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Eingeladener Vortrag
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Effect of Elevated Temperatures on the Mechanical Properties of a Direct Laser Deposited Ti-6Al-4V
(2021)
In the present work, the mechanical properties of the DLD-processed Ti-6Al-4V alloy were obtained by tensile tests performed at different temperatures, ranging from 20 °C to 800 °C. Thereby, the process conditions were close to the conditions used to produce large-sized structures using the DLD method, resulting in specimens having the same initial martensitic microstructure. According to the obtained stress curves, the yield strength decreases gradually by 40% when the temperature is increased to 500 °C. Similar behavior is observed for the tensile strength. However, further heating above 500 °C leads to a significant increase in the softening rate. It was found that the DLD-processed Ti-6Al-4V alloy had a Young’s modulus with higher thermal stability than conventionally processed alloys. At 500 °C, the Young’s modulus of the DLD alloy was 46% higher than that of the wrought alloy. The influence of the thermal history on the stress relaxation for the cases where 500 °C and 700 °C were the maximum temperatures was studied. It was revealed that stress relaxation processes are decisive for the formation of residual stresses at temperatures above 700 °C, which is especially important for small-sized parts produced by the DLD method. The coefficient of thermal expansion was investigated up to 1050 °C.
The application of magnesium (Mg) inevitably involves dissimilar welding with steel. A novel solid state spot welding method, refill friction stir spot welding (refill FSSW), was utilized to weld AZ31 Mg alloy to galvanized DP600 steel. Although Mg/Fe is an immiscible alloy system, defect-free welds with high strength were successfully obtained in a wide parameter window. The results of microstructure, interfacial reactions, and mechanical properties are reported to reveal the underlying joining mechanism. Due to the melting of Zn coating and subsequent Mg-Zn reactions, Mg-Zn eutectic and intermetallic compounds were detected within welds. Heterogeneous interfacial reactions occur along Mg/steel interface, and the relationship between interfacial structure and fracture behavior was investigated. The joining mechanism is associated with Zn coating and Fe-Al layer: 1) the presence of Zn coating is beneficial for achieving high-quality welding between Mg and steel, it protects the interface from oxidation and contributes to brazing of the weld; 2) the Al present in Mg alloy reacts with Fe, resulting in the growth of Fe-Al layer, which contributes to the diffusion bonding in the interface. The overall results clearly show that Refill FSSW is a competitive welding method for joining Mg and galvanized steel.
Iron aluminides depict a sustainable and light-weight material class which could be employed in many applications requiring high strength at intermediate to high temperatures. According to first results, the alloy Fe-26Al-4Mo-0.5Ti-1B surpasses conventional materials in wet corrosion resistance and creep resistance up to 650 °C. For these reasons, the AiF research project “WAFEAL – Materials applications for iron aluminides” was initiated to transfer these findings into a standardised materials dataset and to derive best practices for processing. In the first place, a set of different microstructures adjusted by varying casting methods, wall thicknesses and heat treatments was investigated and correlated with hardness on macro and micro scale. Correlations were drawn between solidification rates and resulting grain sizes and hardness. The effect of vacancy hardening was only verified for wall thickness as low as 2.5 mm. Moreover, a common decrease of macrohardness after a heat treatment at 1000 °C for 100 h was observed irrespective of casting process or wall thickness. This effect was linked with an unexpected decrease of the complex boride phase fraction which acts as a hardening phase.
The development of refractory CCAs has been explored for potential use in high temperature applications. An example of this is the AlMo0.5NbTa0.5TiZr alloy, which resembles the well-known γ/γ’ microstructure in Ni-Base superalloys with cuboidal particles embedded in a continuous matrix. The aim of this work is to evaluate the alloy’s mechanical behavior under tension in the temperature range 800-1000°C, by applying creep tests under vacuum (excluding oxidation effects). Some little temperature influence on minimum creep rate @ 1000 and 1100 °C was found and at a first glance, and Norton plots shows that deformation is probably both diffusion and dislocation controlled. However, further work is needed to stablish deformation and degradation micro mechanisms in the studied creep regime.
The progress in activities on Microbeam Analysis under VAMAS/TWA 37 is reviewed. Particularly the liaison with the new projects within the ISO technical committee TC 202 is presented and discussed with respect to the possibility of identifying and launching related VAMAS projects. Need of more promotion for the engagement of more participants from industry and academia at national, European and international level is highlighted. Also, the competition with the other technical working areas (on 'nano' or materials-related) is critically discussed. Further, a short overview of the VAMAS areas of activities is given where Germany is involved.
Due to its matrix/γ′ structure, the compositionally complex alloy (CCA) Al10Co25Cr8Fe15Ni36Ti6 has excellent properties that fulfill the requirements for a high-temperature material. This base alloy is alloyed with small amounts of high melting elements to a further improvement of its properties, which results in different shapes, fractions and sizes of the two phases γ′ and Heusler after various homogenization and annealing steps. By correlating this microstructure with time independent and dependent mechanical properties, conclusions can be drawn about the effects of the individual phases. The needle-shaped Heusler-phase leads to bad mechanical behavior if its phase fraction is too high. A fraction below 3 vol% is not critical in tensile tests, but it reduces the creep resistance compared to a purely two-phase matrix/γ′-alloy. Sharp-edged cubic γ′-particles and a coarse Heusler-phase without sharp edges in case of the base alloy with 0.5 at.% hafnium lead to the best tensile and creep properties in the high temperature range. At 750 °C, the Hf-containing alloy clearly outperforms two commercially used alloys in the targeted area of application when it comes to creep resistance.
In Additive Manufacturing everybody is talking about Free Form, Unconventional Design, Re-thinking Components, “Think out of the box”.
However, there are a few outstanding question: a) What are the material properties ? They certainly differ from literature values for conventional materials; b) How about the microstructure? It is different from conventional materials. Does it stay so with ageing? How does it form? c) Do we properly take residual stress into account? We often blame them for our ignorance about failure scenarios. d) Do we apply tailored heat treatments? Very often, we follow conventional schedules…
This talk describes the summary of the efforts carried out within the BAM Project AGIL.
At BAM, we aimed to thoroughly investigate the microstructure and how it evolves as a function of load and temperature (service), to determine the material properties after different process and service conditions, to properly determine residual stress and the way it impacts mechanical properties and component performance, to properly quantify the impact of (unavoidable?) defects, and to determine heat treatments tailored to the process-specific material (stress relieve, microstructure homogenization etc.).
The Project AGIL was and is intimately coupled with the project ProMoAM, dealing with online monitoring of AM processes.
Ultra-SAXS can enhance the capabilities of existing synchrotron SAXS/WAXS beamlines. A compact ultra-SAXS module has been developed, which extends the measurable q-range with 0.0015 ≤ q (nm−1) ≤ 0.2, allowing structural dimensions in the range 30 ≤ D (nm) ≤ 4000 to be probed in addition to the range covered by a high-end SAXS/WAXS instrument. By shifting the module components in and out on their respective motor stages, SAXS/WAXS measurements can be easily and rapidly interleaved with USAXS measurements. The use of vertical crystal rotation axes (horizontal diffraction) greatly simplifies the construction, at minimal cost to efficiency. In this paper, the design considerations, realization and synchrotron findings are presented. Measurements of silica spheres, an alumina membrane, and a porous carbon catalyst are provided as application examples.
The supply and processing of materials for highly stressed components are usually cost-intensive. Efforts to achieve cost and resource efficiency lead to more complex structures and contours. Additive manufacturing steps for component repair and production offer significant economic advantages. Machining needs to be coordinated with additive manufacturing steps in a complementary way to produce functional surfaces suitable for the demands. Regarding inhomogeneity and anisotropy of the microstructure and properties as well as production-related stresses, a great deal of knowledge is still required for efficient use by small- and medium-size enterprises, especially for the interactions of subsequent machining of these difficult-to-machine materials. Therefore, investigations on these influences and interactions were carried out using a highly innovative cost-intensive NiCrMo alloy (IN725). These alloys are applied for claddings as well as for additive component manufacturing and repair welding using gas metal arc welding processes. For the welded specimens, the adequate solidification morphology, microstructure and property profile were investigated. The machinability in terms of finishing milling of the welded surfaces and comparative analyses for ultrasonic-assisted milling processes was examined focussing on surface integrity. It was shown that appropriate cutting parameters and superimposed oscillating of the milling tool in the direction of the tool rotation significantly reduce the mechanical loads for tool and workpiece surface. This contributes to ensure a high surface integrity, especially when cutting has to be carried out without cooling lubricants.
Damage tolerance counts as one of the most widespread approach to fatigue assessment and surely as one of the most promising in understanding the process-structure-property-performance relationships in additively manufactured metallic materials. Manufacturing defects, surface roughness, microstructural features, short and long crack fatigue propagation, residual stresses and applied loads can be taken into consideration in a fracture mechanics-based fatigue assessment. Many aspects are crucial to the reliable component life prediction. Among those a prominent role is played by an accurate measurement and modelling of the short crack fatigue behavior, and reliable statistical characterization of defects and residual stresses.
This work aims at addressing the issues related to both experimental testing, fatigue and fatigue crack propagation, and fracture mechanics-based modelling of fatigue lives. Examples will be provided on an additively manufactured AISI 316 L.