Additive Manufacturing Of Ti6Al4V Research Articles

Hybrid additive manufacturing of an electron beam powder bed fused Ti6Al4V by transient liquid phase bonding

Hybrid Additive Manufacturing (HAM) is a production strategy enhancing the flexibility of the already versatile Additive Manufacturing (AM) techniques. AM of Ti6Al4V, on the other hand, has been of great interest to numerous research works, thanks to the unique corrosion, biomedical and mechanical properties of the alloy. Hence, this research marks the first report on the HAM of Ti6Al4V by Transient Liquid Phase (TLP) bonding of an Electron Beam Powder Bed Fused (EB-PBF) sample to a conventional one. A copper interlayer was used for bonding, and the TLP process was performed at 890 °C and 970 °C for 60 min. Shear strength test was carried out and the results showed the highest shear strengths of 579.3 and 662.5 MPa for TLP bonding at 890 °C and 970 °C, respectively. By increasing the bonding temperature to 970 °C, no Cu-rich phases were observed in the microstructure, as opposed to the 890 °C samples, and a complete isothermal solidification without intermetallic phases was achieved. Moreover, the 970 °C TLP sample was featured with a much better microstructural integrity and homogeneity in both the base metals and the bonded zone. TLP bonding at 970 °C resulted in a more ductile fracture surface than that bonded at 890 °C. The strong differences between the two TLP bonds were primarily attributed to the faster diffusion rate of elements along the joint and base metal at higher temperatures.

Thermodynamically consistent phase-field modeling of competitive polycrystalline growth of beta grains during additive manufacturing of Ti6Al4V

In this paper, a thermodynamically consistent multi-phase and multi-component phase-field (PF) model was developed for the prediction of competitive growth of β grains during Ti6Al4V solidification in the directed energy deposition (DED) process. By incorporating the thermodynamic and diffusion mobility databases of the Ti-Al-V system, the concentration evolutions of Al and V solutes were accurately predicted separately for the first time, where the solidified β has a slightly higher Al but lower V than the liquid phase, matching the phase diagrams very well. Influences of cross-diffusion between solutes on concentration distributions were also investigated. The PF modeling predicted elongated β grains of columnar shape during Ti6Al4V solidification, whose growth pattern was highly consistent with the experimental results. A validated DED model was also utilized to acquire the temporal and spatial temperature profiles and heating/cooling rates in the molten pool. By coupling such thermal information, the grain growth and concentration evolutions under the influence of cooling rate were carried out.

Mechanical anisotropy and its evolution with powder reuse in Electron Beam Melting AM of Ti6Al4V

Powder reuse is recognized as a key to industrialization of metal additive manufacturing (AM), which necessitates that changes in the structural behavior of metal with reuse are clearly understood. In this investigation, the mechanical properties of Ti6Al4V resulting from Electron Beam Melting (EBM) were evaluated as a function of powder reuse over 30 build cycles. The metal was characterized in the horizontal and vertical build orientations under uniaxial tension to failure in the as-built and machined conditions. Results showed that there was an increase in strength and decrease in ductility of the metal with powder reuse, resulting from rising oxygen content of the powder. The elongation at failure and tensile toughness of the metal exhibited anisotropy, which increased substantially with powder reuse. For the horizontal orientation, the elongation at failure decreased by over 60% in the 30 builds, which was one and a half times greater reduction than that for the vertical orientation. The anisotropy in ductility with reuse is suspected to result from changes in microstructure with oxygen content, specifically changes in the grain boundary alpha phase. Additionally, the powder recovery system (PRS) was identified as a contributor to powder oxidation.

Additive Manufacturing of Ti6Al4V with Wire Laser Metal Deposition Process

Additive manufacturing (AM) using wire as an input material is currently in full swing, with very strong growth prospects thanks to the possibility of creating large parts, with high deposition rates, but also a low investment cost compared to the powder bed fusion machines. A versatile 3D printing device using a Direct Energy Deposition Wire-Laser (DED-W Laser) with Precitec Coaxprinter station to melt a metallic filler wire is developed to build titanium parts by optimizing the process parameters. The geometrical and metallurgical of produced parts are analyzed. In the literature, several authors agree to define wire feed speed, travel speed, and laser beam power as first-order process parameters governing laser-wire deposition. This study shows the relative importance of these parameters taking separately as well as the importance of their sequencing at the start of the process. Titanium deposit are obtained with powers never explored in bibliography (up to 5 kW), and wire feed speed up to 5 m.min-1 with a complete process repeatability.

Powder reuse and its contribution to porosity in additive manufacturing of Ti6Al4V

Powder reuse has become a central issue in the pursuit to industrialize metal Additive Manufacturing (AM). Furthermore, build porosity is a critical concern to component reliability and damage tolerance of the metal. However, the contributions of powder reuse to metal porosity has received very limited attention. In this study, the porosity resulting from Powder Bed Fusion-Electron Beam Melting (PBF-EB) AM of Ti6Al4V was characterized over a series of 30 build cycles (consisting of ~ 480 h cumulative build time) using X-ray Micro Computed Tomography (μCT). Investigated were the volume fraction of pores (i.e. the porosity), as well as the pore size, shape, and spatial distribution. The most prevalent pores identified were: i) those originating from the gas atomized powder, and ii) those caused by incomplete fusion of the melt pool. Although there was a minor reduction in the pore size distribution with powder reuse, the overall average volumetric porosity was 0.10 ± 0.02% and there was no significant change with increasing reuse. An inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Whereas the greatest pore density was located at the transition between the contour and melt beams, approximately 0.5–1.0 mm adjacent to the surface of parts, the largest pores were located within the interior hatching region. Hence, despite progressive deformation of the particles and an increase in oxidation, there was no discernable change in metal porosity with powder reuse in PBF-EB AM of Ti6Al4V.

Integrated 2D cellular automata-phase field modeling of solidification and microstructure evolution during additive manufacturing of Ti6Al4V

In this work, the as-solidified β-grain microstructure and concentration evolutions for each solute during the directed energy deposition (DED) of Ti6Al4V are successfully simulated via the combination of two predictive models: A validated 3D DED model is first employed to simulate the thermal history in the multi-track Ti6Al4V depositions. By incorporating the extracted quasi-steady-state temperature profiles as inputs, the novel integrated 2D cellular automata (CA)-phase field (PF) model simulates the grain details and solutal evolution on the sub-grain level over the entire fusion zone, which takes advantages of the high calculation efficiency of the CA component and the broad capability of the PF component. In this 2D CA-PF model, the 2D CA component simulates the dendrite growth and solute redistribution while the 1D PF component calculates the growth kinetics including growth velocity and equilibrium solute partition. Besides, The PF component is coupled with the thermodynamic and solute diffusion mobility data of the Ti-Al-V system such that the solutal evolution of Al and V is predicted successfully. The simulation results are compared with experimental observation and 3D CA modeling results.

A process-microstructure finite element simulation framework for predicting phase transformations and microhardness for directed energy deposition of Ti6Al4V

This paper presents a process-microstructure finite element modeling framework for predicting the evolution of volumetric phase fractions and microhardness during laser directed energy deposition (DED) additive manufacturing of Ti6Al4V. Based on recent experimental observations, the present microstructure evolution model is formulated to combine the formation and dissolution kinetics of grain boundary, Widmanstätten colony/basketweave, massive/martensitic alpha and beta phases of Ti6Al4V. The microstructure evolution algorithm is verified and embedded into a three-dimensional finite element process simulation model to simulate thermally driven phase transformations during DED processing of a Ti6Al4V thin-walled rectangular sample. The microhardness values of different locations of the part, which experience different thermal histories, are computed based on the simulated fractions and hardness values of different phases in the final microstructure. The simulated volumetric phase fractions and related microhardness distribution agree reasonably well with experimental measurements performed on the sample. Thus the proposed simulation model could be useful for designers to understand and control process-microstructure-property relationships in a DED-processed part.

Erratum to: Modeling the Microstructure Evolution During Additive Manufacturing of Ti6Al4V: A Comparison Between Electron Beam Melting and Selective Laser Melting

Beam-based additive manufacturing (AM) is an innovative technique in which parts are built layerwise, starting from the material in powder form. As a developing manufacturing technique, achievement of excellent mechanical properties in the final part is of paramount importance for the mainstream adoption of this technique in industrial manufacturing lines. At the same time, AM offers an unprecedented opportunity to precisely control the manufacturing conditions locally within the part during build, enabling local influence on the formation of the texture and microstructure. In order to achieve the control of microstructure by tailoring the AM machine parameters, a full understanding and modeling of the heat transfer and microstructure evolution processes is needed. Here, we show the implementation of the non-equilibrium equations for phase formation and dissolution in an AM modeling framework. The model is developed for the Ti6Al4V alloy and allows us to show microstructure evolution as given by the AM process. The developed capability is applied to the cases of electron beam melting and selective laser melting AM techniques to explain the significantly different microstructures observed in the two processes.

Controlling of residual stress in additive manufacturing of Ti6Al4V by finite element modeling

Minimizing the residual stress build-up in metal-based additive manufacturing plays a pivotal role in selecting a particular material and technique for making an industrial part. In beam-based additive manufacturing, although a great deal of effort has been made to minimize the residual stresses, it is still elusive how to do so by simply optimizing the manufacturing parameters, such as beam size, beam power, and scan speed. With reference to the Ti6Al4V alloy and manufacturing by electron beam melting, we perform systematic finite element modeling of one-pass scanning to study the effects of beam size, beam power density, beam scan speed, and chamber bed temperature on the magnitude and distribution of residual stresses. Our study elucidates both qualitative and quantitative features of the residual stress fields originated by these manufacturing parameters. Our findings can serve as useful guidelines for engineers and designers to deal with residual stress build-up during additive manufacturing of Ti6Al4V.

Modeling the Microstructure Evolution During Additive Manufacturing of Ti6Al4V: A Comparison Between Electron Beam Melting and Selective Laser Melting

Beam-based additive manufacturing (AM) is an innovative technique in which parts are built layerwise, starting from the material in powder form. As a developing manufacturing technique, achievement of excellent mechanical properties in the final part is of paramount importance for the mainstream adoption of this technique in industrial manufacturing lines. At the same time, AM offers an unprecedented opportunity to precisely control the manufacturing conditions locally within the part during build, enabling local influence on the formation of the texture and microstructure. In order to achieve the control of microstructure by tailoring the AM machine parameters, a full understanding and modeling of the heat transfer and microstructure evolution processes is needed. Here, we show the implementation of the non-equilibrium equations for phase formation and dissolution in an AM modeling framework. The model is developed for the Ti6Al4V alloy and allows us to show microstructure evolution as given by the AM process. The developed capability is applied to the cases of electron beam melting and selective laser melting AM techniques to explain the significantly different microstructures observed in the two processes.