Experimental and Numerical Simulation to Study the Reduction of Welding Residual Stress by Ultrasonic Impact Treatment

In this study, the effects of ultrasonic impact treatment (UIT) on the residual stress in a repair welding joint are investigated by experimental and finite element methods. A three-dimensional numerical analysis approach including a thermomechanical-coupled welding simulation and dynamic elastic-plastic UIT simulation is developed, which has been validated by X-ray diffraction measurement and indentation strain method. The results show that longitudinal residual stresses basically turned into the small tensile stress state from the large tensile stress state, and transverse residual stresses have mainly turned into compressive stresses from large tensile stress after the UIT. In the thickness direction, the average decrease of longitudinal residual stress is 259.9 MPa, which is larger than the 149.1 MPa of transverse residual stress. The calculated residual stress distribution after the UIT of the thin plate is compared with that of the thick plate in the literature, with the results showing the stress accumulation layer inside the thick plate. The simulation results show that the elastic strains are decreased slightly and the equivalent plastic strain is increased markedly after UIT, which explains the mechanism of residual stress relaxation.

A Description of Preferential Sites for Vacancy Formation on Grain Boundaries in Copper by Structural Unit Model

The preferential sites for vacancies on a series of symmetric tilt grain boundaries in copper have been investigated by molecular dynamics simulation. The regularity of preferential sites for vacancies on these boundaries can be described by the structural unit model. This is essential because of the correspondence between the geometries of the structural units and the local stress field. The vacancies are energetically preferred at the sites with relatively large tensile stress, and these sites are the corner sites of the structural units. Moreover, these preferential sites are mainly related to the structural unit types irrespective of which grain boundary that the structure units locate in. Therefore, the preferential sites for vacancies on various grain boundaries formed by combinations of certain structural units can be readily described and predicted by the structural unit model.

Modeling and Simulation of Nanofluid Minimum Quantity Lubrication Surface Grinding Thermal Stress

The model of surface grinding with a nanoparticle jet flow of MQL was established. The surface grinding thermal stress of three workpiece materials, namely, 45 Steel, 2Cr13, and nano-ZrO2 dental ceramic, were numerically simulated. Results show that dry grinding generates larger tensile stress, whereas MQL grinding generates larger compressive stress. The finished surface of workpiece produces large tensile stress in grinding direction. With the increase of cutting depth, the time-related variation of thermal stress on finished surface slows down gradually. Residual stress is inversely proportional to cutting depth. With the increase of cutting depth, the finished surface of workpiece is firstly dominated by large tensile stress, which decreases continuously until reaching the maximum compressive stress. Deeper layer is less influenced by temperature field, manifested by smaller stress value and slight variation of the whole stress field.

Erosion Wear Resistance of CWS Laminated Ceramic Nozzles

In coal-water-slurry (CWS) boilers, the nozzle is eroded continuously by the abrasive action of the CWS, and there are high temperature and temperature gradient inside nozzle, which may cause large tensile stress and lead to an increased erosion wear of the nozzle. In this paper, Al2O3/(W,Ti)C+Al2O3/TiC laminated ceramics were developed to be used as nozzles in CWS boilers. The purpose is to reduce the tensile stresses at the nozzle during the CWS burning process. The value of the residual stresses, which arise from a mismatch between the coefficients of thermal expansion of the constituent phases and neighbouring layers, was calculated by means of the finite element method. The erosion wear behavior of the laminated nozzles was investigated and compared with an unstressed reference nozzle. Results showed that the laminated ceramic nozzles exhibited an apparent increase in erosion wear resistance over the unstressed reference one. It is suggested that laminated structures in ceramic nozzles is an effective way to improve their erosion wear resistance in industrial CWS boilers.

Study on Mechanism of Tap Breakage in Low Frequency Torsional Vibration Tapping

Early breakage of tap often occurs when tapping on difficult-to-cut materials with the method of low frequency torsional vibration tapping, which decreases the technological effect and restricts its application. The effect of impact load on tap breakage in vibration tapping was analyzed, and the stress state in tap teeth was simulated by FEM, on this basis, it is concluded that too large tensile stress in tap teeth is the main cause of early breakage of tap. To reduce the tensile stress and enhance the shock resistance of tap teeth, the tap should be ground to minor negative rake angle. Experiments showed that the failure mode of grinded taps changed to wear, and thereby the service life of tap was prolonged significantly.

Motion of a sphere normal to a wall in a second-order fluid

The motion of a sphere normal to a wall is investigated. The normal stress at the surface of the sphere is calculated and the viscoelastic effects on the normal stress for different separation distances are analysed. For small separation distances, when the particle is moving away from the wall, a tensile normal stress exists at the trailing edge if the fluid is Newtonian, while for a second-order fluid a larger tensile stress is observed. When the particle is moving towards the wall, the stress is compressive at the leading edge for a Newtonian fluid whereas a large tensile stress is observed for a second-orderfluid. The contribution of the second-order fluid to the overall force applied to the particle is towards the wall in both situations. Results are obtained using Stokes equationswhen α1+α2=0. In addition, a perturbation method has been utilized for a sphere very close to a wall and the effect of non-zero α1+α2 is discussed. Finally, viscoelastic potential flow is used and the results are compared with the other methods.

Research of Mechanism of Chipping in Step Tapping of Superalloy Based on Wavelet Analysis

Stepping tapping is a new tapping mode for Nickel-based superalloy GH4169. Longer life of tap and higher machining efficiency can be obtained. But chipping of tap is very prevalent. In this paper, the mechanism of chipping in step tapping is investigated. Based on the wavelet analysis of the signals of tapping torque, the load applied to the tap in every stage of the period of motion is studied. It is found that chipping is prone to occur at the moment when the rotating direction of tap shifts from forward to backward, which is caused by the too large and rapidly changed tensile stress on cutting edge. The excessive wear of cutting edge results in the too large tensile stress. To reduce the wear, the backward rotating angle of tap should be selected to be smaller than 100°.

Formation of Tensile Stress Induced Cracks in Flame Hardening Surface Treatment of 12Cr Blade Steel

This paper reports on the formation of tensile stress induced cracks by the application of a flame hardening to 12Cr steels and the monitoring of the flame hardening process at the desirable residual stress state. During the flame hardening of the steels elastic residual tensile stresses were typically generated due to a phase transformation of austenite into martensite and they became greater by increasing both the process temperature and cooling rate. Eventually the cracks were nucleated and propagated across the prior austenite grain boundaries by a generation of large tensile stress, which was accompanied by a drastic decrease in the tensile stress due to a stress relaxation.

The Influence of Thermal History and Alloying Elements on Temporary Strengthening of Thin Al-Cu Films

The influence of Cu on the response of Al-Cu thin films to thermally induced stress is studied. The copper concentration is varied between 0 and 1.15 at. %. It is proposed that copper atoms which have not formed precipitates, largely affect the mechanical behaviour. This idea is supported by the following observations. An isothermal hold results in temporary strengthening of the films. The extent of this strengthening increases with copper concentration, increases with decreasing isothermal hold temperature and saturates with increasing isothermal hold period. Based on these observations the large tensile stress increase below 200 °C is ascribed to the formation of Cottrell atmospheres.

Diffusional Phase Transformation under Induced Thermal Stress

When thin films are deposited on substrates or when compound films are formed through interdiffusion of multi-film structures, intrinsic stress develops in the various films. Thermal mismatch between the expansion coefficients of the substrate and films in multi-film structures gives rise to extrinsic stress at elevated temperaturesBy using Si<100> and rolled Al foil substrates supporting the same multi-film structure SiO2/Si/Co, the effect of extrinsic stress on interdiffusion of thin films is isolated.Silicide growth is found to be inhibited (delayed) when formed on Al substrates compared to that formed on Si substrates. The delay in silicide growth is ascribed to delamination caused by large tensile stress prior to silicide formation. The growth rate of Co2Si is found to be similar on both Al and Si substrates