钛合金表面Ti-Al合金扩散层的摩擦性能研究

利用双层辉光离子渗金属技术,以纯铝作为源极,氩气为工作气体,利用不等电位空心阴极效应在钛合金(Ti6A l4V)表面渗铝,形成均匀致密的钛铝合金扩散层,采用WTM-1型球-盘摩擦磨损试验机对钛合金表面钛铝合金扩散层的摩擦性能进行研究,用显微硬度仪测量渗层的显微硬度,并用扫描电子显微镜和X射线衍射仪观察分析钛铝合金扩散层的截面形貌和相结构.结果表明:在Ti6Al4V合金基材表面形成厚20μm的钛铝合金扩散层,其表面显微硬度达810HV;钛铝合金扩散层主要由金属间化合物Ti3A l和TiAl组成;钛铝合金扩散层可以显著提高钛合金表面的摩擦性能,摩擦系数从0.35左右降至0.15,抗磨损性能大幅提高.

镍镉合金扩散层工艺研究

为满足飞机发动机叶轮耐高温腐蚀要求,研究了镍镉合金扩散层的结构,同时分析了镍镉合金镀液工艺参数对扩散层性能的影响。采用合适的电解镍液或化学镀镍液的配方来制取低应力镍层;采用液体喷丸方法提高零件表面强度及抗应力腐蚀能力。结果表明,该两种方法有效克服了镍镉合金扩散层不耐疲劳和高温应力腐蚀的弊病,即使在工作中承受旋转、弯曲、振动、离心等复杂应力作用的镍镉扩散层,也具有优良的性能。

镍镉合金扩散层内应力的研究

为了满足飞机发动机叶轮耐高温腐蚀性要求,研究了镍镉合金扩散层的结构,采取了两种方法来克服镍镉合金扩散层的弊病。选取合适的镀镍液或化学镀镍液制取低应力镍层;基体用液体喷丸方法提高零件表面强度及抗应力腐蚀能力。同时分析了电镀工艺参数对镍镉扩散层的影响。

压下率对热成形钢铝硅镀层显微组织和性能的影响

轧制差厚板被广泛应用在汽车各零部件中。本文研究了不同压下率轧制对热成形钢铝硅镀层显微组织和性能的影响。结果表明,轧制后的铝硅镀层热成形钢组织形貌保持不变,由外到内依旧保持着铝基体、合金扩散层、硼钢基体的结构特点。随着压下率的增大,试验钢镀层厚度减少,但合金扩散层厚度基本保持不变。压下率为10%~30%时,铝硅镀层及合金扩散层连续性均未被破坏,但合金扩散层内部出现裂纹,裂纹密度随压下率增大而增大。镀层总厚度减薄率略高于压下率;压下为40%时,镀层减薄现象更加明显,部分位置铝基与硼钢基体直接接触。试验钢的抗拉强度、屈服强度、基体维氏硬度随压下率增大而增加,伸长率急剧降低,但断裂伸长率均≥2%。

High temperature oxidation behavior of Ti(Al,Si)_3 diffusion coating on γ-TiAl by cold spray

A Ti(Al,Si)3 diffusion coating was prepared on γ-TiAl alloy by cold sprayed Al?20Si alloy coating, followed by a heat-treatment. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 120 cycles to check the oxidation resistance of the coating. The microstructure and phase transformation of the coating before and after the oxidation were studied by SEM, XRD and EPMA. The results indicate that the diffusion coating shows good oxidation resistance. The mass gain of the diffusion coating is only a quarter of that of bare alloy. After oxidation, the diffusion coating is degraded into three layers: an inner TiAl2 layer, a two-phase intermediate layer composed of a Ti(Al,Si)3 matrix and Si-rich precipitates, and a porous layer because of the inter-diffusion between the coating and substrate.

Effect of α phase on evolution of oxygen-rich layer on titanium alloys

In order to understand the evolution of oxygen-rich layer (ORL) on titanium alloys, the near α titanium alloy TA15 and α+β type titanium alloy TC4 were thermally exposed in air at 850 °C to evaluate the effect of α phase content on formation and evolution of ORL, and the stability and diffusion of oxygen in α- and β-Ti were investigated by first principles calculations to reveal the oxygen diffusion rate. TA15 with more α phases has a higher diffusion coefficient of ORL evolution than TC4, resulting in forming thicker ORL on TA15 under the same thermal exposure condition. The first principles calculations indicate that octahedral interstice of α-Ti is the most stable site for oxygen atom. The nearest neighbor diffusion between octahedral interstices along the [0001] direction in α-Ti presenting the lowest activation energy is the most favorable oxygen diffusion mechanism in α- and β-Ti.

Development of processing windows for diffusion bonding of Ti-6Al-4V titanium alloy and 304 stainless steel with silver as intermediate layer

As titanium alloy is chemically reactive,it is very difficult to join by conventional welding techniques.Titanium alloys can easily pick up nitrogen and oxygen from the atmosphere.In the fusion welding method,brittle intermetallic compounds are formed when joining titanium alloy and stainless steel,which decrease the mechanical behavior of the couples.Hence,for joining of titanium alloy,diffusion bonding is recommended.This work dealt with the measurement of feasible process parameters for diffusion bonding of Ti-6Al-4V and AISI 304 stainless steel with silver as an intermediate layer.The quality of the bonds was confirmed by the lap shear test and microstructural analysis.With the experimental results obtained,diffusion bonding windows were constructed and this will act as reference maps to identify the process parameters for obtaining defect free bond.Bonding was successful in the temperature range of 750-800 °C.Maximum lap shear strength was achieved under a bonding pressure of 5 MPa and holding time of 90 min.

Oxidation behavior and mechanism of porous nickel-based alloy between 850 and 1000 °C

The oxidation behavior and mechanism of a porous Ni?Cr?Al?Fe alloy in the temperature range from850to1000°Cwere investigated by optical microscopy,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS),X-raydiffraction(XRD)analyses and X-ray photoelectron spectroscopy(XPS).The results show that the oxidation kinetics at950and1000°C of this porous alloy is pseudo-parabolic type.Complex layers composed of external Cr2O3/NiCr2O4and internalα-Al2O3areformed on the surface of the oxidized porous alloys.γ?phases favor the formation of NiO/Cr2O3/NiCr2O4during the initial oxidation.Many fast diffusion paths contribute to the development of the oxide layers.The decrease of the open porosity and the permeabilitywith exposure time extending and temperature increasing can be controlled within a certain range.

Effect of postweld heat treatment on interface microstructure and metallurgical properties of explosively welded bronze–carbon steel

The effects of postweld heat treatment on the microstructure and metallurgical properties of a bronze–carbon steel(st37)explosively bonded interface were studied.Explosive welding was done under 1.5-and 2-mm standoff distances and different conditions of explosive charge.Samples were postweld heat treated for 4 and 16 h in the furnace at 250°C and 500°C and then air cooled.Laboratory studies using optical microscopy,scanning electron microscopy,and microhardness testing were used to evaluate the welded samples.Microstructural examinations showed that by increasing the standoff distance and the explosive charge,the interface of bronze to steel became wavier.The microhardness test result showed that the hardness of the samples was higher near the joint interface compared with other areas because of the intensive plastic deformation,which was caused by the explosion force.The results show that increasing the heat treatment temperature and time caused the intermetallic compounds’layer thickness to increase,and,because of the higher diffusion of copper and tin,the iron amount in the intermetallic compounds decreased.Also,because of the increase in heat treatment temperature and time,internal stresses were released,and the interface hardness decreased.

Structure and properties of diffusive titanium nitride layers produced by hybrid method on AZ91D magnesium alloy

A diffusive titanium nitride layers were produced on AZ91 D magnesium alloy by a hybrid method which combines the titanium coating physical vapor deposition（PVD） with subsequent nitriding. The layers are composed of a thin surface zone of titanium nitrides and oxides, an intermediate titanium zone and Ti-Al-Mg type diffusive zone. Since the layers improve the corrosion resistance, the best corrosion resistance is obtained when the layers are produced on aluminum base sub-layer by oxynitriding.

Scope for improved properties of dissimilar joints of ferrous and non-ferrous metals

Dissimilar joints(DSJs)of ferrous and non-ferrous metals have huge technological importance in the frontiers of newdesigns in new machineries and improved design of conventional systems.This investigation was undertaken to improve mechanicalproperties of joints of two dissimilar metals:one is Ti-based and the other is Fe-based.DSJs were processed using bonding pressurefrom1to9MPa in step of2MPa at750°C for60min.Properties of the DSJs of these two metals using different mechanisms andmethods were compared with the present research for verification.Experimental results from the diffusion bonding mechanism forjoining the dissimilar metals validated the improvement in properties.Superior mechanical properties of dissimilar-metals joints wereachieved mainly due to the third non-ferrous metallic foil,Ni of^200-?m thickness,which avoided the formation of brittleFe-Ti-based intermetallics in the diffusion zone.DSJs processed are able to achieve maximum strength of^560MPa along withsubstantial ductility of^11.9%,which is the best ever reported in the literatures so far.Work hardening effect was detected in theDSJs when the bonding was processed at5MPa and above.Bulging ratio of the non-ferrous metal(Ti-based)was much higher thanthat of the ferrous metal(SS)of the DSJs processed.SEM analysis was carried out to know the details of reaction zone,while XRDwas carried out to support the SEM results.Reasons for change in mechanical,physical,and fracture properties of the DSJs with theprocess parameter variations were clarified.