Caiyan Deng

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.

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.

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.

Optimal Preparation Process for Fatigue Specimens Treated by Ultrasonic Peening

This paper addresses the issue of preparing small-scale fatigue specimens for properly evaluating the effect of ultrasonic peening on fatigue life of welded structures and metallic components by laboratory experiment. Axial fatigue test and four-point bend fatigue test were conducted on fatigue specimens including untreated specimens and specimens being peened before and after cutting process. Fracture surfaces of fatigue failure specimens were observed to determine the crack initiation sites. Residual stresses were measured on different widths of cutting pieces to investigate the residual stress relaxation caused by cutting. The experimental results demonstrate that for specimens being cut after peening treatment, residual stresses are relaxed at the cutting edge, which results in significant reduction of fatigue life improvement and potential fatigue failure from the cutting edge. It is concluded that small-scale fatigue specimens should be machined to the final shape first and then applied peening, which not only applies for ultrasonic peening treatment, but for other residual stress-based improvement techniques.

Enhanced wear resistance by combined ultrasonic impact and electro-spark melting

ABSTRACT A novel surface modification method coupling surface melting and ultrasonic impact treatment (UIT) was developed to improve the wear resistance of S355 steel. The principle of ultrasonic impact electro-spark melting treatment (UIEMT) was presented, and the microstructure and surface plastic deformation layer were characterised by scanning electron microscopy, optical microscopy and X-ray diffraction (XRD). The obtained results revealed that UIEMT has more advantage than the UIT method. For UIEMT sample, fine microstructure with the average grain size of less than 2 μm was obtained, and the hardening layer was about 2.75 times deeper than the UIT one. The microhardness profile and residual compressive stress distribution showed a good correlation with the microstructural transition. In addition, no martensite transformation can be found by XRD. It was concluded that UIEMT can significantly improve the wear resistance without changing the chemical composition of the material.