Jianmin Han

Investigation of the Scanning Microarc Oxidation Process

Scanning microarc oxidation (SMAO) is a coating process which is based on conventional microarc oxidation (MAO). The key difference is that deposition in SMAO is achieved by using a stainless steel nozzle to spray an electrolyte stream on the substrate surface as opposed to immersing the workpiece in an electrolyzer. In the present study, SMAO discharge characteristics, coating morphology, and properties are analyzed and compared to results obtained from MAO under similar conditions. Results show that MAO and SMAO have comparable spark and microarc lifetimes and sizes, though significant differences in incubation time and discharge distribution were evident. Results also showed that the voltage and current density for MAO and SMAO demonstrate similar behavior but have markedly different transient and steady-state values. Results obtained from coating A356 aluminum sheet show that oxide thickness and growth rate in SMAO are strongly dependent on interelectrode spacing and travel speed. Analysis of the SMAO coating morphology and structure showed that a denser and slightly harder layer was deposited in comparison to MAO and is attributed to reduced porosity and increased formation of α-Al2O3. Preliminary results indicate that SMAO represents a viable process for coating of aluminum surfaces.

Predicting Distortion in Butt Welded Plates Using an Equivalent Plane Stress Representation Based on Inherent Shrinkage Volume

Due to the adverse effect that distortion has on assembly fit-up and fabrication costs in welded structures, the ability to predict dimensional changes resulting from joining represents an important engineering concern. While distortion can be analyzed using a full 3D finite element (FE) model, this often proves to be computationally expensive for medium and large structures. In comparison, a 2D FE model can significantly reduce the time and effort needed to analyze distortion though such analyses often have reduced accuracy. To address these issues, a plane stress modeling approach based on inherent shrinkage volume is proposed. By inversing the plastic shrinkage zone geometry, an equivalent plane stress representation and eccentric loading condition can be developed and used to predict distortion in butt welded plates. The model was validated using deflection data obtained from welded plates and found to provide good accuracy over the range of thicknesses considered. Results obtained from welding of a large municipal containment tank are also presented and further confirm the utility of the method.

Study of Scanning Micro-arc Oxidation and Coating Development

Micro-arc oxidation (MAO) continues to be the focus of numerous investigations, whereas relatively few studies have considered scanning micro-arc oxidation (SMAO). In the present work, an experimental study was performed using stationary and moving electrodes to investigate coating development in SMAO and discern the effect of key process parameters. Examination of oxide deposits made on A356 aluminum show that coating thickness and growth rate are inversely related to inter-electrode spacing and travel speed. An evaluation of SMAO deposits made by stationary and moving nozzles revealed that coating thickness profiles follow a Gaussian distribution due to the electrolyte flow field in the impingement zone. Hardness surveys and scanning electron microscope analysis of SMAO coatings revealed that micro-hardness distributions and cross-sectional morphology are similar to MAO for a stationary nozzle but that a denser outer layer develops when a moving nozzle is used. This is attributed to a high density of discharge occurring in micropores of the oxide film and remelting which results from the moving electrolyte column. Analysis of voltage–current characteristic curves shows that the resistance of the electrolyte column is essentially linear over the range considered and results indicate that it can be modeled as a variable length resistor. While further testing is needed, results confirm that SMAO is suitable for coating large, planar parts and for repairing worn surfaces.

Analytical modelling of shot-peening residual stress on welding carbon steel surface layer

The residual stress distribution was studied by an analytical model, due to shot peening on the welding carbon steel surface layer. The initial welding residual stresses before shot peening were taken into consideration in this analytical model. The Hertzian elastic contact theory was used to get the elastic compression stress state after impact on the surface layer. The initial welding stress field and the shot peening stress field would superpose and the welding surface layer would yield based on the elastic-plastic evaluation, then the residual stress after shot peening can be achieved. The influence of initial welding residual stress on the stress distribution after shot peening was analyzed and discussed. A series of experiments were carried out and the residual stress on the welding surface was determined by X-ray diffractometer before and after shot peening. The calculation results of the analytical model are consistent with the experimental results. The critical shot velocities when welding surface layer yielded and reverse yielded were calculated. While the welded joint surface material reversely yielded, the maximum compressive residual stress would not obviously increase with the increase of shot velocity, the thickness of the compressive stress layer would be increased. Welding residual tensile stress can enlarge the thickness of the compressive stress layer at the same shot velocity when reverse yield appeared.

An inverse method for searching parameters of combined welding heat source model

The thermo-physical parameters needed to develop accurate welding heat source models for process modelling are not always readily available and must often be developed using empirical methods or through trial and error. An inverse method based on an iterative procedure is presented in this paper, which can be used to optimize the unknown parameters in a combined welding heat source model which is composed of double-ellipsoidal and cylindrical heat sources. In the proposed method, an image of the simulated welding temperature field and the experimental weld cross-section profile of the specimen are digitalized and subtracted in order to generate the level of error between the simulated and experimental weld beads. The Hooke–Jeeves direct search method is then applied to minimize the error and optimize the parameters of interest which include radius and the width and depth and the energy partition coefficient of the combined heat source model. Thus, the parameters needed to describe a combined welding heat source model can be more efficiently obtained using the optimized parameters. The method is validated by a comparison between the measured and calculated temperature curves at a selected point in the heat affect zone.

Residual Stress Reduction in Single Pass Welds Using Parallel Line Reheating

Current postweld heat treatment (PWHT) methods rely mainly on static thermal sources or line heating using dispersed beams which require significant capital investment and often pose limits on weldment size. In the current study, an alternative PWHT method based on line heating is presented and analyzed. The method, which is intended to perform low temperature stress relief, employs parallel oxyacetylene torches to induce a tensile stress in the vicinity of the weld toe. X-ray diffraction (XRD) measurements taken from bead-on-plate (BOP) welds made using ASTM A572-50 showed a 37% decrease in the peak longitudinal stress after parallel line reheating was performed. A corresponding reduction in the stress gradient on the plate surface was also observed. Welding and reheating were also modeled in sysweld to assess how torch placement affected the longitudinal stress distribution and an optimum offset was identified for the 8-mm plate thickness used. Analysis of the thermomechanical history in the vicinity of the weld toe indicates that a tensile stress is superposed during reheating and is concurrent with the reduction in the peak longitudinal stress.

Experimental and Simulation Research on the Influence of Stirring Parameters on the Distribution of Particles in Cast SiCp/A356 Composites

Achieving the uniform distribution of reinforcement particles in MMCs is very important for the effect of stirring parameters and the flow action of the melt, which should be known. The effect of stirring parameters on the distribution of SiC particles in SiCp/A356 composites was studied by the experimental and numerical methods in this paper. The experimental results show the SiC distribution with different stirring parameters. In addition, the effects of the fluid velocity and volume fraction of SiC particle at different position of crucible on the SiC distribution were analyzed by numerical simulation. The velocity magnitude, axial velocity, and radial velocity were analyzed to explain theoretically the particle distribution. The shearing force, moments, and stirring power of the stirring rod were simulated based on CFD code. The numerical results show that the stirring temperature is lower, the shearing force is greater, the stirring time is longer, and particle dispersion gets better. On the other hand, the higher the stirring speed is, the more uniform the radial and axial flow are, and the better the particles were dispersed. The numerical results were in good agreement with the experimental data.

Temperature Effects on the Friction and Wear Behaviors of SiCp/A356 Composite against Semimetallic Materials

Due to the low density and high temperature resistance, the SiCp/A356 composites have great potential for weight reduction and braking performance using the brake disc used in trains and automobiles. But the friction coefficient and braking performance are not stable in the braking process because of temperature rising. In this paper, friction and wear behaviors of SiCp/A356 composite against semimetallic materials were investigated in a ring-on-disc configuration in the temperature range of 30°C to 300°C. Experiments were conducted at a constant sliding speed of 1.4 m/s and an applied load of 200 N. Worn surface, subsurface, and wear debris were also examined by using SEM and EDS techniques. The third body films (TBFs) lubricated wear transferred to the third body abrasive wear above 200°C, which was a transition temperature. The friction coefficient decreased and weight of semimetallic materials increased with the increase of temperature and the temperature had almost no effect on the weight loss of composites. The dominant wear mechanism of the composites was microploughing and slight adhesion below 200°C, while being controlled by cutting grooves, severe adhesion, and delamination above the 200°C.

Microstructures and Mechanical Properties of Weld Metal and Heat-Affected Zone of Electron Beam-Welded Joints of HG785D Steel

Abstract Vacuum electron beam welding (EBW) process was employed to butt weld 10-mm-thick HG785D high-strength steels. The penetration into the steel was adjusted by beam current. Microstructures at weld metal and heat-affected zone (HAZ) regions were comparatively observed. Mechanical properties of the EBWed joints including Vickers hardness, tensile and Charpy impact tests were evaluated. The results indicated that microstructures at the weld metal consisted of coarse lath martensite and a small amount of acicular martensite, while that in the HAZ was tempered sorbite and martensite. The grain size in the weld metal was found to be larger than that in the HAZ, and its proportion in weld metal was higher. The hardness in the weld metal was higher than the HAZ and base metal. The tensile strength and impact toughness in the HAZ was higher than that in the weld metal. All the behaviors were related to microstructure evolution caused by higher cooling rates and state of base metal. The fracture surfaces of tensile and impact tests on the optimized joint were characterized by uniform and ductile dimples. The results differed significantly from that obtained using arc welding process.

Multi-discipline fusion design of structure and process based on simulation technology

The conflict between structure design and casting process of castings has been studied to make them have good casting structure feasibility. In this paper, it takes the use of structure and process integrated design based on simulation technology to resolve the conflict. Casting structure feasibility of aluminum axle box for the high speed train application, as an example, was analyzed by this means.