An experimental study on the effect of Repetitive Non-Equal Channel Angular Pressing (RNECAP) process on the mechanical properties of a biodegradable magnesium alloy

نوع: Type: Thesis

مقطع: Segment: masters

عنوان: Title: An experimental study on the effect of Repetitive Non-Equal Channel Angular Pressing (RNECAP) process on the mechanical properties of a biodegradable magnesium alloy

ارائه دهنده: Provider: Mehdi akbarian

اساتید راهنما: Supervisors: Dr. Faramarz Fereshteh-Saniee

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. Hashem Mazaheri and Dr. Abbas Pak

زمان و تاریخ ارائه: Time and date of presentation: 2025

مکان ارائه: Place of presentation: 55

چکیده: Abstract: In recent years, severe plastic deformation (SPD) processes have been employed to achieve grain refinement for enhancing the mechanical properties of metals. Among the developed methods, Non-Equal Channel Angular Pressing (NECAP) has emerged as a prominent technique, particularly for magnesium and its alloys. Magnesium and its alloys have attracted considerable attention in a wide range of industrial applications due to their low density, high strength-to-weight ratio, and favorable biocompatibility. Furthermore, the simultaneous improvement of mechanical properties and corrosion resistance in these alloys remains a critical requirement for advancing their performance. Various studies have been conducted to enhance the mechanical and corrosion properties of these alloys. In the present research, the Repeated Non-Equal Channel Angular Pressing (RNECAP) process, which has primarily been employed for producing ultrafine-grained materials, was utilized to fabricate multi-layered magnesium-based structures. The main objective was to investigate the microstructural features, bonding characteristics, various mechanical properties, and, in particular, corrosion resistance of the product. To this end, the ZK30 alloy was processed at temperatures of 350°C and 400°C and with two different ram speeds, through successive passes, to produce single, double, and four layered specimens using the RNECAP operation with an extrusion ratio of 2. Force–displacement curves were obtained for all processing conditions, and the influence of each process parameter on the forming force was analyzed. Both the as-cast and processed specimens were evaluated in terms of microstructure, mechanical properties, and corrosion behavior. The results indicated that the processing temperature and the number of passes had significant impacts on the improvement of microstructural homogeneity, ultimate strength, ductility, and microhardness of the extruded bar. Compression tests along the thickness direction revealed that the weld line in the bonding region can independently influence the mechanical response. Moreover, the microstructural analyses showed that the smallest grain sizes achieved after the first, second, and third passes were 22.3 μm, 13.3 μm, and 32.2 μm, respectively, indicating a remarkable refinement compared with the as-cast condition (175 μm). However, during the third pass, a notable grain growth was observed, likely associated with the activation of recovery and grain growth processes at elevated temperatures. In addition, the presence of twins in the microstructure of RNECAPed component has influenced its mechanical behavior. Ultimate strength and ductility increased during the first and second passes due to the processing and bonding between the layers, but decreased after the third pass, due to the grain growth. Examination of the bonding strength between the layers also showed that higher processing temperatures and lower strain rates have led to a higher bonding strength and improved the uniformity. Corrosion tests also revealed that, after the first pass, the corrosion rate was significantly reduced compared with the as-cast sample. However, after the subsequent passes, due to the high grain boundary energy and the presence of the weld line in the bonding region, which acted as preferential sites for corrosion, the corrosion rate increased.