Use of the Cone Expansion Test to Assess the Potential for Longitudinal Seam Failure in Extruded AZ31 and AM30 Mg Alloy Tubes

Magnesium generally has limited ductility which, in the case of extruded tubes, may be further reduced at the extrusion seams. These are locations where the material flow separates and rejoins during extrusion and are unavoidable when a hollow profile product is continuously extruded. When this type of magnesium tube is stressed along the circumferential direction, as in hydroforming, tube ruptures consistently occur along the extrusion seams. The aim of this project was to investigate the propensity to fail at these locations by means of cone expansion tests. These were performed to failure on batches of both magnesium AZ31 and AM30 tubes, at several temperatures and compression rates. In this paper, the role that microstructural changes at the seams such as those associated with precipitate dispersions, local texture changes, grain size changes and entrained material, play in failure is discussed. The sources of these irregularities in the extrusion process are discussed and some means of removing or mitigating them are presented. The most important cause of fracture is shown to be the voids present at the interface between the slowly and rapidly flowing streams of the material within the die. The effects of the extrusion parameters on the extent of such void formation are considered.