Lars Arnberg

Quantitative study on the precipitation behavior of dispersoids in DC-cast AA3003 alloy during heating and homogenization

Abstract The precipitation behavior of dispersoids in DC-cast AA3003 billet during heating and homogenization at 600 °C has been studied by means of TEM, electrical conductivity measurement and image analysis. The orientation relationship between the dispersoids and matrix has been determined. The size, size distribution, number density and Mn/Fe ratio of dispersoids, and the area fraction of PFZ have been measured. The evolution of dispersoids during heat treatment is controlled by nucleation, growth, dissolution and coarsening. A method is proposed to measure the volume fraction of dispersoids from TEM images. The volume fraction of dispersoids precipitated during heating measured from TEM images is in good agreement with that calculated from electrical conductivity data.

Crystal fragmentation and columnar-to-equiaxed transitions in Al-Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

Development of strength in solidifying aluminium alloys

Abstract Quantitative measurements of the strength development in the mushy-zone during solidification have been obtained in two commercial Al Si alloys, AlSi7Mg and AlSi11Mg, an AlCu4 alloy and an AlMg5 alloy with a vane method. The results show that the material does not have any strength prior to dendrite coherency, confirming that the dendrites are free-floating and independent. Grain refining the AlSi7Mg and AlCu4 alloy postpones strength development, and strength increases slowly in the grain refined AlCu4 alloy. Adding Sr to the AlSi7Mg alloy postpones dendrite coherency and strength develops more slowly. Fading is observed to change the strength back towards that of the unmodified alloy with time. Adding 1% Si to the AlMg5 alloy postpones dendrite coherency, but the strength increases rapidly and makes the strength of the AlMg5Si1 alloy converge towards that of the AlMg5 alloy at increasing solid fractions. Strength develops very slowly in the AlSi11Mg alloy. The results are shown to be related to the size and morphology of the growing dendrites.

Effect of iron in silicon feedstock on p- and n-type multicrystalline silicon solar cells

The effect of iron contamination in multicrystalline silicon ingots for solar cells has been investigated. Intentionally contaminated p- and n-type multicrystalline silicon ingots were grown by adding 53 ppm by weight of iron in the silicon feedstock. They are compared to reference ingots produced from nonintentionally contaminated silicon feedstock. p-type and n-type solar cell processes were applied to wafers sliced from these ingots. The as-grown minority carrier lifetime in the iron doped ingots is about 1–2 and 6–20 μs for p and n types, respectively. After phosphorus diffusion and hydrogenation this lifetime is improved up to 50 times in the p-type ingot, and about five times in the n-type ingot. After boron/phosphorus codiffusion and hydrogenation the improvement is about ten times for the p-type ingot and about four times for the n-type ingot. The as-grown interstitial iron concentration in the p-type iron doped ingot is on the order of 1013 cm−3, representing about 10% of the total iron concentra...

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

Abstract Directional solidification of unmodified and strontium modified binary, high-purity, aluminium–7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth interface was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification interface extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified high-purity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy.

Distribution of iron in multicrystalline silicon ingots

The distribution of iron in multicrystalline silicon ingots for solar cells has been studied. A p- and a n-type multicrystalline ingot were intentionally contaminated by adding 53ppmwt (μg∕g) of iron to the silicon feedstock and compared to a reference p-type ingot produced from ultrapure silicon feedstock. The vertical total iron distribution was determined by neutron activation analysis and glow discharge mass spectrometry. For the intentionally Fe-contaminated ingots, the distribution can be described by Scheil’s equation with an effective distribution coefficient of 2×10−5. The interstitial iron concentration was measured in the p-type ingots. In the Fe-contaminated ingot, it is almost constant throughout the ingot and constitutes about 50% of the total concentration, which is in conflict with the previous studies. Gettering had a large impact on the interstitial iron levels by reducing the concentration by two orders of magnitude. Considerable trapping was observed at crystal defects on as-cut wafers...

Grain size distribution in a complex AM60 magnesium alloy die casting

Presolidified equiaxed dendritic crystals are observed in magnesium cold chamber high pressure die castings. Depending on the rate at which new crystals are formed and to what extent they survive in the shot sleeve, a mixture of liquid and crystals is injected into the die cavity resulting in floating crystals in the casting. Box shaped die castings of the AM60 magnesium alloy have been made with a cold chamber high pressure die casting machine. The resulting microstructure is generally observed to consist of (a) a fine grained structure or (b) a mixture of fine grains and coarse grains which is either centred or dispersed in the through thickness cross-section. The prevalence of structures is observed to vary with position in the casting. Close to the gate a coarse grained microstructure dominates, while fine grains dominate further from the gate. The volume fraction of floating crystals in the casting is shown to depend on the initial superheat of the melt. IJCMR/492

In situ investigation of spinodal decomposition in hypermonotectic Al–Bi and Al–Bi–Zn alloys

Spinodal decomposition of hypermonotectic Al-6 wt.%Bi, Al-8 wt.%Bi and Al-6 wt.%Bi-8 wt.%Zn alloys has been investigated using synchrotron radiography. In the case of the 6 and 8 wt.%Bi binary alloys undercoolings of 70 and 110 K, respectively, were required to initiate the L ! L1 + L2 reaction, which appeared to occur very close to the monotectic reaction temperature. The nucleated L2 droplets were set in collective size- dependent motion by forces coupled to external fields (gravity and imposed temperature gradient) as well as forces arising due to internal fluctuations of the system. With experimental conditions similar to those realized during strip casting of the same materials, it was found that the size-dependant droplet velocity field combined with Stokes drag at the L1-L2 interfaces as well as attractive and repulsive diffusion-coupling between adjacent L2 droplets, yield complex meso- to microscale hydrodynamics. The hydrodynamics are the dominating mechanisms for L2 droplet coagulation, and are accordingly decisive for the final size distribution and geometrical dispersion of the soft Bi-rich component in the cast material. A different decomposition mode was observed in the Al-6 wt.%Bi-8 wt.%Zn ternary alloy, with the L2 droplets undergoing an immiscible-miscible-immiscible transition. In contrast to what

An improved method for fluidity measurement by gravity casting of spirals in sand moulds

Abstract This study describes a new equipment for gravity casting of fluidity spirals in a sand mould. The fundamental characteristic of the equipment is a constant pouring temperature, which gives a constant melt superheat and since the molten metal is poured into the spiral sand mould from the same height, the equipment gives a constant initial pressure head and pouring velocity. By comparing data from an earlier version of the equipment, an improvement in reproducibility has been shown. The effect of melt superheat on fluidity has been measured by the new improved equipment and has been confirmed to increase linearly with increasing melt superheat.

The origin of ‘anomalous’ microsegregation in Al–Si foundry alloys—modelling and experimental verification

Abstract Slowly cooled Al–7 wt.% Si foundry alloys show ‘anomalous’ segregation of Si in the dendrite arms, with higher concentrations in the centre of the dendrites than near the eutectic. Calculations and experiments show that this is caused by growth of particles in the eutectic during the cooling period after solidification is completed.