Influence of sintering temperature on the shrinkage and geometrical characteristics of steel parts produced by powder metallurgy

The dimensional and geometrical characteristics of Charpy bars produced with two different steels were investigated to evaluate the effect of increasing the sintering temperature from 1120 °C (conventional sintering temperature) up to 1350 °C. The problem was approached from the Geometric Dimensioning and Tolerancing (GD&T) point of view, referring to the standard ASME Y14.5 (2009). The dimensional and geometrical characteristics were evaluated using a Coordinate Measuring Machine (CMM), measuring the surfaces by scanning mode. The work highlights that the increase in the sintering temperature, aimed at improving the mechanical properties, does not prevent the main benefit of this technology, i.e., the possibility of producing parts with good dimensional and geometrical precision. Moreover, a methodology establishing the measurement procedures and data processing, to be used in future work for the characterisation of more complex shapes, was defined.     

Large-scale aluminum foam plate fabricated by enhanced friction powder compaction process based on sintering and dissolution process

An enhanced friction powder compaction (FPC) process was proposed for fabricating a large plate of aluminum foam by the sintering and dissolution process. In this process, the rotating tool plunged into the die filled with a powder mixture of aluminum and NaCl during the FPC process was made to traverse perpendicularly to the direction of plunging as in the case of friction stir welding. In the enhanced FPC process, no external heat source, such as an electric furnace or a spark plasma sintering, was necessary for fabricating aluminum foam, except for the friction heat generated by traversing the rotating tool. It was found that a long plate of aluminum foam can be fabricated with a length equal to the tool traversing length. By X-ray computed tomography (CT) and scanning electron microscopy (SEM) observations of the pore structures of the fabricated aluminum foam, it was found that the entire sample had a pore structures that was similar to the NaCl morphology, regardless of the position along the traversing direction. The fabricated aluminum foam had a similar stress–strain curve to that of aluminum foam fabricated by spark plasma sintering and exhibited ductile fracture. This is considered to be attributed to the good bonding between aluminum particles in the entire sample. The fabricated aluminum foam exhibited almost the same plateau stress regardless of the position along the traversing direction.     

Friction stir processing of Al/SiC composites fabricated by powder metallurgy

Friction stir processing (FSP) was applied to modify the microstructure of sintered Al–SiC composites with particle concentrations ranging from 4 to 16 vol%. Two SiC particle sizes (490N and 800 grades) were examined. Following FSP, the hardness of the 4 and 8 vol% of 490N grade SiC composites increased from 130 HV and 145 HV to 171 HV and 177 HV respectively. The increase was accounted for by the severe deformation occurring during FSP which uniformly distributed the SiC particles. The composites containing 16 vol% SiC could not be fully consolidated using FSP, and contained residual pores and lack of consolidation which originated from the as-received sintered microstructure. The hardness correlated well with the mean inter-particle spacing for the SiC particles in the case of composites containing 4 and 8 vol% SiC.     

A neural network approach for solution of the inverse problem for selection of powder metallurgy materials

This paper describes research that has been conducted into artificial intelligence techniques for solving the ‘inverse problem’, for assisting with materials selection. The term inverse problem refers to the task of employing process output information (i.e. the required mechanical or physical properties of the final material), in order to recommend suitable input settings for the process concerned. For example, for the powder metallurgy (P/M) process, where parts are manufactured from powdered metals, powder composition, compaction pressure, and sintering conditions are important input parameters that have to be controlled. Previous attempts at solution of the inverse problem have involved the use of statistical methods (such as regression analysis with application of relevant transforms), for fitting curves to the available experimental data. The resulting equations can be combined in a rule-base for generating materials selection advice. While such techniques are useful for identifying general trends in process inputs and outputs, they are subject to a number of disadvantages. P/M manufacture involves multiple process inputs and outputs. Many of the relationships are non-linear, and the experimental data exhibits considerable noise. When fitting curves to non-linear data the selection of transforms is inevitably subjective and becomes very difficult when multiple inputs are involved; also, regression analysis is not well suited to modelling noisy data. These considerations have lead to the identification of the neural network approach as being suitable for P/M modelling for materials selection purposes. Multiple inputs, modelling of highly non-linear responses, and the avoidance of detrimental noise effects have been provided by training a backpropagation neural network with experimental data for ferrous P/M data. The neural network deductions for process inputs were compared to those generated by regression analysis. The network reduced the standard deviation of the errors associated with the inverse solutions by 36%, thereby demonstrating how the technique can improve the accuracy of process recommendations.     

Martensitic transformation of Ti50Ni30Cu20 alloy prepared by powder metallurgy

Phase transformation behavior of Ti50Ni30Cu20 shape memory alloys prepared by powder metallurgy is analyzed with respect to the duration of mechanical alloying. The processed blends were studied by differential scanning calorimetry and room temperature X-ray diffraction. The martensitic transformations evidenced by thermal scans are discussed in correlation with the relative phase content obtained from the refinement of the X-ray diffraction patterns.     

粉末冶金高氮超级奥氏体不锈钢的开发

采用气雾化法制备高氮超级奥氏体不锈钢粉末,利用热等静压成形。结果表明,热等静压后,材料完全致密,而σ及Cr2 N两相的析出导致材料塑性、韧性及耐蚀性显著下降。材料经1200℃×1 h固溶处理后,力学性能及耐蚀性能大大提高,抗拉强度Rm为1050 MPa、屈服强度Rp0.2为735 MPa,伸长率A为57.0%,自腐蚀电位Ecorr为0.946 V。     

装配模型在粉末冶金模具CAD系统开发中的应用

针对粉末冶金模具CAD系统的开发,研究了在Unigraphics NX3.0平台上基于装配模型的CAD系统二次开发方法.详细介绍了装配模型在粉末冶金模具CAD系统中的应用、装配建模的方法,以及系统开发中运用到的关键技术.系统开发实践表明:基于装配模型的开发方法有效地保证了零件设计的可装配性,提高了设计自动化程度.同时,最大限度地保证了零件与装配体修改的同步更新.     

预混合工艺对铁基粉末冶金材料组织和烧结性能的影响

通过加入新型润滑剂制得Fe-2Cu-0.8C预混合铁基粉末,并制备了同成分机械混合粉末进行对比试验。对粉末流动性、松装密度以及压制性能进行了测试,并对烧结体的微观组织进行表征。结果表明:制备的预混合粉末流动性和松装密度均优于机械混合粉。当润滑剂加入量为0.6 mass%时,经600 MPa压力下压制所得的生坯密度为7.01 g/cm^3,烧结体密度为7.11 g/cm^3,批量压制时零件质量变化小于0.15%。通过预混合工艺,使得铜和石墨颗粒粘结到铁颗粒表面上,从而达到防止偏析和提高批次稳定性的目的。使用预混合粉末不仅提高了烧结体的尺寸精度和性能,同时可制备出更光洁的零件表面,进行形状复杂零件生产时更能体现出其在稳定性方面的优势。     

Improving the uniformity in mechanical properties of a sintered Ti compact using a trace amount of internal lubricant

Large sintered powder compacts are likely to be associated with variability in mechanical properties; an improvement of the uniformity of the mechanical properties of sintered powder compacts is important for powder metallurgy. In this work 0.3–1 wt.% stearic acid (SA) or magnesium stearate (MgSt) was added to a 40 mm diameter Ti powder compacts with height to depth (H/D) ratio of unity to give a more uniform green density. Tensile test pieces were cut from selected positions in each sintered compact to obtain the distribution of mechanical properties. Results revealed that variations in mechanical properties are due to the pore morphology with respect to size, aspect ratio and preferred orientation. A trace amount of lubricant significantly improves the uniformity in mechanical properties by optimizing the porosity distribution and minimizing the pore size and aspect ratio of pores after sintering. Such an effect was achieved by reducing the initial green density inhomogeneity and the stress induced by the mismatch of sintering shrinkage. However a relatively high 1 wt.% SA addition with a large particle size created burnt-off pores in the top and bottom zones. MgSt is not recommended since it significantly increases the oxygen content. An addition of 0.6 wt.% SA is the best choice due to the even pore distribution, small pore size and acceptable level of oxygen pick up.     

Phase transformation and damping behavior of lightweight porous TiNiCu alloys fabricated by powder metallurgy process

Porous TiNiCu ternary shape memory alloys (SMAs) were successfully fabricated by powder metallurgy method. The microstructure, martensitic transformation behavior, damping performance and mechanical properties of the fabricated alloys were intensively studied. It is found that the apparent density of alloys decreases with increasing the Cu content, the porous Ti₅₀Ni₄₀Cu₁₀ alloy exhibits wide endothermic and exothermic peaks arisen from the hysteresis of martensitic transformations, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy shows much stronger and narrower endothermic and exothermic peaks owing to the B2-B19 transformation taking place easily. Moreover, the porous Ti₅₀Ni₄₀Cu₁₀ alloy shows a lower shape recovery rate than the porous Ti₅₀Ni₅₀ alloy, while the porous Ti₅₀Ni₃₀Cu₂₀ alloy behaves reversely. In addition, the damping capacity (or internal friction, IF) of the porous TiNiCu alloys increases with increasing the Cu content. The porous Ti₅₀Ni₃₀Cu₂₀ alloy has very high equivalent internal friction, with the maximum equivalent internal friction value five times higher than that of the porous Ti₅₀Ni₅₀ alloy.     

Development of cube texture in pure Ni, Ni–W and Ni–Mo alloys prepared by the powder metallurgy route

Development of textures after heavy cold rolling (95%) and annealing were studied in powder metallurgically prepared pure Ni, Ni–5at.%W and Ni–5at.%Mo alloys. It has been found that W and Mo additions to Ni are beneficial for the development of sharp cube texture, although W has a much more pronounced effect than Mo.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

快速凝固-粉末冶金工艺制备高强阻尼铝合金的阻尼性能(英文)

采用快速凝固-粉末冶金(RS-PM)工艺制备两种不同类型的高强阻尼铝合金(LZ7),一种是直接挤压成型材,另一种是先挤压至棒材再轧制成板材。利用阻尼机械热分析仪(DMTA)在25~300°C下对其进行阻尼性能测试,利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)观察材料的微观组织。结果表明,随着温度的升高材料的阻尼性能提高,在300°C时板材的内耗值高达11.5×10-2,型材的内耗值达到6.0×10-2和7.5×10-2。轧制板材的内部析出相的颗粒形貌较挤压型材的析出相更规则,分布更均匀;板材中颗粒相与基体结合较疏松。基体与颗粒相结合情况的不同可以解释板材在高温阶段(120°C)阻尼性能优于型材阻尼性能的原因。     

Boride coatings obtained by pack cementation deposited on powder metallurgy and wrought Ti and Ti-6Al-4V

Wear resistance of Ti alloys needs to be improved, and an effective way to achieve this is through surface treatment. Boronizing is a surface treatment in which boron diffuses into the surface of Ti leading to the formation of hard and wear-resistant Ti borides. Boronizing of wrought and/or cast Ti alloys by pack cementation has been studied, while similar coatings on Ti alloys produced by powder metallurgy (PM) have not been reported. Also critical process parameters for boronizing Ti alloys, such as pack cementation powder composition and the process temperature have not been systematically studied and analysed. The present work reports on the surface modification of PM Ti and PM Ti-6Al-4V by boronizing, and presents some important thermodynamic aspects of the process comparing it with similar coatings applied to wrought Ti-6Al-4V. The coatings were characterised using scanning electron microscopy and X-ray diffraction. For both Ti and Ti-6Al-4V alloys the use of amorphous B as a B element supplier in the boriding powder pack led to the formation of a uniform external boride layer, while the use of B₄C as a B element supplier in the pack and under the same boronizing conditions, led to the formation of an external TiN layer and an internal layer containing B. The thermodynamic calculations performed proved successful in determining the appropriate conditions for boride coating deposition and estimating the phases likely to be formed. Finally the effect of surface roughness on the coating quality is discussed.     

粉末冶金Cu-Cr合金在0.1MPa纯氧气中的氧化

研究了具有均匀两相组织的粉末冶金Ｃｕ－２５Ｃｒ和Ｃｕ－５０Ｃｒ合 ０．１ＭＰａ氧气中的氧化。与组织不均的铸态Ｃｕ－Ｃｒ合金相似,氧化后两种成分的粉末冶金Ｃｕ－Ｃｒ合鑫匀形成了复杂的混合氧化膜,但未形成连续的Ｃｒ２Ｏ３层。而经机械合经法获和的铬颗粒大小在１μｍ左右的Ｃｕ－２０Ｃｒ合金,氧化时则形成了连续的Ｃｒ２Ｏ３层。档见,在二元双相合金中,合 氧化行为与合金的组织形态,大小和分布密切相关。     

Properties and forming process of prealloyed powder metallurgy Ti-6A1-4V alloy

The properties and forming process of prealloyed powder metallurgy （PM） Ti-6Al-4V alloy were researched for application of high performance of titanium parts. Hot isostatic press （HIP） technology and two kinds of powders were used in the preaUoyed processing to get full density material. Tensile properties, impact toughness and fracture toughness of PM Ti-6Al-4V alloy were studied and discussed. The microstructures were examined with optical microscope and the morphologies of powders were observed by SEM. Forming processes were performed to fabricate PM titanium parts. The experiment results show that the preaUoyed PM Ti-6Al-4V alloy has same good properties as wrought material and the complex shape PM parts can be near-net-shaped. Some of the parts have been commercially used. This indicates that the prealloyed process should have bright prospects in making high-performance, complex shape and low-cost titanium alloy parts.     

The effects of Fe additions on the liquid phase sintering of W–bronze composites

In this investigation, experiments were conducted to evaluate the effects of Fe additives in the range of 1–5 wt.% on the densification of different compositions of W–pre-alloy bronze compacts sintered isothermally at temperatures ranging from 900 °C to 965 °C for 2.30 h. The results showed that substantial improvement in hardness by a factor of two folds and density by 10% was achieved for the W–pre-alloy bronze sintered compacts by the addition of 2–3 wt.% Fe as an activator.     

Microstructure and bonding behavior on the interface of an induction-melted Ni-based alloy coating and AISI 4140 steel substrate

An induction-melted Ni-based alloy (NiCrBSi) coating on AISI 4140 steel was prepared using induction melting under different melting conditions. To determine the microstructure and bonding behavior of their interfaces, the techniques of micro-diffraction, X-ray mapping and wavelength dispersive spectrometry (WDS) were employed. The experimental results showed that a white zone appears on the Ni-based alloy side of the final interface, becoming wider and wider with increasing temperature and time, which was identified as a solid solution of γ-Ni at 1000 °C and 1180 °C. A higher heating temperature and time caused a more severe iron dilution in the Ni-based alloy and decreased the microhardness.     

Some studies on the warm solid- and hollow-extrusion of sintered powder metallurgical copper preforms

The warm extrusion of sintered P/M preforms of electrolytically pure copper has been investigated for extrusion strains of 0.35, 0.5, 0.66 and 0.81 for the solid configuration and 0.43, 0.62, 0.85 and 1.09 for the tubular configuration at temperatures of 303, 363, 433, 493 and 563 K with densities of 7.45 and 7.90 g cm⁻³ for the solid configuration and 7.0 and 7.7 g cm⁻³ for the tubular configuration. Force requirements and microstructural property evaluation have been attempted. Using the standard axial ring-compression test on the initial preforms, the plastic properties were evaluated to enable the computation of theoretical forces. The actual forces were compared with these theoretical forces and a correction factor, φ, then determined. The influence of strain, temperature and the geometry in ensuring near to theoretical density of the extrudates has been assessed.     

热处理对粉末冶金铝铜合金组织性能的影响

以纯铝和氧化铜粉末为原料,运用热等静压原位合成技术,制备了粉末冶金铝铜合金(Al-4.5%Cu)。研究了固溶和时效工艺对其组织和性能的影响,并分析讨论其作用机理。结果表明,粉末冶金法制备的铝铜合金组织均匀、致密,固溶+时效处理使合金硬度、强度明显提高,塑性和韧性略有下降。经过550℃×11 h固溶+150℃×46 h时效处理,合金的维氏硬度HV为1300 MPa,抗拉强度σb为328 MPa,冲击韧性αk为72.85 kJ.m-2,伸长率δ为6.63%。