Andreas Bührig-Polaczek

Influence of convection and grain movement on globular equiaxed solidification

Abstract A two-phase volume averaging model was used to study convection and grain movement, and their influence on the globular equiaxed solidification. Both liquid and solid phases were treated as separate interpenetrating continua. The mass, momentum, species and enthalpy conservation equations for each phase and a grain transport equation were coupled. An ingot casting (Al–4 wt.% Cu) with near globular solidification morphology was simulated. Case studies with different modeling assumptions such as with and without grain movement, and with slip and non-slip boundary conditions for solid phase were presented and compared. Understanding of grain evolution and macrosegregation formation in globular equiaxed solidification was improved.

Some New Observations on Freckle Formation in Directionally Solidified Superalloy Components

To improve the understanding of freckle formation in superalloys, several directional solidification experiments were carried out in a production-scale Bridgman furnace. During the careful inspections of the many cast specimens, several interesting phenomena were observed, indicating new aspects of freckle formation. Accordingly, several new factors influencing the freckle formation, such as the shadow effect, the edge effect, the step effect, and the crystal orientation effect, are proposed. This will contribute to our current understanding of freckle formation and the future development of new measures to minimize freckle defects in directionally solidified components of superalloys.

Semi Solid Casting and Forging of Steel

The semi solid forming of steel has the potential of an innovative technology, which could open a new possibility of metal components production. Earlier research works in the 1990s have already shown the principal feasibility of the process but also revealed the various kinds of technical problems. Since then, research works on steel processing have been continuously made in several locations in Japan, USA and Europe. Currently, the focus of these research activities is the development of suitable tool material and the achievement of high part quality, which is the key point for the success of the process. This paper gives an overview of the previous research works and the current state of the research with an outlook on future tasks for this challenge.

Steels with different carbon content for high pressure die casting in semisolid state

Some excellent projects have been finished successfully since the last conference on the pre-industrial stage showing the great potential of thixoforming technology for steels [1][2]. Experiments presented in this paper on induction heating, process window and mould filling of three widely-used steel alloys offer an even greater field of application for industrial companies. In the past thin walled cutting tools and complex impellers made of cold working steel X210CrW12 were produced at the Foundry Institute [3][4]. Constitutive on this knowledge the field of applications is enlarged by research on two further steel alloys. Processing of 100Cr6 (roller bearing steel) and 42Cr4 (annealing steel) is challenging due to a decreasing window at a higher temperature level with decreasing carbon content. An exact procedure has been worked out to create new control programs for the induction heating unit. It is based on thermo-chemical calculations and delivers billets with feasible temperature distribution and well defined content of liquid phase. Material characterization is performed in a step-die (seven steps between 25 and 0.5mm), especially developed for demands of semi-solid casting. Mould filling capacity, micro structural evolution and mechanical properties are determined on each of the seven steps. For any reproducible process the knowledge of suitable system parameters is essential. To find the limits of the process window sensitive experimental parameters were changed systematically for each of the three steel alloys. In addition to tool temperature the process windows consist principally of content of liquid phase fs, piston velocity vP and pressure during freezing pfr. Characteristic differences appeared between the different steel grades. The thermal simulation was used to find functional dimensions for the gating system and to shorten process times. Simulation of tools system was used to estimate the additional thermal load induced by higher working temperatures. The abrasive wear at the offsets to the next steps was in the same order of magnitude for all examined steels. These results provide the opportunity for commercial steel grades to cast complex steel parts in high pressure die casting.

The Geometrical Effect on Freckle Formation in the Directionally Solidified Superalloy CMSX-4

In order to better understand the geometrical effect on freckle formation in superalloys, samples with uniform increases and decreases in their cross sections were directionally solidified in a Bridgman furnace. In comparison to our conventional knowledge, some new features of freckle appearance have been observed. Freckles could occur at sloped surfaces where the freckle pattern is no longer roughly parallel to the direction of gravity but has the same slope as the surface. At significantly angled surfaces, the freckles appear as discrete flakes, having the shape of tree roots, instead of the long and narrow chains which are usually observed. The component portions having inward sloping surfaces are very freckle prone while those with outward sloping surface are mostly freckle free, although the completely opposite freckling tendency is indicated by the simulated solidification condition. Therefore, as an independent factor the geometrical feature of the components can more effectively affect the freckle formation than the local thermal conditions.

Investigation on the asymmetry of thermal condition and grain defect formation in the customary directional solidification process

In the present study, the non-uniformity of the thermal condition and the corresponding grain defect formation in the customary Bridgman process were investigated. The casting clusters in radial alignment were directionally solidified in a Bridgman furnace. It was found that in the casting cluster, the shadow side facing the central rod was ineffectively heated in the hot zone and ineffectively cooled in the cooling zone during withdrawal, compared with the heater side facing the furnace heater. The metallographic examination of the simplified turbine blades exhibited that the platforms on the shadow side are very prone to stray grain formation, while the heater side reveals a markedly lower tendency for that. The asymmetric thermal condition causes the asymmetrical formation of these grain defects. This non-uniformity of the thermal condition should be minimized as far as possible, in order to effectively optimize the quality of the SC superalloy components.

Biomimetic cellular metals—using hierarchical structuring for energy absorption

Fruit walls as well as nut and seed shells typically perform a multitude of functions. One of the biologically most important functions consists in the direct or indirect protection of the seeds from mechanical damage or other negative environmental influences. This qualifies such biological structures as role models for the development of new materials and components that protect commodities and/or persons from damage caused for example by impacts due to rough handling or crashes. We were able to show how the mechanical properties of metal foam based components can be improved by altering their structure on various hierarchical levels inspired by features and principles important for the impact and/or puncture resistance of the biological role models, rather than by tuning the properties of the bulk material. For this various investigation methods have been established which combine mechanical testing with different imaging methods, as well as with in situ and ex situ mechanical testing methods. Different structural hierarchies especially important for the mechanical deformation and failure behaviour of the biological role models, pomelo fruit (Citrus maxima) and Macadamia integrifolia, were identified. They were abstracted and transferred into corresponding structural principles and thus hierarchically structured bio-inspired metal foams have been designed. A production route for metal based bio-inspired structures by investment casting was successfully established. This allows the production of complex and reliable structures, by implementing and combining different hierarchical structural elements found in the biological concept generators, such as strut design and integration of fibres, as well as by minimising casting defects. To evaluate the structural effects, similar investigation methods and mechanical tests were applied to both the biological role models and the metallic foams. As a result an even deeper quantitative understanding of the form-structure-function relationship of the biological concept generators as well as the bio-inspired metal foams was achieved, on deeper hierarchical levels and overarching different levels.

Potential of the rheocasting process, demonstrated on different aluminum based alloy systems

This work describes the processing of different aluminum cast- and wrought alloys in the Cooling-Channel-Process. The investigations take the castability, the microstructural evolution for the SSM-processing and the resulting properties in the test specimen into account. Beside the alloys AlMg5Si2Mn (M59), AlCu4TiMgAg (AF48), AlSi12CuNiMg (SF70), AlSi17Cu4Mg (SF90), AlZnMgCu1,5 (AA7075) the optimization of the alloy AlSi7Mg0,3 (A356) by the addition of magnesium and copper were a main task of the work.

Undercoolability of Superalloys and Solidification Defects in Single Crystal Components

The undercooling behaviour of the superalloys IN 939 and CMSX-6 during the isothermal heating and cooling processes was investigated. Under the same process conditions, the undercoolability of CMSX-6 was measured to be 50.4 K while that of IN 939 was only 10.1 K. This great difference can be attributed to the amount of alloying elements. During Bridgman directional solidification using the same process parameters, the used alloys exhibited a very different tendency to form structure defects. While the CMSX-6 components were nearly defect-free, the IN 939 samples of the same geometry were very prone to stray grain formation. This difference can be attributed to the different undercoolability of the investigated superalloys. Therefore, the undercoolability is an important factor of castability and the cast quality of single crystal components can be effectively improved by increasing the undercoolability of the superalloys.

Fruit walls and nut shells as an inspiration for the design of bio-inspired impact resistant hierarchically structured materials

Until today the structuring of different types of fruit walls has been used only as an inspiration for packaging when seen from a biomimetic perspective. However, by a detailed investigation of the Macadamia nut with its tough testa, Citrus maxima, possessing a large spongy mesocarp and Cocos nucifera, having a combination of a fi brous mesocarp and a tough endocarp, it becomes evident that those structures also provide excellent biological role models for impactand puncture-resistant materials. Both Citrus maxima and Cocos nucifera are relatively heavy, lack any aerodynamic adaptation and share the same challenge of having to withstand the impact from heights of >10 m. Conducting high-speed camera-controlled free fall experiments of Citrus maxima from 6 m height, we could demonstrate a deceleration of the fruits of 3100 m/s2, which corresponds to 316 g, without any visible damage of the fruit. An analysis using cyclic quasi-static compression tests of the pericarp of Citrus maxima revealed that the material behaves constant in good approximation after the fi rst loading cycle. During the fi rst cycle, almost 75% of the energy is dissipated. The pericarp of Citrus maxima is highly visco-elastic, which causes the samples within 1 min to recover 30% of their initial deformation caused by loading to 40% strain. The mesocarp of Citrus maxima is best described as an open-pore foam with a gradual increase in the pore size. Understanding the principles of as to how combining the structure and material in biological constructions yields a fully functional protection layer will allow us to construct new lightweight bio-inspired materials of high impact and puncture resistance with a combination of high energy dissipation, benign failure and almost complete recovery from large deformations.