造孔剂法制备泡沫钛的研究现状与进展

泡沫钛融合了泡沫结构与金属钛的双重属性,具有出色的力学性能、优异的耐腐蚀性和良好的生物相容性等优点,在航空、航天、海洋工程、生物医学、能源与环保等领域应用前景广阔。基于粉末冶金技术的造孔剂法是目前制备泡沫钛的主流方法,不仅具有操作简单、设备要求低的优点,而且能通过调整造孔剂参数来控制最终制品的结构与性能。本文综述了造孔剂法制备泡沫钛领域的研究现状与进展,通过分析文献、整理数据,讨论了高孔隙率泡沫钛的研究历程和瓶颈问题,指出了泡沫钛孔隙率研究的发展趋势。     

Processing and Characterization of Dual Phase Steel Foams Featured by Different Pore Distribution

Porous materials featuring cellular structures are known to have many interesting combinations of physical and mechanical properties. Some of them have been extensively used in structural applications (i.e. balsa wood), as well as in functional applications (heat exchangers, filters, etc.). Steel foams present promising theoretical properties for both functional and structural applications, but processing such kind of foams is complex due to their high melting temperature. Starting from a technique based on molten metal infiltration into a ceramic space holder, a new process is presented here for open‐cell steel sponges processing. Using a SiC cellular preforms as a space holder, dual phase steels foams with different porosity and steel microstructure were successfully developed. This technique is suitable to obtain steel alloy sponges featured by a relative density equal to 0,6 and interconnected pores. The compression tests indicate that the resulting material features the typical stress‐strain behaviour of classical cellular metals. Moreover mechanical properties, such as Elastic Modulus, σ_plateau, ε_{densification} and E_absorbed, depended on porosity and on martensite fraction, which is a function of the applied intercritical temperature.     

MAX phase foams produced via powder metallurgy process using water soluble space holder

Abstract

There have been few systematic studies of the fabrication and properties of porous MAX phase microstructures despite the potential applications of these materials. A simple, low cost, eco-friendly PM method has been developed to prepare MAX phase foams from commercial Ti₂AlC powder, using crystalline carbohydrate as space holder. This method involves: mixing Ti₂AlC powder with crystalline carbohydrate, pressing to form a green body, removal of the space holder and sintering. Compaction was achieved by uniaxial pressing (UP) and cold isostatic pressing (CIP). Control of porosity was achieved by varying the particle size (three ranges between 250 and 1000?μm), and volume fractions (20, 40 and 60%) of space holder. The foams were characterised and the porosity compared with the expected values. Optimal conditions for this novel processing technique were established with the aim of controlling the final microstructures and properties of the Ti₂AlC foams.     

A Facile Technique to Produce Open Cell Titanium Foams

Nature prefers open cell order in most structures such as trees and skeletons. Engineers have been imitating the nature, to obtain high specific strength, to transfer heat, energy absorbance and filtering for various applications such as filters, heat exchangers, impact absorbers, bone implants. Although ceramic foams have been produced and used as heat shields and filters, recently a great effort has been put on metallic foams produced from aluminium, copper, iron, stainless steel, nickel and titanium. Studies on open cell structured titanium foams are steps forward among these materials due its good biocompatibility and high specific strength. In this study, open cell titanium foams are produced utilizing polymer impregnation process accompanied by a facile sintering method. In traditional foam making, long sintering durations reduce the efficiency as far as cost and delivery time for producers. In order to overcome these problems, an alternative solution is made in the production of open cell titanium foams. A facile method is designed to sinter the impregnated polymers by using induction heating. Titanium foams with open cell structures are successfully produced. SEM, XRD, metallographic characterizations are performed. Average hardness value is calculated as 815.093 ± 6.59 Hv0.1.     

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。     

Effect of spacer type and cold compaction pressure on structural and mechanical properties of porous titanium scaffold

Abstract

Cell shape plays a crucial role on mechanical properties of titanium foam as scaffold in bone tissue engineering. In the present research, titanium foam was prepared using space holder technique. Sodium chloride and ammonium bicarbonate were utilised as spacer agent separately. The effect of cold compaction pressure and spacer agent type on the cell morphology was investigated by scanning electron microscopy (SEM) and optical stereo microscopy. Image analysing technique was performed to evaluate the microscopic images quantitatively. Exact salt leaching time was introduced by a new approach using electrical conductivity measurement. True and apparent porosities and compressive mechanical properties of the synthesised foams were evaluated. Finally, the superior spacer agent and appropriate cold compaction pressure were determined. It was shown that sodium chloride, due to maintaining its morphology during cold compaction pressure and absence of chemical side effects on titanium, is the superior spacer agent.     

Relationship Between Porosity and Spacer Content of Open Cell Metal Foams

Metal foams with high melting points are generally fabricated by the space holder technique. Foams with desirable pore size, shape and distribution can be obtained by using appropriate space holder. However, porosity (P) is always not equal to spacer content (ϕmac), which makes it difficult to obtain the expected porosity. In this study, a model equation for describing the relationship between P and ϕmac was established based on theory, i.e., P = (a1ϕmac + a2)/(a3ϕmac + a4). Each coefficient ai (i.e., a1, a2, a3, a4) could be determined by the porosity of cell walls (Pcell wall) and the volume change rate of macropores (Rmac). The theoretical equation was in accordance with the experimental results by authors and in the studies. It implies that Pcell wall and Rmac can be considered as constants in one certain production condition.     

Enhanced Sintering of TiNi Shape Memory Foams under Mg Vapor Atmosphere

TiNi alloy foams are promising candidates for biomaterials to be used as artificial orthopedic implant materials for bone replacement applications in biomedical sector. However, certain problems exist in their processing routes, such as formation of unwanted secondary intermetallic phases leading to brittleness and deterioration of shape memory and superelasticity characteristics; and the contamination during processing resulting in oxides and carbonitrides which affect mechanical properties negatively. Moreover, the eutectic reaction present in Ti-Ni binary system at 1391?K (1118?°C) prevents employment of higher sintering temperatures (and higher mechanical properties) even when equiatomic prealloyed powders are used because of Ni enrichment of TiNi matrix as a result of oxidation. It is essential to prevent oxidation of TiNi powders during processing for high-temperature (>1391?K i.e., 1118?°C) sintering practices. In the current study, magnesium powders were used as space holder material to produce TiNi foams with the porosities in the range of 40 to 65?pct. It has been found that magnesium prevents secondary phase formation and contamination. It also prevents liquid phase formation while enabling employment of higher sintering temperatures by two-step sintering processing: holding the sample at 1373?K (1100?°C) for 30?minutes, and subsequently sintering at temperatures higher than the eutectic temperature, 1391?K (1118?°C). By this procedure, magnesium may allow sintering up to temperatures close to the melting point of TiNi. TiNi foams produced with porosities in the range of 40 to 55?pct were found to be acceptable as implant materials in the light of their favorable mechanical properties.     

Microstructural design of cellular materials—II: Microsandwich foams

The linear elastic behaviour of foams is primarily controlled by bending of cell walls. The mechanical performance of foams can be improved by increasing the bending stiffness of the cell walls. Recently, our group has developedmicrosandwich foams in which the cell walls are microsandwich elements. The improved bending stiffness of the walls resulting from the sandwich microstructure increases their performance indices. Here, we describe a finite element analysis of the Young's moduli and Poisson's ratio of microsandwich foams. We then use the analysis to perform a parametric study of the way in which the performance of the microsandwich foams depends on microstructural parameters. Finally we compare the results of the analysis with data for three microsandwich foams. The analysis describes the data well.     

Hydroxyapatite fiber reinforced poly(alpha-hydroxy ester) foams for bone regeneration

A process has been developed to manufacture biodegradable composite foams of poly(DL-lactic-co-glycolic acid) (PLGA) and hydroxyapatite short fibers for use in bone regeneration. The processing technique allows the manufacture of three-dimensional foam scaffolds and involves the formation of a composite material consisting of a porogen material (either gelatin microspheres or salt particles) and hydroxyapatite short fibers embedded in a PLGA matrix. After the porogen is leached out, an open-cell composite foam remains which has a pore size and morphology defined by the porogen. By changing the weight fraction of the leachable component it was possible to produce composite foams with controlled porosities ranging from 0.47 +/- 0.02 to 0.85 +/- 0.01 (n = 3). Up to a polymer:fiber ratio of 7:6, short hydroxyapatite fibers served to reinforce low-porosity PLGA foams manufactured using gelatin microspheres as a porogen. Foams with a compressive yield strength up to 2.82 +/- 0.63 MPa (n = 3) and a porosity of 0.47 +/- 0.02 (n = 3) were manufactured using a polymer:fiber weight ratio of 7:6. In contrast, high-porosity composite foams (up to 0.81 +/- 0.02, n = 3) suitable for cell seeding were not reinforced by the introduction of increasing quantities of hydroxyapatite short fibers. We were therefore able to manufacture high-porosity foams which may be seeded with cells but which have minimal compressive yield strength, or low porosity foams with enhanced osteoconductivity and compressive yield strength.     

Quantitative assessment of microstructure and its effects on compression behavior of aluminum foams via high-resolution synchrotron X-ray tomography

Synchrotron X-ray microtomography has been used for the three-dimensional characterization of microstructure in the cell walls of aluminum foams. A combination of high-resolution phase contrast imaging technique and several application techniques has enabled the quantitative image analyses of microstructures as well as the assessment of their effects on deformation behaviors. The application techniques include local area tomography, microstructural gauging and in-situ observation using a specially designed material test rig. It has been clarified that ductile buckling of a cell wall occurs regardless of any of the microstructural factors in the case of a pure aluminum foam, while rather brittle fracture of a cell wall is induced by the existence of coarse micropores and their distribution independently of the intermetallic particles and the grain boundary in the case of aluminum foams alloyed with Zn and Mg. It has also been confirmed that coarse TiH₂ particles, which are a residual foaming agent added to alloy melts, remain intact during the deformation. When cooling rate during foaming is high, however, lower energy absorption might be attributable to the significant amount of residual TiH₂ particle and its inhomogeneous distribution. These tendencies are also confirmed by three-dimensional strain mapping by tracking internal microstructural features.     

Mathematical Model to Estimate the Rate Parameters and Thermal Efficiency for the Reduction of Iron Ore–Coal Composite Pellets in Multi-layer Bed at Rotary Hearth Furnace

Aluminium foams have become popular because of their properties such as high stiffness combined with very low density. The aluminium foams are being used in many applications like automobiles, railways, aerospace, ship building, household applications etc. The development of foam with consistent quality and study of foam structure–property relation is important for both scientific and industrial applications. Metallic foams are commonly produced using hydride and carbonates foaming agents. However carbonate foaming agents are safer to handle than hydrides and produce aluminum foam with a fine, homogenous cell structure, low cost and easily available. The number of pores per inch and relative density of the foam play an important role on their physical and mechanical properties. Hence it is very important to investigate effect of grain size of calcium carbonate foaming agent on pores per inch and relative density. The present work deals with the effect of grain size of the calcium carbonate forming agent on the physical properties of an eutectic Al–Si alloy closed cell foam. The foam was produced with different grain size of calcium carbonate (150, 106, 75, 53 µm) as a foaming agent. The pores per inch and density of the foam produced with different grain size of calcium carbonates as foaming agent are determined. Relative density is in the range of 0.21–0.34, pores per inch is in the range of 11–20 for the produced eutectic Al–Si alloy closed cell foam. It is observed that as grain size of calcium carbonate used for production of aluminium foam increases, the number of pores per inch decreases, relative density decreases and porosity increases.     

NaCl造孔剂添加量对多孔NiTi合金孔结构和力学性能的影响

选用Na Cl作为造孔剂,采用压制+烧结法制备孔结构和弹性模量可控的多孔Ni Ti形状记忆合金,采用SEM,XRD和形状回复率检测等测试手段研究造孔剂添加量对Ni Ti形状记忆合金的孔结构和力学性能的影响。结果表明:随Na Cl添加量增加,多孔体孔隙率从39%上升到72%,孔径大于50μm的孔隙数量明显增加;多孔体主要由Ni Ti奥氏体相(B2)和马氏体相(B19′)组成,并存在少量Ni Ti2,Ni3Ti和Ni4Ti3等相;合金的弹性模量随造孔剂的添加从30%时的10.8 GPa下降到70%时的1.5 GPa;当添加量为50%时,多孔体孔隙分布均匀,大于50μm的孔隙占45%,弹性模量为4.8 GPa,形状回复率达到最高值83%,最适合多孔植入体的要求。     

Synthesis of Biomimetic Superhydrophobic Surface through Electrochemical Deposition on Porous Alumina

The superhydrophobicity of plant leaves is a benefit of the hierarchical structures of their surfaces. These structures have been imitated in the creation of synthetic surfaces. In this paper, a novel process for fabrication of biomimetic hierarchical structures by electrochemical deposition of a metal on porous alumina is described. An aluminum specimen was anodically oxidized to obtain a porous alumina template, which was used as an electrode to fabricate a surface with micro structures through electrochemical deposition of a metal such as nickel and copper after the enlargement of pores. Astonishingly, a hierarchical structure with nanometer pillars and micrometer clusters was synthesized in the pores of the template. The nanometer pillars were determined by the nanometer pores. The formation of micrometer clusters was related to the thin walls of the pores and the crystallization of the metal on a flat surface. From the as-prepared biomimetic surfaces, lotus-leaf-like superhydrophobic surfaces with nickel and copper deposition were achieved.     

Fabrication of Al-3.7?Pct Si-0.18?Pct Mg Foam Strengthened by AlN Particle Dispersion and its Compressive Properties

Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al₃Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al₃Ti particles is a possible cause of this smooth plateau stress region.     

Processing and Pore Growth Mechanisms in Aluminum Gasarites Produced by Thermal Decomposition

Gasarites are a subclass of metallic foams that have a cylindrical pore morphology created by directional solidification of metals saturated with a gas. Thermal decomposition is an alternative process in which the soluble gas is delivered by decomposition of a particulate gas source. Aluminum gasarites formed through decomposition of titanium and zirconium hydrides were studied to both replicate the results of a previous study and discern pore-formation mechanisms. Replication of the previous study was not achieved, and additional processing enhancements were required to produce gasarite pore morphologies. For the first time, zirconium hydride was utilized to produce gasarites, with porosity levels and pore sizes lower than that from titanium hydride. Maximum average porosity levels of 10 and 6 pct were observed for titanium hydride and zirconium hydride, respectively. Pore-formation mechanisms in aluminum gasarite foams created via thermal decomposition of titanium and zirconium hydrides were evaluated through metallographic analysis and scanning electron microscopy. Definitive evidence of gas–metal eutectic pore growth was not found, but pore morphological characteristics and chemical analysis of particulate at pore surfaces support direct gas evolution from the hydride particles as a contributor to pore formation and growth.     

Microstructural and Mechanical Characterization of Two Aluminium Based In Situ Composite Foams

In situ composites are multiphase materials where the reinforcing phase is synthesized within the matrix during composite fabrication. The present paper deals with the processing, microstructural and mechanical characterization of Al?C7Si?C0.3Mg?C10TiB₂ and Al?C4Cu?C10TiB₂ foams. Composite foams with very low relative density (??_r?=?0.17?C0.37) and foams containing uniform cell sizes were successfully processed. Since the TiB₂ particle sizes are less than 2???m and have a good wetting behaviour, TiB₂ can be very good foam stabilizers. Microstructural characterization of the cell walls showed significant grain refinement since TiB₂ is a grain refiner. Elemental mapping clearly showed TiB₂ particles at inter dendritic boundaries. Compression testing of the processed foams showed some interesting features. Stress?Cstrain curve showed a lot of serrations which indicated brittle fracture of the cell walls and edges. Hence, it is observed that a balance should be attained between the grain refinement of ??-Al grains and the amount of TiB₂ particles to obtain desirable mechanical properties. Energy absorbed by the processed foams was calculated and they were observed to be close to that of the commercially available ALPORAS foams.     

A New Approach for Using Interlayer and Analysis of the Friction Welding of Titanium to Stainless Steel

Dissimilar material joining is often more difficult than joining the similar material or alloys with minor differences in physical properties and composition. The formation of deleterious intermediate phases consisting of intermetallic compounds during welding of titanium and stainless steel is a challenge to the welding processes, for decades. Friction welding has been used in an attempt to reduce formation of intermetallic compounds through inserting an interlayer material. In recent years, a number of approaches have been developed to insert interlayer between the substrates to avoid the metallurgical incompatibility. In this research, dissimilar joint of titanium and stainless steel were welded effectively using a new technique of electrodeposited nickel coating on one of the substrate (stainless steel) as interlayer. The bonding interface of the joints was characterized using optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Tensile strength of the nickel interlayer joints was higher than the direct joints. The microstructural characterization in the interface of titanium and stainless steel showed the absence of brittle Fe–Ti intermetallic compounds, which was a condition attributed to the use of interlayer technique. Whereas, the characterization of interface were identified as the presence of Ti–Ni phases which were more plastic than Fe–Ti intermetallic compounds.     

Fabrication of Aluminum Foams with Small Pore Size by Melt Foaming Method

This article introduces an improvement to the fabrication of aluminum foams with small pore size by melt foaming method. Before added to the melt, the foaming agent (titanium hydride) was pretreated in two steps. It firstly went through the traditional pre-oxidation treatment, which delayed the decomposition of titanium hydride and made sure the dispersion stage was controllable. Then such pre-oxidized titanium hydride powder was mixed with copper powder in a planetary ball mill. This treatment can not only increase the number of foaming agent particles and make them easier to disperse in the melt, which helps to increase the number of pores, but also reduce the amount of hydrogen released in the foaming stage. Therefore, the pore size could be decreased. Using such a ball-milled foaming agent in melt foaming method, aluminum foams with small pore size (average size of 1.6 mm) were successfully fabricated.     

Structure and Properties of Detonation Coatings Based on γ-TiAl

Investigations of a coating ― substrate composite before and after oxidation in air at 900°C revealed that the main structural features were: formation of an Al₂O₃ scale on the surface of the TiAlCrSc(γ) coating (as a result of oxidation) and of inner layer at its interface with a 90% titanium substrate (as a result of diffusion in the composite). The observed phenomenon was caused by a Kirkendall effect resulting in the formation of titanium-enriched phases, apparently Ti₃Al and α-Ti, which have a broad homogeneity range. The formation of a diffusion zone in the system with displacement of the Kirkendall plane in the direction of the substrate has a positive effect on the adherence of the coating. Furthermore, the filling of vacancies and pores in the coating with additional material supplied by the diffusion of titanium should have a favorable effect on the strength and durability of the coating, particularly its fatigue resistance.