In this paper, dissimilar resistance spot welding of AISI 1075 eutectoid steel to AISI 201 stainless steel is investigated experimentally. For this purpose, the experiments are designed using response surface methodology and based on four-factor, five-level central composite design. The effects of process parameters such as welding current, welding time, cooling time and electrode force are investigated on the tensile-shear strength of resistance spot welds. The results show that tensile-shear strength of spot welds is increased with increase in the welding current and welding time. Also, it is concluded from results that with increasing the electrode force and cooling time, tensile-shear strength of welded joints is decreased. During tensile-shear tests, three failure modes are observed, namely interfacial, partial pullout and pullout modes. The analysis of variance for the tensile-shear strength indicates that the main effects of welding current, electrode force, welding time, cooling time, second- order effect of the welding current and cooling time, two level interactions of welding current with welding time, welding current with cooling time and electrode force with cooling time are significant model terms. The results of analysis of variance show that the presented model for tensile-shear strength of dissimilar resistance spot welds of AISI 1075 eutectoid steel to AISI 201 stainless steel can predict 95.00% of the experimental data and leave only 5.00% of the total variations as unexplained. پرونده مقاله

In this paper the feasibility of NbAl3/Al2O3 nanocomposite formation through mechanochemical reaction between Al and Nb2O5 and the effect of mechanical activation on the kinetic and reaction mechanism were investigated. Structural and phase evolution during mechanical alloying were studied by employing transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. It was found from the results that the reduction of Nb2O5 by Al takes place in an explosive mode and after 450 min of mechanical alloying leading to the formation a microstructure consisting of NbAl3 matrix and nanocrystalline Al2O3 particles. The results also showed that heating the as-blended powder mixture of Nb2O5 and Al, leads to the formation of NbAl3/Al2O3 composite through two exothermic reaction; A thermite reaction at 880 °C followed by the synthesis of NbAl3 intermetallic at 944 °C. The mechanical activation of Nb2O5 and Al powder for 5h decreased the formation temperature of NbAl3/Al2O3 from 944 °C to 625 °C. Five isoconversional methods, of Kissinger-Akahira-Sunose, Tang, Flynn-Wall-Ozawa, Starink and Friedman were used to determine the activation energy, Eα, for NbAl3/Al2O3 formation reaction and its variation with conversion degree α. The results showed that these methods lead to similar or comparable values of Eα. پرونده مقاله

Mechanical properties optimization of weldment such as tensile strength, microhardness and impact toughness are of prime importance in welding technology. The main aim of this research is to investigate the microstructural evolutions and also optimization of tensile strength in dissimilar laser welding between Inconel 625 superalloy and AISI 430 ferritic stainless steel. It was found from the microhardness measurements that the hardness is increased form AISI 430 base metal toward Inconel 625 superalloy. This can be attributed to the inherent higher hardness of Inconel 625 and formation of very fine structure and also fine precipitations in the weld metal which are distributed uniformly. Formation of very fine structure and precipitations is due to the rapid cooling rate of the weld metal during laser welding process. Design of experiment based on the response surface method was used in order to optimize the tensile strength of the weld joints. Three process parameters i. e. pulse duration, lamping current and also travel speed were chosen as the main parameters which are considerably affecting the tensile strength of the weld joint. It was found that the lamping current has a higher impact on the tensile strength of the weld joints. پرونده مقاله

هدف از انجام این پژوهش، تولید نانوکامپوزیتی با زمینه NbAl3 توسط روش آلیاژسازی مکانیکی به منظور افزایش چقرمگی و استحکام دما بالای آن می‌باشد. به این منظور مقدار 49 درصد وزنی پودر آلومینیوم به همراه 51 درصد وزنی اکسید نیوبیوم مخلوط و آلیاژسازی مکانیکی گردید. تغییرات فازی ذرات پودر و همچنین ریزساختار و مورفولوژی ذرات پودر در زمان های مختلف آسیاب کاری توسط آزمون های پراش پرتو ایکس (XRD)، میکروسکوپ الکترونی روبشی (SEM) و میکروسکوپ الکترونی عبوری (TEM) مطالعه شدند. عملیات آنیل و همچنین آنالیز حرارتی افتراقی (DTA) جهت بررسی رفتار حرارتی ذرات پودر انجام شد. مشاهده شدکه واکنش بین آلومینیوم و اکسید نیوبیوم به صورت انفجاری بوده که در نتیجه این واکنش نانوکامپوزیت زمینه بین فلزی NbAl3 با ذرات تقویت کننده Al2O3 شکل می‌گیرد. مخلوط پودری آسیاب نشده نیز توسط DTA مورد مطالعه قرار گرفت که نتایج نشان داد در این شرایط تشکیل کامپوزیت NbAl3/Al2O3 در دو مرحله صورت می‌گیرد. در مرحله اول اکسید نیوبیوم پنج ظرفیتی توسط آلومینیوم موجود در سیستم احیا شده و نیوبیوم تشکیل می‌شود. در مرحله دوم آلومینیوم باقیمانده از واکنش با نیوبیوم تولید شده وارد واکنش شده که در نهایت کامپوزیت NbAl3/Al2O3 شکل می‌گیرد. مشاهدات صورت گرفته توسط میکروسکوپ الکترونی عبوری، تشکیل ساختار نانومتری و ذرات تقویت کننده نانومتری را تأیید می‌نماید. پرونده مقاله