In the 1936 "Symposium on High-Strength Materials", from the Society for Testing Materials, discussion around the alloys of Aluminum lays out the basic formula of Aluminum-Copper-Silicon and notes that Magnesium is often omitted due to the "difficulty to fabricate" castings containing it. This weakness has long since been overcome. The fact that the "Wedge" of Aiud contains no Magnesium - hints that an older piece of equipment was used in the digging.
See: a dragline shovel made of aluminum, with Aluminum Bucket Teeth.
A further reason that such a piece of equipment has aluminum teeth is to reduce the weight of the apparatus - thereby increasing the load and efficiency of the operation. (Less energy is used just in moving the equipement, and a larger percentage of the rated lift capacity can be used for the load.)