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In situ alloying and reinforcing of Al6061 during selective laser melting
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Procedia CIRP 00 (2017) Procedia CIRP 000–000 74 (2018) 39–43 www.elsevier.com/locate/procedia
10th CIRP 10th CIRPConference Conference on on Photonic Photonic Technologies Technologies [LANE [LANE 2018] 2018]
In situ alloying 28th andCIRP reinforcing of Al6061 during selective Design Conference, May 2018, Nantes, France laser melting a, a b b Sasan Dadbakhsh *, Raya Mertens , Kim the Vanmeensel , Jef Vleugels , Jan Vanarchitecture Humbeeckb, JeanA new methodology to analyze functional and physical of Pierre Krutha
existing products for an assembly oriented product family identification a
PMA, Department of Mechanical Engineering, KU Leuven & Member of Flanders Make, Leuven 3001, Belgium b Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
* Corresponding author. Tel.: +32-163-72866 ; fax: +32-163-22987. E-mail address: [email protected] 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
This work investigates the feasibility of a novel method to simultaneously alloy and reinforce a low alloyed Al alloy (i.e., Al6061) during selective laser melting (SLM) via in situ decomposition of zinc oxide (ZnO). Based on Gibbs free energy calculations, an Al6061+6wt%ZnO Abstract powder mixture is designed and prepared. The thermal decomposition of ZnO, resulting in the formation of Al oxide and free Zn, Thisproduct also provides extra alloys and reinforces the Al matrix. thermal energy that altersDue the todynamics of the melt Insimultaneously today’s business environment, the trend towards more variety and customization is unbroken. this development, thepool needand of necessitates a completelyproduction different set of optimised SLM parameters compared to traditional Al alloys. AfterToSLM, it isand shown that this method agile and reconfigurable systems emerged to cope with various products and product families. design optimize production can successfully thethe Al optimal matrix with numerous nanometer oxide particles ~ 50-120 nm). this methods clear success to systems as well asreinforce to choose product matches, product sized analysis methods are(typically needed. Indeed, most ofDespite the known aim to manufacture in situ reinforced Al composites by SLM, the applied method could not avoid partial Zn evaporation (limiting in situ alloying) and analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and could not successfully suppress the cracking that also occurs after SLM of unreinforced Al6061. nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production © 2018 2018 The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. This This is is an an open open access access article under under the CC CC BY-NC-ND BY-NC-ND license license © system. AThe new methodology is proposed to analyze existing products inarticle view of theirthe functional and physical architecture. The aim is to cluster (http://creativecommons.org/licenses/by-nc-nd/3.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable Peer-review under under responsibility responsibility of of the the Bayerisches Bayerisches Laserzentrum Laserzentrum GmbH. GmbH. Peer-review assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and
a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Keywords: In situ alloying and reinforcing, Aluminium alloy; Metal matrix composites; Laser processing; Powder metallurgy; Microstructure; similarity between product families by providing design 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 This is very important for Al alloys where the equilibrium Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
solid solubility of many elements is very limited [1]. Aluminium metal matrix composites (Al MMCs) are For SLM of MMCs, two distinguished (i) ex situ and (ii) in Keywords: Assembly; Design method; Family identification increasingly demanded from a variety of applications such as situ approaches have been utilised. As an ex situ technique, aerospace industries due to a possible combination of high two (or more) constituents are introduced to the thermal conductivity, high strength, low density, and/or manufacturing process, leading to the main constituent 1.relatively Introduction of the product range andadditives. characteristics manufactured and/or reinforced by the minor For example, Al alloys are low-cost. Due to the wide range of applications for assembled system.reinforcements In this context, such the main challenge in reinforced inbythisvarious as TiC [2, 3], Al MMCs, selective laser melting (SLM) process has also Dueemerged to the fast development in the domain modelling is now notusing only the to cope single SiC analysis or Mg2AlO SLM with process. In Al2O3 [4], and been to manufacture them. In SLM, a laser is used of to 4 [5, 6] communication ongoingcross trendsections of digitization and products, range product families, contrast toa limited ex situ product techniques, in or an existing in situ approach SLM is selectively fuse and a set an of desired layer by layer digitalization, manufacturing enterprises are facing but also be able to control analyze aand to compare products to define used to to trigger and chemical reaction producing in on a metal-based powder bed. The powder based, important layer-bychallenges today’s environments: a continuing new It can be observed that classical existing situ product formed families. reinforcement particles within the matrix. This layer and inlaser fusionmarket principle of SLM are excellent tendency towards reduction of product development and product families are regrouped function of clients or features. approach can provide more in stable reinforcements, cleaner advantages to manufacture composites with a times uniform shortened product lifecycles. In addition, there isgeometry an increasing However, assembly oriented families are hardly to find. particle–matrix interface andproduct stronger interfacial bonding, and distribution of secondary phases in a complex from demand of customization, being at the materials). same time In in addition, a global On the product of family products inside differ mainly in two better distribution fine level, reinforcements the matrix [7any desirable mixture (even reactive competition with competitors all solidification, over the world. This trend, main (i)that the this number of components andsuitable (ii) the 9]. Itcharacteristics: should be noted approach only targets SLM is associated with a rapid modifying the which is inducing the development from refinement, macro to micro type of components (e.g.submicron mechanical, electrical, electronical). reactions for nano and particle reinforced MMCs, microstructure through microstructural solid markets, in diminished lot sizes due tohomogeneity. augmenting Classical methodologies mainly single products being in contrast with thoseconsidering chemical reactions to bind metal solubilityresults extension and increasing chemical product varieties (high-volume to low-volume production) [1]. or solitary, already existing product families analyze the To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 possible © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license identify potentials in Ltd. the This existing difficulties an efficient definition and (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this 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.2018.08.009
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powders to manufacture low function multi-material composites in some early works. One of the earliest reports to manufacture SLM assisted in situ Al composites has been reported by Dadbakhsh and Hao [10-13]. In this method, an exothermic reaction in the mixture of Al/Fe2O3 was triggered and controlled by a laser during the SLM process. As a result, a significant amount of extra energy was released during the creation of new constituents. This led to SLM parts containing a uniform distribution of very fine particles (e.g. 50-2000 nm) ranging from oxides to intermetallics. In another work, Chang et al [14] added SiC particles to AlSi10Mg alloy for the SLM process. This resulted in reaction of some of SiC with AlSi10Mg alloy matrix and partial formation of Al4SiC4 and therefore an Al alloy matrix containing SiC and Al4SiC4 (and also some Mg2Si). In contrast to previous works, this work for the first time attempts to develop a new in situ made composite from a mixture of Al6061 and zinc oxide via the SLM process. Supported by thermodynamic calculations, the experimental results aim at illustrating the potential to manufacture such in situ particle reinforced composites by SLM, highlighting the corresponding microstructural features and outlining the challenges.
2.2. Experimental procedure Al6061 powder (LPW, UK) was provided with a particle size of 15-100 µm. In weight percent, Al6061 contained 0.81.2 Mg, 0.4-0.8 Si, 0.25 Zn, 0.15 Mn, 0.35 Cr, 0.15-0.4 Cu, 0.7 Fe, 0.15Ti, and Al balance. This was mixed with 6wt% zinc oxide (ZnO). ZnO with 99.9% purity was supplied from Sigma-Aldrich (USA) with a particle size below 5µm. Mixing was performed by mechanical mixing in a turbula mixer, sieving (using a 100µm mesh sieve) and once again mechanical mixing. This resulted in a homogeneous mixture in which ZnO particles decorate Al6061 powder surfaces, as seen in Fig. 1. It is should be noted that due to the poor flowability of the powder mixture, no flow could be measured using Hall flow or Carney test equipment.
2. Theoretical and experimental procedure 2.1. Theoretical basis This work is based on a SLM-controlled in situ reaction between Al and ZnO according to standard Gibbs free energy of formation, as below: 2Al (l) +1.5O2 (g) → Al2O3 (s) ΔG0=-1687200+326.8T (J), at 993-2327 K [15]
(1)
Zn (v) + 0.5O2 (g) → ZnO (s) ΔG0=-460200+198T (J), at 1243-1973 K [15]
(2)
2Al (l)+3ZnO (s)→ 3Zn (v)+ Al2O3 (s) ΔG0=-306600-267.2T (J), at 1243-1973 K
(3)
As seen from the Eq. 3 (derived from Eqs. 1-2), the energy for formation of Zn and Al2O3 would be always negative in an Al and ZnO system (at least when Al is liquid). This means that ZnO tends to decompose next to Al during SLM and form Zn and Al2O3. Accordingly, the current work investigates SLM of an Al/ZnO powder system. After SLM, the aim is to dissolve the released Zn in the Al matrix while the in situ formed Al2O3 phase is desired to appear as a strengthening secondary phase. Inspired by the Zn content in a typical high strength Al 7xxx alloy, 6 wt% ZnO was added to the system. This theoretically generates about 4.9 wt% Zn and 2.5 wt% Al2O3 in an Al matrix.
Fig. 1. (a) Al6061 powders decorated by (b) very fine ZnO particles.
An in-house built SLM machine equipped with a 1 kW fibre laser with a beam diameter (d1/e²) of about 60 µm was used to manufacture samples under argon atmosphere. For SLM, a range of different parameters such as laser power (P), scanning speed (v), hatch spacing (h), and layer thickness (t) were practiced. A 90° rotation and a bi-directional scanning were applied. SLM was done on both Al6061 and Al6061/6wt%ZnO powder mixture to give a comparative understanding. The thermal behaviour of Al6061 and Al6061/ZnO powder mixtures was analysed by differential scanning calorimetry (DSC) (Mettler Toledo) operated from RT to 700 °C. The sample cross-sections were viewed using an Axiocam Leica optical microscope after chemical etching with Keller’s reagent at 20°C. The samples were also viewed using a FEI XL30 scanning electron microscope (SEM) and a Scanning EM NanoSEM 450. Zn composition was measured using XRF.
Sasan Dadbakhsh et al. / Procedia CIRP 74 (2018) 39–43 Author name / Procedia CIRP 00 (2018) 000–000
3. Results & Discussion 3.1. DSC To illustrate the effect of ZnO on the thermal behaviour of Al6061/ZnO mixtures, DSC (via heating) was carried out on Al6061, and Al6061/16wt%ZnO powders. Al6061/16wt%ZnO has been selected to generate stronger reactions and to provide a better contrast. As seen from Fig. 2, during heating three reactions can be distinguished for the Al6061+ZnO powder mixture, while only one reaction occurs for the pure Al6061 matrix. Peak (1) and peak (2) appear clearer at higher heating rates (see the curves for Al6061/16wt%ZnO heated by 20 and 30°C/min rates). The origin of the peaks can be speculated according to their shape, location, and their thermodynamic classification. In fact, peak (1), as a wide endothermic peak occurring at lower temperatures, can be associated to the decomposition of ZnO in the presence of aluminium (i.e., ZnO → Zn + 0.5O2). The second peak (which is exothermic) can be attributed to the formation of secondary phases such as alumina just below the melting point of the Al6061 matrix. These mean that the in situ mechanisms can take place even before melting, as a decomposition step followed by formation of constituents. Nevertheless, since the formation of a secondary oxide phase (such as Al oxide) is an exothermic process, the released heat can partially compensate for the required heat of melting (reaction 3). This leads to smaller melting peaks in the presence of ZnO in comparison with only Al6061 (compare Al6061 and Al6061+16wtZnO with 10 °C/min heating rate).
41 3
scanning when most of the in situ reaction products have already been consumed. This drastic change in melting pool size along with the intensive presence of secondary phases stimulate a very dynamic nature into the melt region (as observed by excessive spatter of particles to surrounding areas). Consequently, the consolidation mechanism significantly alters, in such a manner that maximum consolidation is reached at a higher powder layer thickness. Fig. 4 shows that a better density can be achieved when 60µm thick layers and lower scanning speeds (for instance about 200 mm/s) are applied during SLM. This observation is in a clear contrast with the SLM parameters used to process common Al SLM alloys such as AlSi10Mg, in which lower layer thicknesses are used (e.g. 30µm) and higher scan speeds (above 1000 mm/s) are applied [16].
Fig. 3. The visual appearance of parts during SLM of Al6061+6wt%ZnO powders; a typical part during (a) first laser scanning and (b) second laser scanning without changing laser parameters.
Fig. 4. Dimensional density of SLM components made from Al6061+6wt%ZnO at different powder thicknesses. Fig. 2. DSC curves of Al6061 and Al6061+16wt%ZnO powders heated with different rates of 10, 20, and 30 °C/min.
3.2. Process Optimisation Fig. 3 shows the visual conditions during SLM of the Al6061/ZnO system. As seen, addition of 6wt% ZnO has visually enlarged the melting spot about 2-3 times, as the melting spot appears drastically smaller in the second
3.3. Cross-sectional and microstructural analysis After optimisation, the cross sections of the optimised parts were analysed (Fig. 5). As observed, Al6061 has severely cracked after SLM along with many remaining and large pores. In comparison, addition of 6wt% ZnO failed to eliminate the cracking (seemingly from hot tears in Al6061). However, addition of in situ reacting ZnO may reduce the
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porosity content, but in a completely different set of parameters (from speed and layer thickness to hatch spacing and laser powder).
Fig. 5. Cross sectional micrographs of SLM parts made from (a) Al6061 (made by P=275 W, v=1400mm/s, h=105µm, t=30µm) and (b) Al6061+6wt%ZnO (made by P=400 W, v=200mm/s, h=140µm, t=60µm).
Fig. 6 better reveals the microstructural characteristics of the SLM parts made from Al6061+6wt%ZnO. As seen from Fig. 6a, no severe agglomeration is notable and the secondary particles are relatively homogeneously distributed. However, it seems that laser melting dynamics have affected the distribution of particles with different sizes. Different morphologies of secondary phases are better demonstrated in Fig. 6b in which two distinguished morphological secondary phases are recognisable, i.e., i) coarser polygonal phases and ii) finer spherical particles. The former perhaps originates from the tearing of oxide films existing on Al powders. These films can evaporate or tear up and remain in the Al matrix. In contrast, the latter (i.e., fine spherical particles) are the in situ oxides that are forming during SLM. That is why their sizes are extremely small within nano size range (typically around 50 nm to 120 nm), and they appear with a perfectly spherical morphology (Fig. 6c). These nano sized in situ formed reinforcing particles also show a relatively good interface with Al matrix, meaning that they can potentially act as effective strengthening reinforcements. Despite this notable success, very fine submicron pores appear in Al matrix. Based on the very small size and the perfectly spherical morphology of these submicron pores, they can be attributed to the evaporation of Zn after the decomposition of ZnO. This can also be observed as white smoke during SLM processing (see Fig. 3a), resulting in a reduced Zn content in the final component. Despite the fact that this reduces Zn in the material composition, in situ alloying (as one of the main goals of this work) is still effective and Zn can be partially absorbed to the material composite structure.
Fig. 6. Microstructural characteristics of SLM part from Al6061+6wt%ZnO (made by P=400 W, v=200mm/s, h=140µm, t=60µm) at various magnifications. The samples are viewed along building direction.
4. Conclusions SLM was used to trigger and control an in situ reaction in an Al/6wt%ZnO powder mixture. It was found that: - In situ reactions in Al/ZnO mixture could progress via a decomposition and reformation mechanism. This can be done even below the melting point. - The extra energy generated during the exothermic reaction between Al/ZnO completely changed the dynamics of the melt pool. This visually appeared as a much larger melt pool with excessive spattering of particles. - The enlarged melt pool combined with the presence of secondary particles altered the optimum parameters attaining a dense part. In this work, the highest densities have been reached at a low scanning speed and a higher layer thickness in contrast to what has been commonly practiced to reach dense Al parts. - After decomposition, Zn can diffuse into the Al structure, or form fine gas pores, or even leave the Al melt pool as a white smoke. Accordingly, only a part of the designated zinc contributed to the in situ alloying, leading to a final Zn content less than the amount that was primarily intended. - After SLM, very fine reinforcements with a typical size of 50-120 nm and a relatively homogeneous distribution appeared in the Al matrix without any notable agglomeration. This confirms the in situ reinforcing of the described system. - Despite the severe solidification nucleation through the in situ formed secondary phases, laser solidification cracks still appeared in the Al6061 matrix. - Although the cracking and the partial Zn evaporation limited the success from in situ reinforcing, the findings of this work can be used for designing improved material systems.
Sasan Dadbakhsh et al. / Procedia CIRP 74 (2018) 39–43 Author name / Procedia CIRP 00 (2018) 000–000
Acknowledgments This work was supported by the Flemish Agency for Innovation by Science and Technology (IWT) under Strategic Basic Research (SBO) project “MultiMet”, Belgium. The financial support from the internal KU Leuven GOA project, entitled ‘Summa’ is appreciated. References [1] Dadbakhsh S, Hao L, Kruth J-P. Selective laser melting towards manufacture of three dimensional in situ Al matrix composites: A review. in: VRAP, Leiria: CRC Press; 2013. p. 303-8. [2] Yuan P, Gu D, Dai D. Particulate migration behavior and its mechanism during selective laser melting of TiC reinforced Al matrix nanocomposites. Mater Des 2015;82:46-55. [3] Gu D, Wang H, Dai D, Yuan P, Meiners W, Poprawe R. Rapid fabrication of Al-based bulk-form nanocomposites with novel reinforcement and enhanced performance by selective laser melting. Scr Mater 2015;96:258. [4] Han Q, Setchi R, Evans SL. Synthesis and characterisation of advanced ball-milled Al-Al2O3 nanocomposites for selective laser melting. Powder Technol 2016;297:183-92. [5] Manfredi D, Calignano F, Krishnan M, Canali R, Ambrosio EP, Biamino S, Ugues D, Pavese M, Fino P. Additive Manufacturing of Al Alloys and Aluminium Matrix Composites (AMCs). in: Monteiro WA (Ed.) Light Metal Alloys Applications. Rijeka; InTech 2014. p. Ch. 01. [6] Manfredi D, Canali R, Krishnan M, Ambrosio EP, Calignano F, Pavese M, Miranti L, Belardinelli S, Biamino S, Fino P. Aluminium matrix composites (AMCs) by DMLS. in: VRAP, Leiria: CRC Press; 2013, p. 249-54. [7] Fan T, Zhang D, Yang G, Shibayanagi T, Naka M. Fabrication of in situ Al2O3/Al composite via remelting. J Mater Process Technol 2003;142:556-61. [8] Tong XC, Fang HS. Al-TiC composites in situ-processed by ingot metallurgy and rapid solidification technology: Part I. Microstructural evolution. Metall Mater Trans A 1998;29 A:875-91. [9] Yu P, Deng C-J, Ma N-G, Yau M-Y, Ng DHL. Formation of nanostructured eutectic network in α-Al2O3 reinforced Al–Cu alloy matrix composite. Acta Mater 2003;51:3445-54. [10] Dadbakhsh S, Hao L. In situ formation of particle reinforced Al matrix composite by selective laser melting of Al/Fe2O3 powder mixture. Adv Eng Mater 2012;14:45-8. [11] Dadbakhsh S, Hao L. Effect of Al alloys on selective laser melting behaviour and microstructure of in situ formed particle reinforced composites. J Alloys Compd 2012;541:328-34. [12] Dadbakhsh S, Hao L. Effect of Layer Thickness in Selective Laser Melting on Microstructure of Al/5 wt.%Fe2O3 Powder Consolidated Parts. Sci World J 2014;2014:106129. [13] Dadbakhsh S, Hao L. Effect of Fe2O3 content on microstructure of Al powder consolidated parts via selective laser melting using various laser powers and speeds. Int J Adv Manuf Technol 2014;73:1453-63. [14] Chang F, Gu D, Dai D, Yuan P. Selective laser melting of in-situ Al4SiC4+SiC hybrid reinforced Al matrix composites: Influence of starting SiC particle size. Surf Coat Technol 2015;272:15-24. [15] Gaskell DR. Introduction to the thermodynamics of materials. 5th ed. New York: Taylor and Francis; 2008. [16] Mertens R, Clijsters S, Kempen K, Kruth J-P. Optimization of Scan Strategies in Selective Laser Melting of Aluminum Parts With Downfacing Areas. J Manuf Sci Eng 2014;136:061012-7.
43 5
ScienceDirect
Available online atonline www.sciencedirect.com Available at www.sciencedirect.com
ScienceDirect ScienceDirect
Procedia CIRP 00 (2018) 000–000
www.elsevier.com/locate/procedia
Procedia CIRP 00 (2017) Procedia CIRP 000–000 74 (2018) 39–43 www.elsevier.com/locate/procedia
10th CIRP 10th CIRPConference Conference on on Photonic Photonic Technologies Technologies [LANE [LANE 2018] 2018]
In situ alloying 28th andCIRP reinforcing of Al6061 during selective Design Conference, May 2018, Nantes, France laser melting a, a b b Sasan Dadbakhsh *, Raya Mertens , Kim the Vanmeensel , Jef Vleugels , Jan Vanarchitecture Humbeeckb, JeanA new methodology to analyze functional and physical of Pierre Krutha
existing products for an assembly oriented product family identification a
PMA, Department of Mechanical Engineering, KU Leuven & Member of Flanders Make, Leuven 3001, Belgium b Department of Materials Engineering, KU Leuven, Leuven 3001, Belgium
Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat
* Corresponding author. Tel.: +32-163-72866 ; fax: +32-163-22987. E-mail address: [email protected] 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
This work investigates the feasibility of a novel method to simultaneously alloy and reinforce a low alloyed Al alloy (i.e., Al6061) during selective laser melting (SLM) via in situ decomposition of zinc oxide (ZnO). Based on Gibbs free energy calculations, an Al6061+6wt%ZnO Abstract powder mixture is designed and prepared. The thermal decomposition of ZnO, resulting in the formation of Al oxide and free Zn, Thisproduct also provides extra alloys and reinforces the Al matrix. thermal energy that altersDue the todynamics of the melt Insimultaneously today’s business environment, the trend towards more variety and customization is unbroken. this development, thepool needand of necessitates a completelyproduction different set of optimised SLM parameters compared to traditional Al alloys. AfterToSLM, it isand shown that this method agile and reconfigurable systems emerged to cope with various products and product families. design optimize production can successfully thethe Al optimal matrix with numerous nanometer oxide particles ~ 50-120 nm). this methods clear success to systems as well asreinforce to choose product matches, product sized analysis methods are(typically needed. Indeed, most ofDespite the known aim to manufacture in situ reinforced Al composites by SLM, the applied method could not avoid partial Zn evaporation (limiting in situ alloying) and analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and could not successfully suppress the cracking that also occurs after SLM of unreinforced Al6061. nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production © 2018 2018 The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. This This is is an an open open access access article under under the CC CC BY-NC-ND BY-NC-ND license license © system. AThe new methodology is proposed to analyze existing products inarticle view of theirthe functional and physical architecture. The aim is to cluster (http://creativecommons.org/licenses/by-nc-nd/3.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable Peer-review under under responsibility responsibility of of the the Bayerisches Bayerisches Laserzentrum Laserzentrum GmbH. GmbH. Peer-review assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and
a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Keywords: In situ alloying and reinforcing, Aluminium alloy; Metal matrix composites; Laser processing; Powder metallurgy; Microstructure; similarity between product families by providing design 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 This is very important for Al alloys where the equilibrium Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
solid solubility of many elements is very limited [1]. Aluminium metal matrix composites (Al MMCs) are For SLM of MMCs, two distinguished (i) ex situ and (ii) in Keywords: Assembly; Design method; Family identification increasingly demanded from a variety of applications such as situ approaches have been utilised. As an ex situ technique, aerospace industries due to a possible combination of high two (or more) constituents are introduced to the thermal conductivity, high strength, low density, and/or manufacturing process, leading to the main constituent 1.relatively Introduction of the product range andadditives. characteristics manufactured and/or reinforced by the minor For example, Al alloys are low-cost. Due to the wide range of applications for assembled system.reinforcements In this context, such the main challenge in reinforced inbythisvarious as TiC [2, 3], Al MMCs, selective laser melting (SLM) process has also Dueemerged to the fast development in the domain modelling is now notusing only the to cope single SiC analysis or Mg2AlO SLM with process. In Al2O3 [4], and been to manufacture them. In SLM, a laser is used of to 4 [5, 6] communication ongoingcross trendsections of digitization and products, range product families, contrast toa limited ex situ product techniques, in or an existing in situ approach SLM is selectively fuse and a set an of desired layer by layer digitalization, manufacturing enterprises are facing but also be able to control analyze aand to compare products to define used to to trigger and chemical reaction producing in on a metal-based powder bed. The powder based, important layer-bychallenges today’s environments: a continuing new It can be observed that classical existing situ product formed families. reinforcement particles within the matrix. This layer and inlaser fusionmarket principle of SLM are excellent tendency towards reduction of product development and product families are regrouped function of clients or features. approach can provide more in stable reinforcements, cleaner advantages to manufacture composites with a times uniform shortened product lifecycles. In addition, there isgeometry an increasing However, assembly oriented families are hardly to find. particle–matrix interface andproduct stronger interfacial bonding, and distribution of secondary phases in a complex from demand of customization, being at the materials). same time In in addition, a global On the product of family products inside differ mainly in two better distribution fine level, reinforcements the matrix [7any desirable mixture (even reactive competition with competitors all solidification, over the world. This trend, main (i)that the this number of components andsuitable (ii) the 9]. Itcharacteristics: should be noted approach only targets SLM is associated with a rapid modifying the which is inducing the development from refinement, macro to micro type of components (e.g.submicron mechanical, electrical, electronical). reactions for nano and particle reinforced MMCs, microstructure through microstructural solid markets, in diminished lot sizes due tohomogeneity. augmenting Classical methodologies mainly single products being in contrast with thoseconsidering chemical reactions to bind metal solubilityresults extension and increasing chemical product varieties (high-volume to low-volume production) [1]. or solitary, already existing product families analyze the To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 possible © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license identify potentials in Ltd. the This existing difficulties an efficient definition and (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this 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.2018.08.009
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powders to manufacture low function multi-material composites in some early works. One of the earliest reports to manufacture SLM assisted in situ Al composites has been reported by Dadbakhsh and Hao [10-13]. In this method, an exothermic reaction in the mixture of Al/Fe2O3 was triggered and controlled by a laser during the SLM process. As a result, a significant amount of extra energy was released during the creation of new constituents. This led to SLM parts containing a uniform distribution of very fine particles (e.g. 50-2000 nm) ranging from oxides to intermetallics. In another work, Chang et al [14] added SiC particles to AlSi10Mg alloy for the SLM process. This resulted in reaction of some of SiC with AlSi10Mg alloy matrix and partial formation of Al4SiC4 and therefore an Al alloy matrix containing SiC and Al4SiC4 (and also some Mg2Si). In contrast to previous works, this work for the first time attempts to develop a new in situ made composite from a mixture of Al6061 and zinc oxide via the SLM process. Supported by thermodynamic calculations, the experimental results aim at illustrating the potential to manufacture such in situ particle reinforced composites by SLM, highlighting the corresponding microstructural features and outlining the challenges.
2.2. Experimental procedure Al6061 powder (LPW, UK) was provided with a particle size of 15-100 µm. In weight percent, Al6061 contained 0.81.2 Mg, 0.4-0.8 Si, 0.25 Zn, 0.15 Mn, 0.35 Cr, 0.15-0.4 Cu, 0.7 Fe, 0.15Ti, and Al balance. This was mixed with 6wt% zinc oxide (ZnO). ZnO with 99.9% purity was supplied from Sigma-Aldrich (USA) with a particle size below 5µm. Mixing was performed by mechanical mixing in a turbula mixer, sieving (using a 100µm mesh sieve) and once again mechanical mixing. This resulted in a homogeneous mixture in which ZnO particles decorate Al6061 powder surfaces, as seen in Fig. 1. It is should be noted that due to the poor flowability of the powder mixture, no flow could be measured using Hall flow or Carney test equipment.
2. Theoretical and experimental procedure 2.1. Theoretical basis This work is based on a SLM-controlled in situ reaction between Al and ZnO according to standard Gibbs free energy of formation, as below: 2Al (l) +1.5O2 (g) → Al2O3 (s) ΔG0=-1687200+326.8T (J), at 993-2327 K [15]
(1)
Zn (v) + 0.5O2 (g) → ZnO (s) ΔG0=-460200+198T (J), at 1243-1973 K [15]
(2)
2Al (l)+3ZnO (s)→ 3Zn (v)+ Al2O3 (s) ΔG0=-306600-267.2T (J), at 1243-1973 K
(3)
As seen from the Eq. 3 (derived from Eqs. 1-2), the energy for formation of Zn and Al2O3 would be always negative in an Al and ZnO system (at least when Al is liquid). This means that ZnO tends to decompose next to Al during SLM and form Zn and Al2O3. Accordingly, the current work investigates SLM of an Al/ZnO powder system. After SLM, the aim is to dissolve the released Zn in the Al matrix while the in situ formed Al2O3 phase is desired to appear as a strengthening secondary phase. Inspired by the Zn content in a typical high strength Al 7xxx alloy, 6 wt% ZnO was added to the system. This theoretically generates about 4.9 wt% Zn and 2.5 wt% Al2O3 in an Al matrix.
Fig. 1. (a) Al6061 powders decorated by (b) very fine ZnO particles.
An in-house built SLM machine equipped with a 1 kW fibre laser with a beam diameter (d1/e²) of about 60 µm was used to manufacture samples under argon atmosphere. For SLM, a range of different parameters such as laser power (P), scanning speed (v), hatch spacing (h), and layer thickness (t) were practiced. A 90° rotation and a bi-directional scanning were applied. SLM was done on both Al6061 and Al6061/6wt%ZnO powder mixture to give a comparative understanding. The thermal behaviour of Al6061 and Al6061/ZnO powder mixtures was analysed by differential scanning calorimetry (DSC) (Mettler Toledo) operated from RT to 700 °C. The sample cross-sections were viewed using an Axiocam Leica optical microscope after chemical etching with Keller’s reagent at 20°C. The samples were also viewed using a FEI XL30 scanning electron microscope (SEM) and a Scanning EM NanoSEM 450. Zn composition was measured using XRF.
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3. Results & Discussion 3.1. DSC To illustrate the effect of ZnO on the thermal behaviour of Al6061/ZnO mixtures, DSC (via heating) was carried out on Al6061, and Al6061/16wt%ZnO powders. Al6061/16wt%ZnO has been selected to generate stronger reactions and to provide a better contrast. As seen from Fig. 2, during heating three reactions can be distinguished for the Al6061+ZnO powder mixture, while only one reaction occurs for the pure Al6061 matrix. Peak (1) and peak (2) appear clearer at higher heating rates (see the curves for Al6061/16wt%ZnO heated by 20 and 30°C/min rates). The origin of the peaks can be speculated according to their shape, location, and their thermodynamic classification. In fact, peak (1), as a wide endothermic peak occurring at lower temperatures, can be associated to the decomposition of ZnO in the presence of aluminium (i.e., ZnO → Zn + 0.5O2). The second peak (which is exothermic) can be attributed to the formation of secondary phases such as alumina just below the melting point of the Al6061 matrix. These mean that the in situ mechanisms can take place even before melting, as a decomposition step followed by formation of constituents. Nevertheless, since the formation of a secondary oxide phase (such as Al oxide) is an exothermic process, the released heat can partially compensate for the required heat of melting (reaction 3). This leads to smaller melting peaks in the presence of ZnO in comparison with only Al6061 (compare Al6061 and Al6061+16wtZnO with 10 °C/min heating rate).
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scanning when most of the in situ reaction products have already been consumed. This drastic change in melting pool size along with the intensive presence of secondary phases stimulate a very dynamic nature into the melt region (as observed by excessive spatter of particles to surrounding areas). Consequently, the consolidation mechanism significantly alters, in such a manner that maximum consolidation is reached at a higher powder layer thickness. Fig. 4 shows that a better density can be achieved when 60µm thick layers and lower scanning speeds (for instance about 200 mm/s) are applied during SLM. This observation is in a clear contrast with the SLM parameters used to process common Al SLM alloys such as AlSi10Mg, in which lower layer thicknesses are used (e.g. 30µm) and higher scan speeds (above 1000 mm/s) are applied [16].
Fig. 3. The visual appearance of parts during SLM of Al6061+6wt%ZnO powders; a typical part during (a) first laser scanning and (b) second laser scanning without changing laser parameters.
Fig. 4. Dimensional density of SLM components made from Al6061+6wt%ZnO at different powder thicknesses. Fig. 2. DSC curves of Al6061 and Al6061+16wt%ZnO powders heated with different rates of 10, 20, and 30 °C/min.
3.2. Process Optimisation Fig. 3 shows the visual conditions during SLM of the Al6061/ZnO system. As seen, addition of 6wt% ZnO has visually enlarged the melting spot about 2-3 times, as the melting spot appears drastically smaller in the second
3.3. Cross-sectional and microstructural analysis After optimisation, the cross sections of the optimised parts were analysed (Fig. 5). As observed, Al6061 has severely cracked after SLM along with many remaining and large pores. In comparison, addition of 6wt% ZnO failed to eliminate the cracking (seemingly from hot tears in Al6061). However, addition of in situ reacting ZnO may reduce the
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porosity content, but in a completely different set of parameters (from speed and layer thickness to hatch spacing and laser powder).
Fig. 5. Cross sectional micrographs of SLM parts made from (a) Al6061 (made by P=275 W, v=1400mm/s, h=105µm, t=30µm) and (b) Al6061+6wt%ZnO (made by P=400 W, v=200mm/s, h=140µm, t=60µm).
Fig. 6 better reveals the microstructural characteristics of the SLM parts made from Al6061+6wt%ZnO. As seen from Fig. 6a, no severe agglomeration is notable and the secondary particles are relatively homogeneously distributed. However, it seems that laser melting dynamics have affected the distribution of particles with different sizes. Different morphologies of secondary phases are better demonstrated in Fig. 6b in which two distinguished morphological secondary phases are recognisable, i.e., i) coarser polygonal phases and ii) finer spherical particles. The former perhaps originates from the tearing of oxide films existing on Al powders. These films can evaporate or tear up and remain in the Al matrix. In contrast, the latter (i.e., fine spherical particles) are the in situ oxides that are forming during SLM. That is why their sizes are extremely small within nano size range (typically around 50 nm to 120 nm), and they appear with a perfectly spherical morphology (Fig. 6c). These nano sized in situ formed reinforcing particles also show a relatively good interface with Al matrix, meaning that they can potentially act as effective strengthening reinforcements. Despite this notable success, very fine submicron pores appear in Al matrix. Based on the very small size and the perfectly spherical morphology of these submicron pores, they can be attributed to the evaporation of Zn after the decomposition of ZnO. This can also be observed as white smoke during SLM processing (see Fig. 3a), resulting in a reduced Zn content in the final component. Despite the fact that this reduces Zn in the material composition, in situ alloying (as one of the main goals of this work) is still effective and Zn can be partially absorbed to the material composite structure.
Fig. 6. Microstructural characteristics of SLM part from Al6061+6wt%ZnO (made by P=400 W, v=200mm/s, h=140µm, t=60µm) at various magnifications. The samples are viewed along building direction.
4. Conclusions SLM was used to trigger and control an in situ reaction in an Al/6wt%ZnO powder mixture. It was found that: - In situ reactions in Al/ZnO mixture could progress via a decomposition and reformation mechanism. This can be done even below the melting point. - The extra energy generated during the exothermic reaction between Al/ZnO completely changed the dynamics of the melt pool. This visually appeared as a much larger melt pool with excessive spattering of particles. - The enlarged melt pool combined with the presence of secondary particles altered the optimum parameters attaining a dense part. In this work, the highest densities have been reached at a low scanning speed and a higher layer thickness in contrast to what has been commonly practiced to reach dense Al parts. - After decomposition, Zn can diffuse into the Al structure, or form fine gas pores, or even leave the Al melt pool as a white smoke. Accordingly, only a part of the designated zinc contributed to the in situ alloying, leading to a final Zn content less than the amount that was primarily intended. - After SLM, very fine reinforcements with a typical size of 50-120 nm and a relatively homogeneous distribution appeared in the Al matrix without any notable agglomeration. This confirms the in situ reinforcing of the described system. - Despite the severe solidification nucleation through the in situ formed secondary phases, laser solidification cracks still appeared in the Al6061 matrix. - Although the cracking and the partial Zn evaporation limited the success from in situ reinforcing, the findings of this work can be used for designing improved material systems.
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Acknowledgments This work was supported by the Flemish Agency for Innovation by Science and Technology (IWT) under Strategic Basic Research (SBO) project “MultiMet”, Belgium. The financial support from the internal KU Leuven GOA project, entitled ‘Summa’ is appreciated. References [1] Dadbakhsh S, Hao L, Kruth J-P. Selective laser melting towards manufacture of three dimensional in situ Al matrix composites: A review. in: VRAP, Leiria: CRC Press; 2013. p. 303-8. [2] Yuan P, Gu D, Dai D. Particulate migration behavior and its mechanism during selective laser melting of TiC reinforced Al matrix nanocomposites. Mater Des 2015;82:46-55. [3] Gu D, Wang H, Dai D, Yuan P, Meiners W, Poprawe R. Rapid fabrication of Al-based bulk-form nanocomposites with novel reinforcement and enhanced performance by selective laser melting. Scr Mater 2015;96:258. [4] Han Q, Setchi R, Evans SL. Synthesis and characterisation of advanced ball-milled Al-Al2O3 nanocomposites for selective laser melting. Powder Technol 2016;297:183-92. [5] Manfredi D, Calignano F, Krishnan M, Canali R, Ambrosio EP, Biamino S, Ugues D, Pavese M, Fino P. Additive Manufacturing of Al Alloys and Aluminium Matrix Composites (AMCs). in: Monteiro WA (Ed.) Light Metal Alloys Applications. Rijeka; InTech 2014. p. Ch. 01. [6] Manfredi D, Canali R, Krishnan M, Ambrosio EP, Calignano F, Pavese M, Miranti L, Belardinelli S, Biamino S, Fino P. Aluminium matrix composites (AMCs) by DMLS. in: VRAP, Leiria: CRC Press; 2013, p. 249-54. [7] Fan T, Zhang D, Yang G, Shibayanagi T, Naka M. Fabrication of in situ Al2O3/Al composite via remelting. J Mater Process Technol 2003;142:556-61. [8] Tong XC, Fang HS. Al-TiC composites in situ-processed by ingot metallurgy and rapid solidification technology: Part I. Microstructural evolution. Metall Mater Trans A 1998;29 A:875-91. [9] Yu P, Deng C-J, Ma N-G, Yau M-Y, Ng DHL. Formation of nanostructured eutectic network in α-Al2O3 reinforced Al–Cu alloy matrix composite. Acta Mater 2003;51:3445-54. [10] Dadbakhsh S, Hao L. In situ formation of particle reinforced Al matrix composite by selective laser melting of Al/Fe2O3 powder mixture. Adv Eng Mater 2012;14:45-8. [11] Dadbakhsh S, Hao L. Effect of Al alloys on selective laser melting behaviour and microstructure of in situ formed particle reinforced composites. J Alloys Compd 2012;541:328-34. [12] Dadbakhsh S, Hao L. Effect of Layer Thickness in Selective Laser Melting on Microstructure of Al/5 wt.%Fe2O3 Powder Consolidated Parts. Sci World J 2014;2014:106129. [13] Dadbakhsh S, Hao L. Effect of Fe2O3 content on microstructure of Al powder consolidated parts via selective laser melting using various laser powers and speeds. Int J Adv Manuf Technol 2014;73:1453-63. [14] Chang F, Gu D, Dai D, Yuan P. Selective laser melting of in-situ Al4SiC4+SiC hybrid reinforced Al matrix composites: Influence of starting SiC particle size. Surf Coat Technol 2015;272:15-24. [15] Gaskell DR. Introduction to the thermodynamics of materials. 5th ed. New York: Taylor and Francis; 2008. [16] Mertens R, Clijsters S, Kempen K, Kruth J-P. Optimization of Scan Strategies in Selective Laser Melting of Aluminum Parts With Downfacing Areas. J Manuf Sci Eng 2014;136:061012-7.
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