Lars Arnberg

Influence of Grain Refiner Addition on the Precipitation of Fe-Rich Phases in Secondary AlSi7Cu3Mg Alloys

The effect of grain refiner addition on the precipitation of Fe-rich compounds in secondary AlSi7Cu3Mg has been studied. Thermal and metallographic analysis techniques have been used to examine the formation of Fe-bearing intermetallics. The results show that grain refinement with AlTi5B1 can alter the precipitation sequence of Fe-rich phases. In non-grain refined alloy and grain-refined alloy with AlTi10, two different Fe-rich phases, α-Al15(Fe,Mn)3Si2 and π-Al8Mg3FeSi6, form, whereas, with AlTi5B1 addition, the β-Al5FeSi phase also tends to crystallize, nucleating on TiB2 particles. It has also been demonstrated how at high cooling rates, the β-Al5FeSi-involved reactions occur more preferentially, when compared to α-Al15(Fe,Mn)3Si2 phase. This crystallization behaviour is explained in terms of phase diagram relationship and the nucleation kinetics of competing α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phases.

A Synchrotron X-Ray Radiography Investigation of Induced Dendrite Fragmentation in Al-15wt%Cu

The effect of a Pulsed Electro Magnetic Field (PEMF) on the solidification of an Al‑15 wt%Cu alloy was studied in situ by synchrotron X-ray radiography. Samples were solidified with and without the presence of the PEMF while recording radiographs, enabling observation and quantification of dendrite fragmentation by image analysis. Fragmentation increased with a PEMF and was attributed to induced inter-dendritic flow.

Effect of Lanthanum on Grain Refinement of Casting Aluminum-Copper Alloy

The influence of lanthanum (La) addition on the solidification structure of a casting Al-Cu alloy has been studied. It is found that the addition of 0.3 wt.% La has a significant grain refining effect on the alloy. Microstructure characterization reveals that a large number of globular grains have fine block-shaped Al-Ti-V-La particles in the centers. It is suggested that the dispersed Al20(Ti,V)2La particles precipitated in the aluminum melt have acted as heterogeneous nucleation sites for the α-Al grains during solidification. Moreover, during the growth of α-Al grains, La will be accumulated in the liquid of solidification front due to segregation. Another influence of La on the grain refinement is attributed to the restriction effect of La on the grain growth of α-Al grains during solidification.

Correlation between Mechanical Properties and Porosity Distribution of A356 in Gravity Die Casting and Low Pressure Die Casting

Gravity die casting (GDC) and low pressure die casting (LPDC) methods were used to compare the mechanical properties and porosity distribution in a 5-step mould design. Commercially available A356 alloy was used for the experiments. Ar and Ar+H2 mixture were used to achieve two different hydrogen levels, i.e. 0,1 and 0,2 ml/100g Al, respectively. Although the porosity level was lower in LPDC, the tensile properties were lower than GDC due to the fact that LPDC melt had 50 mm bifilm index, whereas GDC melt had 20 mm. This investigation has shown that the metal quality has a larger effect over the mechanical properties than the porosity content.

A Comparative Study of Porosity and Pore Morphology in a Directionally Solidified A356 Alloy

Hydrogen in aluminum alloy melts leads to porosity, generally considered to be the most serious defect affecting both static and dynamic properties of structural aluminum castings. In order to study these effects, an investigation has been undertaken to examine the A356 alloy that is gravity die cast and directionally solidified. Castings were produced with as-molten metal and degassed metal, and with metal up-gassed by various treatments. Hydrogen content during melt treatment was measured with ALSPEK H,® a device based on electrochemical principles. Porous disc filtration apparatus (PoDFA) and reduced pressure tests (RPT) were performed in parallel with casting experiments to assess melt quality. The effect of hydrogen content and melt cleanliness on the amount, size, shape, and location of porosity was characterized, and the results show that amongst all the up-gassing methods used, up-gassing with wood and Ar-water vapor mixture is the most efficient. Pore size and shape factor increased with increasing distance from the chill.

Modification of Eutectic Si in Al-Si Based Alloys

The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To date, impurity-induced twinning mechanism and twin plane re-entrant edge mechanism are the well-accepted theories. However, neither IIT nor TPRE can be used to interpret all modification observations. Therefore, a re-consideration of modification mechanisms is still required. In this contribution, recent advances on the understanding the modification of eutectic Si are reviewed. Two different cases are highlighted. In the case of Sr, Na and Eu addition, eutectic Si was modified from a faceted to a fibrous morphology, which involves the formation of multiple Si twinning. In the case of Yb and Ca addition, eutectic Si was refined to a smaller size, but still maintained a plate-like morphology. The possible modification mechanism was thus discussed in terms of (i) adsorption of atoms at twin re-entrant edge, and (ii) segregation across {111}Si growth planes. Furthermore, solute entrainment of modifying elements (M) was introduced to interpret the formation of Al2Si2M phase or M-rich clusters within Si crystals.

Solidification of Silicon for Solar Cells

Silicon is the dominating material in solar cells. Monocrystalline and multicrystalline cells have approximately equal market shares and are produced from wafers, cut from single crystals produced by Czochralski (CZ) pulling or from polycrystalline ingots made by directional solidification, respectively. The present paper reviews how demands for lower cost, better yield, higher efficiency and use of less pure silicon in solar cells are addressed by advanced solidification processing. In monocrystalline solar silicon, careful growth control results in less point defects, and better efficiency. Continuous- or semi-continuous CZ growth processes are being developed for better productivity and lower cost. In multicrystalline solar silicon, extended defects such as dislocations and grain boundaries decrease efficiency, particularly in combination with new, less expensive, but more contaminated silicon feedstock. This problem is addressed by control of nucleation and growth of ingots with larger grains, preferred grain orientation and lower dislocation density.

Modeling of Lifetime Distribution in a Multicrystalline Silicon Ingot

Lifetime distribution of a multicrystalline silicon ingot of 250 mm diameter and 100 mm height, grown by unidirectional solidification has been modeled. The model computes the combined effect of interstitial iron and dislocation distribution on minority carrier lifetime of the ingot based on Shockley Read Hall (SRH) recombination model for iron point defects and Donolato’s model for recombination on dislocations. The iron distribution model was based on the solid state diffusion of iron from the crucible and coating to the ingot during its solidification and cooling, taking into account segregation of iron to the melt and back diffusion after the end of solidification. Dislocation density distribution is determined from experimental data obtained by PVScan analysis from a vertical cross section slice. Calculated lifetime is fitted to the measured one by fitting parameters relating the recombination strength and the local concentration of iron

Hollow castings produced by interrupted low pressure die casting

Abstract Hollow aluminium castings have been produced by low pressure die casting where the pressure has been released when only a shell has solidified, thereby draining the liquid out of the centre of the casting. The influence of holding time of the melt in the die, the die temperature and the alloy type on the thickness and quality of the solidified shell has been investigated. It has been found that hollow castings can be produced reproducibly with constant wall thickness under constant process conditions. The casting wall thickness increases linearly with pressure holding time. The die temperature has less influence on the wall thickness. The roughness of the inner surface of the castings varies with alloy type. An Al–12 wt-%Si (AlSi12) alloy gives a very uneven rough inner surface whereas an Al–5 wt-%Mg (AlMg5Si) alloy gives a smooth, even surface. Grain refinement also improves the surface quality. Attempts to predict the wall thickness with a commercial casting software package had limited success; the calculations seem to overestimate the thickness.

Impact of buoyancy on the growth of equiaxed grains in Al–Cu

Abstract Equiaxed dendritic growth in grain refined Al–Cu (15, 20, 25 wt-%Cu) has been studied in-situ during directional solidification by means of high resolution synchrotron X-ray video microscopy. At these compositions, the α-Al grains have a lower density than the surrounding melt and experience buoyant forces which affect their growth rates and morphologies. A model has been derived, based on Scheil conditions to account for solute transport across the solid-liquid interface, and a spherical envelope approximation to the dendrite morphology in order to simplify both the interface geometry of the growing crystals and the Stokes drag exerted upon them during motion. The model was compared with experimental results to evaluate its present merits and to devise possible routes for further improvement and development.