Microstructural evolution during direct laser sintering of multi-component Cu-based metal powder

A multi-component Cu-based metal powder was chosen for direct laser sintering. The powder consists of a mixture of high-purity Cu powder, pre-alloyed CuSn and CuP powder. Liquid phase sintering with complete melting of the binder (CuSn) but non-melting of the cores of structural metal (Cu) proves to be a feasible mechanism for laser sintering of this powder system. The microstructural evolution of the sintered powder with variation of laser processing parameters was presented. High sintering activities and sound densification response were obtained by optimizing the laser powers and scan speeds. Using a high laser power accompanied by a high scan speed gives rise to balling effect. At a high laser power with a slow scan speed the sintering mechanism may change into complete melting/solidification, which decreases the obtainable sintered density. The role of additive phosphorus in the laser sintering process is addressed. Phosphorus can act as a fluxing agent and has a preferential reaction with oxygen to form phosphatic slag, protecting the Cu particles from oxidation. The phosphatic slag shows a concentration along grain boundaries due to its light mass as well as the short thermal cycle of SLS.

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.

Ballistic performance of titanium-based layered composites made using blended elemental powder metallurgy and hot isostatic pressing

Metal matrix composites tiles based on Ti-6Al-4V(Ti64)alloy,reinforced with 10,20,and 40(vol%)of either TiC or TiB particles were made using press-and-sinter blended elemental powder metallurgy(BEPM)and then bonded together into 3-layer laminated plates using hot isostatic pressing(HIP).The laminates were ballistically tested and demonstrated superior performance.The microstructure and properties of the laminates were analyzed to determine the effect of the BEPM and HIP processing on the ballistic properties of the layered plates.The effect of porosity in sintered composites on further diffusion bonding of the plates during HIP is analyzed to understand the bonding features at the interfaces between different adjacent layers in the laminate.Exceptional ballistic performance of fabricated structures was explained by a significant reduction in the residual porosity of the BEPM products by their additional processing using HIP,which provides an unprecedented increase in the hardness of the layered composites.It is argued that the combination of the used two technologies,BEPM and HIP is principally complimentary for the materials in question with the abilities to solve the essential problems of each used individually.

Preparation and properties of novel Cu-based bulk metallic glasses Cu_(55-x)Zr_(37)Ti_8In_x

The glassy rods were successfully fabricated in the Cu-Zr-Ti-In alloy system by casting into a copper mold. The value of ATx reaches a maximum of 66 K for the BMG CusoZraTTi8In5 alloy. The reasons for enhancing glass forming ability of Cu-based BMGs with the addition of indium were discussed from atomic size and thermodynamics. Alternatively, the BMG Cu52Zra7Ti8In3 exhibits the highest compressive strength （1981 MPa） and the best plasticity among glassy Cu55-xZra7TisInx （x_〈5）. The total plastic deformation of Cu52Zr37TisIn3 before fracture approaches 1.2%.

Effects of aspect ratio and loading rate on room-temperature mechanical properties of Cu-based bulk metallic glasses

Room-temperature mechanical properties of Cu50Zr40Ti10-xNix（0≤x≤4,mole fraction,%） bulk metallic glasses （BMG） with aspect ratios in the range of 1：1-2.5：1 and loading rates in the range of1×10^-5-1×10^-2s^-1were systematically investigated by room-temperatureuniaxialcompression test.In the condition of an aspect ratio of 1：1, the superplasticity can be clearly observed for Cu50Zr40Ti10BMG when the loading rate is1×10^-4s^-1, while for Cu50Zr40Ti10-xNix（x=1-3, mole fraction, %） BMGs when the loading rate is1×10^-2s^-1. The plastic strain （εp）, yielding strength （σy） and fracture strength （σf） of the studied Cu-based BMGs significantly depend on the aspect ratio and the loading rate. In addition, theσyof the studied Cu-based BMGs with an aspect ratio of 1：1 is close to the σfof those with the other aspect ratios when the loading rate is1×10^-2s^-1. The mechanism for the mechanical response to the loading rate and the aspect ratiowas also discussed.

Microstructure and thermal properties of dissimilar M300–CuCr1Zr alloys by multi-material laser-based powder bed fusion

Multi-material laser-based powder bed fusion (PBF-LB) allows manufacturing of parts with 3-dimensional gradient and additional functionality in a single step. This research focuses on the combination of thermally-conductive CuCr1Zr with hard M300 tool steel.Two interface configurations of M300 on CuCr1Zr and CuCr1Zr on M300 were investigated. Ultra-fine grains form at the interface due to the low mutual solubility of Cu and steel. The material mixing zone size is dependent on the configurations and tunable in the range of0.1–0.3 mm by introducing a separate set of parameters for the interface layers. Microcracks and pores mainly occur in the transition zone.Regardless of these defects, the thermal diffusivity of bimetallic parts with 50vol% of CuCr1Zr significantly increases by 70%–150%compared to pure M300. The thermal diffusivity of CuCr1Zr and the hardness of M300 steel can be enhanced simultaneously by applying the aging heat treatment.

Research progress in CALPHAD assisted metal additive manufacturing

Metal additive manufacturing(MAM)technology has experienced rapid development in recent years.As both equipment and materials progress towards increased maturity and commercialization,material metallurgy technology based on high energy sources has become a key factor influencing the future development of MAM.The calculation of phase diagrams(CALPHAD)is an essential method and tool for constructing multi-component phase diagrams by employing experimental phase diagrams and Gibbs free energy models of simple systems.By combining with the element mobility data and non-equilibrium phase transition model,it has been widely used in the analysis of traditional metal materials.The development of CALPHAD application technology for MAM is focused on the compositional design of printable materials,the reduction of metallurgical imperfections,and the control of microstructural attributes.This endeavor carries considerable theoretical and practical significance.This paper summarizes the important achievements of CALPHAD in additive manufacturing(AM)technology in recent years,including material design,process parameter optimization,microstructure evolution simulation,and properties prediction.Finally,the limitations of applying CALPHAD technology to MAM technology are discussed,along with prospective research directions.

Fabrication of homogeneously dispersed graphene/Al composites by solution mixing and powder metallurgy

Graphene-reinforced aluminum （AI） matrix composites were successfully prepared via solution mixing and powder metallurgy in this study. The mechanical properties of the composites were studied using microhardness and tensile tests. Compared to the pure Al alloy, the graphene/Al composites showed increased strength and hardness. A tensile strength of 255 MPa was achieved for the graphene/Al com- posite with only 0.3wt% graphene, which has a 25% increase over the tensile strength of the pure Al matrix. Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to investigate the morphol- ogies, chemical compositions, and microstructures of the graphene and the graphene/A1 composites. On the basis of fractographic evidence, a relevant fracture mechanism is proposed.

PREPARATION OF NANOSIZED METAL-OXIDE ULTRAFINE POWDERS BY ATOMIZING-COMBUSTION TECHNIQUE

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Effects of Metal and Composite Metal Nanopowders on the Thermal Decomposition of Ammonium Perchlorate (AP) and the Ammonium Perchlorate/Hydroxyterminated Polybutadiene (AP/HTPB) Composite Solid Propellant

Effects of metal (Ni, Cu, Al) and composite metal (NiB, NiCu, NiCuB) nanopowders on the thermal decomposition of ammonium perchlorate (AP) and composite solid propellant ammonium perchlorate/hydroxyterminated polybutadiene (AP/HTPB) were studied by thermal analysis (DTA). The results show that metal and composite metal nanopowders all have good catalytic effects on the thermal decomposition of AP and AP/HTPB composite solid propellant. The effects of metal nanopowders on the thermal decomposition of AP are less than those of the composite metal nanopowders. The effects of metal and composite metal nanopowders on the thermal decomposition of AP are different from those on the thermal decomposition of the AP/HTPB composite solid propellant.

INDUCTION PLASMA REACTIVE DEPOSITION OF TUNGSTEN CARBIDE FROM TUNGSTEN METAL POWDER

Experimental results on the primary carburization reaction between the tungsten powder and methane in the induction plasma, and the secondary carburization of the deposit on substrate at high temperature are reported. Optical microscopy and scanning electron microscopy were used to examine the microstructures of starting tungsten powder, carburized powder, and deposit. X-ray diffraction analysis, thermal gravimetric analysis and microhardness measurement were used to characterize the structures and properties of the powder and the deposit. It is found that the primary carburization reaction in the induction plasma starts from the surface of tungsten particles when the particles are melted. Tungsten particles are partially carburized inside the reactive plasma. Complete carburization is achieved through the secondary carburization reaction of the deposit on substrate at high temperature.

Comparison of Cu−Al−Ni−Mn−Zr shape memory alloy prepared by selective laser melting and conventional powder metallurgy

This work aimed to investigate and critically analyze the differences in microstructural features and thermal stability of Cu−11.3Al−3.2Ni−3.0Mn−0.5Zr shape memory alloy processed by selective laser melting(SLM)and conventional powder metallurgy.PM specimens were produced by sintering 106−180μm pre-alloyed powders under an argon atmosphere at 1060°C without secondary operations.SLM specimens were consolidated through melting 32−106μm pre-alloyed powders on a Cu−10Sn substrate.Mechanical properties were measured through Vickers hardness testing.Differential scanning calorimetry was conducted to assess the martensitic transformation temperatures.X-ray diffraction patterns were collected to identify the metallurgical phases.Optical and scanning electron microscopy was used to analyze the microstructural features.b′1 martensite was found,irrespective of the processing route,although coarser martensitic variants were present in PM-specimens.In conventional powder metallurgy samples,intergranular eutectoid constituents and stabilized austenite also formed at room temperature.PM-specimens showed similar average hardness values to the SLM-specimens,albeit with high standard deviation linked to the porosity.The specimens processed by SLM showed reversible martensitic transformation(T0=171°C).PM-processed specimens did not show shape memory effects.

Impact mechanism of gas temperature in metal powder production via gas atomization

This paper aims at studying the influence mechanism of gas temperatures(300 K,400 K,500 K,and 600 K)on gas atomization by simulating the integral atomization process of the close-coupled nozzle in vacuum induction gas atomization(VIGA).The primary atomization is simulated by the volume of fluid(VOF)approach,and the second atomization is studied by the discrete phase model(DPM)combined with the instability breakage model.The results show that,at an increased gas temperature,the influences of gas-liquid contact angle and gas temperature in the recirculation zone on the primary atomization are virtually negligible.However,increasing the gas temperature will increase the gas-liquid relative velocity near the recirculation zone and decrease the melt film thickness,which are the main reasons for the reduced mass median diameter(MMD,d50)of primary atomized droplets.During the secondary atomization,increasing the gas temperature from 300 K to 600 K results in an increase in the droplet dispersion angle,which is beneficial to the formation of spherical metal powder.In addition,increasing the gas temperature,the positive effect of gas-liquid relative velocity increase on droplets refinement overweighs the negative influence of the GMR decrease,resulting in the reduced MMD and diameter distribution interval.From the analysis of the atomization mechanism,the increase in atomization efficiency caused by increasing the temperature of the atomizing gas,including primary atomization and secondary atomization,is mainly due to the increase in the gas drag force difference between the inner and outer sides of the annular liquid film.

In vitro degradation and cytotoxicity of Mg-5Zn-0.3Ca/n HA biocomposites prepared by powder metallurgy

Mg-5Zn-0.3Ca/nHA biocomposites were prepared from pure Mg,Zn,Ca and nano-hydroxyapatite(nHA)powders by powder metallurgy method.The effect of various mass fractions of nHA(1%,2.5%,5%)as reinforcement on the corrosion properties of Mg-5Zn-0.3Ca alloy was investigated.The corrosion resistance of biocomposite samples was investigated by immersion tests and electrochemical techniques in SBF solution.The results showed that the corrosion resistance of Mg alloy was improved by adding 1%and 2.5%nHA.Bioactive nHA motivated the formation of stable phosphate and carbonate layers on surface and improved corrosion resistance of nanocomposites.However,addition of large contents of nHA in Mg alloy as reinforcement increased the density of this precipitated layer,so gases produced from localized corrosion were accumulated underneath this layer and decreased its adhesiveness and lowered its corrosion resistance.Indirect cytotoxicity evaluation for Mg alloy and its nanocomposites also showed that their extraction was not toxic and nanocomposite with 1%nHA indicated almost similar behavior as negative control.

Numerical investigation on flow process of liquid metals in melt delivery nozzle during gas atomization process for fine metal powder production

Based on volume of fluid(VoF)interface capturing method and shear-stress transport(SST)k-ω turbulence model,numerical simulation was performed to reveal the flow mechanism of metal melts in melt delivery nozzle(MDN)during gas atomization(GA)process.The experimental validation indicated that the numerical models could give a reasonable prediction on the melt flow process in the MDN.With the decrease of the MDN inner-diameter,the melt flow resistance increased for both molten aluminum and iron,especially achieving an order of 10^(2) kPa in the case of the MDN inner-diameter≤1 mm.Based on the conventional GA process,the positive pressure was imposed on the viscous aluminum alloy melt to overcome its flow resistance in the MDN,thus producing powders under different MDN inner-diameters.When the MDN inner-diameter was reduced from 4 to 2 mm,the yield of fine powder(<150μm)soared from 54.7%to 94.2%.The surface quality of powders has also been improved when using a smaller inner-diameter MDN.

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.

Corrosion properties in a simulated body fluid of Mg/β-TCP composites prepared by powder metallurgy

Magnesium matrix composites （MMC） reinforced with 5wt% tricalcium phosphate （TCP） particles were prepared by powder metallurgy. Pure magnesium （CP-Mg） was fabricated by the same procedure for comparison. Scanning electron microscopy and en- ergy-dispersive X-ray spectroscopy analyses revealed that TCP particles were distributed homogeneously in the MMC. In order to investi- gate the corrosion properties, MMC samples were immersed in a simulated body fluid （SBF） at 310~0.5 K for 72 h. The mass loss of the samples in SBF and the pH values of the SBF were evaluated. Moreover, electrochemical measurements were conducted in the SBF. It was shown that the corrosion rate of the MMC decreased with the addition of TCP compared with CP-Mg. Hydroxyapatite was formed on the surface of MMC samples after immersion in the SBF for 72 h but not on the surface of CP-Mg.

Densification Model for Porous Metallic Powder Materials

A new mechanical model for powder metallurgy compaction is presented. In this model, various amount of voids can be introduced into a continuous solid, therefore porosity can be conveniently controlled. The elastic-plastic finite element method was used to analyze the sintered powder material. The model was used to simulate compressing of a sintered cylinder. MSC.Marc of MSC. Software Corporation was applied here, and the sintered powder model was built in MSC.Mentat. The sintered cylindrical powder metallurgy part is treated as a piece of normal metal with pores in the model. The metal block is considered as cylinder with a radius of 6.0 mm and a total height of 10.0 mm. Young’s module was assumed to be 4 000 MPa. Poisson’s ratio was 0.269. The initial yield stress is 210 MPa. Friction coefficient used for the upper and lower contact surfaces is 0.3. Coulomb principle is adopted. Considering axisymmetricity, just half a section is analyzed. Totally there are 1 240 elements. Experiment was carried out by a computer controlled a universal tensile testing machine. During the experiment, the sample was prepared from highly compressible water atomized iron powder with 0.6wt% polymeric lubricant. Particle size is about 100～150 μm. The comparison was performed using a sintered cylindrical sample. The green compact was sintered at 1 140 ℃ for 2 hours. Initially, H0 is 10.20 mm, Φ0 is 12.01 mm and the initial relative density is 0.789. After pressing, H is 7.30 mm, Φ1 is 13.10 mm, Φ2 is 14.64 mm and relative density is 0.88. The load-displacement curves agree with the experimental results very well. Plastic deformation of metallic material is mostly caused by the slipping of crystal lattice. Although very small, a metal powder particle is composed of metallic crystal. Mechanical properties of a powder particle should be very close to their as solid metal counterpart.

IGNITING SHS BY LASER AND ITS APPLICATION TO SELECTIVE LASER SINTERING OF METALLIC POWDER MATERIAL

How to directly fabricate metallic functional parts with selective laser sintering (SLS) process is a potential technique that scientists are researching. Existent problems during directly fabricating metal part by use of SLS are analyzed. For the sake of solving the problems, a new idea of adding self-propagating high-temperature synthesis (SHS) material into metallic powder material to form new type of SLS metallic powder material is put forward. This powder material can release controllable amount of heat during its interaction with the laser beam energy to reduce the requirement to laser power during directly sintering metallic part, to prolong the time of metallic liquid phase existing, and to improve the intensity and accuracy of SLS part. For this reason, SHS material′s interaction with the CO2 laser beam energy is researched, which proves that CO2 laser beam energy may instantly ignite SHS reaction. On the basis of the above-mentioned researches, the effect of sintering the metal powder material mixing SHS material with CO2 laser is also researched, which shows: there is an optimal blending ratio of various material in the new metallic powder material. Under the optimal blending ratio and SLS process parameters, this new metallic powder material can indeed release amount of heat and SHS reaction may be controlled within the laser sintering. This research result makes it possible that the metallic part is directly sintered with small CO2 laser (less than 50W), which may greatly reduce the volume, cost and running expenditure of SLS machine, be propitious to application.

Combined powder metallurgy routes to improve thermal and mechanical response of Al−Sn composite phase change materials

Powder metallurgy processes are suitable to produce form-stable solid−liquid phase change materials from miscibility gap alloys.They allow to obtain a composite metallic material with good dispersion of low-melting active phase particles in a high-melting passive matrix,preventing leakage of the particles during phase transition and,therefore,increasing the stability of thermal response.Also,the matrix provides structural properties.The aim of this work is to combine conventional powder mixing techniques(simple mixing and ball milling)to improve active phase isolation and mechanical properties of an Al−Sn alloy.As matter of fact,ball milling of Sn powder allows to reduce hardness difference with Al powder;moreover,ball milling of the two powders together results in fine microstructure with improved mechanical properties.In addition,different routes applied showed that thermal response depends on the microstructure and,in particular,on the particle size of the active phase.In more detail,coarse active phase particles provide a fast heat release with small undercooling,while small particles solidify more slowly in a wide range of temperature.On the other hand,melting and,consequently,heat storage are independent of the particle size of the active phase.This potentially allows to“tailor”the thermal response by producing alloys with suitable microstructure.