芦苇秆粉末高密度材料无胶温压成形与表征

为寻求高质清洁利用农林木质剩余物的新途径,运用响应面实验设计与分析方法对芦苇秆粉末实施无胶温压成形,制备高密度木质材料,用三维立体数码显微镜、扫描电子显微镜、傅里叶红外光谱、核磁共振光谱、热重分析和热解析-气相色谱/质谱联用分析技术对试件的微观结构、物相、纤维素结晶度、热脱附挥发物进行研究。结果表明,芦苇秆粉末粒度对试件性能的影响不明显;芦苇秆粉末在压力、温度、时间的复合作用下,木质素发生软化与流展,纤维素的结晶度得以提高,粉末颗粒间发生化学反应并产生化学链接;芦苇秆粉末的最优无胶温压工艺条件:成形压力为70 MPa,成形温度为160℃,保温保压时间为30 min;在最优工艺条件下制备的木质材料塑化明显,质硬而耐磨,具有韧性断裂特征,且在40、60、90、160℃环境温度下的挥发物中,有益、有害成分的种类和芦苇杆原粉相当,但有益成分的总含量远高于原粉,表明温压成形工艺具有环境友好性,无胶温压成形可以获得高品质人工木质材料。     

杨木粉压坯表面的化学成分和颜色变化规律

依据标准色度学系统和表面粉末抽提实验,研究了在不同的成形温度下杨木粉压坯表面的化学成分和颜色的变化规律。实验结果表明:在成形温度升高过程中,多糖类物质和半纤维素在高温下发生降解,压坯表面生成了导致颜色加深的有色抽提物;随着成形温度的升高,压坯表面的颜色随之加深;成形温度在160～180℃区间时,总体色差值增加,明度值下降趋势显著,该成形温度区间对压坯表面颜色影响较大;压坯样件粉末的含水量随成形温度升高而降低;在同等温度条件下,不同的抽提方法导致抽提物的含量发生较大变化,1%氢氧化钠法的抽提物含量最高,热水法的抽提物含量较冷水法略多,苯醇法的抽提物含量最低。     

温度对杨木粉末温压成形表面颜色的影响

采用CIE(1976) L~*a~*b~*系统和傅里叶红外光谱技术(FTIR),研究温压成形过程中温度对木材颜色和表面化学成分的影响规律。结果表明,成形温度升高可导致试件表面颜色加深,明度值变化范围为37. 5%～79. 2%,红绿色品指数变化范围为53. 8%～207. 7%,黄蓝色品指数变化范围为22. 4%～110. 3%;成形温度在180～200℃时ΔE~*相差1. 1,在180℃之前温度对杨木粉末的影响较为显著;随着温度的升高,半纤维素和木质素开始软化,导致试件表面颜色加深;这2类物质的降解主要存在侧链上,半纤维素降解程度高于纤维素和木质素。     

温压法制备铜铅轴承合金的研究

通过温压工艺制备了铜铅轴承合金材料 ,研究了温压成形温度和压力等因素对压坯致密化和烧结体性能的影响。结果表明 ,温压成形时可用经典的压制方程来描述粉末体的压形规律。温压温度的选择对压坯及烧结体的性能都有明显的影响 ,在合适的工艺条件下 ,温压法较冷压烧结法可制得更高密度和性能的铜铅轴承合金材料。     

温压法制备铜铅轴承合金的研究

通过温压工艺制备了铜铅轴承合金材料，研究了温压成型温度和压力等因素对压坯致密化和烧结体性能的影响，结果表明，温压成形时可经典的压制议程来描述粉末体的压形规律，温压温度的选择对压坯及烧结体的性能都有明显的影响，在合适的工艺条件下，温压法较冷压烧结法可制得更高密度的铜铅轴承合金材料。     

杨木粉温压成形过程的数值模拟

为了探寻木质粉体温压成形致密化规律,基于杨木粉温压成形实验数据,借助有限元法分析成形压力对木质粉体压坯密度的影响,并通过理论计算与实验验证相结合的方法对成形压力与压坯密度的关系进行回归分析。结果表明:Shima模型特别适用于杨木粉常温成形过程的数值模拟,与实验结果十分吻合;在温压成形工艺条件下的模拟结果与实验结果存在明显偏差,但当成形压力不小于50 MPa时,通过理论计算、实验检测与分析修正获得的由理论方程与修正项组成的杨木粉温压成形的压力-密度模型与实验结果高度吻合。     

粉末短流程成形固结技术的研究及展望

针对粉末冶金行业最新发展的几种短流程成形固结新技术,结合华南理工大学近十多年来在粉末材料-工艺-装备-零件一体化方面开展的研究,重点阐述了粉末冶金温压成形、高速压制成形、喷射成形、多场作用下粉末成形与烧结一体化技术的研究进展及应用情况。指出在粉末冶金成形固结研究领域,合理拓展现有粉末冶金技术规范的空间,有望给传统粉末冶金成形固结技术注入新的活力。粉末冶金成形固结新技术的不断出现,必将促进先进制造业和高技术产业的快速发展,也必将给材料工程和制造业带来更加光明的前景。     

木材形状记忆效应与机理的研究进展

形状记忆聚合物(SMPs)是一种改变初始形状并固定后,通过外部刺激又恢复到原始形状的高分子材料.水是容易获得、环境友好的刺激物,水/水热响应的形状记忆材料成为近年来研究的焦点.木材是具有形状记忆效应的聚合物基天然高分子智能材料,可以通过压缩或弯曲等方法固定成临时形状,然后在水热作用下恢复到其永久形状.然而,与具有简单结构的SMPs相比,天然木材的微观构造由不同的组织结构、细胞形态和孔隙结构组成,化学结构由纤维素、半纤维素、木质素以氢键、共价键和物理结合相互嵌入渗透组成.木材复杂的微观构造和化学结构增加了表征形状记忆特性、构建架构模型、揭示记忆机理的难度.近年来,研究人员在干燥木材的过程中发现木材形状记忆效应基于准残余冻结应变的可逆应变,木材湿-热-力模型表征形变回弹率的Rr和形变固定率的Rf是冻结应变的函数.木材形状记忆编程过程有弯曲形状记忆、拉伸形状记忆、压缩形状记忆,定量表征SMPs的方法可应用于木材.聚合物形状记忆模型有交联网络模型、超分子网络模型、渗流网络模型、综合架构模型,其中综合架构模型由开关单元(Switch)和网络节点(Net points)组成,可用来全面解释SMPs的结构.在特定温度-湿度-机械力作用下,木材中的半纤维素最先降解,纤维素结晶度增加,木质素产生交联,可用细胞壁微形态变形理论、纤维素应力松弛理论和疏水化理论在分子水平上揭示消除形状记忆的机理.鉴于以上内容,本文以冻结应变为研究木材形状记忆效应的基础,结合形状记忆编程中定量评价的方法,分析形状记忆材料的架构模型以及木质材料的空间结构,并阐述了在特定温度-湿度-机械力耦合作用下消除木质材料形状记忆效应的机理.     

木质纤维素基高分子材料的研究进展

木质纤维素现已成为制备高分子材料的重要原料。木质纤维素可以用作高分子复合材料中的增强剂或填料;木质纤维素经液化进行适度降解,再与其它物质进一步反应,可以制备聚氨酯、环氧树脂、酚醛树脂等高分子材料;经过适当的化学处理制备纺丝液,利用熔融纺丝技术纺丝再通过炭化处理可以制得炭纤维;浸渍热固性树脂后,在隔绝空气的条件下,高温炭化可以制得木质陶瓷;经组分分离和双亲改性后,使用化学交联剂交联可以制备水凝胶。     

复配改性工业木质素/木纤维复合材料的制备与表征

以H₂O₂为氧化剂对工业木质素进行改性,将H₂O₂氧化改性工业木质素（OMIL）与聚乙烯亚胺（PEI）复配制得复配改性工业木质素（OMIL-PEI）。以木纤维（WF）为基体,OMIL-PEI为黏结相,采用高速混合-平板热压工艺制备了环保型OMIL-PEI/WF复合材料。采用正交试验设计方法研究了H₂O₂用量、氧化时间、OMIL与PEI质量比及复配剂OMIL-PEI用量对该复合材料物理力学性能的影响,探索了复合材料的最优制备工艺参数,并采用FTIR、XRD、DMA和SEM对复合材料的结构和性能进行了表征。结果表明：最优工艺条件为,H₂O₂用量20wt%,氧化时间120 min,OMIL与PEI质量比7：1,OMIL-PEI用量20wt%,所制备的复合材料各项理化性能满足GB/T 11718-2009干燥状态下使用的承重型中密度纤维板的性能要求;OMIL-PEI能够与WF在热压过程中形成良好的化学键;优化工艺条件下的OMIL-PEI/WF复合材料的木质纤维素的晶形结构保持不变,相对结晶度从60.24%（纯WF）升高到72.91%;OMIL-PEI提高了OMIL-PEI/WF复合材料的动态储能模量,对材料的热稳定性影响不大,且各组分之间分布均匀,交织致密,界面粘结性能良好。     

Bulk Ceramic Composites Derived from a Preceramic Polysilazane with Alumina and Zirconia Fillers

Bulk composites for the production of green‐machinable, complex‐shaped ceramic parts with tailorable properties for applications under mechanical stress are fabricated by a novel combination of a poly(vinyl)silazane‐derived ceramic matrix with Al₂O₃ or ZrO₂ fillers. Green‐machinable composites were produced by coating of the filler powders with the preceramic polymer, followed by subsequent warm‐pressing. The final ceramic materials are obtained by a thermal conversion process. A systematic variation of preceramic polymer content, pressure during warm‐compaction, or pyrolytic conversion temperature is used to identify the main factors responsible for differences in densification behavior, microstructure, composition, and mechanical properties. The procedure of composite powder preparation as well as the optimization of the subsequent warm‐pressing step are found to be decisive in the successful production of crack‐free bulk ceramic composites.     

Microstructure evolution and densification behavior of TiC/316L composite powders during cold/warm die compaction and solid-state sintering: 3D particulate scale numerical modelling and experimental validation

To identify the microstructure evolution and densification behavior of TiC/316L composites in powder metallurgy (PM) process, 3D particulate scale numerical simulations were conducted to reproduce the cold/warm compaction and solid-state sintering of TiC/316L composite powders with corresponding physical experiments being carried out for model validation. The effects of compaction parameters and sintering temperature on the densification behavior of TiC/316L composite powders were systemically investigated. The particle deformation and morphology, stress/strain and microstructure evolutions, and grain size distribution in the whole process were characterized and compared to further illustrate the densification behavior and the underlying dynamics/mechanisms. The results show that compared with the cold compaction, the warm compaction can not only achieve higher relative density, smaller and more uniform equivalent stress, and weaker spring back effect, but also improve the friction condition among powder particles. The plastic deformation of 316L particles is the main densification mechanism during compaction. In the solid-state sintering of TiC/316L compacts, the densification is mainly indicated by shrinkage and vanishing of large residual pores along with the growth of the sintering necks, accompanied by the particle movement and growth along the boundary regions. Meanwhile, the particle displacement and grain size distribution are more uniform in the warm compacted TiC/316L component. Moreover, the equivalent (von Mises) stress in 316L particles is smaller than that in TiC particles.     

Recent advances in ferrous powder metallurgy

Ferrous Powder Metallurgy (P/M) has advanced significantly over the past thirty years in providing opportunity for a parts designer and producer to make high strength net shape parts. Currently, for example, transmission gears in U.S. built cars use P/M parts. This review addresses recent advances in the area of ferrous powder for the P/M industry. Development of molybdenum prealloyed powder for increased density, Cr-Mn containing powder for through hardenability and improved wear resistance, binder treated powders for higher control of alloying ingredients and increasing compaction rates are presented. Two major innovations, namely achievement of higher compaction densities through warm compaction and improved magnetic properties by dielectric coating on iron powders are presented.     

Comparative study on the densification process of different titanium powders

In the powder metallurgy of titanium and titanium alloys, titanium powders produced through hydrogenation/dehydrogenation (HDH) approach and titanium hydride powder are extensively used. The choice of initial powder greatly influences the properties and performance of as-sintered materials. In the present study, comparative experiments were performed on two powders of various sizes to elucidate the peculiarities of their densification process and the characteristics (as-sintered density, impurity content, and tensile properties) of the processed materials. As expected, the sintering performance of both powder-type compacts were greatly affected by the specific surface and contact areas, so finer powders and higher compaction pressures were used to achieve higher densities upon sintering. However, the systematic results clearly indicated the advantage of using titanium hydride powder as a starting material in titanium powder metallurgy. At equal size, compaction, and sintering parameters, materials processed from titanium hydride powder had higher density and lower impurity content, thereby providing better balance of tensile properties compared with materials processed from HDH titanium powder. This advantage is explained by the higher relative density of green compacts made of brittle titanium hydride powder and by the higher sintering ability of such compacts activated by powder-released hydrogen.     

Modeling of hot isostatic pressing of metal powder with temperature–dependent cap plasticity model

In this paper, the coupled thermo–mechanical simulation of hot isostatic pressing (HIPing) is presented for metal powders during densification process. The densification of powder is assumed to occur due to plastic hardening of metal particles. The constitutive model developed is used to describe the nonlinear behavior of metal powder. The numerical modeling of hot powder compaction simulation is performed based on the large deformation formulation, powder plasticity behavior, and frictional contact algorithm. A Lagrangian finite element formulation is employed for the large powder deformations. A modified cap plasticity model considering temperature effects is used in numerical simulation of nonlinear powder behavior. The influence of powder-tool friction is simulated by the use of penalty approach in which a plasticity theory of friction is incorporated to model sliding resistance at the powder-tool interface. Finally, numerical examples are analyzed to demonstrate the feasibility of the proposed thermo–mechanical simulation using the modified cap plasticity model in the hot isostatic forming process of powder compaction.     

Studying the influence of initial powder characteristics on the properties of final parts manufactured by the selective laser melting technology

Selective laser melting (SLM) is an additive manufacturing process that enables direct manufacturing of 3D complex shape parts and internal architecture from powder materials. The SLM technology is characterised by high temperature gradients and solidification rates that have a significant effect on the microstructures and properties of final parts. The present paper aims at understanding the influence of the initial properties of various martensitic stainless steel powders on the final microstructures and mechanical properties of parts manufactured using the same optimised SLM process parameter settings. The results obtained show that for applied optimum process parameters, the thermal effects are the same for all martensitic powders used. Besides, the final microstructures and properties are different. The results clearly show the effect of the initial complex chemical composition of the martensitic precipitation hardening powder on the microstructures of final parts, and consequently, on their properties.     

Powder compaction properties of sodium starch glycolate disintegrants

The compaction behavior of three “as supplied” commercially available grades of sodium starch glycolate (SSG), Explotab, Primojel, and Vivastar P, was investigated at compression speeds of 0.17 and 30 mm/sec. The results suggested that the three “as supplied” materials exhibit different compression and compaction behavior. Primojel and Explotab exhibited similar compactibility, whereas Vivastar P produced compacts of poor integrity. This behavior was not mirrored in the compressibility of the powders, where Vivastar P and Explotab exhibited similar performance. The materials were studied using x-ray diffraction, scanning electron microscopy, Carr's compressibility index, and swelling volume. In terms of material characteristics, all the products exhibited similar swelling in water. Primojel and Explotab retained most of the crystallographic order from the parent potato starch and exhibited comparable particle surface topographies. Vivastar P contained the lowest moisture level. However, it is not clear if the poor compactibility of Vivastar P is due to differences in moisture content, the reduced surface topography, or subtle differences in the SSG polymer structures (substitution, cross-linking, and crystallinity). Overall, even though the three commercial grades of sodium starch glycolate are successfully used as disintegrants, they do exhibit differences in their “as supplied” powder mechanical properties: Primojel and Explotab exhibit similar compactibility, whereas Vivastar P is poorly compactable but exhibits similar compressibility to Explotab. These observations may have implications when formulating poorly compactable or moisture-sensitive drugs.     

Compressional Behaviour of an Agglomerated Cellulose Powder

The ability of an agglomerated cellulose powder to total and plastic deformation was evaluated and compared with those of Avicel PH 101, Emcocel and an experimental depolymerized cellulose powder. The elastic recovely of compressed cellulose tablets was also measured. The effects of deformation of the material during the tableting process and recovery of tablet after maximum compression on the mechanical strength of tablets were also discussed.

The apparent net work done into tablets during compression as well as the yield pressures to total and plastic deformation, determined from the Heckel treatment, showed no great differences between the agglomerated cellulose powder and the other cellulose powders. Thus all the cellulose materials studied had rather similar ability to total, i.e. elastic and plastic, deformation and to permanent, i.e. pure plastic, deformation. The obvious fragmentation of the agglomerated cellulose powder already at low compressional pressure, however, seemed to be advantageous for the formation of strong compacts.