Pressureless Infiltration and Resulting Mechanical Properties of Al-AlN Preforms Fabricated by Selective Laser Sintering and Partial Nitridation

A novel manufacturing process has recently been developed for the fabrication of intricate Al-AlN composite parts. The process involves green shape formation by selective laser sintering, preform development by nitridation, and net shape forming by pressureless infiltration. The infiltration atmosphere has an important influence on the final fabrication and mechanical properties. This work presents a detailed investigation on the infiltration of Al-AlN preforms with AA 6061 at various temperatures above its liquidus under nitrogen, vacuum, and argon. The green shapes are formed by selective laser sintering of a premix of AA 6061-2Mg-1Sn-3Nylon (wt pct) powders. They are then partially nitrided to create a rigid, 2- to 3-μm-thick AlN skeleton for subsequent infiltration. Nitrogen infiltration results in the highest density (2.4 gcm⁻³) and best tensile properties (UTS: 214 MPa; elongation: 2.5 pct), while argon infiltration gives the lowest density. Fractographs confirmed the difference in density arising from the use of different atmospheres where small pores are evident on the fracture surfaces of both argon and vacuum-infiltrated samples. The molten AA 6061 infiltrant reacts with nitrogen during infiltration leading to a 5-μm-thick AlN skeleton compared to the original 2- to 3-μm-thick skeleton in both argon and vacuum-infiltrated samples. Transmission electron microscope (TEM) examination revealed inclusions of Mg₂Si and Mg₂Si _x Sn_1−x in both nitrogen- and argon-infiltrated samples but not in vacuum-infiltrated samples. Vacuum infiltration is slower than nitrogen and argon infiltration. The mechanisms that affect each infiltration process are discussed. Infiltration under nitrogen is preferred.     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Dry sliding wear behavior of A356-15 Pct SiC p composites under controlled atmospheric conditions

The present investigation was carried out to provide a deeper insight into the mechanism of wear behavior of A356-15 vol pct SiC _p composite under controlled argon and oxygen atmospheres through a detailed characterization of worn surfaces and subsurfaces. Dry sliding wear tests were performed for both as-cast and T6-treated specimens using a pin-on-disc machine with three sliding velocities (0.5, 1, and 2 ms⁻¹) and three loads (1, 2, and 3 MPa). The wear rate of A356-15 vol pct SiC _p composite was lower by nearly one order of magnitude under argon atmosphere compared to the specimens tested under oxygen atmosphere for all experimental conditions. Under argon atmosphere, the mechanism of material removal was by delamination wear and did not change within the parametric regime. In the case of the specimen tested under oxygen atmosphere, the wear behavior of the composite depended on the experimental conditions. At low load and low sliding velocity, the material removal was by abrasion. While at high load and high sliding velocity, the material removal mechanism was by delamination wear. Further, the mechanical mixed layer (MML) formed under argon atmosphere was more stable and homogenous compared to that formed under oxygen atmosphere. The MML formed under both atmospheres revealed much less in Fe content.     

Bond strengths of fluoride-releasing restorative materials

PURPOSE: To compare the shear bond strengths to dentin of four tooth-colored fluoride-releasing restorative materials that utilize different mechanisms of dentin-bonding. MATERIALS AND METHODS: Four materials were tested: a chemically-cured glass ionomer (Ketac-Fil); a light-cured resin-modified glass ionomer (Photac-Fil); a light-cured resin-modified glass ionomer in conjunction with a resin dentin-bonding agent (Vitremer); and a light-cured resin composite in conjunction with a resin dentin-bonding agent (Tetric). The enamel was removed from four sides of the twenty human noncarious extracted molars which had their roots embedded in acrylic blocks. Cylindrical samples of the materials were prepared in plastic molds and bonded to the dentin surface according to the manufacturers' instructions. After 24 hours of storage in a humidor, the samples were sheared with an Instron testing machine at a crosshead speed of 0.5 mm/minute. RESULTS: Statistical evaluation (ANOVA and Duncan's test) suggests that the fluoride-releasing resin composite material utilizing a resin dentin-bonding agent provided a significantly greater mean shear bond strength (16.5 MPa) after 24 hours storage than any of the selected glass ionomer materials. Of the three glass ionomer materials compared, the light-cured resin-modified material with a resin dentin-bonding agent provided a significantly greater mean shear bond strength (6.7 MPa) than the light-cured resin-modified material without a resin dentin-bonding agent (3.0 MPa) or the chemically-cured material (3.8 MPa).     

氮对316LN奥氏体不锈钢力学性能和耐蚀性的影响

试验用316LN钢(/%:0.015C、0.65Si、0.90Mn、17.3Cr、12.8 Ni、2.6Mo、0.018~0.200N)由50 kg真空感应炉冶炼,破真空后加入氮化铬铁,铸锭锻成Φ20 mm棒材和热轧成4 mm板材,并分别经1 100℃30 min和10min水淬固溶处理。研究结果表明,316LN不锈钢每增加0.010%的氮,抗拉强度提高9 MPa,屈服强度提高7 MPa;伸长率降低0.55%,氮含量对断面收缩率没有影响,约保持在72.5%;氮强烈提高316LN不锈钢的耐点腐蚀性能,每增加0.010%的氮,其点蚀击穿电位提高7 mV;添加适宜的氮(0.079%N),可以改善316LN不锈钢的耐晶间腐蚀性能,过高的氮含量(超过0.120%N)对晶间腐蚀性能有害。     

冷等静压法制备Mo-30Cu合金的组织与轧制性能

采用冷等静压法(cool isostatic pressing,CIP)制得大尺寸钼骨架,对骨架进行渗铜制备Mo-30Cu合金,并在350℃进行温轧,研究CIP压力及熔渗温度和熔渗时间对合金致密度的影响以及合金的轧制性能。结果表明:采用冷等静压法在120~180 MPa压力下可制备孔隙分布均匀,无分层等缺陷的钼骨架,熔渗后坯料的线收缩率随CIP压力增加而逐渐降低,最佳CIP压力为160 MPa;在一定范围内升高熔渗温度与延长保温时间均有助于提高合金致密度;冷等静压–溶渗法制备的高致密Mo-30Cu合金具有较好的温轧性能,有效提高了大尺寸试样的加工性能。CIP压力为160 MPa压制的骨架在1 350℃渗铜6 h后相对密度达到99%以上,合金的温轧变形量可达到65%。     

Evolution of submicrocrystalline iron containing dispersed oxides under mechanical milling followed by consolidation

The structural changes in an Fe-0.6 pct O alloy during mechanical milling followed by consolidation through rolling were studied. The iron-iron oxide powders were mechanically milled in an argon atmosphere for various times from 20 to 300 hours. The powders were then canned into a steel pipe and multiple rolled at 700 °C for consolidation. The microstructure of the final product depended significantly on the milling time. The volume fraction of the dispersed oxides (10 nm in diameter) increased from about 0.3 to 2.5 pct when the milling time was increased from 20 to 300 hours. The relatively short milling time of 20 hours resulted in the evolution of elongated grains (an average size of about 1.2 μm) with a large fraction of low-angle grain boundaries after consolidation. In contrast, much finer grains (about 0.2 μm in size) with a near random grain-boundary misorientation distribution evolved in the samples milled for 300 hours.     

Surface treatment techniques for resin composite repair

PURPOSE: To compare bond strengths of fresh resin composites to previously polymerized ("aged") composites following various surface treatments. MATERIALS AND METHODS: Eighty Pertac Hybrid (PH) and an equal number of Silux Plux (SLX) specimens were fabricated and stored for 1 week prior to surface treatment. The specimens were then polished and stored for an additional 24 hours prior to final surface treatment. The surface treatments included use of one of the following: (1) diamond bur (DB), (2) microetcher with 50 microns Al2O3@80 psi pressure (ME), (3) high-pressure air abrasion with 27 microns Al2O3@psi, (KCP), or (4) low-pressure silicate ceramic deposition using 30 microns particles@34 psi (CJ-S) with a microetcher. Half of the samples were treated with a silanating agent. Fresh resin composite (same type as used for the aged specimen) was bonded to the treated surfaces, and specimens were then stored 24 h and thermocycled 300 x at 5 degrees and 55 degrees C prior to testing for shear bond strength. Two-way ANOVA was used to determine significant differences between mean shear bond strengths for both composite materials. RESULTS: Significant differences were found between the groups for both surface treatment and silane use (P < 0.05). The interaction between the two main effects was also significant (P < 0.05). Overall, the highest bond strengths were found when the low-pressure silicate ceramic deposition system (CJ-S) was used, with or without silane.     

Mn18Cr18N高氮奥氏体不锈钢的冶炼工艺

为了研究Mn18Cr18N高氮奥氏体不锈钢的冶炼工艺,采用加压感应炉+保护气氛电渣重熔工艺进行Mn18Cr18N高氮奥氏体不锈钢的冶炼工艺试验,冶炼过程中采用氮化合金与加压氮气渗入相结合的方法增氮。结果表明,随着氮气分压的增加,钢中氮质量分数随之增大;当冶炼过程中氮气分压提高0.03 MPa时,能够成功抑制皮下气泡的生产;试验钢经保护气氛电渣重熔后,钢中夹杂物会普遍降低,主要为细小的方形TiN夹杂。     

The influence of volume fraction of SiC particles on the properties of Al/SiC<Subscript>p</Subscript> nanocomposites produced by powder metallurgy with high energy ball milling

In this research, the objective is to investigate the effect of the volume percentage of SiC nanoparticles on the physical and mechanical properties of Al/SiC composite produced by powder metallurgy. For the mixing and the proper dispersion of SiC nanoparticles with 0, 1, 3 and 5% volume fraction of SiC in the aluminum matrix, a high energy planetary ball mill with the speed of 350 rpm and ball to powder weight ratio of 10: 1 was used under argon atmosphere. The powder samples were pressed in an onside template under a pressure of 500 MPa and sintered in a tube furnace under argon atmosphere for 2 h at 610°C. FE-SEM image and EDS analysis revealed that SiC nanoparticles represent uniformly dispersion in the aluminum matrix and the density of all the samples have increased while the porosity percentages have dramatically decreased after the sintering process. Furthermore, with the increase of the volume fraction of SiC nanoparticles the hardness also increases.     

Synthesis and characterisation of Al2O3/Ni-type composites obtained by spark plasma sintering

This paper presents the influence of sintering on the structure, morphology and compressing strength of alumina/nickel composite compacts obtained by spark plasma sintering (SPS). Al₂O₃/Ni composites were prepared by SPS in argon atmosphere at temperatures in the range of 1000–1200 –C with a holding time of 2, 5 and 10?minutes. The heating rate was 200 C?min^?1. These composites have been characterised by X-ray diffraction, SEM and EDX. The relative density and compressive strength of the as-obtained compacts were determined. The results showed that the alumina particles are uniformly dispersed in a quasi-continuous Ni network, and there was no sign of phase changes during sintering. The maximum strength of the alumina/nickel composite with a content of 75 vol. ? Al₂O₃ and 25 vol. ? Ni was about 240?MPa for the samples sintered at 1200?C for 10?minutes.

Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.     

高氮Fe-Cr-Mn-Ni系奥氏体不锈钢的加压感应熔炼

采用MgO坩埚高频真空感应炉在氮气压力0.45~1.0MPa、温度1640~1700℃下,对加压感应熔炼高氮Fe-Cr-Mn-Ni系奥氏体不锈钢进行了实验研究。结果表明,1913K、1.0MPa氮气氛中Cr12、Cr17Mn5Ni5、Cr19Mn15和Cr20Mn8不锈钢中氮的溶解度分别为0.391%、0.692%、1.120%和0.899%,氮在液态不锈钢中的溶解与Sievert定律有所偏离;氧浓度在350×10^-6内,1913K、1.0MPa氮气氛中Cr20Mn8钢液的吸氮反应仍为一级反应,其传质系数为0.023cm·s^-1;随钢中氧浓度的增加,液态钢的吸氮速率和钢液中的平衡氮含量显著降低。     

Fracture and fatigue-crack growth behavior in ductile-phase toughened molybdenum disilicide: Effects of niobium wirevs particulate reinforcements

A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pct Nb-wire mesh reinforcements (Nb _m /MoSi₂); results are compared with monolithic MoSi₂ and MoSi₂ reinforced with 20 vol pct spherical Nb particles (Nb _p /MoSi₂). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi₂, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb _m /MoSi₂ composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa , compared to unstable fracture atK _c values below 5 MPa in unreinforced MoSi₂ or Nb _p /MoSi₂. Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix, which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be far superior in the wire-reinforced composite over pure MoSi₂ and Nb _p /MoSi₂. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of the Nb _m /MoSi₂ composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along the (Nb,Mo)₅Si₃ reaction layer interface.     

The fracture characteristics of Al-9Ti/SiC p metal matrix composites

A fracture mechanics approach was used to determine the plane strain fracture toughness (K _IC) of a mechanically alloyed Al-9Ti 20 vol pct cobalt sol-gel-coated SiC particle-reinforced composite. Processing defects consisting of clumped SiC particulate, bonded by the sol-gel, initiated failure in tensile tests. The defects were measured and the fracture toughness was calculated using the Irwin relation. The value ofK _IC for the as-received material was determined to be equal to 4.7 MPa·m^1/2 at room temperature. Annealing the material for 120 hours and 400 hours at 500 °C increased the fracture toughness. This can be attributed to coarsening of an Al₃Ti strengthening phase. Tensile tests conducted at 200 °C show thatK _IC decreases at that temperature for each annealing condition. The sensitivity to the presence of the defects is greatest for samples annealed at 500 °C for 120 hours. The effect of the defects on the failure mechanism of the composite material as a function of temperature was determined. At room temperature, the Co/SiC processing defects provide low-energy paths for crack propagation; at 500 °C, the defects serve as void nucleation sites.     

Activation Energy of Nitriding Medium Carbon Ferromanganese Alloy

Investigation of some kinetics aspects of the reaction between nitrogen and medium carbon ferromanganese (MC-FeMn) was made. Nitriding process of fine medium carbon ferromanganese was carried out at temperature ranging from 973 to 1 223 K and time up to 480 min. Nitriding was carried out under nitrogen and hydrogen gas pressures. At temperature of 573 K, hydrogen gas was injected with pressure of about 0.2 MPa followed by injection of nitrogen gas up to 1.2 MPa. Sample mass was 35 g, nitrided in cylindrical chamber with 34 mm in inner diameter and 1 200 mm in length. The change in nitrogen pressure was taken as an indication for nitrogen pickup. The mass gain i.e. nitrogen pickup in kilograms per surface area (m2) was determined by time at different temperatures. Nitriding rate constants were calculated and the activation energy of nitriding process was derived from Arrhenius equation. The nitriding rate constant was found to be increased by increasing temperature of the reaction. The activation energy of nitriding process of fine medium carbon ferromanganese at time ranging up to 28 800 s is around 140 kJ/mol. It was found that the rate controlling step of the nitriding process of MC-FeMn is diffusion mechanism.     

烧结气氛对钢渣陶瓷晶相和性能的影响

利用钢渣制备陶瓷材料是钢渣资源化大宗利用的一条新途径.开展不同烧结气氛对钢渣陶瓷影响规律的研究,对推动钢渣陶瓷技术的应用具有重要意义.以20%钢渣和80%黏土为原料,分别在空气和氮气气氛下,制备了钢渣陶瓷样品,分析了其晶相转变和性能变化规律,并定量研究了氧分压对钢渣陶瓷中铁元素价态转变的影响机理.研究表明,在空气条件下烧结时,原料中的Fe2+发生氧化形成赤铁矿相,烧结样品物理性能要优于在氮气条件下烧结的样品,其抗压强度和吸水率为310 MPa和3.7%;而在氮气条件下烧结时,Fe2+形成铁铝尖晶石和铁辉石,烧结样品中形成的气孔大小和数量要大于和多于空气条件下的样品,这是导致其力学性能较差的一个主要原因.铁元素赋存晶相转变的氧分压临界范围为0.5%~0.75%:当分压低于0.5%时,可以获得以铁铝尖晶石和铁辉石为主的黑色或褐色陶瓷样品;当氧分压超过0.75%时,Fe2+开始发生氧化并形成Fe3+,逐渐形成赤铁矿并带来样品颜色为褐黄色或褐红色.增加烧结环境中氧气分压量是减少钢渣陶瓷产品黑心的一个重要手段.      

Polymerization contraction stress in thin resin composite layers as a function of layer thickness

OBJECTIVES: In the present study, the effect of layer thickness on the curing stress in thin resin composite layers was investigated. Since the value of the contraction stress is dependent on the compliance of the measuring equipment (especially for thin films), a method to determine the compliance of the test apparatus was tested. METHODS: A chemically initiated resin composite (Clearfil F2, Kuraray) was inserted between two sandblasted and silane-coated stainless steel discs in a tensilometer. The curing contraction of the cylindrical samples was continuously counteracted by feedback displacement of the tensilometer crosshead, and the curing stress development was registered. After 20 min, the samples were loaded in tension until fracture. The curing stress was determined for layer thicknesses of 50, 100, 200, 300, 400, 500, 600, 700 microns, 1.4 mm and 2.7 mm. The compliance of the apparatus was calculated with the aid of a non-linear regression analysis, using an equation derived from Hooke's Law as the model. RESULTS: None of the samples fractured due to contraction stress prior to tensile loading. The contraction stress after 20 min decreased from 23.3 +/- 5.3 MPa for the 50 microns layer to 5.5 +/- 0.6 MPa for the 2.7 mm layer. The compliance on the apparatus was 0.029 mm/MPa. SIGNIFICANCE: A measuring method was developed which was found to be suitable for the determination of axial polymerization contraction stress in this films of chemically initiated resin composites. The method makes it possible to estimate the stress levels that occur in resin composite films in the clinical situation.     

Preparation and properties of Al-alloy coconut shell char particulate composites

A technique to produce cast Al-11.8 pct Si alloy composites containing up to 40 vol pct (15 pct by weight) dispersions of 125 μm size coconut shell char particles is described. The technique consists of stirring shell char particles into the vortex created by mechanical stirring of melts and subsequent casting of composite melts in suitable molds. The composite melts were also pressure die cast at a pressure of 100 MPa into cylindrical castings. The incorporation of large volume fraction of shell char particles is aided by (a) preheating of the particles to about 500 ° to 600 ° for two hours before introduction into the melts, and (b) alloying of Al-11.8 pct Si melts with 3 to 6 pct Mg. Electron Probe Micro Analysis (EPMA) analysis indicated an Mg enriched region around dispersed char particles in the composite indicating that prealloying with Mg probably improves wetting between char particles and the melt. Dispersions of 15 pct wt of char particles lead to decreases in hardness (from 85 BHN to 55 BHN), compression strength (from 542.30 MPa to 218.68 MPa), U. T. S., (from 164.16 MPa to 63.75 MPa), and electrical conductivity (from 27.8 pct IACS to 11 pct IACS). However, since these decreases are accompanied by a decrease in density, specific strength values of Al-11.8 pct-shell char composites are adequate for a variety of applications. Adhesive wear rates and friction coefficient values at low sliding speeds (0.56 m per second, and at loads of 10 N and 60 N) decrease with increase in wt pct of char particles under dry conditions. M. K. SURAPPA and P. K. ROHATGI, both formerly with Regional Research Laboratory, Trivandrum.     

Characterization of Ti4AlN3

Bulk samples of Ti₄AIN₃ were fabricated by reactive hot isostatic pressing (hipping) of TiH₂, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain size of ≈20 μm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm³), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive stress of ≈450 MPa. Ti₄AlN₃ thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2 × 10⁻⁶ °C⁻¹. The room-temperature electrical conductivity is 0.5 × 10⁶ (Θ · m)⁻¹. The resistivity increases linearly with increasing temperature. Ti₄AlN₃ is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C. graduated from the Department in June of 1999 with an MS thesis.     

Structure of a Fe-Cr-Mn-Mo-N alloy processed by mechanical alloying

Elemental Fe, Cr, Mn, and Mo powders were processed by mechanical alloying to develop a nanostructured Fe-18Cr-11Mn-5Mo alloy under a N₂ atmosphere. It was found that the nitrogen contents in the as-milled powder mixture increased up to 1.6 wt pct after 190 hours processing time. The as-milled powders were then annealed under vacuum at either 1173 or 1473 K to promote the formation of the resultant equilibrium phases. In the annealed powder mixtures, depending on the temperature and nitrogen content, the phases identified by X-ray diffraction were either austenite, ferrite, or chromium nitrides. Annealing at 1173 K promoted the development of γ-Fe, α-Fe, and Cr₂N for all the nitrogen contents considered (0.5 to 1.6 wt pct). The volume fractions of the various phases formed were found to be strongly influenced by the nitrogen content and annealing temperature. In addition, the levels of nitrogen absorbed during processing were retained after annealing. Finally, the outcome indicates that a fully austenitic structure can be obtained by annealing powder mixtures at 1473 K with maximum nitrogen contents of up to 1 wt pct.