Laser powder bed fusion of a Ni3Al-based intermetallic alloy with tailored microstructure and superior mechanical performance

Ni3Al-based alloys are excellent candidates for the structural materials used for turbine engines due to their excellent high-temperature properties.This study aims at laser powder bed fusion and post-hot isostatic pressing(HIP)treatment of Ni3Al-based IC^(-2)21 M alloy with a highγ0 volume fraction.The as-built samples exhibits unavoidable solidification cracking and ductility dip cracking,and the laser parameter optimization can reduce the crack density to 1.34 mm/mm^(2).Transmission electron microscope(TEM)analysis reveals ultra-fine nanoscaleγ0 phases in the as-built samples due to the high cooling rate during rapid solidification.After HIP treatment,a fully dense structure without cracking defects is achieved,which exhibits an equiaxed structure with grain size~120-180μm and irregularly shapedγ0 precipitates~1-3μm with a prominently high fraction of 86%.The room-temperature tensile test of as-built samples shows a high ultimate tensile strength(σUTS)of 1039.7 MPa and low fracture elongation of 6.4%.After HIP treatment,a significant improvement in ductility(15.7%)and a slight loss of strength(σUTS of 831.7 MPa)are obtained by eliminating the crack defects.Both the as-built and HIP samples exhibit retained highσUTS values of 589.8 MPa and 786.2 MPa,respectively,at 900C.The HIP samples exhibita slight decrease in ductility to~12.9%,indicating excellent high-temperature mechanical performance.Moreover,the abnormal increase in strength and decrease in ductility suggest the critical role of a highγ0 fraction in cracking formation.The intrinsic heat treatment during repeating thermal cycles can induce brittleness and trigger cracking initiation in the heat-affected zone with notable deteriorating ductility.The results indicate that the combination of LPBF and HIP can effectively reduce the crack density and enhance the mechanical properties of Ni_(3)Al-based alloy,making it a promising material for high-temperature applications.

Fuzzy neural network analysis on gray cast iron with high tensile strength and thermal conductivity

To develop a high performance gray cast iron with high tensile strength and thermal conductivity, multivariable analysis of microstructural effects on properties of gray cast iron was performed. The concerned parameters consisted of graphite content, maximum graphite length, primary dendrite percentage and microhardness of the matrix. Under the superposed influence of various parameters, the relationships between thermal conductivity and structural characteristics become irregular, as well as the effects of graphite length on the strength. An adaptive neuro-fuzzy inference system was built to link the parameters and properties. A sensitivity test was then performed to rank the relative impact of parameters. It was found that the dominant parameter for tensile strength is graphite content, while the most relative parameter for thermal conductivity is maximum graphite length. The most effective method to simultaneously improve the tensile and thermal conductivity of gray cast iron is to reduce the carbon equivalent and increase the length of graphite flakes.

Plasma Treatment Induced Chemical Changes of Alkali Lignin to Enhance the Performances of Lignin-Phenol-Formaldehyde Resin Adhesive

Alkali lignin was processed by plasma and then used in modification of phenol formaldehyde resin in this study.Chemical structural changes of lignin which was processed by plasma as well as bonding strength,tensile property,curing performance and thermal property of the prepared phenol formaldehyde resin which was modified by the plasma processed lignin were analyzed.Results demonstrated that:(1)Alkali lignin was degraded after the plasma processing.The original groups were destroyed,and the aromatic rings collected abundant free radicals and oxygen-containing functional groups like hydroxyls,carbonyls,carboxyls and acyls were introduced into increase the reaction activity of lignin significantly.(2)The introduction of alkali lignin decreased the free formaldehyde content and increased bonding strength and toughness of the prepared phenol formaldehyde resin,especially after the introduction of lignin treated with plasma.(3)The introduction of alkali lignin led to high curing temperature for the prepared phenol formaldehyde resin,but that was reduced by the plasma processed alkali lignin.(4)The introduction of alkali lignin could also increase thermal stability of phenol formaldehyde resin,but that was modified by plasma processed alkali lignin was better than the unprocessed lignin.Based on the results,the plasma processed lignin was used to modify phenol formaldehyde resin,which could increase the strength and toughness of phenol formaldehyde resin significantly.

Influence of shrinkage-reducing admixture on drying shrinkage and mechanical properties of high-performance concrete

High-performance concrete （HPC） has specific performance advantages over conventional concrete in strength and durability. HPC mixtures are usually produced with water/binder mass ratios （mW/mB） in the range of 0.2-0.4, so volume changes of concrete as a result of drying, chemical reactions, and temperature change cannot be avoided. For these reasons, shrinkage and cracking are frequent phenomena. It is necessary to add some types of admixture for reduction of shrinkage and cracking of HPC. This study used a shrinkage-reducing admixture （SRA） for that purpose. Concrete was prepared with two different mW/mB （0.22 and 0.40） and four different mass fractions of SRA to binder （w（SRA） = 0%, 1%, 2%, and 4%）. The mineral admixtures used for concrete mixes were： 25% fly ash （FA） and 25% slag by mass of binder for the mixture with mW/mB = 0.40, and 15% silica fume （SF） and 25% FA for the mixture with mW/mB = 0.22. Tests were conducted on 24 prismatic specimens, and shrinkage strains were measured through 120 days of drying. Compressive strength, splitting strength, and static modulus of elasticity were also determined. The results show that the SRA effectively reduces some mechanical properties of HPC. The reductions in compressive strength, splitting tensile strength, and elastic modulus of the concrete were 7%-24%, 9%-19%, and 5%-12%, respectively, after 90 days, compared to concrete mixtures without SRA. SRA can also help reduce drying shrinkage of concrete. The shrinkage strains of HPC with SRA were only as high as 41% of the average free shrinkage of concrete without SRA after 120 days of drying.

Influence of Assembly Parameters on Tensile Behavior of Hybrid CMC and Superalloy Bolted Joints by Progressive Damage Analysis

With the considerable applications of ceramic matrix composites(CMC) in aircraft engineering, the design of CMC bolted joint gains paramount attention because of its capacity to to improve load-bearing efficiency of aircraft key structure. In this work, a 3 D finite element model was established to predict tensile performance and failure modes of single-lap, single-bolt 2 D C/SiC composite, and superalloy joint, which considers the progressive damage behavior of 2 D woven C/SiC composites. On the basis of the developed progressive damage model, a parametric study was carried out to illustrate the effects of bolt preload and bolt-hole clearance on mechanical behaviors of the hybrid bolted joint. It was found that the increase in the value of bolt preload made the failure load grow first and then drop, and the optimum value of bolt preload 5.00 kN generated 56.47% rise in the initial failure load and 22.83% rise in the final failure load for the bolted joint in comparison with zero preload case. As the clearance increased from 0 to 2.00%, the initial and final failure loads respectively declined by 45.88% and 24.02% for 2.00% bolt-hole clearance relative to the neat-fit case. The loss in failure loads can be reduced to compressive stress concentration around the fastening hole-edge area, leading to the appearance of earlier damages by the introduction of increasing bolt hole clearance.

Mechanical Performances and Morphology of LDPE/UHMWPE Single-Polymer Composites Produced by Extrusion-Calendering Method

Extrusion-calendering method was developed to prepare single-polymer composites（SPCs） of ultrahigh molecular weight polyethylene（ UHMWPE） fabric reinforcing low density polyethylene（LDPE）.Differential scanning calorimeter（DSC） experiments were executed to determine the setup of extrusion temperature.Effects of the die temperature on the tensile and interfacial performances of SPCs were studied through the tensile and T-peel tests,respectively. The results showed that both tensile strength and modulus increased initially and decreased afterwards as the temperature increased. The peak values of tensile strength and modulus of PE SPCs,which are 10. 8 and 3. 5 times as high as those of the unreinforced LDPE respectively,were obtained at 150 ℃. Higher temperatures also give a positive effect on peel strength. Scanning electron microscopy（ SEM） and camera were also used to observe the morphology of the SPCs samples.

Improve Needle Abrasion Performance of Insulated Cables by Using Coextrusion Technology

在电线电缆耐磨方面，已经有过许多研究，特别是在汽车行业和石油开采业，在近海石油工业中，电缆必须能够抵御来自延伸到井里面的检测工具的破坏。在汽车工业，耐磨要求处理主要是由于电线线束安装或组装的需要；为了避免由于直接或者间接的耐磨原因导致电线绝缘破坏使设计失效。

掺轻质砂超高性能混凝土的轴拉力学性能

为研究轻质砂对不同试件尺寸下超高性能混凝土(ultra high performance concrete,UHPC)拉伸应变强化性能的影响,采用轻质砂对原黄砂进行等体积替代,完成9组不同轻质砂体积率(0~35%)和不同试件厚度(30~100 mm)的单轴拉伸试验,并同步进行声发射实时探伤测试。结果表明:轻质砂体积率对UHPC弹性极限点对应应力和对应应变的影响较小,但轻质砂体积率由0增加到35%时,UHPC的极限抗拉强度和极限拉应变分别由10.6 MPa和2.35×10^(-3)提高到了19.4 MPa和4.3×10^(-3);轻质砂体积率大于15%时,UHPC的应变强化程度得到显著提升,试件内部产生的损伤点数更多且分布更均匀,展现出良好的裂缝控制能力;在相同轻质砂体积率下,UHPC的应变强化程度随试件厚度的增加而降低,且试件内部的损伤点趋于集中,表现出明显的尺寸效应。

钢结构模块新型自锁-可解锁式连接节点的抗拉研究

为提高建筑用钢结构模块的施工效率、实现模块单元的快捷装配与拆卸,提出一种新型自锁-可解锁式连接节点方法,用Abaqus数值模拟软件建立了节点的精细化与半精细化有限元模型,探究了摩擦因数对节点抗拉承载性能的影响。选取上限位板厚度、下限位板厚度和上部滑块宽度为关键参数进行有限元模拟,研究各参数对节点抗拉承载力及破坏模式的影响。最后与实际节点抗拉承载力进行对比。结果表明:当摩擦因数为0.3时,新型节点承受最大拉力为1345.49 kN,较角件旋转式连接节点提升约为152%。节点抗拉承载力受上部滑块宽度影响较大,受上限位板厚度与下限位板厚度影响相对小。当上、下限位板厚度均为25mm时,在10~12mm内增加上部滑块宽度可提升节点承载拉力。

单板焊接空心球节点受拉承载性能研究

单板焊接空心球节点常用做弦支穹顶上部结构连接拉杆或拉索的节点,单板主要承受拉力并传递至焊接空心球,为了满足工程设计的要求,需对其受拉承载性能进行研究.通过试验,研究了不同参数的单板焊接空心球节点轴拉状态下的受力性能和破坏机理,建立有限元模型并进行验证,该模型能较好地反映单板焊接空心球节点轴拉性能,通过有限元模型参数化分析揭示了节点尺寸、加劲肋、单板尺寸等参数对其力学性能的影响规律,并提出单板焊接空心球轴拉承载力计算公式.研究结果表明:单板焊接空心球在轴拉作用下为应力集中导致的剪切强度破坏,破坏发生在单板端部与空心球连接处,表现为焊接空心球被撕裂;有限元应力云图得出破坏时沿着单板平面内拉力方向的剪应力起主要作用;单板焊接空心球节点的受拉承载力随单板宽度及空心球壁厚的增大而明显增大,空心球直径的增大会使承载力略有降低,单板的厚度增加使节点承载力略有提高,单板的高度对节点承载力的影响较小,空心球内设置加劲肋能使承载力提高至少20%;基于参数化提出的单板焊接空心球节点的受拉承载力计算公式能简便地计算出单板焊接空心球节点的承载力,相对误差在±15%以内,满足工程设计误差要求.研究工作可为今后外接单板的焊接空心球节点分析设计提供参考.

基于DIC的冲击劈拉荷载作用下UHPC动态抗拉力学性能研究

超高性能混凝土(UHPC)在大跨度桥梁、机场路面及抗爆结构中广泛应用,此类结构常受到高应变率冲击荷载作用,故研究UHPC在冲击劈拉荷载下的抗拉力学性能至关重要。基于数字图像相关技术(DIC),利用分离式霍普金森杆(SHPB)装置开展不同冲击气压下的动态劈拉试验,研究UHPC在冲击荷载下的力学性能与损伤规律,分析不同冲击气压下UHPC的破坏形态和损伤演化过程,分析UHPC的动态应力–应变关系曲线,讨论动态抗拉强度、动态弹性模量、动态提高因子、动态耗散能与应变率的关系,建立UHPC极限抗拉强度的动态提高因子模型。研究结果表明,不同应变率劈拉荷载下UHPC的损伤形式略有不同:在冲击劈拉荷载作用下,随着应变率的增大,试件最终被破坏时主裂缝扩展程度增大;UHPC试件极限抗拉强度均随着应变率的增大而增大;UHPC耗散冲击能量有基于混凝土基体破坏、通过拔出或拔断钢纤维两种方式;UHPC在冲击作用下强度大,变形能力强,可有效耗散冲击能量;UHPC的抗冲击性能及极限抗拉强度的应变率敏感性均高于普通混凝土;UHPC的动态耗散能随应变率的增大而增加,但能量耗散比却呈现出先增长后降低的趋势。

砂土中螺旋桩在斜拉荷载作用下的承载性能

为研究斜拉荷载倾斜角度和叶片埋深对螺旋桩极限承载力的影响,基于ABAQUS软件构建了砂土中螺旋桩的有限元模型,通过螺旋桩水平承载机理砂箱试验与竖向拉拔试验数据,对模型的准确性和可行性进行验证。设置4组不同叶片埋深的螺旋桩模型,分别施加不同角度的斜拉荷载,探讨了砂土地基中螺旋桩的承载性能与斜拉角度、叶片埋深的相关性。结果表明:砂土中螺旋桩的极限承载力随着斜拉角度的增大不断减小;当斜拉角度小于30°时,螺旋桩极限承载力与斜拉角度的关系曲线近似呈线性;螺旋桩极限承载力存在一个临界埋深比,当埋深比大于等于4时,承载力不随埋深比的增大而变化。推导了深埋状态下螺旋桩受斜拉荷载作用的极限承载力公式,经验证该公式能较好反映螺旋桩极限承载力随斜拉角度增大而逐渐下降的规律。

硬脂酸钙/硅酮粉改性PE-UHMW/PE-HD共混物的制备

超高分子量聚乙烯(PE-UHMW)与高密度聚乙烯(PE-HD)熔融共混,可提高PE-UHMW熔体加工性能,但其工业化生产效率仍然较低,无法满足熔体纺丝工业生产的需求。针对此问题,通过双螺杆挤出机熔融共混与单螺杆挤出机挤出实验,研究了硬脂酸钙(CaSt_(2))、硅酮粉及其复合助剂对质量配比为60∶40的PE-UHMW/PE-HD共混物加工流动性及拉伸性能的影响。加工扭矩与初生丝表面状态结果表明,CaSt_(2)、硅酮粉及其复合助剂均能降低加工扭矩,提高熔体挤出稳定性。黏流活化能结果表明这两种添加剂具有内润滑效果。在高剪切速率下,CaSt_(2)表现出较强的外润滑效果,能进一步降低加工扭矩,提高熔体挤出的稳定性。CaSt_(2)与硅酮粉协同使用,可以进一步提高助剂内润滑作用,降低挤出扭矩并提高挤出稳定性。差式扫描量热分析表明,改性剂可促进共混物结晶,从而使材料拉伸强度提高。研究结果表明,质量分数0.5%CaSt_(2)和0.5%硅酮粉复合改性剂表现出内外润滑的协同作用,与未改性共混物相比,协同改性使PE-UHMW/PE-HD共混物挤出合格初生丝最高螺杆转速从5 r/min升高到20 r/min,加工扭矩降低21.4%,拉伸强度提高11.9%。

正交实验设计优选FDM_3D打印工艺参数

由于FDM_(3)D打印工艺参数对制件力学性能有较大影响,采用正交实验设计方法,系统地探讨了聚乳酸(PLA)材料在熔融沉积成型3D打印过程中,不同工艺参数对试样拉伸性能的影响程度。实验结果表明,在4个工艺参数中,填充密度对PLA试件的拉伸性能影响最为显著,而打印速度对拉伸强度的影响则相对较小。通过极差分析得出,当打印层厚为0.25 mm,填充密度为70%,填充速度为50 mm/s,并采用内六角填充模式时,可以制得拉伸性能最优的PLA制件,方差分析表明填充密度对抗拉强度影响较显著(P<0.05),层高、打印速度、填充单元对抗拉强度影响均不显著。最后,通过实验证实了上述工艺下的试件具有最高的拉伸强度。这一发现对于优化FDM 3D打印工艺参数,提升PLA制件力学性能具有重要的指导意义。

EPS外墙外保温系统聚氨酯粘结胶加固后受力性能试验研究

通过EPS保温板抗拉试验、疲劳试验、加固疲劳试验,研究粘贴率、粘贴方式对EPS板及聚氨酯粘结胶加固试样的抗拉承载力和疲劳性能影响。结果表明:除满粘外,在相同条件下,EPS板抗拉承载力平均值由大到小为:条粘>点粘>十字粘>点框粘;EPS板疲劳次数与粘贴率呈正相关,条粘法疲劳性能最好;采用聚氨酯粘结胶加固后的试样抗拉承载力平均值均符合要求,且达到满粘抗拉承载力的90%;初始疲劳对加固效果的影响最大仅7%,加固后点粘、条粘、十字粘、点框粘的疲劳次数分别达到满粘的87%、89%、87%、75%;聚氨酯粘结胶加固后累计疲劳次数更多,EPS板整体使用寿命得到延长,加固方法较为可靠。

含三氟甲基二胺单体对聚酰亚胺薄膜性能的影响

以2,2'-双(三氟甲基)-4,4'-二氨基联苯(TFMB)和2,2-双[4-(4-氨基苯氧基)苯基]-1,1,1,3,3,3-六氟丙烷(HFBAPP)两种含三氟甲基的二胺单体与4,4'-联苯醚二酐(ODPA)进行三元共聚,通过改变两种二胺单体的物质的量比,制备了6种含三氟甲基的透明聚酰亚胺(PI)薄膜,研究了TFMB和HFBAPP两种单体对PI薄膜的结构、热性能、力学性能和光学性能的影响。结果表明,所得的PI薄膜已经完全热亚胺化并引入了含氟基团,具有较好的热性能,玻璃化转变温度为230.5~245.2℃,5%热失重温度为480.9~503.7℃,10%热失重温度517.3~534.0℃,当TFMB和HFBAPP的物质的量比为8∶2时,薄膜的热性能表现最佳;TFMB的加入可以提高薄膜的拉伸强度,当TFMB和HFBAPP的物质的量比为6∶4时,薄膜的拉伸强度达到最大(95.26 MPa),断裂伸长率为4.40%;所得薄膜的光学性能表现良好,紫外-可见光范围内的最大透过率均在86.47%以上,当TFMB和HFBAPP的物质的量比为5∶5时,薄膜的光学性能最优,450 nm处的透过率为87.28%,紫外-可见光范围内的最大透过率为91.34%。以上结果表明,在HFBAPP中适量引入TFMB有利于提高PI薄膜的性能。

超高性能混凝土拉伸与疲劳性能研究进展

超高性能混凝土(UHPC)是一种具有超高抗拉强度、优异抗疲劳性能的材料,结合国内外相关文献,利用其突出的性能,对近几年UHPC的拉伸行为及疲劳性能的研究进展进行综述。在拉伸性能方面,总结得出试件的形状、尺寸、钢纤维以及材料配比等都不同程度地影响着UHPC的抗拉性能;合理优化钢纤维的掺量、形状、级配及排列顺序是提高抗拉性能的重要因素,对钢纤维表面进行处理同样可以提高抗拉强度;在疲劳性能方面,对UHPC抗弯疲劳性能、抗压疲劳性能、抗拉疲劳性能进行总结得出:钢纤维对疲劳性能影响最大,温度、UHPC的强度、应力水平以及加入其他纤维与配筋都不同程度地影响着钢纤维的疲劳性能。最后,从工程应用角度出发,介绍了UHPC相关规范,介绍了UHPC在抗拉、抗疲劳性能及桥梁方面的应用实例,并提出合理的建议。

10 kV配网耐张型复合绝缘横担研究综述

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