Papers by Author: Jing Long Li

Abstract: The co-continuous AlN composites with different Al contents were fabricated through the squeeze casting of porous AlN preform with varied porosities obtained by carbothermal reduction. The effects of volume fraction of Al phase and the heat treatment temperature on the mechanical properties of composites were investigated. The change of the mechanical properties of composites with the Al content was in line with the mixed rule. With an increase in the Al content, the fracture toughness increased, the Vickers hardness and the flexural strength decreased. The toughen mechanism of composites included ductile rupture and microcrack toughening. The avoiding of excessive interface reaction between AlN and Al was beneficial to the mechanical properties of composites. With an increase in the heat treatment temperature, the stress and the dislocation due to mismatch in the coefficients of thermal expansion increased, the fracture toughness decreased, the vickers hardness and the flexural strength increased.

Abstract: A new method for preparing porous AlN ceramics with high porosity had been developed by carbothermal reduction of die-pressed green bodies composed of alumina, carbon, sintering additives and AlN seeds. The influences of sintering additives and sintering temperature on the microstructure and mechanical properties of porous AlN ceramics were investigated. XRD analysis proved that complete formation of AlN phase except for minor of glass phase. SEM analysis showed that the resultant porous AlN ceramics occupied fine microstructure and a uniform pore structure. Porous AlN ceramics with a porosity of 41~66% and a strength of 2.7~ 51.8 MPa were obtained.

Abstract: According to the microstructure defects of laser claded nickel alloy, such as crack and nets brittle phase, the friction stir processing (FSP) method is utilized in this paper to modify microstructure defects by the coupled effect of thermal and stress of FSP. For this purpose, two key scientific problems of large deformation condition and microstructure evolution theory during FSP, are proposed in this paper. Based on this, the following issues are highlighted in this work. Firstly, obtain the friction condition of larger deformation by continuous dirved friction experiment. Secondly, study on the temperature (T), stress (σ), and strain velocity under different processing parameters via self-developed physical simulation apparatus. Thirdly, disclose the effects of T、σ、 on the microstructure evolution and then obtain the microstructure evolution equation by dynamic re-crystallization. The above work could provide theory support for the new technique of laser cladding and FSP for preparing high performance coating.

Abstract: Pure titanium TA2 and 1Cr18Ni9Ti stainless steel were vacuum diffusion bonded using Nb + Ni foils as interlayers, where Nb foil was adjacent to TA2 and Ni foil adjacent to 1Cr18Ni9Ti. The samples were heated to a temperature of 800°C at a pressure of 10 MPa and kept for 60 min, and then to a temperature of 900°C at a pressure of 1 MPa and kept for 30 min. The mechanical properties of the joint were characterized by the tensile strength test. The average tensile strength of the joints was about 398 MPa. The fracture characteristics of the joint were investigated using scanning electron microscope. The fracture of tensile test samples occurred in the Ni3Nb intermetallic compound formed in the interlayers.

Abstract: The investigation of the plunge stage of the tool during friction stir welding (FSW) is the basis of understanding the nature of FSW, which is especially important for friction stir spot welding. In this study, the plunge stage of FSW tool was numerically examined by dynamic explicit finite element method with more attention to the temperature evolution. It was found that the high temperature region around the stirred zone extended with increasing the rotation speed. The heat affected zone expanded with the increase of inserting time, but the highest temperature around the stirred zone changed little. The range of the stirred zone changed little with the variations of the rotation speed and inserting time. The simulation result on the temperature field was in comparable agreement with the experimental one.

Abstract: Ti-6Al-4V was successfully jointed by linear friction welding (LFW). A sound weld of thickness of 65-115 $m was obtained under the present conditions. The weld consisted of the superfine α+β structure, which is associated with the rapid heating and cooling processes involved in LFW. The thermomechanically affected zone (TMAZ) was limited and comprised of the severely deformed α and β grains. In the flash, a gradual microstructure was formed from the weld edge to the flash edge. A martensite structure (α’) was formed besides the acicular α+β in the flash edge owing to the fast cooling rate.

Abstract: In this paper, the preparation of mullite suspension for electrophoretic deposition (EPD) is presented. It shows that the successful deposition of mullite coating is closely related to the weight ratio of mullite powder to ethanol solvent, and the concentration of phosphate ester (PE) in ethanol solvent, which is employed to positively charge the mullite particles in suspension. The polyvinyl butyral (PVB) is necessary as binder in the suspension. With increasing the powder:ethanol weight ratio, the content for the PVB in the suspension will also increase in order to make the deposited layer adhere to the substrate. The compositions of the mullite suspension were optimized and the process was determined based on the surface quality. It provides a new route to develop mullite coating on SiC or Si3N4 ceramic materials for high temperature application.

Abstract: 2D Fiber Reinforced carbon-carbon composites (C/C) was successfully bonded to itself with Ti-Ni-Ti sandwich interlayer by transient liquid phase bonding at 1050°C for 20~100 min under axial pressure of 0.1 MPa. The microstructures of joints were investigated and bonding strengths were examined, fracture surfaces were characterized by SEM and XRD. The results show that Ti-Ni eutectic liquid infiltrates into the interconnected capillaries of C/C through the open pores of the C/C boding surface. The presents of the breakpoints in joint alloy and the reaction between Ti and C are beneficial to the improvement of bonding strength. The shear strength of joint is as high as 37.4 MPa.

Abstract: Ba-ferrite particles were dispersed into distilled water to make stable slurry, which was then slip cast in transverse magnetic field. The water drainage from the slurry was performed by vacuum evacuation to solidify the slurry into a cake – like sample. To obtain high degree of alignments, the slurries were slip cast in static – / pulsed – magnetic field and by using two separate steps of magnetic orientation and drainage. The particles turned their faces to the magnetic field and formed long chains stacked immediately while the magnetic field was applied, of which high induction density led to high degree of alignments. It is necessary to correspond with the drainage, gravity and magnetism so as to suppress the tendency of distortion or cracking of the sample as the aligned long chains of the particles tend to break into pieces and cave in randomly, which eventually destroys the particle alignments.

Abstract: Water-based slurries containing barium ferrite particles have been prepared and slip cast in magnetic field. This paper presents the characteristics of the suspensions in terms of Iso-Electric Points (IEP) and zeta potential that were evaluated through pH titration and polymer adsorption. Both enlarging the specific surface area of particles by planetary milling and adjusting the pH to low value apparently increase the zeta potentials. Stable slurry was obtained by adding polyethylene glycol (PEG) into the suspension at pH = 2 ~ 3.5. The steric repulsion plays key role in dispersion and PEG films served as insulative layers and mechanically kept particles from contact each other. The barium ferrite particles formed many stacks of plates during slip casting, which either aligned randomly without magnetic field applied or regularly aligned to form textured structure when magnetic field was applied.