Iranian Journal of Materials Forming
Volume:7 Issue: 1, Winter and Spring 2020

Johnson-Cook constitutive equation is one of the most famous constitutive equations that have ever been developed to model the hot deformation flow curves of different materials. This equation is a predefined model in the traditional finite element codes to describe the material behavior in applications such as simulating the manufacturing processes. In this work, two different genetic algorithm-based (GA) optimization procedures, referred to as free and constrained optimization procedures, were proposed to find the constants of the Johnson-Cook constitutive equation. The proposed procedures were applied to fit the Johnson-Cook constitutive equation on the experimental flow curves of API X65 pipeline steel. According to the obtained constants, the modeling performances of the proposed procedures were compared with each other and with the modeling performance of the conventional procedure of finding the constants of the Johnson-Cook equation. Root mean square error (RMSE) criterion was used to asses and to compare the performances of the examined procedures. According to the obtained results, it was determined that the proposed free GA based optimization procedure with the RMSE value of 7.2 MPa had the best performance, while the performance of the conventional procedure was the worst.

Laser tube bending is a flexible forming process. Two irradiation methods of axial and circumferential scanning are generally used to form metal tubes. The two most important disadvantages of circumferential scanning are its lower bending angle and the time interval required between each scan for cooling purpose. In this research, a novel cooling strategy during laser tube bending with circumferential scanning is proposed to eliminate these disadvantages. The effects of this method on the bending angle and total production time are experimentally investigated. Also, the changes in the microstructure of the tube after bending are studied. The bending angle obtained at each scan using this strategy was increased more than 1.5 times with much less production time and energy consumption. Besides, the undesired effect of HAZ was significantly reduced. It is shown that this new cooling technique can highly improve the efficiency of laser tube bending by the circumferential scanning method.

In the present study, the finite element analysis of the hot deep drawing process of commercially pure (CP) titanium has been performed without the blank holder in order to investigate the influence of temperature (T), die radius (Rd) and blank diameter (D) on the maximum punch force (Fp) and minimum thickness of the blank (t). Tensile tests were first conducted to extract the mechanical properties of CP titanium sheets at various temperatures to simulate the hot deep drawing process. The results of the numerical simulation were used to perform the experimental tests at the optimal condition of the parameters. The experimental results of the process at the optimal condition of the parameters indicated that there is good agreement between the numerical and experimental investigations. The results indicated that the hemisphere of titanium without any wrinkling, tearing, and without any oxidation can be obtained by a blank diameter of 580 mm and forming temperature of 400°C.

In order to improve the properties of aluminum and its alloys, some various approaches (e.g. reduction of grain size, addition of alloying elements and composite manufacturing) have been considered. Among all these processes, the use of solid-state processes such as the friction stir processing (FSP) is highly convenient to create surface composites at temperatures below the melting point. Therefore, in this research, considering the FSP’s ability as a thermo-mechanical process and its advantages in the production of surface composites, the Al7075 surface composites were produced using reinforcing particles (Al2O3) and based on the FSP process in accordance with the design of experiments (DOE) approach. So, the response surface methodology (RSM) was selected as the experiment design method and variable factors such as: tool rotational speed, tool feed rate, diameter of tool shoulder and size of reinforcing particles were determined as the input variables. Statistical analysis and optimization of those parameters which affect the mechanical properties (yield strength and hardness) of surface composite Al7075/Al2O3 were performed. The results of the ANOVA and regression analysis of the experimental data approved the accuracy of regression equations and showed that the linear, interactional and quadratic terms of the input variables affect the yield strength and hardness of the composite specimens. Also, the optimal condition of the input variables was determined using the desirability method. In addition to the high values of desirability function (0.835, 0.822, 0.764), it could be found that the procedure of optimization has well fulfilled the pre-determined targets. In addition, the optimal condition has been confirmed by implementing the verification test.

In this paper, the laser bending process of a circular tube made of mild steel has been investigated experimentally. For this purpose, the effects of the irradiating length and the number of irradiating passes on the main bending angle of a laser bent tube are studied. In addition, the main defects of the laser tube bending process such as lateral bending angle, ovality and thickness variation are examined. Hence the effects of irradiating length and number of irradiating passes on the lateral bending angle, ovality and thickness variation of a laser bent tube are investigated. The results show that with an increase in the irradiating length, the main bending angle, the ovality percentage and the thickness variation increase and also the lateral bending angle will decrease. In addition, it is concluded that with an increase in the number of irradiating passes, main and lateral bending angles, ovality percentage and thickness variation increase.

The weldability of GTD-111 nickel-based superalloy by pulsed Nd:YAG laser welding with an average power of 250 W was studied, and the microstructural evolution and cracking characteristics were also investigated. The solidification cracking of the fusion zone (FZ) and the intergranular liquation cracking in the heat affected zone (HAZ) were observed in the joint. Solidification cracking was caused by the residual liquid metal originated from the segregation of Ti, Nb and Al elements in the interdendritic region at the last stage of solidification. And the HAZ liquation cracking was associated with the constitutional liquation of , MC carbides, and the melting of Cr-rich boride. Ti was introduced as the most important factor in the formation of the liquation cracks in HAZ by reducing the start temperature of  eutectic reaction and increasing the dissolution temperature. Chemical analysis of the crack edges at HAZ revealed the presence of high amounts of Ti and Al elements which can be attributed to  partial melting. Gleeble physical simulation revealed that in casting the sample, the liquation started at significantly lower temperatures than in the 1200  solution heat treated samples. This is attributed to the boride and intermetallic particles, which had dissolved by the 1200  heat treatment. The formation of fine grains due to the high cooling rate of the weld as well as the formation of dispersed carbides in the fusion and heat affected zones led to an increase in the microhardness by about 130 HV compared to the base metal.

The Rotary-Draw Bending (RDB) process is a distinguished process employed for the precision cold bending of a hollow tube with small radius bends using Numerical Control (NC) machines. This research presents an analytical model based on the power law hardening model for the RDB processing of rectangular thin tubes. Based on the constraints of the tube in the dies and its thinness, the plane stress assumption in the thickness direction and the plane strain assumption in the transverse direction for all faces of the tube are used. By assuming the proportionality of the process and using the Levy–Mises flow rule hypothesis, the stress filed of the tube is predicted and the balance of the forces in all aspects of the tube is used to identify the position of the neutral axis. Then the bending moment as well as the amount of the spring back is developed analytically. By analyzing the geometry of the tube and the process, a new mandrel is designed and constructed. The process has been modeled and analyzed using the explicit finite element ABAQUS commercial code and also carried out experimentally. Comparison of the analytical, numerical and experimental strain fields shows good agreement.

In the current study, the recrystallization behavior of 75% cold-rolled Fe-22Mn-10Al-1.4C steel during annealing at 750, 770, 790, 810, and 830°C was studied. X-ray diffraction patterns and optical microscopy were used to characterize microstructures. The Vickers Micro-hardness test was used to characterize recrystallization kinetics during annealing. Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to evaluate the experimental data. The as-homogenized microstructure illustrated only austenite with a high fraction of annealing twins, and austenite to martensite phase transformation was not observed after quenching at a high temperature and also until high thickness reduction. Avrami exponent was decreased from 0.76 to 0.42, with increasing the annealing temperature from 750 to 830°C. The activation energy value was determined to be ~175 kJ/mol, which was slightly higher than the diffusion activation energy of carbon in austenite.

Microstructural evolutions during annealing of cold rolled AISI 304L and AISI 316L stainless steels were studied. Cold rolled AISI 304L alloy was fully martensitic but cold rolled AISI 316L alloy was partially martensitic due to the higher stability of the austenite phase in the latter. During continuous heating to elevated temperatures, the complete reversion of strain-induced martensite at 750°C and an average austenite grain size of 0.4 µm was achieved in AISI 304L alloy. However, the complete reversion in AISI 316L alloy was observed at 800°C, but the recrystallization of the retained austenite was achieved at 900°C. The latter requirement for the formation of an equiaxed microstructure resulted in a much coarser average austenite grain size (2.3 µm). Annealing up to higher temperatures resulted in the grain growth of both alloys. The transformation-induced plasticity (TRIP) effect was found to be a major factor in dictating the mechanical properties, where lower stability of the austenite phase and the pronounced TRIP effect in AISI 304L alloy resulted in higher ductility in the tension test, higher friction stress in the Hall-Petch plot for hardness, and deeper dimples on the fracture surface.

Coating plays a significant role in surface engineering, which leads to the improvement of the mechanical and metallurgical properties of products. It also brings about economic benefits thanks to the cost savings of the improved properties of the surface of a product in particular. Friction surfacing is a relatively new way to create a homogeneous, fine-grained coating with amended resistance to wear and corrosion. In this study, the deposition of Al7075-T6 coating on Al2024-T351 substrates is investigated. Response surface methodology is implemented to study the effects of the rotational speed, axial force, and feed rate on the mechanical properties and microstructure of the specimens. Coating width, coating thickness, ultimate tensile strength, and grain size of coating are considered as the output responses. The input parameters are optimized to attain a wider and thicker coating with higher ultimate tensile strength and of course smaller grain size.  Results display the joining of two materials without any porosity at the interface. Moreover, an entirely fine-grained homogeneous microstructure of the deposition is observed. Furthermore, the average grain size of the deposition is diminished by 65% compared to the consumable rod.