Co-extrusion process to produce Al–Mg eutectic clad magnesium products at elevated temperatures

Abstract

A novel and effective method to co-extrude metallic alloys is described using a direct extrusion method. This co-extrusion method was used to experimentally produce Al–Mg eutectic clad rods of magnesium using a laboratory scale extruder at the University of Waterloo. In addition a mathematical model of the co-extrusion process using DEFORM 2D was developed to gain insight and a scientific understanding of the material flow and metallurgical reactions that occurred between the aluminum and magnesium during the co-extrusion process. The co-extrusion method involved, filling a feeder pocket of the extrusion die with a disc of one material (Al), whereas the billet material had a different composition (magnesium). As the ram pushes the billet into the die, it punches through the disc and the resulting extrudate is clad in a eutectic alloy of the disc and billet material. An added benefit to this process is that with the choice of the correct material compositions and process conditions, the extrusion loads can be substantially lowered as a eutectic composition is formed between the billet and disc materials such that the interface becomes molten at the extrusion temperatures while the parent billet and disc remain solid. This greatly aids in the formation of a high quality interface with respect to bonding between the eutectic clad layer and core material of the extrudate. Subsequent extrusions done using the die with an already punched disc in the feeder pocket (a donut) were also able to be clad and showed significantly lower loads (∼30% difference in peak load) during extrusion as compared to billet materials that were extruded with an empty die feeder pocket. In this research, extrusion trials were performed with a magnesium billet and aluminum disc at 350°C (below the Al–Mg eutectic point) and 450°C (about the Al–Mg eutectic point). At 350°C, no significant cladding of the aluminum on the core Mg alloy occurred, but at 450°C, the co-extrusion process was successful with an Al–Mg eutectic clad rod of magnesium being produced. Subsequent co-extrusion trials were also done using the Al-donut in the feeder pocket of the die and also resulted in significantly lower loads and an Al–Mg eutectic clad extrudate. The integrity of the interface was examined and deemed to be of high quality based on visual and microscopic examinations. Both the Finite Element Method (FEM) simulation using DEFORM 2D and extrusion load measurements showed a significant drop in extrusion pressure using this co-extrusion process.