An extended laser beam heating model for a numerical platform to simulate multi‑material selective laser melting

A laser beam heating model (LBHM) is an important part of a platform for numerical modelling of a multi-material selective laser melting process. The LBHM is utilised as a ray-tracing algorithm that is widely applied for rendering in different applications, mainly for visualisation and very recently for laser heating models in selective laser melting. The model presented in this paper was further extended to transparent and translucent materials, including materials where transparency is dependent on the material temperature. In addition to refection and surface absorption, commonly considered in such models, phenomena such as refraction, scattering and volume absorption were also implemented. Considering associated energy transfer, the model represents a laser beam as a stream of moving articles, i.e. photons of the same energy. When the photons meet a boundary between materials, they are reflected, absorbed or transmitted according to geometric and thermal interfacial characteristics. This paper describes the LBHM in detail, its verification and validation, and also presents several simulation examples of the entire selective laser melting process with implemented LBHM

Interfacial oxidation in processing of nanocrystallised metallic materials using duplex technique - experimental and modelling aspects

Duplex techniques are attempted to be developed combining nanocrystallisation processes with a subsequent thermomechanical processing in order to produce multilayered bulk structures with improved yield and ultimate tensile strengths, while conserving an acceptable elongation to failure. However, bonding imperfections at the interfaces due to interfacial oxidation among other reasons during the duplex process can significantly influence the final properties. Moreover, the interface oxidation occurring during duplex processes influences microstructure development around the interfaces depending on whether the oxide scale is a continuous layer or a layer of discontinuous oxide clusters with heterogeneous thicknesses. Effectively the oxide scale becomes a part of microstructure development in such nano-crystallised multilayered structures. This paper deals with understanding of the underlying events around the highly reactive interfaces explaining the microstructure evolution applying advanced experimental and numerical modelling techniques. The research encompassed surface mechanical attrition treatment followed by constrained compression testing and hot rolling of the assembly of steel strips supported by multilevel numerical analysis using combined finite element (FE) and cellular automata (CA) methods. Shear banding has been observed near metal-metal contact between the oxide clusters at the interfaces. The shear banding can be considered as bonding enhancement creating channels for the base metal of the different laminates to come into contact through the oxidised interface. Temperature, texture and grain refining are among the factors influencing the shear banding. In the simulations, the meso-level of the developed multi-level FE-based model is combined with the advanced 3D frontal CA numerical model allowing for both appearance of the new boundaries and rotation of dislocation cells (sub-grains and grains) simultaneously

Additive manufacturing of multi layered bioactive materials with improved mechanical properties: modelling aspects

Multilayered laminate structures obtained by coating of ultrafine-grained metallic materials with bioactive and multifunctional composite coatings are considered for biomedical applications. Laser-assisted densification of multiple materials using laser cladding and selective laser melting is an alternative route to reduce the risk of early implant failure allowing for faster and cheaper fabrication. To understand the cooperative relationships between different factors that cam influence the manufacture of such bioactive laminates reflecting in their bioactivity and mechanical properties, the multi scale numerical modelling is applied. This work presents resent advances on development of integrated numerical models including generation, melting and solidification of the powder bed, considering surface flow, wettability, surface tension and other physical phenomena, specific mechanical and thermo-mechanical aspects and microstructure evolution

Inhomogeneity of plastic deformation in austenitic stainless steel after surface mechanical attrition treatment

Inhomogeneity of microstructure evolution in cold-rolled austenitic stainless steel after surface mechanical attrition treatment (SMAT) was investigated. A characteristic deformation pattern was obtained for all studied specimens. Selected areas were examined through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Estimation of the α′-martensite volume fraction below the treated surfaces showed, that some of the studied areas are characterised by significant different amount of this phase (from 10 to 22% - BB, from 19 to 31% - GIXD). The performed finite element (FE) numerical analysis showed, that the reason for this may be the presence of an air gap between the impacted material and fixation and also relatively short high stress time duration due to surface inclination during the surface treatment. Annealing at 550 and 650 °C greatly increased the volume fraction of α′-martensite (up to 47% - BB) and formed Fe2O3 as well as Fe3O4, whereas annealing at 700 °C resulted in both disappearance of α′-martensite and in reduction of oxides

Bioactive glass S520 laser cladding on ultrafine-grained pure titanium substrates

Nowadays, titanium alloys are commonly used for different biomedical applications instead of pure titanium because of their superior mechanical properties. Presence of some alloying elements, such as aluminium and vanadium, can be harmful to human health, and can be considered as disadvantage in long term applications. Potentially, there is a possibility of replacing the commercial titanium alloys with ultrafine-grained commercially pure Ti (cpTi). The yield and ultimate strength of cpTi can exceed 1000 MPa [1]. When manufacturing medical devices, laser cladding is known as one of the most promising methods for manufacturing of modern medical implants with improved osseointegration, where bioactive glass coatings are imposed on metallic substrates [2, 3]. Experimental Methods: In this work, S520 bioactive glass was imposed on ultrafine-grained cpTi using laser cladding technique. Cross-sectional SEM images of titanium substrate and bioactive glass were analyzed. The interface between bioactive glass and metallic titanium substrate was also studied using SEM/EDX. Results and Discussion: The refined microstructure of cpTi was locally modified in the areas affected by the laser beam. Figure 1 shows the cross-section of the ultrafine-grained cpTi substrate after the laser cladding process. The cross-section of the cladded bioactive glass is presented in figure 2. Some pores of up to 200 µm diameter were found within. Conclusion: The S520 bioactive glass was successfully cladded onto the ultrafine-grained cpTi substrate. The application of cpTi allows for exclusion of potential toxic elements from the human body and its refined microstructure allows to achieve strength properties similar to those of Ti6Al4V alloy

Nanocrystallised multilayered metalic materials for structural applications: properties and processing

Polish: W artykule przedstawiono charakterystykę procesu wytwarzania nanokrystalicznych wielowarstwowych materiałów konstrukcyjnych. Omówiono zalety i wady poszczególnych rozwiązań technologicznych, a także potencjalne zastosowania wyrobów gotowych. Przeprowadzono analizę numeryczną procesu „duplex” z wykorzystaniem metody elementów skończonych i modelowania wieloskalowego oraz uzyskano rozkład naprężeń i odkształceń wokół utelniających się powierzchni rozdziału. Uwzględniono obecność klastrów zgorzelinowych o różnych wymiarach. Wyniki numeryczne są zbliżone z wynikami badań doświadczalnych. English: Processing of the nano-crystallised multilayered metallic materials designed for structural applications is discussed in the work. Advantages and disadvantages of different technological approaches as well as the potential product applications are discussed. As an example, numerical analysis of a duplex technique is presented. The analysis is based on application of the finite element multi-level methodology. The stress and strain distributions around the oxidised interface of the multilayered metallic material are analysed. Numerical results show good agreement with available experimental data

Characterization of nanocrystallised multilayered metallic materials produced by the SMAT followed by constrained compression

Nanocrystallised multilayered metallic material was obtained via duplex technique combining the surface mechanical attrition treatment (SMAT) with a novel constrained compression (CC) process. At the initial stage the 1 mm thick sheets of 316L austenitic stainless steel were processed by the SMAT in order to form a nanocrystalline structure. At the final stage disc-shaped plates excised from SMATed sheets, were assembled in a package and compressed in order to produce metallurgical bonding between individual plates. The characterization of such a multilayered structure was studied both experimentally and numerically. The microscopic examination revealed that the bonding occurred in the central portions of the package where the oxide scale covering each plate was fragmented by high shear strains. The numerical analysis confirmed that the strains at the interior interfaces are significantly higher than at the external ones. A high degree of structural inhomogeneity was observed via TEM studies in the regions where the successful bonding was achieved. Regions characterised by fine band structure with the presence of α′-martensite phase as well as coarse cellular structure within a single γ-austenite phase were identified

Analysis of the porosity degree during laser-assisted cladding of bioactive glass on titanium substrates with highly refined grain structure

Titanium alloys, due to their exceptional mechanical properties and biocompatibility, are commonly used to produce medical implants nowadays. However, the presence of such elements as aluminium and vanadium can be harmful to human health. One of the possible solutions could be replacing the titanium alloys with commercially pure titanium (cpTi) with highly refined grain structure. One of the most promising methods in manufacturing medical implants with improved biological fixation is laser cladding in which bioactive glass coatings are imposed on metallic substrates. The aim of this work is to present a 3D numerical modelling of the above mentioned additive manufacturing process. The obtained model is able to predict the stress-strain and temperature distributions as well as porosity degree during the processing. Porosity affects the bioactivity of medical implants as it significantly improves their ability to bonding with host tissues

Towards Processing of Multilayered Metallic Materials - Constrained Compression Testing

The complex analysis of the interface behaviour has been performed during constrained compression testing of 316L steel plates as a way towards processing of the multilayered nanostructured metallic materials. The approach is based on a combination of experiments under appropriate operating conditions and computer modelling based on finite element (FE) methodology for interpretation of the test results. Multilayered metallic structure was successfully obtained using the constrained compression testing technique. The specially designed die and compression specimens allowed for joining of the steel plates together even at room temperatures. The performed numerical analysis using the ABAQUS/Standard FE software revealed the strain and stress localisation areas within the multilayered structure among other features that are described in the paper. The results are in agreement with experimental observations

Numerical modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Microstructure evolution around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials have been investigated and discussed in the present work. Conditions leading to grain refinement during co-rolling stage of the duplex processing technique are analysed using the multi-level finite element based numerical model combined with three-dimensional frontal cellular automata. The model was capable to simulate development of grain boundaries and changes of the boundary disorientation angle within the metal structure taking into account crystal plasticity formulation. Appearance of a large number of structural elements, identified as dislocation cells, sub-grains and new grains, has been identified within the metal structure as a result of metal flow disturbance and consequently inhomogeneous deformation around oxide islets at the interfaces during the co-rolling stage. These areas corresponded to the locations of shear bands observed experimentally using SEM-EBSD analysis. The obtained results illustrate a significant potential of the proposed modelling approach for quantitative analysis and optimisation of the highly refined non-homogeneous microstructures formed around the oxidised interfaces during processing of such laminated materials