Significant Improvement in Extrudability of Mg-9Al-0.8Zn-0.9Ca-0.6Y Alloy Through Mischmetal Addition

Abstract

The combined addition of small amounts of Ca and Y to Mg-Al-Zn alloys has recently been found to greatly improve the ignition resistance, corrosion resistance, and mechanical properties of these alloys. However, Mg-Al-Zn-Ca-Y alloys with high Al content show poor extrudability, which is an obstacle to their use as extruded components in industrial applications. This study aims to improve the extrudability of a Mg-Al-Zn-Ca-Y alloy through the addition of a small amount of mischmetal (MM). To this end, a trace amount (0.5 wt%) of Ce-rich MM is added to a Mg-9Al-0.8Zn-0.9Ca-0.6Y (AZXW9110) alloy. MM addition leads to significant grain refinement of the homogenized billet, from 326 to 180 mu m. When the MM-added alloy is extruded into a sheet at 350 degrees C with an extrusion ratio of 22.9, the maximum ram speed at which it is extrudable without the occurrence of hot cracking is 12 mm/s, substantially higher than the 2 mm/s possible without MM addition. The Ce and La atoms decomposed from the MM are dissolved in the Mg17Al12, Al8Mn4Y, Al2Ca, and Al2Y phases, apparently leading to an increase in the thermal stability of the phases and, consequently, to an improvement in the AZXW9110 alloy's extrudability. At ram speeds above 2 mm/s, many small-sized cracks form at the edges of the extruded AZXW9110-0.5MM sheets. As the ram speed increases up to 6 mm/s, the number density of edge cracks gradually increases while their average spacing decreases, but beyond 6 mm/s both remain almost unchanged.