Volume 95, Issue 12

NiAl alloys are very promising candidates for high-temperature applications such as turbine blades of gas turbines. For such turbine blades, investment casting is the common manufacturing process. Up to now, the processing of NiAl alloys in general and in particular via investment casting is hardly possible due to the very high melting temperature, hardness, reactivity of the melt, and a characteristic ductile-to-brittle transition temperature. In this publication, the development of an investment casting technology will be summarised including the shell mould and core development, casting process development, the advancement of numeric simulation tools, and the formulation of a complete data set.

The steady-state creep behavior of NiAl was studied by constant strain-rate tests in the strain-rate regime 1.5 × 10 −4 ≤ ɛ̇ ≤ 1.0 × 10 −2 s −1 .Primary creep of NiAl was investigated at different stresses (10 MPa−40 MPa) in the temperature range from 1200 to 1400°C. The steady-state creep rate was fitted to a temperature-compensated power-law expression ɛ̇ = B ( σ/μ ) n exp(− Q/RT ), from which a stress exponent n≈3.5 and an activation energy Q ≈ 398 kJ/mol were derived, respectively. The obtained activation parameters indicate that dislocation creep dominates the creep behavior of NiAl at temperatures above 1200°C up to 1400°C. The transient creep behavior at 40 MPa could be fitted to a power-law equation ε = ε 0 + bt m with b α σ 3:5 and 0.52< m <0.68. The creep data of NiAl obtained in this study were utilized for modeling and simulation of the consolidation process of NiAl composites during hot pressing by finite-element analysis.

A new interest in magnesium alloys has arisen since lightweight constructions have become more and more attractive. Products that are currently available are mostly cast, forged, and extruded. A cost-effective production of sheet metals will render new perspectives in the design of lightweight constructions [1, 2]. The development of new casting technologies like thin slab casting or strip casting with an in-line rolling process is expected to result in a shorter processing time featuring a high potential of saving energy, time, and cost [3, 4, 5, 6, 7, 8, 9]. Particular attention is paid to the casting process parameters such as casting speed and solidification speed. This article deals with some aspects of a magnesium-adapted singlebelt caster. Results of casting experiments and material examination are shown as well as some advices given concerning safety and handling of liquid magnesium [10].

The numerical analysis of the thermo-mechanical interaction between the cast part and mould in casting processes requires a strong coupling between the thermal and mechanical simulation. This includes a 3-D frictionless contact algorithm and a model to describe the local heat transfer between part and mould as a function of the contact pressure or the formation of a local gap between both domains. Several features suitable for the analysis of solidification processes have been introduced in the classical master/slave contact algorithm like a temperature-dependent penalty factor and implemented in the software package CASTS. Benchmark tests and two industrial casting applications illustrate the correctness of the developed thermo-mechanical program and the ability to simulate complex cast parts.

This paper presents a scientific contribution to the development of lightweight/high-performance Al –Li alloys suitable for semi-solid processing. Thermodynamic calculations identified the most promising compositions with focus on the solidus – liquidus interval, fraction of solid-versus-temperature and phase reactions. The synthesis of Al –Li precursor billets was performed by overpressure induction melting in controlled atmosphere. DTA and microstructure investigations on Al –Li specimens were carried out as well as thixocasting trials of demonstrator components. New rheocasting of Al –Li alloys was investigated to identify the potential of this alternative precursor material route. It is shown that specifically developed Al –Li alloys offer great potential for semi-solid manufacturing.

The current study on a medium-carbon steel (0.36% C) with deformation-induced spheroidized cementite produced by heavy warm deformation (HWD) was carried out in order to investigate an alternative microstructural route when compared to conventional quenching and tempering (QT) or soft annealing (SA) processes. After austenite deformation, the HWD samples were cooled and heavily deformed at temperatures below the γ – α transformation followed by a simulated coiling treatment. In particular, the study addresses the influence of adiabatic heating due to the heavy deformation on the microstructure and properties. The mechanical properties after the various treatments show that the strength – ductility relation after HWD and QT are superior to those after continuous cooling (CC) or soft annealing (SA). In the microstructures obtained from the CC and SA treatments we observed lamellar pearlite which deteriorated the mechanical properties. A similar effect was observed after the adiabatic HWD treatment, which also entails some lamellar pearlite. HWD at 670 °C with subsequent coiling produced a microstructure with dispersed spheroidized cementite distributed homogeneously in the fine-grained ferrite matrix. The mechanical properties of the HWD samples are well comparable to those observed after the QT process owing to the resemblance in microstructure.

The Dy–Mg system has been critically assessed by means of the CALPHAD technique. The solution phases (liquid,body-centered cubic, and hexagonal close-packed) were modeled with the Redlich – Kister equation. The intermetallic compounds DyMg, DyMg 2 , and Dy 5 Mg 24, which have some homogeneity ranges, were treated as the formula (Dy, Mg) (Mg, Va), (Dy, Mg) (Dy, Mg) 2 , and (Dy, Mg) 5 (Dy, Mg) 24 by a two-sublattice model with Dy and Mg on the first sublattice, Mg and vacancies, denoted Va, or Dy and Mg on the second one, respectively. The compound DyMg 3 was treated as a stoichiometric compound. A set of self–consistent thermodynamic parameters of the Dy–Mg system was obtained.

Ball indentation technique has been employed to explore the possibilities for revealing the effect of pre-strain on mechanical properties of two groups of high-strength low-alloy steels. These steels were pre-strained up to 5%. Results are presented here for 1, 3, 4, and 5% of pre-strain effect on these steels. The dependence of yield strength, ultimate tensile strength, strength coefficient, strain-hardening component and true stress–strain curves on pre-straining effect are available from the ball indentation test, and they are validated with the standard mechanical test results. It is found that the increment of the pre-straining effect results in an increment of the yield strength and ultimate tensile strength values, while strength coefficient and strain-hardening component values are decreased. The existence of Lüders bands in the conventional stress–strain curves is also revealed in ball indentation test by the presence of the Lüders strain surrounding the indentation profile. In the conventional test it is found that the amount of Lüders bands is decreasing with increasing pre-strain. Similarly, Lüders strain as well as pile-up/sink-in of materials surrounding the indentation decrease as the pre-straining increases. It is found that ball indentation technique can be effectively used to determine the minute changes of mechanical properties due to introducing small amount of pre-straining.

Interpenetration Al 2 O 3 /Al(Si) composites were fabricated by reactive infiltration of molten Al into the preforms with SiO 2 –4.6 mol.% cordierite at 1150 °C in air. The formation kinetics and microstructures of the composites were investigated. Experimental results show that the addition of cordierite in SiO 2 is favorable for wetting of molten Al on the preform, leading to high growth rate of the composite (8.76 mm min –1 ). The growth rate of the composite was controlled by the infiltration of molten Al into the preform through the open pores and cracks formed during the preform fabrication or through microcracks created during the infiltration process. Since the addition of cordierite in SiO 2 can lower the preform sintering temperature, one of its applications is used for ceramic – ceramic or ceramic – metal joints. A strong Al 2 O 3 –Al 2 O 3 joint with a shear strength of 109 MPa was fabricated and investigated using the Al 2 O 3 /Al(Si) interlayer formed by this method.

Solidification in binary alloy melts was simulated with two different three-dimensional models, that differ in the way the role of the solid/liquid interface energy on the solidification is expressed: either surface rearrangement or a surface curvature-dependent attachment probability was applied. In the melt, the simultaneous diffusion of all diffusing particles (atoms) was taken into account. Cellular –dendritic growth modes, as observed in practice, could be well simulated. The surface rearrangement model is essential for exhibiting details (as facetting) and the effect of the next-nearest neighbour interaction.

The aim of the work presented in this report was to produce age-hardenable aluminium alloys containing scandium and zirconium by a casting process with similar cooling conditions like an industrial casting process. Microstructure, precipitation structure and age-hardening response of different alloys with up to 0.4 wt.% Sc and Zr were investigated. Age-hardening experiments from the as-cast condition without solution annealing showed a significant increase of hardness of about 100% for Sc-rich alloys and of 50% for Zr-rich alloys compared to the as-cast condition. TEM investigations revealed the formation of precipitates of ternary Al 3 (Sc x Zr 1– x ) phases with a cubic cP4 crystal structure. In addition to the strengthening effect, a high thermal stability especially of the precipitates in Zr-rich alloys up to 400 °C let these alloys look very promising for high-temperature applications.