Lars Arnberg

In-situ observation of transient columnar dendrite growth in the presence of thermo-solutal convection

The time evolution of tip radii and velocities of growing columnar dendrites have been extracted from an image sequence measured in situ by synchrotron X-ray video microscopy during directional solidification of Al-15%Cu-9%wtSi alloy. At the scale of the primary dendrites, a highly transient growth was observed in conjunction with liquid flow caused by modest thermo-soutal convection. The nominal spatial and temporal resolutions employed in the experiment were 1.4 μm at approximately 100ms frame rate, respectively. A very prominent mutual inter-relation between the solute field and flow velocity field and the dendrite growth rate and morphology is evident in these observations. Image processing allowed extraction of the corresponding projection averaged solute field in each frame, and an estimate of the solute boundary velocity field by frame-to-frame tracking of its motion. These measurements may eventually support development of statistical dendrite kinetics models for transient states in alloy solidification processes that could be applied in microscopic scale solidification models such as phase field.

Depth profile analysis of solar cell silicon by GD-MS

In this work we have assessed the capability of depth profile analysis by glow discharge mass spectrometry (GD-MS) for several impurities relevant for solar cell silicon. Fast-flow direct-current high resolution GD-MS has been used. Six multicrystalline p-type silicon samples with contamination of B, P, Ti, Fe and Cu have been investigated. Ion implantation has been used for impurity contamination with a target depth of 3 μm. The acquisition time was approximately 30 seconds, giving a depth resolution of approximately 0.5 μm. The GD-MS concentration profiles of the samples contaminated with B, P and Ti agreed well with the levels implanted. Because Fe and Cu are fast diffusers, their distribution deviates from the target implantation. This indicates that for fast diffusing transition metallic impurities, such as Fe and Cu, different impurity distribution mechanisms occur and should be taken into account when analysing their depth profiles.

Influence of Oxide Additions on the Porosity Development and Mechanical Properties of A356 Aluminium Alloy Castings

Three A356 aluminium alloy melts with different oxide contents were produced by mixing alloy turnings with ingot material. The melts had a constant hydrogen level and were cast in a step mould steel die. The resulting casting porosity was characterized using the Archimedes’ method and image analysis. Melt quality was accessed using a Porous Disc Filtration Apparatus (PoDFA) and the Reduced Pressure Test (RPT). It was shown that oxide additions lead to higher amounts of fine structured oxide films resulting in an increase of the fraction of small pores. Shrinkage porosity with its inherent partially elongated and interdendritic pore morphology was predominant. Ultimate tensile strength (UTS) and elongation from thin and thick sections in the step castings deteriorated with increased oxide content; UTS decreased from 3–18 % and elongation from 18–38 % with oxide additions as the section thickness increased. The melt cleanliness analyses from PoDFA were consistent with the oxide additions.

Fluidity evaluation methods for Al–Mg–Si alloys

Abstract Much work has been carried out to assess the fluidity of casting alloys by various fluidity test methods. This study compares two tests which have been used to evaluate the fluidity of three Al– Mg–Si alloys for High Pressure Die Casting (HPDC) processes: Al–5Mg–2.5Si, Al–5Mg–1.5Si, and Al–3Mg–0.5Si (compositions in wt-%). The fluidity of the alloys has been measured using the commercially available strip fluidity test method and an experimental test method using a spiral sand mould designed by SINTEF (Norway). Reproducibility and fluidity measurements obtained using the two methods are reviewed and discussed. The experimental results show that both fluidity test methods give the same trends and internally consistent results.

Measurement of grain bridging in some Al-Cu and Al-Sn alloys

A quantitative investigation of interdendritic bridging in some Al-Cu and Al-Sn alloys is presented. The experimental measurement methods and bridging fractions at different eutectic liquid fractions are given. The different bridging fractions in Al-Cu and Al-Sn of different compositions have been found, and the effect of grain refinement on the bridging between the grains is analysed. The measurement is based on metallographic investigation of samples that have been quenched just above the eutectic temperature. Electron microprobe analyses across the bridges have been done and the diffusion of Cu in Al has been calculated in order to rule out alternative interpretations of experimental investigations. Finally, the possibilities of incorporating the experimental results into hot tearing models are discussed. IJCMR/501

Anomalous microsegregation in Al-Si foundry alloys

Abstract Measurements of Si and Mg concentrations across dendrites in a commercial Al–Si–7Mg alloy have shown a higher Si concentration in dendrite centers decreasing towards the dendrite boundaries, which is opposite to what would be expected from the phase diagram.

Investigating thermal donors in n-type Cz silicon with carrier density imaging

A new method to map the thermal donor concentration in silicon wafers using carrier density imaging is presented. A map of the thermal donor concentration is extracted with high resolution from free carrier density images of a silicon wafer before and after growth of thermal donors. For comparison, free carrier density mapping is also performed using the resistivity method together with linear interpolation. Both methods reveal the same distribution of thermal donors indicating that the carrier density imaging technique can be used to map thermal donor concentration. The interstitial oxygen concentration can also be extracted using the new method in combination with Wijaranakulas model. As part of this work, the lifetime at medium injection level is correlated to the concentration of thermal donors in the as-grown silicon wafer. The recombination rate is found to depend strongly on the thermal donor concentration except in the P-band region.

Precipitation of Dispersoids in DC-Cast AA3103 Alloy during Heat Treatment

The precipitation behavior of dispersoids in DC cast AA3103 alloy during heating and homogenization at 600°C has been studied by means of TEM, electrical conductivity measurement and image analysis. During heating, α-Al(Mn,Fe)Si is the first phase to precipitate in the alloy. When heated to higher temperature, long rod like and plate like Al6(Mn,Fe) dispersoids precipitate in the alloy. During homogenization, the size of Al6(Mn,Fe) dispersoids grows while a-Al(Mn,Fe)Si dispersoids dissolve quickly. The evolution of dispersoids during heat treatment is mainly controlled by nucleation, coarsening and dissolution. The size, and number density of dispersoids have been measured. The volume fraction of dispersoids formed during heating, measured from TEM images, has the same trend as the volume fraction calculated from the electrical conductivity of the alloy.

Effect of grain refinement and dissolved hydrogen on the fluidity of A356 alloy

Abstract The influence of grain refinement and dissolved hydrogen on the fluidity of A356 alloy has been investigated. A spiral casting test method, recently developed, has been used to measure fluidity in a reproducible way. The grain refinement reduces the grain size of the spirals, particularly at the tip, but no significant influence on the fluidity has been revealed. The hydrogen additions in the melt have not affected the fluidity but have, of course, significantly increased the porosity.

Influence of TiB2 particles on growth of eutectic silicon in aluminium–silicon alloys

Abstract Large amounts of an Al–5 wt-%Ti–1 wt-%B grain refiner master alloy have been added to an Al–12 wt-%Si alloy. It has been found that high concentrations of TiB2 particles influence the growth of the Si crystals in the Al–Si eutectic. Not only are the sizes of the particles reduced, but the morphology is also changed towards a more rounded or fibrous shape, similar to Al–Si eutectics that have been modified by Sr or Na. Addition rates between 0·5 and 10 wt-% of master alloy have been used in the present investigation, in contrast to normal addition rates which are on the order of 0.1–0.5 wt-%. The microstructure, and particularly the eutectic growth morphology was investigated by optical microscopy, scanning electron microscopy and electron backscattering diffraction. In addition to the eutectic silicon modification effect, which was highly reproducible, observations were made about the settling, agglomeration and distribution of TiB2 particles and the association between TiB2 particles and areas of modified eutectic growth. Different hypotheses about the mechanism of Si refinement, like physical blocking, nucleation of the Si or eutectic Si colonies by TiB2 were investigated. In addition, some experiments were preformed with Ti addition; TiAl3 crystals were found to nucleate the Si crystals.