Additive Manufacturing Viewed from Material Science: State of the Art & Fundamentals J.Y. Hascoet 1.a, K.P. Karunakaran 2.b and S.Marya 3.

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1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Additive Manufacturing Viewed from Material Science: State of the Art & Fundamentals J.Y. Hascoet 1.a, K.P. Karunakaran 2.b and S.Marya 3.c 1 IRCCyN, Ecole Centrale Nantes, France 2 Dept. of Mechanical Engineering, Indian Institute of technology Bombay, INDIA 3 GeM, Ecole Centrale de Nantes, France a Jean-yves.hascoet@ec-nantes.fr, b karuna@iitb.ac.in c Surendar.marya@ec-nantes.fr Keywords Additive Manufacturing, Rapid Prototyping, 3D printing, LENS, Laser Melting Abstract Additive Manufacturing (AM), also designated as 3D Printing (3DP), is one of the most visionary and friendly approaches for flexible manufacturing with conservation of energy and material resources. It is a factory in a box that can generate multiple objects. It requires little manpower to bring virtual innovations into the real world. AM for metals can be mechanistically associated with welding. The technique employs a variety of energy sources (laser, electron beam, electric Arc, ), feed stocks (powder, wire and ribbon) and motion kinematics & control (articulated robot and 3-5 axes CNC machine ). From the materials perspectives, akin to fusion welding in many respects, AM involves a multitude of complex and interacting physical phenomena such as heat transfer, fluid flow, discrete and continuum mechanics, sintering, melting, solidification, solid state transformations, grain growth, diffusion, textures etc. The desired process performance can be achieved by controlling the parameters of energy, feed stock and motion. The effect of successive thermal cycles along with the epitaxial relations between substratum and deposits constitute some of the challenging tasks for developing optimized parts. This paper reviews the state of the art and presents some challenges facing metal product development for service applications. Introduction Manufacturing had been dominated by subtraction, viz., material removal, for centuries till Not only the machining processes, but also the formative processes of casting and forming were dependent on material removal to produce their tooling such as dies, molds and patterns. Manufacturing underwent a sign change from negative to positive, viz., material addition, with the launch of the first Rapid Prototyping machine by 3D Systems, USA in 1987 Ϯ. Additive manufacturing is a step forward from rapid prototyping as its main goal is to directly manufacture fully functional parts from digital models using real materials, opposite to rapid prototyping that aimed nonfunctional parts [1,2]. Figure 1 depicts the basic schematics of the process chain of additive manufacturing. Fig.1 Schematics of the basic chain for AM In AM, parts are generated by selectively adding detailed features to a functional substrate thanks to a computer-controlled deposition technique. Additive manufacturing is thus a free forming technology and has become a new paradigm as it opens new design possibilities that include shapes and multi-material parts to engineers without having to worry about the manufacturing routes. Physically, this involves a heat source, a feedstock, a computer controlled All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,17:51:53)

2 2348 THERMEC 2013 machine that authorizes the deposition /fusion of the feedstock in locations predefined by the digital object model. The result is a near net shape preform that still may need additional processing to optimize product quality for service standards. AM has revealed as an effective tool to compress product development time, reduce raw material usage and/or manufacturing cost without sacrificing the performance of the end product. The technology seems to give a competitive edge in case of high tech materials and complex part shapes that would be impossible or difficult to produce such as: Conformal cooling channels, Assemblies without joints, Gradient matrix, Non-equilibrium matrix, Extremely complex Shapes, Customized solutions. The growing interest for AM is supported by recent reports that precise $2.2 billion revenues generated by AM in Of this, some 28% concerned directly the final product rather than prototypes or non- functional parts. US leads in installed AM systems (38%) followed by Japan (9,7%), Germany (9,4%) and China. Sixteen companies in Europe, seven in China, five in US are now selling industrial AM systems [3]. Star applications of AM that include fuel injector for rocket engines (Aerojet Rocketdyne) and nozzles for jet engines are being planned by GE for its LEAP project within the next three years. Briefly resumed, AM is emerging as a lead vector in manufacturing business and explains ensuing academic interest in comprehending the basics of the physical processes that would yield best product properties. After a brief description of the technology, this paper would present a review of mechanisms and constraints that would affect the overall behavior of the AM part. In fact, AM technology, in spite of its significant economic impact in manufacturing has not yet drawn strong interest from material science perspectives. General Approach for metallic parts Powder-bed technologies, Deposition technologies and to a lesser extent Laminated manufacturing are the most popular approaches for metallic objects. In a powder-bed technology, the layer of powder is spread on which the required adhesive jet(s) or energy beam(s) writes the cross-section of the object so as to join the particles of the layer. Adhesive joining results in poor green strength and hence is not suitable for metals. Joining using energy beam is popular among metals. The layer may be formed either through sintering or melting. The metal deposition technology invariably uses an energy beam that may be an arc, electron beam or laser. While powder-bed technology necessarily uses powder form of raw material, the deposition technology can use powder, wire as well as strip forms of raw material. When the feedstock is powder in a deposition technology, it is also known as powder-feed technology. In a deposition technology, the material is fed along with the energy beam. The sintered or molten metal by the energy beam coming out of the nozzle is moved in contouring and area-filling paths so as to weave the layer. Table 1 Comparison of Powder-bed and Deposition Technologies Characteristics Powder-Bed Technologies Deposition Technologies Raw material - Only powder + Powder, wire and strip Material feeding Coarse powder particles (> 20µm) requiring mechanical spreading. - If powder, particles are fine; so, fluidized feeding using Argon is used. + Wire/strip is easy to feed using Support mechanism + Inherent - Explicit support mechanism is required. The authors use 5-axis! Gradient matrix - Not readily amenable for FGM + Readily amenable for FGM (exception: 3DP) Bonding Fusion using Laser/ EB/ Arc Fusion using Laser/ EB/ arc Table 1 presents a comparison of powder-bed and deposition technologies. While the simplicity of support mechanism is the significant advantage of powder-bed technology, amenability for gradient deposition goes to the credit of deposition technology. Here the substrate can be a forging, a wrought product or even a damaged part requiring refection.

3 Materials Science Forum Vols Solid form of feedstock is the most suitable for deposition technologies. These forms are powder, wire, strip or ribbon, sheet and roll. Powder form alone is used in laser and electron beam processes so far despite a poor yield of 10-15% as it is difficult to dispense other material forms exactly below these very narrow beams. With the advent of excellent beam shaping, researchers are attempting to use wire as well as strip forms even in laser cladding. It is very easy to feed wire and strip forms and control their flow rate accurately. In short, the fusion based AM process that are outlined here are built on multi pass welding technology. Additive Manufacturing Using Laser-Weld Deposition Laser additive manufacturing remains the most dominant AM process due to the inherent characteristics of laser beams such as narrow and controllable spot size, energy densities beyond a million watt par sq cm, low divergence and a variety of wave lengths and sources. Most widely used lasers are either CO2 or fiber lasers. Generally, the powder feedstock is used and melted in the laser beam and deposited at pre- defined locations to manufacture the part. The general principle of LAM is depicted in Fig.2a.The main advantage lays in the compactness of the AM machine. Figure 2b shows the CLAD machine based on a 4KW fiber laser and equipped with two nozzles to supply multiple powders to make complex and functional gradient parts [4]. Laser power, powder feed rate, travel speed and the height shift during layer by layer deposition are some of the important process parameters (Table 2). The effect of travel speed at a given laser power and powder feed rate for a single layer and 10 layer deposits as shown in Fig.3 reveals that higher weld speeds generate lower heights, but the surface finish is better [5]. Surface roughness Ra between 12 to 25 microns can be attained by the choice of powders and process parameters. At lower speeds, the amount of molten powder is important due to higher beam powder interaction times and as anticipated the molten metal flows down to give a more rough surface finish. Other considerations that need careful thoughts are relevant to ensuing cooling rates that would affect the grain structures, phases and mechanical properties. This is discussed in subsequent sections. Fig. 2a Principle of Laser additive Manufacturing 2b Clad Machine (top), Clad parts (below) Table 2. Process parameters and input materials for CLAD (LAM) Process Parameters Input Materials Laser (Power, Wave Length, spot size, delivery system) Power density Powder (Flow rate, Single, multiple, coaxial or offset) Deposit layer thickness and line spacing Substrate (Chemistry, microstructure, texture..) Substrate (wrought, cast, forged..) Substrate temperature Shielding gas flow Post process treatment (HIP ) Feedstock (Powder, wire..)

4 2350 THERMEC 2013 Fig.3 Cross sections of laser tracks in case of 1 and10 layers with six different velocities (a)500, (b)700, ( c)900, (d)1100, (e)1300 and (f)1500 mm/min. Fixed laser power and powder flow rate [5]. Additive Manufacturing Using Arc-Weld Deposition Arc weld deposition is faster and more economical by an order of magnitude than laser and electron beam processes (50-130g/min as against 2-30g/min). The surface quality is in the decreasing order for laser, electron beam and arc. If finish machining is necessary, then these marginal quality differences do not matter. Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW) are two of the potential techniques copied from welding for their simplicity and lower cost [6]. GMAW uses consumable electrode and so it is impossible to control independently the energy flow and mass flow. Therefore, the process tends to become unstable even for the slightest oxide layer. Furthermore, the feature size realizable is greater than 2mm and only materials that are available in wire or ribbon form can be used. GMAW-based is limited to large tools and parts. In Gas Tungsten Arc Welding (GTAW), the energy and mass flows can be controlled independent of each other, energy by controlling the current flow through the fixed tungsten electrode and mass flow by controlling the flow of the external consumable wire. Apart from this independent control which will lead to more homogeneous matrix and enable feature resolution of less than 1mm, it will be possible to employ differentially fed multiple wires/strips so as to build gradient matrix. Plasma arc laser deposition with wire addition has also been experimented in order to produce bulky parts faster with wire as feedstock [7]. Still not sure if arc processes would compete with laser or electron beam additive manufacturing, particularly as the price of laser based systems is coming down with their market extension. Metallurgical Features in Additive manufacturing Additive manufacturing is akin to multi-pass fusion welding as the part is constituted by layerby-layer deposition and depending on the process parameters and feedstock, the grain structures, phases and textures would be different and dependent on the lay out of the process itself [8]. From process perspectives itself, additive manufacturing brings in to foresight a number of complex interacting physical phenomena such as: Heat transfer, Fluid mechanics, continuum mechanics, melting, solidification, solid state phase transformations, grain growth, textures, diffusion. Further complexity arises as the number of process variables that include input materials interact to control the deposit characteristics such as: deposit dimensions, stress state, extent of lack of fusion, gas porosity, structure & chemical gradients. Heat is transferred to the powder from the laser beam and the molten droplets are transferred to a predefined point before solidification. The beam- powder interaction may result in evaporation of some of high vapor pressure elements contained in feed stock Further, any oxide on the feedstock or formed in its melting and solidification phases would be remnant in the AM product itself with subsequent consequences on the ductile characteristics of

5 Materials Science Forum Vols the part. The role of feedstock powders needs careful investigations as the grain homogeneity and shape are known to affect the profile of the deposit bead and the extent of projections that diminish the surface quality as well as the process stability. The microstructures and phase transformations are complex to predict without full knowledge of local thermal cycles and the structural characterization of the substrate. Besides difference in chemistry of feedstock and the substrate, their frame of reference relevant to the mechanical anisotropy, for instance rolling direction for a plate substrate, would be different from that of AM typically built around the laser axis. Further the dilution itself, which depends on the parameters selected during the process design, would generate chemical heterogeneity in the first build up passes. Additionally, the contours of the laser tracks and their order during product built up would influence the homogeneity, residual stresses and distortions of the AM product. Based on AM mechanisms and highlights discussed above, it seems important to list and examine some of the points from material perspectives. The contribution of substrate structure on the overall structure property relation. Relation between the feedstock and homogeneity of the deposit. Mechanical anisotropy along with structures in different regions of the deposit: close to substrate, middle and top of the deposit. For AM functional gradient materials, the transition zone strategy in relation with alloy compositions and process parameters. Selection of process parameters designed during AM fabrication: Laser power, power density, laser height/stitching shifts, travel speed and feedstock feed rate in relation with bead characteristics and surface finish. Establish methodologies in view of laser contours and sequences with critical focus on structure property relations. The above list is not exhaustive but proposes some of the points that merit investigations to elucidate and apprehend the basics of process design and its relation with the properties of the AM products. In fact, not only AM needs to be cost and time competitive with conventional processes, it must also deliver parts with sufficient and repeatable mechanical performance. Going a step forward, monitoring the process via temperature control followed by online NDT of the AM part would ensure confidence in the technology. An important point that needs to be considered is whether the conventional wrought alloys elaborated for thermal mechanical processing are viable candidates for AM technology. For example, Table 3 compares the tensile results on specimens machined from AM built up and those from a 316L plate. As anticipated from differences in their thermal mechanical treatments, AM specimens comparatively show lower ductility and lower mechanical strength. Theoretically, it looks be better to confine AM to alloys designed for casting. Or it may be necessary to design specific alloys for AM, but that would be time consuming and it s not sure if then the technology would be cost competitive with regards to the conventional manufacturing technologies. An intermediate solution may consist in working with conventional alloys and design post AM treatments such as HIP to improve mechanical characteristics and structural homogeneity. Table 3. Comparative tensile properties of AM and wrought commercial 316L stainless steel Yield Strength, MPa Ultimate Strength, MPa Elongation % AM 316L 401/ /771 43/ Plate 131/ /393 68/68

6 2352 THERMEC Conclusions The sign change in manufacturing led to phenomenal shortening of product launch times due to the total automation of the process. In conjunction with appropriate pre-/in-situ/post-build processes and treatments, it is today possible to obtain objects of almost any complexity and matrix. From material perspectives, the complexity of the process itself that brings in to play Fluid mechanics, continuum mechanics and thermic in first instance and then melting, solidification, and microstructural evolution during product geometrical evolution, requires more investigations to quantify the essential process parameters. As the essence of the technology to generate products as good as other conventional manufacturing technologies, the paper proposes a set of investigations that would and could facilitate the real product development apt for service via AM technology. References [1] G.N.Levy, R.Schindel, J.P. Kruth, Rapid manufacturing and rapid tooling with layer manufacturing (LM) technologies, State of the art and future perspectives, CIRP Annals - Manufacturing Technology, Volume 52, Issue 2, 2003, [2] G. Levy, Additive Manufacturing and Electro Physical & Chemical Processes, CIRP ICME 12-8th CIRP Conference on Intelligent Computation in Manufacturing engineering, Innovative and Cognitive Production Technology Systems,18-20 July 2012, Ischia (Naples), Italy [3] Further information from [4] Further information from Irccyn, Ecole Centrale Nantes (France), [5] H.El Cheikh, B.Courant, S.Branchu, X.Huang, J.Y. Hascoet, Direct laser fabrication process with coaxial projection of 316L steel. Geometrical Characteristics and microstructure characterization of wall structures, Optics & laser Engineering 50 (2012), [6] S.Akula, K.P.Karunakaran, Hybrid adaptive layer manufacturing: an intelligent art of direct metal rapid tooling process. Robotics and Computer-Integrated Manufacturing 2 2,(2006) [7] F. Martina, J. Mehnen, S.W. Williams, P. Colegrove, and F. Wang, Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti 6Al 4V, Journal of Materials Processing Technology 212 (2012) [8] J.P.Kruth, G.Levy, F.Klocke, T.H.C.Childs, Consolidation phenomena in laser and powder-bed based layered manufacturing. CIRP Annals: Manufacturing Technology 56, (2007)