDM) [21]. Schematic diagram describing the SLM process is shown in Figure 2.
Figure 2: Schematic diagram selective laser melting process [21] Numerous research has been conducted with the application of selective laser melting of stainless steel in the literature. Jandin et al, 2005 investigated the influence of laser power strength on the porosity of component built. In their study, ytterbium fibre laser with a wavelength of 1065 nm was used to process the 316L stainless steel powder. The experiment showed that low laser power and high scanning speed caused incomplete melting of the powder material and resulted in high porosity in the components. This can be improved by increasing the laser power, and decreasing the scanning speed. Wang et al, [22] investigated selective laser melting of stainless steel 316L with low porosity and high build rates by employing fast scanning speeds to fabricate high-density stainless steel 316L (SS316L) parts. The aim of the study was to improve the production rate while maintaining a low porosity for the selective laser melting-built parts. The study shed light on the improvement of selective laser melting build rates without any decrease in the mechanical properties or any loss of parts density of stainless steel 316L. Miranda et al, [23] investigated and
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developed models for predicting the physical and mechanical properties of 316L stainless steel produced by selective laser melting. The influence of various processing parameters (laser power, scanning speed and scanning spacing) on density, hardness and shear strength of 316L stainless steel were studied using statistical analysis (ANOVA) in order to significantly determine main factors and their interactions. Six different models were developed as a predictive design tool to determine the influence of these processing parameters on the shear strength, hardness and density. Wang et al, [24] investigated the development of grain structure mechanism of 316L stainless steel fabricated by selective laser melting (SLM) and mechanical property characterization. The grain structure mechanism was studied using finite element analysis (FEA) in order to reveal the growth mechanism of grains under rapid solidification condition. A detailed analysis of crystal orientation of formed dendrite was performed using geometrical analysis in collaboration with experimental findings. It was discovered that rapid solidification caused by high-speed scanning resulted into sub-micron grains within the final solidified microstructure. It was also detected that grain size and densification was a dependant on high volume energy density of the laser which will significantly affect the mechanical properties of the final product formed after solidification. Casati et al, [25] studied the microstructure and fracture behaviour of 316L austenitic stainless steel produced by selective laser melting and discovered that severe thermal gradients and high cooling rates affects the crystal growth and orientation of grain structure after solidification. This causes material spattering and microstructure defects like pores and incomplete melted particles. The influence of effect of different distribution of defects on mechanical response and failure mechanism were investigated using 316L bars with microstructure and texture built along two different orientations. It was concluded that semi-molten metallic powder particles of stainless steel 316L were responsible for the scattering and reduced strength of the material after solidification. Liu et al, [26] investigated the spatter behaviour of stainless steel 316L during selective laser melting process. It was discovered that spatter is caused as a result of negative impact of laser on the building of parts in successive layers during selective laser melting process. Two types of spatter were identified which were droplet spatter, generated by the tearing behaviour of molten metal and powder spatter, which are produced when non-metallic powder particles around the molten pool are blown away as a result of metallic vapour impact. It was discovered that oxygen composition increase during spatter and X-ray diffraction shows that diffraction peaks of austenite content and ferrite are low due to the formation of iron oxides. Li et al, [27] investigated the deformation behaviour of stainless steel micro-lattice structures produced by selective laser melting. Macroscopic deformation of micro-lattice structures and microscopic stress and strain evolution were studied using a full scale 3D finite element model. The finite element prediction revealed that deformation of micro-lattice is significantly affected by applied boundary conditions and constitutive properties (young modulus) of the selective laser melted parent material. Zhao et al, [28] studied the influence of stainless steel decarburization on its young’s modulus (E) hardness and tensile strength during selective laser melting process. The study was investigated using evolution mechanism of the chemical element during SLM. It was discovered that during decarburization process 21% of carbon composition was lost and as a result, it reduces the young modulus and hardness of the molten pool boundary as well as the tensile property of the stainless steel. Selective laser melting also been used to process martensitic (AISI 420) grade of stainless steel. Krakhmalev et al, [29] investigated the evolution of microstructure in AISI 420 martensitic stainless steel during selective laser melting. It was discovered that several upper layers which are in austenite phase posses hardness value higher than the final bulk microstructure of thermally decomposited martensite. Also, numerical simulation results of thermal cycles discovered that thermal process can be controlled by variation of laser energy input. The tribology of selective laser melting of 316Lstainless steel as a processed part under lubricated conditions was studied by Zhu et al, [30] The friction and wear behaviours of 316L stainless steel produced both by selective laser melting and traditional methods were studied using a ring on-disc rig under lubricated conditions. It was discovered that the tribological performance of SLM stainless steel sample will be better if the pores can be drastically reduced with refined grains. Cherry et al, [31] investigated how processing parameters affects the microstructural and physical properties of 316L stainless steel by selective laser melting. After systematic characterization of porosity and microstructure, it was discovered that porosity is highest at lower laser energy and decreases at higher laser energy and also laser energy density alteration resulted in production of dense parts. Material hardness was also increased due to reduction in porosity.
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Most of the research that has been done focused mostly on austenitic stainless steel grade, 316L and also on martensitic grade AISI 420. Selective laser melting has been extensively explored with stainless steel but further study need to be done with the employment of other grades of stainless steel. 4. Summary Stainless steel and stainless steel composites are extensively used engineering materials which finds its applications in variety of sectors because of its excellent properties but further studies need to be carried out in order to fully harness its potential through powder bed laser additive manufacturing This study presents an overview of selective laser sintering and selective laser melting, powder bed laser additive manufacturing process of stainless steel. Numerous studies and discoveries using powder bed laser additive manufacturing of stainless steel were also presented and the need for further investigations in its application were also highlighted Acknowledgements This work was supported by the University of Johannesburg research council (URC) fund. References [1] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana (2014). Revolutionary additive manufacturing: an overview. Lasers in Engineering, vol. 27, pp. 161- 178. [2] M.R. Mahamood, (2017), Laser Metal Deposition Process of Metals, Alloys, and Composite Materials, Springer. [3] Wong KV, Hernandez A. A review of additive manufacturing. ISRN Mech Eng. 2012. [4] Panda S.K, Padhee S, Sood A.K, Mahapatra S.S. 2009. Optimisation of Fused Deposition Modelling Process Parameters using bacterial Foraging technique. Journal of scientific research .P 89-87. [5] Ibrahim K.A, Wu B, Brandon N.P. 2016. Electrical conductivity and porosity in stainless steel 316L scaffolds for electrochemical devices fabricated using selective laser sintering. Journal of Materials and Design. 106. http://dx.doi.org/10.1016/j.matdes.2016.05.096, pp 51 – 59. [6] Xie F, Heb X, Caoa S, Qua X. 2013. Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering. Journal of Material processing Technology.213. http://dx.doi.org/10.1016/j.jmatprotec.2012.12.014, pp 838 – 843. [7] Pal S, Tiyyaguraa H.R, Drstvenšeka I, Kumar CS. 2016. Properties of Stainless Steel PH1 product produced by Direct Metal Laser Sintering. International Conference on Manufacturing Engineering and Materials, ICMEM. Procedia Engineering 149. Pp 359 – 365. [8] Chauke C.F , KF Leong. 2014. 3D Printing and additive manufacturing principles and applications, 4th Edition. World Scientific Singapore, p518. [9] AlMangour B, Yang J.M. 2016. Understanding the deformation behavior of 17-4 precipitate hardenable stainless steel produced by direct metal laser sintering using micropillar compression and TEM. Journal of Advanced Manufacturing Technology. DOI 10.1007/s00170-0169367-9. [10] Vaupotic B, Brezocnik M, Balic J. Use of Polyjet technology in manufacture of new product. Journal of achievement in Materials and Manufacturing Engineering. 18, pp 319 – 322, 2006. [11] Bryden B.G, Pashby I.R, Wimpenny D.I, Adams C. 2000. Laminated steel tooling in the aerospace industry. Journal of Materials and Design. 21 . Warwick Manufacturing Group, University of Warwick, Co¨entry CV4 7AL, UK. Vol. 21. Pp 403 – 408. [12] M.R. Mahamood and E.T. Akinlabi (2017), Functionally Graded Materials, Springer Science Publisher, Switzerland. [13] R.M. Mahamood, E.T. Akinlabi, (2015), Effect of Processing Parameters on Wear Resistance Property of Laser Material Deposited Titanium -Alloy Composite, Journal of Optoelectronics and Advanced Materials (JOAM), Vol. 17, No. 9-10, Pp. 1348 - 1360. [14] R.M. Mahamood, E.T. Akinlabi , (2017), Laser Power and Powder flow rate influence on the metallurgy and microhardness of Laser metal Deposited Titanium alloy, Materials Today Proceedings, 4 (2), 3678–3684 [15] R.M. Mahamood, E.T. Akinlabi, (2016), Process Parameters Optimization for Material Deposition Efficiency in Laser Metal Deposited Titanium Alloy, Lasers in Manufacturing and Materials Processing, 3(1), pp. 9-21. DOI 10.1007/s40516-015-0020-5. [16] R.M. Mahamood, E.T. Akinlabi , (2015), Effect of laser power and powder flow rate on the wear resistance behaviour of laser metal deposited TiC/Ti6Al4V composites, Materials Today: Proceedings, Volume 2, Issues 4–5, 2015, Pages 2679-2686 [17] R.M. Mahamood, E.T. Akinlabi, (2015), Laser metal deposition of functionally graded Ti6Al4V/TiC, Materials & Design, Volume 84, pp. 402-410 [18] R. M. Mahamood, E. T. Akinlabi and S. A. Akinlabi (2015). Laser Power and scanning speed influence on the Mechanical property of laser metal deposited Titanium-alloy, Lasers in Manufacturing and Materials Processing, Volume 2, Issue 1, pp 43-55. [19] Bakshi K.R, Mulay A.V. 2006. A Review on selective Laser Sintering: A Rapid Prototyping Technology. IOSR. Journal of Mechanical and Civil Engineering (IOSR JMCE). Vol. 5, P 53 – 57.
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[20] Yan C, Shi Y, Hao L. 2011. Investigation into differences in selective laser sintering between amorphous and semi-crystalline polymers. Journal of Polymer Processing. Vol. 4. DOI 10.3139/217.2452. Pp 416 – 423. [21] Udroiu R, 2012. Powder bed additive manufacturing systems and its applications. Academic Journal of Manufacturing Engineering. Vol. 10. P 129. [22] Wang X , Deng D, Qi M, Zhang H. 2016. Influences of deposition strategies and oblique angle on properties of AISI316L stainless steel oblique thin-walled part by direct laser fabrication. Optics and Laser Technology. Vol. 80. http://dx.doi.org/10.1016/j.optlastec.2016.01.002. Pp 138 – 144. [23] Miranda G, Faria S, Bartolomeu. F, Pinto E, Madeira S, Mateus A ,Carreira , N. Alves F.S.Silva, O. Carvalho. 2016. Predictive models for physical and mechanical properties of 316L stainless steel produced by selective laser melting. Journal of Material Science and Engineering.657. http://dx.doi.org/10.1016/j.msea.2016.01.028. pp 43 -56. [24] Wang D , Song C, Yang Y. 2016. Investigation of crystal growth mechanism during selective laser melting and mechanical property characterization of 316L stainless steel parts. Journal of Materials and Design. 100. Pp 291 -299. WCE 2016, June 29 - July 1, 2016, London, U.K.ISBN: 978-988-14048-0-0. [25] Casati R , Lemke J, Vedani M. 2016. Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel Produced by Selective Laser Melting. Journal of Material Science and Technology. 32. http://dx.doi.org/10.1016/j.jmst.2016.06.016. pp 738 - 744. [26] Liu Y, Yang Y, Mai S , Di Wang, Song C. Investigation into spatter behavior during selective laser melting of AISI 316L stainless steel powder. Journal of Materials and Design. 87. http://dx.doi.org/10.1016/j.matdes.2015.08.086. pp 797 - 806. 2015. [27] Li. P, Wang. Z, Petrinic N, Siviour . 2014. Deformation behavior of stainless steel micro lattice structures by selective laser melting. Journal of Material Science and Engineering. 614. http://dx.doi.org/10.1016/j.msea.2014.07.015. Pp 116-121. [28] Zhao X, Song B, Zhang Y , Zhu X, Wei Q, Shi Y. 2015. Decarburization of stainless steel during selective laser melting and its influence on Young's modulus, hardness and tensile strength. Journal of Material Science and Engineering. 647 http://dx.doi.org/10.1016/j.msea.2015.08.061 pp 58 – 61. [29] Krakhmalev P, Yadroitsava I, Fredriksson G, Yadroitsev I. 2015. In situ heat treatment in selective laser melted martensitic AISI 420 stainless steels. Journal of Materials and Design. 87. http://dx.doi.org/10.1016/j.matdes.2015.08.045 pp 380 – 385. [30] Zhu Y, Zoum J, Chen X , Yang H. 2015. Tribology of selective laser melting processed parts: Stainlesssteel 316L under lubricated conditions. Wear 350 - 351 http://dx.doi.org/10.1016/j.wear.2016.01.004 pp 46 – 55. 2015. [31] Cherry J.A, Davies H.M, Mehmood, N. P. Lavery, S. G. R. Brown, J. Sienz. 2014. Investigation into the effect of process parameters on microstructural and physical properties of 316L stainless steel parts by selective laser melting. Journal of Advanced Manufacturing Technology. 76. DOI 10.1007/s00170-014-6297-2. Pp 869 – 879.
ScienceDirect Materials Today: Proceedings 5 (2018) 18510–18517
www.materialstoday.com/proceedings
ICMPC_2018
Powder Bed Based Laser Additive Manufacturing Process of Stainless Steel: A Review Adebola Adeyemi, Esther T. Akinlabi, Rasheedat M. Mahamood,* Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park, Kingsway Campus, Johannesburg, 2006, South Africa
Abstract This study presents a review on powder bed laser additive manufacturing of stainless steel. The powder bed laser additive manufacturing processes that are presented in this paper are the selective laser sintering and selective laser melting. The powder bed laser additive manufacturing process of stainless steel are reviewed in this paper. The process parameters was found to plat an important role in the evolving properties of the powder bed based laser additive manufacturing process. The paper ends with the perspective of future research directions. . © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Additive manufacturing;, laser additive manufacturing;
selective
laser melting; selective laser sintering ; Materials
characterization
1. INTRODUCTION Steels are important engineering material invented by mankind because of their extreme multiplicity in properties. Stainless steel originated from steel as a result of the addition of chromium. The percentage composition of the chromium is sufficient enough to prevent rusting in corrosive environment. Different grades of stainless steel includes, Martensitic stainless steel, Ferritic stainless steel, Austenitic stainless steel, superferritic stainless steel, duplex stainless steel, precipitation hardening stainless steel and super austenitic stainless steel [1]. *Corresponding author. E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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Asides from its high corrosion resistance property, stainless steel are malleable enough to be bent, folded, welded, machined and deep drawn, they also have a high heat conductivity and high strength. Stainless steels find extensive applications that include: chemical equipments, food processing equipments, cryogenic vessels, X-ray tube bases, heat exchangers, cutleries, jet-engine parts, automotive fasteners, valves, brewing equipments, and aircraft fittings. Methods of fabrication of stainless steel include hot forming processes and cold forming processes. Complex parts are broken down into smaller parts when these conventional manufacturing processes are used. These does not only make the process to be cumbersome but also heavier because of extra materials that are used in joining the several parts together. Additive manufacturing processes is an advanced manufacturing process that can produce complex parts no matter the complexity as a single unit part [2]. Additive manufacturing is a modern method of fabrication process which is used in producing a functional engineering metallic components one layer at a time from computer aided design (CAD) model data [1]. There are various types of additive manufacturing technology, which include: vat photopolymerization (stereolithography), fused deposition modelling, selective laser sintering/melting, laminated object manufacturing and Laser metal deposition that is also referred to as (Laser engineered net shaping) [3] . Vat photopolymerization (stereolithography) is the first commercial additive manufacturing method that is used to create a layer of solidified material using ultraviolet radiation to selectively polymerize a curable resin until a complete part is formed (Cooper et al, 2001). Its advantages includes high building speed and flexibility. Major disadvantages are high cost of materials and process errors due to over curing [2, 3]. Fused deposition modelling additive manufacturing is also referred to as material extrusion process, this process is used for fabricating 3D parts by deposition of laser heated thermoplastic filaments in a layer wise manner [4]. With this method, complex durable parts can be easily manufactured with high accuracy. Draw-backs of this additive manufacturing technology include poor surface finish, time consumption and high porosity of manufactured parts. This method has been applied in automotive , aerospace, medical and plastic industries. Selective laser sintering/malting is a powder bed additive manufacturing method that involves atomic fusion/melting of metallic powder deposited in form of layers [1]. Parts can be easily processed within a short time frame, it is flexible and accurate. The main challenge of this method is that the process is difficult to control due to large number of parameters involved. This additive manufacturing method has been mostly used with metals such as stainless steel 316L austenitic grade, precipitated hardened stainless steel [5-6]. Selective laser sintering has an extensive application in the field of aerospace, medical and automotive engineering due to the ability to control the stiffness of components in a desired model. Selective laser melting is a powder based bed fusion that is used to produce metallic components by deposition of a thin metallic powder on a substrate and using a high intensity laser beam to melt and fuse selective region of metallic powder according to the computer aided design data (CAD) in a layer-wise fashion [7]. This method has an advantage of less porosity of built parts with better mechanical property, manufacturability of complex shapes and excellent scanning efficiency. The disadvantage is similar to that of the process discuss earlier in terms of process control challenge due to too many parameters and also there is a material wastage. Selective laser melting has been extensively used with the employment of stainless steel [9]. Selective laser melting has an extensive application in the field of aerospace, medical, automotive engineering and medical health care sectors [1]. Benefits of this methods include good efficiency of material usage, parts with complicated shapes can be built, high strength material can be achieved, materials can be customized, it takes less time and it eliminated oxide impurities due to vacuum environment . Disadvantages of this methods are high price of set-up due to integration of vacuum with the machine for good thermal and impurity free environment, X-rays are formed during the process which is detrimental to human health. Laminated object additive manufacturing is an additive-subtractive rapid prototyping manufacturing process where 3D objects are manufactured by metal sheet material that are bonded together by thermally activated adhesive coating layer by layer. Each layer is formed from a sheet of paper coated with a thermoplastic adhesive and sheet is bonded together by using a heated stainless steel roller after which a CO2 laser cuts cross-section into a layer of paper according to the information from the CAD model repeatedly until the required object is formed and lamination is actualized [10]. The process is simple and faster since the laser doesn’t have to scan the entire area of the cross section. Merits of this method includes that fact that large size parts can be built, it is cheap, no microstructural alteration during the process, and it is flexible in the sense that component does not need support structures. Setbacks of this methods includes wastage of material during the subtractive (actual forming) process,
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complex internal cavities and hollow parts are difficult to build, and it has poor surface finishing [11]. Laminated object manufacturing has been demonstrated in aerospace and tool design industries . Laser Metal Deposition, an additive manufacturing process, is used in building parts by melting a metal powder (either in composite or single metal) that is injected into a specific location by mean of a high power laser beam [12-14]. The process of solidification and cooling occur in a closed chamber in an argon atmosphere so as to prevent oxidation of the melt pool. This process permits the use of high variety of metals and composites such as stainless steel mostly austenitic grades, which are mainly 316, SS 316L, SS 304L and other exotic metals such as titanium and its alloys, composites and functionally graded materials [15-18]. A major benefit of laser additive manufacturing technology is that, it provide new chances for customization of metallic components in terms of material composition manipulation and properties, improvements in product performance, and lower overall manufacturing costs due to its unique capabilities. Lots of research and discoveries has been achieved with laser metal deposition as an additive manufacturing method with the employment of stainless steel and stainless steel composite with much effort in enhancing wear resistance and strength property. This paper presents an overview of selective laser sintering and selective laser melting process of stainless steel. Some recent research works on stainless steel using these powder bed additive manufacturing techniques are presented and future research need are also proposed. 2. Selective Laser Sintering of Stainless Steel This additive manufacturing process is a powder based layer-additive manufacturing process where metallic components are built section by section. A moderately low laser power is used in this process as metallic material never reach a liquid phase during the heating process. The process occurs at a faster rate at high temperature which is why it involves heating a powder [10]. The process is achieved when the laser scan powder material deposited on the substrate on the 2D cross-section of the part created in 3D geometrical shape [20]. The process is repeated in which after the first laser scan, the powder bed is lowered by the amount of thickness of the layer produced initially, and then a new layer of powder material is spread on the powder bed again on top of the initial scanned layer until a fully dense built part is produced. Selective laser sintering (SLS) process has the potential to become one of the most valuable additive manufacturing techniques, because it has potential to easily produced complex shapes. The Figure 1 shows the schematic diagram of the SLS process.
Fig. 1. Schematic diagram of selective laser sintering process [20]
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Few research investigations and studies have been done in the application of selective laser sintering with stainless steels. Stainless steel grades that has been involved in this process are the austenitic type mainly SS 316L and the precipitated hardened stainless steel grade PH . Ibrahim et al, [16] studied the fabrication of a novel porous electrode scaffold made from stainless steel 316L powder using selective laser sintering by careful selection of process parameters and also how the property such as porosity, electrical conductivity and optical microscopy measurements were used to investigate the properties of the fabricated sample. In this investigation stainless steel, SS316L with particle size of 25 to 50 micro-meter were built with 30 W laser power and 1500 mm/s scanning speed. Density and porosity properties were investigated and it was discovered that high porosity metal parts can be produced by using a low laser power and high scan speed. This study also revealed the feasibility of producing porous metal sintered parts for electrochemical devices using the right processing parameters. Xie et al, [6] studied the mechanical and structural characteristics of porous 316L stainless steel fabricated by indirect laser sintering. In this investigation, a simple encapsulated method was developed to coat 316L SS powder fabricated by indirect SLS process, with ethylene-vinyl acetate copolymer (EVA) resin. In this experiment, a water atomized 316L stainless steel powder with particle size of 45 micro-meter and ethylene-vinyl acetate copolymer (EVA) resin were used to encapsulate the stainless steel metallic particle together . The selective laser sintering method was performed in a pure argon environment on a WYS600 SLS equipment with powder bed temperature 5 0 C below the melting temperature of ethylene-vinyl acetate copolymer (EVA) resin. The processing parameters employed were scan spacing of 0.10, 0.15 and 0.20 mm, laser power ranges between 10 and 35 W at interval of 5 W, scanning speed was set between 1000 and 2000 mm/s at difference of 200 mm/s and layer thickness was set at 0.15 mm. Before characterization of the EVA resin, post-processing was carried out in pure hydrogen contained furnace. The EVA resin was then characterized in terms of its thermal behaviour, density, porosity, average pore size. Mechanical test was performed on a CMT4305 electronic testing universal testing machine with a purpose of investigating the yield strength and young modulus. It was discovered that the laser power and the sintering temperature are determining factor on the reduction in the porosity of the material. It was concluded that the characteristics of the sintered porous stainless steel 316 L produced can be use as a substitute for bones in biomedical applications. Pal et al , [7] investigated the effect of post-processing and machining process parameters on the mechanical properties of stainless steel (precipitation hardened, PH1) product produced by direct laser sintering. A sample of stainless steel was fabricated in an argon atmosphere in a 40 degree centigrade pre-heating machining chamber with a fibre laser system (EOSINT-M270) having beam diameter of 0.1 mm. Processing parameters of tensile specimens were 195 W laser power, scan speed of 900 mm/s with 40 micro-meter thickness layer. Tensile test was then performed with specimen dimensions 80 mm in total length, 40 mm gauge length, 5 mm gauge diameter, length of holding part was set at 20 mm and diameter 6 mm. It was discovered that the tensile strength of the DLMS part increased after heat treatment. Residual stresses remain in the het treated part with increased tensile strength due to rapid cooling without undergoing any post-processing stage. It was discovered after the analysis that the energy density will determine the mechanical property which implies that tensile strength of the stainless steel (precipitation hardened, PH 1) can be controlled by the combination of the machining parameters and energy density. Laser power and scanning speed will also determine the extent of surface roughness of the stainless steel. AlMangour et al, [9]. studied the deformation mechanism of 17-4 precipitated hardened stainless steel fabricated by direct metal laser sintering using micro pillar compression testing and transmission electron microscopy. 17- 4 stainless steel were first produced using direct metal laser sintering system ( EOSINT M 280, EOS GmbH) in an argon atmosphere with spherical size of approximately 15 – 45 micro-meter in diameter. Scanning speed was 750 mm/s with scanning direction made 67 degrees between successive building layers with hatch spacing was 0.11 mm. micro compression properties of the 17-4 precipitation hardened stainless steel was evaluated. Outcome revealed that the microstructure and properties of the 17-4 stainless steel stainless steel specimens vary significantly from those produced by conventional manufacturing methods because of fine grain evolution that emerged. .
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3. Selective Laser Melting of Stainless Steel Selective Laser Melting process is an additive manufacturing technology that can be used to produce solid metallic components from metallic powder by using a high intensity laser to melt and fuse selective region of the metallic powder layer by layer according to the computer aided design data (CAD). A new layer of metal powder is applied and the build platform is being lowered by the amount of thickness of one layer. The process involves building of component layer by layer by depositing a thin metallic powder on a substrate. A high intensity power laser is then used to melt and fuse together a specific area of the metallic powder according to the data from the 3D CAD. Once the laser scanning is completed, succeeding layer of metallic powder is deposited on top and laser scans another new layer until the required component is completely built after repeated successive layer of metallic powder is deposited [3]. Once the laser scanning processes completed, loose powders are removed from the building chamber and the component can be separated from the substrate plate manually or by electrical discharge machining (EDM) [21]. Schematic diagram describing the SLM process is shown in Figure 2.
Figure 2: Schematic diagram selective laser melting process [21] Numerous research has been conducted with the application of selective laser melting of stainless steel in the literature. Jandin et al, 2005 investigated the influence of laser power strength on the porosity of component built. In their study, ytterbium fibre laser with a wavelength of 1065 nm was used to process the 316L stainless steel powder. The experiment showed that low laser power and high scanning speed caused incomplete melting of the powder material and resulted in high porosity in the components. This can be improved by increasing the laser power, and decreasing the scanning speed. Wang et al, [22] investigated selective laser melting of stainless steel 316L with low porosity and high build rates by employing fast scanning speeds to fabricate high-density stainless steel 316L (SS316L) parts. The aim of the study was to improve the production rate while maintaining a low porosity for the selective laser melting-built parts. The study shed light on the improvement of selective laser melting build rates without any decrease in the mechanical properties or any loss of parts density of stainless steel 316L. Miranda et al, [23] investigated and
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developed models for predicting the physical and mechanical properties of 316L stainless steel produced by selective laser melting. The influence of various processing parameters (laser power, scanning speed and scanning spacing) on density, hardness and shear strength of 316L stainless steel were studied using statistical analysis (ANOVA) in order to significantly determine main factors and their interactions. Six different models were developed as a predictive design tool to determine the influence of these processing parameters on the shear strength, hardness and density. Wang et al, [24] investigated the development of grain structure mechanism of 316L stainless steel fabricated by selective laser melting (SLM) and mechanical property characterization. The grain structure mechanism was studied using finite element analysis (FEA) in order to reveal the growth mechanism of grains under rapid solidification condition. A detailed analysis of crystal orientation of formed dendrite was performed using geometrical analysis in collaboration with experimental findings. It was discovered that rapid solidification caused by high-speed scanning resulted into sub-micron grains within the final solidified microstructure. It was also detected that grain size and densification was a dependant on high volume energy density of the laser which will significantly affect the mechanical properties of the final product formed after solidification. Casati et al, [25] studied the microstructure and fracture behaviour of 316L austenitic stainless steel produced by selective laser melting and discovered that severe thermal gradients and high cooling rates affects the crystal growth and orientation of grain structure after solidification. This causes material spattering and microstructure defects like pores and incomplete melted particles. The influence of effect of different distribution of defects on mechanical response and failure mechanism were investigated using 316L bars with microstructure and texture built along two different orientations. It was concluded that semi-molten metallic powder particles of stainless steel 316L were responsible for the scattering and reduced strength of the material after solidification. Liu et al, [26] investigated the spatter behaviour of stainless steel 316L during selective laser melting process. It was discovered that spatter is caused as a result of negative impact of laser on the building of parts in successive layers during selective laser melting process. Two types of spatter were identified which were droplet spatter, generated by the tearing behaviour of molten metal and powder spatter, which are produced when non-metallic powder particles around the molten pool are blown away as a result of metallic vapour impact. It was discovered that oxygen composition increase during spatter and X-ray diffraction shows that diffraction peaks of austenite content and ferrite are low due to the formation of iron oxides. Li et al, [27] investigated the deformation behaviour of stainless steel micro-lattice structures produced by selective laser melting. Macroscopic deformation of micro-lattice structures and microscopic stress and strain evolution were studied using a full scale 3D finite element model. The finite element prediction revealed that deformation of micro-lattice is significantly affected by applied boundary conditions and constitutive properties (young modulus) of the selective laser melted parent material. Zhao et al, [28] studied the influence of stainless steel decarburization on its young’s modulus (E) hardness and tensile strength during selective laser melting process. The study was investigated using evolution mechanism of the chemical element during SLM. It was discovered that during decarburization process 21% of carbon composition was lost and as a result, it reduces the young modulus and hardness of the molten pool boundary as well as the tensile property of the stainless steel. Selective laser melting also been used to process martensitic (AISI 420) grade of stainless steel. Krakhmalev et al, [29] investigated the evolution of microstructure in AISI 420 martensitic stainless steel during selective laser melting. It was discovered that several upper layers which are in austenite phase posses hardness value higher than the final bulk microstructure of thermally decomposited martensite. Also, numerical simulation results of thermal cycles discovered that thermal process can be controlled by variation of laser energy input. The tribology of selective laser melting of 316Lstainless steel as a processed part under lubricated conditions was studied by Zhu et al, [30] The friction and wear behaviours of 316L stainless steel produced both by selective laser melting and traditional methods were studied using a ring on-disc rig under lubricated conditions. It was discovered that the tribological performance of SLM stainless steel sample will be better if the pores can be drastically reduced with refined grains. Cherry et al, [31] investigated how processing parameters affects the microstructural and physical properties of 316L stainless steel by selective laser melting. After systematic characterization of porosity and microstructure, it was discovered that porosity is highest at lower laser energy and decreases at higher laser energy and also laser energy density alteration resulted in production of dense parts. Material hardness was also increased due to reduction in porosity.
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Most of the research that has been done focused mostly on austenitic stainless steel grade, 316L and also on martensitic grade AISI 420. Selective laser melting has been extensively explored with stainless steel but further study need to be done with the employment of other grades of stainless steel. 4. Summary Stainless steel and stainless steel composites are extensively used engineering materials which finds its applications in variety of sectors because of its excellent properties but further studies need to be carried out in order to fully harness its potential through powder bed laser additive manufacturing This study presents an overview of selective laser sintering and selective laser melting, powder bed laser additive manufacturing process of stainless steel. Numerous studies and discoveries using powder bed laser additive manufacturing of stainless steel were also presented and the need for further investigations in its application were also highlighted Acknowledgements This work was supported by the University of Johannesburg research council (URC) fund. References [1] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana (2014). Revolutionary additive manufacturing: an overview. Lasers in Engineering, vol. 27, pp. 161- 178. [2] M.R. Mahamood, (2017), Laser Metal Deposition Process of Metals, Alloys, and Composite Materials, Springer. [3] Wong KV, Hernandez A. A review of additive manufacturing. ISRN Mech Eng. 2012. [4] Panda S.K, Padhee S, Sood A.K, Mahapatra S.S. 2009. Optimisation of Fused Deposition Modelling Process Parameters using bacterial Foraging technique. Journal of scientific research .P 89-87. [5] Ibrahim K.A, Wu B, Brandon N.P. 2016. Electrical conductivity and porosity in stainless steel 316L scaffolds for electrochemical devices fabricated using selective laser sintering. Journal of Materials and Design. 106. http://dx.doi.org/10.1016/j.matdes.2016.05.096, pp 51 – 59. [6] Xie F, Heb X, Caoa S, Qua X. 2013. Structural and mechanical characteristics of porous 316L stainless steel fabricated by indirect selective laser sintering. Journal of Material processing Technology.213. http://dx.doi.org/10.1016/j.jmatprotec.2012.12.014, pp 838 – 843. [7] Pal S, Tiyyaguraa H.R, Drstvenšeka I, Kumar CS. 2016. Properties of Stainless Steel PH1 product produced by Direct Metal Laser Sintering. International Conference on Manufacturing Engineering and Materials, ICMEM. Procedia Engineering 149. Pp 359 – 365. [8] Chauke C.F , KF Leong. 2014. 3D Printing and additive manufacturing principles and applications, 4th Edition. World Scientific Singapore, p518. [9] AlMangour B, Yang J.M. 2016. Understanding the deformation behavior of 17-4 precipitate hardenable stainless steel produced by direct metal laser sintering using micropillar compression and TEM. Journal of Advanced Manufacturing Technology. DOI 10.1007/s00170-0169367-9. [10] Vaupotic B, Brezocnik M, Balic J. Use of Polyjet technology in manufacture of new product. Journal of achievement in Materials and Manufacturing Engineering. 18, pp 319 – 322, 2006. [11] Bryden B.G, Pashby I.R, Wimpenny D.I, Adams C. 2000. Laminated steel tooling in the aerospace industry. Journal of Materials and Design. 21 . Warwick Manufacturing Group, University of Warwick, Co¨entry CV4 7AL, UK. Vol. 21. Pp 403 – 408. [12] M.R. Mahamood and E.T. Akinlabi (2017), Functionally Graded Materials, Springer Science Publisher, Switzerland. [13] R.M. Mahamood, E.T. Akinlabi, (2015), Effect of Processing Parameters on Wear Resistance Property of Laser Material Deposited Titanium -Alloy Composite, Journal of Optoelectronics and Advanced Materials (JOAM), Vol. 17, No. 9-10, Pp. 1348 - 1360. [14] R.M. Mahamood, E.T. Akinlabi , (2017), Laser Power and Powder flow rate influence on the metallurgy and microhardness of Laser metal Deposited Titanium alloy, Materials Today Proceedings, 4 (2), 3678–3684 [15] R.M. Mahamood, E.T. Akinlabi, (2016), Process Parameters Optimization for Material Deposition Efficiency in Laser Metal Deposited Titanium Alloy, Lasers in Manufacturing and Materials Processing, 3(1), pp. 9-21. DOI 10.1007/s40516-015-0020-5. [16] R.M. Mahamood, E.T. Akinlabi , (2015), Effect of laser power and powder flow rate on the wear resistance behaviour of laser metal deposited TiC/Ti6Al4V composites, Materials Today: Proceedings, Volume 2, Issues 4–5, 2015, Pages 2679-2686 [17] R.M. Mahamood, E.T. Akinlabi, (2015), Laser metal deposition of functionally graded Ti6Al4V/TiC, Materials & Design, Volume 84, pp. 402-410 [18] R. M. Mahamood, E. T. Akinlabi and S. A. Akinlabi (2015). Laser Power and scanning speed influence on the Mechanical property of laser metal deposited Titanium-alloy, Lasers in Manufacturing and Materials Processing, Volume 2, Issue 1, pp 43-55. [19] Bakshi K.R, Mulay A.V. 2006. A Review on selective Laser Sintering: A Rapid Prototyping Technology. IOSR. Journal of Mechanical and Civil Engineering (IOSR JMCE). Vol. 5, P 53 – 57.
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