مصطفی جعفرزادگان

The applicability of the diffusion brazing technique for joining Nimonic105 superalloy using a BNi-2 interlayer was assessed. The phases formed during the brazing process in the isothermally solidified zone (ISZ), athermally solidified zone (ASZ) and diffusion affected zone (DAZ) were studied. The formation of Ni-Cr-Mo boride precipitates in DAZ was determined in this region. The ISZ was a precipitation-free zone, mainly consisting of gamma solid solution. ASZ contains the matrix solid solution of gamma-nickel, Ni-rich boride, and Cr-rich boride and Cr-rich intermetallic compounds. The investigation confirms higher bonding time (60 and 240 min) led to a more uniform hardness profile across the joint and a more homogenous interface region via dissolution of intermetallic phases. The highest shear strength and ductility of the brazed joint can be up to 92.2 MPa and 19%, respectively for the bonding time of 60 min. This high strength and ductility can be attributed to the control of formation of deleterious intermetallic compounds in ASZ and at the same time limited time for grain growth.

This study was conducted to investigate the effect of Si addition (1, 2, 3, and 5 wt. %) on the microstructure and castability of hypoeutectic Al-4.5Cu-Si alloys. According to the results, Si addition increased the average size and volume fraction of eutectic Si platelets in the microstructure. Adding Si also improved the fluidity and decreased the porosity content of the alloy, and enhanced its resistance against hot tearing. According to the constrained rod test results, the hot tearing sensitivity index (HTS) of the alloys containing 1, 3, and 5 wt. % Si, was decreased by 48, 78, and 88%, respectively. In agreement with hot tearing testing results, the extensive existence of dendrites and shrinkage micropores on the hot torn surface of the alloy without Si implies on its low potential in feeding the solidification shrinkages and healing hot cracks. Adding Si decreased the amount of shrinkage micropores on the hot torn surface and due to improving the fluidity as well as increasing the amount of Al-Cu-Si ternary eutectic at the last stage of solidification, improved the feeding characteristics of alloy. Therefore, the amount of free dendrite arms was significantly decreased on the fracture surface. However, adding 5 wt. % Si increased the amount of eutectic Si on the hot torn surface giving rises to a more brittle fracture mode.

In this research, dissimilar joints between copper and stainless steel 316by gas-tungsten arc welding using copper and monel filler metals and also the similar welding of steel 316 have been investigated. The microstructure was studied by OM, SEM and EDS. Tensile and impact tests were done to evaluate the mechanical properties. In order to investigate the sigma phase formation, the specimens were heat treated. The microstructure of the dissimilar welds contained an iron rich and a copper or monel phase. After heat treatment, no sigma phase was observed in the dissimilar joint that it seems that the presence of copper and nickel as an austenite stabilizer elements in the microstructure prevents the sigma phase formation.

Joining of stainless steels to carbon steels is one of the most widely used types of dissimilar joints. In this research, the microstructure and mechanical properties of dissimilar shielded metal arc welding of austenitic stainless steel AISI 316L to carbon steel St-37 is studied. This joining has made by E316L electrode and three heat inputs of 0.72, 0.84 and 1.0 kJ/mm. The microstructure of the joints was characterized by optical and scanning electron microscopy. The mechanical properties of the joints were evaluated by tensile and microhardness tests. The solidification microstructure of the weld metals contained mainly austenite and some skeletal delta ferrite. There was an epitaxial growth at the interface of the weld metal with the AISI 316L steel HAZ, and at the interface of the weld metal with the St-37 HAZ, there is an un-etched narrow area which is suggested to have austenitic-martensitic structure. The results of hardness tests showed that the hardnessof the interface of the weld metal with the St-37 steel was maximum which is attributed to the formation of the martensitic narrow band. The longitudinal tensile results showed that the increasing of the heat input decreaes the tensile strength and increases the elongation of the joints.

In this paper, the distribution of temperature and thermal cycle in gas tungsten arc welding of stainless steel 304 is studied. The welding operation was performed under different heat inputs of 540 and 782 kJ/mm on the plates with 2 mm thickness. The Abaqus software was employed for simulation. The volumetric heat distribution, according to the double ellipsoidal (Goldak) model, was considered as the heat source of the welding and DFLUX subroutine was coded by FORTRAN. The simulation results were compared with the Rosenthal and Adams models and experimental results. The simulation results with both heat inputs are well compatible with the cross section dimensions of the welds, while the Rosenthal model predicts a smaller welds’ pool size. The temperature-time curves obtained from the simulation and Rosenthal model have considerable differences but their predicted cooling time and rate in the temperature range of 800 to 500 °c, especially in heat input of 782 kJ/mm, are close to each other (the difference is less than 7.2%). The results also showed that the maximum temperature of the regions farther from the fusion boundary in Adams model is predicted higher than that of simulation and Rosenthal model, while the results of simulation and Rosenthal model are closer to each other.

Thermal spraying is economical and rapid coating process that creates a rough and clean surface. As a result it can be used for applying the interlayer in transient liquid phase bonding. In the present study, transient liquid phase bonding Al 2024 to Ti-6Al-4V was investigated using brass interlayer, where the interlayer was Cu-Zn foil Cu-Zn thermal spray coat on Al substrate, respectively. The results show that by using thermal spray coat as interlayer, because of the formation of different defects that can be considered as diffusion channels, diffusion potential of Ti and Al becomes higher at the interface. It is concluded that the mechanism of bonding formation involves the diffusion of Cu into Al and Ti base materials and formation of TiAl، TiAl، Cu2 Al and AlCuMg phases and also diffusion of Cu through Al grain boundaries and formation of eutectic phases across the grain boundaries. The formation of these intermetallic phases was confirmed by energy dispersive spectroscopy and X-ray diffraction. Dissolution of the base metals in the joint area and the isothermal solidification process of the thermal sprayed interlayer is more and faster than the foil interlayer. The joint with thermal spray brass coat as interlayer, gives higher shear strength of 25 MPa in comparison with the case of using brass foil as interlayer (14.6 MPa). The decrease in bond strength can be attributed to aggregation and growth of the brittle intermetallics near the joint interface due to lower diffusion potential of Ti and Al in the joint zone.

Friction stir welding is a new and effective method for joining the alloys with welding problems and also for the dissimilar alloys. In the present study, friction stir welding is used to join st37 low carbon to 304 stainless steel plates at different tool rotation speeds. The stir zone in the 304 steel shows evidence of dynamic recrystallization with a moderate dislocation density. The stir zone in the st37 steel appears to experience dynamic recrystallization too, although the allotropic transformation during cooling cycle of the welds removes the features of dynamic recrystallization and produces a fine ferrite-pearlite microstructure with a low dislocation density. The relationship between hardness and microstructure is investigated through Hall-Petch equation for the stir zone of both steels. The results of weighted least-squares fit also show that the average hardness of austenite in the 304 steel and ferrite in the st37 steel inside the stir zones has a reverse relation with hardness according to the Hall-Petch equation. The hardness of the base metal and the stir zones of 304 steel does not stand on a same Hall-Petch line that can be attributed to the relatively higher dislocations due to the dynamic recrystallization of stir zones.

Friction stir processing as a relatively new and effective solid-state process is used for surface modification of metals and alloys in order to modify their microstructure and properties. In this study, the effect of friction stir processing on the microstructure and mechanical properties of an Al-0.9Ni-Fe alloy was investigated. The process was performed under the rotational speeds of 600-2000 rpm and the traverse speed of 25 mm/min. According to the results, the severe surface plastic deformation refined the intermetallic compounds present in the microstructure, improved their distribution within the matrix, and substantially reduced the amount of casting defects. As a result of these microstructural variations, the tensile strength and percent elongation of the base alloy are increased by almost 40 and 205%, respectively. The fracture toughness is also improved by 205%. It was also found that the hardness of processed alloy is about 24% higher as compared to the base alloy.

In this study, plates of 304 stainless steel and st37 steel were welded together by friction stir welding which has been set at a welding speed of 50 and 100 mm/min and tool rotational speed of 400, 600 and 800 rpm in order to study the effect of parameter variations on micro structure and corrosion resistance. Samples were cut from the cross section of the joint and metallographic imaging was performed after etching. For the corrosion resistance evaluation, samples were immersed in Ferric Chloride and acetic acid 2M and then were studied by macroscopic imaging and stereograms also Tafel polarization and electrochemical impedance spectroscopy (EIS) tests have been performed in 3.5% NaCl solution in order to verify weight loss results. The results showed that the corrosion rate of the St37 base metal is more than other parts of the joint. On the other hand, all parts of stainless steel 304 was galvanically protected. The best corrosion resistance among all the weld areas were determined for the weld at the tools rotational speed of 800 rpm due to minimum grain growth by heating.