Ingvar L Svensson

Strength-Ductility Behaviour of Al-Si-Cu-Mg Casting Alloys in T6 Temper

A comparative study of the mechanical properties of 20 experimental alloys has been carried out. The effect of different contents of Si, Cu, Mg, Fe and Mn, as well as solidification rate, has been assessed using a strength-ductility chart and a quality index-strength chart developed for the alloys. The charts show that the strength generally increases and the ductility decreases with an increasing content of Cu and Mg. Increased Fe (at Fe/Mn ratio 0.5) dramatically lowers the ductility and strength of low Si alloys. Increased Si content generally increases the strength and the ductility. The increase in ductility with increased Si is particularly significant when the Fe content is high. The charts are used to show that the cracking of second phase particles imposes a limit to the maximum achievable strength by limiting the ductility of strong alloys. The (Cu + Mg) content (at.%), which determines the precipitation strengthening and the volume fraction of Cu-rich and Mg-rich intermetallics, can be used to select the alloys for given strength and ductility, provided the Fe content stays below the Si-dependent critical level for the formation of pre-eutectic α-phase particles or β-phase plates.

Polysilicate equilibria in concentrated sodium silicate solutions

Polysilicate equilibria in concentrated sodium silicate solutions have been studied using potentiometric (glass electrode) and 29Si n.m.r. methods. Experimental data cover the ranges: 11.36 ⩽ pH ⩽ 14.09, 2.4 ⩽[Na+]tot/mol dm–3⩽ 9.6, 2.1 ⩽[Si]tot/mol dm–3⩽ 9.3 with a variation of SiO2/Na2O between 3.3 and 1.0. From available pH and n.m.r. data an equilibrium model is derived describing polymerization and protonation of silicate species in these water-glass solutions. With SiO2/Na2O = 1.0, i.e. in the most alkaline solutions (pH ≳ 13), dimeric as well as cyclic forms of tri- and tetra-meric silicate species predominate. At higher SiO2/Na2O, polymerization leads to the formation of polysilicate species containing branching groups with nuclearities 6–8 dominating. With SiO2/Na2O ≳ 3.3, i.e. close to the precipitation boundary of amorphous silica, cage-like polysilicate species dominate.The average charge per Si atom for the different polysilicate species varied between –2 in the most alkaline solutions down to –0.5 close to the precipitation boundary. The buffering capacity of the different water-glass solutions was found to decrease with increasing SiO2/Na2O.The change in the different formation constants (log βpq) due to variation in total Si and SiO2/Na2O, was found to be linear with respect to the Na+ concentration, i.e. log βpq= log β°pq+k[Na+].Distribution diagrams describing speciation and equilibria in water-glass solutions of different SiO2/Na2O and total Si concentrations are derived using the computer program SOLGASWATER.

Thermomechanics of the cooling stage in casting processes: Three-dimensional finite element analysis and experimental validation

A thermomechanical three-dimensional (3-D) finite element analysis of solidification is presented. The heat transfer model is based on a multidomain analysis accounting for noncoincident meshes for the cast part and the different mold components. In each subdomain, a preconditioned conjugate gradient solver is used. The mechanical analysis assumes the mold is rigid. A thermoelastic-viscoplastic rheological model is used to compute the constrained shrinkage of the part, resulting in an effective local air gap width computation. At each time increment, a weak coupling of the heat transfer and mechanical analyses is performed. Comparisons of experimental measurements and model predictions are given in the case of a hollow cylindrical aluminum alloy part, showing a good quantitative agreement. An application to an industrial aluminum casting is presented, illustrating the practical interest of thermomechanical computations in solidification analysis.

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.

Modeling of ferrite growth in nodular cast iron

In nodular cast iron, ferrite forms around the graphite nodules and growth proceeds until pearlite nucleates and consumes the remaining austenite. In order to simulate the structure, it is therefore necessary to have accurate models for the ferrite growth. Some investigators have proposed that the growth is completely governed by carbon diffusion through the ferrite shell. In the present work, it is shown that the ferrite growth in nodular cast iron can be divided into three different stages where the growth initially is governed by carbon diffusion in the austenite until the graphite nodule is entirely enveloped by a ferrite shell. During the second stage, it is proposed that the growth is controlled by the incorporation rate of carbon atoms on the graphite nodule. During the later stages of the transformation, the diffusion distance has increased considerably, and therefore, the diffusion of carbon through the ferrite shell will determine the growth rate.

Modelling the effect of graphite morphology on the modulus of elasticity in cast irons

Abstract Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.

Inoculation of primary austenite in grey cast iron

Abstract Primary austenite is the least investigated microstructural component of grey cast iron. It is the first phase to be nucleated in the liquid metal and it grows in a dendritic manner, followed by growth of the eutectic phase. This paper describes an investigation on three potential nucleating agents to promote the nucleation of primary austenite in grey cast iron under industrial conditions. A direct austempering after solidification (DAAS technique) was used to determine the number of primary austenite grains. A colour etching technique was used to reveal the primary austenite grains and the eutectic cells. The results show that properly chosen nucleation agents significantly promote the number of primary austenite grains. Furthermore, the authors show that the eutectic cell size and the morphology of the graphite are obviously influenced by the primary solidification in grey cast iron. The increased nucleation of austenite grains resulted in an increased number of eutectic cells.

Thermal conductivity–structure relationships in grey cast iron

Abstract The thermal conductivity of grey iron cast with different solidification conditions and treated with different inoculants has been evaluated by the laser flash technique. Colour etching has been utilized to reveal the influence of the fraction of primary phase and the eutectic cell diameter. The thermal conditions during solidification together with inoculation largely affect the heat conductivity of grey iron. The results show that an increased fraction of primary austenite depresses the thermal conductivity, while straight, long graphite flakes of type A are favourable with regard to the ability to transfer heat.

Effects of nodularity on thermal conductivity of cast iron

Abstract The thermal transport properties of five predominately pearlitic grades of grey, compacted graphite and spheroidal graphite iron have been investigated by the laser flash technique. Samples have been taken from cylinders cast in controlled thermal environments designed to produce three dissimilar cooling rates. Digital image analysis has been utilised in order to characterise the different graphite morphologies. The results indicated linear relationships between the thermal transport properties and the roundness of the graphite and the nodularity for compacted graphite and spheroidal graphite iron. A pronounced decrease in the thermal conductivity occurred when the lamellar graphite structure was transformed into compacted graphite. The thermal conductivity of compacted and spheroidal graphite iron has been recalculated with good accuracy over a temperature range of 25–500°C by means of regression analysis.

Comparison of mechanical properties in flake graphite and compacted graphite cast irons for piston rings

Abstract Cast iron is often used as the piston ring material in marine diesel engines. The mechanical properties of cast irons are to a great extent governed by the size, distribution and shape of the incorporated graphite particles. In a set of experiments, the mechanical properties of a pearlitic grey cast iron and a pearlitic compacted graphite cast iron are compared. Both cast iron grades have a eutectic composition. The experiments confirm the importance of micro-yielding of the matrix at the tip of the graphite particles on the macroelastic behaviour of the studied cast irons. This applies especially for the flake graphite cast iron where the graphite tip is sharper and the matrix bridges between the graphite particles are shorter than in the case of the compacted graphite cast iron resulting in micro-yielding at the graphite tip at a very low macro-stress and macro-strain. The high local stresses at the graphite tips also result in the opening of the graphite cavities which is much more severe in the flake graphite cast iron than in the compacted graphite cast iron. The mechanical properties of the eutectic flake graphite cast iron are largely affected by the size and amount of the graphite particles. The smaller the graphite particles (faster solidification) in the microstructure, the lower the values of the mechanical properties. In compacted graphite cast iron, the macro-elastic behaviour is influenced by the matrix and the overall coarseness of the microstructure to a greater extent and the effect of the incorporated graphite particle size is much less pronounced.