زهرا عباسي

Additive manufacturing methods specially LPB-F has a high ability to manufacture Near net shape complex geometries with favorable mechanical properties. These methods have been used to produce all kinds of steel and superalloy parts for use and service at high temperatures. Therefore, eva‎luation of high temperature mechanical properties and thermal stability of parts made by additive manufacturing methods has great importance. in this research, 316L stainless steel parts produced by selective laser melting at temperatures of 700, 800, 900 and 1000 ℃ and strain rates of 0.1, 0.01, and 0.001s^(-1) up to a strain of 0.6 were got subjected to hot pressure testing. Jonas-Polyak hardening-softening curves based on flow stress curve indicates the occurrence of dynamic recrystallization in all thermomechanical conditions. The observation of a broad stress peak with a low softening fraction has been attributed to the activity of the continuous dynamic recrystallization mechanism. The fraction of grains with a size less than 10 μm due to recrystallization increases from 2% in the printed structure to 16.8% and 24% in the deformed structures at temperatures of 700℃ and 1000℃, respectively. The average grain size decreases from 73 μm in the printed structure to 45 μm in the deformed structure at 700℃. A similar trend regarding the deformed structures can be traced at temperatures of 800 ℃, 900 ℃, and 1000 ℃, but compared to the temperature of 700 ℃, the average grain size increases so that at 1000 ℃, the average grain size reaches 72 μm. Based on the evolution of the sub-boundary network and the structure of the sub-grains, the continuous dynamic recrystallization mechanism has been introduced as the main mechanism of recovery during the hot deformation of this steel. The extent of continuous dynamic recrystallization has been attributed to the presence of cellular substructure in the printed prototype after selective laser melting and before deformation. The average grain size decreases from 73 μm in the printed structure to 45 μm in the deformed structure at 700℃. A similar trend regarding the deformed structures can be traced at temperatures of 800 ℃, 900 ℃, and 1000 ℃, but compared to the temperature of 700 ℃, the average grain size increases that at 1000 ℃, the average grain size reaches 72 μm. Based on the evolution of the sub-boundary network and the structure of the sub-grains, the continuous dynamic recrystallization mechanism has been introduced as the main mechanism of recovery during the hot deformation of this steel. The extent of continuous dynamic recrystallization has been attributed to the presence of cellular substructure in the As-printed after selective laser melting and before deformation. The pre-existing sub-grains act as the primary nucleus for the nucleation of dynamic recrystallization, and as a result, the process of converting low-angle boundaries into large-angle ones is facilitated and the rate and extent of recrystallization increases. Also, grain boundary blistering and the formation of a necklace structure have been identified as a confirmation of the activity of the discontinuous recrystallization mechanism, and grain boundary deformation and their intertwining have been identified as a sign of geometric recrystallization activity in deformed structures. After deformation the texture changes also indicates the formation of a preferred texture with <111> orientation in the deformed structures at temperatures of 700℃, 800℃, and 900℃, but increasing temperature to 1000℃, the structure loses its preferred texture. The observed microstructural and texture changes and curves interpreted the softening behavior of the material.

فولاد زنگ نزن , ذوب ليزر انتخابي , تبلورمجدد , نرم شوندگي , پايداري حرارتي

Stainless steel , selective laser melting , Recrystallization , Softening , Thermal stability

Zahra Abbasi

Dr. Hamid Reza Abedi - Dr. Mohmad Taghi Salehi