Flow properties of tool steel powders for selective laser melting – influence of thermal and mechanical powder treatments

In powder bed fusion additive processes the flow properties of the powder influence the quality of the final component and the efficiency of the process. In this investigation an attempt is made to identify flowability indicators which can describe the flow performance of the powder during the powder layering (i.e. recoating) step; common to all powder bed fusion processes. To this end, shear tests were performed by means of a powder rheometer. Bulk density, flow function and degree of cohesion were measured. The results suggest that there is a good correlation between the aforementioned parameters and the flowability of the powder during SLM processing. In addition, it was found that thermal treatments and tumbling enhance flowability. Thermal treatments were performed at 150, 200 and 250°C for a period of 10?min and in air.     

Properties and sintering behaviour of fine spherical iron powders produced by new hydrogen reduction process

Abstract

The most important use of fine spherical iron powders is for metal injection moulding (MIM). For many applications, the high costs of powder based on the carbonyl or atomising production route are a limiting factors. An alternative two-step hydrogen reduction process using a granulated hematite powder, which is a recycling product from steelmaking, has been developed to produce <25 µm spherical powder. The morphology and properties of the powder have been found to depend strongly on the second temperature step of the reduction process. A further important step is enclosed powder processing by milling and sieving to remove agglomerates. The powder properties and sintering behaviour as a function of heat treatment and processing parameters are reported and discussed.     

Physical simulation of converter steelmaking with powder injection

A water model of top and bottom blown converter with top lance powder injection and bottom tuyere powder injection was established to investigate the powder injection. The results show that the powder penetration ratio under the condition of top lance injection is greater than that under the condition of bottom tuyere powder injection. In both cases, the powder penetration ratio increases with the increase of solid/gas ratio and powder particle size. Powder uniform dispersion time with top lance powder injection is longer than that with bottom tuyere powder injection. Top lance powder injection, lance height of 258?mm, bottom blowing rate of 1·96?Nm³?h^?1 and powder particle size of 0·212–0·380?mm are suggested as the optimum powder injection operation under the experimental condition. The corresponding optimum operation for prototype is top lance powder injection with lance height of 1550?mm, powder size of 1–5?mm and bottom blowing rate of 450?Nm³?h^?1.     

A Novel Method for Direct Synthesis of WC-Co Nanocomposite Powder

In this study, a novel method, termed dielectric-barrier-discharge-plasma (DBDP) assisted ball milling and low-temperature carburization, was used to synthesize WC-Co nanocomposite powder. X-ray diffraction, scanning/transmission electron microscopy, and differential scanning calorimetry were used to characterize the microstructure of powders. Starting from W, Co, and graphite powder mixtures, the DBDP-milled W-C-10Co powder exhibited a flakelike morphology with very fine lamellar structure. The WC-Co composite powder was synthesized at 1273 K (1000 °C), which is much lower than the requisite temperature for the conventional carburizing method. The obtained WC-Co composite powder had a nanocomposite microstructure in which fine WC particles were bounded by homogenously distributed Co phase, and the WC crystals had a slablike morphology with a planar size of about 200 nm and <100-nm thickness. The combinational effect of the milling and the plasma in the DBDP milling caused a unique fine flakelike morphology and high-density interfaces in the W-C-10Co powder mixture, which is responsible for the reduced carburization temperature and the nanocomposite structure of WC-Co powder.     

Surface inclusions and their evolution on strip rolled on CSP line and in following cold rolling process

Abstract

Hot and cold rolled strip samples with surface defects such as black pockmark, sliver and central cracking were collected from a compact strip production line followed by cold rolling. Microstructures and chemical compositions of the strip with defects were studied by scanning electronic microscopy and energy dispersive X-ray spectroscopy. Four typical types of defects on strip including surface spherical mould powder entrapment, centre spherical mould powder entrapment, surface linear mould powder entrapment and centre linear mould powder entrapment were studied by hot and cold rolling experiment in laboratory. It was observed that spherical mould powder entrapment was transformed into the surface black pockmark after the hot rolling and cold rolling process. Centre spherical mould powder entrapment led to central fracture in the hot and cold rolling process. Surface linear mould powder entrapment resulted in sliver and centre linear mould powder entrapment led to severe sliver.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

氧乙炔燃流氧化处理制备氧化物包覆ZrB_2/SiC核壳结构粉末特征与机理的研究

ZrB_2/SiC复合粉体熔点高,在等离子喷涂过程中难以获得良好的熔融状态,变形颗粒间存在大量缺陷,导致涂层抗氧化性能急剧下降。为此,本文设计了氧化物包覆ZrB_2/SiC复合粉末,并探索采用氧乙炔火焰热处理方法进行核壳结构粉末的制备。采用扫描电镜对不同试验阶段粉体表面及截面进行观察,发现氧化热处理后的粉体可以得到明显核壳结构的团聚粉,粉体的致密性也得到了明显的提高。采用EDS对三种粉体的成分分布进行了观察,探究了原位氧化法制备得到的核壳结构的粉体中Zr、Si、O主要元素的分布情况,结合XRD进行的物相分析,进一步证实了扫描电镜观察的结果,并对结果形成原因进行了分析。     

Annealing Behavior of a Nanostructured Fe1.5Mo Alloy

In-situ synchrotron radiation X-ray powder diffraction is shown to be complementary to thermal analysis for the study of the annealing of a ball-milled nanocrystalline Fe 1.5 wt pct Mo powder. The evolution of domain size distribution and defects is quantified via whole powder pattern modeling (WPPM) of the diffraction data. A possible annealing mechanism is proposed for the powder.     

铁粉类型对铜基粉末冶金摩擦材料性能的影响

采用粉末冶金工艺分别制备含还原铁粉、泡沫纤维铁粉和铁合金粉的铜基摩擦材料,研究了铁粉种类对摩擦材料摩擦磨损性能的影响.当摩擦转速从3000 r·min-1提升至6200 r·min-1,用还原铁粉制备的样品,其摩擦因数随速度的升高出现严重衰退;含泡沫纤维铁粉的样品具有稳定的摩擦因数,试验范围内其波动值不超过0.024,但是磨损严重;采用铁镍合金粉制备的样品可有效减缓高速阶段摩擦因数的衰退,高速下摩擦因数波动低于0.027.以铁铬合金粉制备的样品,其磨耗随摩擦速度的增加几乎不发生变化,抗磨损能力最佳.      

Solid state sintering and consolidation of Al powders and Al matrix composites

As an attempt to depart from conventional transient liquid phase sintering practice, solid state vacuum sintering was studied in loose powder and in hot quasi-isostatically forged samples composed of commercial inert gas atomized (CIGA) or high purity Al powder. The high purity Al powder was generated by a gas atomization reaction synthesis (GARS) technique that results in spherical powder with a far thinner surface oxide. After vacuum sintering at 525 °C for up to 100 h, SEM results showed that the GARS Al powder achieved significantly advanced sintering stages, compared to the CIGA Al powder. Tensile results from the forged samples also showed that although its ultimate tensile strength is lower, 95 vs. 147 MPa, the ductility of the GARS pure Al sample is higher than the CIGA Al sample. Forging also consolidated a model powder-based composite system composed of an Al matrix reinforced with quasi-crystalline Al–Cu–Fe powders, where the same powder synthesis methods were compared. Auger surface analysis detected evidence of increased matrix/reinforcement interfacial bonding in the composite sample made from GARS powder by alloy interdiffusion layer measurements, consistent with earlier tensile property measurements. The overall results indicated the significant potential of using Al powders produced with a thin, high purity surface oxide for simplifying current Al powder consolidation processing methods.     

Synthesis of ultrafine WC/Co powder by mechanochemical process

Abstract

A new approach to produce ultrafine WC/Co powder by a mechanochemical process was made to improve the mechanical properties of advanced hardmetals and to cut production costs. For powder preparation, the water soluble salts containing W and Co components were used as starting materials. After synthesis of the precursor powder from an aqueous solution by a spray drying technique, a salt removing heat treatment in air atmosphere was carried out to prepare the oxide powder. The oxide powder was mixed with carbon black by ball milling and this mixture was converted at 800^oC to the nanophase WC/Co powder in H₂ and N₂ atmospheres. The average size of the WC particle was 100-150 nm. The possibility of achieving high density sintered material with an ultrafine and homogeneous microstructure using grain growth inhibitors, such as tantalum and vanadium carbides, has been shown.     

Density variation during the deformation of powders in an open volume

Conclusions The density of a powder poured with a spreader approaches its tap density. For the majority of powders the densities in the deformed condition differ but little from the volumetric densities. The densification of a powder in the section H_p apparently begins from its volumetric density. The law of constancy of mass in powder rolling may be written in differential form. The change in porosity experienced by a powder during deformation in an open volume may be employed as a quantitative indicator in the prediction of the properties of rolled powder strip.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 10–18, April, 1980.     

Mechanical alloying of FeAl intermetallic powder for metal injection moulding process

Abstract

The Fe–48 at.-%Al powder used for powder injection moulding (PIM) was prepared by mechanical alloying in a high energy planetary ball mill and subsequent vacuum annealing. The effects of stearic acid (SA) as the process control agent on powder characteristics were investigated using X-ray diffractometer, laser particle size analysis and scanning electron microscopy. The dependence of solvent debinding efficiency of PIM feedstock on the powder characteristics was also studied. The results indicate that a low SA content for ball milling helps to prepare near spherical, coarse powder particles that exhibit good solvent debinding efficiency. On the contrary, a high SA content leads to thin layered, small particles with poor PIM solvent debinding efficiency. An intermediate SA content such as 1 wt-% makes the ball milled powder combine the advantageous characteristics for PIM process.     

Investigation of the process of grinding reduced sponge iron and development of the technology of producing iron powders of different bulk density

Summary The following grinding technology is recommended for the purpose of increasing the fine-fraction yield of powder: a) grinding of reduced briquets in a hammer mill and the sifting of powder into coarse and fine fractions; b) grinding of the coarse fraction in a ball mill over a period of 16 h and the sifting of powder by means of a vibrating screen; c) mixing of the fine fractions obtained in the first and the second screenings.In order to increase the bulk density of the powder, it is recommended to perform triple grinding of the powder in a hammer mill or to grind it in a ball mill.     

SINTERING KINETICS OF OXIDE-FILMCOATED COPPER POWDER

Abstract

The sintering kinetics of pure copper powder and of copper powder coated with a critical oxide (CU₂O) film thickness (～,500Å), has been studied by following the densification of the compacts as a function of temperature and time in pure dry hydrogen and in vacuum. The activation energy for the sintering of pure copper powder in hydrogen was 55,000 cal/mole, suggesting that the volume self-diffusion mechanism predominates during the sintering process. In the case of the oxide-coated powder the corresponding value was 37,000 cal/mole. The high rate of sintering of the coated powder in hydrogen and in vacuum is explained in terms of an activated sintering mechanism.     

FRICTION INDEX AND PRESSING CHARACTERISTICS OF SPHERICAL STAINLESS-STEEL POWDER AND OXIDIZED COPPER POWDER

Abstract

A procedure is described for accumulating data on the friction index and compression ratio of a copper powder at different stages of oxidation and of screened fractions of a stainless-steel powder. The dimensionless and nameless powder property, friction index divided by the compression ratio, was constant under certain conditions for a regular series of size fractions of stainless-steel powder. Calculations were made of the coefficient of compression and the limiting value of the degree of volume change by the method due to Kawakita.     

Mould powder consumption of continuous casting operations

Abstract

Mould powder consumption is an important parameter in continuous casting. Previous investigations, involving regression analysis or estimation of powder consumption, have identified the factors controlling powder consumption. However, those data were obtained for casting speeds less than 2 m min ^-1, and some relate to speeds less than 1 m min ^-1. Sumitomo Metal Industries has developed a medium thick high speed continuous casting operation for speeds between 2 and a maximum of 8 m min ^-1. Some of the powder consumption data obtained using the new process could not be explained on the basis of the former investigations: these data are discussed in the present report. The conclusions drawn are: (i) gravity is the main driving force for inflow of liquid powder between mould wall and solidified shell; (ii) to increase the powder consumption per unit strand surface area for speeds of ~8 m min ^-1 is difficult; and (iii) a powder consumption of under 0·1 kg m ^-2 is sufficient when casting at 8 m min ^-1.     

Reduction Annealing for Cast Iron Powder and Its Effect on Sintered Antifriction Material Properties

The effect of reduction annealing for powder obtained from cast iron shavings on powder specimen properties is studied. An approximate mechanism is suggested for the oxidation-reduction process during annealing of cast iron powder. An increase in annealing temperature leads to an increase in carbon gasification rate and change in absolute powder weight. Prior reduction annealing for cast iron powder improves its compactability, and increases the mechanical properties of sintered material. The best mechanical properties were obtained for specimen made from cast iron powder reduced at 1000°C. Alloying an iron-cast iron charge with 3 mass% of nickel-chromium alloy leads to a marked improvement in the mechanical properties of a sintered composite.     

在高炉喷吹煤粉中添加高炉除尘灰的试验研究

通过高炉喷吹煤粉中添加一定比例除尘灰的试验发现,随着煤粉中除尘灰含量的增加,灰分含量（Ad）明显增加,挥发份（Vdaf）略有减少,固定碳含量（FCad）也呈减少趋势;煤粉的灰熔点总体呈升高趋势,煤粉的发热量明显降低;煤粉的燃烧速度明显加快;除尘灰中的Fe得以顺利回收。从而实现了除尘灰更有效地回收和利用,为高炉节能减排开辟一条新的途径。