Prof Lars Arnberg - NTNU, Norway
Influence on trace element on the microstructure of Al-Si foundry alloys
Thomas Hartmut Ludwig1, Paul Louis Schaffer2 and Lars Arnberg1
1Norwegian University of Science and Technology, Norway
2Hydro Aluminium, Norway
Recent estimates predict a decrease of the quality of raw materials for aluminium production and an increased use of scrap metal in primary metal production. Therefore higher levels of trace elements must be tolerated in future aluminium alloys, including Al-Si foundry alloys. The present paper presents investigations of the influence of calcium, phosphorous, nickel and vanadium on the microstructure of hypoeutectic Al-Si foundry alloys.
It has been found that phosphorous in concentrations down to 3 ppm form aluminium phosphide particles (AlP) that nucleate eutectic silicon very efficiently at low undercooling and therefore has a major influence on the Al-Si eutectic microstructure – eutectic silicon forms coarse plates at low undercooling. Calcium in amounts exceeding 40 ppm form the intermetallic phase Al2Si2Ca that nucleates on the aluminium phosphides and deactivates the phosphides, preventing them from nucleating eutectic silicon resulting in a reduction of the eutectic temperature and a refinement of the Al-Si eutectic. Calcium does not, however, cause a flake-to-fibrous transition (chemical modification) of the eutectic silicon.
Nickel partitions into the Al3Ni phase, which is well distributed in the Al-Si eutectic and forms a close system with the eutectic Si. In commercial Al-Si alloys where Fe is present, Ni partitions into the Al9FeNi phase.
Vanadium also dissolves into the Fe-rich phases of commercial Al-Si alloys. Traces of V were found in β-Al5FeSi and α-Al8Fe2Si phases. When the V-concentration exceeded 0.2 wt%, most of the vanadium formed a pre-eutectic polyhedral Si2V phase.