Hasse Fredriksson

The solidification sequence in an 18-8 stainless steel, investigated by directional solidification

The directional solidification technique was applied in order to investigate the complicated solidification sequence in a commercial austenitic stainless steel which was known to yield a primary precipitation of § ferrite when cast into a 5 tons ingot. Three stages of solidification were found. The first precipitation of § ferrite was interrupted by precipitation of austenite and at the end of the solidification there was a transition back to precipitation of § ferrite. The competition between the first two stages is affected by the cooling rate and the nitrogen content. The precipitation of austenite from the melt results in the usual coring whereas ô ferrite forms with a very homogeneous composition, presumably due to rapid diffusion in this phase. On cooling austenite forms from the § ferrite and this reaction also results in coring, presumably due to rapid diffusion in § ferrite.

The influence of microgravity on the solidification of Zn-Bi immiscible alloys

Four experiments with alloys from the immiscible Zn-Bi system have been performed under microgravity conditions. Two alloys with a composition slightly within the miscibility gap (8 pct Bi) were cooled through the miscibility gap with two different cooling rates. It was found that Bi droplets were rather homogenously distributed in the samples, but the sizes of the droplets were somewhat larger than what would be expected from diffusion controlled growth. In samples with higher Bi contents (24 pct and 38 pct) larger areas of Bi-rich phase appeared in the center of the samples and as a layer around the samples. Calculations of the droplet size have been performed and compared with the experimental found droplet size. The calculated size was smaller and the difference was shown to depend on a collision coalescence. The layer around the samples was explained by a flotation of Bi-droplets to the surface.

On the reaction between aluminium, K2TiF6 and KBF4

The reaction between molten Al and KBF4 and K2TiF6 was analyzed. Additions of the two salts separately, consecutively and simultaneously were made at 800 and 1000 °C. The phases formed were identi ...

On the mechanism of pore formation in metals

The formation of different types of gas pores has been investigated by directional solidification experiments. A mathematical model of pore growth has been derived and the calculated pore growth has been compared with experimental data and a good correlation was found. The nucleation process of pores has also been treated. It was shown that micropores can be homogeneously nucleated in an interdendritic area according to the pressure drop caused by the solidification shrinkage.

Reactions during infiltration of

The infiltration sequence of graphite fibers with liquid aluminum alloyed with titanium was studied. The titanium concentration was chosen such that a severe reaction occurred between the fibers and the melt. Aluminum carbide and titanium carbide, as well as an aluminide phase were formed. The phenomenon occurring during the infiltration sequence was explained with the aid of the ternary-phase diagram Al-Ti-C. The effect of the reaction on the infiltration height is discussed.

On the thermodynamics and kinetics of carbides in the aluminium-rich corner of the AlTiC phase diagram

Aluminium composites have a great potential because of their high strength-to-weight ratio. Carbides will in the future be powerful reinforcements. To be able to manufacture these it is of importan ...

On The Formation of Macrosegregations in

The effect of the natural convection on the formation of macrosegregations has been experimentally and theoretically analyzed. The Sn-10 pct Pb and Pb-15 pct Sn alloys were unidirectionally solidified. The temperature gradient and the solidification direction were perpendicular to the gravity vector. The concentration and temperature gradients in the samples cause natural con- vection. Large macrosegregations were observed in the sample. Two different convection modes were found in the two alloys caused by different density gradients in the two-phase region. This difference in convection causes an enrichment of Pb in the lower part of the last solidified regions in the Sn-10 pct Pb alloy and an enrichment of Sn in the upper part in the Pb-15 pct Sn alloy. Computer simulation of the convection mode models the convection pattern and the solidification process during which macrosegregation occurs.

Comparison of numerical simulation models for predicting temperature in solidification analysis with reference to air gap formation

Abstract As a result of its influence on heat transfer between cast part and mould, air gap formation is an important problem for many casting processes. The general explanation for gap formation is that, as a result of stresses and distortions that are created from inhomogeneous cooling, shrinkage of the casting and expansion of the mould occur. In this paper, different thermomechanical approaches are applied to a well defined casting process using three commercial and one inhouse codes and their predictions are compared with experimental findings. The experimental data were obtained from the solidification and subsequent cooling of cylindrical castings of eutectic Al–13%Si and ternary Al–7%Si–0.3%Mg alloys. Based on these findings, the major differences between the predictions of the models and the actual formation of the air gap are discussed.

Physics of functional materials

Preface. 1. Structures of Melts and Solids. 1.1 Introduction. 1.2 X-ray Analysis. 1.3 The Hard Sphere Model of Atoms. 1.4 Crystal Structure. 1.5 Crystal Structures of Solid Metals. 1.6 Crystal Defects in Pure Metals. 1.7 Structures of Alloy Melts and Solids. Summary. Exercises. 2. Theory of Atoms and Molecules. 2.1 Introduction. 2.2 The Bohr Model of Atomic Structure. 2.3 The Quantum Mechanical Model of Atomic Structure. 2.4 Solution of the Schrodinger Equation for Atoms. 2.5 Quantum Mechanics and Probability. Selection Rules. 2.6 The Quantum Mechanical Model of Molecular Structure. 2.7 Diatomic Molecules. 2.8 Polyatomic Molecules. Summary. Exercises. 3. Theory of Solids. 3.1 Introduction. 3.2 Bonds in Molecules and Solids. Some Definitions. 3.3 Bonds in Molecules and Non-Metallic Solids. 3.4 Metallic Bonds. 3.5 Band Theory of Solids. 3.6 Elastic Vibrations in Solids. 3.7 Influence of Lattice Defects on Electronic Structures in Crystals. Summary. Exercises. 4. Properties of Gases. 4.1 Introduction. 4.2 Kinetic Theory of Gases. 4.3 Energy Distribution in Particle Systems. Maxwell-Boltzmanns Distribution Law. 4.4 Gas Laws. 4.5 Heat Capacity. 4.6 Mean Free Path. 4.7 Viscosity. 4.8 Thermal Conduction. 4.9 Diffusion. 4.10 Molecular Sizes. 4.11 Properties of Gas Mixtures. 4.12 Plasma - The Fourth State of Matter. Summary. Exercises. 5. Transformation Kinetics: Diffusion in Solids. 5.1 Introduction. 5.2 Thermodynamics. 5.3 Transformation Kinetics. 5.4 Reaction Rates. 5.5 Kinetics of Homogeneous Reactions in Gases. 5.6 Diffusion in Solids. Summary. Exercises. 6. Mechanical, Thermal and Magnetic Properties of Solids. 6.1 Introduction. 6.2 Total Energy of Metallic Crystals. 6.3 Elasticity and Compressibility. 6.4 Expansion. 6.5 Heat Capacity. 6.6 Magnetism. Summary. Exercises. 7. Transport Properties of Solids. Optical Properties of Solids. 7.1 Introduction. 7.2 Thermal Conduction. 7.3 Electrical Conduction. 7.4 Metallic Conductors. 7.5 Insulators. 7.6 Semiconductors. 7.7 Optical Properties of Solids. Summary. Exercises. 8 Properties of Liquids and Melts. 8.1 Introduction. 8.2 X-ray Spectra of Liquids and Melts. 8.3 Models of Pure Liquids and Melts. 8.4 Melting Points of Solid Metals. 8.5 Density and Volume. 8.6 Thermal Expansion. 8.7 Heat Capacity. 8.8 Transport Properties of Liquids. 8.9 Diffusion. 8.10 Viscosity. 8.11 Thermal Conduction. 8.12 Electrical Conduction. Summary. Exercises Answers to Exercises. Index.

Theory of hot crack formation

Abstract The hot crack sensitivity in metals is suggested to be caused by the supersaturation of vacancies created during the solidification process. Equations have been derived to predict the nucleation and growth of cracks by the condensation of vacancies. The transition temperature from brittle to ductile fracture was found to be related to the decrease in the supersaturation of vacancies due to an annealing process. The hot crack sensitivity was observed to be related to the supersaturation of vacancies, the diffusion rate, and the structure coarseness. The effect of surface active elements such as phosphorous and sulphur in steel alloys is discussed.