Additive manufacturing of low alloyed steels
Weld-microstructure and post treatment in stainless steel in continuous TIG-welding
Fabrication and characterization of Al/B4C metal matrix composites
Al/B4C metal matrix composites are fabricated by gas pressure infiltration (GPI) and squeeze casting (SC) of liquid pure Al or A6061 alloy into B4C preforms of variable degree of pre-sintering. The composites are characterized with regard to their thermophysical and mechanical properties. Particular focus was laid on the changes in formation of reaction products depending on the matrix metal used and the processing. For GPI it is found that the A6061 alloy leads to stronger reaction than the pure aluminium. Reaction products can essentially be suppressed if the infiltration is performed by squeeze casting. The reaction products are detrimental to the overall performance of the composite.
Title: Investigation on thermomechanical properties of high-nitrogen nickel-free austenitic stainless steels, commercially pure Titanium and Ti-6Al-4V
The goal of the project was to study the influence of slightly increased temperature on the rolling characteristics (deformation step width, formability, need of annealing, appearance of cracks) for two stainless steels and two titanium alloys. A moderate increase of rolling temperature to 200 to 400°C is found to have a significant effect on the ductility and the resistance against crack formation, as well as on the forces required for rolling.
Kinetics of precipitation in 6xxx aluminium alloys
AA6111 is a copper rich heat treatable aluminium alloy. The sequence of precipitation and its kinetics at low temperature was studied and compared to the literature. A model called Quench Factor Analysis (QFA) based on the nucleation and growth of precipitate phases can be used to evaluate selected properties as a function of the thermal history. The observation of the microstructure to characterise the precipitation and its sequence can support the results obtained with the QFA model.
Comportement mécanique des phases intermétalliques dans les alliages Al-Cu-Li pour applications aéronautiques
The influence of fragile intermetallics in Li-containing A2xxx alloys on the reduction in Young’s modulus after rolling has been investigated and compared to a A7050 reference material. It turned out that Young’s modulus would still decrease with increasing strain in a subsequent tensile test, yet to variable extent depending on the orientation of the sample with regard to the rolling direction.
A novel Platinum-based Metallic glass
A novel platinum-based metallic glass was developed based on the ternary system Pt-B-Si. The effect of various minor additions of other alloying elements on the glass forming ability and the characteristic parameters i.e. the melting point, the glass transition temperature and the crystallisation temperature was assessed. A critical diameter in excess of 1 mm was found for some of the compositions classifying those novel metallic glasses as bulk metallic glasses. A distinct feature of these glasses is their remarkably high hardness compared to standard Platinum based metallic glasses derived from the Pt-P system.
Student: Karel Lietaert
Title: Production and characterisation of porous magnesium alloys as degradable orthopaedic implants
The project aimed at evaluating the potential of cellular magnesium-based material as degradable orthopaedic implant material. The material was produced by liquid metal infiltration in NaCl preforms, the latter being leached of after infiltration. Pure magnesium and M2, ZM21 and AZ63 was used as matrix material. The compressive strength of the cellular material was assessed before and after exposure to simulated body fluid for various times. Hydrogen production as a measure of degradation was monitored for the various samples.
The study of the influence of the crystalline orientations and the dependence to the test done on the measure of failure stress of an anisotropic brittle material were done in this study. The material selected is a pure monocrystalline silicon. Three crystalline orientations and two types of tests were investigated in this project, a four-point bending test according a standard and a more exotic one, a nut-cracker like test. The results gave no significant difference between crystalline orientations (which can be entirely explained by the error made during experimens). First, a difference appeared between the two test which gave a Weibull modulus of 5.5 for nut-cracker test and 7.5 according the standard and a sigma0 of 151 and 178 MPa respectively. This difference, was later explained not by the test itself but by the way of calculation of the Weibull parameters and the presence of different surface flaws distributions. This leads to the conclusion than failure stress of the monocrystalline silicon is independent from the way of measuring it and the the crystalline orientations but further analysis need to be done for eliminate all the doubts about it.
The goal of this work was to develop methods for the production of cast parts out of copper in the micrometer range. For the replication of the shape of an organic precursor three approaches are explored, namely (i) Cold Isostatic Pressing (CIP), (ii) investment casting and (iii) casting. After burn out of the microtemplate, the cavities in the preforms were pressure infiltrated.
Smooth embedding of templates and dense preforms are essential for good replication and surface quality. Problems encountered were low wetting, preform drying, cracking and dissolution.
CIP allowed reproduction of fibers with a diameter of 15μm. The surface roughness for fibers produced by this method is in the order of 3μm.
For casting and investment casting different slurries are developed. Two are able to reproduce the exact shape of the template. The first one is a combination of a commercially available ceramic glue and fine alumina powder. The second one is
based on a phosphate salt (K3PO4) with high melting temperature and mixed with MgO. Comparing the two casting methods, it is seen, that the obtained surface roughness does not depend on the applied technique but on the material of the preform.
Surface roughness obtained using ceramic molds is of 2μm approximately. The disadvantage of this slurry is in the mold dissolution in HF. Consequently particular safety regulation must be taken. Micromolds of K3PO4 used in casting permits rapid dissolution of the preform in phosphoric acid after copper infiltration. Specimens with surface roughness as low as 0.5μm are repeatedly produced.
“Densification of High Al Aluminium Bronze Melt-spun Ribbons“
Aluminium bronzes of 13.7 & 14.9 %wt Al are melt spun and characterised by XRD, optical Microscopy, SEM and microhardness. Final melt spun ribbons show thicknesses between 20µm and 30µm and grain sizes of about 10 µm. Their microhardness is approximately 450 HV, which is 50% higher than cast ingots. They also contain less gamma phase, compared to cast and water quenched ingots. The ? phase, known to be brittle and prone to corrosion, is located at grain boundaries. Cu 13.7%wt Al ribbon are exposed to heat treatments at different temperatures up to 900°C and characterised again. Thermal stability is low and sample heated over 150°C lose their fine microstructure. Ribbons containing 13.7%wt Al were compacted into copper containers and extruded from a diameter of 19cm to 5.5cm in two steps. Cold extruded samples had no cohesion and a high porosity. Their global Vickers hardness was around 100 HV10. Samples extruded at 900°C were dense, composed of a homogenous dispersion of the different phase in grains of 2 to 20 microns, and reach a hardness of about 300 HV30. A tensile test could be done on a sample machined out of a hot extruded sample.
“Metal laminated composites”
“Crack initiation and propagation in AA6XXX”
“High temperature toughness and deformation mechanisms in TiAl Alloys”
The mechanical behaviour of three γ base TiAl alloys, namely Ti-46Al-7Nb, Ti-46Al-8Nb and Ti-46Al-8Ta (atomic %) was studied from 293 K up to 1073 K. Ti-46Al-7Nb shows a duplex microstructure, the other two alloys are characterized by a convoluted microstructure. The plastic deformation mechanisms were assessed by carrying out repeated relaxations. The global trends show that plastic deformation is controlled by Peierls lattice friction from 293 K to 673 K, interactions with defects from 673 K to 923 K and glide/climb of the screw/edge dislocations components at higher temperatures. Differences in the temperature dependence of flow stress were observed. They were attributed to the alloys composition and microstructure. The second part of this work consists in observations of cracks propagation during toughness tests carried out in a SEM. Two different alloys were tested at room temperature: Ti-46Al-8Ta and Ti-46Al-8Nb. It was observed that crack propagation occurs principally by interlamellar decohesion.
“Influence of quench rate and thermomechanical treatments on mechanical properties and microstructure of AA6XXX aluminum alloys”
“Precipitate tailoring in an Al – 6.5 Li – 0.06 Sc – 0.02 Yb (at%) alloy” (carried out at Northwestern University Evanston (Chicago), Illinois)
This work proposes a thorough microstructural study of an Al – 6.5 Li – 0.06 Sc – 0.02 Yb (at%) quaternary alloy undergoing a 2 steps heat-treatment in order to obtain complex core/shell precipitates. In a preliminary step, considerations concerning homogenization of the alloy and setting running parameters for one of the main characterization tool used in this study, namely a 3D Atom Probe Tomograph (APT) are addressed. The alloy is then investigated by APT in as- homogenized state. Then a first aging step is chosen by construction of an isothermal hardness aging curve at 325 °C. APT is then applied on a sample heat treated to peak hardness. In second aging step, an other isothermal hardness aging curve is built for 170 °C after a 8 h preliminary treatment at 325 °C. APT and various TEM techniques are applied on the samples obtained. The as homogenized state shows a clustering of Yb atoms in an L12 structure, with Li substituting for a fraction of Al atoms. The first aging step shows nucleation and growth of core / shell L12 precipitates, with an Yb-rich core and Sc-rich shell. The precipitates obtained are small but numerous compared to results on ternary alloys from the literature, suggesting slow diffusion kinetics caused by the presence of Li. The observations made on samples having undergone the second aging step show that part of the precipitates obtained in the first step show an increased complexity of the structure with appearance of an Al3Li δ’ phase. The result obtained opens exciting opportunities for precipitate tailoring in aluminum alloys.
“Etude de la remise en solution des intermétalliques dans un acier superausténitique 904L” (2007).
Resumé: Ce travail est une étude de la mise en solution de phases intermétalliques dans l’acier inoxydable superausténitique 904L.
Une caractérisation des phases présentes dans cet alliage est et comparée à une simulation réalisée grâce au logiciel Thermo-Calc. La dépendance entre la macro-ségrégation et la présence d’intermétalliques est mise en évidence par microscopie optique et analyses EDX. Un calcul d’homogénéisation préconise la température et la durée du traitement de mise en solution des phases intermétalliques. Des traitements thermiques de mise en solution sont alors effectués entre 1050°C et 1225°C.
Une confrontation entre les phases prédites par modélisation, les résultats expérimentaux et la littérature définit σ comme la phase majoritaire dans l’alliage 904L étudié. Une analyse de la taille de grain et de la présence résiduelle d’intermétalliques après traitement de mise en solution indiquent finalement que les exigences sont atteintes et qu’une application industrielle est possible.
“Comportement en fluage de mousses d’aluminium produit par le procédé de réplication” (2006).