Experimental and numerical investigations on radial rolling process at elevated temperatures with two typical aluminum and magnesium hollow circular specimens produced via centrifugal casting process

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Experimental and numerical investigations on radial rolling process at elevated temperatures with two typical aluminum and magnesium hollow circular specimens produced via centrifugal casting process

ارائه دهنده: Provider: Danesh M. Rahmani

اساتید راهنما: Supervisors: Dr. Faramarz Fereshteh-Saniee

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. Mehran Kadkhodayan - Dr. Hamed Deylami Azodi - Dr. Abbas Paakk

زمان و تاریخ ارائه: Time and date of presentation: ساعت 14 - 1403/6/27

مکان ارائه: Place of presentation: Faculty of Engineering

چکیده: Abstract: Ring rolling is an advanced and complex technology in the field of metal forming. This operation is widely used to manufacture seamless rings with precise dimensions, such as anti-friction bearings, ring gears, nuclear reactors parts, connecting flanges, railway tires, as well as rings required in the chemical, aerospace, and automotive industries. This thesis investigates the effects of radial-axial ring rolling (RARR) process at elevated temperatures on AM60 magnesium and AL6063 aluminum rings initially produced by centrifugal casting. In horizontal centrifugal casting, the centrifugal forces and Coriolis acceleration move the melt along the inner rotating surface of the mold to form a hollow product. This approach forms a finer equiaxed grain structure to obtain more homogeneous and isotropic mechanical properties. Therefore, this casting method can improve the melt flow rate, save the raw material, reduce the production cost, increase the casting accuracy, and enhance the mechanical properties. In this research, several tubes were first prepared using the centrifugal casting. Then, the cast tubes were machined into primary rings for the rolling process. In order to perform the RARR tests, considering the dimensions and materials of the samples that affect the size of the rolls and bearings of the rolling machine and determine the minimum force required for the operation, a lab-scale horizontal rolling machine was designed and manufactured. To prevent the rings from breaking during the RARR due to high loading rate and to avoid considerable temperature drop of the sample and rolls, the duration of all the ring rolling tests, was considered constant (20 sec). The RARR experiments were performed at three high temperatures, including 200, 250, and 300 °C, and with three different rotational speeds of 40, 60, and 80 RPM for the main roll. To derive the stress-strain curves of the cent-cast as well as rolled rings, several compression tests were performed with cylindrical specimens cut from the rings in different directions, including Rolling Direction (RD), Transverse Direction (TD) and Normal Direction (ND). The experimental findings revealed that for both the AM60 and AL6063 alloys, the yield and ultimate strengths increased in all directions after the radial-axial ring rolling process, although the increase in strength in the RD direction was greater than the other directions. The highest increase in the ultimate strength occurred at a rotational speed of 40 RPM and temperature of 200 oC (S40-T200) in the RD direction, namely 47.9% and 30.6% for Al and Mg rings, respectively. The highest yield stress enhancements in RD direction and condition of S40-T200 for AM60 and AL6063 rings were respectively 110.3% and 79.3%, compared with the relevant cast alloys. The experimental results showed that both the initial ring temperature and main roll rotational speed had inverse influences on the mechanical properties such as strength and hardness, although the effect of the former was greater. In other words, a 50% temperature change was more effective for strength enrichment than 100% variation in the main roll rotational speed. The relationship between the average grain size (AGS) of the deformed rings and their mechanical properties was in agreement with the Hall-Petch relation. With this regard, the findings indicated that the average grain size of the rolled AM60 sample was much larger than the related AL6063 ring rolled under the same conditions. The ultimate strength, fracture strain and toughness of rolled aluminum rings were also greater than the corresponding deformed Mg samples, although the specific strength of the rolled AM60 specimens was much greater than that of the aluminum ones. Moreover, the rolled Mg rings showed more isotropic properties compared with the relevant aluminum parts. The design of experiment (DOE) method was used to evaluate the main effects and the interaction of the process temperature and main roll rotational speed on the mechanical and microstructural properties of both AL6063 and AM60 rolled rings. In addition, several high precision regression models were generated by Minitab software to predict different properties of the rolled rings versus the temperature and the main roll speed. The influences of the ring temperature on the strain hardening behaviors of these two alloys were also dissimilar. Contrary to AL6063 samples, the DOE analyses revealed a nonlinear effect of temperature on the strength of AM60 rings. Finally, the radial-axial ring rolling process was simulated in Abaqus software employing finite elements (FE), 3D Dynamic/Explicit solver, with ALE meshing. Due to symmetry of the RARR process in the axial direction and to reduce the numerical calculations and CPU time, only the upper half of the ring and upper rolls of the real machine were modeled. Moreover, these rolls were defined as rigid bodies in the Abaqus. In the beginning of the research, the FE simulation was carried out for a high-strength Al alloy, namely AL2024, at the room temperature and a rotational speed of 40 RPM. As a preliminary factor of safety, the output forces for this Al alloy were used to design the radial-axial rolling machine required for doing the RARR practical tests. After performing the main RARR experiments, the stress-strain curves of AL6063 and AM60 alloys at different temperatures and strain rates were extracted from the literature to separately input to the software and conduct the FE simulations of these tests. With this regard, distributions of several field variables such as equivalent stress and equivalent strain across the transverse cross section of the rolled rings as well as the rolling force (exerting by the main roll) were extracted from the software and then investigated.

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