Residual oxygen content and powder recycling: effects on microstructure and mechanical properties of additively manufactured Ti-6Al-4V parts

The laser-based powder bed fusion of metals (PBF-LB/M) offers a variety of advantages over conventional processing techniques and the possibility to recycle and reuse powder increases its sustainability. However, the process and resulting part properties are influenced by a variety of factors including powder recycling grade and residual oxygen content of the process atmosphere. Especially in terms of reactive materials like Ti-6Al-4V, oxidation during processing and recycling determines process stability and reproducibility. This work therefore focusses on the influence of the conventionally varied processing parameters as well as atmosphere residual oxygen content process and powder recycling on the microstructure and mechanical properties. For this purpose, the design of experiments approach is used and by evaluation of regression models, effect sizes and interactions are given. Additionally, two different etching techniques were employed to reveal different aspects of the microstructure. While no significant influence of powder recycling and residual oxygen on the microstructure could be observed, they both significantly influence the mechanical properties. A maximum hardness of 470 HV0.1, a maximum ultimate tensile strength of 1252.3 MPa, and a maximum elongation at break of 17.8 % were obtained. The results demonstrate the importance of the processing atmosphere’s residual oxygen content and of taking into account the changing powder characteristics during recycling as well as its effect on the part properties

Ad sollemnia Caesareae Universitatis Dorpatensis quae quo die haec academia olim condita est die XII mensis Decembris anni MDCCCLXXX hora XII in Aula Magna oratione Hermani Emminghaus P. P. O. et renuntiatione victorum in eruditionis certaminibus praemia adeptorum publice agentur rite concelebranda doctores omnium ordinum amplissimos et commilitones humanissimos et quicumque rebus nostris litterarumque studiis bene volunt omni qua par est observantia invitamus rector et senatus. Subiectae sunt G. Loeschckii Observationes archaeologicae

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Determination of optimum process parameters for different Ti-6Al-4V powders processed by Laser-based Powder Bed Fusion

The PBF-LB (laser-based powder bed fusion) process is subject to a large number of variables, including the characteristics of the processed powder. Since a powder with a given specification can be supplied by various powder manufacturers, the transferability of optimized parameter settings and statistical processing models is of major interest. This work therefore investigates the processing windows of two Ti-6Al-4V powders supplied by different manufacturers following the Design of Experiments (DoE) approach. The fitted regression models for porosity and roughness demonstrate a significant influence of the powder and its size distribution. Further, the powder type significantly interacts with laser power, scanning speed and hatch spacing. It is shown that an increase of the powder size distribution quantiles by less than 10 µm leads to a shift of optimum settings towards a higher volume energy density by 6.4 J/mm3 as well as to higher roughness on the top and side surfaces

Laser-based powder bed fusion of Ti-6Al-4V structures with different surface-area-to-volume ratios in oxygen-reduced and oxygen-free environment

Titanium alloys, such as Ti-6Al-4V, are particularly susceptible to oxidation, which is why their processing in the laser-based powder bed fusion process is carried out conventionally in a protective gas atmosphere. However, this atmosphere still contains critical residual oxygen levels, which are to be eliminated as part of a new approach. This approach envisages doping the argon protective gas atmosphere with small amounts of the highly reactive gas silane (ratio < 1:1000). The residual oxygen content is particularly critical in filigree and thin-walled structures that have a high surface-area-to-volume ratio and are a typical field of application for this additive manufacturing process. Therefore, this work focuses on the manufacturing of Ti-6Al-4V structures with different surface-area-to-volume ratios in conventional argon (< 200 ppm residual oxygen) and argon-silane atmospheres (< 10-14 ppm residual oxygen) on an innovative laboratory machine. After processing, the specimens are analyzed for surface topography, microstructure, and Vickers hardness. In addition, energy-dispersive X-ray spectroscopy and X-ray diffraction measurements are carried out to further investigate the chemical composition and present phases in the as-built specimens. The influence of the different atmospheres and their residual oxygen content, the surface-to-volume ratio, and possible interactions between them are discussed

Influence of Silane-doped Argon Processing Atmosphere on Powder Recycling and Part Properties in L-PBF of Ti-6Al-4V

In the additive manufacturing of metal powders, the residual oxygen in the processing atmosphere plays a crucial role, especially in highly reactive materials like titanium alloys. Besides oxidation of the built parts, it leads to oxygen pick-up into the unmolten powder. Since oxidized particles cannot be removed during recycling, the powder properties deteriorate after multiple uses. In this work, Ti-6Al-4V powder was processed under conventional argon atmosphere (residual oxygen content < 0.01 vol%) as well as silane-doped argon atmosphere (< 0.001 vol% silane in argon). The silane-doping leads to a residual oxygen content of < 10-20 vol%. The powder was sieved and used 5 times for each atmosphere. The powder properties morphology, chemical composition and flowability were analyzed for virgin as well as reused powder. Furthermore, the roughness and relative density of the built parts were evaluated. It was hypothesized that oxygen-free production improves recyclability and thus resource efficiency.Mechanical Engineerin

Towards experimental process parameter development for Ti-6Al-4V TPMS lattice structures with application to small scale dental implants using micrographs

Ti-6Al-4V is a widely used alloy in implant engineering and lattice structures can be applied to locally match the stiffness of the implant to the stiffness of bone. Triply periodic minimal surface (TPMS) structures are popular due to their curved surface, which supports a good manufacturability and osseointegration of the implant. Tests with different TPMS structures showed a strong interaction between design factors and manufacturing parameters resulting in the need for individual parameter development. However, to the best of our knowledge, the most work in the current literature focusses on mechanical and biological examinations of TPMS structures manufactured with standard parameters. As process parameters influence the structural properties, the optimum values for further analysis may not have been investigated (e.g., their influence on microstructure and mechanical properties). In this work, a design of experiments approach is used to develop process parameters. As computer tomography scans are resource intensive for large scale parameter development, a sparser approach using micrographs for porosity analysis is introduced. Small structures with unit cell size as small as 1.0 mm are fabricated on a laser powder bed fusion industrial machine. Our initial studies show that the design factor pore size is negligible in comparison to wall thickness when optimizing internal porosity.Mechanical Engineerin

Approach for the monetary evaluation of process innovations in early innovation phases focusing on manufacturing and material costs

In early innovation phases, the monetary evaluation of process innovations is a challenge for companies due to a lack of data. However, an innovation evaluation is essential in an early innovation phase to ensure that process innovations deliver economic value added (EVA) in early innovation phases and to channel technology transfer expenditures in a goal-oriented manner. This paper presents an approach for a semi-quantitative procedure for the monetary evaluation of process innovations in the early innovation phase focusing on manufacturing and material costs. Exemplarily, the approach is applied to process innovations of the Collaborative Research Center 1368 on oxygen-free production. In order to ensure the net present value orientation within the innovation evaluation, the procedure developed is based on a driver tree of the EVA. To link value drivers of the EVA and innovation-driven factors influencing EVA, the EVA driver tree is further systematized with a focus on manufacturing and material costs using a literature-based impact model. Based on the last level of the impact model, a guideline for a semi-structured expert interview is developed. Using this interview guideline, data is collected in the form of innovation-driven influencing factors, which represent the input for the final monetary innovation evaluation. An adapted weighted scoring model is used to draw a semi-quantitative conclusion regarding the EVA achieved by the process innovation. The practical application of the approach developed to process innovations in oxygen-free production has shown that, in the context of three process innovations under consideration, their implementation with the aim of achieving an EVA through reduced manufacturing and material costs at the current innovation status is not effective. However, based on the impact model developed, corresponding levers can be identified to positively influence the EVA and thus also the industrialization of the process innovation. Finally, further necessary steps are identified to evolve the presented approach into a complete method for monetary innovation evaluation in early innovation phases

Corrosion and corrosion fatigue properties of additively manufactured magnesium alloy WE43 in comparison to titanium alloy Ti-6Al-4V in physiological environment

Laser powder bed fusion (L-PBF) of metals enables the manufacturing of highly complex geometries which opens new application fields in the medical sector, especially with regard to personalized implants. In comparison to conventional manufacturing techniques, L-PBF causes different microstructures, and thus, new challenges arise. The main objective of this work is to investigate the influence of different manufacturing parameters of the L-PBF process on the microstructure, process-induced porosity, as well as corrosion fatigue properties of the magnesium alloy WE43 and as a reference on the titanium alloy Ti-6Al-4V. In particular, the investigated magnesium alloy WE43 showed a strong process parameter dependence in terms of porosity (size and distribution), microstructure, corrosion rates, and corrosion fatigue properties. Cyclic tests with increased test duration caused an especially high decrease in fatigue strength for magnesium alloy WE43. It can be demonstrated that, due to high process-induced surface roughness, which supports locally intensified corrosion, multiple crack initiation sites are present, which is one of the main reasons for the drastic decrease in fatigue strength

Mental health of refugees following state-sponsored repatriation from Germany

von Lersner U, Elbert T, Neuner F. Mental health of refugees following state-sponsored repatriation from Germany. BMC Psychiatry. 2008;8(1): 88.BACKGROUND: In recent years, Voluntary Assisted Return Programmes (VARPs) have received increasing funding as a potential way of reducing the number of refugees in EU member states. A number of factors may affect the mental well-being of returnees. These include adjustment to the home country following return, difficult living conditions, and long-term effects resulting from the severe traumatic stress that had originally driven the affected out of their homes. Little is known about the extent to which these and other factors may promote or inhibit the willingness of refugees to return to their country of origin. The present pilot study investigated refugees who returned to their country of origin after having lived in exile in Germany for some 13 years. METHODS: Forty-seven VARP participants were interviewed concerning their present living conditions, their views of their native country, and their attitudes towards a potential return prior to actually returning. 33 participants were interviewed nine months after returning to their country of origin. Mental health and well-being were assessed using the questionnaires Posttraumatic Stress Diagnostic Scale (PDS) and EUROHIS and the structured Mini International Neuropsychiatric Interview (M.I.N.I.).Our objectives were to examine the mental health status of refugees returning to their home country following an extended period of exile. We also aimed to assess the circumstances under which people decided to return, the current living conditions in their home country, and retrospective returnee evaluations of their decision to accept assisted return. RESULTS: Prior to returning to their home country, participants showed a prevalence rate of 53% for psychiatric disorders. After returning, this rate increased to a sizeable 88%. Substantial correlations were found between the living situation in Germany, the disposition to return, and mental health. For two thirds of the participants, the decision to return was not voluntary. CONCLUSION: Psychological strain among study participants was of a considerable magnitude. As a result of traumatic stress experienced during war and refuge, victims were vulnerable and not well equipped to cope with either post-migration stressors in exile or with a return to their country of origin. It is noteworthy that the majority returned under pressure from immigration authorities. Living conditions after return (such as housing, work, and health care) were poor and unstable. Participants also had great difficulty readapting to the cultural environment after having lived abroad for an average of 13 years. Current VARPs do not take these factors into account and are therefore not able to assist in a humanitarian reintegration of voluntary returnees