Papers by Author: Jun Chen

Abstract: To predict material’s formability in the hydroforming processes, the plane stress assumption would be invalid. The instability perturbation approach proposed by Hu et al. [1] is extended with the through-thickness normal stress by combining Hill’48 and Hosford’s yield criteria. The influences of through-thickness normal stress on the predicted forming limit strains in the forms of traditional Forming Limit Diagram (FLD) and equivalent plastic strain (EPS) based FLD (epFLD) are investigated. The results show that forming limit curves (FLCs) in both forms of FLD enhance with increasing through-thickness normal stress under proportional and non-proportional loadings. This new model can be utilized to study the effects of fluid pressure on the formability of orthotropic thin sheets.

Abstract: In this paper, the effects of temperature and initial strain rate on the superplasticity of as-received Ti₂AlNb alloy were studied by uniaxial tensile tests. Temperature from 870°C to 1030°C with an interval of 40°C and initial strain rate range of 10^-2 s^-1 to 10^-4 s^-1 were selected. The optimal superplasticity of 190.3% was obtained at 990°C with initial strain rate of 10^-3s^-1. The superplastic properties were deteriorated at 1030°C due to serious grain coarsening. In order to improve superplastic properties, the as-received alloy was hydrogenated with different hydrogen contents. It was found that hydrogen addition can significantly decrease flow stress and increase elongation. A higher elongation occurs at 910°C in hydrogenated alloy.

Abstract: Hydroforming has been used widely across many industrial fields. Large applied pressure during hydroforming makes it necessary to consider the influence of normal stress in the thickness direction, while in FE simulation, the use of traditional shell element based upon plane-stress assumption is not appropriate in such cases. Here, the traditional shell element is modified by changing the constitutive relation which took into account the normal stress in the thickness direction, and the modified shell element formula is combined with Yld91 yield function to simulate the forming process of Aluminium alloy. Then the element formulation and material model is implemented into the FE code Ls-Dyna by means of USER interface. Two examples are carried out and good correlations are obtained when compared to the traditional shell element and solid element.

Abstract: Incremental sheet forming (ISF) is a highly versatile and flexible process for rapid manufacturing of complex sheet metal parts. Comparing to conventional sheet forming processes, ISF is of a clear advantage in manufacturing small batch or customized products such as cranial implant. Although effort on cranial reconstruction by using incremental sheet forming approach has been made in recent years, research has been mostly based on the single point incremental forming (SPIF) strategy and there are still considerable technical challenges for achieving better geometric accuracy, thickness distribution and complex cranial shape. In addition, the use of a backing plate or supporting die reduces the process flexibility and increases the cost. To overcome these limitations, double side incremental sheet forming (DSIF) process is employed for forming Grade 1 pure titanium sheet by using different toolpath strategies. The geometric accuracy and thickness distribution of the final part are evaluated so the optimized tool path strategies are developed. This leads to an assessment of the DSIF based approach for the application in cranial reconstruction.

Abstract: The microstructure and mechanical properties of Mg-6Al-1Zn-0.9Y-1.8Gd alloy have been studied by micro-analysis and tensile tests. The results showed that the alloy mainly consists of Mg matrix, Al₂Y, Mg₁₇Al₁₂ and A_l2Gd. The best tensile strength of the alloy was 255 Mpa at room temperature, and the alloy still had the very high mechanical property at high temperature.

Abstract: The stress-strain data from hot compression tests over a wide range of temperatures (1173–1473 K at an interval of 100 K) and strain rates (0.01, 0.1, 1 and 10 s^-1) were conducted using Gleeble-1500D thermo-mechanical simulator. A modified Zerilli-Armstrong constitutive model was developed using the experimental data of 70Cr3Mo back-up roll steel. The predictable efficiency of this model was evaluated by correlation coefficient and the value was 0.9902.

Abstract: By combining carbon woven fabric with thermoplastics grains, a thermo-stamping process is proposed for forming parts with complex double curvatures in one step, to implement the affordable application of fiber reinforced composites in high volume merchandises such as automotive industry. In the proposed thermo-stamping process, laminated carbon woven fabrics with thermoplastic grains are heated, and then transferred rapidly to a preheated mould for thermo-stamping, and cooled down to form the carbon fiber reinforced composite part. Various thermoplastics such as PP, PA6 and ABS are used as matrix material in the composite part. Experimental results including shear angle distribution in the fabric, deformed boundary profile of fabric with different original fiber orientation and forming defects are presented. It is demonstrated that high quality parts can be obtained with the proposed forming process, and defects are controllable. By using the proposed process and laminated structures, it is feasible to implement the high-volume and low-cost manufacturing of fiber reinforced composite parts.

Abstract: Metal-complex hydrides Li3AlH6 and V-doped Li3AlH6 nanoparticles were synthesized by solid reactions of LiH and LiAlH4 in the absence and in the presence of VCl3, respectively. X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Brunauer- Emmett-Teller sorption, thermogravimetry and differential thermal analysis have been used to investigate the phase composition, microstructure and surface properties. Not only the nanocrystalline Li3AlH6, but also the coexisting catalyst with “valence-transfer” state can influence the dehydrogenation kinetics. The extension of the catalytic mechanism is attractive for reversible hydrogen storage of the alanate system.

Abstract: The complex superplastic forming (SPF) technology applying gas pressure and compressive axial load is an advanced forming method for bellows made of titanium alloy, which forming process consists of the three main forming phases namely bulging, clamping and calibrating phase. The influence of forming gas pressure in various phases on the forming process are analyzed and models of forming gas pressure for bellows made of titanium alloy are derived according to the thin shell theory and plasticity deformation theory. Using model values, taking a two-convolution DN250 bellows made of Ti-6Al-4V titanium alloy as an example, a series of superplastic forming tests are performed to evaluate the influence of the variation of forming gas pressure on the forming process. According to the experimental results models are corrected to make the forming gas pressures prediction more accurate.