S.H. Seyedein

Effects of welding parameters on weld pool characteristics and shape in hybrid laser-TIG welding of AA6082 aluminum alloy: numerical and experimental studies

The effects of three important welding parameters including laser power, welding current and welding speed on the weld pool characteristics, shape and dimensions in hybrid laser-TIG welding of AA6082 aluminum alloy are studied by numerical, experimental, and statistical approaches. For this aim, first, a 3D numerical model is used to simulate heat transfer and fluid flow in the weld pool and then resultant weld shape for various welding conditions. Besides, a set of experiments are performed to validate and calibrate the model. Finally, analysis of variance (ANOVA) method is applied to investigate more precisely how welding parameters affect weld dimensions. The simulation results show with increasing the laser power and welding current and decreasing the welding speed, the Marangoni and buoyancy forces increase. With increasing the laser power, the weld depth increases more significantly than the weld width. The weld half width increases with increasing the welding current, whereas the weld pool depth is relatively unchanged. Furthermore, with increasing the welding speed, both weld pool depth and half width decrease with similar slope. Generally, the presented model showed a good capability to predict the weld geometry and characteristics under various applied welding conditions which can reduce number of needed experiments.

Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel Prior to Direct Quenching

Direct quenching and tempering (DQ-T) of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving. Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products. The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components. These models were mostly used to predict austenite grain size (AGS), dynamic, meta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process. The hot compression tests were carried out on 42CrMo4 steel in the temperature range of 900–1100 °C and the strain rate range of 0.05–1 s−1 in order to study the high temperature softening behavior of the steel. For the exact prediction of flow stress, the effective stress-effective strain curves were obtained from experiments under various conditions. On the basis of experimental results, the dynamic recrystallization fraction (DRX), AGS, hot deformation and activation energy behavior were investigated. It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation.

Fluid Flow and Heat Transfer Modeling of AC Arc in Ferrosilicon Submerged Arc Furnace

A two-dimensional mathematical model was developed to describe the heat transfer and fluid flow in an AC arc zone of a ferrosilicon submerged arc furnace. In this model, the time-dependent conservation equations of mass, momentum, and energy in the specified domain of plasma zone were numerically solved by coupling with the Maxwell and Laplace equations for magnetic filed and electric potential, respectively. A control volume-based finite difference method was used to solve the governing equations in cylindrical coordinates. The reliability of the developed model was checked by experimental data from the previous available literature. The results of present model were in good agreement with the given data comparing with other models, because of solving the Maxwell and Laplace equations simultaneously in order to calculate current density. In addition, parametric studies were carried out to evaluate the effects of electrical current and arc length on flow field and temperature distribution within the arc. According to the computed results, a lower power input led to a higher arc efficiency.

Mathematical Modeling and Simulation of the Interface Region of a tri-layer Composite Material, brass-steel-brass, Produced by Cold Rolling

The object of this study was to find the optimum conditions for the production of a sandwich composite from the sheets of brass-steel-brass. The experimental data obtained during the production process were used to validate the simulation program, which was written to establish the relation between the interface morphology and the thickness reduction amount of the composite. For this purpose, two surfaces of a steel sheet were first prepared by scratching brushing before inserting it between two brass sheets with smooth surfaces. Three sheets were then subjected to a cold rolling process for producing a trilayer composite with various thick-nesses. The sheet interface after rolling was studied by different techniques, and the bonding strength for each rolling condition was determined by peeling test. Moreover, a relation between interfacial bonding strength and thickness reduction was found. The simulation results were compared with the experimental data and the available theoretical models to modify the original simulation program with high application efficiency used for predicting the behavior of the interface under different pressures.

Comparative study on dissimilar welding of Inconel 718 and Udimet 500 precipitation strengthened nickel based superalloys

Abstract Microstructural, mechanical and weldability aspects in the similar and dissimilar welds of alloy 718 and alloy 500 nickel based superalloys have been investigated. Alloy 500 weld metal showed high tendency of titanium to the segregation. Coalescence of the microvoids led to propagation of hot solidification microfissures. The alloy 718 weld metal displayed the formation of Nb rich low melting eutectic type morphologies, which can reduce the weldability. The microstructure of dissimilar weld metal with dilution of 65 wt-% displayed semideveloped dendritic boundaries. The less segregation and decrease in the low melting eutectics caused less susceptibility of dissimilar weld to solidification cracking. The segregation elimination phenomenon has occurred in the heat affected zone of alloy 500. In the partially melted zone, remelted and resolidified regions have been observed. These locations provided sites for nucleation of liquation cracks. For the alloy 718 heat affected zone, dissolution of γ″-Ni3Nb needle-like precipitations has taken place. It was the chief reason for sharp decline of the microhardness. The heat affected zone of alloy 500 revealed intense liquation cracking, in which the crack is initiated at the partially melted zone. The hot liquation cracking in the heat affected zone of Alloy 718 was observed as a result of γ″-Ni3Nb dissolution.

New approach for assessing the weldability of precipitation-strengthened nickel-base superalloys

A new procedure was proposed for evaluating the weldability of nickel-base superalloys. The theory is on the basis of two microstructural patterns. In pattern I, the weld microstructure exhibits severe alloying segregation, many low-melting eutectic structures, and low weldability. The weld requires a weaker etchant and a shorter time for etching. In pattern II, the weld microstructure displays less alloying segregation, low quantity of eutectic structures, and high weldability. The weld needs a stronger etchant and a longer time for etching. Five superalloys containing different amounts of Nb and Ti were designed to verify the patterns. After welding operations, the welds were etched by four etchants with different corrosivities. The weldability was determined by TG-DSC measurements. The metallography and weldability results confirmed the theoretic patterns. Finally, the etchant corrosivity and etching time were proposed as new criteria to evaluate the weldability of nickel-base superalloys.

Modeling of steady state hot flow behavior of API-X70 microalloyed steel using genetic algorithm and design of experiments

Comparison between measured and predicted stress in various temperatures, grain sizes and strain rates.Display Omitted Hot torsion test was performed to study hot flow behavior of API-X70 steel.Genetic algorithm was used for the first time to model steady state hot flow behavior of API-X70 steel.Taguchi Design of Experiments method was used to reach an optimal value for Genetic Algorithm parameters.The model extracted from GA has higher accuracy with respect to the conventional methods.The GA models take the effect of metallurgical phenomena and predict hot flow behavior with good accuracy. API-X70 microalloyed steel is one of the most conventional materials that has been used to produce the pipelines used in oil and gas industry. This steel is produced by thermo mechanical processing (TMP). Prediction of steady state hot flow behavior of metals during TMP, for design of its forming process is of great importance. In this research, flow curves of API-X70 were obtained using hot torsion test at temperature range of 9501150C and strain rates of 0.0013s1. Genetic algorithm (GA) was used to find parameters of steady state stress semi-empirical model in the way that minimizing the difference between experimental data and model output. The optimal combination of GA parameters were chosen by Taguchi design of experiments(DOE) method in order to increase efficiency of GA. Accuracy of developed model to predict flow stress in steady state region was evaluated through statistical methods. Results showed a good agreement between developed model and experimental data with R2=0.99 and this model can predict steady state flow stress well.

Process Control Strategies for Dual-Phase Steel Manufacturing Using ANN and ANFIS

In this research, a comprehensive soft computational approach is presented for the analysis of the influencing parameters on manufacturing of dual-phase steels. A set of experimental data have been gathered to obtain the initial database used for the training and testing of both artificial neural networks (ANN) and adaptive neuro-fuzzy inference system (ANFIS). The parameters used in the strategy were intercritical annealing temperature, carbon content, and holding time which gives off martensite percentage as an output. A fraction of the data set was chosen to train both ANN and ANFIS, and the rest was put into practice to authenticate the act of the trained networks while seeing unseen data. To compare the obtained results, coefficient of determination and root mean squared error indexes were chosen. Using artificial intelligence methods, it is not necessary to consider and establish a preliminary mathematical model and formulate its affecting parameters on its definition. In conclusion, the martensite percentages corresponding to the manufacturing parameters can be determined prior to a production using these controlling algorithms. Although the results acquired from both ANN and ANFIS are very encouraging, the proposed ANFIS has enhanced performance over the ANN and takes better effect on cost-reduction profit.

Fabrication of Single-Phase NiTi by Combustion Synthesis of Mechanically Activated Powders

Single-phase NiTi was fabricated through the thermal explosion mode of combustion synthesis of mechanically activated powders. Combustion and ignition temperatures of combustion synthesis were investigated in different milling times. In this process, equiatomic powder mixtures of nickel and titanium were activated by planetary ball mill and pressed into disk-shaped pellets then heated in a tube furnace, while temperature-time profile was recorded. X-ray diffraction analysis (XRD) was performed on milled powders as well as synthesized samples. Scanning electron microscopy (SEM) was also used to study the microstructural evolution during milling. The results showed that there was a threshold milling time to obtain single-phase NiTi. It was also seen that the ignition temperature and combustion temperature were reduced significantly by increasing milling time.

Mathematical Modeling of Transport Processes in Funnel Shaped Mold of Steel Thin Slab Continuous Caster

In this work a three-dimensional fluid flow and heat transfer model was developed to predict the flow pattern and superheat dissipation in funnel shaped mold of a thin slab continuous caster with a novel tetrafurcated design for the submerged entry nozzle. Low Reynolds k―e turbulent model was adopted to account for the turbulent effect. The transport equations were solved numerically using finite volume method. The results were compared with a full scale water model of the caster. Good agreement between mathematical and physical models was obtained. Parametric studies were carried out to evaluate the effect of casting speed, nozzle submergence depth, and inlet temperature on the superheat dissipation, flow pattern, and surface turbulence in the mold region. The results indicate a special flow pattern and heat distribution in the caster while using a tetrafurcated nozzle. Aiming to achieve more product capacity, in the case of casting with lower superheat temperature, a higher casting speed, together with higher submergence depth, is recommended in order to avoid surface turbulence and high heat flux across the narrow face.