Microstructure of Zn-Al4 alloy microcastings by micro precision casting based on metal mold

Metal mold micron scale precision casting technology was developed successfully,and three-dimension complicated microgear castings in micron scale were produced.Evolvement regularity of microgear castings were observed by optical microscope and scanning electron microscope.Compared with conventional casting,microcasting is characterized by typical nonequilibrium solidification,for example,its grain size can be refined significantly,eutectic structure is transformed from lamellar morphology to rod eutectic,and the ratio of primary phase is increased.This kind of microstructure can promote mechanical properties of microcasting.

Oxygen variation in titanium powder and metal injection molding

The control of oxygen is paramount in achieving high-performance titanium(Ti)parts by powder metallurgy such as metal in-jection molding(MIM).In this study,we purposely selected the Ti and Ti-6Al-4V powders as the reference materials since these two are the most representative Ti materials in the industry.Herein,hydride-dehydride(HDH)Ti powders were pre-oxidized to examine the ef-fect of oxygen variation on the characteristics of oxide layer on the particle surface and its resultant color feature.The results indicate that the thickness and Ti oxide level(Ti^(0)→Ti^(4+))of the oxide layer on the HDH Ti powders increased as the oxygen content increased,lead-ing to the transition of color appearance from grey,brown to blue.This work aids in the powder feedstock selection at the initial stage in powder metallurgy.In addition,the development of oxygen content was comprehensively studied during the MIM process using the gas-atomized(GA)Ti-6Al-4V powders.Particularly,the oxygen variation in the form of oxide layer,the change of oxygen content in the powders,and the relevant parts were investigated during the processes of kneading,injection,debinding,and sintering.The oxygen vari-ation was mainly concentrated in the sintering stage,and the content increased with the increase of sintering temperature.The variation of oxygen content during the MIM process demonstrates the crucial role of powder feedstock and sintering stage in controlling oxygen con-tent.This work provides a piece of valuable information on oxygen detecting,control,and manipulation for the powder and processing in the industry of Ti and its alloys by powder metallurgy.

Evolution of stresses in metal injection molding parts during sintering

The evolution of stresses due to inhomogeneity in metal injection molding （MIM） parts during sintering was investigated. The sintering model of porous materials during densification process was developed based on the continuum mechanics and thermal elasto-viseoplastic constitutive law. Model parameters were identified from the dilatometer sintering experiment. The real density distribution of green body was measured by X-ray computed tomography （CT）, which was regarded as the initial condition of sintering model. Numerical calculation of the above sintering model was carried out with the finite element soRware Abaqus, through the user-defined material mechanical behavior （UMAT）. The calculation results showed that shrinkages of low density regions were faster than those of high density regions during sintering, which led to internal stresses. Compressive stresses existed in high density regions and tensile stresses existed in low density regions. The densification of local regions depended on not only the initial density, but also the evolution of stresses during the sintering stage.

Effective metal mold method for the production of bionic adhesives based on electrochemical modifications

Bionic adhesives with tip-expanded microstructural arrays have attracted considerable interest owing to their high adhesive performance at low preloads.Their mainstream manufacturing method is molding.Due to most molds are made of silicon or silicon-based soft templates,and have poor wear resistant or vulnerability to high temperature,limiting their use in large-scale manufacturing.Nickel is widely used as an embossing mold in the micro/nano-imprint industrial process owing to its good mechanical properties.However,the processing of metal molds for the fabrication of tip-expanded microstructural arrays is extremely challenging.In this study,using electrodeposition techniques,the shape of the micropores is modified to obtain end-controlled pores.The effect of the non-uniformity of the electric field on the microporous morphology in the electrodeposition process is systematically investigated.Furthermore,the mechanism of non-uniformity evolution of the microporous morphology is revealed.The optimized microporous metal array is used as a mold to investigate the cavity evolution laws of the elastic cushions under pre-load during the manufacturing process.As a result,typical bionic adhesives with tip-expansion are obtained.Moreover,corresponding adhesion mechanics are analyzed.The results show that electrochemical modifications have broad application prospects in the fabrication of tip-expanded microstructures,providing a new method for the large-scale fabrication of bionic adhesives based on metal molds.

Fabrication and sintering behavior of high-nitrogen nickel-free stainless steels by metal injection molding

High-nitrogen nickel-free stainless steels were fabricated by the metal injection molding technique using high nitrogen alloying powders and a mixture of three polymers as binders.Mixtures of metal powders and binders with various proportions were also investigated, and an optimum powder loading capacity was determined as 64vol%.Intact injection molded compacts were successfully obtained by regulating the processing parameters.The debinding process for molded compacts was optimized with a combination of thermo-gravimetric analysis and differential scanning calorimetry analysis.An optimum relative density and nitrogen content of the specimens are obtained at 1360℃,which are 97.8%and 0.79wt%,respectively.

Biocompatibility of vascular stents manufactured using metal injection molding in animal experiments

This study aimed to evaluate the feasibility and safety of a novel stent manufactured by metal injection molding(MIM)in clinical practice through animal experiments.Vessel stents were prepared using powder injection molding technology to considerably improve material utilization.The influence of MIM carbon impurity variation on the mechanical properties and corrosion resistance of 316L stainless steel was studied.In vitro cytotoxicity and animal transplantation tests were also carried out to evaluate the safety of MIM stents.The results showed that the performance of 316L stainless steel was very sensitive to the carbon content.Carbon fluctuations should be precisely controlled during MIM.All MIM stents were successfully implanted into the aortas of the dogs,and the MIM 316L stents had no significant cytotoxicity.The novel intravascular stent manufactured using MIM can maintain a stable form and structure with fast endothelialization of the luminal surface of the stent and ensure long-term patency in an animal model.The novel intravascular stent manufactured using MIM demonstrates favorable structural,physical,and chemical stability,as well as biocompatibility,offering promising application in clinical practice.

Porous titanium implants fabricated by metal injection molding

Sodium chloride(NaCl)was added as a space holder in synthesis of porous titanium by using metal injection molding(MIM)method.The microstructure and mechanical properties of porous titanium were analyzed by mercury porosimeter, scanning electron microscope(SEM)and compression tester.The results show that the content of NaCl influences the porosity of porous titanium significantly.Porous titanium powders with porosity in the range of 42.4%-71.6%and pore size up to 300μm were fabricated.The mechanical test shows that with increasing NaCl content,the compressive strength decreases from 316.6 to 17.5 MPa and the elastic modulus decreases from 3.03 to 0.28 GPa.

Antioxidation Study of Sm(Co, Cu, Fe, Zr)_z-Sintered Permanent Magnets by Metal Injection Molding

Antioxidation effects on Sm （Co, Cu, Fe, Zr）z-sintered magnets treated by different methods were studied through TGA and DTA. Microstructure of Sm（Co, Cu, Fe, Zr）z-sintered magnets was analyzed through SEM and EDS. The results indicate that the antioxidation effect of the alloy powder treated in silane solution is better than that of the other methods. The alloy powders treated in stearic acid （SA） solution and polymethyl methacrylate （PMMA） solution can prevent powders from oxidation for a short period of time. Silane solution is not suitable for metal injection molding （MIM） because it severely damages the magnetic properties and microstructure of Sm（Co, Cu, Fe, Zr）z-sintered magnets. SA solution can not only prevent powders from oxidizing in MIM, but also does not damage magnetic properties and microstructure of Sm（Co, Cu, Fe, Zr）z magnets. The oxygen content of Sm（Co, Cu, Fe, Zr）z-sintered magnets by MIM is 3300μg·g^-1.

RHEOLOGICAL PROPERTIES OF METAL INVECTION MOLDING BINDER AND FEEDSTOCK

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Phase Behaviors in Bi-phase Simulation of Powder Segregation in Metal Injection Molding

Powder segregation induced by mold filling is an important phenomenon that affects the final quality of metal injection molding （MIM）. The prediction of segregation in MIM requires a bi-phase flow model to describe distinctly the flows of metallic powder and polymer binder. Viscous behaviors for the flows of each phase should hence be determined. The coefficient of interaction between the flows of two phases should also be evaluated. However, only viscosity of the mixed feedstock is measurable by capillary tests. Wall sticking is supposed in the traditional model for capillary tests, while the wall slip is important to be taken into account in MIM injection. Objective of the present paper is to introduce the slip effect in bi-phase simulation, and search the suitable way to determine the viscous behaviors for each phase with the consideration of wall slip in capillary tests. Analytical and numerical methods were proposed to realize such a specific purpose. The proposed method is based on the mass conservation between the capillary flows in mono-phase model for the mixed feedstock and in bi-phase model for the flows of two phases. Examples of the bi-phase simulation in MIM were realized with the software developed by research team. The results show evident segregation, which is valuable for improving the mould designs.

Synthesis and properties of nanocrystalline nonferrous metals prepared by flow-levitation-molding method

Nanocrystalline nonferrous metals (Cu, Al, and Ag) were synthesized by flow-levitation-molding method. The microstructure of the as-prepared nanocrystalline metals was characterized by XRD and FESEM. The microhardness and electrical resistivity were tested by the HMV-2 type Microhardness Tester and 6157 type Electrometer, respectively. The synthesis process was also studied. The results show that the spheriform particles in nanocrystalline metals have average grain size of 20-30 nm. The relative density of nanocrystalline Cu, Al, and Ag are 95.1%, 98.1% and 98.3%, respectively. The microhardness of nanocrystalline Cu, Al and Ag are 2.01, 2.11 and 1.26 GPa respectively, which are larger than those of their coarse-grained counterparts by the factor of 4.5, 14, and 2.5, respectively. The electrical resistivity of nanocrystalline Cu at room temperature is 1.5×10-7 Ω·m, which is higher than coarse-grained Cu by a factor of 7.5. The pressure is the predominant factor influencing the density of the as-prepared nanocrystalline nonferrous metals.

Mechanical properties and microstructure of Ti-6Al-4V compacts by metal injection molding

Ti-6Al-4V compacts were fabricated by metal injection molding(MIM). Influence of vacuum sintering time on mechanical properties and microstructure of the sintered compacts at 1 260 ℃ were investigated. The experimental results show that the compacts sintered at 1 260 ℃ for 36 h, which was made from hydrogenation-dehydrogenation(HDH) powder(average particles size is 45 μm), have a relative density of 95.6% 96.7%, ultimate tensile strength of 648686MPa and 0.2% yield strength of 526615MPa; but a lower elongation(<4%) and that the compacts sintered at 1 260 ℃ for 26 h, which was made from 90% gas-atomized powder(average particles size is 32.5 μm) and 10% HDH powder, have higher relative density(>95%), ultimate tensile strength of 800848MPa, 0.2% yield strength of 712762MPa and high elongation (7.4%9.5%). When the sintering time is increased, porosity decreases and microstructure of sintered products changes from equiaxed to typical Widmanstatten, the average sizes of prior β grains, α colonies and α phase thickness in the β grains increase accordingly. After HIP treatment, pores obviously become less, microstructure of alloy is refined and mechanical properties are greatly improved.

New development of powder metallurgy in automotive industry

The driving force for using powder metallurgy(PM)mostly relies on its near net-shape ability and cost-performance ratio.The automotive application is a main market of PM industry,requiring parts with competitive mechanical or functional performance in a mass production scale.As the automobile technology transforms from traditional internal combustion engine vehicles to new energy vehicles,PM technology is undergoing significant changes in manufacturing and materials development.This review outlines the challenges and opportunities generated by the changes in the automotive technology for PM.Low-cost,high-performance and light-weight are critical aspects for future PM materials development.Therefore,the studies on PM lean-alloyed steel,aluminum alloys,and titanium alloy materials were reviewed.In addition,PM soft magnetic composite applied to new energy vehicles was discussed.Then new opportunities for advanced processing,such as metal injection molding(MIM)and additive manufacturing(AM),in automotive industry were stated.In general,the change in automotive industry raises sufficient development space for PM.While,emerging technologies require more preeminent PM materials.Iron-based parts are still the main PM products due to their mechanical performance and low cost.MIM will occupy the growing market of highly flexible and complex parts.AM opens a door for fast prototyping,great flexibility and customizing at low cost,driving weight and assembling reduction.

Preparation of Ti-Mo getters by injection molding

Ti-Mo getters have been fabricated via metal injection molding (MIM) using three kinds of Ti powders with different mean particle sizes of 46 μm,35 μm and 26 μm,respectively. The surface morphology,porosity,and hydrogen sorption properties of Ti-Mo getters formed by MIM using paraffin wax as a principal binder constituent were examined. It has been proven that the powder injection molding is a viable forming technique for porous Ti-Mo getters. The particle size of Ti powders and the powder loading influence...

Rheological, mechanical and corrosive properties of injection molded 17-4PH stainless steel

The feedstock based on the binder 65%PW-30%EVA-5%SA has the best general rheological properties for the 17-4PH stainless steel powder. The 17-4PH stainless steel compacts sintered at 1 380 ℃ for 90 min have the best mechanical properties and the good microstructure with homogeneously distributed pore structure and the moderate-sized grains. Whereas the compacts sintered for 60 min and 120 min show an inadequate and an over-sintered microstructure respectively. The compacts sintered at 1380 ℃ for 90 min have the density of 7.70 g/cm^3, the strength of 1 275 MPa, the elongation of 5%, and hardness of HRC36. With the increase of sintering temperature, the density, strength and hardness increase, while the elongation decreases. The 17-4PH stainless steel has good corrosion resistance, showing an activation-passivation polarization curve. But the passivation potential range is narrow and the spot corrosion potential is low, indicating a low anti-spot corrosive properties.

Effect of annealing processing on microstructure and properties of Ti-6Al-4V alloy by powder injection molding

Ti6A14V alloy parts were prepared by metal injection molding. Brown parts were densified at 1 200-1 260℃for 2-4 h in vacuum atmosphere. The as-sintered specimens were treated through Hot-Isostatic Pressure(HIP) at 960℃and 140 MPa. Ti6A14V alloy compacts were annealed at 720-760℃for 1 h. The results show that binder in the parts can be removed by solvent debinding and thermal debinding process. Ti6A14V alloy has an uniform duplex microstructure with many equiaxedαgrains and a littleβgrains. When the annealing temperature is higher than 800℃, T16A14V alloy has lower mechanical properties.After solution treatment and aging, a typical martensite microstructure can be achieved.

基于Moldflow的带金属嵌件打印机板浇口优化设计

针对带金属嵌件打印机板使用传统注塑模具设计方法时,存在开发周期长、成本高且质量不易保证等缺点,采用计算机辅助设计软件Pro/E和有限元分析软件Moldflow相结合的方法对制品进行不同浇口位置方案下的"填充+保压+翘曲"模拟分析,从充填时间、流动前沿温度、气穴、熔接痕和翘曲变形等方面进行了综合比较和评测,确定了最佳浇口设计方案。结果表明,应用Moldflow软件进行模拟仿真,找出了制件在不同浇口设置环境下缺陷产生的原因,从而为避免或消除制件上的缺陷,进一步优化模具设计、降低生产成本,为指导实际生产提供合理的依据。

Investigation on the Moldability of HDDR NdFeB Bonded Magnet Prepared by Compression Molding Method

The phenomenon of uneven density distribution is inevitable in the process of preparing bonded magnets with complicated shape or with large geometric size by compression molding due to the friction among magnetic powder grains and between magnetic powder and the die wall, which reduces the pressure along the compression direction. In order to improve the density distribution homogeneity, the thin wall rings with composition of HDDR NdFeB prepared by compression molding were selected as the investigated object in this study. It was systematically investigated on the effect of addition level of lubricants and the addition methods of lubricants and strengthener on magnet density, density distribution and on magnet strength. By means of joint addition of lubricants and strengthener, which increases the magnet density and improves the magnet density distribution under the precondition of keeping the magnet strength unreduced. Thus the moldability of the bonded magnets is improved.

Injection molding of micro-porous titanium alloy with space holder technique

The powder space holder （PSH） and powder injection molding （PIM） methods have an industrial competitive advantage because they are capable of the net-shape production of micro-sized porous parts. In this study, micro-porous Ti6Al4V alloy （Ti64） parts were produced by the PSH-PIM process. Ti64 alloy powder and spherical polymethylrnethacrylate （PMMA） particles were used as a space holder material. After molding, binder debinding was performed by thermal method under inert gas. Debinded samples were sintered at 1250℃ for 60min in a vacuum （10-4 Pa）. Metallographic studies were conducted to determine densification and the corresponding microstructural changes. The surface of sintered samples was examined by SEM. The compressive stress and elastic modulus of the rificro-porous Ti64 samples were determined. The effects of fraction of PMMA on the properties of sintered micro-porous Ti64 alloy samples were investigated. It was shown that the fraction of PMMA could be controlled to affect the properties of the Ti alloy.

Development of wax-oil-polyethylene binder for Fe-2Ni powder injection molding

A new wax oil polyethylene binder was developed for Fe 2Ni powder injection molding. The advantages of both wax based and oil based binders were acquired. Moreover, the debinding solvent could be recycled. The miscibility of the binder ingredients and the characteristics were evaluated. The results show that the maximum powder loading can reach 60%(volume fraction), and the two direction debinding rate is over 2 mm/h. The drying time for 6.37 mm thick samples is less than 20 min. The recycle ratio of the debinding solvent is over 95%.