Effects of Pore Geometry on the Fatigue Properties of Electron Beam Melted Titanium-6Al-4V

Current percent-porosity based quantification of pores in additively manufactured parts does not provide information about the size, shape, and distribution of pores throughout a build. Such information is necessary to understand the conditions under which the part was printed as well as its mechanical reliability. This research, through a combination of fatigue testing and microstructural characterization demonstrates a method by which the internal porosity can be characterized and using the knowledge of the pores differing formation mechanisms to inform future design and build strategies. Though the test bars were printed under nominally identical conditions, ignoring lack-of-fusion, batch 1 had 34 pct fewer lenticular pores and 147 pct more spherical pores than batch 2 which shows that the actual print conditions of these parts varied substantially as would their as-printed mechanical reliability. To quantify this difference extensive optical, SEM, and EBSD metallographic studies were conducted on several samples from these bars as well as the fracture surfaces to gain an understanding of the porosity’s shape, size, and location. The comparison of these datasets along with knowledge of the pore’s evolution allows for the optimization of future build strategies and the more accurate prediction of the resulting as-built mechanical properties.

     

Mechanics of sintering materials with bimodal pore distribution. II. Mean-square strain rate for the solid phase of a biporous body and instability of the monomodal pore structure during constrained sintering

Sintering of materials with a bimodal porous structure under conditions of external kinematic limitations that make their macroscopic deformation impossible is considered. It is established that in contrast to materials that contain pores of the same size, with sintering of materials that have a bimodal pore distribution under the conditions indicated above there is deformation within the limits of the matrix material. It is demonstrated that a monomodal pore structure under the same conditions is not stable, in the sense that small disturbance of the uniform pore distribution with respect to size may lead to marked stratification during sintering. Tangential stresses that arise in the solid phase as a result of the existence of two types of pores are evaluated. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 36–44, January–February, 2006.     

Finite Element and Experimental Analysis of Closure and Contact Bonding of Pores During Hot Rolling of Steel

The closure and contact bonding behavior of internal pores in steel slabs during hot rolling was studied using experiments and the finite element method (FEM). Effects of pore size and shape were investigated, and three different cases of pore closure results were observed: no closure, partial closure, and full closure. The FEM results well reproduced various closure events. Bonding strengths of unsuccessfully closed pores, measured by tensile tests, showed critical effects. Also, there was a difference in bonding strengths of several fully closed pores. Fracture surfaces showed that welded regions could be divided into three (not, partially, and perfectly) welded regions. The pressure–time curves obtained from the FEM results indicate that pore surface contact time and deformed surface length are important parameters in pore welding. Pore size, pore shape, time of pressure contact, and deformed surface length should be considered to completely eliminate pores in final products.     

粉末烧结法制备开孔泡沫铝压缩性能的研究

采用粉末烧结工艺制备开孔泡沫铝并研究了其压缩性能，不同形态的尿素和氯化钠颗粒作为造孔剂使泡沫铝的孔隙度控制在70％。结果表明：粉末烧结法制备的泡沫铝呵以容易地控制孔隙度及孔径的大小，并且孔结构很好地保持了造孔剂的形状。不同的孔结构对泡沫铝的压缩性能具有显著影响，球形孔结构得到了最佳的压缩效果。     

Simulation of microporosity formation in modified and unmodified A356 alloy castings

In order to comprehensively model both the performance and inspectability of early design stage safety critical aluminum castings, the size, shape, and location of defects such as pores should be determined by simulation. In this article, a two-dimensional (2-D) model to predict grain size, pore size, pore morphology, and location is presented. The proposed model couples hydrogen gas evolution and microshrinkage pore formation mechanisms with a grain growth simulation model. The nucleation and growth of grains are modeled with a probabilistic method that uses the information from a macroscale heat transfer simulation to determine the rules of transition for grain evolution. Microshrinkage pores and the combination of microshrinkage and gas pores are addressed. The proposed model and postprocessing can provide direct simulated views of the microstructure of the solidifying casting. In the present work, the effect of Sr modifier and hydrogen content on pore size and morphology for equiaxed aluminum alloy A356 is modeled. The simulation results correlate well with the experimental observation of cast structures and other published data. In addition, Sievert’s law and the conditions for spontaneous growth of a gas pore are derived from first principles in the Appendix.     

Pore filling process in liquid phase sintering

Models for liquid flow into isolated pores during liquid phase sintering are described qualitatively. The grains are assumed to maintain an equilibrium shape determined by a balance between their tendency to become spherical and a negative capillary pressure in the liquid due to menisci at the specimen surface and the pore. With an increase of grain size, the grain sphering force decreases while the radius of liquid menisci increases to maintain the force equilibrium. When grain growth reaches a critical point, the liquid menisci around a pore become spherical and the driving force for filling the pore rapidly increases as liquid flows into it. The critical grain size required for filling a pore increases linearly with pore size. Experimentally, filling of isolated pores has been investigated in Fe-Cu powder mixture after liquid phase sintering treatment and after dipping into a molten matrix alloy. The observed pore filling behaviors agree with the qualitative predictions based on the models. In Fe-Cu alloy, pore filling is terminated by gas bubbles formed in liquid pockets. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.     

Modeling the mechanical behavior of tantalum

A crystal plasticity model is proposed to simulate the large plastic deformation and texture evolution in tantalum over a wide range of strain rates. In the model, a modification of the viscoplastic power law for slip and a Taylor interaction law for polycrystals are employed, which account for the effects of strain hardening, strain-rate hardening, and thermal softening. A series of uniaxial compression tests in tantalum at strain rates ranging from 10⁻³ to 10⁴ s⁻¹ were conducted and used to verify the model’s simulated stress-strain response. Initial and evolved deformation textures were also measured for comparison with predicted textures from the model. Applications of this crystal plasticity model are made to examine the effect of different initial crystallographic textures in tantalum subjected to uniaxial compression deformation or biaxial tensile deformation.     

Quantification and Taxonomy of Pores in Thermal Spray Coatings by Image Analysis and Stereology Approach

Porosity is one of the most important microstructural features in thermal spray coatings and has been actively studied and measured by many methods. Image analysis techniques have become popular techniques in determining porosity in coatings because of simplicity, accessibility, and an ability to measure both open and closed porosities as well as pore characteristics such as size, shape, orientation, and spatial distribution. In the current study, an image analysis technique has been complemented by several stereology procedures to determine the porosity level and characteristics of pores within coatings. Stereology protocols such as Delesse, DeHoff, and Cruz-Orive analyses were used to derive the porosity level, pore size, and shape distributions, and the effectiveness of each stereology protocol was compared. Standoff distance (SOD) and annealing process did not alter the distribution trend of number of pores but influenced the distribution of pore volume fractions significantly. The bivariate size–shape distribution of the pores was used to predict the dominant pore type and fractions of pores that arose from different formation mechanisms. It was found that nearly spherical pores that originated from gas bubbles and entrapped gas pockets dominate at shorter SOD, while the different types of pores become more evenly distributed when the SOD was increased.     

Deformation Behavior Estimation of Aluminum Foam by X-ray CT Image-based Finite Element Analysis

Aluminum foam is a lightweight material owing to the existence of a large number of internal pores. The compressive properties and deformation behavior of aluminum foam are considered to be directly affected by the shape and distribution of these pores. In this study, we performed image-based finite element (FE) analyses of aluminum foam using X-ray computed tomography (CT) images and investigated the possibility of predicting its deformation behavior by comparing the results of FE analyses with those of actual compressive tests. We found that it was possible to create an analytic model reflecting the three-dimensional (3D) pore structure using image-based modeling based on X-ray CT images. The stress distribution obtained from image-based FE analysis correctly indicates the layer where deformation first occurs as observed in actual compressive tests. Also, by calculating the mean stress of each plane perpendicular to the direction of compression based on the stress distribution obtained from image-based FE analysis, it was found that deformation begins in the layer containing the plane with maximum stress. It was thus possible to estimate the layer where deformation begins during the compression of aluminum foam.     

Parametric Studies on the Behavior of Reinforced Soil Retaining Walls under Earthquake Loading

The finite element procedures are extremely useful in gaining insights into the behavior of reinforced soil retaining walls. In this study, a validated finite element procedure was used for conducting a series of parametric studies on the behavior of reinforced soil walls under construction and subject to earthquake loading. The procedure utilized a nonlinear numerical algorithms that incorporated a generalized plasticity soil model and a bounding surface geosynthetic model. The reinforcement layouts, soil properties under monotonic and cyclic loadings, block interaction properties, and earthquake motions were among major variables of investigation. The performance of the wall was presented for the facing deformation and crest surface settlement, lateral earth pressure, tensile force in the reinforcement layers, and acceleration amplification. The effects of soil properties, earthquake motions, and reinforcement layouts are issues of major design concern under earthquake loading. The deformation, reinforcement force, and earth pressure increased drastically under earthquake loading compared to end of construction.     

Normalizing Behavior of Unsaturated Granular Pavement Materials

One of the important components of a flexible pavement structure is granular material layers. Unsaturated granular pavement materials (UGPMs) in these layers influence stresses and strains throughout the pavement structure, and have a large effect on asphalt concrete fatigue and pavement rutting (two of the primary failure mechanisms for flexible pavements). The behavior of UGPMs is dependent on water content, but this effect has been traditionally difficult to quantify using either empirical or mechanistic methods. This paper presents a practical mechanistic framework for determining the behavior of UGPMs within the range of water contents, densities, and stress states likely to be encountered under field conditions. Both soil suction and generated pore pressures are determined and compared to confinement under typical field loading conditions. The framework utilizes a simple soil suction model that has three density-independent parameters, and can be determined using conventional triaxial equipment that is available in many pavement engineering laboratories.     

热等静压对铸件致密化及组织演变机理的影响研究

研究了铸件制品的热等静压致密化机理,分析了铸件制品热等静压的致密化模型,综合应用合金的蠕变理论和粉末冶金的烧结理论,利用现有的物理和数学模型对铸件在热等静压过程中的内部孔洞类缺陷闭合过程机理进行了阐述。确认了热等静压过程中起主要致密化作用的孔洞形变机制,为选择制定合适的热等静压工艺参数提供了理论参考。     

SHPB加载作用下考虑节理分形的闭合变形性质研究

为了探讨动载荷作用下的节理闭合变形性质,用水泥砂浆制作出含节理的人工试样,并采用SHPB装置对具有不同分形（JRC）的含节理试样进行不同加载率作用下的冲击试验,获得了不同加载率下具有不同Barton标准 JRC值的节理动态闭合变形曲线。冲击后试样及其结果分析显示：随着加载率的增加,碎裂的块数明显增加,表现出较强的加载率相关性;节理闭合变形曲线的变化趋势基本不受加载率的影响,节理变形量δ均随着应力σ的增大而增大。在加载速率相同的情况下,不同节理形貌试样的节理法向变形量存在较大差异;具有相同节理的试样在不同加载率作用下的闭合变形曲线也是显著不同的;无论加载率多大,δ和σ之间均具有很强的指数关系,可用统一的公式表达。该研究成果为进一步开展动载荷作用下节理闭合变形本构方程研究提供了试验基础。     

The deformability of porous sintered materials in tension

Summary Formulas are derived for evaluating the deformation of porous sintered materials, taking into account the properties of the dense materials, the variation in the number of pores in various specimen cross sections, pore shape, and the loosening of materials during loading. The formulas obtained for the total strain, residual strain after fracture, and the limit of proportionality are compared with experimental data. Good agreement between experimental and calculated data was found.     

The use of microstructural gradients in hot gas-pressure forming of zn-ai sheet

The gas-pressure forming of Zn-22 pct Al sheet containing regions of different initial microstructure, introduced by selected-area heat treatment, at elevated temperature and low strain rates has been evaluated. The heat treatments used converted a fine, spheroidal, phase mixture into a lamellar one, and the fraction of transformed material could be controlled reasonably well. The mechanical response of the material, determined by tensile testing, was correlated with the microstructure and its evolution during deformation. The material behavior was formalized as a simple constitutive equation which was used in finite-element modeling of the forming process. It was possible to influence the thickness distribution of a formed shape in a controlled manner using this approach.     

Dynamic Analysis of Multilayered Soils to Water Waves and Flow

In nature, a soil profile generally consists of several heterogeneous layers. This study is aimed at discussing the interactive problem of oscillatory water waves and flow passing over multilayered soils. The soil behavior is considered as viscoelastic in the present mathematical model modified from Biot’s poroelastic theory. Employing this model, the dynamic response including the profiles of pore water pressure and effective stress in the multilayered soils is discussed. The results reveal that the perturbed pore pressure is different from that inside a single-layered soil where the thickness of the first soil layer is less than the water wavelength. The discrepancy of the vertical effective stresses between multilayered and single-layered soils is even much more apparent under the same conditions. Moreover, seepage force is examined and is found to be larger near the bed surface and the bottom of the first soil layer where soils are easily disturbed by external disturbance. The locations where soil failure might happen are found near the troughs of surface water waves.     

渗流作用下风化壳淋积型稀土矿细观孔隙结构演化特征

为研究风化壳淋积型稀土矿浸出过程中溶液渗流作用对孔隙结构的影响,以去离子水为溶浸液开展浸矿实验。对浸出前后矿样进行显微CT扫描,获取了试样内部结构图像,利用阈值分割算法得到了浸出前后稀土矿样的孔隙结构图像。进而,研究了溶液渗流作用下试样孔隙结构的变化特征,分析了渗流作用对试样孔隙率、孔隙体积、孔隙长度、孔隙宽度和孔隙方位角等参数的影响。结果表明:稀土矿孔隙形状和尺寸在渗流作用下发生显著变化,且在粗细颗粒接触区最为明显;溶液渗流作用使得稀土矿孔隙率增大,孔隙总数量减少,孔隙总体积增大。渗流作用下矿样中小孔隙数量减少,大孔隙数量增多,各尺寸区间的孔隙数量变化率随孔隙尺寸的增大呈现先增大后减小的趋势。溶液渗流作用下孔隙长宽比分布更加集中,孔隙方位角在各角度区间的分布更加均匀,孔隙各向异性增强。      

Analyzing Dynamic Behavior of Geosynthetic-Reinforced Soil Retaining Walls

An advanced generalized plasticity soil model and bounding surface geosynthetic model, in conjunction with a dynamic finite element procedure, are used to analyze the behavior of geosynthetic-reinforced soil retaining walls. The construction behavior of a full-scale wall is first analyzed followed by a series of five shaking table tests conducted in a centrifuge. The parameters for the sandy backfill soils are calibrated through the results of monotonic and cyclic triaxial tests. The wall facing deformations, strains in the geogrid reinforcement layers, lateral earth pressures acting at the facing blocks, and vertical stresses at the foundation are presented. In the centrifugal shaking table tests, the response of the walls subject to 20 cycles of sinusoidal wave having a frequency of 2 Hz and of acceleration amplitude of 0.2g are compared with the results of analysis. The acceleration in the backfill, strain in the geogrid layers, and facing deformation are computed and compared to the test results. The results of analysis for both static and dynamic tests compared reasonably well with the experimental results.     

The densification of powders by power-law creep during hot isostatic pressing

Hot isostatic pressing (HIP) is potentially a cost-effective and efficient process for the manufacture of high quality metal components from powders. The densification of the powder during HIP proceeds in stages marked by changes in the geometry of the pores and by the dominance of different densification mechanisms. When the density is high and the pores are isolated and roughly spherical, the densification rate under a compressive load due to creep can be approximated by the densification rate of a sphere of creeping material containing a single, centered void. The densification rates predicted by such a model are significantly increased by small deviations of the load from purely hydrostatic compression. Thus, a careful account of the coupling between the hydrostatic and deviatoric stresses is important in the accurate modelling of the process and the design of an efficient HIP cycle; this can be achieved through the introduction of an approximate strain rate potential for the porous body.