Analytische Chemie
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Avoiding the formation of defects such as keyhole pores is a major challenge for the production of metal parts by Laser Powder Bed Fusion (LPBF). The use of in-situ monitoring by thermographic cameras is a promising approach to detect defects, however the data is hard to analyze by conventional algorithms. Therefore, we investigate the use of Machine Learning (ML) in this study, as it is a suitable tool to model complex processes with many influencing factors. A ML model for defect prediction is created based on features extracted from process thermograms. The porosity information calculated from an x-ray Micro Computed Tomography (µCT) scan is used as reference. Physical characteristics of the keyhole pore formation are incorporated into the model to increase the prediction accuracy. Based on the prediction result, the quality of the input data is inferred and future demands on in-situ monitoring of LPBF processes are derived.
i-TRIBOMAT will provide the world's first Open Innovation Test Bed dedicated to analysing and simulating materials in tribological systems. In order to make necessary results of tribological tests comparable, harmonised ways to produce data are mandatory. Many sources influence the outcome of a Tribological test. Different approaches of institutes were evaluated in best practices via quality management tools minimising the impact on results.
Ferritic-austenitic chromia-forming alloys are frequently used as boiler tubes and heat exchanger materials for fossil-,biomass, and co-fired power plants. In all applied environments several strongly corrosive gaseous species such as CO2, SO2, SO3, H2O, O2 exist, causing materials degradation by high-temperature corrosion. The elucidation of degradation mechanisms introduced by multiple gases is challenging due to the presence of different oxidizing agents contributing to the competing reactions for oxidation, sulfurization or carburization. The degradation processes can be divided into initial stages, a transitional stage and the further proceeding steady-state oxidation reaction. Especially the long-term steady-state oxidation and further materials’ life-time are strongly dependent on the initial stages. The adsorption and absorption of the reactive species at the alloy surface and the growing oxide in the initial reaction is further influenced by dissociation and re-reactions of the gas phase molecules. To understand these mechanisms from a fundamental point of view in more detail, dedicated experiments and advanced characterization techniques on various length scale need to be applied. Real-time approaches using highly energetic synchrotron X-ray diffraction showed a high potential to enlighten competitively mechanisms by following the corrosion reactions in-situ in the environment they occur. Despite various other thin film characterization techniques, time of flight secondary ion mass spectroscopy (ToF-SIMS) is a powerful tool to visualize light atoms or labeled isotopes enabling the Differentiation between different oxidizing species. It was especially shown to be applicable in challenging atmospheres containing KCl deposits or in CO/CO2/O2 environments. The present study analyses the competing oxidation/sulfidation process in a humid atmosphere on two ferritic alloys with 2 and 9 % in weight chromium by in situ energy dispersive X-ray diffraction (EDXRD) and comparative tube furnace exposure using S16O2 and H2 18O atmosphere.
HEAs represent a relatively new class of materials. The the alloy concept is fundamentally different from the most conventional materials and alloys that are used today. Recently, the focus of HEA designs is more application-based. For that purpose, the elements of interest are carefully selected and multiple phases as well as micro-structures are deliberately adjusted. Currently, only limited attention has been paid to weldability of HEA. This encompasses possible effects on metallurgy and its influence on the desired properties. It remains open if welding causes e.g. considerable number of intermetallic phases or segregations and their effect on weld joint properties. For that reason, the scope of this study is to summarize already available studies on welding of HEAs with respect to the HEA-type, the applied welding process and its influence on the weld joint properties.
Hydrogen was once called “the versatile embrittler” [1], which summarizes very well the effect on reduction of ductility and/or toughness in technical alloys like steel. In that connection, welding is one of the most important component fabrication technologies. During welding, hydrogen can be transferred to the weld pool from manifold sources (like contaminations, residuals at the surface, etc.). As hydrogen embrittles a material, the safety of welded components with hydrogen is always a critical issue. Weld heat input causes additional changes in the microstructure like grain growth or partial dissolution of precipitates and many more. All these things influence the mechanical properties and also represent hydrogen traps. These traps decrease the hydrogen diffusion compared to the ideal lattice. The result can be so-called delayed hydrogen assisted cracking (HAC) of the weld joint due to the significantly decreased diffusivity by trapped hydrogen. This is often an underestimated risk as those cracks can appear in the weld joint even after some days!
It is essential to know about hydrogen ingress during welding and the microstructure specific hydrogen diffusion. Both are depended on weld parameter influence and the chemical composition of the base material and weld metal. For that purpose, gas analytic methods like solid-state carrier gas hot extraction (CGHE) are useful tools to: (1) identify detrimental hydrogen concentrations from weld joints, (2) binding energies from hydrogen traps by thermal desorption analysis or (3) high-temperature diffusion coefficients. Those values are extremely important for welding practice in terms of recommendations on realistic hydrogen removal heat treatment (HRHT) after welding. Considering the increasing use of “digital” experiments, the data is also needed for reliable numerical simulations of HAC process or HRHT-effectiveness.
The present contribution gives an overview on the influence of hydrogen on weld joints, the necessity, methods and standards for hydrogen determination (CGHE) with the aim of fabrication of safe welded and crack-free components.
[1] R. A. Oriani (1987), Corrosion 43(7):390-397. doi: 10.5006/1.3583875
Due to its low mass and high diffusivity in presence of compositional, thermal and mechanical gradients, hydrogen within a metallic microstructure can result in severe loss in ductility even at low concentrations and might lead eventually to a catastrophic and unpredictable failure of structural components during service. In this context, hydrogen mapping at the microscale is still considered among the most important challenges on the pathway towards a better understanding of the hydrogen transport and assisted cracking phenomena in metals, specifically in structural components, e.g. steels.
Among the very few available techniques to localize hydrogen at the microscale, Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was proven to be a reliable tool. Based on the assumption that deuterium influence the microstructure similarly to hydrogen, in the following contribution ToF-SIMS was applied as the main technique to detect and locally map the deuterium distribution in several alloys: lean 2101 and standard 2205 duplex stainless steel (DSS), AISI 304L austenitic stainless steel and titanium 6Al-4V alloy. These alloys were selected as case studies in this work due to the wide use of them in many applications and environments which frequently provide critical conditions for hydrogen absorption and assisted degradation.
The innovative design of in-situ and ex-situ experiments enabled us to elucidate the permeation, transport and trapping of deuterium in the microstructure in sub-micron resolution for the first time. In addition to the novel experimental setups, further progress was gained by applying computational multivariate data analysis (MVA) on the raw data and data fusion with high resolution structural characterization methods (scanning electron microscopy and electron back-scattered diffraction – SEM/EBSD). This combination allowed us to correlate the deuterium distribution and the influence on the microstructure.
In the presented research, the high potential and abilities of secondary ion mass spectrometry (ToF-SIMS) to detect and locally map the hydrogen distribution in two types of duplex stainless steels are shown. The research validates certain proposed mechanisms by combining ToF-SIMS with high-resolution scanning electron microscopy and electron-backscattered diffraction. The combination of data from several techniques on the same region was conducted in this field for the first time by applying data treatment of the ToF-SIMS raw data and data fusion approach. This powerful combination of methods allows reviewing of the occurring processes related to hydrogen assisted cracking. The step beyond the state of the art in this field was gained here by developing permeation and mechanical loading experiments within the ToF-SIMS during chemometric imaging of the hydrogen distribution in the microstructure. The research presents the necessary correlation between the hydrogen distribution and the resulted structural changes, the diffusion behavior in a duplex microstructure and stress induced diffusion of hydrogen by applying external load at the microscale.
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in welded joints using a thermal conductivity device (TCD) for quantitative measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries and factors that additionally influence hydrogen determination. They are namely: specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PID-furnace controller as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up the reach the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by evaluation of the recorded data. Generally, independent temperature measurement with calibration specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
Acoustic emission testing
(2018)