SiO2对铝合金熔体直接氧化的影响

本文研究了在高温空气氛中涂覆在Al-Mg-Si合金表面的SiO₂对铝合金直接氧化的影响规律。实验揭示了SiO₂对Al-Mg-Si合金熔体直接氧化生长表面的形态的影响规律,发现SiO₂有助于Al₂O₃/Al复合材料以光滑的方式进行氧化生长,提高了材料的致密度。实验还发现,SiO₂可消减Al-Mg-Si合金熔体直接氧化所需的孕育期,缩短Al₂O₃/Al复合材料的生长时间。     

SiO2玻璃原位反应合成Al/Al2O3复合材料

利用SiO₂玻璃具有易近成型、致密及各向同性的特点,通过SiO₂玻璃与铝熔体间的反应合成了Al/Al₂O₃复合材料,克服长期以来在合成Al/Al₂O₃复合材料时均采用颗粒反应物的局限。反应产物是一种组织均匀致密的Al 与Al₂O₃互为网络的Al/Al₂O₃陶瓷基复合材料。反应温度升高,整个反应产物中的Al的体积分数上升。Al/Al₂O₃复合组织在三维空间的真实形态中存在着Al相被Al₂O₃完全包围的形态,证明了网络状Al₂O₃组织形成的烧结机理。与合成Al/Al₂O₃的其它工艺相比,本工艺可在1000℃的较低温度进行,并具有反应速度快、断裂韧性和抗弯强度值高的特点。     

纳米-Al2O3/SiO2加入量对MgO-Al2O3-SiO2复相陶瓷烧结机理的影响

为了探究纳米-Al₂O₃/SiO₂加入量对MgO-Al₂O₃-SiO₂复相陶瓷烧结行为的作用机理。以微米级MgO、纳米级Al₂O₃和SiO₂为主要原料制备陶瓷基复合材料。通过XRD和 SEM等检测手段对烧后试样的物相组成和微观结构进行测试与表征,重点研究Al₂O₃/SiO₂的加入对复相陶瓷物相组成、微观结构及烧结性能的影响。结果表明:随着Al₂O₃/SiO₂加入量的增大,试样烧后相对密度和烧后线变化率呈先增大后减小再增大的趋势,加入15%Al₂O₃/SiO₂(质量分数)的试样经1 500 ℃烧结后,其相对密度可以达到94%。引入的Al₂O₃/SiO₂与基体中的MgO生成镁铝尖晶石与镁橄榄石相,原位反应伴随的体积膨胀,抵消部分烧结过程中的体积收缩。Al₂O₃/SiO₂加入量为75%(质量分数)的试样经1 400 ℃烧结后,基体中有大量堇青石相生成,随着煅烧温度提高到1 500 ℃,堇青石分解所产生的高温液相促进了试样的烧结收缩。     

补加合金成分对铝合金熔体直接氧化生长的影响

为了控制DIMOX工艺中复合材料的生长速度和体积,采用压差法补加合金,使剩余铝合金熔体的成分得到调整并与正在氧化生长的Al₂O₃/Al复合材料层保持连续接触。试验研究了补加合金成分对Al-3Mg-10Si合金氧化生长的影响作用。结果表明:补加纯Al或低Mg、Si含量的铝合金,能够降低合金熔体的含镁量,缩短材料生长前沿合金熔体成分到达Al₂O₃-(Al,Mg)两相区的时间,促进传质过程,加速材料生长,有利于获取较大体积的复合材料。     

Al-TiO2系XD合成的动力学分析

通过XD(Exothermic Dispersion)法原位反应生成Al₂O₃与Al₃Ti增强的铝基复合材料,对反应过程动力学机理进行了探讨。结果表明Al/TiO₂经充分混合、挤压成坯后,在真空炉中以一定的升温速率加热至800℃左右时发生剧烈的化学反应。SEM、XRD及能谱分析可知:生成物Al₂O₃呈等轴颗粒状,Al₃Ti呈棒状,并进一步说明了升温速率、铝含量、压坯的预紧实度等因素对反应结果的影响。     

不同纳米材料与石墨混合填充PTFE复合材料摩擦磨损性能

利用MM-200型磨损试验机考察了载荷对纳米SiO₂、TiO₂、Al₂O₃与石墨混合填充PTFE复合材料摩擦磨损性能的影响,采用扫描电子显微镜观察分析磨损表面形貌及磨损机理。结果表明,纳米材料及其与石墨混合都可以不同程度地提高PTFE的耐磨性,而它们对PTFE耐磨性的提高程度各不相同,其中以纳米SiO₂-石墨填充PTFE复合材料的磨损质量损失最小,纳米Al₂O₃-石墨填充PTFE复合材料的磨损质量损失较大;填充PTFE复合材料同钢对磨时的摩擦系数表现出不同的性能,纳米SiO₂-石墨填充PTFE的摩擦系数与纯PTFE相差不大。     

Lanx ide Al2O3/Al 复合材料的自润滑特性及磨损机制研究

利用熔融铝合金的直接氧化反应制备了具有含油自润滑特性的Al₂O₃/Al 复合材料。通过对磨面的SEM 观察及能谱分析, 测定了金属相和显微孔隙对复合材料磨损性能的影响, 初步探讨了Al₂O₃/Al 复合材料的磨损机制。     

硼酸铝晶须增强6061 铝复合材料的界面改性

采用溶胶凝胶C₂Al₂O₃ 涂层涂覆硼酸铝晶须方法来改善硼酸铝晶须增强6061Al 复合材料的界面性能。结果表明: 试验采用的涂覆工艺可以在硼酸铝晶须表面获得40 nm 厚的氧化铝涂层。硼酸铝晶须与铝合金基体在压铸过程中生成的几乎连续的脆性尖晶石产物使晶须连续性结构损伤严重。C₂Al₂O₃ 涂层虽然不能完全阻止界面反应的发生, 但是却有效地抑制了尖晶石界面反应, 保持了硼酸铝晶须的结构完整性。     

硅酸铝纤维增强铝基复合材料的疲劳断裂特征

采用压力铸造法, 制得Al₂O₃?SiO₂短纤维增强的铝合金复合材料, 对其弯曲疲劳性能进行了测试, 并详细观察了疲劳裂纹的形成及扩展方式。结果表明: Al₂O₃?SiO₂f/ ZL 108复合材料存在 多种疲劳源; 疲劳裂纹的扩展是通过主裂纹与裂尖前方孔洞的相互联接而进行的, 是不连续的, 沿着纤维及渣球密集的路径扩展; 疲劳过程中主裂纹的形成消耗了大部分的疲劳寿命, 一旦主裂纹形成就快速扩展瞬间断裂。该复合材料的断裂宏观上是脆性的, 但微观上显示出塑性的特征。     

20wt% SiO2/Al-Mg复合材料的界面反应及其微结构

采用粉末冶金法制备了20wt% SiO₂/Al-Mg复合材料。研究了SiO₂和基体元素Al,Mg反应机制,研究表明:在原SiO₂颗粒内,形成MgAl₂O₄,MgO,Mg₂Si和少量Al和Si。MgAl₂O₄呈不规则形状,而且MgAl₂O₄往往和Al相邻;MgO和Mg₂Si形成片层状共析体;经620℃烧结30min,SiO₂被完全反应掉。反应生成物Si多数被排到Al基体中;原Al-Mg基体中主要物相为:Al,Mg₂Si和Si,Mg₂Si颗粒的尺寸小于0.2μm。原Al-Mg基体中,单质Mg已不存在,Mg反应形成Mg₂Si。     

Solute Redistribution with Shear Flow in Molten Pool of Ni-Cr Alloy

Columnar grain growth with shear flow in molten pool of Ni-Cr alloy was simulated with a coupled model of grain growth and solute transport.The results indicate that shear flow alters solute distribution at the vicinity of columnar grains.The solute concentration gradient on the upstream side is greater,while that on the downstream side is smaller,leading to asymmetrical growth of columnar grains.In the interior of a columnar grain,solute concentration increases from the bottom to the dendrite tip,but the rate of increase tends to be reduced.The simulated results are consistent with the experiments.     

High Cycle Fatigue Property of Al–Mg–Sc–Zr Alloy Fabricated by Laser Powder Bed Fusion

Laser powder bed fusion (LPBF) of high-strength Al–Mg–Sc–Zr alloy possesses great potential application in the aerospace industry. However, fatigue performance has become a very important issue in the safety and durability design of engineering structures. Herein, the high cycle fatigue property of LPBF-fabricated Al–Mg–Sc–Zr alloy and its correlation with defects, microstructure, and precipitated phases are studied. The LPBF-manufactured Al–Mg–Sc–Zr alloy appears heterogeneous structure composed of equiaxed grains at the molten pool boundary and columnar grains at the inner of the molten pool. After aging treatment (325 °C/4 h), the nanosized Al₃Sc intragranular particles and Mn-rich intergranular particles are precipitated, leading to more difficult movement of dislocations that favors the fatigue strength. The ultimate tensile strength of the samples after aging treatment is 507.05 MPa and corresponding 10⁷ cycle fatigue strength (R = −1) is 106 MPa. The fracture morphology of the fatigue specimens shows that the fatigue cracks start from the surface defects with strong stress concentration, especially the lack of fusions, and then expand through the surplus part.     

Effect of substrate preset temperature on crystal growth and microstructure formation in laser powder deposition of single-crystal superalloy

A successful repair of single-crystal components needs to avoid the stray grain formation and achieves continuous epitaxial growth of columnar dendrites in the repaired zone. In this study, the effect of substrate preset temperature on crystal growth and microstructure formation in laser powder deposition of single-crystal superalloy was studied through an improved mathematical model and corresponding experimental approaches. The results indicated that the variation of substrate preset temperature between ?30?°C and +210?°C changes the molten pool morphology little, but obviously affects the columnar-to-equiaxed transition conditions. The preheating of substrate facilitates the stray grain formation and enlarges the primary columnar dendrite arm spacing, while the situation for precooling of substrate is opposite. Under the specific processing conditions, the critical condition for continuous epitaxial growth is that the substrate preset temperature T_sub?≤?+90?°C. When the substrate preset temperature T_sub is below +90?°C, the height ratio of melting depth to total height of the molten pool is larger than that of stray grain, ensuring that stray grains can be completely remelted and the continuous columnar dendrites during the multi-layer laser powder deposition process on (001) surface of single-crystal substrate can be achieved.     

Direct fabrication of single-crystal-like structure using quasi-continuous-wave laser additive manufacturing

The synchronously periodic re-melting of molten pool was firstly introduced in additive manufacturing to promote the epitaxial growth of columnar structure using a novel quasi-continuous-wave(QCW)laser.The epitaxial growth of columnar structure was intensified and the single-crystal-like sample with highly oriented "zigzag" columnar grains was produced.The modified molten-pool geometry and the synchronously high-frequency re-melting of the molten pool contribute to the formation of singlecrystal-like structure.This work reports a new route to promote the continuously epitaxial growth of dendrites for fabrication of single-crystal-like sample.     

化学气相沉积制备钼钨合金

以MoF6、WF6以及H2为原料,用化学气相共沉积的方法,成功地在铜基片表面沉积出钼钨合金.实验分析表明:沉积层结构为单相均匀固溶体.在钼含量高时沉积层显微组织呈细晶层状结构,在钼含量较低时沉积层显微组织呈柱状晶生长.改变反应气体中WF6、MoF6相对含量可实现膜层成分可控.采用化学气相沉积法沉积钼钨等难熔金属合金,沉积纯度高,设备简单,沉积速率快,在高温抗烧蚀涂层及耐腐蚀涂层方面具有广泛的应用前景.     

Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation – Part I: Model description

Part I of this two-part investigation presents a volume-averaging multiphase solidification model that accounts for mixed columnar-equiaxed solidification, non-dendritic and dendritic crystal growth, nucleation of equiaxed grains, columnar primary dendrite tip tracking, melt flow, sedimentation of equiaxed crystals, and their influence on macrostructure and macrosegregation. Five distinct thermodynamic phases (phase regions) are defined: solid dendrites in equiaxed grains, the interdendritic melt between equiaxed dendrites, solid dendrites in columnar trunks, the interdendritic melt between trunk dendrites, and the extradendritic melt. These five phase regions are quantified by their volume fractions and characterized by their solute concentrations. The five phase regions are grouped into three hydrodynamic phases: equiaxed grains consisting of solid dendrites and interdendritic melt, columnar trunks consisting of solid dendrites and interdendritic melt, and extradendritic melt. The extradendritic melt is separated from the interdendritic melt with a grain envelope, whose profile connects the primary, secondary or tertiary dendrite tips to form a ‘natural’ enclosure of the equiaxed grains or columnar trunks. The envelope is further simplified as a volume-equivalent sphere for equiaxed grains, or as volume-equivalent cylinder for columnar trunks by use of morphological shape factors. Expansion of the envelopes during solidification is determined by dendrite growth kinetics, using the Kurz–Giovanola–Trivedi model for growth of columnar primary dendrite tips and the Lipton–Glicksman–Kurz model for growth of columnar secondary dendrite tips (radial growth of the columnar trunk) and equiaxed primary dendrite tips. The solidification of the interdendritic melt is driven by the supersaturation of the interdendritic melt and governed by the diffusion in the interdendritic melt region. Illustrative process simulations and model verifications are presented in Part II.     

采用金属镓层氮化技术在石英衬底上生长多晶GaN

随着多晶GaN材料发光研究的不断深入 ,大面积、价格低廉的多晶GaN基光电器件的研制已成为工业生产中一个重要的研究领域。石英玻璃以其自身特有的优势 ,成为生长多晶GaN材料的较为理想的衬底。本文采用一种新的金属镓层氮化技术 ,使用无定形石英作衬底 ,在常压下制备出多晶GaN。经分析测试表明 ,生长出的多晶GaN为六方结构且质量较好 ,并观察到针状的表面结构     

对流扩散-多相相变体系内柱状晶/等轴晶形成过程的数值模拟

采用扩散支配相变动力学方法对Fe-Bi-Mn系易切削合金侧向快速凝固过程进行数值研究。建立对流扩散-多相相变体系三维凝固模型,考虑固、液、气三相扩散流动相变对合金凝固的影响,模拟研究合金中MnS和Bi(易切削相)的柱状晶/等轴晶形成过程。结果表明:合金凝固过程中MnS和Bi的柱状晶/等轴晶形成模式强烈受对流扩散和多相相变影响;对流扩散为正值处,溶质的多相质量相变速率较大且富集程度较低,流动稳定易形成柱状晶;对流扩散为负值处,溶质的多相质量相变速率较小且富集程度较高,当晶尖处溶质富集到一定程度,对流扩散与多相相变产生的紊流使柱状晶尖端断裂,成为等轴晶形核中心,此处为等轴晶稳定形成区域。     

Microstructural refinement of 6061 and 5052 aluminium alloys by arc oscillation

The mechanism of the microstructure refinement of metals and alloys (5052 and 6061) by arc oscillating was studied and discussed via the grain size of regular welds and those welded at different oscillation frequencies. In 5052 alloy, fine columnar crystals appeared near the ‘internal fusion line’ during the oscillating arc welding and the grains have been significantly refined by oscillating arc with low and high frequency. However, the grain refinement of 6061 Al alloy was limited during arc oscillating welding due to the different crystal growth pattern at the fusion line and material properties. It is not caused by changing of the grain refinement mechanism but by the difference in the material properties.     

超声辅助CMT电弧增材制造TC4钛合金微观组织和力学性能研究

目的研究超声辅助对CMT电弧增材制造钛合金TC4微观组织及力学性能的影响。方法 CMT增材制造TC4钛合金的同时利用超声辅助设备进行振动辅助,采用不同的振动功率和不同的振动位置对增材后的TC4力学性能和微观组织进行对比分析。结果 600 W超声辅助振动基板时的钛合金试样成形美观,力学性能优异。水平方向上,抗拉强度平均值为952.7 MPa,伸长率平均值为7.46%;垂直方向上,抗拉强度平均值为905.83 MPa,伸长率平均值为11.03%,相较未施加超声振动增材试样的力学性能有明显提高。结论超声辅助的引入有效提高了熔池的深宽比,加快了熔池冷却速度,柱状晶尺寸也明显减小,针状马氏体数量增多,得到的钛合金力学性能和微观组织均良好。