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2018 Vol. 23, No. 6
Published: 2018-12-20
Engineering and Technology
547
Effects of the magnesium content on the microstructure and mechanical properties of Al-Cu-Mg sintered alloy
XIE Jiaoya, LIU Rutie, CHEN Jie, XIONG Xiang, LUAN Huaizhuang
Al-3.9Cu-Mg alloy with Mg content (mass fraction) of 0.5%, 0.8%, 1%, 1.2% and 1.6% were prepared by powder metallurgy using high-purity aluminum powder, copper powder and Mg powder as raw materials. The microstructural analysis and mechanical property tests were carried out to study the effects of Mg content on the microstructure and mechanical properties. The results show that the optimum sintering temperature is 620 ℃ when the Mg content is less than 2%. The microstructure of the alloy tends to be uniform with increasing the content of Mg. The density, hardness, and tensile strength of the alloy first increase and then decrease with increasing Mg content. The optimal properties of Al-3.9Cu-1Mg alloy with a density of 98.1%, a hardness (HBW) of 93.1, and a tensile strength of 242 MPa is obtained when the Mg content is 1%.
2018 Vol. 23 (6): 547-552 [
Abstract
] (
409
)
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766
)
553
Preparation and tensile behavior of SiC
f
/SiC minicomposites with (PyC/SiC)
n
multilayered interphases
YANG Ping, ZHANG Ruiqian, LI Yue, CHEN Zhaoke, HE Zhongbei, LIU Guiliang, FU Daogui, SUN Wei, WANG Yalei, XIONG Xiang
(PyC/SiC)
4
or (PyC/SiC)
8
multi-layer interphases were introduced into two types of SiC fiber bundles by CVI method and further densified with SiC to obtain SiC
f
/SiC minicomposites. The effects of fiber types and interface types on mechanical properties and fracture mechanism of SiC
f
/SiC minicomposites were studied. The results show that, the densified SiC
f
/SiC minicomposite is a whole. In the minicomposites, a clear interface layer with an uniform thickness can be observed between fiber and matrix. The maximum tensile strength of A/(PyC/SiC)
4
/SiC, B/(PyC/SiC)
4
/SiC and A/(PyC/SiC)
8
/SiC minicomposites are 466, 350 and 330 MPa respectively, with the ultimate tensile strain of 0.519%, 0.219% and 0.330%, respectively. In addition, the fracture morphologies of minicomposites and the length of pull-out fiber with different types of reinforced fiber and interface are quite different, indicated a different fracture mode. The A/(PyC/SiC)
4
/SiC fractures in Model Ⅱ, B/(PyC/SiC)
4
/SiC and A/(PyC/SiC)
8
/SiC fracture in ModelⅠ.
2018 Vol. 23 (6): 553-561 [
Abstract
] (
376
)
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919
)
562
Microstructure and mechanical properties of Ni particles reinforced AZ61 composites
FENG Yan, CHEN Chao, WANG Richu, WANG Xiaofeng
Ni particle reinforced AZ61 composites with the Ni mass fraction of 3%,6% and 9% respectively were prepared by closed semi-solid stirring casting and hot extrusion, and the microstructure, mechanical properties and strengthening mechanism were studied. The results show that the reinforcement phases of Ni
p
/AZ61 composite are the fine Mg
2
Ni phase and large AlNi phase, and their sizes are 2-4 μm and 10-20 μm, respectively. The Mg
2
Ni and AlNi phases can effectively refine the grains, promote recrystallization, and have a good bonding with the magnesium matrix. Dislocation pile-up are present around the particles and twins are present in the matrix. Both Mg
2
Ni and AlNi phases can effectively improve the hardness, elastic moduous and tensile strength, but the elongation decreases. 9%Ni
p
/AZ61 has tensile strength 286.5±1.2 MPa and elongation of and 4.3%±0.9%. The strengthening mechanism of Ni
p
/AZ61 are fine grain strengthening, Orowan strengthening and load transfer effect.
2018 Vol. 23 (6): 562-568 [
Abstract
] (
394
)
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1125
)
569
Effects of carbon additive amount on the microstructure and properties of TiC-based cermets
YAO Songsong, CHENG Huichao, FAN Jinglian, LIU Tao
Using TiC powder, Mo powder, WC powder, and Ni powder as raw materials, 0, 1%, 2%, and 3% (mass fraction) of C powder were respectively added to prepare TiC-WC-Ni-Mo cermets, and the effects of C additive amount on the microstructure and properties of TiC based cermets were studied. The results show that the TiC-based cermets shows a typical black-core and grey-rim structure the main component of the core is TiC, and the rim is composed of Ti, W, Mo and other elements. With the increase of C additive amount, the grain size of the cermets decreases, but the porosity increases, the rim becomes thinner, the hardness of the material decreases, and the bending strength increases first and then decreases. When the C additive amount is 2%, the bending strength of the material reaches the maximum value of 1 061 MPa, and the hardness is 14.29 GPa.
2018 Vol. 23 (6): 569-574 [
Abstract
] (
362
)
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(589 KB) (
1020
)
575
Microstructural evolution of a commercial pure Ni processed by ambient and cryogenic rolling
LI Yan, NI Song, CHEN Gang, SONG Min
The hardness and microstructural evolutions of a commercial pure nickel subjected to rolling (thickness reduction is 20%, 40% and 60%, respectively) at room temperature and cryogenic temperature were investigated using transmission electron microscope (TEM), optical microscope and Vickers microhardness testing. The results show that, the dislocation slip dominates the deformation process at small strain when the sample is rolled at ambient temperature. As the strain increases, dislocation and twins appeared to coordinate the deformation. When the sample is rolled at cryogenic temperature, a larger density of dislocations and more twins can be observed at small strain, compared to the sample rolled at the ambient temperature. In addition, the grain refinement process of the cryogenically treated sample is much faster than the room temperature rolled sample because the interaction between dislocations and MBs/twins is more severe due to the low speed of dynamic recovery. Both samples have a sharp hardness increase at small strain, then the hardness increases softly with increasing the strain. The cryogenically treated sample always has a higher hardness due to a high density of the dislocations.
2018 Vol. 23 (6): 575-581 [
Abstract
] (
424
)
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(549 KB) (
1395
)
582
Effects of processing parameters on microstructure and mechanical properties of Co-25Cr-5Mo-5W alloys by selective laser melting
HUANG Zonglian, WANG Bo, LIU Fei, MA Qing, LIU Shaojun
The Co-25Cr-5Mo-5W alloys were fabricated by selective laser melting (SLM) technique using gas atomization Co-25Cr-5Mo-5W powders as raw materials. The effects of the laser power, scanning speed, scanning space and laser energy density on the phase composition, microstructure and mechanical properties of SLM-ed Co-25Cr- 5Mo-5W alloys were investigated systematically by combining XRD, SEM, TEM and tensile testing. The results show that the laser power is positively associated with the sintering density of SLM Co-25Cr-5Mo-5W alloys. In contrast, it is negatively associated with the scanning speed (excluding 300 mm/s) and scanning space. The γ(
fcc
) and ε(
hcp
) phases are observed in the SLM Co-25Cr-5Mo-5W alloys. They present SN (Shoji-Nishiyama) orientation relationship The fine and uniform columnar crystals, cellular crystal and nanosized carbide precipitates result in the improvement of the mechanical properties of SLM-ed alloy. Co-25Cr-5Mo-5W alloys by SLM with high strength and good ductility are achieved when the laser power, the scanning speed, the scanning space and the laser energy density are 160 W, 400 mm/s, 0.07 mm, and 190.4 J/mm
3
, respectively. The relative density, tensile strength, yield strength and elongation are 98.6%, 1 284 MPa, 934 MPa, and 16% respectively.
2018 Vol. 23 (6): 582-590 [
Abstract
] (
358
)
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(0 KB)
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(805 KB) (
794
)
591
Effect of pre-oxidation treatment time on C/C composites and its SiC anti-oxidation coating
WANG Yujie, ZHANG Mingyu, SU Zhean, TONG Kai, HUANG Qizhong
The SiC coating with embedded interface was prepared on the surface of C/C composites by pre-oxidation treatment and chemical vapor deposition. The effects of pre-oxidation time on the microstructure of C/C composites and SiC coating were investigated, and the anti-oxidation test at 1 500 ℃ of the coated sample was carried out. The results show that the surface of the pre-oxidized material possesses porous structure, and the oxidation rate of matrix is greater than that of fiber. As the pre-oxidation time increases, the surface roughness of C/C composites and the embedded depth of the SiC coating increase, the coating crack narrows. In the high temperature oxidation experiments, the mass loss rates of the coated samples prepared by pre-oxidation treatment of 6 min and 9 min are 2.34% and 1.89% respectively after oxidation for 21 h. The samples prepared by 6min pre-oxidation show better oxidation resistance, and the mass loss rate is 4.11% after static oxidation for 26 h. These two samples can both produce “pinning effect” on the matrix in the first 21 h oxidation test. Nevertheless after 21 h, the thermal stress damage of the laminar pyrolytic carbon in the samples prepared by 9min pre-oxidation treatment lead to the decrease of interfacial bonding strength, which is the main reason for its rapid failure.
2018 Vol. 23 (6): 591-599 [
Abstract
] (
370
)
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(1111 KB) (
1217
)
600
Optimization design of the VVT-Cam sprocket
SHEN Xiaoping, HUANG Yongqiang
VVT-Cam sprocket is a key part of automobile engine, which consequently require a relatively high matching precision as an automobile transmission mechanism. In this paper, based on the continuum powder material model, a simulation analysis of the density distribution of the green compact under three schematic designs was carried out using the finite element software- DEFORM. The structural and dimensional optimization of the mold and the improvement of the uniformity of density distribution were realized through the numerical simulation analysis. The results prove that the optimal design can satisfy the accuracy, density and performance requirements for the sprocket manufactured by the advanced net forming method of powder metallurgy. A upper and a bottom die structure with a powder leaking hole at the corresponding position of the keyhole is designed, so that the supersaturated powder at the keyhole can be moved and the phenomenon of overpressure can be prevented. When the size of leaking hole is small, the overpressure phenomenon can not be alleviated. When the size of leaking hole is large, the powder around the keyhole loses, forming a low density area, resulting in uneven overall density distribution. When the volume of the powder leakage hole is designed as 2/3 of the keyhole volume, the blank with the most uniform density distribution can be obtained.
2018 Vol. 23 (6): 600-606 [
Abstract
] (
411
)
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(458 KB) (
1324
)
607
Effect of reducing agent concentration on the growth of Sn-Ag nanoparticles synthesized by chemical co-reduction method
WANG Yao, MA Yunzhu, LIU Wensheng, TANG Siwei, HUANG Yufeng
Sn-3.5%Ag (mass fraction, %) nanoparticles were synthesized by chemical co-reduction method and the morphology, phase composition and thermal behavior were studied. The effects of reducing agent concentration on nanoparticles size and the mechanism of nucleation and growth in the solution were studied. The results show that the concentration of reducing agent controls the rate of particle nucleation and growth in the solution, and the average particle size decreases with increasing the reducing agent concentration. The average radius of the nanoparticles reaches a minimum value of about 13nm when the reducing agent concentration is 0.25 mol/L. The nanoparticles consist of β-Sn and Ag
3
Sn phases. The minimum melting point is about 206 ℃. The increasing concentration of the reducing agent consumes a large amount of metal ions for instantaneous nucleation and inhibits further growth of the nanoparticles. Ag
3
Sn nucleates and grows in the solution preferentially. With the decrease of Ag atoms saturation in the solution, the composition of the nanoparticles is dominated by β-Sn. Finally forms nano-alloy consisting of Ag
3
Sn and β-Sn phases.
2018 Vol. 23 (6): 607-613 [
Abstract
] (
415
)
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1468
)
614
Effects of particle size and dispersion of TiC on the microstructure and tensile strength of Mo alloy
CHEN Jiyong, FAN Jinlian, CHENG Huichao
Two Mo-TiC alloys with TiC mass fraction of 6% were prepared by powder metallurgy using ultrafine TiC powder with a particle size of 0.5 μm and a nanometer TiC powder with a particle size of 0.05 μm as raw materials. The effects of particle size and dispersion of TiC on the microstructures and tensile strength of Mo-TiC alloys were studied. The results show that, compared to the addition of ultrafine TiC powder, the addition of nanometer TiC powder has a significant improvement on the tensile strength of the alloy, but because the agglomeration of nano TiC powder in the matrix. When sintered at 1 950 ℃, the tensile strength of Mo-nano TiC alloy reaches 515 MPa, which is more than 30% higher than that of Moultrafine TiC alloy. After adding organic compound as a dispersant, the TiC aggregates in the Mo base are transformed from the spheroid to elongated shape, and some of the nanoscale TiC are fully dispersed the alloy grain is further refined and the strength of the alloy is improved. The tensile strength of the material sintered at 1 850 ℃ reaches 615 MPa.
2018 Vol. 23 (6): 614-618 [
Abstract
] (
343
)
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(357 KB) (
687
)
619
Modification of oil-wax-polypropylene based binder by EPDM for MIM
ZHANG Chenming, LU Renwei, YANG Zhongchen, LI Duxin
EPDM was used as modifier in oil-wax-polypropylene based binder system for MIM in this paper. Four formulations of binder and 17-4PH stainless steel powder were mixed to prepare feedstocks and then injection molding. The effects of EPDM content on the rheological characteristics of feedstocks, properties of green parts as well as degreasing properties were studied. The results show that the viscosity of feedstock increases with EPDM addition. With the increase of EPDM content, the impact energy of green parts increases as well as bending strength decreases. The optimal mechanical properties of green parts are obtained when EPDM content is 2%. After seven cycles, the impact energy is 2.048 J, the bending strength is 6.3 MPa. In addition, the speed of solvent debinding increases with EPDM addition. Carbon content and oxygen content of debound parts can keep in reasonable range.
2018 Vol. 23 (6): 619-623 [
Abstract
] (
424
)
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(0 KB)
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(407 KB) (
920
)
624
Syntheses and characteristics of ultrafine AlN powder by carbothermal reduction-nitridation
WEI Xin, ZHANG Hao, SUN Dengqiong, TIAN Chenguang, LIU Xiangyu, CUI Song, TANG Wenming
The porous precursors were fabricated by extruding the mixture of superfine Al
2
O
3
powder, nano carbon black and organic binder, and then adhesives-discharging. The precursors were calcined at high temperature to form AlN via the carbothermal reduction-nitridation reaction. Finally, the residual carbon was removed from the AlN calcined bodies and the AlN powder was obtained. Morphology, crystallinity, grain size, average particle size and the O, N and C contents of the AlN powder were investigated. The results show that the precursor has a porous structure with a high special surface area and spherical pores after adhesives-discharging at 360 ℃ for 2 h. After calcining at 1 650 ℃ and above, the product is composed of AlN, no Al
2
O
3
residues is detected, indicating fulfillment of the AlN synthesis process. This experimental result agrees well with the thermodynamic calculation. The decarburization temperature has the greatest effect on the C content of the AlN powder and the feed rate is in the second place. The AlN powder synthesized by calcining at 1 700 ℃ for 12 h and decarburizing at 700 ℃ under the conditions of the pure oxygen atmosphere and the feed rate of 4 kg/h has a composition of 0.77% O, 33.42%N and 323×10
-6
C. The powder also has a high degree of sphericility, a narrow particle size distribution and an average particle size (
d
50
) as low as 1.36 μm. Such characteristics of the AlN powder are close to those of the foreign similar products.
2018 Vol. 23 (6): 624-631 [
Abstract
] (
534
)
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(648 KB) (
1829
)
632
Microstructure and mechanical properties of in-situ (Y
2
O
3
+TiB)/Ti-6Al-4V composites
HU Jun, LIU Huiqun, WANG Bin, YI Danqing, TIAN Yu, WANG Nanhai
The rare earth compound YB
4
was added into Ti-6Al-4V alloy fabricated using Ti and Al-40V alloy powders as raw materials to prepare in-situ (Y
2
O
3
+TiB)/Ti-6Al-4V composites by the high temperature thermal decomposition reaction of YB
4
with titanium matrix. The effects of the amount of YB
4
addition on the microstructure and mechanical properties of the composite were studied. The microstructures and phase compositions of the composites were analyzed by X-ray diffractometer (XRD), SEM and EPMA. The mechanical properties of materials were tested by electronic stretcher. The results show that the YB
4
reacts with titanium matrix and in-situ generate Y
2
O
3
particles and TiB whiskers. With increasing YB
4
additive amount, Y
2
O
3
particles gradually agglomerate and TiB whiskers grow. The minor content of YB
4
in (Y
2
O
3
+TiB)/Ti-6Al-4V composites can slightly increase the tensile strength and significantly improve the elongation. In-situ synthesized (TiB+Y
2
O
3
)/Ti-6Al-4V composites achieved both high tensile strength (924.0 MPa) and high tensile ductility (9.13%) with adding mass fraction of 0.263% YB
4
powder.
2018 Vol. 23 (6): 632-639 [
Abstract
] (
384
)
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(692 KB) (
3494
)
640
Prepared process and mechanism of spherical iron nano-powder by the hydrogen reduction of calcium ferrites
ZHANG Yuqi, GAO Zili, CHEN Xingui, WU Aihua, YE Nan, WU Jiajing, TANG Jiancheng
Three kinds of percursors powders including single phase CaFe
2
O
4
, single phase CaFe
2
O
4
and mixed phase Ca
2
Fe
2
O
5
/CaO were prepared by calcine using CaO and Fe
2
O
3
as raw materials. Nanometer Fe powders were prepared by reducing percursors powders under hydrogen. The effects of the type of percursors on the reducing temperature, the morphology and particle size of Fe powders were studied. The formation mechanism of spherical nanometer Fe powders prepared by reducing calcium ferrite was discussed. The results show that the reducing temperature increases, the particle size of Fe powders decreases and the morphology of Fe powders change from irregular to sphere, with increasing the ratio of CaO. The spherical Fe powders with the average particle size of 34 nm are obtained by reducing Ca
2
Fe
2
O
5
/CaO percursor at 950 ℃ under hydrogen, the particle size distribution is uniform and the didpersity is good. The CaO plays a role in dispersing and promoting the nucleation of Fe powder in the preparation of nano Fe powder.
2018 Vol. 23 (6): 640-646 [
Abstract
] (
341
)
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(568 KB) (
892
)
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