A356铸造铝合金的疲劳裂纹萌生及短裂纹扩展研究

研究了A356-T6铸造铝合金的缺口疲劳裂纹萌生与早期扩展行为及机制.结果表明,热等静压试样的疲劳抗力优于非热等静压试样.对于钝缺口试样,疲劳裂纹萌生于缺口根部附近的多个平面,最终哪个裂纹源扩展成主裂纹取决于局部微观组织.对于缺口几何形状不同的热等静压和非热等静压疲劳试样,在疲劳过程中,不管是在高应力状态下,还是在低应力状态下,都出现了铝基体的循环塑性变形和共晶硅粒子断裂导致疲劳裂纹萌生.对于非热等静压试样,铸造缩孔在构件的疲劳过程中起着重要作用,但即使缺口根部存在较大尺度的铸造缩孔,导致了疲劳裂纹萌生,但也同时观察到疲劳裂纹从共晶硅粒子、金属间化合物、铝基体的滑移带和铁基金属间化合物等处萌生.对于脆性的A356铸造铝合金可采用修正的断裂力学参量ΔKn、局部应力范围Δσ或局部应变幅Δε/2作为控制参量来表征疲劳裂纹萌生行为,而缺口有效应力强度因子范围ΔKneff和ΔJs参量可用来表征缺口场中短裂纹扩展行为.     

热等静压扩散连接CuAgZn/GH909接头组织及力学性能

通过热等静压-扩散连接工艺直接连接CuAgZn和GH909,利用扫描电镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)观察接头的显微组织和成分,并通过测试显微硬度和剪切强度研究接头的力学性能.结果表明,CuAgZn/GH909结合界面紧密完整,接头密实、均匀,无未连接的缺陷.接头的最大显微硬度为HV 443,高于...     

TC11钛合金粉末涡轮盘热等静压成形数值模拟研究

为了成形复杂结构的涡轮盘零件,采用粉末热等静压工艺(HIP),设计了粉末净成形和粉-固扩散连接成形两种方案,使用MSC.MARC软件,在Shima模型中嵌入TC11钛合金材料参数进行数值模拟研究。模拟结果显示:粉末净成形的模具变形是粉-固扩散连接成形的2倍左右;粉末净成形的粉末体在热等静压后,关键部位出现明显的密度梯度分布,相对密度较低,粉-固扩散连接成形的相对密度分布的均匀性优于粉末净成形,所以粉-固扩散连接成形方案优于粉末净成形方案。试验结果显示:热等静压工艺可以实现闭式涡轮盘的净成形。     

Hot isostatic pressing of plasma sprayed Ni-base alloys

This article reports the effects of hot isostatic pressing (HIPing) on the microstructure and properties of plasma sprayed Ni-based alloy coatings. Hot isostatic pressing was used as a post- spray treatment on plasma sprayed Ni-5Al, Ni-20Al, and NiCrAl coatings. The aim was to densify the coatings and modify physical properties such as strength, amount of porosity, and hardness. The coatings were HIPed at 750 to 950 ‡C at pressures of 50 to 200 MPa and held for 1 h. The treated coatings were examined by optical microscopy and scanning electron microscopy (SEM). Coating porosity was determined using a combination of an image analyzer and SEM. Near- zero porosity levels could be obtained, and HIP treatment at increasing temperatures and pressures changed the microstructure and increased the microhardness of the coatings. Mechanical testing of the coatings was performed on a Dynamic Mechanical Analyzer (DMA) from ambient to — 1000 ‡C. The results showed that the elastic modulus of HIPed coatings was greater than as-sprayed coatings up to — 750 ‡C. These changes can be related to plastic flow, interlamellar diffusion, and creep that occur at increased temperatures and pressures.     

The effect of hot isostatic pressing on cast aluminum

Aluminum-silicon alloys are the most important commercial aluminum casting alloys, primarily because of their superior casting characteristics. This article discusses the effect of hot isostatic pressing on the mechanical properties of cast aluminum alloy A356. The effect of low-cost Densal hotisostatic pressing is also examined. C.S.C. Lei earned his Ph.D. in mechanical engineering/solid mechanics at Drexel University in 1987. He is currently a visiting scientist at the Naval Air Warfare Center, Aircraft Division. W.E. Frazier earned his Ph.D. in materials engineering at Drexel University in 1987. He is currently the head of metal, ceramics, and nondestructive evaluation at Naval Air Warfare Center, Aircraft Division. Dr. Frazier is also a member of TMS. E.W. Lee earned his Ph.D. in materials engineering at the Georgia Institute of Technology in 1982. He is currently propulsion materials team leader at Naval Air Warfare Center, Aircraft Division.     

Effects of solidification structure on fatigue crack initiation and fatigue strength in Al−Si−Mg cast alloys

Rotating-bending uniaxial fatigue tests and micro-fatigue crack initiation tests were carried out using a permanent mold cast (PMC) and semi-solid die cast (SDC) with Al−7%Si−0.35%Mg composition in order to examine the relationship between solidification structures and fatigue behaviors. The crack length was measured using a replication method. Fatigue strength was improved in SDC, which was almost consistent with the predicted fatigue strength using the size of Si particle cluster. Resistance to fatigue crack initiation and fatigue strength were improved in SDC owing to the finer Si cluster and to higher ultimate tensile strength. Fatigue crack in PMC was preferentially initiated at pores. For SDC, the fatigue crack was initiated at the Si particle/matrix interface, and then sucessively grew along eutectic cell boundaries.     

预烧结温度对热等静压法制备Co基金刚石串珠性能的影响

采用合适的工艺流程及参数,将热等静压(Hot Isostatic Pressing,HIP)技术应用于Co基金刚石串珠制备。分析了HIP后胎体性能的变化,并通过扫描电镜(Scanning Electron Microscope,SEM)进行断面微观分析。结果表明:相比较热压烧结,HIP更有助于改善Co基胎体各项性能,对于实验中各组Co基胎体,750℃预烧结胎体的性能提高最多,其中胎体抗弯强度提高了25.2%,胎体对钢基体的夹持力提高了37.9%。显然,HIP可用于Co基金刚石串珠的制备,而预烧结温度对HIP后胎体性能有着重要影响,在合适的预烧结温度下可以获得最佳性能的胎体。     

基于Ti-6Al-4V的热等静压与温压成形数值模拟对比研究

为对比研究钛合金的热等静压和温压成形工艺特点,以典型圆柱件成形为例,采用MSC.MARC软件,选取了Shima模型嵌入Ti-6Al-4V材料模型进行数值模拟.通过分析两种工艺过程中粉末体的流动情况和致密度分布,得出粉末体的热等静压是一个先向外膨胀再向中心收缩,最终达到全致密的过程,致密度分布均匀;而温压成形是沿着冲头方向直接进行收缩,致密度不高并存在明显的梯度现象.通过两种成形特点的对比,能分别预测成形方法的变形以及指导两种成形方法的选取.     

热等静压处理ZTC4钛合金低温热处理过程中组织演变

研究了低温热处理后热等静压处理的ZTC4的组织演变规律。利用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)分析了不同尺度的组织特征。研究结果表明:原始组织由粗大的β晶粒和不同取向的α、β片层组成;温度由100℃升高到300℃,组织形态基本没有变化;当热处理温度在400℃时,α、β片层的形态由均匀变为不均匀。元素的微区扩散是造成这种现象的主要原因。     

大尺寸TiB2-Cu-Ni金属陶瓷块体的燃烧合成及准热等静压致密化

利用燃烧合成结合准热等静压 (PHIP)技术成功制备了直径为 2 40mm的大尺寸TiB2 Cu Ni金属陶瓷 ,对合成产物进行了X射线衍射 (XRD)、扫描电镜 (SEM)和力学性能等实验研究。XRD结果表明反应产物中只有TiB2 和 (Cu ,Ni)两相。SEM分析发现增强相TiB2 陶瓷颗粒已经形成骨架状 ,其间为 (Cu ,Ni)基体相 ,TiB2 颗粒形貌为近等轴状 ,尺寸细小且较均匀。合成产物的致密度为 94.2 % ,弯曲强度和断裂韧性分别达到 5 99.4MPa和12 .5 6MPa·m1/ 2 。     

热等静压及后续热处理对MIM418涡轮合金组织与性能的影响

比较了热等静压前后MIM418合金组织与致密度的变化,研究了不同淬火(水冷和空冷)、时效工艺对热等静压后固溶态MIM418合金微观组织及各种析出相的影响,利用扫描电镜、透射电镜、能谱分析等手段分析了合金中主要强化相γ'和碳化物二者的形态、大小和分布的规律。结果表明,热等静压可明显提高MIM418合金的致密度,但对显微组织影响不大,"固溶+中温时效"可以获得较好的硬化效果,主要析出强化相γ'和二次碳化物的析出受固溶冷却方式的影响较大,γ'相纳米化和二次碳化物的析出或将有益于MIM418硬度的大幅度提高。     

A7N01S-T5铝合金焊接热影响区腐蚀疲劳裂纹萌生行为研究

以高速列车常用的A7N01S-T5铝合金型材焊接热影响区为研究对象,分析析出相的SEM组织及化学成分,采用超景深显微镜观察不同时间节点腐蚀疲劳裂纹萌生过程,并统计分析点蚀坑的面积及最大径长。结果表明,热影响区的腐蚀面积率、腐蚀速率、蚀坑最大径长均随着试验时间的增加而增加,腐蚀坑主要沿着挤压方向分布的大颗粒、富含Fe的二相粒子周围萌生,在交变应力的促进作用下,热影响区腐蚀疲劳裂纹在腐蚀坑最大径向方向沿着挤压方向萌生、扩展。     

热等静压处理温度对一种高性能粉末高温合金组织和性能的影响

采用光学显微镜、扫描电镜和拉伸试验等研究了不同温度热等静压处理(HIP)对一种高性能粉末高温合金显微组织和力学性能的影响。结果表明:在γ′相溶解温度以下进行热等静压处理,合金中存在较多的原始粉末颗粒边界(PPB)组织,并且含有大量的残留枝晶,热处理后PPB不能完全消除,以至于影响合金的室温和高温塑性;在γ′相溶解温度以上进行热等静压处理,基本消除了PPB组织,残留枝晶含量较少,热处理后得到完全再结晶组织,不含有PPB组织,同时晶粒并未快速长大。     

高速热模锻模具热疲劳裂纹萌生期预测

在考虑高速热模锻凹模的材料性能和工况条件基础上，由Ｃｏｆｆｉｎ－Ｍａｎｓｉｏｎ公式导出了热疲劳裂纹萌生期预测公式，并采用这一公式对高速热模锻凹模进行了热疲劳裂纹萌生期的预测。通过试验发现预测结果与试验结果基本吻合。     

Effects of hot isostatic pressing temperature on casting shrinkage densification and microstructure of Ti6Al4V alloy

The Ti6Al4V alloy castings were produced by the investment casting process, and the hot isostatic pressing(HIP) was used to remove shrinkage from castings. The processing pressure and holding time for HIP were 150 MPa and 20 min, respectively. Four different HIP temperatures were tested, including 750 ℃, 850 ℃, 920 ℃ and 950 ℃. To evaluate the effects of temperature on densification and microstructure of Ti6Al4V alloy treated by HIP, non-destructive testing and metallographic observation was performed. The experimental results show that the shrinkage was completely closed at 920 ℃ and 950 ℃. The densification of Ti6Al4V alloy increased as the HIP temperature increased below 920 ℃. The lamel ae were more uniform, the thickness of lamel ae was obviously broadened and the structure was coarsen. Besides, the Norton creep equation was used to simulate the effect of different temperatures on the densification of Ti6Al4V alloy during HIP. The simulation results were in good agreement with the experimental results. It was also found that 920 ℃ is a suitable temperature for HIP for Ti6Al4V alloy.     

Microstructure and mechanical properties of hot isostatically pressed cermets with TiN coatings

To increase the adhesion strength between the coating and the substrate, sintered Ti（C,N）-based cermets were selected and deposited with monolayer TiN using a multiarc ion-plating technique; subsequently, hot isostatic pressing （HIPhag） treatment was performed at 1000℃ using nitrogen pressure up to 110 MPa. The mechanical properties of cermets after a coating process and subsequent HIPing treatment have been evaluated with respect to the hardness, the residual stress, and the coating adhesion. The results show that atter the HIPing process, there was a higher increase ha critical load ha the TiN-coated cermets with lower surface roughness compared with those with higher surface roughness. In all cases, the residual stress was found to be compressive. The effects of substrate surface roughness and posttreatment on the adhesion strength of the coatings were thus investigated. It was also fotmd that the HIPing posttreatment process is well suited for hacreasing the adhesion strength between the coating and the substrate.     

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.     

Using equal-channel angular pressing for the production of superplastic aluminum and magnesium alloys

Equal-channel angular pressing (ECAP) is a useful tool for achieving exceptional grain refinement in bulk metallic alloys. Typically, the grain sizes produced through ECAP are in the submicrometer range, and thus they are smaller by up to an order of magnitude than the grain sizes attained through typical thermomechanical treatments. As a consequence of these ultrafine grains, the as-pressed alloys may exhibit superplastic ductilities at faster strain rates than in conventional superplastic alloys. This work initially describes the application of ECAP to two different alloys. First, results are presented for a commercial Al-2024 alloy where this alloy was selected because it contains no minor additions of either zirconium or scandium to assist in restricting grain growth. The results show that superplasticity is achieved through the use of ECAP. Second, results are described for a Mg-0.6%Zr alloy where this alloy was selected because it is the optimum composition for achieving a high damping capacity. Again, processing by ECAP produces superplastic ductilities not attained in the cast alloy. The second part of this work demonstrates that processing by ECAP may be extended from conventional rod or bar samples to samples in the form of plates. This is a very attractive feature for industrial superplastic forming applications. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

热压烧结对GdSiGeDy合金室温磁制冷性能的影响

用机械合金化制备的GdSiGeDy系合金粉末作为真空热压烧结的原料,研究了真空热压烧结温度、烧结时间及升温速率对烧结体磁制冷性能的影响,并用VSM测出的数据作了磁熵变曲线图.经实验研究后得出结论:升温速率对磁熵变的影响较大,且随升温速率减小,最大磁熵变增加;随着烧结温度的提高,样品的最大磁熵变是先增加,后减小,到达1050 ℃时,样品失去铁磁性.