Additive manufacturing of zirconia parts by indirect selective laser sintering

Thermally induced phase separation (TIPS) was used to produce spherical polypropylene–zirconia composite powder for selective laser sintering (SLS). The influence of the composition of the composite starting powder and the SLS parameters on the density and strength of the composite SLS parts was investigated, allowing realizing SLS parts with a relative density of 36%. Pressure infiltration (PI) and warm isostatic pressing (WIPing) were applied to increase the green density of the ZrO₂–PP SLSed parts. Infiltrating the SLS parts with an aqueous 30 vol.% ZrO₂ suspension allowed to increase the sintered density from 32 to 54%. WIPing (135 °C and 64 MPa) of the SLS and SLS/infiltrated complex shape green polymer–ceramic composite parts prior to debinding and sintering allowed raising the sintered density of the 3 mol Y₂O₃ stabilized ZrO₂ parts to 92 and 85%, respectively.     

Distribution Features of the Residual Stress For T-Joints by Laser Welding

The residual stress of T-joints for SUS304 stainless steel by YAG laser welding was measured by the method of hole-drilling,and the effects of heat treatment and hardness distribution of weld joints on the distribution of residual stress for T-joints were analyzed.The results showed that the maximum of longitudinal residual tensile stress for T-joints was about 140MPa.The maximum of residual stress was not more than 40MPa after heat treatment,the peak of residual stress reduced obviously.The gradient of residual stress distribution was also reduced significantly.Strain hardening phenomenon occurred for T-joints,and the hardness of weld and heat affected zone was both higher than the hardness of base metal,the peak of hardness occurred in the fusion line.The effect of strain hardening phenomenon of weld and heat affected zone on the residual stress distribution of T-joints for SUS304 stainless steel was obvious,which made longitudinal residual tensile stress become higher.     

镍基高温合金电子束焊接接头疲劳性能

对GH4169高温合金板材电子束焊接试样进行了金相分析和高、低周疲劳试验,分析GH4169合金的高、低周疲劳性能及疲劳断口形貌.结果表明,GH4169合金电子束焊接接头焊缝区的特点是树枝晶比较明显,为比较发达的树枝晶和明显的二次枝晶.在较高放大倍数下发现,母材和热影响区也为树枝晶结构.接头低周疲劳性能不低于母材,随应变幅降低,相同应变幅下焊接接头疲劳寿命变得略高于母材.接头高周中值疲劳强度比母材高约80 MPa.     

Grain selection and growth orientation of prior-β phase for Ti-6-4 during additive manufacturing: insights from a modeling perspective

The microstructure of Ti-6-4 components produced by additive manufacturing suffers from the coarse and elongated prior-β grain, which leads to a decrease of the tensile behavior and the occurrence of anisotropy. To understand and control the grain evolution, a multiscale simulation is applied to investigate the relationship between the grain selection, growth orientation, and the molten pool morphology with the different deposition layer numbers and processing parameters. The accuracy of the simulation is validated by experiments in both qualitative and quantitative ways. Results show that when the grain with unfavorable orientation loses the competitive growth with its neighbors, there will be a great chance that the blocked grain is eliminated in the following layer-and-layer deposition, which leads to the increase of the grain width. The size of the molten pool increases remarkably as the layer number increases, which lays a heavy burden on the stability of the molten pool. The analytical relationship between the molten pool morphology and the grain growth orientation is also deduced. The flat molten pool causes the grains with the <001> direction close to the building direction to have greater survival potential. Besides, decreasing the line power energy shows little effect on the stability of the molten pool and the grain growth orientation, especially when the deposited layer number is large. The revealing mechanisms will help in understanding and further controlling the grain evolution.

     

Characterising the behaviour of composite single lap bonded joints using digital image correlation

Three-dimensional and two-dimensional Digital Image Correlation (DIC) have been used to evaluate the evolution of deformation and strain in composite single lap bonded joints prior to failure. In general, composite components are increasingly being joined using structural adhesives for aerospace and other safety critical applications. Reliable design requires that the mechanical behaviour of composite bonded joints is well understood. In this respect, experimental tests are crucial to (a) characterise the deformation and strains induced under load and (b) develop and validate realistic numerical models. Although modern numerical models contain many degrees of freedom, only a few degrees of freedom are typically measured using conventional instrumentation such as strain gauges and extensometers. However, 3D DIC provides an opportunity to measure full-field deformations and surface strains. In the current study, 3D DIC was successfully used to measure full-field in-plane surface strains and out-of-plane surface deformations for composite single lap bonded joints (adherends manufactured from both fibre preimpregnated resin (pre-preg) and resin infused non-crimp-fabric (NCF)). Moreover, strategically located strain gauges were used to validate the strains measured by 3D DIC. Finally, 3D DIC measurements may be useful in detecting subcritical damage as shown in the case of the pre-preg joint. The specific location and magnitude of the maximum principal strain in the adhesive fillet region were determined using high magnification 2D DIC.     

Mechanical and failure behaviour of hybrid polymer–metal staked joints

Structural applications that use multi-material structures in the transportation industry have increased in recent years. Weight reduction in order to avoid excessive emissions is the driving force of this trend. The current joining technologies for such complex structures have potential for engineering and performance improvement. This preliminary study shows an alternative joining method for hybrid structures, the so-called Injection Clinching Joining (ICJ) [Abibe et al., J Thermoplast Compos 2011;24(2): 233–49], based on the principles of staking, injection moulding, and mechanical fastening. The main objectives of the paper are to exploit the mechanical behaviour of overlap joints produced by this proposed method and assess its potential as an applicable technology. The measurements used in this research are optical and scanning electron microscopy, X-ray computer microtomography, lap-shear strength testing and in situ strain distribution. Different failure modes were found, depending on the joining conditions. Net tension failure had a brittle and catastrophic nature, while rivet pull-out presented a more desirable slow ductile failure mode. The joint strengths were good, ranging from 35.9% to 88.5% of the base material’s experimental ultimate tensile stress. Although there is a lack of studies on structural staking applications, this paper shows potential for these joining techniques and introduces ICJ as a potential focus of future research.     

Dissolution behavior of hollow hydroxyapatite microspheres immersed in deionized water

Hollow hydroxyapatite (HA) microspheres were fabricated by plasma spraying method. These microspheres were immersed in deionized water for different periods of time to investigate their dissolution behavior with the starting HA powders as contrast. The results showed that although having a relatively lower crystallinity, the dissolution rate of the HA microspheres is somewhat lower than that of HA powders, which was due to the very dense surface layer of the microspheres. Moreover, open pores were seen on the surface of hollow microspheres after immersion. These results suggest that there might be a novel application in drug delivery for the hollow HA microspheres.     

MoSx-Ta composite coatings on steel by d.c magnetron sputtering

Sputter-deposited MoS₂ films have been often used as dry lubricant in various industrial fields, such as space application and much attention has been paid to reduction of friction coefficient and improvement of mechanical properties in recent decades. One way to achieve this is to deposit a MoS₂ film doped with another metal. The MoS_x-metal films were found to be denser, more adhesive and more oxidation-resistant than pure MoS₂. In this study, MoS_x-Ta composite films were synthesized by Electron Cyclotron Resonance microwave source enhanced DC sputtering with different target powers. The effects of doping Ta on mechanical properties of MoS_x-Ta films were investigated. The morphology and structure of films were investigated using a scanning electron microscope (SEM), X-ray diffraction (XRD) and atomic force microscopy (AFM). The microhardness was evaluated using microhardness test instrument, and the adhesion strengths were obtained using a scratch tester. The results showed that the S/Mo ratio was influenced by the dc sputtering target power. Typical MoS₂ (100) (103) (002) orientations were present in pure MoS_x films, but disappeared with the increase in doped Ta, with the S/Mo content ratios decreasing from 1.52 to 0.84, and the hardness increasing from 3.55 to 15.23 GPa. The roughness and surface topography, friction coefficient and adhesion were significantly affected by the Ta, Mo and S content. The content of doped Ta plays a dominant role on the change in the Mo/S ratio, thereby influencing the mechanical and tribological properties of the MoS_x-Ta composite films.     

Effects of extrusion temperature on the microstructure and tensile properties of Al–16 wt% Al4Sr metal matrix composite

This study was undertaken to investigate the effect of extrusion temperature on the microstructure and tensile properties of Al metal matrix composite (MMC) containing 16 wt% Al₄Sr intermetallic. Microstructural examinations were assessed by the use of optical microscope, scanning electron microscope (SEM) and X-ray diffractometry (XRD). The results showed that hot extrusion with the ratio of 18:1 at 420 °C reduces the maximum length of Al₄Sr particles from 222 μm to 35 μm. It was found that by applying extrusion parameters in optimum conditions, uniform distribution of fine Al₄Sr intermetallic in Al matrix is obtained. Microstructural evolution also intensified the ultimate tensile strength (UTS) values of the MMC from 54 MPa to 145 MPa. Remarkable result of this study revealed that hot extrusion improves the ductility of the MMC significantly. Fractographic examinations of the composite in as-cast condition showed a complete cleavage fracture surface that changes to more homogenous dimples after hot extrusion process.     

Nd:YAG激光焊接对铝锂合金组织和织构的影响

采用Nd:YAG激光进行了5A90铝锂合金薄板的对焊实验,借助光学显微镜、扫描电镜及EDS能谱、背散射衍射技术测试了焊缝的显微组织、合金元素分布及焊缝中的微观织构,并与母材进行了比较。结果表明:Nd:YAG激光焊接使5A90铝锂合金的微观组织和微观织构发生了很大的变化。焊缝区呈现出大量的等轴枝晶组织,这是由于焊缝中存在较多的异质形核点和较高的成分过冷度。焊缝中织构呈随机分布的状态,激光焊接完全改变了母材面心立方金属的冷轧织构组织。