جستجوی مقالات مرتبط با کلیدواژه « welding residual stress » در نشریات گروه « مکانیک »

Residual stress is one of the most substantial defects of welded parts caused by intensive thermal gradient. In this study, different mechanical and thermal techniques for reducing residual stresses have been investigated and the effectiveness of contributing parameters has been discussed afterwards. Subsequently, some equations have been proposed for welding energy and exergy efficiency and the effects of parallel flame heating, vibration method, and hammer working method on reducing welding residual stresses are expressed. The results show that by using parallel heating technique, the enhancement of flame power would result in reducing both energy and exergy efficiencies. However, the decremental rate of the two efficiencies would slow down and they approach to an asymptotic value. Increasing the speed of welding improves two efficiencies more than 2 times. On the other hand, the normalized entropy is reduced by increasing the heat input of the flames. This fact is an indicator of a reduction in welding residual stress. This reduction is more at high speeds. Eventually, the ratio of the two efficiencies shows that in this study, economical power was about 1800j/s. The reduction of normalized entropy for the vibration, hammering, and parallel flame methods are 0.001, 0.1, and 10, respectively. Overall, it is expected that thermal methods are more efficient than mechanical methods in reducing residual stresses.

One of the abilities of Ultrasonic method which has been less investigated, in comparison with other residual stress measurement methods, is the residual stress measurement capability in specimen depths. this capability is due to ultrasonic waves penetration in different depths of specimen, according to their transmittance frequency. In this research, the capability of Ultrasonic method in measurement of longitudinal welding residual stress in specimen depth has been studied. For this goal, four series of Ultrasonic sensors, including 1, 2, 4 and 5 MHz were used in order to reach four depths of pieces of aluminum alloy series 5000 joined by TIG welding. Welding residual stresses were simulated in Abaqus software. Then, simulation model was validated by comparing with the results of X-ray diffraction experimental method. The validated FEM model with x-ray diffraction method was used for validation of residual stress obtained from Ultrasonic method in four depths. Finally, good agreement was observed. Ultrasonic method could measure longitudinal residual stresses with the maximum error of 12% of yield stress of aluminum plate. The accuracy of this method in measurement of the maximum longitudinal residual stress was greater than 92%.

Welding residual stresses, especially longitudinal ones, have a significant role in the fatigue and fracture of the structures. The common residual stress measurement methods have an important restriction that they are incapable in measuring the residual stresses versus depth. In this study, the combination of FEM simulation and X-ray diffraction experimental method has been used for residual stress measurement along specimen depth. Residual stresses were induced by the TIG welding of two 5086 aluminum plates with a thickness of 8 mm. The welding process was simulated in Abaqus and then the simulation results were validated with X-ray diffraction ones. It was observed that in weld centerline, by increasing depth, maximum longitudinal residual stresses decreased by 37 MPa while this reduction in heat affected zone and in the distance of 10 to 25 mm from the weld centerline was 70 MPa. At farther points from the weld centerline, the longitudinal residual stresses decreased and the compressive residual stresses were formed in the specimen depths. With respect to significant variation of residual stress along the depths of the specimen, residual stress measurement along specimen depths seems to be necessary for performing accurate analysis.

In recent years, measurement of residual stress by ultrasonic method is developing because of nondestructive nature, portable equipment and being cheap and fast. In this research, the Capability of ultrasonic method by using longitudinal critically refracted or Lcr wave in measurement of longitudinal welding residual stress has been scrutinized. For this purpose, two plates of aluminum alloy series 5000 were joined by TIG welding method. Measurement of longitudinal residual stress by ultrasonic method was done in closeness of surface via 5 MHz transducers based on acoustoelasticity theory. In order to create Lcr wave and transmit it into specimen, an ultrasonic wedge was made based on Snell’s law. Also a triaxial table was used to control the wedge movement and keep the pressure on it fixed. In order to calculate residual stress and increase in accuracy, acoustoelastic constant for each three welding zones, including weld metal, HAZ and base metal was obtained separately from uniaxle tension test. In order to validate ultrasonic method results, measured longitudinal residual stress by x-ray diffraction method in 5 points on the specimen surface was used. Finally after comparing the results of two used methods with each other, good agreement was seen which indicates the good ability of ultrasonic method in measurement of longitudinal residual stress.