Abstract - University of Liverpool



The Three-Prong Method: A Novel Assessment of Residual Stress in Laser Powder Bed Fusion S.A. Sillarsa,c, C.J Sutcliffeb,c, A.M. Philoa,c, S.G.R. Browna, J. Sienza, N.P. LaveryaStuart.Sillars@Swansea.ac.uka College of Engineering, Swansea University, Bay Campus, Crymlyn Burrows, Swansea, SA1 8EN, UKb School of Engineering, University of Liverpool, Brownlow Hill, Liverpool, L69 3GH, UKc Renishaw Plc, Additive Manufacturing Products Division, Whitebridge Park, Stone, Staffordshire, ST15 8LQ, UKAbstractResidual stress is a major problem for most metal based Laser Powder Bed Fusion (L-PBF) components. Residual stress can be reduced by appropriate build planning and post-process heat treatments; however, it is not always avoidable and can lead to build failures due to distortion and cracking. Accurate measurement of residual stress levels can be difficult due to high equipment set-up costs and long processing times. This paper introduces a simple but novel method of measuring residual stresses via a three-pronged cantilever component, the three-prong method (TPM). The method allows for a quick and easy characterisation of residual stress for a wide range of machine parameters, build strategies and materials.Many different cantilever designs have been used to indicate residual stress within additive manufacturing techniques, such as the ones presented by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "author" : [ { "dropping-particle" : "", "family" : "Yadroitsava", "given" : "I", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yadroitsev", "given" : "I", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Engineering", "given" : "Mechatronic", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "State", "given" : "Free", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "id" : "ITEM-1", "issue" : "2010", "issued" : { "date-parts" : [ [ "2012" ] ] }, "page" : "614-625", "title" : "RESIDUAL STRESS IN METAL SPECIMENS PRODUCED BY DIRECT METAL LASER SINTERING", "type" : "article-journal" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.4028/MSF.828-829.305", "ISBN" : "9783038355625", "ISSN" : "1662-9752", "author" : [ { "dropping-particle" : "", "family" : "Yadroitsava", "given" : "Ina", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Grewar", "given" : "Stephen", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Hattingh", "given" : "Daniel", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yadroitsev", "given" : "Igor", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Materials Science Forum", "id" : "ITEM-2", "issued" : { "date-parts" : [ [ "2015" ] ] }, "page" : "305-310", "title" : "Residual Stress in SLM Ti6Al4V Alloy Specimens", "type" : "article-journal", "volume" : "828-829" }, "uris" : [ "" ] }, { "id" : "ITEM-3", "itemData" : { "DOI" : "10.1007/s11740-009-0192-y", "ISSN" : "09446524", "abstract" : "For establishing Selective Laser Melting (SLM) in production technology, an extensive knowledge about the transient physical effects during the manufacturing process is mandatory. In this regard, a high process stability for various alloys, e.g. tool steel 1.2709 (X3NiCoMoTi 18-9-5), is realisable, if approaches for the virtual qualification of adequate process parameters by means of a numerical simulation based on the finite element analysis (FEA) are developed. Furthermore, specific methods to evaluate and quantify the resulting residual stresses and deformations due to the temperature gradient mechanism (TGM) are required. Hence, the presented work contains particular approaches using the FEA for the simulation of transient physical effects within the additive layer manufacturing (ALM) process. The investigations focus on coupled thermo-mechanical models incorporating specific boundary conditions and temperature dependant material properties to identify the heat impact on residual stresses and deformations. In order to evaluate the structural effects and simultaneously validate the simulation, analysis on residual stresses based on the neutron diffractometry as well as considerations concerning part deformations are presented.", "author" : [ { "dropping-particle" : "", "family" : "Zaeh", "given" : "Michael F.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Branner", "given" : "Gregor", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Production Engineering", "id" : "ITEM-3", "issued" : { "date-parts" : [ [ "2010" ] ] }, "page" : "35-45", "title" : "Investigations on residual stresses and deformations in selective laser melting", "type" : "article-journal", "volume" : "4" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Zaeh and Branner, 2010; Yadroitsava <i>et al.</i>, 2012, 2015)", "plainTextFormattedCitation" : "(Zaeh and Branner, 2010; Yadroitsava et al., 2012, 2015)", "previouslyFormattedCitation" : "(Zaeh and Branner, 2010; Yadroitsava <i>et al.</i>, 2012, 2015)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Zaeh and Branner, 2010; Yadroitsava et al., 2012, 2015). All of which share the same short-coming, that they indicate stress in one direction. If the principal component of stress is not aligned with the beam geometry it will underestimate peak stress values.A novel Three-Prong design is proposed which covers 2 dimensions by utilising 3 adjoined cantilever beams, a configuration which echoes that of hole-drilling where three measurements are used to calculate the stress field around a drilled hole. Each arm of the component resembles a curved bridge-like structure; one end of each bridge is cut away from the base plate leaving the centre intact. Deformation of the beams are then measured using a Co-ordinate Measurement Machine. Stress profiles are then estimated using finite element analysis by meshing the deflected structure and forcing it back to its original shape. In this paper, the new Three-Prong method is used to compare the residual stress levels of components built in Ti-6Al-4V with different hatch patterns, powers and exposure times.IntroductionLaser Powder Bed Fusion (L-PBF) is an additive layer manufacturing (ALM) technique whereby layers of metal powder are deposited onto a substrate and melted by a focused laser beam. The substrate is lowered and the process is repeated to form a 3D component. This manufacturing technique has allowed designers to create functional, detailed components, quickly and efficiently, leading to cost savings in many areas such as manufacturing tooling for injection moulding and die casting machines, as well as short manufacturing run components. Near net-shape components are created reducing material waste, which in turns allows for the use of more expensive materials such as Ti-6Al-4V. The design process for additive manufacturing is opposite to that of conventional CNC methods, as designers must plan where to add material, instead of planning where to cut away material. This naturally leads to more optimised designs further reducing the amount of raw material used and reducing the weight of the end component. This is especially important in the aeronautical and astronautically industries, where even the most minor weight savings are worth pursuing. Many aerospace companies are already using ALM to create prototypes components, Stratasys ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "URL" : "", "author" : [ { "dropping-particle" : "", "family" : "Hiemenz", "given" : "Joe (Stratasys)", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "0" ] ] }, "title" : "Additive Manufacturing Trends in Aerospace : Leading the Way", "type" : "webpage" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Hiemenz, no date)", "plainTextFormattedCitation" : "(Hiemenz, no date)", "previouslyFormattedCitation" : "(Hiemenz, no date)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Hiemenz, no date) reported that NASA used 70 different ALM components on its Mars rovers, but a handful have already committed to using ALM in final production. Up to May, 2017 Thales Alenia space ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "URL" : "", "author" : [ { "dropping-particle" : "", "family" : "Alenia", "given" : "Thales", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2017" ] ] }, "title" : "THALES ALENIA SPACE, THE WORLD CHAMPION IN 3D-PRINTED PARTS IN ORBIT!", "type" : "webpage" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Alenia, 2017)", "plainTextFormattedCitation" : "(Alenia, 2017)", "previouslyFormattedCitation" : "(Alenia, 2017)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Alenia, 2017) had reportedly used 79 components created using ALM into space, on their Telkom 3S, SGDC and KOREASAT-7 satellites. Although ALM has already made its way into final production there are still inherent problems to overcome. In L-PBF laser spot sizes usually have a diameter of between 60-80?m. Meaning that a single layer of a component will be made up of millions of individual spots (in the case of a modulated laser). As the area undergoes thermal expansion when irradiated by the laser beam, the material surrounding it is compressed. The compression causes a part elastic part plastic strain, when the area cools to the surrounding temperature the plastic strain causes a tensile strain in the iradiated zone and a compressive strain in the surrounding area as shown by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.addma.2016.04.007", "ISSN" : "22148604", "abstract" : "Measurements of the temperature and distortion evolution during laser powder bed fusion (LPBF) are taken as a function of time. In situ measurements have proven vital to the development and validation of FE (finite element) models for alternate forms of additive manufacturing. Due to powder obscuring all but the top layer of the part in LPBF, many non-contact measurement techniques used for in situ measurement of additive manufacturing processes are impossible. Therefore, an enclosed instrumented system is designed to allow for the in situ measurement of temperature and distortion in an LPBF machine without the need for altering the machine or the build process. By instrumenting a substrate from underneath, the spread powder does not affect measurements. Default processing parameters for the EOS M280 machine prescribe a rotating scan pattern of 67?? for each layer. One test is completed using the default rotating scan pattern and a second is completed using a constant scan pattern. Experimental observations for the build geometry tested showed that for Inconel?? 718 and a constant scan pattern produce results in a 37.6% increase in distortion as compared with a rotated scan pattern. The in situ measurements also show that the thermal cycles caused by the processing of a layer can impact the distortion accumulated during the deposition of the previous layers. The amount of distortion built per layer between the rotating and constant scan pattern cases highlights inter-layer effects not previously discovered in LPBF. The demonstrated inter-layer effects in the LPBF process should be considered in the development of thermo-mechanical models of the LPBF process.", "author" : [ { "dropping-particle" : "", "family" : "Dunbar", "given" : "A. J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Denlinger", "given" : "E. R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Heigel", "given" : "J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Michaleris", "given" : "P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Guerrier", "given" : "P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Martukanitz", "given" : "R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Simpson", "given" : "T. W.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Additive Manufacturing", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2016" ] ] }, "page" : "25-30", "title" : "Development of experimental method for in situ distortion and temperature measurements during the laser powder bed fusion additive manufacturing process", "type" : "article", "volume" : "12" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Dunbar <i>et al.</i>, 2016)", "plainTextFormattedCitation" : "(Dunbar et al., 2016)", "previouslyFormattedCitation" : "(Dunbar <i>et al.</i>, 2016)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Dunbar et al., 2016). The co-efficient of thermal expansion, melting temperature, yield stress, thermal diffusivity of the material and processing parameters such as power and exposure time will control the level of residual stress created by the laser melting process. As the strains caused by the shrinkage achieve higher stress values than the yield stress, the maximum stress will be limited by the yield stress of the material. As the yield strenTherefore the temperature gradients surrounding the melt pool is the greatest influence on residual stress. The chamber is usually heated on most powder-fusion machines but with melt pools with peak temperatures of above 2000°C as shown in the modelling by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "author" : [ { "dropping-particle" : "", "family" : "Cheng", "given" : "Bo", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Chou", "given" : "Kevin", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Solid Freeform Fabrication Symposium", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2015" ] ] }, "page" : "1182-1194", "publisher-place" : "Austin, Texas", "title" : "Meltpool Evolution in Selective Laser Melting", "type" : "paper-conference" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Cheng and Chou, 2015)", "plainTextFormattedCitation" : "(Cheng and Chou, 2015)", "previouslyFormattedCitation" : "(Cheng and Chou, 2015)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Cheng and Chou, 2015) the chamber temperature is a small fraction of this, usually in the range of 100-200°C. Higher chamber temperatures, while having the benefit of reducing residual stress, will cause slower cooling rates, which if high enough can lead to undesirable mechanical properties, such as a reduction in yield strength as shown by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1115/1.4028513", "ISSN" : "1087-1357", "abstract" : "Cracks and delamination, resulting from residual stresses, are a barrier in the world of additive manufacturing and selective laser melting (SLM) that prohibits the use of many metals in this field. By preheating the baseplate, thermal gradients are lowered and stresses can be reduced. In this work, some initial tests were performed with M2 high speed steel (HSS). The influence of preheating on density and mechanical and physical properties is investigated. The paper shows many promising results for the production of SLM parts in materials that are very sensitive to crack formation and delamination. When using a preheating of 200 \u00b0C, crack-free M2 HSS parts were produced with a relative density of 99.8%.", "author" : [ { "dropping-particle" : "", "family" : "Kempen", "given" : "Karolien", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Vrancken", "given" : "Bey", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Buls", "given" : "Sam", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Thijs", "given" : "Lore", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Humbeeck", "given" : "Jan", "non-dropping-particle" : "Van", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kruth", "given" : "Jean-Pierre", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Journal of Manufacturing Science and Engineering", "id" : "ITEM-1", "issue" : "6", "issued" : { "date-parts" : [ [ "2014" ] ] }, "page" : "061026-1 - 061026-6", "title" : "Selective Laser Melting of Crack-Free High Density M2 High Speed Steel Parts by Baseplate Preheating", "type" : "article-journal", "volume" : "136" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Kempen <i>et al.</i>, 2014)", "plainTextFormattedCitation" : "(Kempen et al., 2014)", "previouslyFormattedCitation" : "(Kempen <i>et al.</i>, 2014)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Kempen et al., 2014), ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.msea.2017.04.033", "ISBN" : "0921-5093", "ISSN" : "09215093", "abstract" : "During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800\u00a0\u00b0C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570\u00a0\u00b0C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of \u03b1\u2032 martensitic microstructure into an equilibrium \u03b1+\u03b2 microstructure. At 570\u00a0\u00b0C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively.", "author" : [ { "dropping-particle" : "", "family" : "Ali", "given" : "Haider", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ma", "given" : "Le", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ghadbeigi", "given" : "Hassan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Mumtaz", "given" : "Kamran", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Materials Science and Engineering A", "id" : "ITEM-1", "issue" : "February", "issued" : { "date-parts" : [ [ "2017" ] ] }, "page" : "211-220", "title" : "In-situ residual stress reduction, martensitic decomposition and mechanical properties enhancement through high temperature powder bed pre-heating of Selective Laser Melted Ti6Al4V", "type" : "article-journal", "volume" : "695" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Ali <i>et al.</i>, 2017)", "plainTextFormattedCitation" : "(Ali et al., 2017)", "previouslyFormattedCitation" : "(Ali <i>et al.</i>, 2017)" }, "properties" : { "noteIndex" : 4 }, "schema" : "" }(Ali et al., 2017) and ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1007/s11665-013-0658-0", "ISBN" : "1166501306580", "ISSN" : "10599495", "abstract" : "This work compares two metal additive manufacturing processes, selective laser melting (SLM) and electron beam melting (EBM), based on microstructural and mechanical property evaluation of Ti6Al4V parts produced by these two processes. Tensile and fatigue bars conforming to ASTM standards were fabricated using Ti6Al4V ELI grade material. Microstructural evolution was studied using optical and scanning electron microscopy. Tensile and fatigue tests were carried out to understand mechanical properties and to correlate them with the corresponding microstructure. The results show differences in microstructural evolution between SLM and EBM processed Ti6Al4V and their influence on mechanical properties. The microstructure of SLM processed parts were composed of an alpha' martensitic phase, whereas the EBM processed parts contain primarily alpha and a small amount of beta phase. Consequently, there are differences in tensile and fatigue properties between SLM- and EBM-produced Ti6Al4V parts. The differences are related to the cooling rates experienced as a consequence of the processing conditions associated with SLM and EBM processes.", "author" : [ { "dropping-particle" : "", "family" : "Rafi", "given" : "H. 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The temperature gradients of the irradiated area and surrounding area are anisotropic due to location on the surface as well as scan strategy. Areas in front and to the sides of the melt pool will be cooler than that behind the melt pool. With hatch strategies being based upon arbitrary starting positions and rotational angles, the principal direction of stress is difficult to predict. While many components already exist to analyse residual stress they usually take the form of a singular cantilever beam like the ones presented by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1007/s11740-009-0192-y", "ISSN" : "09446524", "abstract" : "For establishing Selective Laser Melting (SLM) in production technology, an extensive knowledge about the transient physical effects during the manufacturing process is mandatory. In this regard, a high process stability for various alloys, e.g. tool steel 1.2709 (X3NiCoMoTi 18-9-5), is realisable, if approaches for the virtual qualification of adequate process parameters by means of a numerical simulation based on the finite element analysis (FEA) are developed. Furthermore, specific methods to evaluate and quantify the resulting residual stresses and deformations due to the temperature gradient mechanism (TGM) are required. Hence, the presented work contains particular approaches using the FEA for the simulation of transient physical effects within the additive layer manufacturing (ALM) process. The investigations focus on coupled thermo-mechanical models incorporating specific boundary conditions and temperature dependant material properties to identify the heat impact on residual stresses and deformations. In order to evaluate the structural effects and simultaneously validate the simulation, analysis on residual stresses based on the neutron diffractometry as well as considerations concerning part deformations are presented.", "author" : [ { "dropping-particle" : "", "family" : "Zaeh", "given" : "Michael F.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Branner", "given" : "Gregor", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Production Engineering", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2010" ] ] }, "page" : "35-45", "title" : "Investigations on residual stresses and deformations in selective laser melting", "type" : "article-journal", "volume" : "4" }, "uris" : [ "", "" ] } ], "mendeley" : { "formattedCitation" : "(Zaeh and Branner, 2010)", "plainTextFormattedCitation" : "(Zaeh and Branner, 2010)", "previouslyFormattedCitation" : "(Zaeh and Branner, 2010)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Zaeh and Branner, 2010). This configuration can only analyse stress acting lengthways, while the principal component may be misaligned and therefore under-estimated. Design and Experimental ProcedureThe component proposed within this paper comprises of three cantilever beams connected to a central pin. This configuration gives a better indication of stress distribution within the component. Stress throughout the component is anisotropic as temperature gradients around the melt pool will vary due to geometry and scan strategy. Sometimes the melt pool will be surrounded by cold material, while others it could be bordered on 2 sides by recently melted material due to component geometry.The geometry was re-designed from a pin supported cantilever to an arched bridged design much like the bridge curvature method (BCM) design proposed by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1177/0954405412437085", "ISBN" : "0954405412", "ISSN" : "0954-4054", "abstract" : "During selective laser melting, the irradiated material experiences large temperature fluctuations in a short time which causes unwanted thermal stresses. In order to assess thermal stresses in a simple and fast way, a new pragmatic method is developed, namely the bridge curvature method. The bridge curvature method is used to assess and qualitatively compare the influence of different laser scan patterns, laser parameter settings and more fundamental process changes on residual stresses. The results from the experiments, as well as the findings from literature, lead to two general conclusions: changes that reduce the high temperature gradient, like using short scan vectors and preheating of the base plate, reduce the thermal stresses. And, thermal stresses in a particular direction can be reduced by optimal choice of the orientation of scan vectors. The experiments indicate the reliability of the bridge curvature method. Statistical analysis is used to check the repeatability of the method and to quantify the uncertainties during measurement.", "author" : [ { "dropping-particle" : "", "family" : "Kruth", "given" : "J.-P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Deckers", "given" : "J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yasa", "given" : "E.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Wauthle", "given" : "R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture", "id" : "ITEM-1", "issue" : "6", "issued" : { "date-parts" : [ [ "2012" ] ] }, "page" : "980-991", "title" : "Assessing and comparing influencing factors of residual stresses in selective laser melting using a novel analysis method", "type" : "article-journal", "volume" : "226" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Kruth <i>et al.</i>, 2012)", "plainTextFormattedCitation" : "(Kruth et al., 2012)", "previouslyFormattedCitation" : "(Kruth <i>et al.</i>, 2012)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Kruth et al., 2012). This optimises the design for electric discharge machining (EDM) over rotatory saw cutting, speeding up the analysis process. The geometry allows for an automated EDM program to easily cut the supports from the base plate. The deflections are then measured using an automated co-ordinate measuring machine (CMM) program, taking several points at the end of each arm, comparing these deflections to heights measured before cutting.Figure SEQ Figure \* ARABIC 1 - Three Prong Method Component Dimensions, Mesh and Stress AnalysisMeasured deflections are then used in ANSYS to calculate stress profiles. The central pin is fixed while displacements are added to the end of each prong, downwards towards the baseplate. This gives the stress profile apparent in the part before it was cut from the base-plate, an example of this is shown in REF _Ref486576763 \h Figure 1 along with the mesh used. Ti64 builds were created on a Renishaw RenAM500 using Ti64 powder produced by LPW Technology Ltd. Cheshire, UK with a particle size distribution of 15-48.5?m conforming to ASTM B348 Grade 23 (6%Al, 4%V). The build parameters for 200W build were; 60?m point distance, 70?s exposure time and 95?m hatch spacing. The build parameters for 400W build were; 80?m point distance, 60?s exposure time and 100?m hatch spacing. Both with a layer depth of 60 ?m.Results and DiscussionInitial studies showed that re-scanning the top skin of the component to improve surface finish affected the stress profile. Most CAD slicers will apply a range of parameters to different parts of the component depending on geometry; overhangs, edges and top skins will be given extra passes with the laser or different build parameters all together to improve surface finish and reduce porosity. In this instance, the top skin has been given an extra pass with the laser to induce re-melting, which produces a smoother surface finish. This final pass with the laser travels perpendicular to beam C, causing larger deflections to be seen in beams A and B. Therefore, to mitigate this effect, top skins rescans were turned off for all preceding builds. As residual stress is built up in the component through thermal expansion, a pore will give the material a void into which it can expand and contract without causing plastic deformation, leading to a reduction in residual stress. This effect has been mitigated by repeating each set of machine parameters across several positions in different areas of the base plate.Laser PowerTwo laser powers are compared in this study, 200W and 400W both using the meander hatch pattern. These laser powers were chosen because suitable build parameters, optimised for highest density already existed. It has been shown by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "author" : [ { "dropping-particle" : "", "family" : "Dilip", "given" : "J J S", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Anam", "given" : "Ashabul", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Pal", "given" : "Deepankar", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Stucker", "given" : "Brent", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Solid Freeform Fabrication Symposium", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2016" ] ] }, "page" : "1644-1659", "title" : "A short study on the fabrication of single track deposits in SLM and characterization", "type" : "paper-conference" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "author" : [ { "dropping-particle" : "", "family" : "Tadamalle", "given" : "A.P", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Reddy", "given" : "Y.P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Ramjee", "given" : "E.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Advances in Production Engineering & Management", "id" : "ITEM-2", "issue" : "1", "issued" : { "date-parts" : [ [ "2013" ] ] }, "page" : "52-60", "title" : "Influence of laser welding process parameters on weld pool geometry and duty cycle", "type" : "article-journal", "volume" : "8" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Tadamalle, Reddy and Ramjee, 2013; Dilip <i>et al.</i>, 2016)", "plainTextFormattedCitation" : "(Tadamalle, Reddy and Ramjee, 2013; Dilip et al., 2016)", "previouslyFormattedCitation" : "(Tadamalle, Reddy and Ramjee, 2013; Dilip <i>et al.</i>, 2016)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Tadamalle, Reddy and Ramjee, 2013; Dilip et al., 2016) that laser power has the greatest effect upon weld height, while exposure time has the greatest effect on the weld depth as shown. The calculated stress results are shown in REF _Ref486434103 \h Figure 2, 400W showed an average mean increase of 5% in calculated max stress when compared to 200W, well within the error bars of the data. Figure SEQ Figure \* ARABIC 2 – Calculated Stress Results of TPM StudyExisting Hatch PatternsMany different scan strategies have been suggested in literature, but the most commonly used scan strategies are meander, chessboard and stripes, therefore these are the ones chosen for this study. Some others of note a fractal and spiral as presented by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1007/s00170-006-0665-5", "author" : [ { "dropping-particle" : "", "family" : "Ma", "given" : "Liang", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Bin", "given" : "Hongzan", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Int J Adv Manuf Technol", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2007" ] ] }, "page" : "898-903", "title" : "Temperature and stress analysis and simulation in fractal scanning-based laser sintering", "type" : "article-journal" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Ma and Bin, 2007)", "plainTextFormattedCitation" : "(Ma and Bin, 2007)", "previouslyFormattedCitation" : "(Ma and Bin, 2007)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Ma and Bin, 2007). Chessboard was created by Concept Laser GmbH to reduce vector length; the chessboard pattern divides the slice into squares of user-defined dimensions. Each square is filled with lines at differing angles, this aims to reduce the number of aligned scan vectors on any given slice. As temperature gradient mechanism (TGM) is assumed to be the driving factor behind residual stress in L-PBF components, the chessboard scan strategy would theoretically decrease the beam deflection by reducing the scan vector lengths as shown by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1108/13552540610707013", "ISBN" : "13552546 (ISSN)", "ISSN" : "13552546 (ISSN)", "abstract" : "Purpose - This paper presents an investigation into residual stresses in selective laser sintering (SLS) and selective laser melting (SLM), aiming at a better understanding of this phenomenon. Design/methodology/approach - First, the origin of residual stresses is explored and a simple theoretical model is developed to predict residual stress distributions. Next, experimental methods are used to measure the residual stress profiles in a set of test samples produced with different process parameters. Findings - Residual stresses are found to be very large in SLM parts. In general, the residual stress profile consists of two zones of large tensile stresses at the top and bottom of the part, and a large zone of intermediate compressive stress in between. The most important parameters determining the magnitude and shape of the residual stress profiles are the material properties, the sample and substrate height, the laser scanning strategy and the heating conditions. Research limitations/implications - All experiments were conducted on parts produced from stainless steel powder (316L) and quantitative results cannot be simply extrapolated to other materials. However, most qualitative results can still be generalized. Originality/value - This paper can serve as an aid in understanding the importance of residual stresses in SLS/SLM and other additive manufacturing processes involving a localized heat input. Some of the conclusions can be used to avoid problems associated with residual stresses. \u00a9 Emerald Group Publishing Limited.", "author" : [ { "dropping-particle" : "", "family" : "Mercelis", "given" : "P", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kruth", "given" : "J.-P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Rapid Prototyping Journal", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2006" ] ] }, "page" : "254-265", "title" : "Residual stresses in selective laser sintering and selective laser melting", "type" : "article-journal", "volume" : "12" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Mercelis and Kruth, 2006)", "plainTextFormattedCitation" : "(Mercelis and Kruth, 2006)", "previouslyFormattedCitation" : "(Mercelis and Kruth, 2006)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Mercelis and Kruth, 2006).Figure SEQ Figure \* ARABIC 3 – Beam deflection across beams A, B and C for different scan strategiesHowever, on any given slice there will be several islands with vectors that align with each other. The longer the geometry is in any given direction, increases the number of scan vectors which will align. In the same way that meander strategy aligns with the longest geometrical spans on some layers, chessboard does partially on every single layer. This shows in the results in REF _Ref486434103 \h Figure 2, meander and chessboard have similar values of peak stress along with stripe scan strategy that is functionally identical to meander.It can be seen in REF _Ref486501410 \h Figure 3 that meander and chessboard scan strategies give similar values of beam deformation, across all 3 beams, while stripes show the greatest variation.Long scan vectorsIt has been shown by ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1177/0954405412437085", "ISBN" : "0954405412", "ISSN" : "0954-4054", "abstract" : "During selective laser melting, the irradiated material experiences large temperature fluctuations in a short time which causes unwanted thermal stresses. In order to assess thermal stresses in a simple and fast way, a new pragmatic method is developed, namely the bridge curvature method. The bridge curvature method is used to assess and qualitatively compare the influence of different laser scan patterns, laser parameter settings and more fundamental process changes on residual stresses. The results from the experiments, as well as the findings from literature, lead to two general conclusions: changes that reduce the high temperature gradient, like using short scan vectors and preheating of the base plate, reduce the thermal stresses. And, thermal stresses in a particular direction can be reduced by optimal choice of the orientation of scan vectors. The experiments indicate the reliability of the bridge curvature method. Statistical analysis is used to check the repeatability of the method and to quantify the uncertainties during measurement.", "author" : [ { "dropping-particle" : "", "family" : "Kruth", "given" : "J.-P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Deckers", "given" : "J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Yasa", "given" : "E.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Wauthle", "given" : "R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture", "id" : "ITEM-1", "issue" : "6", "issued" : { "date-parts" : [ [ "2012" ] ] }, "page" : "980-991", "title" : "Assessing and comparing influencing factors of residual stresses in selective laser melting using a novel analysis method", "type" : "article-journal", "volume" : "226" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "author" : [ { "dropping-particle" : "", "family" : "J.-P. Kruth, E. Yasa", "given" : "J. Deckers", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "14\u00e8mes Assises Europ\u00e9ennes du Prototypage & Fabrication Rapide, 24-25 Juin", "id" : "ITEM-2", "issued" : { "date-parts" : [ [ "2009" ] ] }, "page" : "1-11", "title" : "Experimental investigation of laser surface re-melting for the improvement of selective laser melting process", "type" : "article-journal" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(J.-P. Kruth, E. Yasa, 2009; Kruth <i>et al.</i>, 2012)", "plainTextFormattedCitation" : "(J.-P. Kruth, E. Yasa, 2009; Kruth et al., 2012)", "previouslyFormattedCitation" : "(J.-P. Kruth, E. Yasa, 2009; Kruth <i>et al.</i>, 2012)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(J.-P. Kruth, E. Yasa, 2009; Kruth et al., 2012) among others ADDIN CSL_CITATION { "citationItems" : [ { "id" : "ITEM-1", "itemData" : { "DOI" : "10.1016/j.addma.2016.04.007", "ISSN" : "22148604", "abstract" : "Measurements of the temperature and distortion evolution during laser powder bed fusion (LPBF) are taken as a function of time. In situ measurements have proven vital to the development and validation of FE (finite element) models for alternate forms of additive manufacturing. Due to powder obscuring all but the top layer of the part in LPBF, many non-contact measurement techniques used for in situ measurement of additive manufacturing processes are impossible. Therefore, an enclosed instrumented system is designed to allow for the in situ measurement of temperature and distortion in an LPBF machine without the need for altering the machine or the build process. By instrumenting a substrate from underneath, the spread powder does not affect measurements. Default processing parameters for the EOS M280 machine prescribe a rotating scan pattern of 67?? for each layer. One test is completed using the default rotating scan pattern and a second is completed using a constant scan pattern. Experimental observations for the build geometry tested showed that for Inconel?? 718 and a constant scan pattern produce results in a 37.6% increase in distortion as compared with a rotated scan pattern. The in situ measurements also show that the thermal cycles caused by the processing of a layer can impact the distortion accumulated during the deposition of the previous layers. The amount of distortion built per layer between the rotating and constant scan pattern cases highlights inter-layer effects not previously discovered in LPBF. The demonstrated inter-layer effects in the LPBF process should be considered in the development of thermo-mechanical models of the LPBF process.", "author" : [ { "dropping-particle" : "", "family" : "Dunbar", "given" : "A. J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Denlinger", "given" : "E. R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Heigel", "given" : "J.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Michaleris", "given" : "P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Guerrier", "given" : "P.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Martukanitz", "given" : "R.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Simpson", "given" : "T. W.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Additive Manufacturing", "id" : "ITEM-1", "issued" : { "date-parts" : [ [ "2016" ] ] }, "page" : "25-30", "title" : "Development of experimental method for in situ distortion and temperature measurements during the laser powder bed fusion additive manufacturing process", "type" : "article", "volume" : "12" }, "uris" : [ "" ] }, { "id" : "ITEM-2", "itemData" : { "DOI" : "10.1007/s11661-014-2549-x", "ISSN" : "10735623", "abstract" : "Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45\u00a0deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension. \u00a9 2014, The Minerals, Metals & Materials Society and ASM International (outside the USA).", "author" : [ { "dropping-particle" : "", "family" : "Wu", "given" : "Amanda S.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Brown", "given" : "Donald W.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Kumar", "given" : "Mukul", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "Gallegos", "given" : "Gilbert F.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" }, { "dropping-particle" : "", "family" : "King", "given" : "Wayne E.", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science", "id" : "ITEM-2", "issue" : "13", "issued" : { "date-parts" : [ [ "2014" ] ] }, "page" : "6260-6270", "title" : "An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel", "type" : "article-journal", "volume" : "45" }, "uris" : [ "" ] }, { "id" : "ITEM-3", "itemData" : { "DOI" : "10.3850/978-981-09-0446-3", "ISBN" : "9789810904463", "author" : [ { "dropping-particle" : "", "family" : "Ina Yadroitsava", "given" : "Igor Yadroitsev Central", "non-dropping-particle" : "", "parse-names" : false, "suffix" : "" } ], "container-title" : "Proceedings of the International Conference on Progress in Additive Manufacturing", "id" : "ITEM-3", "issue" : "January 2014", "issued" : { "date-parts" : [ [ "2014" ] ] }, "page" : "364-369", "title" : "Evaluation of Residual Stress in Selective Laser Melting of 316L Steel", "type" : "article-journal" }, "uris" : [ "" ] } ], "mendeley" : { "formattedCitation" : "(Ina Yadroitsava, 2014; Wu <i>et al.</i>, 2014; Dunbar <i>et al.</i>, 2016)", "plainTextFormattedCitation" : "(Ina Yadroitsava, 2014; Wu et al., 2014; Dunbar et al., 2016)", "previouslyFormattedCitation" : "(Ina Yadroitsava, 2014; Wu <i>et al.</i>, 2014; Dunbar <i>et al.</i>, 2016)" }, "properties" : { "noteIndex" : 0 }, "schema" : "" }(Ina Yadroitsava, 2014; Wu et al., 2014; Dunbar et al., 2016), that longer scan vectors cause higher values of stress aligned with the scan vector when compared to short scan vectors or scan vectors perpendicular to the stress measurement. This has been attributed to the increased temperature of the area of recently irradiated metal behind the melt-pool. As the melt pool will be elongated in the scan direction, the shrinkage in this longitudinal direction will be greatest, while transversal shrinkage will be lesser. It is therefore advantageous to limit the amount of aligned scan vectors within a given layer. A way to achieve this is by aligning the scan vector with the shortest dimension of the current layer. Two scan strategies were tested, a short scan vector (SSV) and a long scan vector (LSV). The three prongs of the TPM component were separated and hatch patterns applied independently, short scan vectors were orientated 90 degrees to the prong, while long scan vectors were applied in line with the prong.The resulting measured beam deflections can be seen in REF _Ref486501410 \h Figure 3, the short scan vector shows decreased beam deflection when compared to the long scan vector (LSV). While there is a variance in the 3 prongs, possibly due to the 90°-135°-135° angular arrangement, it is obvious that the LSV is the worst-case scenario of scan strategies. Comparing calculated stress values in REF _Ref486434103 \h Figure 2 shows the LSV has ~40% increase in mean stress values over the SSV and a ~25% increase over traditional scan strategies. While the SSV shows a decrease of ~15% over traditional scan strategies. This shows that although these scan strategies have improved on the worst-case scenario they are by no means the complete solution.SummaryIt can be seen from the results shown in REF _Ref486434103 \h Figure 2 that stress in as built components, still attached to the base plate is close to that of the yield strength of the material. In this case the highest calculated stress was in the long scan vector scenario at 1000MPa, with Ti-6Al-4V having a yield strength of 1000-1100 MPa. It is expected that taller wall like structures would achieve even higher values of stress, eventually leading to cracking. No great difference was found in residual stress levels between the most common scan strategies, however the Short Scan Vector (SSV) showed a ~15% improvement while the Long Scan Vector (LSV) which showed a large increase in beam deflection. ConclusionIt is clear from the results that improvements to residual stress can be made through smart scan strategies. As hardware improvements become harder and harder to achieve, more emphasis should be put on software. Currently most slicing software takes an STL file and splits them into a series of 2D geometric shapes, then an arbitrary scan strategy is applied. Slicing software should be integrated into CAD software to investigate each slice and apply a scan strategy which considers geometry, possibly guided by a high efficiency multi-scale computational models which can operate in real time. If a slice was split up into a number of squares, with each square being investigated for maximum vector length, the hatch start angle could be set as perpendicular, creating a smart chessboard scan strategy.AcknowledgementsThe authors would like to thank the Additive Manufacturing Products Division at Renishaw Plc., and the Engineering Doctoral Training schemes MATTER (ESPRC funded) and Materials and Manufacturing Academy M2A (European Social Fund). In addition, the authors would like to acknowledge the Centre for Materials Advanced Characterisation (MACH1) for the use of state-of-the-art equipment funded by the Welsh Government and the Advanced Sustainable Manufacturing Technologies (ASTUTE 2020) funded by the Welsh European Funding Office.ReferencesADDIN Mendeley Bibliography CSL_BIBLIOGRAPHY Alenia, T. (2017) THALES ALENIA SPACE, THE WORLD CHAMPION IN 3D-PRINTED PARTS IN ORBIT! Available at: , H. et al. (2017) ‘In-situ residual stress reduction, martensitic decomposition and mechanical properties enhancement through high temperature powder bed pre-heating of Selective Laser Melted Ti6Al4V’, Materials Science and Engineering A, 695(February), pp. 211–220. doi: 10.1016/j.msea.2017.04.033.Cheng, B. and Chou, K. (2015) ‘Meltpool Evolution in Selective Laser Melting’, in Solid Freeform Fabrication Symposium. Austin, Texas, pp. 1182–1194.Dilip, J. J. S. et al. (2016) ‘A short study on the fabrication of single track deposits in SLM and characterization’, in Solid Freeform Fabrication Symposium, pp. 1644–1659.Dunbar, A. J. et al. (2016) ‘Development of experimental method for in situ distortion and temperature measurements during the laser powder bed fusion additive manufacturing process’, Additive Manufacturing, pp. 25–30. doi: 10.1016/j.addma.2016.04.007.Hiemenz, J. (Stratasys) (no date) Additive Manufacturing Trends in Aerospace?: Leading the Way. Available at: Yadroitsava, I. Y. C. (2014) ‘Evaluation of Residual Stress in Selective Laser Melting of 316L Steel’, Proceedings of the International Conference on Progress in Additive Manufacturing, (January 2014), pp. 364–369. doi: 10.3850/978-981-09-0446-3.J.-P. Kruth, E. Yasa, J. D. (2009) ‘Experimental investigation of laser surface re-melting for the improvement of selective laser melting process’, 14èmes Assises Européennes du Prototypage & Fabrication Rapide, 24-25 Juin, pp. 1–11. Available at: , K. et al. (2014) ‘Selective Laser Melting of Crack-Free High Density M2 High Speed Steel Parts by Baseplate Preheating’, Journal of Manufacturing Science and Engineering, 136(6), pp. 61026-1-61026–6. doi: 10.1115/1.4028513.Kruth, J.-P. et al. (2012) ‘Assessing and comparing influencing factors of residual stresses in selective laser melting using a novel analysis method’, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 226(6), pp. 980–991. doi: 10.1177/0954405412437085.Ma, L. and Bin, H. (2007) ‘Temperature and stress analysis and simulation in fractal scanning-based laser sintering’, Int J Adv Manuf Technol, pp. 898–903. doi: 10.1007/s00170-006-0665-5.Mercelis, P. and Kruth, J.-P. (2006) ‘Residual stresses in selective laser sintering and selective laser melting’, Rapid Prototyping Journal, 12, pp. 254–265. doi: 10.1108/13552540610707013.Rafi, H. K. et al. (2013) ‘Microstructures and mechanical properties of Ti6Al4V parts fabricated by selective laser melting and electron beam melting’, Journal of Materials Engineering and Performance, 22(12), pp. 3872–3883. doi: 10.1007/s11665-013-0658-0.Tadamalle, A. ., Reddy, Y. P. and Ramjee, E. (2013) ‘Influence of laser welding process parameters on weld pool geometry and duty cycle’, Advances in Production Engineering & Management, 8(1), pp. 52–60.Wu, A. S. et al. (2014) ‘An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel’, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 45(13), pp. 6260–6270. doi: 10.1007/s11661-014-2549-x.Yadroitsava, I. et al. (2012) ‘RESIDUAL STRESS IN METAL SPECIMENS PRODUCED BY DIRECT METAL LASER SINTERING’, (2010), pp. 614–625.Yadroitsava, I. et al. (2015) ‘Residual Stress in SLM Ti6Al4V Alloy Specimens’, Materials Science Forum, 828–829, pp. 305–310. doi: 10.4028/MSF.828-829.305.Zaeh, M. F. and Branner, G. (2010) ‘Investigations on residual stresses and deformations in selective laser melting’, Production Engineering, 4, pp. 35–45. doi: 10.1007/s11740-009-0192-y. ................
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