MICROSTRUCTURE OF MAGNESIUM ALLOY AZ31 AFTER LOW-SPEED EXTRUSION

Abstract

1. INDUSTRIAL MOTIVATION Extrusion of magnesium alloys, in general, does not differ a lot from extrusion of other metals. Magnesium alloys are warm and hot extruded. Deformation dependence on tempe-rature allow obtaining wide range of mechanical properties due to temperature variation from hot working range, where the material is entirely recrystallised to warm and cold working range, where deformation takes place below recrystallization temperature and so significant amount of deformed material remains in work-hardened condition. The latter makes it possible to control and influence the final properties of extrudates, therefore lowered temperatures are often used to attain higher strength, which is of great significance, as magnesium alloys, contrary to aluminium alloys or steel that undergo additional treat-ment after extrusion, are provided their final properties directly after extrusion [1]. Although higher loads are needed, lower extrusion temperatures make it also possible to increase extrusion rate preventing from surface defects formation in the aftermath of defor-mation heat generated. In extrusion magnesium alloys working speeds are lower than with aluminium alloys. The highest speeds (50 m/min) are used for AZ31 magnesium alloy in hydrostatic extrusion processes. However, good surface quality can provide extrusion speed below 34 m/min [2, 3]. At 50 m/min so called " orange skin " effect occurs. AZ31 magnesium alloy is the most ductile and the most popular amongst AZ wrought alloys (Mg-Al-Zn group). Soft as it is, this alloy offers good combination of strength and ductility for structutral application after deformation with severe reductions. Parameters of main groups of products made of this alloy are presented in Table 1. 114 Table 1. Mechanical properties of AZ31B magnesium alloy [1] Sometimes extrusion is carried out in order to prepare the material for further deforma-tion, for instance, impression-die forging. In such cases, the extrusion process is not aimed at obtaining required level of strength, but appropriate quality of the forging stock. The criteria of determining extrusion process conditions, such as temperature and/or reduction ratio, is then providing proper size and shape of grains as well as microstructure uniformity and homogeneity, with care of β-phase precipitates, which are known to adversely affect plasticity [4, 5]. Generally, it is maintained that to complete breakdown of a cast structure throughout the whole cross-section, reduction ratio of at least 50 should be assumed [6]. Provision of such large deformations significantly affects production rate, as well as cross-sectional dimensions of produced extrudates due to limited capacities of available equipment. Also tool life is shorter. Lower reduction ratios in extrusion can improve process efficiency of both extrusion and die forging processes, if properties requirements are met. As mentioned above, despite providing higher production rate, higher speeds are accompanied by significant temperature increase. Therefore, lower process temperatures are assumed so that the increase in temperature during extrusion due to deformation heat should not exceed the incipient melting point. In result of lower temperatures higher loads are ob-served. In addition, high speeds are found to influence the mechanical properties of extru-dates in an adverse way. For that reason application of lower working speed to obtain higher strength is addressed in the paper with microstructural changes taken into consideration. The effect of extrusion process conditions on the final properties and microstructure of profiles extruded of magnesium alloys has been a subject of numerous studies. However, results presented in papers most often concern problems of extrusion of pipes or profiles as a finished products or focus on the issues of producing fine-grained structures to employ superplastic flow conditions. Since extrudates of smaller dimensions are characterized by more uniform profile of deformation in a cross-section, process conditions used there may not be suitable for bigger cross-sections. As such, extrusion of bars designed for forging stock call for determination of conditions which will guarantee microstructure providing satisfactory plasticity and required level of final properties of final parts.