Magnus Wessén

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.

Influence of alloying elements on microstructure and mechanical properties of CGI

Abstract The influence of alloying elements and cooling conditions on compacted graphite iron is investigated. To investigate the influence of different alloying elements, a sampling cup designed for thermal analysis and tensile test bars were cast. A total of 19 casting trials were performed, for each of the trials; three different cooling conditions were obtained for both the sampling cup and the tensile test bars. Mg has a major influence on the graphite morphology and causes a substantial increase in nodularity. Cu, Si and Sn only cause minor changes in the nodularity, but on the other hand, affect the mechanical properties primarily by altering the pearlite content of the material. Cr and Mo are investigated mainly with respect to their carbide promoting properties. However, no columnar white structure was found in the microstructure. The effect of the microstructure on the thermal history caused by differences in alloying content is also covered.

Determining effect of slurry process parameters on semisolid A356 alloy microstructures produced by RheoMetal process

Abstract The RheoMetal process, based on the rapid slurry forming (RSF) technology, is a new rheocasting process enabling the production of semisolid slurry of high quality and with high efficiency. The RSF technology is based on an enthalpy exchange between two alloy systems rather than applying external cooling as is done in many other rheocasting processes. In this work, several process parameters important for the RSF technology have been investigated on aluminium A356 alloy slurry microstructures. During slurry preparation, the thermal history for the different parameters studied was also recorded. It was found that the process parameters tested (amount of solid addition, melt superheat, rotation speed) have an effect on the thermal history during processing and on the microstructures formed after slurry formation.

Influence of Microstructural Inhomogeneity on Fracture Behaviour in SSM-HPDC Al-Si-Cu-Fe Component with Low Si Content

In the current paper, a low-Si containing aluminium alloy (1.4-2.2% Si) was used to fabricate a complex shape telecom component using Semi-Solid High-Pressure Die Cast (SSM-HPDC), process. Microstructure and fracture characteristics were investigated. The cast material exhibited microstructural inhomogeneity, in particular macrosegregation in the form of liquid surface segregation bands in addition to sub-surface pore bands and gross centre porosity. Tensile specimen were taken from the cast components. Elongation and microstructural inhomogeneity were investigated and correlated. Fracture surfaces of the tensile specimen were examined under scanning electron microscope (SEM). The study showed that both near surface liquid segregation bands and subsurface porosity strongly affected the fracture behaviour. Dominant for loss of ductility were gross centre porosity. This centre porosity was found to be a combination of trapped gas and insufficient, irregular feeding patterns.

Microstructure and material soundness in liquid and rheocast AM50 and effect of section thickness

Abstract Commercial grade AM50 magnesium alloy was diecast to varying wall thickness using both the RheoMetal process and conventional liquid casting. The resulting microstructure and pore structure were analysed. The rheocast material had larger pores than the conventionally cast material, scattered over the central 50% of the cross-section, while the pores in the conventionally cast material was concentrated to a segregation band at a depth corresponding to approximately 30% of the wall thickness. In the Rheocast samples, there was a correlation between thicker sections and larger primary α-Mg globules with a lower shape factor.

Effect of injection velocity on porosity formation in rheocast Al component using RheoMetal process

Abstract Rheocasting is on its way to establish as a cost efficient method for producing high integrity aluminium components. There is still, however, a need to learn more about the possibilities and limitations of this relatively new process in industrial use. This especially relates to ingate design and machine process parameters, e.g. shot profile, die temperature and intensification pressure, for obtaining sound quality. A component was produced by rheocasting at varying phase II velocities in a normal high pressure die casting machine supplied with a RheoMetal station. The component quality was investigated in the most massive section of the casting, using X-ray and metallographic techniques. The pore content and its distribution were found to strongly depend on the injection velocity where a low velocity is preferred due to the low risk for air entrapment and the higher fraction solid obtained at completed die filling, thereby reducing the solidification shrinkage.

The influence of copper on microstructure and mechanical properties of compacted graphite iron

Abstract The influence of copper content (0·26 to 1·31 wt-%) on microstructure formation and mechanical properties of compacted graphite iron (CGI) has been evaluated through standard metallographic analysis, colour etching techniques and tensile testing of machined test bars. The properties investigated are yield strength, tensile strength and elongation. The castings were made in an industrial environment from a combination of CGI returns, pig iron, cast iron- and steel scrap. A total of four heats were cast in specially designed sampling cups (3 different cooling rates), chill wedges as well as tensile test bars machined from sand moulded cylinders (20, 45 and 85 mm in diameter). The results clearly illustrate the combined effect of copper and cooling rate on nodularity, chilling tendency as well as pearlite content. A discussion concerning the effect of graphite morphology on the ferrite growth is also included.

Influence of Antimony on Microstructure Formation and Mechanical Properties in Thick-walled Ductile Iron Castings

A series of cylinders (Ø 30–480 mm) were cast at three different Sb-levels. The microstructures as well as the mechanical properties were studied at different locations in the castings. It was shown that Sb increases both graphite nodularity and pearlite content, but that the positive effect on hardness and strength can be somewhat reduced as a result of formation of deteriorated graphite morphologies in the grain boundaries.

Effect of Superheat on Melting Rate of EEM of Al Alloys during Stirring Using the RheoMetal Process

The RheoMetal process (previously called the Rapid S- and RSF- process) is a novel method to produce cost effective, high quality, semisolid slurries for component casting. The RheoMetal process uses an Enthalpy Exchange Material (EEM) as cooling agent to absorb heat and produce a slurry. Critical process parameters to create a slurry by robust melting of the EEM are alloy content, stirring speed, EEM to melt ratio, EEM temperature, EEM microstructural characteristics and melt superheat. In this paper, the melting sequence and melting rate of the EEM was studied experimentally. The effect of EEM composition, as well as superheat, on evolution of shape and dimension of the EEM during stirring was investigated. Initial material freezing onto the EEM was observed, followed by a stationary phase with subsequent gradual melting of the EEM. It was shown that the characteristics of freeze-on layer were strongly correlated to melt superheat, EEM temperature, as well as material composition, hence also has significant influence on the melting sequence.

Shear band formation in shaped rheocast aluminium component at various plunger velocities

Significant progress has been made in recent years in understanding and modelling the rheology of semi-solid metals. These models show the effects of the microstructure in terms of size and morphology of globules on the material response. More recently it has been shown that semi-solid metals can behave as compacted granular materials such as sand. A particular signature of such deformation is that the deformation becomes concentrated into shear bands which are 10-20 grains wide. Such bands have also been observed in a range of cast products. Recently, it has been clearly shown that shear bands in high pressure die cast (HPDC) products are also the results of Reynolds dilatancy. Shear bands are also known to be a common feature in semi-solid metal products. The segregation banding in semi-solid metal (SSM) material and its dependence of plunger velocity were investigated. Shaped castings were made with the RHEOMETAL™ process with a range of different plunger velocities. The microstructural characteristics were investigated, with a particular emphasis on shear bands. It is shown that ingate velocities influence the location and characteristics of the shear bands.