Ductility in hot isostatically pressed 250-Grade maraging steel

Prealloyed 250-grade maraging steel powder produced by the rotating electrode process was fully consolidated by hot isostatic pressing (HIP) at 1100 and 1200°C. The strength following aging (3 h at 480°C) equalled that of wrought material; however, ductility was negligible. This lack of ductility in the powder metallurgy product was traced to titanium segregation which occurred at the powder surface during powder production. The formation of a titanium intermetallic at the prior particle boundaries during aging caused failure at low plastic strains. Altered aging treatments successfully broke up the embrittling film and resulted in a significant ductility recovery for the HIP material. Analysis of the fracture process indicates that further ductility gains are possible by reducing the titanium content, refining the particle size, and optimizing the thermal cycles. Formerly with Sandia Laboratories, Livermoe, California     

TA7ELI粉末冶金材料力学性能与显微组织

以TA7 ELI钛合金棒为原料,用等离子旋转电极工艺制备出高品质钛合金球形粉末.采用热等静压成形工艺,将粉末压制成钛合金材料,并研究了材料的组织和力学性能.结果表明:等离子旋转电极工艺制备的钛合金球形粉末具有非常高的球形度和振实密度,粒度分布比较窄,非金属夹杂含量非常低;热等静压制备的低温钛合金达到全致密,其组织均匀细...     

Structure property correlations in hot isostatically pressed AISI-304 stainless steel

Abstract

Stainless steel components are usually fabricated by conventional manufacturing methods. This route becomes uneconomical for highly intricate shapes. Therefore, processing of AISI-304 stainless steel has been carried out by powder metallurgy and hot isostatic pressing (HIP) route. The microstructure and mechanical properties of the hipped steel produced from inert gas atomised powder were studied. The steel has shown a homogeneous and fine grained microstructure containing annealing twins and the absence of undesirable prior particle boundaries. This has led to higher yield strength, ultimate tensile strength and the ductility than that of the conventionally processed steel. The tensile deformation behaviour of hipped steel was found to be similar to that of the wrought material. Impact strength and fracture toughness of the hipped steel were superior to that of the wrought material; hence the hipped steel could be recommended for manufacture of components for critical applications.     

Powder metallurgy T15 tool steel: Part II. Microstructure and properties after heat treatment

The effect of powder particle size and heat treatment on the micro structure and properties of hot isostatically pressed (“hipped”) T15 tool steel has been evaluated. Gas-atomized powder was screened into size fractions covering the range of ≤44 to 1200 /i-m and hipped at 1130 ‡C or 1195 ‡C. The consolidated powders were austenitized at 1175 ‡C or 1225 ‡C and tempered at 538 ‡C, 552 ‡C, or 565 ‡C to control prior austenite grain size, carbide type, carbide volume fraction, and carbide size distribution. Properties measured were bend strength, C-notch impact toughness, and hot hardness. Prior austenite grain size increases with hot isostatic pressing (“hipping”) temperature and austenitizing temperature but is independent of the particle size; similarly, the influence of austenitizing temperature on dissolution of MC and M₆C is independent of the particle size. In each particle size fraction, the volume fraction and size distribution of MC are independent of the tempering temperature. For M₆C, the volume fraction increases and the size distribution is skewed to coarser sizes with increasing tempering temperature. No significant differences in strength and toughness were detected as a function of particle size. Hot hardness is not affected by the particle size. The hot hardness of a powder blend (≤1200 Μm) hipped at 1130 ‡C was superior to that of commercial powder metallurgy (PM) T15 tool steel hipped at 1195 ‡C; this is attributed to a finer carbide size in the noncommercial material. It is established that the subcommercial hipping temperature (1130 ‡C) results in significant microstructural refinement; there is an associated small amount of residual porosity, and this controls the mechanical properties.     

Production of Extra Low Oxide Beryllium Powder by the Rotating Electrode Process

The Rotating Electrode Process (REP) is a method for producing high quality spherical metal powders. REP powder is made by melting the end of a cylindrical bar, which is rotated at high speed about its longitudinal axis, using an arc source for melting. As the end of the bar is melted, metal droplets are ejected by centrifugal force and solidify during flight through a helium atmosphere. The advantage of REP for producing powders of reactive metals is that molten metal is never in contact with other metals or refractories. This factor reduces the likelihood of contamination and makes possible the production of powder that is close to the input material in cleanliness and composition. The plasma rotating electrode process (PREP) is an extension of REP in which the tungsten cathode heat source is replaced by a plasma torch. Spherical Be powders can be produced using either REP or PREP, although extra low oxide powders having BeO contents less than 300 PPM are produced via PREP. Mechanical properties of low oxide PREP beryllium powder consolidated by hot isostatically pressing, while lower than those of commerical low oxide grades, still compare favorably. Yield strengths are in the range of 140 to 204 MPa (20 to 30 ksi) and tensile strengths 173 to 259 MPa (25 to 37 ksi) depending on HIP conditions. Microyield strength is between 34 and 40 MPa (5 to 6 ksi), which is exceptionally high considering the coarse grain si2e and low strength of the material.     

航空航天用高品质3D打印金属粉末的研究与应用

作为金属3D打印的主要耗材,金属粉末对打印产品的质量有着至关重要的影响,航空航天、国防、医疗等领域精密复杂零件的3D打印对粉末性能,如粒度、形貌和纯净度等有着较高的要求。研究并介绍了航空航天领域3D打印用高品质镍基、钴基合金及钛合金等金属粉末的基本要求及主要制粉工艺;对两种常用的高质量金属粉末制备工艺真空感应熔炼氩气雾化法(VIGA)和等离子旋转电极法(PREP)进行了比较,指出VIGA法细粉收得率高,但存在空心粉和卫星粉;PREP粉球形度高、表面光洁、粉末粒度分布窄、流动性好、陶瓷夹杂少,在金属3D打印领域具有独特的优势。为进一步提高PREP粉的质量,应开发更新一代等离子旋转电极雾化制粉技术及装备,提高细粉收得率和生产效率。     

Effect of Yttrium on the Fatigue Behavior of Investment-Cast and Wrought Ti-6Al-4V

The effect of yttrium (Y)-containing particles on the microstructure and fatigue behavior of investment-cast and hot isostatically pressed (HIP) Ti-6Al-4V has been investigated. Elemental Y was added to the Ti-6Al-4V melt to obtain a concentration of 200 ppm Y, which is 4 times the amount allowed by current industrial specifications. The observed particles were between 100 nm and 1 μm in diameter, were widely dispersed, and did not agglomerate. High-cycle fatigue tests were conducted and an extensive fractography study revealed that the effect of the particles was negligible compared to the large colony size, and therefore slip length, in the cast material. The fatigue lives were consistent with those in the literature for tests conducted on specimens containing standard concentrations of Y. A section of the casting was extruded to refine the grain size in order to study the effects of the particles on a microstructure with shorter slip lengths. The results of the fractography study on these specimens were not as straightforward. A fine particle was observed at the crack-initiation site of one of the specimens of extruded material and shallow depressions on the size scale of the particles were present in other samples. However, no evidence of Y was detected by energy-dispersive spectroscopy (EDS) and the fatigue lives were on the high end of the scatter band typically encountered in wrought and annealed Ti-6Al-4V.     

Manufacture of metallurgical semifinished products from carbonyl nickel powder. Part 1

Summary It has been established in principle that carbonyl nickel powder can be processed into rod and strip by the technique of plastic working of sintered billets, and an appropriate technological process has been developed. The powder is pressed isostatically under a pressure of 9 kg/mm² into cylindrical billets, which are then sintered in hydrogen with a dew point between –20 and –30°C at a temperature of 1350°C for 5 h. To obtain sheet, the cylindrical billets are forged over the temperature range 1200-900°C into bars, which are first subjected to surface milling or planing and then to hot and cold rolling into sheet and strip. The resulting strip has high mechanical properties. The yield of sound metal in the processing of the powder to strip is 85%.Further work is in progress on the use of pressed sheet bars, which may be expected to shorten and simplify the fabrication process and raise the yield of sound material to 95%.Translated from Poroshkovaya Metallurgiya, No. 4 (64), pp. 29–36, April, 1968.     

PREP工艺参数对FGH95高温合金粉末特性的影响

研究了采用等离子旋转电极工艺制取FGH95高温合金粉末时各主要工艺参数对粉末平均粒度及其分布的影响。粉末的平均粒度主要取决于棒料的转速，且随棒料转速的增加而减小。粉末粒度的分布则与棒料转速、等离子弧电流强度以及等离子枪与棒料端部的距离等工艺参数有关。提高棒料转速、减小等离子弧电流强度或等离子枪与棒料端部的距离，均可使粉末粒度的分布范围变窄。     

Inconel 625 合金粉末的雾化制备方法对比

分别采用旋转电极、等离子雾化、无坩埚雾化、真空气雾化、水雾化制备出了Inconel 625粉末。对粉末的球形度、流动性、松装密度、氧含量等性能进行了对比。结果表明,旋转电极方法制备的Inconel625粉末球形度最好,均匀性最佳,但是较难制备出细粒径粉末。无坩埚雾化制备出的粉末性能综合性能相对较好,能够制备出球形度较好的较西粒径粉末。水雾化法制备出的粉末氧含量及球形度等性能最差。     

THE INFLUENCE OF EXTRUSION-CONSOLIDATION VARIABLES ON THE INTEGRITY AND STRENGTH OF THE PRODUCT FROM PREALLOYED 7075 ALUMINIUM POWDER

Abstract

A study has been made of extrusion-consolidation processing variables for the production of sound material from spherical 7075 aluminium alloy powder (median particle size 132 μm) canned in evacuated cylinders at ～60% initial density. Maximum product integrity and tensile properties were obtained by extrusion at 644K (700°F)–700K (800°F) and 6:1–10:1 reduction ratio. At lower reduction ratios (2:1 and 3:1) the product exhibited gross cracking and was not completely dense. At a reduction ratio of 40:1, it had significantly poorer tensile properties, attributable to the formation, during extrusion and heat-treatment, of longitudinal cracks at the particle boundaries and to the microstructure produced within the particle grains by the thermomechanical conditions. In general, processing behaviour and product properties were either inferior to or, in some cases, equal to those of wrought material extruded for comparison under the same conditions. Inferior behaviour of the metal powder during processing and tensile testing results from the presence of a brittle oxide film on the surfaces of the particles. Suggestions for improving the processing behaviour of the metal powder and the properties of the extruded product are made.     

Intergranular fracture of gamma titanium aluminides under hot working conditions

A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results.     

Pressure effects on flow and fracture of Be-Al alloys

The flow and fracture behavior of Be-Al alloys were determined in tension with different levels of superimposed pressure. The Be-Al alloys were prepared by Brush Wellman, Inc. (Cleveland, OH) from prealloyed powders processed to either a hot isostatically pressed (“hipped”) or cold isostatically pressed and extruded condition. Significant effects of pressure on both the flow and ductility have been observed at room temperature, with implications on the formability of these materials. The effects of changes in processing conditions and stress state on the flow and fracture behavior are summarized in addition to both optical and scanning electron microscopy (SEM) examination of the fracture surfaces. Separate other studies on the alloy constituents (e.g., Al and Be) are also reported. The results are also compared to previous works on monolithic materials and composites tested with high pressure.     

Microporosity in hot isostatically pressed Ti-6Al-4V powder compacts

The existence of micropores of less than 100 nm size is reported in hot isostatically pressed Ti-6Al-4V compacts that were expected to be fully dense. The micropores are in a film-like arrangement which traverses individual alpha grains. Several possible origins of the micropores are discussed. Of these, the dissolution of oxide film on prior powder particles appears most probable. Formerly Graduate Student at Case Western Reserve University, Cleveland, OH 44106.     

Plastic flow and microstructure evolution during thermomechanical processing of laser-deposited Ti-6Al-4V preforms

Plastic flow behavior and microstructure evolution during hot working and heat treatment of Ti-6Al-4V synthesized via a laser-deposition, Laser Engineered Net Shaping (LENS?), process were established. To this end, isothermal, hot compression tests were conducted on samples in either a deposited + stress relieved condition or a deposited + hot isostatically pressed (hipped) condition. The starting microstructures consisted of columnar grains with fine or coarse Widmanstätten (basketweave) alpha platelets. At subtransus temperatures, the flow curves of both microstructural conditions exhibited a peak stress at low strains followed by extensive flow softening; these curves were almost identical to previous measurements on ingot-metallurgy (IM) Ti-6Al-4V with similar transformed microstructures. In addition, the kinetics of globularization of the alpha phase during subtransus deformation or subsequent static heat treatment were found to be the same as for IM Ti-6Al-4V with comparable alpha-platelet thicknesses. During supertransus heat treatment, moderately fine beta-grain microstructures were developed in samples that had been predeformed below the beta transus. Such a heat treatment for samples previously deformed above the transus gave rise to a nonuniform distribution of coarse beta grains, an effect attributed to critical grain growth.     

Microstructure Characterization and Mechanical Properties of FGH 95 Turbine Blade Retainers

FGH 95 is a powder metallurgy (P/M) processed superalloy,which was developed in the 1980s in China.One of the applications of FGH 95 was high pressure turbine blade retainers.The manufacturing processes used to produce FGH 95 blade retainers consisted of atomization by plasma rotating electrode process (PREP),hot isostatic pressing (HIP) at super-solvus temperature and a sub-solvus solution heat treatment.The material had an equiaxed grain structure (ASTM 6.5-7.5).The γ′ precipitates in as-HIP FGH 95 showed a tri-model distribution.Carbides in the alloy were MC type and precipitated at grain boundaries.The prior particle boundaries (PPB) in the material originated mainly from γ′ phase.Statistics of the mechanical properties data from batch production of the FGH 95 blade retainers were investigated.The as-HIP FGH 95 blade retainers showed high strength at room temperature and 650 ℃,excellent creep resistance and outstanding stress rupture strength at 650 ℃.     

Tensile Fracture Behavior of 316L Austenitic Stainless Steel Manufactured by Hot Isostatic Pressing

Herein we investigate how the oxygen content in hot isostatically pressed (HIP’d) 316L stainless steel affects the mechanical properties and tensile fracture behavior. This work follows on from previous studies, which aimed to understand the effect of oxygen content on the Charpy impact toughness of HIP’d steel. We expand on the work by performing room-temperature tensile testing on different heats of 316L stainless steel, which contain different levels of interstitial elements (carbon and nitrogen) as well as oxygen in the bulk material. Throughout the work we repeat the experiments on conventionally forged 316L steel as a reference material. The analysis of the work indicates that oxygen does not contribute to a measureable solution strengthening mechanism, as is the case with carbon and nitrogen in austenitic stainless steels (Werner in Mater Sci Eng A 101:93–98, 1988). Neither does oxygen, in the form of oxide inclusions, contribute to precipitation hardening due to the size and spacing of particles. However, the oxide particles do influence fracture behavior; fractography of the failed tension test specimens indicates that the average ductile dimple size is related to the oxygen content in the bulk material, the results of which support an on-going hypothesis relating oxygen content in HIP’d steels to their fracture mechanisms by providing additional sites for the initiation of ductile damage in the form of voids.     

The Fracture Toughness and Toughening Mechanism of Commercially Available Unalloyed Molybdenum and Oxide Dispersion Strengthened Molybdenum with an Equiaxed,Large Grain Structure

Commercially available molybdenum and oxide dispersion strengthened (ODS) molybdenum produced by powder metallurgy (PM) methods were subjected to tensile testing, fracture toughness testing, and examination of the toughening mechanism. Both PM and ODS molybdenum have an equiaxed grain size that is larger in scale than comparable wrought products. This results in lower tensile strength and a higher tensile ductile-to-brittle transition temperature (DBTT) for PM and ODS molybdenum compared to wrought product forms. Although the grain size for PM molybdenum is large and the oxygen content is relatively high, both attributes tending to embrittle molybdenum, the transition temperature and fracture toughness values are comparable to those observed for wrought molybdenum. Crack initiation at grain boundaries and the center of grains where pores are present were observed to leave ligaments for the PM molybdenum that are similar in scale to those observed for wrought molybdenum. This is a similar toughening mechanism to the ductile laminate mechanism observed for wrought molybdenum. The larger oxide particle size for PM ODS molybdenum produces larger cracks that result in lower fracture toughness values and a higher DBTT in comparison to PM molybdenum. The impact of the grain size, grain shape, and oxide particles on the toughening mechanism and resulting properties is discussed.     

The Origin of Microstructural Diversity,Texture, and Mechanical Properties in Electron Beam Melted Ti-6Al-4V

An additive layer manufacture (ALM) technique, electron beam melting, has been used for the production of simple geometries, from prealloyed Ti-6Al-4V powder. Microstructure, texture, and mechanical properties achieved under standard operating conditions have been investigated. Three transitional regions are observed with a change in microstructural formation dependent on the thermal mass of deposited material. Prior β-phase reconstruction, from room temperature α-phase electron backscatter diffraction (EBSD) data, reveals a strong texture perpendicular to the build axis. Variation of build temperature within the processing window of 898 K to 973 K (625 °C to 700 °C) is seen to have a significant effect on the properties and microstructure of both as-deposited and hot isostatically pressed (HIP) samples.     

超高转速等离子旋转电极工艺制备钬铜球形粉末的研究

采用超高转速等离子旋转电极工艺(supreme-speed plasma rotating electrode process, SS-PREP)制备韧性金属间化合物钬铜(HoCu)球形粉末, 粉末粒度在15~106μm之间。利用X射线衍射仪、扫描电子显微镜、能谱分析及光学显微镜分析了SS-PREP钬铜球形粉末的粒度分布、松装密度、振实密度及霍尔流速等粉末特性, 比较了不同试验方法对粒度分布的表征。结果表明, SS-PREP钬铜粉末主要由CsCl结构的RM型B2相构成, 不同粒度的HoCu球形颗粒化学成分基本一致, 随着粉末粒度增大, HoCu球形粉末的非球形颗粒比例呈现下降趋势。