熔模精密铸造TiAl基金属间化合物研究进展

TiAl基金属间化合物作为一种新型轻质高温结构材料,在航空航天和汽车等领域具有广阔的应用前景。熔模精密铸造是当前普遍采用的制备TiAl基金属问化合物的方法。主要介绍了熔模精密铸造TiAl基合金的铸件以及型壳用粘结剂及耐火材料的发展现状,TiAl合金的熔炼技术及最新研究进展,并对TiAl基金属间化合物熔模精密铸造技术的不足进行了分析并提出了展望。     

TiAl基金属间化合物的研究现状与发展趋势

系统地总结了TiAl基金属间化合物结构材料的研究现状、存在的问题以及在航空航天等领域的应用情况。对TiAI基金属间化合物的组织控制与性能研究、冶金熔炼、成形加工等进行了归纳,结合TiAl基金属间化合物材料与应用研究取得的新进展,预测了TiAl基金属间化合物轻质结构材料在今后一段时期的发展趋势。     

TiAl基金属间化合物的发展

介绍了普通TiAl基金属间化合物的发展过程,特别是北京科技大学开发的高Nb—TiAl金属间化合物的成分-组织-性能-制备关系,Nb对TiAl合金相关系和抗氧化性的影响,Nb的强化机理和工程合金的制备和性能。最后简单总结了TiAl基金属问化合物的发展趋势。     

原位自生TiAl3增强Al基复合材料的组织结构及形成机理研究

采用冷喷涂技术沉积Ti-80Al（wt.%）复合涂层,通过热处理获得了原位自生TiAl3金属间化合物颗粒增强Al基复合材料涂层。采用SEM、EDS和XRD等分析了冷喷涂Ti/Al复合涂层在不同热处理温度下的组织结构演变规律及Ti、Al粒子间原位扩散反应过程,并对TiAl3金属间化合物的形成机理进行了探讨。结果表明,冷喷涂Ti/Al复合涂层组织致密,其相结构与喷涂粉末完全相同,450℃热处理后涂层局部区域发生Ti、Al间的固态扩散反应,并在Ti、Al粒子界面原位形成TiAl3金属间化合物,随着热处理温度升高,TiAl3金属间化合物的含量显著增加,600℃热处理后,Ti/Al复合涂层中的Ti粒子全部转变为TiAl3金属间化合物,获得原位自生TiAl3颗粒增强的Al基复合材料.     

轻质γ-TiAl金属间化合物的研究进展

γ-TiAl金属间化合物是一种新型轻质的高温结构材料,是当代航空航天工业、民用工业等领域的优秀候选高温结构材料之一,具有重要的工程化应用潜力。介绍了γ-TiAl基金属间化合物的发展过程,以及成分-组织-性能-制备关系,产业化和应用状况。特别指出,北京科技大学发展的高Nb,TiM金属间化合物为国内外TiAl基金属间化合物发展的重点方向。最后总结了TiAl基金属间化合物的国家需求和发展趋势。     

TiAl基合金的工艺—显微组织—力学性能关系

因密度、比刚度、高温比强度和阻燃性等方面的优势,TiAl基金属间化合物被认为是最有应用潜力的新一代结构材料。室温脆性以及延性、蠕变性能和其它性能的平衡是阻碍TiAl基合金作为高温结构材料实际应用的主要障碍。在分析合金化对组织与性能的影响及机理、加工／热处理 对组织与性能的影响、变形和断裂机制、显微组织与拉伸性能的关系、抗氧化性及改善、蠕变性能与蠕变机制的基础上,论述了TiAl基合金的工艺－显微组织－力学性能关系。     

TiAl基合金双态组织的控制

研究了TiAl合金双态组织的显微组织参数对力学性能的影响 ,通过对Ti 46 5Al 2Cr 1 5Nb 1V金属间化合物进行特定的热处理 ;分别获得了具有相同晶粒大小、不同片层晶粒体积分数 ,以及相同片层晶粒体积分数、不同晶粒大小显微组织的两个系列的双态复相TiAl合金。研究结果表明 :对于双态复相组织 ,晶粒尺寸由预备热处理过程中的退火时间来控制 ;而片层晶粒体积分数则通过后续热处理温度来控制 ,不同片层晶粒含量双态组织的晶粒尺度可以通过后续热处理时间来控制     

TiAl基合金的激光表面处理

研究了激光表面合金化和表面熔覆技术在γ-TiAl基金属间化合物上的应用，从激光表面处理技术的特点和其在TiAl合金表面改性上的最新研究结果入手，探讨了激光表面层及表面处理工艺参数对TiAl合金表面组织与高温氧化性能和耐磨性的影响及其机理，研究结果表明激光表面处理在改善TiAl合金高温抗氧化性能和提高耐磨性方面有着广阔的应用前景。     

反应扩散连接高铌TiAl合金(英文)

采用Al箔作中间层在1200℃/2 h条件下通过反应扩散连接成功实现了高铌TiAl合金(TAN)的焊接,深入研究了接头的界面微观组织结构和连接机理。结果表明:连接过程中Al箔熔化成液相后与高铌TiAl反应在接头中形成了连续的TiAl3化合物层;在高温扩散作用下,TiAl3化合物逐渐转变为γ-TiAl相;最后经焊后热处理形成了γ+α2层片组织。另外,当直接采用高铌TiAl合金的热处理工艺进行焊接时,亦可以获得具有层片组织的接头。     

Reaction-Diffusion Bonding of High Nb Containing TiAl Alloy

采用Al箔作中间层在1200℃/2 h条件下通过反应扩散连接成功实现了高铌TiAl合金(TAN)的焊接,深入研究了接头的界面微观组织结构和连接机理。结果表明:连接过程中Al箔熔化成液相后与高铌TiAl反应在接头中形成了连续的TiAl3化合物层;在高温扩散作用下,TiAl3化合物逐渐转变为γ-TiAl相;最后经焊后热处理形成了γ+α2层片组织。另外,当直接采用高铌TiAl合金的热处理工艺进行焊接时,亦可以获得具有层片组织的接头。     

循环处理对铝合金尺寸稳定性的影响

研究了循环处理对ＬＤ１０合金的尺寸稳定性畸变的影响,并通过残余应力和组织结构的变化分析了作用机理,探讨了工艺路线,工艺参数影响规律,得出增强ＬＤ１０合金稳定性降低残余应力的循环处理优化工艺。     

Microhardness and Stress Analysis of Laser-Cladded AISI 420 Martensitic Stainless Steel

Laser cladding is a surface treatment process which is starting to be employed as a novel additive manufacturing. Rapid cooling during the non-equilibrium solidification process generates non-equilibrium microstructures and significant amounts of internal residual stresses. This paper investigates the laser cladding of 420 martensitic stainless steel of two single beads produced by different process parameters (e.g., laser power, laser speed, and powder feed rate). Metallographic sample preparation from the cross section revealed three distinct zones: the bead zone, the dilution zone, and the heat-affected zone (HAZ). The tensile residual stresses were in the range of 310–486 MPa on the surface and the upper part of the bead zone. The compressive stresses were in the range of 420–1000 MPa for the rest of the bead zone and the dilution zone. The HAZ also showed tensile residual stresses in the range of 140–320 MPa for both samples. The post-cladding heat treatment performed at 565 °C for an hour had significantly reduced the tensile stresses at the surface and in the subsurface and homogenized the compressive stress throughout the bead and dilution zones. The microstructures, residual stresses, and microhardness profiles were correlated for better understanding of the laser-cladding process.     

硬质合金基体上金刚石膜的XRD研究

采用直流等离子体射流CVD法在硬质合金 (WC + 8wt.%Co)基体上生长金刚石膜 ,主要借助XRD分析方法对CVD金刚石膜的微观结构和残余应力进行了研究 ,探讨了CVD金刚石膜中残余应力的形成及其对金刚石膜粘附性能的影响。研究结果表明 :CVD金刚石膜中通常存在GPa数量级的残余压应力 ,膜中存在适中的残余压应力 ,有利于CVD金刚石膜获得最佳的粘附性能     

The effect of thermo-mechanical treatment on The pitting corrosion of reinforcing carbon steel bars

Steel reinforcing rods with varying amounts of cold work and carbon concentrations (0.06 to 0.45% C) exposed in soil embankments has shown various degrees of pitting corrosion. To understand this pitting process, laboratory investigations on residual stresses, microstructures and potentiodynamic cyclic polarization were undertaken. Analysis of residual stresses in the steels indicated low value of compressive stresses in hot rolled steels and high value of tensile stresses in the cold worked counterparts. Hot rolled steels displayed a slightly better pitting corrosion resistance than cold worked samples which is consistent with the above internal stress pattern. No definite correlation was obtained between the percentage of carbon in the steels and pitting susceptibility. An attempt has been made to define the role of the complex steel-soil system for pitting corrosion behavior.     

Residual stress evaluation in small diameter pipes welded using the orbital TIG process

The objective of this work is to evaluate the effect of orbital TIG welding on the level of residual stresses in the joint, on the resultant microstructure and on the surface appearance of the weld bead, considering the effects of the welding energy, of the number of passes and of the pulse type (current and rotation) in tubes of small diameter and to contribute to the analysis of decisions of whether or not to apply hydrostatic tests or thermal treatment post welding. The test pieces, welded with different levels of energy, pulses and number of passes, were submitted to measurement of residual stresses in an X-ray diffractometer. The samples were also submitted to metallographic analyses. It was concluded that the residual stresses on the external surface of the tubes are compressive in nature. The level of residual stresses in the tubes falls with the increase in welding energy and is lower for tubes welded using pulsing than for those welded without pulsing and, for this work, was not influenced substantially by the number of passes. The samples analysed presented conventional microstructures for the steel employed.     

Performance Optimization of Cold Rolled Type 316L Stainless Steel by Sand Blasting and Surface Linishing Treatment

Sand blasting followed by a surface linishing treatment was applied to optimize the near-surface microstructure of cold rolled type 316L stainless steel. The introduction of cold rolling led to the formation of α-martensite. Specimens with large thickness reductions (40, 53%) were more susceptible to localized corrosion. The application of sand blasting produced a near-surface deformation layer containing compressive residual stresses with significantly increased surface roughness, resulting in reduced corrosion resistance. The most resistant microstructure was obtained with the application of a final linishing treatment after sand blasting. This treatment produced microstructures with compressive near-surface residual stresses, reduced surface roughness, and increased resistance to localized corrosion.     

Microscopic phase-dependent residual stresses in the machined surface layer of two-phase alloy

Residual stresses in the machined surface layer, which affect fatigue crack nucleation and stress corrosion cracking especially in aerospace engines and gas turbines for power generation, depend on microstructures in case of machining a multiple-phase alloy. Hence, the microscopic phase-dependent residual stresses should be known when a machined part is used under critical stress conditions and circumstances. In the present paper, finite element modeling of machining two-phase alloys has been developed for obtaining the residual stresses in the machined surface layer. Iron and steels, which consist of different volume fractions of ferrite and eutectoid pearlite, were selected as work materials to be machined. First, it was confirmed that the calculated results agree well in chip formation and cutting forces with experimental ones. Then, residual stresses in the machined surface layer were obtained for different carbon contents and regular/random arrangements of microstructure. As a result, it is found that the microstructure of the workpiece has a great influence on the residual stress distribution on the machined surface and that tensile surface residual stress on pearlite is much larger than that on ferrite. Finite element machining of the work material with stripe arrangement of ferrite and pearlite revealed that the peak of residual stress would be reduced by decreasing the width of stripes of ferrite and pearlite.     

Electrochemical noise characterisation of base metal and weld metal of an ASTM 516 steel in H2S containing environment

The inspection of steel structures exposed to sour environments has revealed a higher incidence of cracks at welded joints. This tendency is associated to several factors, including residual stresses, different microstructures and dissimilar electrochemical behaviour between base metal and weld metal. The crack incidence in general is attributed to hydrogen absorption due to the cathodic reduction of this species. In this work, the electrochemical noise behaviour of weld and base metals of an ASTM 516, C‐Mn steel exposed to a H₂S containing environment is studied. Current and voltage fluctuations are measured and the spectral noise impedance (R_sn) is determined. The current fluctuations are measured by means of a Zero Resistance Ammeter and voltage fluctuations with respect to a calomel reference electrode. In this case, residual stresses and microstructures of both materials can be assumed as secondary factors to hydrogen cracking. However, electrochemical noise measurements indicated the existence of different electrochemical processes for base metal and weld metal in the environment considered.     

Contributions of numerical modelling to the optimisation of welding processes

The welding industry has not been excluded from the beneficent invasion of numerical simulation which, by allowing modelling of metal and mechanical behaviour, predicts the microstructures that will be obtained and the real residual stresses. Based on codes of calculation on finite elements, these evaluations can even establish the risk of brutal rupture or the fatigue behaviour of a given structure. As a consequence, the prediction of residual distortions opens the way for the creation of qualitatively optimised structures, created by means of optimised welding processes.     

Effects of zonal heat treatment on residual stresses and mechanical properties of electron beam welded TC4 alloy plates

Zonal heat treatment(ZHT) was conducted in situ to 14.5 mm-thick TC4 alloy plates by means of defocused electron beam after welding. The effects of ZHT on residual stresses, microstructures and mechanical properties of electron beam welded joints were investigated. Experimental results show residual stresses after welding are mostly relieved through ZHT, and the maximum values of longitudinal tensile stress and transverse compressive stress reduce by 76% and 65%, respectively. The tensile strength and ductility of welded joint after ZHT at slow scanning velocity are improved because of the reduction of residual stress and the microstructural changes of the base and weld metal. ZHT at fast scanning velocity is detrimental to the ductility of welded joint, which is resulted from insufficiently coarsened alpha phase in the fusion zone and the appearance of martensite in the base metal.