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Annular laser beam based direct metal deposition
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Procedia CIRP 00 (2017) 000–000 Procedia CIRP 74 (2018) 222–227 www.elsevier.com/locate/procedia th 10th CIRPConference Conference on on Photonic PhotonicTechnologies Technologies [LANE [LANE 2018] 2018] 10 CIRP
Annular laser beam based direct metal deposition 28th CIRP Design Conference, May 2018, Nantes, France a,
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Edvard Govekar *, Andrej Jeromen , Alexander Kuznetsov , Matjaž Kotar , Masaki Kondo A new methodology to analyze the functional and physical architecture of University of Ljubljana, Faculty of Mechanical Engineering, Aškečeva cesta 9, SI-1000 Ljubljana, Slovenia existing products for, CO., anLTD., assembly orientedNagoya product family DMG Mori 2-35-16 Meieki, Nakamura-ku, City Aichu 450-0002, Japan identification aa
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* Corresponding author. Tel.:+ 38-614-771-606 ; fax: +38-614-771-651. E-mail address: [email protected]
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
- Invited Paper École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC -EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France * Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected]
Abstract The paper provides a description of a direct annular laser beam based metal deposition head, which makes use of axial material feeding and Abstract variation of the laser beam intensity distribution (LBID) on the surface of the workpiece. The application and advantages of the developed head are presented by processes of direct metal droplet, metal wire and powder deposition. Due to the achieved process symmetry, the stability and Inrobustness today’s business environment, the trend This towards more product unbroken. Due towhich this development, the unstable need of of the processes are increased. is the most clearlyvariety seen inand thecustomization case of metal is droplet deposition, is an inherently agile and In reconfigurable production systems emerged to of cope variousofproducts and product families. To designisand optimize production process. the case of wire deposition, a precise pre-set the with proportion energy input into the workpiece surface enabled, which has been shown to anasimportant affecting thematches, stabilityproduct and process outcome. In are the needed. case of laser powder themethods benefits aim of the systems as be well to chooseparameter the optimal product analysis methods Indeed, most deposition, of the known to analyze a product or one product on the physical level. Different of thethe number and developed head are reflected in family achieved powder catchment efficiencyproduct greaterfamilies, than 80however, % and inmay the differ head'slargely ability in toterms influence deposited nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production layer properties by adjusting the LBID. © 2018AThe Authors. Published by Elsevier Ltd. This is an open access CC BY-NC-ND license system. new methodology is proposed to analyze existing products in article view ofunder theirthe functional and physical architecture. The aim is to cluster © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review underBased responsibility of Flow the Bayerisches Bayerisches Laserzentrum GmbH. assembly systems. on Datum Chain, the Laserzentrum physical structure of the products is analyzed. Functional subassemblies are identified, and Peer-review under responsibility of the GmbH. a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Keywords:between Annular laser beam;families direct metal similarity product by deposition; providing process design stability support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. © 2017 The Authors. Published by Elsevier B.V. 1. Introduction parameters, governed2018. by the geometrical relations of the laser Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference
beam and of the fed wire or powder stream. Apart from the laser beam caustics and the geometry of the fed wire or technology which can be used to repair and to rebuild highpowder stream, the geometrical relations are primarily defined value damaged parts, to apply functional coatings, and to by the design of the deposition head which is used to supply manufacture complex 3D, free-form components [1]. It the metal material into the melt pool created by the laser 1.involves Introduction of the product range and characteristics manufactured and/or the continuous supply of a metallic material in the beam. Such a head consists of laser beam guiding and assembled in this system. In this context, the main challenge in form of a wire or powder and the delivery of a localised high focusing optics, and a metal wire or powder delivery nozzle, Due energy to thesource. fast The development in isthe of modelling and analysis is now not only to cope with single density energy source useddomain to heat and and forms one of components of LDMD systems which has communication and surface an ongoing trend of digitization and products, a limited product range or existing product families, melt the workpiece and supplied metal material. Due the greatest influence on the process. Depending on the digitalization, manufacturing enterprises are facing important but also to be able to analyze and to compare products to define to the many advantages which arise from such a highlyapplication, in the case of commercially available LDMD challenges environments: a continuing new product families. canaxial be observed thatlaser classical controlled in andtoday’s localisedmarket heat input, a laser beam is most systems, a head withIt an Gaussian beamexisting and a tendency towards reduction of product development times and product families are regrouped in function of clients frequently used as the high density energy source, so that single nozzle from one side (Fig. 1a) is usually usedortofeatures. deliver shortened product addition,has there is an increasing However, oriented product hardly to find. laser direct metal lifecycles. depositionIn (LDMD) been accepted as a the metal assembly material into the melt poolfamilies [3]. Theare main advantage demand of customization, being at the same time in a global On the product family level, products differ mainly in two promising additive manufacturing technology [2]. of a single side nozzle is its simple geometry and design. competition with competitors all over the world. This trend, main characteristics: (i) the number of components and (ii) the In LDMD the geometrical, metallurgical and mechanical However, the related deposition process is asymmetrical with which is inducing the development from macro to micro type of components (e.g. mechanical, electrical, electronical). properties of the deposited layer and the manufactured part respect to the laser beam and therefore dependent on the markets, results by in the diminished sizes due– tofedaugmenting Classicaldirection. methodologies considering products are influenced complex lot laser beam material deposition In order to ensure mainly processsingle symmetry and product varieties (high-volume to low-volume production) [1]. or solitary, already existing product families analyze the workpiece interactions. These are, in addition to the process a free shape, in the case of direction independent metal powTo cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which identify potentials in Ltd. the This existing difficulties an efficient definition and © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license 2212-8271 possible production system, it is important to have a precise knowledge comparison of different product families. Addressing this (http://creativecommons.org/licenses/by-nc-nd/3.0/) DirectAssembly; metal Design deposition an identification additive manufacturing Keywords: method;isFamily
Peer-review under responsibility of the Bayerisches Laserzentrum GmbH. 2212-8271 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2212-8271 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of scientific the Bayerisches Laserzentrum GmbH. Peer-review under responsibility of the committee of the 28th CIRP Design Conference 2018. 10.1016/j.procir.2018.08.099
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laser beam intensity distribution (LBID) from a Ring shape to a Gaussian like shape [8]. This brings additional advantages which are due to the possibility of adjusting the LBID towards a particular application optimum. In the following text the newly developed annular laser beam head is described, and its application to various processes of direct material deposition, including metal droplet, metal wire and powder deposition, all of which can be performed with minor modifications of the head. In addition to the achievement of a general increase in process stability, the observed and particular potential advantages of such an annular laser beam application are presented and discussed. 2. The annular laser beam direct metal deposition head The scheme of the novel annular laser beam (ALB) direct metal deposition head is presented in Fig. 2a. It consists of a laser beam shaping unit, a guiding mirror, a laser beam focusing unit, a material feeding unit, an axial material delivery nozzle, and a coaxial shielding gas nozzle. In the laser beam shaping unit, a collimated beam is shaped into an ALB. By means of the mirror the ALB is then guided coaxially to the axis of the material delivery tube into the focusing unit, so that it is focused onto the workpiece surface. The laser beam shaping unit, which consists of two axicons and a lens in between them can be used to shape various ALB caustics by varying the position of the lens. An example of an ALB caustic measured by the IR camera on a thin metal foil, with the related LBIDs vs. the laser beam focal plane lfw, is shown in Fig. 2b. In Fig. 2b θ denotes the convergence and γ the wedge angle of the caustic, which are defined based on the detection of the 1/e2 intensity boundaries of the LBID IR images at different workpiece standoff positions lfw. It can be seen that, by varying the position lfw, the LBID and the related laser beam diameter do can be changed. In general, by varying the positions lfw and the position of the lens between the axicons, various LBIDs can be generated, from Ring to Gaussian- like, at various diameters do. This property, together with the presented design of the head, makes the ALB direct metal deposition head applicable for various applications. This is because, by replacing the material feed-
Fig. 1. Deposition head- material feeding nozzle concepts: (a) Lateral with an axial beam; (b) Coaxial with an axial beam; (c) Axial with three laser beams; (d) Axial with an annular laser beam.
der deposition, a coaxial nozzle with multiple jets or a continuous annular stream is applied (Fig. 1b). A common property of a deposition head with a side or coaxial nozzle is that the Gaussian or Tophat laser beam is usually delivered along the axis of the head. Application of side or coaxially fed powder can cause overheating at the centre and laser beam power attenuation and insufficient heating at the edge of the deposited layer. This, together with higher powder density and higher cooling rates at the edge compared to the centre can lead to deep, non-uniform and even asymmetrical dilution with a weak bond at the edge of the deposited layer [4]. Additionally, side and coaxial powder feeding are accompanied by relatively low powder catchment efficiency. In order to increase powder catchment efficiency, and to deposit layers having the desired properties, the laser beam intensity and powder density distribution need to be adjusted properly so as to achieve a uniform heat input over the width of deposited layer. In order to avoid the previously mentioned drawbacks of heads which make use of an axial laser beam and a side or coaxial material delivery, recently a novel design with material supply along the head's axis and the coaxial delivery of multiple beams (Fig. 1c) [5], or with an annular laser beam, has been developed (Fig. 1d) [6,7]. In addition to axial feeding of the material, the latter enables variation of the .
Fig. 2. (a) Scheme of the ALB, direct metal deposition head, (b) Annular laser beam caustic and related LBIDs, (c) Wire and powder feeder unit
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ing unit (Fig. 2c) and making some minor adjustments to the head, a metal material in the form of a wire or powder can be applied, and the ALB direct metal deposition head can be used either for deposition of droplets generated from a metal wire, or for direct deposition i.e. laser cladding of metal wire or powder, as presented in the following. 3. Droplet deposition In metal droplet based manufacturing a molten droplet is deposited on the workpiece surface to form a bond with the surface [9]. The unique properties of metal droplet deposition, including a minimal heat load on the workpiece and discrete material deposition, can be employed in various applications such as joining [10], surface cladding, and additive manufacturing [11], especially when temperature-sensitive materials and components, e.g. ceramic substrates, thin layers or foils, are involved. Apart from conventional liquid jetting and electric arc based droplet deposition, laser based droplet deposition provides benefits due to the well-defined spatial and temporal energy input, including a low heat load of the surroundings, individual droplet size, and deposition timing control, and the ability to employ materials with a high melting point [12]. However, as described in the following, for the drop-on-demand and continuous droplet generation and deposition process to be stable, the axially symmetrical energy input which can be achieved by the annular laser beam focused onto the axially fed wire, as well as precise synchronization between the laser power and wire feeding velocity, are of great importance [13]. 3.1. Experimental In laser droplet deposition experiments, a Nd:YAG pulsed laser with a wavelength of 1064 nm was used as the energy source. The laser emits pulses with a time-dependent profile of power in the range from 0.5 to 8 kW and a duration from 0.3 to 20 ms. As a fed material in drop-on-demand deposition (DoD-D) and continuous droplet deposition (CD-D) a Ni wire of 0.6 and 0.25 mm diameter was used. The wire feeding unit shown in Fig. 2c consisted of a wire spool, a wire straightener, and a DC motor feeder, which was capable of 20 µm displacement resolution, a max. 30 m/s2 acceleration, and a max. velocity of 0.35 m/s. DoD-D consists of two phases: 1) formation of a pendant droplet, and 2) pendant droplet detachment from the solid wire-end, and deposition. The corresponding time course of the laser pulse power P and the wire feeding velocity vw are presented schematically in Fig. 3a. In order to form an initial,
Fig. 3. Laser pulse power P(t) and wire feeding velocity vw(t) in: (a) Drop-ondemand deposition, (b) Continuous droplet deposition.
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Fig. 4. Example of selected IR images of the droplet detachment phase, with induced Rayleigh-Plateau instability a t = 0.75 ms.
small-diameter pendant droplet, the wire-end is melted by the first part of the laser pulse, delivering the energy E1. Then, to increase the pendant droplet diameter to a pre-set value, the second part of the laser pulse delivers the energy E2 to melt an additional length of the wire, which is synchronously fed downwards at a feeding velocity of vw. In order to detach the formed pendant droplet, the wire is fed further downwards to position the droplet neck at the proper distance in relation to the laser beam focus, which has been shown to be a highly influential parameter [13]. The laser pulse with energy Ed is then used to force detachment by inducing Rayleigh-Plateau instability of the molten column of wire above the droplet, as illustrated in the example of the IR image sequence in Fig. 4. Using a 0.6 mm diameter Ni wire, droplets with diameters dd ranging from 0.85 to 1.25 mm can be generated, with a resolution of 50 µm and a standard deviation of 15 µm [13]. Besides the droplet-based joining applications presented in [10], in Fig. 5a various examples of different 2D patterns manufactured on a room-temperature Ti workpiece of 0.4 mm thickness are shown. They were produced in Ar protective atmosphere using a DoD-D at a standoff distance of 1 mm to demonstrate droplet diameter dd flexibility, repeatability, and accuracy of deposition. Additionally, in Fig. 5b an example DoD-D of a simple 3D structure i.e. pyramid is shown. Sequential metal droplet deposition, as required by certain applications, can be achieved by repeated DoD-D with implemented control, based on the initial temperature of the wire-end. Alternatively, continuous droplet deposition (CDD) can be used, where periodic laser pulses of frequency fp and duration τ are applied with a constant wire feeding velocity of vw (see Fig. 3b). In CD-D, the mechanism of the pendant droplet detachment and the corresponding detached
Fig. 5. Examples of deposited droplets: (a) Various 2D patterns with different dd, (b) A 3D structure, (c) Column and a porous wall structure.
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Fig. 7. (a) Definition of WIP, (b) Continuous and , (c) Broken molten bond.
Fig. 6. Influence of the laser pulse frequency fp on the generated droplet diameter dd (blue dots).
droplet diameter dd depend on the laser pulse frequency fp, as shown in Fig. 6. The presented results were obtained for the case of a 0.25 mm diameter Ni wire, with a feeding velocity of vw = 0.06 m/s, a pulse duration of τ = 0.8 ms, and using an average laser power of 120 W. At a low frequency fp = 50 Hz, the detachment of the smallest droplets occurs due to Rayleigh-Plateau instability, whereas at higher frequencies fp the detachment of larger droplets is triggered by resonant oscillation of the pendant droplet's eigenmodes with polar wavenumbers n = 1, 2, and 3, as has been proved theoretically in [13]. At high frequencies of fp > 250 Hz, spontaneous detachment of the largest droplets is caused by gravity alone. As can be seen in Fig. 6, the most stable detached droplet diameter dd is caused by the n = 1 eigenmode and by spontaneous detachment at the laser pulse frequencies fp = 130 and 300 Hz, respectively. The corresponding mean droplet diameters dd with standard deviation intervals were (1.18 ± 0.01) and (2.97 ± 0.02) mm, and the mean detachment frequencies were (3.79 ± 0.06) and (0.254 ± 0.006) Hz. An example of a column and a porous structure produced by CDD of 1.18 mm diameter droplets using a 0.25 mm diameter Ni wire and a laser pulse frequency of fp = 140 Hz is shown in Fig. 5c. 4. Wire direct deposition In laser direct metal deposition a material in the form of a wire or a powder can be used. In comparison with the use of a powder, one of the main advantages of laser direct wire deposition (LDWD) are the 100 % efficiency of the wire material, the high deposition rates, and the lower prices of metal wire [2]. On the other hand, the stability of the direct powder deposition process is fairly robust, whereas in the case of wire deposition the energy input into the workpiece surface and the wire has to be very well defined and synchronized with the wire vw and workpiece vwp feeding parameters. Thus, in general, process stability depends on many parameters, including the wire initial position and the inherent process parameters: laser beam power P, wire vw and workpiece vwp feeding velocity, which define the input energy. One of the advantages of annular laser beam direct wire deposition is that the workpiece illumination proportion (WIP) and the related energy input into the workpiece and wire surface can be influenced by variation of the annular beam caustic and the position of the workpiece surface hwf with respect to the ALB, as shown in Fig. 7a. A WIP value of
100 % means that all the laser beam energy is introduced into the workpiece surface. In the following text some results concerning the influence of WIP on the stability of the annular laser beam wire deposition process and its outcome are presented. 4.1. Experimental In the performed annular LDWD experiments, a 99.6 % pure Ni wire of 0.6 mm diameter was deposited on a 3.0 mm thick AISI 304 stainless steel workpiece. A CW diode laser, with a wavelength range of 900-1100 nm, was used as the energy source. The experiments involving single layer deposition were performed using a constant wire vf = 13 mm/s and workpiece feeding velocity vwp = 5 mm/s, and a laser beam power P in the range between 900 W and 2500 W, with steps of 100 W. In order to investigate the influence of WIP on the process, three different values of WIP, i.e. 7 %, 49 %, and 99 %, were selected. The process stability was analysed based on process visualisation using a CMOS visual camera. The related process outcome was characterised based on the geometrical characteristics of the deposited layer, i.e. its width w, height h, and maximum penetration depth hmd, all of which were extracted from photos of the deposited layer crosssections taken by an optical microscope. If the LDWD process is to be stable, a continuous molten bond has to be established, at the start, between the melt pool on the workpiece surface and the fed wire, and then maintained without breaking throughout the process. Photos of characteristic examples of a continuous and broken molten bond are shown in Fig. 7b and 7c. Based on this criterion, at a wire feeding velocity of vw = 13 mm/s and a scanning velocity of vwp = 5 mm/s, a stability diagram corresponding to the annular LDWD process in the space of the laser beam power P and WIP was defined, as is shown in Fig. 8, where markers denote the stable process windows. It can be seen that the WIP strongly influences the laser beam power P stability
Fig. 8. Stable process windows vs. P at values of WIP = 7, 49, and 99 %.
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Fig. 10. (a) IR camera based images of LBID, (b) related intensity profiles averaged over 360°.
5.1. Experimental Fig. 9. (a) Deposited wire cross sections and (b) cross section width w, height h and maximal penetration depth hmd vs. WIP at laser power P = 1.5 kW.
windows. At higher WIPs process stability is achieved at a higher laser beam power P, whereas the window width is narrower at lower WIPs and is nonlinearly dependent on the WIP. In addition to process stability the WIP also influences the process outcome. In Fig. 9 examples of photos of the deposited wire cross section at a laser beam power of P = 1.5 kW and WIPs of 7, 49, and 99 %, with the related values of the deposited layer's width w, height h, and maximum penetration depth hmd are shown. Closer examination of the photos of the cross-sections shown in Fig. 9a reveals that, at the lowest WIPs, the largest dilution area and wetting angle α are observed, which decrease as the WIP is increased. Based on the graphs shown in Fig. 9b, although not yet statistically confirmed, the nonlinear dependence of h, a linear increase in w, and a linear decrease in hmd with increasing WIPs, from 0 towards 100 %, could be assumed. As presented in [14], the observed relations do not qualitatively change with variation of the annular laser beam power P. 5. Powder deposition One advantage of laser powder deposition is the diversity of available powder materials, including pure metals, alloys, super alloys, ceramics and composites, which are not available in the form of a wire. In laser beam powder direct deposition the key problem is how to deliver a proper heat input by means of which, taking into account the laser beampowder-workpiece interactions, the process will run in a stable manner, and the related deposited layer or built-up part of the desired geometrical, metallurgical and mechanical properties will be produced. In comparison with the use of a wire, the stability of laser powder deposition is more robust. However, in the case of powder, as well as process stability and the mechanical properties of the manufactured part, the powder catchment efficiency ηp is an important process characteristic. All of these characteristics, in addition to the inherent process parameters including the laser beam power P, the powder mass flow Φm, and the workpiece feeding velocity vf,, can be influenced by the design of the head, which defines the powder supply and the related laser beam / powder / workpiece interactions, and the laser beam intensity distribution itself. The latter has been shown to be very important in many applications [15], including SLM [16].
In the laser metal powder deposition experiments a CW diode laser with a wavelength range of 900-1100 nm was used. Using SS316L powder with a particle size spanning from 53 to 125 µm, the deposition was performed on a SS304 workpiece with the dimensions (60 x 25 x 10) mm. Several experiments were performed using four different LBIDs, i.e. Ring, Tophat(-), Tophat(+) and a Gaussian like, with a 4σdiameter 3.0 mm, shown in Fig. 10. In order to monitor the process a CMOS visual camera was used. The related process outcome was characterized by the geometrical properties of the deposited layer and the powder catchment efficiency ηp. In Fig. 11a examples of the deposited layer's cross sections obtained at different LBIDs at a laser power P of 2 kW, a powder mass flow Φm = 8 g/min, and a workpiece feeding velocity vf = 10 mm/s, are shown. It can be seen that the shape of the deposited layer and the dilution are strongly influenced by the LBID. In the presented case the depth hmd of the Ushaped dilution increases with variation of the LBID from Ring towards Gaussian-like. At LBIDs with a high intensity at the center, due to the low LBI and the higher heat conduction into the workpiece at the edges of the deposited layer, a lack of dilution and insufficient melting can be observed at the edges of the cross-section. Additionally, in the case of the Tophat(+) and Gaussian-like LBID, an undesirable plasma plume above the melt pool was observed. The related mean values and the ±σ error bars of the geometrical characteristics of the deposited layers are presented in Fig. 11b. The clad wi-
Fig. 11. (a) Deposited layer cross sections at P =2.0 kW and (b) related geometrical characteristics of deposited layer vs. LBID.
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stable process does not necessarily result in the desired properties of the deposited layer. At present, one of the observed benefits of the annular laser beam and axially fed powder is high, above 80 % powder catchment efficiency. Additionally important benefits are expected from the influence of LBID, which can be varied from Ring to Gaussian-like, on the properties of the deposited layer Fig. 12. Powder catchment efficiency ηp at various LBID.
dth w remains almost constant, whereas the height h and the maximum penetration depth hmd increase as the LBID changes from Ring towards Gaussian-like. The results of powder catchment efficiency ηp including ±σ error bars, achieved at a laser beam power of P = 2.0 kW, with various LBIDs and process parameters, are shown in Fig. 12. The results showed that, in general, in the axially fed powder deposition process, a very high powder catchment efficiency ηp above 80 % can be achieved. Additionally, as expected, an increase in the feed velocity vf causes a decrease in the ηp, whereas an increase in the powder mass flow Φm results in an increase of ηp. With respect to the LBID, the ηp increases when the LBID is varied from Ring towards Gaussian-like. However, with an increase in the laser beam power P, the observed LBIDs influence becomes less pronounced. Consequently, additional qualitative changes of the process and potential benefits of the annular laser beam based variation of LBID in the case of axial powder feeding are expected at larger laser beam diameters do. 6. Discussion and conclusions In the paper an annular laser beam (ALB) based deposition head, which enabled axial delivery of the metal material, was presented together with its application to metal droplet, wire and powder direct deposition. In general, application of the annular laser beam and an axially fed material, results into increase of deposition process symmetry, which contributes towards higher stability and robustness of the process. This is especially evident in the case of metal droplet generation and deposition, which is a dynamically inherently unstable process [17] and has been stabilized by means of the application of an annular laser beam. Additionally, ALB based droplet generation, in comparison to other metal droplet generation processes, offers flexibility in the diameter, temperature and deposition time for each individual droplet, as well as flexibility in the used wire diameter and material, including materials with high melting points. In the case of laser direct wire deposition besides to the process symmetry an important advantage arising from of the application of an ALB and axial wire feeding is that the proportion of the energy input into the workpiece (WIP) and the wire can be well defined. It has been shown that WIP strongly influences the process stability and deposited layer geometrical properties. The highest process stability and robustness were achieved in the case of the largest investigated WIPs. From the stability point of view, laser beam direct powder deposition appears to be the most robust process. However, a
Acknowledgements The authors acknowledge the financial support received from the Slovenian Research Agency (core funding No. P2 0241) and DMG Mori Co., Ltd. References [1] Schmidt M, Merklein M, Bourell D, Dimitrov D, Hausotte T, Wegener K, Overmeyer L, Vollertsen F, Levy GN. Laser based additive manufacturing in industry and academia. CIRP Ann-Manuf Techn 2017;66(2):561-583. [2] Thompson SM, Bian L, Shamsaei N, Yadollahi A. An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics. Addit Manuf 2015;8:36–62. [3] Lamikiz A, Tabernero I, Ukar E, Martinez S, de Lacalle LNL. Current Designs of Coaxial Nozzles for Laser Cladding. Recent Pat Mech Eng 2011;4:29–36. [4] de Lange DF, Hofman JT, Meijer J. Influence of intensity distribution on the meltpool and clad shape for laser cladding. In: Proc. Third Int. WLT – Conference on Lasers in Manufacturing 2005.: 323-327. [5] Ocylok S, Leichnitz M, Thieme S, Ocylok S. Investigations on laser metal deposition of stainless steel 316L with coaxial wire feeding. 9th Int. Conf. on Photonic Techn. LANE 2016 :1–4. [6] Kuznetsov A, Jeromen A, Levy G, Fujishima M, Govekar E. Annular Laser Beam Cladding Process Feasibility Study. Phys Proc 2016;83:647– 656. [7] Fujishima M, Govekar E, Levy GN. Head for additive processing, processing machine, and processing method. Patent WO2017115406 2017. [8] Govekar E, Jeromen A, Kuznetsov A, Levy G, Fujishima M. Study of an annular laser beam based axially-fed powder cladding process. Submitted to CIRP Ann-Manuf Techn 2018. [9] Chun JH, Passow CH, Suh NP. Droplet-Based Manufacturing. CIRP AnnManuf Techn 1993;42:235–238. [10] Govekar E, Jerič A, Weigl M, Schmidt M. Laser droplet generation: Application to droplet joining. CIRP Ann-Manuf Techn 2009;58:205– 208. [11] Murr LE, Johnson WL. 3D metal droplet printing development and advanced materials additive manufacturing. J Mater Res Technol 2017;6:77–89. [12] Govekar E, Kuznetsov A, Jerič A. Drop on demand generation from a metal wire by means of an annular laser beam. J Mater Process Tech 2016;227:59–70. [13] Kuznetsov A, Jeromen A, Govekar E. Droplet detachment regimes in annular laser beam droplet generation from a metal wire. CIRP AnnManuf Techn 2014;63:225–228. [14] Kotar M, Govekar E. The influence of the workpiece illumination proportion in annular laser beam wire deposition process. 10th Conference on Photonic Technologies – LANE 2018 [15] Duocastella M, Arnold CB. Bessel and annular beams for materials processing. Laser Phot Rev 2012;6:607-621. [16] Roehling TT, Wu SSQ, Khairallah SA, Roehling JD, Soezeri SS, Crumb MF, Matthews MJ. Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing. Acta Materialia 2017;128:197–206. [17] Krese B, Perc M, Govekar E. The dynamics of laser droplet generation. Chaos 2010;20:01312.
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Procedia CIRP 00 (2017) 000–000 Procedia CIRP 74 (2018) 222–227 www.elsevier.com/locate/procedia th 10th CIRPConference Conference on on Photonic PhotonicTechnologies Technologies [LANE [LANE 2018] 2018] 10 CIRP
Annular laser beam based direct metal deposition 28th CIRP Design Conference, May 2018, Nantes, France a,
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Edvard Govekar *, Andrej Jeromen , Alexander Kuznetsov , Matjaž Kotar , Masaki Kondo A new methodology to analyze the functional and physical architecture of University of Ljubljana, Faculty of Mechanical Engineering, Aškečeva cesta 9, SI-1000 Ljubljana, Slovenia existing products for, CO., anLTD., assembly orientedNagoya product family DMG Mori 2-35-16 Meieki, Nakamura-ku, City Aichu 450-0002, Japan identification aa
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* Corresponding author. Tel.:+ 38-614-771-606 ; fax: +38-614-771-651. E-mail address: [email protected]
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
- Invited Paper École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC -EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France * Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected]
Abstract The paper provides a description of a direct annular laser beam based metal deposition head, which makes use of axial material feeding and Abstract variation of the laser beam intensity distribution (LBID) on the surface of the workpiece. The application and advantages of the developed head are presented by processes of direct metal droplet, metal wire and powder deposition. Due to the achieved process symmetry, the stability and Inrobustness today’s business environment, the trend This towards more product unbroken. Due towhich this development, the unstable need of of the processes are increased. is the most clearlyvariety seen inand thecustomization case of metal is droplet deposition, is an inherently agile and In reconfigurable production systems emerged to of cope variousofproducts and product families. To designisand optimize production process. the case of wire deposition, a precise pre-set the with proportion energy input into the workpiece surface enabled, which has been shown to anasimportant affecting thematches, stabilityproduct and process outcome. In are the needed. case of laser powder themethods benefits aim of the systems as be well to chooseparameter the optimal product analysis methods Indeed, most deposition, of the known to analyze a product or one product on the physical level. Different of thethe number and developed head are reflected in family achieved powder catchment efficiencyproduct greaterfamilies, than 80however, % and inmay the differ head'slargely ability in toterms influence deposited nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production layer properties by adjusting the LBID. © 2018AThe Authors. Published by Elsevier Ltd. This is an open access CC BY-NC-ND license system. new methodology is proposed to analyze existing products in article view ofunder theirthe functional and physical architecture. The aim is to cluster © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable (https://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review underBased responsibility of Flow the Bayerisches Bayerisches Laserzentrum GmbH. assembly systems. on Datum Chain, the Laserzentrum physical structure of the products is analyzed. Functional subassemblies are identified, and Peer-review under responsibility of the GmbH. a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Keywords:between Annular laser beam;families direct metal similarity product by deposition; providing process design stability support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. © 2017 The Authors. Published by Elsevier B.V. 1. Introduction parameters, governed2018. by the geometrical relations of the laser Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference
beam and of the fed wire or powder stream. Apart from the laser beam caustics and the geometry of the fed wire or technology which can be used to repair and to rebuild highpowder stream, the geometrical relations are primarily defined value damaged parts, to apply functional coatings, and to by the design of the deposition head which is used to supply manufacture complex 3D, free-form components [1]. It the metal material into the melt pool created by the laser 1.involves Introduction of the product range and characteristics manufactured and/or the continuous supply of a metallic material in the beam. Such a head consists of laser beam guiding and assembled in this system. In this context, the main challenge in form of a wire or powder and the delivery of a localised high focusing optics, and a metal wire or powder delivery nozzle, Due energy to thesource. fast The development in isthe of modelling and analysis is now not only to cope with single density energy source useddomain to heat and and forms one of components of LDMD systems which has communication and surface an ongoing trend of digitization and products, a limited product range or existing product families, melt the workpiece and supplied metal material. Due the greatest influence on the process. Depending on the digitalization, manufacturing enterprises are facing important but also to be able to analyze and to compare products to define to the many advantages which arise from such a highlyapplication, in the case of commercially available LDMD challenges environments: a continuing new product families. canaxial be observed thatlaser classical controlled in andtoday’s localisedmarket heat input, a laser beam is most systems, a head withIt an Gaussian beamexisting and a tendency towards reduction of product development times and product families are regrouped in function of clients frequently used as the high density energy source, so that single nozzle from one side (Fig. 1a) is usually usedortofeatures. deliver shortened product addition,has there is an increasing However, oriented product hardly to find. laser direct metal lifecycles. depositionIn (LDMD) been accepted as a the metal assembly material into the melt poolfamilies [3]. Theare main advantage demand of customization, being at the same time in a global On the product family level, products differ mainly in two promising additive manufacturing technology [2]. of a single side nozzle is its simple geometry and design. competition with competitors all over the world. This trend, main characteristics: (i) the number of components and (ii) the In LDMD the geometrical, metallurgical and mechanical However, the related deposition process is asymmetrical with which is inducing the development from macro to micro type of components (e.g. mechanical, electrical, electronical). properties of the deposited layer and the manufactured part respect to the laser beam and therefore dependent on the markets, results by in the diminished sizes due– tofedaugmenting Classicaldirection. methodologies considering products are influenced complex lot laser beam material deposition In order to ensure mainly processsingle symmetry and product varieties (high-volume to low-volume production) [1]. or solitary, already existing product families analyze the workpiece interactions. These are, in addition to the process a free shape, in the case of direction independent metal powTo cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which identify potentials in Ltd. the This existing difficulties an efficient definition and © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license 2212-8271 possible production system, it is important to have a precise knowledge comparison of different product families. Addressing this (http://creativecommons.org/licenses/by-nc-nd/3.0/) DirectAssembly; metal Design deposition an identification additive manufacturing Keywords: method;isFamily
Peer-review under responsibility of the Bayerisches Laserzentrum GmbH. 2212-8271 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2212-8271 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of scientific the Bayerisches Laserzentrum GmbH. Peer-review under responsibility of the committee of the 28th CIRP Design Conference 2018. 10.1016/j.procir.2018.08.099
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laser beam intensity distribution (LBID) from a Ring shape to a Gaussian like shape [8]. This brings additional advantages which are due to the possibility of adjusting the LBID towards a particular application optimum. In the following text the newly developed annular laser beam head is described, and its application to various processes of direct material deposition, including metal droplet, metal wire and powder deposition, all of which can be performed with minor modifications of the head. In addition to the achievement of a general increase in process stability, the observed and particular potential advantages of such an annular laser beam application are presented and discussed. 2. The annular laser beam direct metal deposition head The scheme of the novel annular laser beam (ALB) direct metal deposition head is presented in Fig. 2a. It consists of a laser beam shaping unit, a guiding mirror, a laser beam focusing unit, a material feeding unit, an axial material delivery nozzle, and a coaxial shielding gas nozzle. In the laser beam shaping unit, a collimated beam is shaped into an ALB. By means of the mirror the ALB is then guided coaxially to the axis of the material delivery tube into the focusing unit, so that it is focused onto the workpiece surface. The laser beam shaping unit, which consists of two axicons and a lens in between them can be used to shape various ALB caustics by varying the position of the lens. An example of an ALB caustic measured by the IR camera on a thin metal foil, with the related LBIDs vs. the laser beam focal plane lfw, is shown in Fig. 2b. In Fig. 2b θ denotes the convergence and γ the wedge angle of the caustic, which are defined based on the detection of the 1/e2 intensity boundaries of the LBID IR images at different workpiece standoff positions lfw. It can be seen that, by varying the position lfw, the LBID and the related laser beam diameter do can be changed. In general, by varying the positions lfw and the position of the lens between the axicons, various LBIDs can be generated, from Ring to Gaussian- like, at various diameters do. This property, together with the presented design of the head, makes the ALB direct metal deposition head applicable for various applications. This is because, by replacing the material feed-
Fig. 1. Deposition head- material feeding nozzle concepts: (a) Lateral with an axial beam; (b) Coaxial with an axial beam; (c) Axial with three laser beams; (d) Axial with an annular laser beam.
der deposition, a coaxial nozzle with multiple jets or a continuous annular stream is applied (Fig. 1b). A common property of a deposition head with a side or coaxial nozzle is that the Gaussian or Tophat laser beam is usually delivered along the axis of the head. Application of side or coaxially fed powder can cause overheating at the centre and laser beam power attenuation and insufficient heating at the edge of the deposited layer. This, together with higher powder density and higher cooling rates at the edge compared to the centre can lead to deep, non-uniform and even asymmetrical dilution with a weak bond at the edge of the deposited layer [4]. Additionally, side and coaxial powder feeding are accompanied by relatively low powder catchment efficiency. In order to increase powder catchment efficiency, and to deposit layers having the desired properties, the laser beam intensity and powder density distribution need to be adjusted properly so as to achieve a uniform heat input over the width of deposited layer. In order to avoid the previously mentioned drawbacks of heads which make use of an axial laser beam and a side or coaxial material delivery, recently a novel design with material supply along the head's axis and the coaxial delivery of multiple beams (Fig. 1c) [5], or with an annular laser beam, has been developed (Fig. 1d) [6,7]. In addition to axial feeding of the material, the latter enables variation of the .
Fig. 2. (a) Scheme of the ALB, direct metal deposition head, (b) Annular laser beam caustic and related LBIDs, (c) Wire and powder feeder unit
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ing unit (Fig. 2c) and making some minor adjustments to the head, a metal material in the form of a wire or powder can be applied, and the ALB direct metal deposition head can be used either for deposition of droplets generated from a metal wire, or for direct deposition i.e. laser cladding of metal wire or powder, as presented in the following. 3. Droplet deposition In metal droplet based manufacturing a molten droplet is deposited on the workpiece surface to form a bond with the surface [9]. The unique properties of metal droplet deposition, including a minimal heat load on the workpiece and discrete material deposition, can be employed in various applications such as joining [10], surface cladding, and additive manufacturing [11], especially when temperature-sensitive materials and components, e.g. ceramic substrates, thin layers or foils, are involved. Apart from conventional liquid jetting and electric arc based droplet deposition, laser based droplet deposition provides benefits due to the well-defined spatial and temporal energy input, including a low heat load of the surroundings, individual droplet size, and deposition timing control, and the ability to employ materials with a high melting point [12]. However, as described in the following, for the drop-on-demand and continuous droplet generation and deposition process to be stable, the axially symmetrical energy input which can be achieved by the annular laser beam focused onto the axially fed wire, as well as precise synchronization between the laser power and wire feeding velocity, are of great importance [13]. 3.1. Experimental In laser droplet deposition experiments, a Nd:YAG pulsed laser with a wavelength of 1064 nm was used as the energy source. The laser emits pulses with a time-dependent profile of power in the range from 0.5 to 8 kW and a duration from 0.3 to 20 ms. As a fed material in drop-on-demand deposition (DoD-D) and continuous droplet deposition (CD-D) a Ni wire of 0.6 and 0.25 mm diameter was used. The wire feeding unit shown in Fig. 2c consisted of a wire spool, a wire straightener, and a DC motor feeder, which was capable of 20 µm displacement resolution, a max. 30 m/s2 acceleration, and a max. velocity of 0.35 m/s. DoD-D consists of two phases: 1) formation of a pendant droplet, and 2) pendant droplet detachment from the solid wire-end, and deposition. The corresponding time course of the laser pulse power P and the wire feeding velocity vw are presented schematically in Fig. 3a. In order to form an initial,
Fig. 3. Laser pulse power P(t) and wire feeding velocity vw(t) in: (a) Drop-ondemand deposition, (b) Continuous droplet deposition.
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Fig. 4. Example of selected IR images of the droplet detachment phase, with induced Rayleigh-Plateau instability a t = 0.75 ms.
small-diameter pendant droplet, the wire-end is melted by the first part of the laser pulse, delivering the energy E1. Then, to increase the pendant droplet diameter to a pre-set value, the second part of the laser pulse delivers the energy E2 to melt an additional length of the wire, which is synchronously fed downwards at a feeding velocity of vw. In order to detach the formed pendant droplet, the wire is fed further downwards to position the droplet neck at the proper distance in relation to the laser beam focus, which has been shown to be a highly influential parameter [13]. The laser pulse with energy Ed is then used to force detachment by inducing Rayleigh-Plateau instability of the molten column of wire above the droplet, as illustrated in the example of the IR image sequence in Fig. 4. Using a 0.6 mm diameter Ni wire, droplets with diameters dd ranging from 0.85 to 1.25 mm can be generated, with a resolution of 50 µm and a standard deviation of 15 µm [13]. Besides the droplet-based joining applications presented in [10], in Fig. 5a various examples of different 2D patterns manufactured on a room-temperature Ti workpiece of 0.4 mm thickness are shown. They were produced in Ar protective atmosphere using a DoD-D at a standoff distance of 1 mm to demonstrate droplet diameter dd flexibility, repeatability, and accuracy of deposition. Additionally, in Fig. 5b an example DoD-D of a simple 3D structure i.e. pyramid is shown. Sequential metal droplet deposition, as required by certain applications, can be achieved by repeated DoD-D with implemented control, based on the initial temperature of the wire-end. Alternatively, continuous droplet deposition (CDD) can be used, where periodic laser pulses of frequency fp and duration τ are applied with a constant wire feeding velocity of vw (see Fig. 3b). In CD-D, the mechanism of the pendant droplet detachment and the corresponding detached
Fig. 5. Examples of deposited droplets: (a) Various 2D patterns with different dd, (b) A 3D structure, (c) Column and a porous wall structure.
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Fig. 7. (a) Definition of WIP, (b) Continuous and , (c) Broken molten bond.
Fig. 6. Influence of the laser pulse frequency fp on the generated droplet diameter dd (blue dots).
droplet diameter dd depend on the laser pulse frequency fp, as shown in Fig. 6. The presented results were obtained for the case of a 0.25 mm diameter Ni wire, with a feeding velocity of vw = 0.06 m/s, a pulse duration of τ = 0.8 ms, and using an average laser power of 120 W. At a low frequency fp = 50 Hz, the detachment of the smallest droplets occurs due to Rayleigh-Plateau instability, whereas at higher frequencies fp the detachment of larger droplets is triggered by resonant oscillation of the pendant droplet's eigenmodes with polar wavenumbers n = 1, 2, and 3, as has been proved theoretically in [13]. At high frequencies of fp > 250 Hz, spontaneous detachment of the largest droplets is caused by gravity alone. As can be seen in Fig. 6, the most stable detached droplet diameter dd is caused by the n = 1 eigenmode and by spontaneous detachment at the laser pulse frequencies fp = 130 and 300 Hz, respectively. The corresponding mean droplet diameters dd with standard deviation intervals were (1.18 ± 0.01) and (2.97 ± 0.02) mm, and the mean detachment frequencies were (3.79 ± 0.06) and (0.254 ± 0.006) Hz. An example of a column and a porous structure produced by CDD of 1.18 mm diameter droplets using a 0.25 mm diameter Ni wire and a laser pulse frequency of fp = 140 Hz is shown in Fig. 5c. 4. Wire direct deposition In laser direct metal deposition a material in the form of a wire or a powder can be used. In comparison with the use of a powder, one of the main advantages of laser direct wire deposition (LDWD) are the 100 % efficiency of the wire material, the high deposition rates, and the lower prices of metal wire [2]. On the other hand, the stability of the direct powder deposition process is fairly robust, whereas in the case of wire deposition the energy input into the workpiece surface and the wire has to be very well defined and synchronized with the wire vw and workpiece vwp feeding parameters. Thus, in general, process stability depends on many parameters, including the wire initial position and the inherent process parameters: laser beam power P, wire vw and workpiece vwp feeding velocity, which define the input energy. One of the advantages of annular laser beam direct wire deposition is that the workpiece illumination proportion (WIP) and the related energy input into the workpiece and wire surface can be influenced by variation of the annular beam caustic and the position of the workpiece surface hwf with respect to the ALB, as shown in Fig. 7a. A WIP value of
100 % means that all the laser beam energy is introduced into the workpiece surface. In the following text some results concerning the influence of WIP on the stability of the annular laser beam wire deposition process and its outcome are presented. 4.1. Experimental In the performed annular LDWD experiments, a 99.6 % pure Ni wire of 0.6 mm diameter was deposited on a 3.0 mm thick AISI 304 stainless steel workpiece. A CW diode laser, with a wavelength range of 900-1100 nm, was used as the energy source. The experiments involving single layer deposition were performed using a constant wire vf = 13 mm/s and workpiece feeding velocity vwp = 5 mm/s, and a laser beam power P in the range between 900 W and 2500 W, with steps of 100 W. In order to investigate the influence of WIP on the process, three different values of WIP, i.e. 7 %, 49 %, and 99 %, were selected. The process stability was analysed based on process visualisation using a CMOS visual camera. The related process outcome was characterised based on the geometrical characteristics of the deposited layer, i.e. its width w, height h, and maximum penetration depth hmd, all of which were extracted from photos of the deposited layer crosssections taken by an optical microscope. If the LDWD process is to be stable, a continuous molten bond has to be established, at the start, between the melt pool on the workpiece surface and the fed wire, and then maintained without breaking throughout the process. Photos of characteristic examples of a continuous and broken molten bond are shown in Fig. 7b and 7c. Based on this criterion, at a wire feeding velocity of vw = 13 mm/s and a scanning velocity of vwp = 5 mm/s, a stability diagram corresponding to the annular LDWD process in the space of the laser beam power P and WIP was defined, as is shown in Fig. 8, where markers denote the stable process windows. It can be seen that the WIP strongly influences the laser beam power P stability
Fig. 8. Stable process windows vs. P at values of WIP = 7, 49, and 99 %.
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Fig. 10. (a) IR camera based images of LBID, (b) related intensity profiles averaged over 360°.
5.1. Experimental Fig. 9. (a) Deposited wire cross sections and (b) cross section width w, height h and maximal penetration depth hmd vs. WIP at laser power P = 1.5 kW.
windows. At higher WIPs process stability is achieved at a higher laser beam power P, whereas the window width is narrower at lower WIPs and is nonlinearly dependent on the WIP. In addition to process stability the WIP also influences the process outcome. In Fig. 9 examples of photos of the deposited wire cross section at a laser beam power of P = 1.5 kW and WIPs of 7, 49, and 99 %, with the related values of the deposited layer's width w, height h, and maximum penetration depth hmd are shown. Closer examination of the photos of the cross-sections shown in Fig. 9a reveals that, at the lowest WIPs, the largest dilution area and wetting angle α are observed, which decrease as the WIP is increased. Based on the graphs shown in Fig. 9b, although not yet statistically confirmed, the nonlinear dependence of h, a linear increase in w, and a linear decrease in hmd with increasing WIPs, from 0 towards 100 %, could be assumed. As presented in [14], the observed relations do not qualitatively change with variation of the annular laser beam power P. 5. Powder deposition One advantage of laser powder deposition is the diversity of available powder materials, including pure metals, alloys, super alloys, ceramics and composites, which are not available in the form of a wire. In laser beam powder direct deposition the key problem is how to deliver a proper heat input by means of which, taking into account the laser beampowder-workpiece interactions, the process will run in a stable manner, and the related deposited layer or built-up part of the desired geometrical, metallurgical and mechanical properties will be produced. In comparison with the use of a wire, the stability of laser powder deposition is more robust. However, in the case of powder, as well as process stability and the mechanical properties of the manufactured part, the powder catchment efficiency ηp is an important process characteristic. All of these characteristics, in addition to the inherent process parameters including the laser beam power P, the powder mass flow Φm, and the workpiece feeding velocity vf,, can be influenced by the design of the head, which defines the powder supply and the related laser beam / powder / workpiece interactions, and the laser beam intensity distribution itself. The latter has been shown to be very important in many applications [15], including SLM [16].
In the laser metal powder deposition experiments a CW diode laser with a wavelength range of 900-1100 nm was used. Using SS316L powder with a particle size spanning from 53 to 125 µm, the deposition was performed on a SS304 workpiece with the dimensions (60 x 25 x 10) mm. Several experiments were performed using four different LBIDs, i.e. Ring, Tophat(-), Tophat(+) and a Gaussian like, with a 4σdiameter 3.0 mm, shown in Fig. 10. In order to monitor the process a CMOS visual camera was used. The related process outcome was characterized by the geometrical properties of the deposited layer and the powder catchment efficiency ηp. In Fig. 11a examples of the deposited layer's cross sections obtained at different LBIDs at a laser power P of 2 kW, a powder mass flow Φm = 8 g/min, and a workpiece feeding velocity vf = 10 mm/s, are shown. It can be seen that the shape of the deposited layer and the dilution are strongly influenced by the LBID. In the presented case the depth hmd of the Ushaped dilution increases with variation of the LBID from Ring towards Gaussian-like. At LBIDs with a high intensity at the center, due to the low LBI and the higher heat conduction into the workpiece at the edges of the deposited layer, a lack of dilution and insufficient melting can be observed at the edges of the cross-section. Additionally, in the case of the Tophat(+) and Gaussian-like LBID, an undesirable plasma plume above the melt pool was observed. The related mean values and the ±σ error bars of the geometrical characteristics of the deposited layers are presented in Fig. 11b. The clad wi-
Fig. 11. (a) Deposited layer cross sections at P =2.0 kW and (b) related geometrical characteristics of deposited layer vs. LBID.
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stable process does not necessarily result in the desired properties of the deposited layer. At present, one of the observed benefits of the annular laser beam and axially fed powder is high, above 80 % powder catchment efficiency. Additionally important benefits are expected from the influence of LBID, which can be varied from Ring to Gaussian-like, on the properties of the deposited layer Fig. 12. Powder catchment efficiency ηp at various LBID.
dth w remains almost constant, whereas the height h and the maximum penetration depth hmd increase as the LBID changes from Ring towards Gaussian-like. The results of powder catchment efficiency ηp including ±σ error bars, achieved at a laser beam power of P = 2.0 kW, with various LBIDs and process parameters, are shown in Fig. 12. The results showed that, in general, in the axially fed powder deposition process, a very high powder catchment efficiency ηp above 80 % can be achieved. Additionally, as expected, an increase in the feed velocity vf causes a decrease in the ηp, whereas an increase in the powder mass flow Φm results in an increase of ηp. With respect to the LBID, the ηp increases when the LBID is varied from Ring towards Gaussian-like. However, with an increase in the laser beam power P, the observed LBIDs influence becomes less pronounced. Consequently, additional qualitative changes of the process and potential benefits of the annular laser beam based variation of LBID in the case of axial powder feeding are expected at larger laser beam diameters do. 6. Discussion and conclusions In the paper an annular laser beam (ALB) based deposition head, which enabled axial delivery of the metal material, was presented together with its application to metal droplet, wire and powder direct deposition. In general, application of the annular laser beam and an axially fed material, results into increase of deposition process symmetry, which contributes towards higher stability and robustness of the process. This is especially evident in the case of metal droplet generation and deposition, which is a dynamically inherently unstable process [17] and has been stabilized by means of the application of an annular laser beam. Additionally, ALB based droplet generation, in comparison to other metal droplet generation processes, offers flexibility in the diameter, temperature and deposition time for each individual droplet, as well as flexibility in the used wire diameter and material, including materials with high melting points. In the case of laser direct wire deposition besides to the process symmetry an important advantage arising from of the application of an ALB and axial wire feeding is that the proportion of the energy input into the workpiece (WIP) and the wire can be well defined. It has been shown that WIP strongly influences the process stability and deposited layer geometrical properties. The highest process stability and robustness were achieved in the case of the largest investigated WIPs. From the stability point of view, laser beam direct powder deposition appears to be the most robust process. However, a
Acknowledgements The authors acknowledge the financial support received from the Slovenian Research Agency (core funding No. P2 0241) and DMG Mori Co., Ltd. References [1] Schmidt M, Merklein M, Bourell D, Dimitrov D, Hausotte T, Wegener K, Overmeyer L, Vollertsen F, Levy GN. Laser based additive manufacturing in industry and academia. CIRP Ann-Manuf Techn 2017;66(2):561-583. [2] Thompson SM, Bian L, Shamsaei N, Yadollahi A. An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics. Addit Manuf 2015;8:36–62. [3] Lamikiz A, Tabernero I, Ukar E, Martinez S, de Lacalle LNL. Current Designs of Coaxial Nozzles for Laser Cladding. Recent Pat Mech Eng 2011;4:29–36. [4] de Lange DF, Hofman JT, Meijer J. Influence of intensity distribution on the meltpool and clad shape for laser cladding. In: Proc. Third Int. WLT – Conference on Lasers in Manufacturing 2005.: 323-327. [5] Ocylok S, Leichnitz M, Thieme S, Ocylok S. Investigations on laser metal deposition of stainless steel 316L with coaxial wire feeding. 9th Int. Conf. on Photonic Techn. LANE 2016 :1–4. [6] Kuznetsov A, Jeromen A, Levy G, Fujishima M, Govekar E. Annular Laser Beam Cladding Process Feasibility Study. Phys Proc 2016;83:647– 656. [7] Fujishima M, Govekar E, Levy GN. Head for additive processing, processing machine, and processing method. Patent WO2017115406 2017. [8] Govekar E, Jeromen A, Kuznetsov A, Levy G, Fujishima M. Study of an annular laser beam based axially-fed powder cladding process. Submitted to CIRP Ann-Manuf Techn 2018. [9] Chun JH, Passow CH, Suh NP. Droplet-Based Manufacturing. CIRP AnnManuf Techn 1993;42:235–238. [10] Govekar E, Jerič A, Weigl M, Schmidt M. Laser droplet generation: Application to droplet joining. CIRP Ann-Manuf Techn 2009;58:205– 208. [11] Murr LE, Johnson WL. 3D metal droplet printing development and advanced materials additive manufacturing. J Mater Res Technol 2017;6:77–89. [12] Govekar E, Kuznetsov A, Jerič A. Drop on demand generation from a metal wire by means of an annular laser beam. J Mater Process Tech 2016;227:59–70. [13] Kuznetsov A, Jeromen A, Govekar E. Droplet detachment regimes in annular laser beam droplet generation from a metal wire. CIRP AnnManuf Techn 2014;63:225–228. [14] Kotar M, Govekar E. The influence of the workpiece illumination proportion in annular laser beam wire deposition process. 10th Conference on Photonic Technologies – LANE 2018 [15] Duocastella M, Arnold CB. Bessel and annular beams for materials processing. Laser Phot Rev 2012;6:607-621. [16] Roehling TT, Wu SSQ, Khairallah SA, Roehling JD, Soezeri SS, Crumb MF, Matthews MJ. Modulating laser intensity profile ellipticity for microstructural control during metal additive manufacturing. Acta Materialia 2017;128:197–206. [17] Krese B, Perc M, Govekar E. The dynamics of laser droplet generation. Chaos 2010;20:01312.