Macroscopic illustration of Zn evaporation during liquid phase sintering of Cu–28Zn prepared from prealloyed powder

Abstract

The formation of a liquid phase during sintering of prealloyed Cu–28Zn powder is potentially attractive for densification, but the effect of gravity results in graded densification which tends to result in heterogeneous cross-sections. The formation of a necklace structure along grain boundaries due to the accumulation of liquid phase with high Zn content, especially at higher temperatures, is also of interest. Evaporation of zinc is another essential feature to consider, since loss of Zn during sintering can influence strongly the mechanical properties of brass products. A macroscopic visualisation of Zn evaporation has been achieved using a copper substrate placed within the gas stream near the sample. FEGSEM observation and XRD analysis of the deposited white mass revealed the formation of nanocrystalline ZnO as a consequence of Zn evaporation. It is proposed that this method could usefully show the evaporation of other alloying elements during sintering of similar alloys.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT

The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30?min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.     

Microstructural development during liquid phase sintering of Fe and Fe–Mo alloys containing elemental boron additions

Abstract

The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm^?3 were obtained for these alloys.     

Particle rearrangement during liquid phase sintering of Cu–20Zn and Cu–10Sn–10Pb prepared from prealloyed powder

Abstract

It would be useful to be able to produce brass and bronze components made from prealloyed powders by supersolidus liquid phase sintering. The microstructures obtained in such alloys are sensitive to constituent alloying elements and small change in sintering temperature. Although the formation of liquid during sintering is potentially attractive for densification, the effects of gravity on the liquid phase can result in graded densification. Evaporation of alloying elements and their solubility in the base metal also affect the extent to which heterogeneous cross-sections are obtained. The aim of the present study was to examine the effect of alloying and sintering temperature on the mode of particle rearrangement, and consequently on graded densification, by microstructural and fractographic analysis. Comparing the fracture morphology from top to bottom of the fracture surface is also helpful in developing a model to describe the phenomena during sintering of similar alloys.     

Suitability of nickel as alloying element in titanium sintered in solid state

Abstract

The suitability of nickel as an alloying element in titanium alloys produced using the blended elemental powder metallurgy approach has been explored. Nickel initially accelerates sintering, providing greater densification at lower temperature than observed for unalloyed titanium. However, it provides only a minor improvement in the density achieved after long sintering times or at high solid state sintering temperatures. Swelling is observed under liquid phase sintering conditions. The highest density was achieved by sintering at just below the solidus temperature. Nickel also accelerates the Ostwald ripening of the pore structure and the conversion of open porosity into closed porosity.     

Production and properties of brass-base P/M constructional materials. A review

Conclusions The best processing properties are exhibited by brass powders manufactured by the diffusional impregnation technique, using a zinc powder, brass swarf, or a copper-zinc master alloy as a point source. However, as this is a very labor-intensive process, normally preference should be given to melt atomization as a method of manufacture of brass powders. Brass P/M parts produced by the conventional method consisting of pressing a powder and sintering the resultant compacts have porosities of not less than 7–10%, and consequently this method is not widely used for the production of constructional brass parts. The sintering of compacts from copper and copper-zinc master alloy powders gives more stable zinc contents compared with the sintering of compacts from copper and zinc powders; the greatest stability of chemical composition is exhibited by sintered compacts from a homogenized brass powder. The formation of diffusional porosity accompanying the evaporation of zinc may be prevented by performing sintering in the presence of a liquid phase (which appears in the presence of a phosphorus or lead addition), saturating the sintering atmosphere with zinc vapor, and adding carbonates or halides of alkali and rare-earth metals to starting powders. The mechanical properties of materials can be markedly improved by eliminating their porosity. This may be achieved by subjecting porous preforms to hot forging, which enables brass P/M parts to be obtained whose mechanical properties are comparable to those of cast parts.Translated from Eoroshkovaya Metallurgiya, No. 3(255), pp. 56–64, March, 1984.     

Microstructural coarsening during supersolidus liquid phase sintering of alpha brass

Coarsening of the grains and pores during sintering has a pronouncedly negative effect on the densification of prealloyed brass powder compacts. This investigation examines the role of sintering variables in realising the complicated effects on densification and microstructure. Experiments were designed to model and evaluate the effect of sintering parameters such as temperature, time and furnace atmosphere on densification, grain and pore intercept as well as pore number. The study of microstructures suggests that there is a good correlation between grain and pore intercepts. It is concluded that pore coarsening is a result of supersolidus liquid phase sintering of Cu28Zn powder, and it can retard densification, which is in acceptable agreement with the experimental data.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5–FeAl2 powders Part 2 – Densification mechanism analysis

Abstract

The use of Fe₂Al₅–FeAl₂ prealloyed powders and heating rates >150 K min^?1 overcomes the formation of density restricting Kirkendall porosity in the Fe–Al system. X-ray diffraction, electron probe micro analysis and differential thermal analysis suggest that the absence of a persistent liquid, experienced when liquid phase sintering with elemental powders, is overcome. Homogenisation is greater during heating at a rate of 20 K min^?1 than for 150, 250 or 400 K min^?1 and homogenous Fe₃Al forms across the compact at temperatures below the melting point of the liquid forming constituent, indicating that a liquid will not form under such processing conditions. The maximum density achieved under the processing conditions in the present study is 92% of theoretical density. The presence of large pores shortly after liquid formation suggests that the remaining porosity is largely due to powder agglomeration during mixing.     

THE MEASUREMENT AND FORM OF POROSITY IN THE LOOSE SINTERING OF COPPER COMPACTS

Abstract

Changes in surface area of specimens of loose-sintered –300-mesh spherical copper powder, measured by the BET gas-adsorption technique, are given for sintering temperatures of 700,800,900, and 1000°C under furnace atmospheres of hydrogen and argon, for times of 0–24 h. Porosity determinations, using a xylene-impregnation technique, show that the porosity is composed entirely of interconnected pores at 700, 800, and 900°C, connected porosity occurring only after 14 h sintering at 1000°C. Determinations of pore-size distribution are also given, measured by a technique based on a “capillary rise of a liquid in a porous material”. Results indicate that for specimens sintered under a hydrogen atmosphere, an overall increase in pore size occurs, whereas for specimens sintered at 800 and 900°C under an argon atmosphere the size of the majority of the pores remains constant, whilst a small percentage of extremely large pores is developed. Permeability coefficients calculated from surface-area and pore-size distribution data are compared with the experimental values.     

IRON-COPPER-TIN SINTERED COMPACTS

Abstract

A wide range of copper and tin powder additions to iron powder sintered compacts hasbeen studied. From mechanical-property tests it has been shown that when using sinteririg temperatures of 900–1100°C in nitrogen/10% hydrogen atmospheres there is an optimum copper: tin ratio of 15:2. The mechanical properties obtained from compacts pressed from iron mixed with 4% copper+tin in this ratio and sintered at 900°C were similar to those obtained from iron ?l0% copper powder compacts sintered at 1100°C. Moreover, the iron-copper-tin components showed improved dimensional accuracy.

In a further series of experiments, it was shown that tin additions to iron-copper alloy compacts increased the solubility of iron in the liquid phase at the sintering temperature and simultaneously decreased the rate of diffusion of copper into the iron particles. At the same time, tin improved the wettability of the liquid, reducing its surface tension and allowing it to disperse more completely throughout the matrix. The mechanical properties of compacts containing larger amounts of tin were decreased by the presence of brittle compounds, although the sintering rate was increased. It is concluded that the optimum properties of iron-copper-tin compacts are obtained by making correct additions of copper and tin to the iron powder and giving careful consideration to the sintering atmosphere.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Effect of residual pressure on vacuum sintering of Ti–Ni alloys

Abstract

In earlier work the authors examined the sintering of Ti–Ni alloys by means of dilatometry of mixed elemental powders. Some notable differences were observed when heat treatments were carried out using a vacuum tube furnace rather than the dilatometer: higher sintered density was achieved due to a combination of lower heating rate and lower residual pressure, and swelling during liquid phase sintering was greatly reduced. This observation is consistent with the idea that gas pressure within closed pores causes swelling during liquid phase sintering and retardation of shrinkage in solid state sintering. In addition to the results of measurements of density and open and closed porosity as a function of Ni content and sintering temperature, macrographs and optical micrographs of the sintered compacts are presented, and the effects of heating rate and compaction pressure are described.     

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract

Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering. This was made possible by the use of the Fe-P-C system with or without Cu. Without post-sintering treatment, a brittle microstructure was obtained. By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved. Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided. In this way, iron phosphide is not formed on grain and/or particle boundaries. Presence of pores is confirmed to be beneficial for grain growth control.     

CHARACTERIZATION OF THE DEGREE OF MIXING IN LIQUID-PHASE SINTERING EXPERIMENTS

Abstract

Homogeneity of mixing plays an important role in liquid-phase sintering. In order to describe quantitatively the dependence of shrinkage on the degree of mixing, six different tungsten-copper powder mixtures were prepared. These powder blends were either presintered or sintered in the presence of liquid phase. The degree of mixing in both the presintered and liquid-phase sintered samples was determined by quantitative microstructural analysis. The developed method is well able to characterize the different state of mixing in the six powder blends on a microscopic scale. It is shown that a close relation exists between the microscopically characterized degree of mixing and densification during liquid-phase sintering.     

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.     

THE DRIVING FORCE FOR SHRINKAGE IN COPPER POWDER COMPACTS DURING THE EARLY STAGES OF SINTERING

Abstract

The shrinkage behaviour of compacts from irregular copper powder during the initial stages of sintering has been determined by a dilatometric method. The effects of compacting pressure and of external load during sintering at a constant heating rate of 3°C/min upon shrinkage were observed. The residual stresses present on the surface of compacts heated at the same rate to temperatures of 200, 300, 400, 500, and 600°C were also measured. It was observed that shrinkage starts at temperatures where considerable residual stresses in the surface of the compacts are still present, and that this temperature also depends upon the external stress applied during sintering. Residual and externally applied stresses complement each other in shifting the temperature of start of shrinkage to lower values with increasing stress. It is concluded that, in the low-temperature range up to 400°C, residual and externally applied stresses, rather than surface-tension forces, cause shrinkage.     

The founding of EPMA

Abstract

Liquid phase sintering is commonly used in powder metallurgy to improve physical properties through densification enhancement. With the aim of combining the advantages of liquid phase sintering and the use of promising alloying elements such as Mn and Si, liquid promoters with complex compositions were designed to provide a low melting point to form a liquid phase below the common sintering temperatures. The properties of these liquid phases were characterised in terms of contact angle, spreading evolution and infiltration. Using a Krüss drop shape analysis system, both wetting angle experiments and infiltration experiments were performed by changing the substrate characteristics from sintered to green iron specimens respectively. The discussion is based on the different features found for these liquids compared with copper, which is a well known liquid phase former used for improving the properties of low alloy steels. Simulations of the thermodynamic and kinetic processes taking place were performed by combining ThermoCalc and DICTRA software analysis.     

Dilatometric analysis of thermal debinding of injection moulded iron compacts

Abstract

The less than desired tolerance control of powder injection moulded compacts is a result of inconsistent dimensional changes in the compacts accumulated during moulding, debinding, and sintering. This study investigated the in situ length changes and their causes during thermal debinding on compacts which have been solvent debound. The dilatometric analysis showed that the specimen shrank in the early stage between 250 and 370°C, not because of sintering, but through the loss of N, C, and O in the carbonyl iron powder. At temperatures between 370 and 450°C, the specimen expanded owing to the carburisation of the iron powder. The length change was also influenced by the heating rate, debinding atmosphere, and the amount of the backbone binder. These dilatometric results are helpful in establishing the guidelines in designing binder compositions and debinding schedules.     

EURO PM2009 in cool Copenhagen

Abstract

The effect of additions of silicon powder on the sintering behaviour and microstructure of compacted 304L stainless powder has been studied. The shrinkage ratio increases substantially with silicon content. Silicon profoundly activates the sintering process through the formation of a eutectic and/or δ ferrite, which is pseudoperitectically formed during sintering. The sintering behaviour is closely related to the microstructures, which depend upon the amount of silicon addition. Ostwald ripening is encountered in the liquid phase sintered specimens (Si≤3 wt-%). The solid phase sintered materials (Si≥ wt-%) containing δ ferrite densify more rapidly than the liquid phase sintered ones. The densification kinetics are governed by the wetting characteristics of the eutectic liquid and the formation of ferrite. As a result of the silicon addition, the austenitic stainless steel powder aggregates are sintered into duplex stainless steels with austenite-ferrite structures. PM/0395