Dongpo Wang

Finite element simulation of thermal stress during diffusion bonding of Al2O3 ceramic to aluminium

AbstractThe development of thermal stress during the diffusion bonding of Al2O3 ceramic to aluminium is analysed via the finite element analysis method using a commercial software package, namely, MARC/MENTAL. Before diffusion bonding, a 0.2 mm copper layer is sintered onto the ceramic. The results show that shear stress concentration exists in the ceramic near the corner of the Al2O3/Cu interface, and tensile stress concentration occurs near the corners of the Al2O3 outer surface when the sample cools from bonding temperature to room temperature. When the concentrated tensile stress on the Al2O3 undersurface increases to a certain value, the ceramic will deform gradually, which results in stress relaxation. The stress relaxation originates in the stress concentration region of the Al2O3 outer surface.

Comparison of two types of low-transformation-temperature weld metals based on solidification mode

ABSTRACT Two types of low-transformation-temperature weld metals were devised, one associated with primary austenite solidification, the other primary ferrite solidification. The martensite start temperature of both low-transformation-temperature weld metals was about 125°C. Experimental results showed that low-transformation-temperature weld microstructure associated with primary austenite solidification was martensite with 8.0% retained austenite, whereas that one related to primary ferrite solidification primarily consisted of martensite and δ-ferrite. Accordingly, both welded joints had little distinction between distortion and residual stress, indicating that weld metal associated with primary ferrite solidification played the same function as primary austenite solidification on residual stress reduction. Moreover, the low-transformation-temperature weld metal associated with primary ferrite solidification had higher tensile strength and hardness than that based on primary austenite solidification.

Correction of buckling distortion by ultrasonic shot peening treatment for 5A06 aluminum alloy welded structure

Abstract Ultrasonic shot peening treatment (USPT) was proposed to correct welding buckling distortion. The residual stress distribution along the depth direction of the peened zone was measured by an X-ray diffractometer. The microstructure of the treated specimens was investigated by scanning electron microscopy (SEM). The Vickers microhardness was measured in different areas of welded joint before USPT and along the depth direction of the weld after USPT. The experimental results indicated that the welding buckling distortion of 5A06 aluminum alloy butt joint can be essentially corrected by USPT; the average correction rate reached 90.8% in this study. Furthermore, USPT enhanced specimens by work hardening. The microstructure of the peened zone was improved; moreover, the distribution of the precipitates and grains presented an apparent orientation.

Study on fabrication of ceramic coatings on Ti–6Al–4V alloy by combined ultrasonic impact treatment and electrospark

A method combining ultrasonic impact treatment with electrospark was developed to fabricate ceramic coatings on Ti–6Al–4V alloy. In this paper, the mechanism of the combined process is described in detail. The microstructure, chemical states, phase composition and surface residual stress of the ceramic coated titanium alloy were presented. The coatings mainly consist of carbides of titanium. The hardness of the coating increased significantly owing to the existence of the ceramic phase. Further, the impact process and the narrow space that the shock ball traverses decrease the surface roughness of the sample after the combined process. Finally, the surface residual stress of the sample treated by the combined process is compressive stress.

Effective FE model to predict surface layer characteristics of ultrasonic surface rolling with experimental validation

Abstract The present paper presents a numerical approach to investigate the surface layer characteristics after ultrasonic surface rolling process (USRP), which is a severe plastic deformation process involving complex static extrusion and dynamic impact. Efforts are made in the development of finite element model (FEM), accompanied by assessment of material constitutive relation and meshing accuracy as well as definition of loading condition according to the technological principle of USRP. The majority of processing parameters have been taken into account, and results are discussed in terms of residual stress, surface roughness and workhardening. As supplied 40Cr disc was subjected to ultrasonic surface rolling for validation of FEM, and the residual stresses predicted from the FEM are in good agreement with the measurements obtained by X-ray diffraction method, indicating that the present FEM can be used as a fast prediction tool for the design of ultrasonic surface rolling parameters.

Comparison of Microstructure and Residual Stress Between TIG and MAG Welding Using Low Transformation Temperature Welding Filler

A Cr–Ni type of low transformation temperature (LTT) welding filler was devised in the present study. The LTT weld microstructures of the tungsten inert gas (TIG) and metal active gas (MAG) weldings were investigated by using electron-backscattered diffraction and orientation imaging microscopy. The results showed that the LTT weld microstructures prepared by TIG and MAG weldings were primarily martensite with 17.5% and 8.0% retained austenite, respectively. The LTT weld metal using TIG welding had larger grain size than using MAG. In addition, based on the Taylor factor calculation, the weld metal using MAG welding was more competent in repressing fatigue crack initiation. Meanwhile, the high angle and coincidence site lattice grain boundaries were dominant in the LTT weld metal using MAG welding. Moreover, the hardness of the LTT weld metal using MAG welding was higher than that of using TIG. Based on heat input and phase transformation, finite element method was applied to analyzing the tensile residual stress (RS) reduction in welded joints prepared by both conventional and LTT welding fillers, respectively. The corresponding outcome confirmed that the LTT weld metal using MAG welding was more beneficial to tensile RS reduction.

Behaviours of ultrasonic peening treatment induced effects near the edges

ABSTRACT The behaviours of ultrasonic peening treatment induced effects near the edges of weld toe were investigated. The deformed weld toe profiles were observed and numerical models were established to calculate the stress concentration factor for the middle and the edges of the specimen. Micro-hardness profiles were also measured in the subsurface layer. The results revealed that comparing with the middle position the stress concentration is lower, the macro deformation is larger, and the strain hardening characterised by the increase of hardness is less significant at the edges. Finite element analysis was employed to predict the residual stress distribution under different peening sequences and it was found that tensile stresses appeared at the starting edge, and relatively smaller compressive stresses existed at the ending edge. The optimal peening strategy should be starting the treatment inside and moving out towards the edges.

Finite element analysis of residual stress evolution with multiple impacts on one point in ultrasonic impact treatment process

The ultrasonic impact treatment process is widely used to improve the fatigue life of the weldments by inducing compressive residual stresses at the sub-surface. The purpose of the article is to conduct the dynamic elastic–plastic finite element analysis of multiple impacts on 5A06 aluminum alloy with different controlled parameters. The numerical model was validated by pin drop test. The changes in penetration depth, maximum compressive residual stress, and surface residual stress were obtained by analyzing the residual stress field and equivalent plastic strain. The effect of impact times, impact velocities, pin shapes, and impact angles on the residual stress was investigated so that the ultrasonic impact treatment parameters could be controlled to obtain expected residual stress distributions.

Effects of ultrasonic peening treatment on hydrogen sulphide corrosion behaviour

ABSTRACT After being subjected to ultrasonic peening treatment (UPT), the resistance of X65 pipeline steel to hydrogen sulphide corrosion was investigated through electrochemical, immersion and slow-strain-rate testing. In addition, the UPT-induced compressive residual stresses layer and related changes in the steel near-surface microstructure and hardness were examined by scanning electron microscopy, optical microscopy and X-ray diffraction. The obtained results revealed that UPT was able to significantly improve the corrosion potential of X65 pipeline steel and reduce its hydrogen sulphide corrosion rate, while the formation of the compressive stress layer increased the steel’s tensile strength and inhibited hydrogen blistering.

Numerical assessment of fatigue design curve of welded T-joint improved by high-frequency mechanical impact (HFMI) treatment

Abstract In the paper, the fatigue performances of as-welded T-joint and T-joint improved by high frequency mechanical impact (HFMI) were numerically investigated using structural hot spot stress approaches: linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effects of main plate thickness and material strength for HFMI-treated joints were investigated. The results showed that the TTWT method was more effective to study the effect of thickness on T-joints improved by HFMI treatment than LSE method. For as-welded T-joints, the thickness correction exponent n = 0.04 was obtained when the attachment plate thickness was set as constant. For HFMI-treated T-joints, a reverse thickness effect was observed with negative thickness correction exponents, and the thickness correction exponents increased with material strength. In addition, the adoption of S–N slope varying with yield strength was proven to be more proper for HFMI improvement assessment.