Papers by Author: Arash Rezaei

Abstract: In this investigation, the dynamic recrystallization phenomenon (DRP) of an Al-Li-Cu-Mg-Zr alloy was studied at two temperatures, 350°C and 400°C. Wedge samples were subjected to hot rolling deformation in both temperature and one passes. For wedge specimens, reduction up to 70% was considered. Results showed that the grain size of the specimens after hot rolling decreases from 100 μm to 30 μm because of DRP. Furthermore, it is observed that critical reduction for starting DR at 400°C is 40% and at 350°C reduction must increase to 50%.

Abstract: Aluminum-Lithium alloys were developed as major replacements for existing aluminum alloys to reduce the weight of aircraft and aerospace structures. Mechanical properties of Al-Li alloys greatly depend on solidification conditions. Other than reducing the microsegregation, homogenization treatment has other effects on the microstructure of Al-Li ingots. In this research, effects of homogenization treatment at constant temperature (500°C) on the precipitation in the microstructure of Al-1Li-3Cu-0.1Zr (wt %) and Al-2Li-3Cu- 0.1Zr (wt %) specimens have been investigated. Results show that homogenization at 500°C for 24 hours not only increases the hardness and phases precipitating in grain but also reduces microsegregation of Fe in grain boundaries.

Abstract: Surface alloying is a new method of surface treatment by which an element is diffused into the surface of another element causing the formation of a solid solution on its surface. Using this method, the problems of casting methods have been eliminated. In this method, the sample is sunk into an aqueous solution and is connected to the cathode. Applying enough voltage, lithium separated from lithium sulphat-sulfuric acid solution diffuses into the aluminum surface. The depth of diffusion into the aluminum is measured to be 50 to 500μm according to the diffusion time which is selected to be 1 to 24 hours. Experiments showed that the formed layer is very uniform. It was also concluded from the experiments that hardness is increased extensively, with the maximum value of 98HV, and fatigue life is increased by the factor 4. It was also shown that the toughness, yield stress and ultimate tensile strength of samples are the same as those of pure aluminum.

Abstract: To respond the need of industries to the new materials with higher specific modulus and lower density than those of the older Al alloys, aluminum- lithium alloys have been invented and improved. Aging process is one of the most important methods to improve the mechanical properties of aluminum- lithium alloys which are in the heat treatable category of aluminum alloys. Low temperature and natural aging processes cause the "short range diffusion" of Cu atoms in the aluminum and the formation of the GP zones. In this research, stability of GP zones and the effects of these areas on physical and mechanical properties of AA2090 alloy were investigated by hardness, electrical resistance, DSC (differential scanning calorimetric) and tensile tests. Results show that endothermic effect in the DSC diagrams of AA2090 alloy at 180°C to 240°C can be related to the enthalpy of GP zones dissolution. Formation of GP zones in the structure increases hardness, tensile strength and electrical resistance of Al- Li C u (2090) alloys.

Abstract: The precipitation of T1 phase during the ageing of an Al- Li- Cu -Zr alloy sheet was studied by DSC (Differential Scanning Calorimetric) technique. Results showed that precipitation of T1 phase occurred in temperature range of 250°C to 300°C whereas its dissolution occurred within the temperature of 450°C to 530°C. Furthermore, activation energies for precipitation and dissolution of T1 phase were determined 122.1 kJ/mol and 130.3 kJ/mol, respectively.

Abstract: In this study, diffused carbon in workpiece surface with Plasma Electrolysis Carburizing (PEC) treatment applied to harden SAE 1015 steel is investigated. The workpiece is connected cathodically to a high current DC power supply that applies high voltage and uses stainless steel as the anode. After water ionization with this voltage, a hydrogen film forms around the workpiece. This film acts as a thermal and electrical insulation layer which increases the electric resistance around the workpiece and makes it being heated. The surface of the workpiece is transformed to austenite and after the disconnection of the current, the workpiece is quenched in aqueous solution. By the aid of this method, the workpiece can be hardened to 1200HV or more. In this paper, the effect of time and solution concentration on the depth of diffused carbon was studied. The hardness of workpiece was measured and the variation of the diffusion depth versus time and solution concentration was investigated.

Abstract: Aluminum-Lithium alloys are attractive choices for weight saving of aerospace structures. By addition of Lithium to Aluminum alloys, the mechanical and physical properties are improved, density decreases up to 10% and module of elasticity increases up to 15%. As-cast Al-Li ingots include some continues brittle phases in grain boundaries that decrease formability by crack initiation during hot rolling. In the present work, influence of homogenization on hot rolling of Al- Li alloys was studied. Homogenization temperature determined based on DTA test and homogeneity properties were characterized by microstructures observation. One specimen was cast in conventional method and the other was cast by direct casting (DC) method. Homogenization was performed at four different times: 10, 24, 50 and 100 hours at 500°C also no cracking has been observed in both ingots in 100 hr and 24 hr at 500°C respectively due to eliminate the brittle phases.

Abstract: In the present study, solidification and segregation of Al-Li-Cu-Zr alloys have been investigated. Results show that shape of ingots and amount of lithium can change solidification essence. Increasing solidification rate, not only eliminates grain-refining effect of zirconium but also causes coarse segregation of copper in grain boundaries (negative segregation). Increasing amount of lithium in Al-Cu-Li-Zr alloys can change the solubility of other alloying elements (Cu, Zr) and increase solidification range and microsegregation of copper in structure.