Papers by Author: M. Haddad-Sabzevar

Abstract: An internal cooling agent is used in rapid slurry forming (RSF) process to produce a high solid fraction slurry for a short period of time. In the process used in this research, the swarf which is known to be a low enthalpy material was added to the melt as the internal cooling agent. During the process, the swarf started to melt and a semi-solid slurry with a relatively high solid fraction was formed. This slurry was formed by exchanging the enthalpies between the low and high enthalpy materials. A commercial Al-Si-Cu alloy, i.e. AS9U3 Aluminum alloy, was used in this investigation. The microscopic examination showed that the Al-Si eutectic colonies start to melt during the melting process of swarf material resulting in the formation of globular Alpha-Al grains due to the multiplication of secondary dendrites arms. The fracture of dendrites arms and the subsequent spheroidization were suggested to be the origin of non-dendritic globular grains in the final microstructure. The amount of primary globular Alpha-phase was measured by the image analysis software. The results showed that during high pressure die-casting of AS9U3 Aluminum alloy using 4 mm thick samples, around 35 percent solid has been formed at the temperature of 580 oC.

Abstract: Non-isothermal crystallization kinetics of Co₆₇Fe₄Cr₇Si₈B₁₄ amorphous ribbons was studied by differential scanning calorimetry (DSC) technique under 10, 20, 30, 40 and 80 °Cmin^-1 heating rates. It is found that Co₆₇Fe₄Cr₇Si₈B₁₄ amorphous alloy exhibits two-stage crystallization on heating. The two crystallization peaks shift to higher temperatures with increasing heating rate. The apparent activation energies (E_C) for the first stage of crystallization were determined as 443.44 and 434.47 kJmol^-1 by using the Kissinger and Ozawa equations, respectively. Frequency factor (A) estimated to be 1.084×10²⁶ s^-1 using Kissinger equation. Kinetics parameters such as Crystallization exponent (n) and dimensionality of growth (N_dim) were determined using JMA (Johnson-Mehl-Avrami) method. Details of the nucleation and growth behaviours during the non-isothermal crystallization were studied in terms of local activation energy E_C(x) by the OFW (Ozawa, Flynn and Wall) method. Also the activation energy for nucleation (E_n) and growth (E_g) separately estimated.

Abstract: The application of hot deformation during Friction Stir Welding (FSW) gives rise to produce a weld nugget consisting usually of dynamically recrystallized grains whose size is substantially lower than that of the base material. In the present study, several specimens with different conditions were first prepared using 2024 and 5083 aluminum alloys, which were then welded with FSW method. The microstructures of weld nugget in both series of specimens were examined using optical and scanning electron microscopes. The specimen of 2024 with anneal (O) and artificial aging (T6) conditions, and type 5083 with anneal (O), 30% and 50% cold work conditions were friction stir welded. Grain size distribution, hardness and temperature profiles in the welded zones were determined in order to obtain the relationship between the grain structure and the hardness profile in these regions. In each alloy, the average grain size in the weld nuggets was identical. The hardness of nugget zones in each alloy was the same due to their similar microstructures. According to the results obtained in this investigation, the initial microstructure showed no considerable effect on final microstructure and hardness of the weld nuggets, which this effect may be attributed to continuous dynamic recrystallization phenomenon.

Abstract: In order to investigate the crystallization behavior of the Co67Fe4Cr7Si8B14 amorphous metallic alloy, ribbons of this alloy were prepared by planar flow melt spinning process (PFMS). Differential scanning calorimetery (DSC) and differential thermal analyzer (DTA) were used to analyze the thermal properties and crystallization behavior of the samples at three heating rates of 10, 20 and 30º C/min. The experimental data were fitted to the Avrami model to determine the crystallization behavior. The results showed that the crystallization exotherm became wider and shifted toward a higher temperature range as the heating rate increased. The Avrami analysis also showed that n is about 1, which is related to the same transformation mechanism at different heating rates. The Kissinger method was used to determine the activation energy for the first crystallization peaks. The measured value is approximately 332.67 kJ/g.