Producing nanocrystalline bulk Mg-3Al-Zn alloy by powder metallurgy assisted hydriding-dehydriding

Nanocrystalline bulk Mg-3Al-Zn alloy with an average grain size of 48 nm has been prepared by powder metallurgy assisted hydriding-dehydriding. Evolutions of nanograined structure powders and bulk alloy have been investigated by TEM, SEM and XRD, respectively. The results showed that by milling in hydrogen for 60 h, as-hydriding powder possessed an average grain size of 5.9 nm. After a subsequent process of desorption-recombination treatment (at 350 °C) and consolidation process (extruded at 200 °C) resulted in bulk samples with an average crystallite size of 48 nm and MgH₂ was fully turned into Mg. The consolidated samples of 60 h milled powder had a final density of 1.77663 ± 0.006 g/cm³, which corresponded to 97.57 ± 0.3% of theoretical density. The highest microhardness of the nanocrystalline bulk alloy reached about 872.5 MPa, which is about three times higher than that of the coarse-grained AZ31.     

Bulk Mg-3Al-Zn alloy with ultrafine grain size produced by powder metallurgy

Ultrafine-grained Mg-3Al-Zn alloys with an average grain size of 180 nm have been made by powder metallurgy. First, the nanocrystalline powders with mean grain size of 45 nm were produced by ball milling under argon atmosphere, and then through vacuum hot pressing at 633 K for 40 min and warm extrusion at 373 K, bulk solid samples were compacted successfully from the mechanically milled powders, and the relative density of the samples was about 98.87% (1.8003 g/cm³). XRD, SEM and TEM analysis showed that the microstructure of the samples consists of homogeneous equiaxed grains and grain growth has taken place during the consolidation process.     

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.     

The effect of particle size,sintering temperature and sintering time on the properties of Al–Al2O3 composites,made by powder metallurgy

Al₂O₃ is a major reinforcement in aluminum-based composites, which have been developing rapidly in recent years. The aim of this paper is to investigate the effect of alumina particle size, sintering temperature and sintering time on the properties of Al–Al₂O₃ composite. The average particle size of alumina were 3, 12 and 48 μm. Sintering temperature and time were in the range of 500–600 °C for 30–90 min. A correlation is established between the microstructure and mechanical properties. The investigated properties include density, hardness, microstructure, yield strength, compressive strength and elongation to fracture. It has been concluded that as the particle size of alumina is reduced, the density is increased followed by a fall in density. In addition, at low particle size, the hardness and yield strength and compressive strength and elongation to fracture were higher, compared to coarse particles size of alumina. The variations in properties of Al–Al₂O₃ composite are dependent on both sintering temperature and time. Prolonged sintering times had an adverse effect on the strength of the composite.     

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.     

Influence of sinter-cooling rate on intergranular corrosion of powder metallurgy superaustenitic stainless steel

Powder metallurgy superaustenitic stainless steel sintered in N₂–H₂ atmosphere (95%–5%) was obtained. Three different processes were applied by controlling the sinter-cooling rate (furnace, gas and water). Intergranular corrosion resistance was evaluated. Furthermore, microstructure, density, porosity, macro- and microhardness were investigated. Microstructures were characterised by optical microscopy and scanning electron microscopy with energy dispersive analysis of X-rays. Intergranular corrosion behaviour was evaluated by immersion and by single and double loop electrochemical potentio-kinetic reactivation tests. The superaustenitic steel after fast cooling by water quenching showed the best intergranular corrosion behaviour, the highest microhardness values and the most homogeneous microstructure.     

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.     

一种新型镍基粉末高温合金挤压工艺研究

针对一种新型粉末高温合金WZ-A3进行了一系列的热挤压工艺试验,探究了挤压温度、挤压速度、挤压比对棒材显微组织的影响以及棒材的整体组织均匀性。结果表明：在恒定挤压速度35 mm/s、挤压比4.7:1条件下,挤压温度为1110 ℃时,合金棒材已发生完全动态再结晶,当挤压温度继续增加,晶粒发生明显长大。在恒定挤压温度1110 ℃、挤压速度35 mm/s条件下,挤压比为2.1:1时,合金再结晶不完全而且存在较多原始颗粒边界;当挤压比增加至4:1~4.7:1时,再结晶程度完全充分,PPB也完全消除。在恒定挤压温度1130 ℃、挤压比4.7:1条件下,在20~50 mm/s范围内随着挤压速度的增加,晶粒呈长大趋势。在挤压温度1110 ℃、挤压速度35 mm/s、挤压比4.7:1条件下整个挤压棒材的组织较为均匀,从棒材头部到尾部,晶粒略微细化,并且棒材边缘的组织较心部和1/2R更细小。     

Zn对粉末冶金法制备Mg-Zn合金组织与性能的影响

以Mg粉和Zn粉为初始原料,采用粉末冶金技术制备Mg-Zn合金。研究了Zn含量对Mg-Zn合金烧结密度、显微组织、物相组成、弯曲强度和显微硬度的影响。测量了Mg-Zn合金的耐腐蚀性,探讨了Zn元素在粉末冶金过程中的作用机理。研究结果表明当添加Zn元素后,烧结产物的晶粒细小,烧结密度提高。此外,随着Zn含量的增加,烧结产物的致密度持续增加。XRD分析表明Mg-3 wt%Zn合金主要由α-Mg相组成,而Mg-4 wt%Zn合金由α-Mg 和 MgZn₂两相组成。随着Zn含量的增加,Mg-Zn合金的弯曲强度先增加而后降低,但是显微硬度持续增加。Mg-3 wt% Zn合金的抗弯强度为123.6 MPa,显微硬度为101.7 HV,分别为纯Mg样品高出58%和45%。耐腐蚀性能测试表明当添加Zn元素后,Mg-Zn合金的腐蚀速率降低,Mg-3 wt%Zn合金具有最低的腐蚀速率和最佳的耐腐蚀性能。     

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.     

Preparation of superfine-grained high entropy alloy by spark plasma sintering gas atomized powder

In this work, nanocrystalline CrMnFeCoNi HEAs were prepared by powder metallurgy. It was found mechanical milling can further refine the microstructures and morphologies of the gas-atomized powder, and increase the sintering ability. The HEAs sintered from the mechanically milled powder have much finer microstructures than that from the gas-atomized powder. The original morphology and defects in both the gas-atomized and the mechanically milled powders can be inherited to the bulk forms after the SPS. The SPSed HEAs have a tensile strength as high as 1000 MPa at room temperature and reasonable ductility. The strengthening mechanism can be attributed to the nanocrystalline microstructures, in which grain boundaries block the movement of dislocations. Powder metallurgy can be taken as a promising way for preparing HEAs with high mechanical properties.     

粉末冶金制备TNZS基生物材料的体外组织相容性

采用高能球磨与冷压烧结相结合的粉末冶金法制备了TNZS、5%TiO_2/TNZS及5%HA/TNZS(质量分数,下同)生物材料,并研究了TiO_2和HA的添加对TNZS体外组织相容性的影响。结束表明:3种TNZS基生物材料均无细胞毒性;5%TiO_2/TNZS和5%HA/TNZS表面在1,3,5和7 d的细胞相对增殖抑制率CPIR值大幅低于TNZS组,TiO_2和HA的添加显著提高了TNZS材料表面的细胞增殖速度,增强了其细胞增殖能力,更有助于诱导成骨细胞的体外增殖;3种TNZS基材料表面贴附的成骨细胞伪足伸展状态良好,而5%TiO_2/TNZS表面贴附的成骨细胞分布更加均匀。     

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.     

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.     

A feasibility study of W-Cu composites production by high pressure compression of tungsten powder

For manufacturing a heavy duty W-Cu composite, a porous tungsten skeleton is required; which later can be filled by molten copper via infiltration technique. The compression force usually up to 200 MPa can be provided by cold isostatic press (CIP) and the temperatures used for sintering the green compacts are more 2000 °C. However, in this research, high pressure within the range of 200 to 663 MPa was used to produce high density green specimens (60-80%) by CIP while sintering was carried out at a moderate temperature of 1550 °C. The tungsten skeletons were infiltrated with molten copper at 1300 °C.The reduction of sintering temperature from over 2000 °C to 1550 °C for a highly densified W-skeleton not only resulted into a successful production of W-Cu composites but also the obtained physical and mechanical properties of these composites are comparable to those obtained for lower compaction pressures and sintering temperature higher than 2000 °C.