نشریه مهندسی مکانیک مدرس
سال بیست و دوم شماره 8 (امرداد 1401)

In this study, the effect of calcium and boron addition has been investigated on the microstructure and compression properties of AZ91 alloy. Initially, in order to determine the optimal calcium amount to reduce the grain size and the presence of secondary phases, 0.1, 0.5, 0.8 and 1 weight percent of calcium were added to the AZ91 alloy. The microstructure evaluation showed that an increase of calcium up to 0.5wt. % decreased the average grain size from 105 μm to 65 μm without the formation of Al2Ca phase. The similar tendency on grain boundary phases are also observed when increasing quantity of Ca added.  The microstructure and mechanical properties of the alloy was then evaluated by adding 0.03, 0.09 and 0.12wt. % B in the form of Al-3B master alloy. The results showed that the optimum concentration of boron is 0.09wt.% in order to achieve the lowest average grain size. In addition, the measured compression properties of samples containing 0.5wt. %   calcium with different amounts of boron showed that the alloy with 0.9wt. % B   presented the best mechanical properties.

In the present study, solid particle erosion of Ti-6Al-4V alloy under the impact of spherical alumina particles with a diameter of 85 microns was analyzed using experimental studies and smoothed particle hydrodynamics (SPH) modeling. The erosive behavior of this alloy was simulated as impacts on micro-scale and based on Johnson-Cook constitutive equations. This research focuses on the effect of particle velocity and impact angle on erosion rate as the most critical factors. Additionally, the results of this model are validated by empirical results under-considered conditions. At the end of the article, based on the alloy properties, the velocity of particles, and impact angle, a prediction equation was presented on erosion rate in the studied range of velocity and impact angle. This study indicates a power-law equation between the velocity of particles and the erosion rate, where the power is independent of impact angle. Furthermore, in all the velocity and angle ranges, the maximum erosion rate was associated with the angle of 45o. Therefore, the critical angle in erosion is also independent of the velocity of particles.

The deformation behavior of the material in micro-forming processes is different from macro-scale due to the size effect. The size effect in micro-scale appears due to few grains in the deformation region and causes the material behavior to be affected by the thickness and grain size of the sheet. Because of this, conventional constitutive models are not suitable for predicting the material behavior in micro-forming processes. In this paper, a new constitutive model based on the Swift equation and considering size effect in micro-scale is presented to describe the strain-hardening behavior of the stainless steel 304 foil. Comparison of flow stress curves of specimens with different grain sizes showed that the prediction of material flow stress with the new constitutive model is improved compared to the existing model, especially at high strains, so that the average and maximum error of the new model is less than one-third and less than half of the conventional model error, respectively. Finite element simulation of the micro-tensile test was performed using the new constitutive model to investigate the size effect on the deformation behavior of the specimens. The new constitutive model was verified by comparing the results of experimental tests and finite element simulation of sheets with different grain sizes. Also, the results revealed that the estimation of the forming force using the new constitutive model is done with higher accuracy than the conventional and existing model for sheets with different grain sizes and high strain ranges.

Tool wear has a significant influence on the turning process. Investigations on tool wear monitoring through various methods and sensors have been widely conducted to determine and predict the tool wear. In this study sound generation mechanisms during turning process have been investigated comprehensively and three sound generation sources have been determined and distinguished. Sound generation mechanisms which originated from tool vibration, deformation in the workpiece and vibration at the contact zones (friction), have been investigated and frequency range of the sound generated through each mechanism has been determined. It has been shown that these mechanisms produce sound in 10s hertz, kilohertz and megahertz respectively. Then the mechanism which is appropriate for tool condition monitoring has been studied and suggested. Then the relation between the sound generation mechanisms and chip formation has been studied during machining. Hence, a deep understanding about the machining process has been brought out. Findings could lead to an effective approach to monitoring the machining process, not only using mathematical signal processing methods, but also through a physical comprehension background. Experimental studies have been conducted to evaluate developed theories and models. Experimental results have shown effectiveness of the proposed approach.

The series elastic actuators make more comfort in the use of assistive exoskeletons. In this paper, an assistive controller is designed for a series-elastic-actuator-driven knee exoskeleton to restore normative mobility of individuals with weak muscles. The main target of the proposed controller is to modify the dynamics performance of the coupled human-exoskeleton system. In other words, the proposed controller modifies the relationship between the net muscle torque exerted by the human and the resulting angular motion. There are fewer sensors in the proposed intent-independent method relative to other methods. Moreover, there are less controller coefficients to regulate where these coefficients are extracted from a type zero Takagi-Sugeno-Kang fuzzy system. The performance of the controller is evaluated by simulations and experiments. The amplitude of the EMG signals decreased in a healthy person worn the SUT-KneeExo. Moreover, the proposed algorithm has a better performance in comparison with integral admittance shaping mothed and output feedback assistive controller. In other words, the amplitude of the integral admittance is more and the phase lag is less than other methods.

Due to the importance of the joints in the pressurized instruments and the abilities of the laser welding in this study, the welding of AISI316L tubes have been studied and analyzed. In this regard a fixture has been drawn in order to positioning of the tubes and fixed on the welding desk. The welding input parameters includes the laser welding adjusting variables, includes (welding current, welding pulse width, and welding frequency). Moreover, the effect of the two other variables (rotating speed and the force applied to the welding seam) has also been studied. The welding output characteristics comprises the welding width, depth of penetration and welding strength. The experimental data has been collected using L27 Taguchi design. The relation between the process input variables and output characteristics has been established using different regression models. Based on the analysis of variance (ANOVA) results, pulse width and welding current with 70% contribution have an influential effect on all the three response characteristics. Moreover, the seam force has only the influential effect on the depth of penetration and strength. Next, in optimization step based on the importance of the process characteristics (strength, depth of penetration, welding width), the optimized levels have been determined. At the end, the optimized condition has been conducted using laser welding, in comparison of which the samples in the design matrix, the welding depth has a close relation with the thickness of the wall, the welding boundaries smother and the strength has a close value to the base metal.