Effect of welding sequence on residual stress and deformation of 6061-T6 aluminium alloy automobile component

Four different welding sequences of double-pulse MIG welding were conducted for 6061-T6 aluminum alloy automobile bumpers by using nonlinear elastoplasticity finite element method based on ABAQUS software.The post-welding residual stress and deformation were definitely different among the four welding sequences.The results showed that the highest temperature in Solution A was approximately 200℃higher than the melting point of base metal.High residual stress was resulted from this large temperature gradient and mainly concentrated on the welding vicinity between beam and crash box.The welding deformation primarily occurred in both of the contraction of two-ends of the beam and the self-contraction of crash box.Compared with other welding sequences,the residual stress in Solution A was the smallest,whereas the welding deformation was the largest.However,the optimal sequence was Solution B because of the effective reduction of residual stress and good assembly requirements.

FE analysis of extrusion defect and optimization of metal flow in porthole die for complex hollow aluminium profile

To solve the defects of bottom concave appearing in the extrusion experiments of complex hollow aluminium profiles,a 3D finite element model for simulating steady-state porthole die extrusion process was established based on HyperXtrude software using Arbitrary Lagrangian–Eulerian(ALE)algorithm.The velocity distribution on the cross-section of the extrudate at the die exit and pressure distribution at different heights in the welding chamber were quantitatively analyzed.To obtain an uniformity of metal flow velocity at the die exit,the porthole die structure was optimized by adding baffle plates.After optimization,maximum displacement in the Y direction at the bottom of profile decreases from 1.1 to 0.15 mm,and the concave defects are remarkably improved.The research method provides an effective guidance for improving extrusion defects and optimizing the metal flow of complex hollow aluminium profiles during porthole die extrusion.

Mechanical Stimulus Inhibits the Growth of a Bone Tissue Model Cultured In Vitro

Objectives To construct the cancellous bone explant model and a method of culturing these bone tissues in vitro, and to investigate the effect of mechanical load on growth of cancellous bone tissue in vtro. Methods Cancellous bone were extracted from rabbit femoral head and cut into I-ram-thick and 8-ram-diameter slices under sterile conditions. HE staining and scanning electron microscopy were employed to identify the histomorphology of the model after being cultured with a new dynamic load and circulating perfusion bioreactor system for 0, 3, 5, and 7 days, respectively. We built a three-dimensional model using microCT and analyzed the loading effects using finite element analysis. The model was subjected to mechanical load of 1000, 2000, 3000, and 4000 με respectively for 30 minutes per day. After 5 days of continuous stimuli, the activities of alkaline phosphatase （AKP） and tartrate-resistant acid phosphatase （TRAP） were detected. Apoptosis was analyzed by DNA ladder detection and caspase-3/8/9 activity detection. Results After being cultured for 3, 5, and 7 days, the bone explant model grew well. HE staining showed the apparent nucleus in cells at the each indicated time, and electron microscope revealed the living cells in the bone tissue. The activities of AKP and TRAP in the bone explant model under mechanical load of 3000 and 4000 με were significantly lower than those in the unstressed bone tissues （all P〈0.05）. DNA ladders were seen in the bone tissue under 3000 and 4000με mechanical load. Moreover, there was significant enhancement in the activities of caspase-3/8/9 in the mechanical stress group of 3000 and 4000 με （all P〈0.05）. Conclusions The cancellous bone explant model extracted from the rabbit femoral head could be alive at least for 7 days in the dynamic load and circulating perfusion bioreactor system, however, pathological mechanical load could affect the bone tissue growth by apoptosis in vitro. The differentiation of osteobiasts and osteoclasts might be inhibited after the model is stimulated by mechanical load of 3000 and 4000 με.

Study on Phase Transformation Behavior of Strain-induced PLLA Mesophase by Polarized Infrared Spectroscopy

The structural transformation of mesophase to crystalline phase of strain-induced poly(L-lactic acid) has been investigated by differential scanning calorimetry(DSC) and in situ temperature dependent polarized Fourier transform infrared(FTIR) spectroscopy. It is found that, as the drawing temperature increases, melting of strain-induced mesophase in the heating process can remarkably interfere the crystallization behavior subsequently. Coupling with in situ polarized FTIR, from 60 °C to 76 °C, the mesophase melts partially rather than completely melting, and changes immediately to three-dimensional ordered structure. Of particular note, through monitoring the subtle spectral change in the critical phase transformation temperature from 60 °C to 64 °C, it is clearly demonstrated that relaxation of oriented amorphous chains initially takes place prior to the melting of mesophase.

High Performance of PVA Nanocomposite Reinforced by Janus-like Asymmetrically Oxidized Graphene:Synergetic Effect of H-bonding Interaction and Interfacial Crystallization

Macromolecule nanocrystal network and strong interfacial interaction are always beneficial to enhance the mechanical property of polymer-based nanocomposites.Poly(vinyl alcohol)(PVA),a typical biocompatible semicrystalline polymer,is an ideal candidate for preparing high performance polymer-based nanocomposites.However,the rich hydrogen bonds between PVA matrix and graphene oxide(GO)can disrupt the formation of PVA nanocrystal network.Thus,it remains a great challenge to achieve both strong and tough PVA-GO nanocomposites.Herein,by introducing a novel Janus-like amphiphilic graphene oxide(JGO),both hydrogen bonding and interfacial crystallization have been constructed between JGO sheets and PVA matrix.Benefiting from amphiphilic interfacial interaction and the enhanced crystal network,both PVA-JGO dried films and their swollen hydrogel films show superior mechanical properties than those of traditional PVA-GO nanocomposites.PVA-JGO dried films exhibit a 264%improvement of toughness at a JGO loading of 1 wt%.Meanwhile,the corresponding PVA-JGO swollen hydrogel films display simultaneous improvement of nearly 8 times increase of tensile strength and 20 times increase of toughness compared to traditional PVA-GO nanocomposite.This work indicates multiple interfacial interactions and macromolecule crystal networks can be concurrent in PVA nanocomposites by innovative modification of nanofillers,providing a new strategy to construct PVA nanocomposites with high strength and high toughness.The integration of outstanding mechanical and swelling resistance properties on PVA-JGO nanocomposite films render their promising applications,such as packaging and toughening hydrogel materials.

Cellulose Nanocrystals-mediated Phase Morphology of PLLA/TPU Blends for 3D Printing

Incorporation of nanoparticles into polymer blend to obtain finely dispersed morphology has been considered as an effective strategy to prepare nanocomposites. Owing to the renewable and degradable characters, cellulose nanocrystals(CNCs) have been proposed to tailor the phase morphology of poly(L-lactic acid)(PLLA) blend for producing high-performance fused deposition modeling(FDM) consumables. However,the main challenge associated with the ternary systems is the dispersion of the highly hydrophilic CNCs in non-polar PLLA blend by industrial melt blending without involving solution. Herein, with poly(vinyl acetate)(PVAc) modified CNCs powder(a mixture of PVAc grafted from CNCs and PVAc homopolymer latex), the selective dispersion of CNCs in PLLA has been achieved by simple melt processing of PLLA/TPU(polyether polyurethane)/CNCs blend. This results in the ultra-fine TPU droplets at nanoscale in PLLA and improves the melt processibility of composites in FDM due to the decreased viscosity ratio of the dispersed/matrix and the enhanced melt elasticity of PLLA. Combined with the intensive shear and continuous stretch effect during FDM, aligned TPU nanofibers(TNFs) were in situ formed along the elongational flow direction during deposition, which in turn contributed to the improvement of PLLA/TPU/CNCs with 5 wt% filler loading in tensile ductility by 418%, inter-layer adhesion strength and notched impact toughness by 261% and 210%, respectively, as well as achieved good dimensional accuracy and very fine surface quality.

Enhanced Reproducibility of Positive Temperature Coefficient Effect of CB/HDPE/PVDF Composites with the Addition of Ionic Liquid

Developing an effective method for improving the reproducibility of positive temperature coefficient(PTC)effect is of great significance for large-scale application of polymer based PTC composites,owing to its contribution to the security and reliability.Herein,we developed a carbon black(CB)/high density polyethylene(HDPE)/poly(vinylidene fluoride)(PVDF)composite with outstanding PTC reproducibility,by incorporating 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([OMIm][NTf2])into the composite.After multiple repeated temperature cycles,the PTC performance of as-prepared material keeps almost unchanged and the varition of resistance at room temperature is less than 7%.Our studies revealed that[OMIm][NTf2]contributes to the improvement of PTC reproducibility in two ways:(i)it acts as an efficient plasticizer for refining the co-continuous phase morphology of HDPE/PVDE blends;(ii)it inhibits the crystallization of PVDF through the dilution effect,leading to more overlaps of the volume shrinkage process of HDPE and PVDF melt which results in the decrease of interface gap between HDPE and PVDF.This study demonstrated that ionic liquids as the multifunctional agents have great potential for improving the reproducibility in the application of the binary polymer based PTC composites.