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See detailProcessing of alloy Ti-6Al-4V and of stainless steel 316L by Laser Beam Melting
Mertens, Anne ULg; Paydas, Hakan ULg; Reginster, Sylvie ULg et al

Conference (2014, May)

Additive manufacturing processes such as Selective Laser Melting (SLM) appear very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic ... [more ▼]

Additive manufacturing processes such as Selective Laser Melting (SLM) appear very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic materials such as Ti alloys and stainless steels. Practically, in SLM, a metallic powder is deposited layer-by-layer in a powder bed and then molten locally according to the desired shape. An important feature of this process is that the structure undergoes an ultra-fast cooling once the beam leaves the working zone, thus giving rise to strongly out-of-equilibrium microstructures. In the case of Ti alloy Ti-6Al-4V, in particular, the microstructural anisotropy resulting from the epitaxial growth of the newly deposited layer on the material previously solidified has been shown to exert a very strong influence on the mechanical properties [1] In the present work, the thermophysical behaviour of Ti-alloy Ti-6Al-4V and of stainless steel 316L has been characterised in details, in order to reach a better understanding of the phenomena controlling the microstructures and mechanical properties of parts. In particular, the thermal conductivity of Ti-alloy Ti-6Al-4V and of stainless steel 316L at high temperature has been determined by combining dilatometry, Differential Scanning Calorimetry (DSC) and laser flash diffusivimetry based on Laplace’s equation. Since Ti-alloy Ti-6Al-4V and stainless steel 316L exhibit quite different physical behaviours, their careful comparison is shown to shed more light into the role of phenomena such as epitaxial growth, out-of-equilibrium phase transformations and/or internal stresses in the additive manufacturing of metallic materials. [less ▲]

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See detailMechanical properties of alloy Ti-6Al-4V and of stainless steel 316L processed by Selective Laser Melting: Influence of out-of-equilibrium microstructures
Mertens, Anne ULg; Reginster, Sylvie ULg; Paydas, Hakan ULg et al

in Powder Metallurgy (2014), 57(3), 184-189

Ti-6Al-4V and stainless steel 316L have been processed by selective laser melting under similar conditions, and their microstructures and mechanical behaviours have been compared in details. Under the ... [more ▼]

Ti-6Al-4V and stainless steel 316L have been processed by selective laser melting under similar conditions, and their microstructures and mechanical behaviours have been compared in details. Under the investigated conditions, Ti-6Al-4V exhibits a more complex behaviour than stainless steel 316L with respect to the occurrence of microstructural and mechanical anisotropy. Moreover, Ti-6Al-4V appears more sensitive to the build-up of internal stresses when compared with stainless steel 316L, whereas stainless steel 316L appears more prone to the formation of “lack of melting” defects. This correlates nicely with the difference in thermal conductivity between the two materials. Thermal conductivity was shown to increase strongly with increasing temperature and the thermophysical properties appeared to be influenced by variations in the initial metallurgical state. [less ▲]

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See detailMicrostructures and Mechanical Properties of Stainless Steel AISI 316L Processed by Selective Laser Melting
Mertens, Anne ULg; Reginster, Sylvie ULg; Contrepois, Quentin et al

in Materials Science Forum (2014), 783-786 (2014)

In this study, samples of stainless steel AISI 316L have been processed by selective laser melting, a layer-by-layer near-net-shape process allowing for an economic production of complex parts. The ... [more ▼]

In this study, samples of stainless steel AISI 316L have been processed by selective laser melting, a layer-by-layer near-net-shape process allowing for an economic production of complex parts. The resulting microstructures have been characterised in details in order to reach a better understanding of the solidification and consolidation processes. The influence of the processing parameters on the mechanical properties was investigated by means of uniaxial tensile testing performed on samples produced with different main orientations with respect to the building direction. A strong anisotropy of the mechanical behaviour was thus interpreted in relation with the microstructures and the processing conditions. [less ▲]

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See detailMeasuring the thermophysical properties of materials at high temperature - Application to the additive manufacturing of alloy Ti-6Al-4V and of stainless steel 316L
Mertens, Anne ULg; Paydas, Hakan ULg; Reginster, Sylvie ULg et al

Conference (2013, November 07)

In view of optimising the microstructures of metallic materials and obtaining the desired properties, the accurate characterisation of the thermophysical behaviour of these materials has long been ... [more ▼]

In view of optimising the microstructures of metallic materials and obtaining the desired properties, the accurate characterisation of the thermophysical behaviour of these materials has long been considered of paramount importance e.g. by allowing for the in-situ study of phase transformations, by providing data for numerical simulations and, essentially, by contributing to a better understanding of the fundamental mechanisms at play during processing. In the present work, the thermophysical behaviour of Ti-alloy Ti-6Al-4V and of stainless steel 316L has been characterised in details, in order to reach a better understanding of the phenomena controlling the microstructures and mechanical properties of parts made by additive manufacturing techniques that appear nowadays very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic materials. In particular, the thermal conductivity of Ti-alloy Ti-6Al-4V and of stainless steel 316L at high temperature has been determined by combining dilatometry, Differential Scanning Calorimetry (DSC) and laser flash diffusivimetry based on Laplace’s equation : k(T)=α(T)ρ(T)Cp(T) where k(T) is the thermal conductivity (W/m*K) α(T) is the thermal diffusivity (mm2/s) ρ(T) is the specific mass (g/cm3) Cp(T) is the specific heat capacity (J/g*K). Since Ti-alloy Ti-6Al-4V and stainless steel 316L exhibit quite different physical behaviours, their careful comparison is shown to shed more light into the role of phenomena such as epitaxial growth, out-of-equilibrium phase transformations and/or internal stresses in the additive manufacturing of metallic materials. [less ▲]

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See detailOn the Role of Out-of-Equilibrium Microstructures in Ti-6Al-4V and in Stainless Steel 316L Processed by Selective Laser Melting in Determining their Mechanical Properties
Mertens, Anne ULg; Reginster, Sylvie ULg; Paydas, Hakan ULg et al

Conference (2013, September 12)

Additive manufacturing processes such as Selective Laser Melting (SLM) appear very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic ... [more ▼]

Additive manufacturing processes such as Selective Laser Melting (SLM) appear very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic materials such as Ti alloys and stainless steels. Practically, in SLM, a metallic powder is deposited layer-by-layer in a powder bed and then molten locally according to the desired shape. An important feature of this process is that the structure undergoes an ultra-fast cooling once the beam leaves the working zone, thus giving rise to strongly out-of-equilibrium microstructures. In the case of Ti alloy Ti-6Al-4V, in particular, the microstructural anisotropy resulting from the epitaxial growth of the newly deposited layer on the material previously solidified has been shown to exert a very strong influence on the mechanical properties [1]. In the present work, the microstructures and mechanical properties of Ti-6Al-4V and of stainless steel 316L processed by SLM have been characterised in details. Since these two materials exhibit quite different physical behaviours, their careful comparison might shed more light into the role of phenomena such as epitaxial growth, out-of-equilibrium phase transformation and/or internal stresses in determining the mechanical properties of metallic parts processed by SLM. [less ▲]

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See detailProcessing of Ti alloys by additive manufacturing: a comparison of the microstructures obtained by laser cladding, selective laser melting and electron beam melting
Reginster, Sylvie ULg; Mertens, Anne ULg; Paydas, Hakan ULg et al

in Materials Science Forum (2013), 765

Additive manufacturing processes such as laser cladding (LC) or selective laser melting (SLM) appear very promising in view of an economic near-net-shape production – and also, in the case of LC, the ... [more ▼]

Additive manufacturing processes such as laser cladding (LC) or selective laser melting (SLM) appear very promising in view of an economic near-net-shape production – and also, in the case of LC, the restoration - of complex and (almost) fully dense parts from Ti alloys. Both techniques involve the melting of a metallic powder with a laser. In the SLM process, the metallic powder is deposited layer by layer in a powder bed and then molten locally according to the desired shape, whereas in LC, the metallic powder is directly projected onto a substrate through a nozzle coaxial with the laser beam. The present research aims at comparing Ti-6Al-4V samples processed by these two techniques with reference samples produced by electron beam melting (EBM), another well established additive manufacturing process (patented by Arcam AB Company) in which a powder bed is molten locally by means of an electron beam. In all three processes, the melt pool undergoes an ultrafast cooling and solidifies very rapidly once the beam has left the area, thus giving rise to strongly out-of-equilibrium microstructures. Yet, each one of these processes also has its own specificities e.g. in terms of scanning strategy and of working atmosphere (low vacuum vs. protective Ar flow). In the present work, the microstructures obtained by these three processes have been compared in details, with a particular attention for characteristics such as porosity, grain size, and the various phases present. Since epitaxial growth of the newly deposited layer on the material previously solidified has been shown [1, 2] to exert a strong influence on the microstructure and on the resulting mechanical properties, great care has been taken to study the microstructural anisotropy associated with each one of the three processes. [less ▲]

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See detailTi alloys processed by selective laser melting and by laser cladding: microstructures and mechanical properties
Mertens, Anne ULg; Contrepois, Quentin ULg; Dormal, Thierry ULg et al

in proceedings of 12th EUROPEAN CONFERENCE ON SPACECRAFT STRUCTURES, MATERIALS & ENVIRONMENTAL TESTING, Noordwijk 20-23 mars 2012 (2012, March)

Selective laser melting (SLM) and laser cladding were developed in the late 1990s as economic layer-by-layer near-net-shape processes allowing for the production – and also, in the case of laser cladding ... [more ▼]

Selective laser melting (SLM) and laser cladding were developed in the late 1990s as economic layer-by-layer near-net-shape processes allowing for the production – and also, in the case of laser cladding, the restoration - of complex parts. Both techniques involve the melting of a metallic powder with a laser. In the case of SLM, the metallic powder is deposited layer by layer and then molten locally according to the desired shape, whereas in laser cladding the metallic powder is projected onto a substrate through a tube coaxial with the laser. In both processes, the metallic melt pools then cool down and solidify very rapidly, thus producing strongly out of equilibrium microstructures that might exhibit high internal stresses. In the present work, efforts have been made to enhance the flexibility of the laser cladding process: a second laser with a maximum power of 300W was installed beside the original laser (with a higher maximum power of 2000W), thus allowing for the processing of parts with thinner walls and/or coatings. Moreover, flexibility was also improved in relation with the geometry of the parts by use of a 5-axes displacement control. Samples of alloy Ti-6Al-4V, that is widely used in the aeronautic industry due to its high specific strength, have been processed both by SLM and by laser cladding. The resulting microstructures have been characterised in details by optical microscopy, SEM and EBSD so as to allow for a better understanding of the solidification process and of the subsequent phase transformations taking place upon cooling for both techniques. The influence of processing parameters such as the orientation of the deposition of the successive powder layers on the mechanical properties was also investigated by means of uniaxial tensile testing performed on samples with different deposition orientations in regard to the direction of mechanical solicitation. Moreover, some of the samples for mechanical testing had undergone an annealing treatment at 640°C for 4 hours to relieve internal stresses, in order to assess more precisely the effect of those stresses on the tensile properties. [less ▲]

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