Sequence of Stages

Densification during liquid-phase sintering commonly occurs in three stages after the liquid forms rearrangement, solution-reprecipitation, and final-stage sintering. However, in many systems there is a strong solid-state diffusion contribution to the overall densification. A brief description of these stages follows. Solid-State Sintering Prior to Liquid Formation. The same solubility, segregation, and diffusivity criteria that are favorable for LPS can lead to enhanced solid-state sintering...

Powder Properties

Metal powder characteristics directly determine the parameters of the CIP process. In mold filling, particle size distribution and particle shape are controlling factors in determining tap density of the filled mold. For billet molds, where final shape is determined by deformation processing operations, exact fill is not as important as in near-net-shape parts. Nevertheless, reproducible particle size and shape factors would be required to ensure repeatable post-CIP processing. In the case of...

Material Selection

Once the die-compaction processing method has been selected the designer must consider the product configuration and material requirements. Previous sections have discussed the design constraints regarding shape and configuration. The next step in the procedure (see Fig. 5) is comparing the critical part performance requirements with the available P M materials. Reference 5 serves as a very useful source of material property data. In many cases, more than one material can satisfy the design...

Net Shape Capability

A major advantage of the P M approach is the net-shape capability, particularly for high-strength materials. This has led to continuing developments in P M technologies for parts production. Warm Compaction. One recent development in P M production is warm compaction, which allows the production of higher-density ferrous P M parts via a single compaction process. The process utilizes heated tooling and powder during the compaction step. The powder and tooling are typically heated between 130...

Other Gas Atomization Methods

Ultrasonic gas atomization (Fig. 1e and 19) is claimed to allow finer particles, but no published comparative work has clearly demonstrated significant advantage over similar close-coupled nozzles. No commercial operator is known to use the method, and the largest melter used has been in the 10 to 50 kg size range. Internal Mixing Nozzles. Typical values of k in Eq 6 shown in Fig. 22 range from 40 to 60 Jm for close-coupled gas nozzles. Far higher efficiencies are obtained in another type of...

If Bt

Where X is the neck diameter, D is the particle diameter, t is the isothermal sintering time, and B is a term that collects material and geometric constants that are specified in Table 3. The values of n, m, and B change with time and degree of sintering, but for many analyses, they have been assumed constant with the mechanism of mass transport. Many experiments have been analyzed using the exponent n to identify the sintering mechanism. However, in cases where the grain boundary energy is...

Introduction

POWDER METAL PARTS are sometimes classified by density, where part production technologies may have low-density and high-density variants. Proponents of a particular technology for high-density parts may refer to the technology as a high-density or full-density process without explicit definition of the term for potential customers or comparison with other processes or materials. For the purposes of this article, higher-density consolidation means effectively 100 dense material that will...

References cited in this section

Bourell, Processing of Nanocrystalline Zirconia and Zirconia Alumina Composite Powder by Hot Pressing and HIP, Synthesis and Processing of Nanocrystalline Powder, D.L. Bourell, Ed., TMS, 1996, p 181-191 3. A.C.F. Cocks, The Structure of Constitutive Laws for the Sintering of Fine Grained Materials, Acta Metall. Mater., Vol 42 (No. 7), 1994, p 2191-2210 4. H. Gleiter, Nanocrystalline Materials, Prog. Mater. Sci., Vol 33 (No. 4), 1989, p 223-315 5. M. Mayo, Processing of...

M2aq H2S MSs 2Haq

Metals can be selectively removed from solution by controlling pH of the sulfide precipitation reaction. The pH is controlled by the addition of ammonia to neutralize the acid generated in the precipitation reaction The nickel and cobalt sulfides are then returned to the leach, the zinc sulfide (which is precipitated independently of nickel and cobalt sulfide) is sold as a by-product, and the barren liquor (metal free ammonium sulfate solution) proceeds to ammonium...

Compaction to Higher Density

Compacting Explosives

Many methods have been investigated and developed for pressing metal powders to higher densities. Most of the commercial methods involve high-temperature compaction (such as hot isostatic pressing and powder forging, as described in detail in other articles in this Section). In general, these methods are used to develop fully dense or nearly fully dense P M products. For example, conditions for full-density iron and steel by HIP are summarized in Table 2. Table 2 Hot isostatic pressing...

CuCOCl

Heterocarbonyls also have been produced, including (CO)5MnCo(CO)4 (CO)5Re 2Fe(CO)4 Re2Ru(CO)i2 Ru2Os(CO)i2 Metal carbonyl carbides also exist, such as Fe5(CO)15C and the dicarbide clusters Ru10C2(CO)24, Co13C2(CO)24, and Rh15C2(CO)23. Another example of metal carbonyl is the dinitrogen derivative A complete list of all the metal carbonyls that can be formed is beyond the scope of this article. The above examples are provided to illustrate the diversity of the chemical forms of metal carbonyls.

Processing Sequence

Manufacturing a consolidated, fully dense beryllium product involves ingot melting and casting, powder manufacture, and consolidation. The consolidated product usually is a right circular cylinder that is made by vacuum hot pressing. This vacuum hot pressed billet can be readily machined into parts for satellites, aircraft, optical systems, or guidance instruments. Beryllium is particularly well suited to these applications because of its superior strength, modulus, and density. The ductility...

Green Strength

Green strength refers to the mechanical strength of a cold pressed powder compact. Adequate mechanical strength of a green part permits cold pressing of the powder, ejection of the green part from the die, and transfer of the green part to the sintering furnace without breakage. Strength of green compacts results mainly from mechanical interlocking of irregularities on the particle surfaces, which is promoted by plastic deformation during pressing. Figure 12 shows green strengths of various...

Selective Laser Sintering

Pease III, Powder-Tech Associates, Inc. Selective laser sintering (SLS), developed by DTM of Austin, Texas (Ref 3), uses laser energy to transform metal powders into useful tooling and parts. Figure 10 is a diagram of the first part in the process, wherein a loose, polymer coated metal powder is laser fused into the required shape. A CAD model of the part is numerically transformed into thin slices, 75 to 250 Jm (0.003 to 0.010 in.). In the machine shown in Fig. 10, a very thin layer...

Guidelines for Heat Treating PM Parts

Whether heat treating P M parts is done in-house or by an outside commercial heat treater, some guidelines need to be followed to ensure that parts are properly treated. In the past, heat treatment of P M parts was primarily specified to improve wear resistance. With the advent of high-compressibility powders and high-temperature sintering, heat treatments are now added to provide improved dynamic properties as well. Recommendations for heat treating porous P M parts are Always degrease parts...

Micromerographs

This equipment is a sedimentation balance occasionally used for determining the particle size distribution of subsieve metal powders. The powder is suspended in air by projecting the sample with a burst of nitrogen through a deagglomerating device consisting of a conical annulus into the settling chamber. The chamber consists of a thermally insulated vertical aluminum tube with a 10 cm (3.94 in.) inside diameter that is 2.5 m (8.20 ft) high. The pan of an automatic balance that weighs the...

Sampling Stored Nonflowing Material

Nonflowing material is a term for very fine cohesive powders, sticky material, moist material, or fibrous solids. These may be stored in small containers such as drums or bags, or in large containers such as trucks or railway wagons. Surface sampling is usually carried out with a scoop because of its simplicity A presupposition is that the powder at the sampling point is representative of the bulk (i.e., that the powder was mixed before storage). Accuracy is increased by taking more than one...

Gas Adsorption

Giesche, School of Ceramic Engineering and Sciences, Alfred University The Brunauer-Emmett-Teller (BET) (Ref 1) method of measuring specific surface area is based on the determination of the amount of gas that is adsorbed on the surface of the sample. The specific surface area (m2 g) determined by this method includes the external as well as internal (pores) surface area. The surface area of closed pores cannot be determined because the adsorbing gas molecules have no physical path to that...

Sampling Flowing Streams

Most powder systems are transported at some time during their manufacture as flowing streams Hoppers are emptied by screw or belt conveyors, powders are transferred to bagging operations by screw or pneumatic conveyors, and many solids are transported through pipes. A general rule in all sampling is that whenever possible, the sample should be taken while the powder is in motion. This is usually easy with continuous processes with consignment sampling it may be possible during filling and...

Krypton for Low Surface Area Determination

The use of krypton or argon rather than nitrogen for adsorbate improves the accuracy of low specific surface area measurements. Assume the sample has a total surface area of 1 m2 and the volume of the sample tube is 20 cm3. The saturation pressure for nitrogen is about P0(N2) 760 torr and for krypton it is P0(Kr) 2.5 torr at liquid nitrogen temperature (assuming a super-cooled liquid state for the krypton). The single-point BET equation for those gases yields

General Description

Relationship between Pore Radii and Intrusion Pressure. Mercury porosimetry is based on the capillary rise phenomenon whereby an excess pressure is required to cause a nonwetting liquid to enter a narrow capillary. The pressure difference across the interface is given by the equation of Young (Ref 31) and Laplace (Ref 32), and its sign is such that the pressure is less in the liquid than in the gas (or vacuum) phase if the contact angle 01S greater than 90 or more if 0is less than 90 . If the...

Measurement Techniques

Particle diameters can be measured directly in an optical microscope with the use of a filar micrometer eyepiece. This eyepiece contains a scale and a movable cross hair that is operated by a calibrated knob on the side of the eyepiece. A particle is moved so that one side touches one of the fixed scale markings, and the cross hair is moved to touch the other side of the particle. The difference between the two readings is the Feret's diameter of the particle. The eyepiece...

Precipitation from Solution

Production of metal powders by hydrometallurgical processing is based on leaching an ore or ore concentrate, followed by precipitating the metal from the leach solution. Although basic precipitation reactions have been known for more than 100 years, commercial use of this process did not flourish until the 1950s, as a result of increasing interest and work on lower-grade ores. Metal precipitation from solution can be accomplished directly by electrolysis, cementation, or chemical reduction....

Powder Compaction Methods

Powders are compacted under high pressure by various methods (Table 1) where the application of pressure packs the powders and reduces porosity. Unlike shaping methods, compaction techniques cause particle deformation. Many compaction methods are used, but the most prevalent method for P M parts production is uniaxial compaction in a rigid die. This method is cost effective with relatively straightforward tooling. However, compaction methods inherently produce parts with density variations that...

Composite Bearings

Roll compacting also can be used for producing composite, or sandwich, materials. An example of such a roll-compacted composite material is bimetallic strip used in producing main and connecting rod bearings (Ref 11). A change in automotive emission standards led to the use of these new materials to replace traditional copper-lead sleeve-bearing components. The rolled strip consists of a layer of Al-8.5Pb-4.0Si-1.5Sn-1.0Cu prealloyed powder sandwiched to a pure aluminum layer. The production...

Calibration of Material Parameters for an Iron Powder Blend

The procedures of the previous section are applied below to the calibration of the two-state variable model introduced earlier. The data is for a powder blend comprising 99.5 by weight of Distalloy AE, 0.5 by weight of graphite, and 1 wax Hoechst micropulver. This last component is admixed as internal lubricant. Distalloy AE is a diffusion alloyed iron powder with composition 4 wt Ni, 1.5 wt Cu, and 0.5 wt Mo. Particle sizes for this powder range from 20 to 180 m. The apparent density of the...

Cementation

The precipitation of a metal from its solution by the addition of another less noble metal is known as cementation. The fundamental equation for copper recovery from copper-bearing pregnant liquors on iron is In practice, the copper bearing solution is passed over scrap iron, such as detinned and shredded cans. Subsequent separation, washing, reduction, and pulverizing produces a copper powder that contains considerable amounts of iron and acid insolubles, such as alumina and silica....

Carbon Control

Tight control of carbon levels in powder metal parts is one of the distinguishing attributes of the high-quality P M producer. The carbon content affects many of the properties of the finished part such as dimensional tolerances and mechanical properties. The pick up of carbon also decreases the corrosion resistance of stainless steel. Carbon control is achieved primarily through a fine tuning of the sintering atmosphere, as it relates to temperature, admixed carbon, carbon and oxygen content...

Powder Forging

The design issues in P F are similar to the requirement of any precision, closed-die forging. The difference is the starting preform in the case of P F, the preform is a sintered powder metal part, typically 80 to 85 of theoretical density, with a shape similar to the final part configuration. By contrast, in a precision closed-die forging the preform is a wrought steel blank with very little shape detail. Preform design for P F fabrication determines the extent of product shape detail required...

Fine Metal Powders

Hershberger, UltraFine Powder Technology Inc. Fine metals have been commercially available for years by various methods. One of the earliest methods was the precipitation of elemental precious metal powders. Fine refractory metal powders are also produced by milling. Elemental iron or nickel powders are also produced to fine particle distributions with the carbonyl process. On a tonnage basis, carbonyl iron still accounts for a large percentage of fine powders used for metal injection...

Accuracy

The high degree of accuracy attainable in some branches of chemistry and physics is out of the question where evaluation of specific surface from adsorption data is concerned. Even in the favorable case of nitrogen or argon, a divergence of at least 10 from the actual area of the solid should be expected (owing to theoretical factors not yet supported by accurate quantitative assessments). In addition, there are several nonnegligible experimental uncertainties, which can be strikingly...

Production of Copper Powder by the Reduction of Copper Oxide

Reduction of copper oxide is the oldest and (in the United States) still one of the main commercial processes for making copper powder, along with water atomization. In this process, particulate copper oxide is converted to copper at elevated temperature by reducing gases. The product, a sintered porous cake, is then milled to a powder. The starting material for reduction was originally copper millscale and cement copper. However, as demand for copper powder exceeded supply and higher purity...

References

Sci, Vol 21 (No. 11), 1966, p 1025 2. C.C. Harris, Powder Technol., Elsevier Sequoia S-A, Vol 12 (No. 85), 1975 3. A. Brouthman, G.N. Wollan, and S.M. Feldman, Chem. Metall. Eng., Vol 52 (No. 4), 1945, p 102 4. Pulvermetallurgie Sinter und Verbundwerkstoffe, W. Schatt, Ed., VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1979 5. W. Volk, Industrial Statistics, Chem. Eng., March 1956 6. S.S. Weidenbraum, Mixing of Solids, Advances in Chemical Engineering, Academic...

Surface Area Determination

Under the assumption of a specific pore geometry, one can calculate the surface area of a sample from mercury porosimetry measurements. Usually the cylinder pore model is used. Rootare and Prenzlow (Ref 45) derived the following equation This equation is equivalent to the cylinder pore model. The later derivation does not contain an assumption about the pore geometry it does however, assume that the movement of the mercury meniscus is reversible. For an interconnected pore system, this is not...

Extrusion

Extrusion is the pressing of the feedstock into and through a die of size and cross-sectional shape appropriate to that of the product. The specific steps (Ref 1) used in binder-assisted extrusion are (1) feedstock paste delivery into extruder, (2) consolidation and flow of feedstock paste through the barrel, (3) deformation and shaping of feedstock paste as it flows through the die, (4) flow of feedstock paste through the die land, and (5) part ejection. For the feedstock to flow through the...

Comparison of Mechanical and Hydraulic Presses

In terms of partmaking capability, no distinct advantage is gained by using either a mechanical press or a hydraulic press. Any part can be produced to the same quality on either type of machine. However, the following parameters influence press drive selection. Production Rate. A mechanical press produces parts at a rate one and one-half to five times that of a hydraulic press as a result of inherent design of the energy transfer systems and stroke length. Operating cost of a hydraulic press...

Part Classification

The Metal Powder Industries Federation has classified P M parts according to complexity. Class I parts are the least complex, and class IV parts are the most complex. To better understand the types of commercially available P M compacting presses, and their advantages and limitations, an understanding of P M part classification and tooling systems used to produce parts is necessary. Part thickness and number of distinct levels perpendicular to the direction of powder pressing determine...

Inspection Considerations

Inspection techniques for finished parts should be based on concurrence between the customer and the supplier. There are many ways to gage most part print specifications, and the specific technique used can have a large effect on the results reported. A great many P M part attributes may be measured with the same techniques used in other industries. There are, however, significant exceptions as briefly described below. Hardness. In powder metallurgy there are generally two types of hardness...

Powder Characterization and Testing

Reliable techniques for powder characterization and testing are required to evaluate the chemical and physical properties of metal powders, both as individual particles and in bulk form. This is important to ensure adequate control of powder production processes and to ensure that the required properties for parts manufacturing are met. This Section also features a series of articles on characterization and testing of powders. The representativity of samples taken to test metal powders is key...

Production of Copper Powder by Hydrometallurgical Processing

Hydrometallurgical methods may be used to produce a number of metal powders, including copper, cobalt, and nickel. The basic processing steps consist of preparing pregnant liquor by leaching ore or another suitable raw material, followed by the precipitation of the metal from this solution. For copper, the most important precipitation methods are cementation, reduction with hydrogen or sulfur dioxide, and electrolysis. Use of several leach-precipitation steps or the inclusion of flotation,...

Fig 3 Feedstock pellets and worms for molding

Pelletized feedstock is injection molded into the desired shape by heating it in the molding machine and hot ramming it under pressure into the tool cavity. By virtue of the binder, the feedstock becomes low enough in viscosity that it can flow into the die cavity under pressure. Cooling channels in the die extract heat and solidify the polymer to preserve the molded shape. The shaping equipment is the same as that used for plastic injection molding. It consists of a die filled through a sprue,...

Materials Processed by PIF

A range of materials have been processed by PIF for a variety of applications. Early applications of the technique were for automotive applications for iron-base parts where high performance in either mechanical or magnetic properties was sought. Various aluminum applications for automotive use were also evaluated. In both applications, near-full density (98 or better) was achieved. Nickel and nickel aluminides have also been consolidated by PIF with good success (100 dense) by direct...

Process Variables

Conventional atomization pressures are typically in the range 0.5 to 4 MPa (70 to 600 psi), and gas velocities in the nozzles range from Mach 1 to 3. However, in free-fall atomizers, measured gas velocities in the impingement area normally have fallen to 50 to 150 m s (for air nitrogen). Typically, gas-atomized powder is spherical with a log normal size distribution. Mean particle size is usually in the range 10 to 300 m with a standard deviation of about 2. Oxygen content is about 100 ppm....

Monolithic Materials

Tungsten, molybdenum, rhenium, niobium, superalloys (nickel, iron, and cobalt base), zinc, aluminum, bronze, cast iron, mild and stainless steels, NiCr and NiCrAl alloys, cobalt-base Stellites, cobalt nickel-base Tribaloys, and NiCrBSi selffluxing Colmonoy have all been successfully thermal spray consolidated either as coatings or structural deposits. Recently Tribolite (FeCrNiBSi) and AmaCor (amorphous) alloys have also been developed for spraying and exhibit excellent wear and...

Mixtures and Segregation

Most P M powders are multicomponent systems and, therefore, are subject to segregation. Segregation is even possible in a one-component metal powder if, for instance, coarse and fine powder particles demix as a result of vibration. The opportunity for powder segregation exists in processes such as shipping and the filling of hoppers and compaction dies, where individual components exhibit different flow rates due to differences in particle size, shape, density, surface roughness, and other...

Specialty Applications

Roll compacting facilitates the production of small coils in which minor compositional changes can be made during blending. Because roll compacting produces materials with uniform and reliable properties, many specialty materials are produced by these methods (Ref 6). Specialty P M strip produced by roll compacting includes various compositions of nickel-iron strip produced for controlled expansion properties and special copper-nickel-tin alloys. The latter must meet stringent property...

Prealloyed Bronzes

Sintered bronze alloys are rather uncommon in powdered metal usage. This is primarily due to their relatively high cost compared to low-alloy steels. However, sintered bronze properties can be advantageous for non-magnetic applications that require very good corrosion resistance, good mechanical strength, and excellent ductility. Prealloyed 80Cu-9Sn-2Zn bronze powders with a select lubricant are intended for the fabrication of high-density P M structural components. Unlike many elemental...

Example 2 Machinability Evaluation Drilling of Sintered Steel

Blind hole drilling test was performed on a wide spectrum of P M qualities. Cylindrical blanks with a diameter of 80 mm and a height of 10 mm are used in the test. A survey test is performed in order to select the type of drill and cutting conditions. The main comparisons were made under dry condition using a HSS drill with a diameter of 4 mm and a point angle of 118 . Total breakdown of the drill is chosen as criterion, based on the fact that drilling is commonly used as a bulk removal...

Colloidal Stability

Because slip casting of metals is a wet process, some understanding of the interaction between the material and water must be established. Most materials when immersed in a polar solvent become charged. The different mechanisms by which the surface acquires such charge depend greatly on the type of material used. These mechanisms include ion dissolution and ion adsorption (Ref 4, 7). Ionic materials acquire their surface charge through preferential dissolution. A very well known example is Agl,...

Elastoplastic Constitutive Behavior

The constitutive behavior is discussed within the framework of small deformation theory of plasticity, which allows the decomposition of the total strain into elastic and plastic components and development of relatively simple expressions for the constitutive response. In practice, large plastic straining of a body can occur during the compaction process. The general forms of expression are still applicable, provided appropriate measures of stress, strain, stress-rate, and deformation-rate for...

Pressure Assisted Reactive Sintering

Even though many of the examples discussed above can be reactively sintered to densities greater than 97 theoretical, this small amount of porosity can be detrimental to the mechanical properties--specifically tensile and fracture behavior--of these compounds. Commonly, a postreactive sintering routine, such as hot isostatic pressing (HIP), is used to fully densify these materials. Alternatively, the powders can be sintered under pressure by RHP or RHIP. Greater densities can be achieved in the...

Production of Copper Powder by Atomization

Water atomization of copper--that is, the disintegration of a molten stream of high-quality copper with high-pressure water jets--produces copper powders of compacting-grade quality. The resulting dried powder then may be subjected to an elevated-temperature treatment that further modifies its characteristics and engineering properties. Nearly spherical powders result from the atomization of liquid copper with inert gas or air. These powders are used in copper flake production and other...

Stainless Steel Powders

Water-Atomized Stainless Steel Powders. Early experiments concentrated on producing stainless steel powders from elemental components and from alloy powders by sensitization embrittlement and grinding of stainless steel sheet. Water atomization became the established process in the 1950s for producing stainless steel powders for conventional die compaction and sintering. Over the years, slight modifications to the existing wrought compositions led to improved compacting properties. Recently,...

Macroexamination

Macroexamination of sintered materials is not commonly practiced. In wrought or ingot-base materials, forging flow lines, oxide segregation, and stringers are studied extensively. These features are not usually found in P M materials, but there are certain other uses for macroexamination. In sectioning a heat treated P M steel, care must be taken not to overheat the specimen and temper or reharden it locally. The etching performed during grinding and polishing to help open the pores will...

Stainless Steel and High Alloy Powders

Samal, OMG AMERICAS Most stainless steel and other high-alloy powders are made by water atomization or by inert gas and centrifugal atomization. Information on these processes can be found in the article Atomization in this Volume. Water-atomized powders generally are of irregular particle shape and widely used for cold compaction and sintering. The densities of the resulting sintered parts are typically below about 95 of theoretical. It is for this reason that dynamic...

Bulk Deformation Processes

In powder forging, a preform shape is cold pressed to between 75 and 85 of full density, sintered, heated to a forging temperature, and then forged in trapped dies using one blow to produce a fully dense net or near-net shape. The sintering step is optional, but it is normally included as a particle surface cleaning step (deoxidation) and to improve the workability of the porous preform. Powder forging fundamentals and general applications are presented in the article Powder...

Activated Liquid Phase Sintering

Activated LPS can be used to improve the sintering behavior of systems with little or no solubility of the solid in the liquid. Densification is enhanced by the addition of elements that segregate to grain boundaries and lower the activation energy for solid-state diffusion. Such elements can be identified by phase diagram characteristics. Limited additive solubility in the liquid helps to ensure segregation to grain boundaries, while the formation of low-melting temperature intermetallic...

Normalized resolution

Fig. 19 Structural boundary fractal dimension (< 5S) relates to the gross morphology of the fine particle. Textural boundary fractal (< ST) relates to the smaller features of the profile. Fig. 20 Individual powder grain of Fig. 10(a) characterized by its fractal dimension and combined with size and chunkiness to form a single point in three-dimensional space. When a group of powder grains are treated in this way, they produce a cloud of points characteristic of the powder.

Applications

The physical and economic differences between the two types of powders result in clearly defined applications for each. Fine powders are used in both consolidated and unconsolidated forms. Markets include metal injection molding (MIM), electronic pastes and inks, thermal spray, additives, radar absorption coatings, and filtration. Metal Injection Molding. The growth forecasts for MIM encouraged development of fine powder production processes. The first MIM parts were made from available powders...

Sintering of Ferrous Materials

Pressed and sintered iron-base materials represent the largest segment of the P M parts industry. Ferrous P M parts are made from iron powders, alloy steel powders, or mixtures of (a) iron and graphite powders, (b) iron (or steel) and copper powders, or (c) iron, copper, and graphite powders (with or without powders of other metals). The overall sintering process consists of several stages and temperature regimes (Table 2). Excluding lubricant burnoff, the following reactions occur during...

Conventional Shape Factors

Particle shape is a fundamental characteristic of powder particles and thus influences the properties of particulate systems. Various shape terms have been proposed to quantitatively represent particle shape. Early systems tended to measure one specific feature of a particle. Table 3 (Ref 23, 24, 25, 26, 27, 28, 29, 30) lists some of the most frequently used shape terms. The applicability and or limitations of various shape factors, also referred to as shape parameters, are discussed in the...

Particle Shape Factors

The most common approach to describe and differentiate particle shapes has been the use of qualitative concepts. Two fundamental concepts have been used (a) the dimensionality of the particle and (b) the surface contour of the particle. By the use of these concepts, a model system of shape characterization is presented in Fig. 21. Photomicrographs of several types of loose powders described in the International Standards Organization standard ISO 3252 are shown in Fig. 22. Basic shapes are...

Other Infiltration Products

Influence of Liquid Metal Infiltration on the Superconducting Characteristics of Niobium Nitride, L.T. Summers, J.R. Miller, M.J. Strum, R.J. Weimer, and D.E. Kizer, Advances in Cryogenic Engineering, Plenum Press, Vol 34, 1988, p 835-842. A fully stabilized multifilamentary BN superconductor was prepared using a combination of physical vapor deposition of NbN on graphite followed by liquid metal infiltration using copper or aluminum. The resulting conductor assumed a finely divided...

Wet Bag Isostatic Pressing

In wet-bag pressing, the mold is filled, evacuated (if this step is used), and sealed outside the pressure vessel. Because these operations occur in a powder-handling area, contamination of external mold surfaces with metal powder can easily occur. Introduction of dirty molds into the vessel results in working fluid contamination with long-term effects similar to mold tearing and gross metal powder spillage. Thorough mold washing as an integral part of the process flow chart is necessary to...

Thermodynamic and Kinetic Factors

When a liquid forms during sintering, the microstructure consists of solid, liquid, and vapor phases. Successful LPS requires a reduction in surface energy as the liquid spreads across the surface of the solid particles. Thus, the solid-liquid surface energy must be less than the solid-vapor surface energy. In this case, the liquid wets the solid and provides a bonding force between the particles to aid densification. Wetting is aided by solubility of the solid in the liquid,...

Induction Hardening

Spur gears, bevel gears, splined hubs, and cams are ideal components to utilized P M production techniques. These parts usually require hard, wear-resistant surfaces in some areas, with retention of the ductility of the sintered matrix in the remainder of the part. Induction hardening is commonly specified for these applications. The process can be placed in an automated machining line to reduce handling and be a cost-effective hardening treatment when high volumes of parts are being produced....

Powder Fill

The important consideration in P M part production is the fill ratio required to produce parts to a density that is compatible with end use requirements. The fill ratios must remain constant for a given part to maintain dimensional reproducibility. Parts can be of single-level or multilevel design. Single-level parts, designated as class I by the Metal Powder Industries Federation (MPIF), present the least difficulty to the tool designer, regardless of the size or part configuration. The main...

Energy Relationship

When a metal is plastically deformed by cold working, most of the mechanical energy of the deformation process is converted into heat. However, a small amount (approximately 5 ) is stored in the metal, thereby raising its internal energy. The energy associated with permanent lattice strain or cold work is minimal for hard, brittle particles, but can be large for ductile materials. The energy expended to overcome the friction between particles is translated to heat and performs no useful work in...

Production of Tungsten and Tungsten Carbide Powders

Lux, Vienna University of Technology B. Zeiler, Wolfram Bergbau- und Huetten-GmbH Nfg KG Tungsten gained major industrial importance at the beginning of the 20th century due to its technical application as alloying element in high-speed steel and as filament wire in incandescent lamps (Ref 1, 2, 3). For both applications, pure tungsten was produced in powder form. From 1900 to 1912 only pure tungsten powders were used in high amounts for high-speed steels containing 16 to 24 tungsten (Ref...

Process Description

A flow diagram of the PIF process is shown in Fig. 1. Beginning on the left, P M materials are prepared in one of two forms a preform or a loose powder. A preform is consolidated to a near-net- or net-shape workpiece by one of several approaches, including conventional die pressing, cold isostatic pressing (CIP), or metal injection molding. The green pieces are often sintered before they are introduced to the PIF process. As a preform, the workpiece is prepared for PIF processing by a range of...

Binder Treatment of Metal Powder Mixes

This technique consists of binding the fine additives to the coarser ones by adding a small quantity of binding agent to the powder. This binder creates a thin film that covers the particles causing the fine additives to adhere to the base powder. Binders can be of two types liquid or solid (Ref 1). In the case of liquid binder (Fig. 5a), a dry mix is produced and the liquid binder is subsequently added to the mixture. Further homogenization is required to ensure that the liquid is evenly...

Die Compaction

The major difficulty with die compaction traces to die wall friction. This friction inhibits ejection and, more importantly, causes density gradients in the green compact. Punch motion against the powder is similar to plowing snow. Close to the punch, the packing is dense, but far removed from the punch, the powder is unaffected. This pressure decay with distance is because the powder spreads load to the die wall in the form of friction. Green density increases with compaction pressure....

Variables Affecting Green Strength

Many theories have been proposed to explain the precise mechanics of green strength. Because the strength of green compacts results mainly from mechanical interlocking of particle surface irregularities, particle shape is the most important factor contributing to green strength. Powders with irregularly shaped particles produce compacts of higher green strength than powders with spherical particle shape. Spherical particles provide the lowest degree of mechanical strength because of low initial...

Selecting a Dispersing Agent

The selection process suggested by Nelson (Ref 17) will soon be superseded by an ISO Standard 14887, Dispersing Procedures for Powders in Liquids, currently in preparation with the author as convenor of the working group. A brief summary of the considerations involved in finding an effective dispersing agent is given below for the major chemical categories of particles. Particles having a natural surface charge may be electrostatically stable to flocculation with no further treatment. If the...

Sampling Techniques

The most important step in any particle size analysis, but especially by microscopic techniques, is sampling from the bulk. Because an extremely small quantity of material is used to determine the particulate size, an accurate analysis cannot be obtained if the bulk material is not properly sampled. Particles tend to segregate according to size. If handling has caused vibration of the sample, coarse material tends to collect near the surface. When free-flowing material is poured into a pile,...

Sinter Hardening

Powder metallurgy alloys that can transform to the hard martensite phase upon cooling from the sinter temperature have been commercialized. This technique, termed sinter hardening, is rapidly gaining popularity in applications requiring high strength and hardness immediately after sintering. This manufacturing route has proven to be cost effective for a number of parts because it eliminates the need for a postsinter heat treatment. Prealloyed powders have been proven useful for this purpose....

Sintering

Most commercial sintering of stainless steel parts is completed in belt, pusher, walking beam, and vacuum furnaces. A wide range of processing parameters is common. Typical sintering atmospheres include hydrogen, dissociated ammonia, H2-N2 mixtures, and all of low dew point, as well as vacuum. Sintering temperatures range from 1120 to 1344 C (2050 to 2450 F), and sintering times range from 20 to 60 minutes. Insufficient sintering, either too short a time or at too low a temperature, will result...

Compressibility

In most cases, the bulk density of a material varies continuously as a function of the consolidating pressure acting on it. Therefore, it is not sufficient to describe a material simply in terms of its apparent density or tap density. Instead, this density-to-pressure relationship can be measured (Ref 7), and the results are often expressed as a straight line on a log-log plot (Fig. 18). In the bulk solids literature, this relationship is often called compressibility, although this term has...

Production of Silver Powders

Silver powders are used in a variety of electrical, electronic, and industrial applications. In powder metallurgy, silver powders are used in the manufacture of electrodes for primary batteries and storage cells, and as mixtures with other metals that are used in the preparation of electrical contacts and other P M parts containing silver. These mixtures include silver-tungsten, silver-iron, silver-tungsten carbide, silver-molybdenum, silver-cadmium oxide, silver-graphite, and silver nickel...

Secondary Operations

A variety of secondary manufacturing and finishing operations may be required to complete the part, to improve properties, or to calibrate dimensional tolerances. Because die-compacted parts have residual porosity that may affect the response to these secondary operations, several guidelines are provided in this section. Repressing. Die-compacted parts can be repressed in a second tool set in order to reduce the amount of porosity or reduce dimensional variation. By increasing the part density,...

Equipment for Blending and Premixing

Many different types of machines are available for blending and premixing solids. The high density, abrasive nature, frictional characteristics, and tendency to segregate of metal powders, however, reduce the suitability of some machines. A blender suitable for metal powders should Achieve a maximum blending in minimum time Have a gentle mixing action to avoid particle degradation Provide repeatability of blending Provide for complete batch discharge Provide accessibility for ease of cleaning...

Thermal Spray Processing Introduction

The term thermal spray describes the family of four or five processes that use the heat energy generated by chemical (combustion) or electrical (plasma or arc) methods to melt, or at least soften, and accelerate fine dispersions of droplets to velocities in the range 50 to > 1000 m s. The high particle temperatures and velocities achieved result in significant droplet deformation on impact at a surface, producing thin layers or lamellae, often called splats, which conform and adhere to the...

Production Methods

The production techniques used for fine (micron scale) powders generally fail in the submicron (nanometer) range. Methods like gas or water atomization have a lower particle size limit of to 5 'm. Milling is used for particle size reduction of very brittle materials like tungsten carbide or ceramics, but fail for ductile materials like precious metals. A variety of methods are available for the production of ultrafine powders with submicron particle size. Gas phase reactions, spray drying, or...

A typical HIP cycle for a coldloading coldwall autoclave is as follows

The powder-filled compacts are placed on fixturing designed to withstand the load weight and elevated temperature to which it will be subjected. This is usually done at a station that can also accommodate the furnace. To maximize efficiency, the load is built in a duplicate base heater while the vessel is actively running a cycle. 2. The furnace is placed over around the compacts and the base heater. The entire load is lifted and placed into the HIP vessel as a module. The base heater engages...

Solvent Based Binders for Room Temperature Extrusions

Solvent-based binder systems are used for room-temperature extrusion. Typically, a water soluble clay and or an organic polymer binder is dissolved in water (Ref 1, 3, 5, 17). The water uniformly disperses the binder throughout the powder and provides the rheological characteristics necessary for proper extrusion. The organic binder is used to increase the viscosity of the feedstock and to change the flow characteristics of the powder water feedstock from Newtonian to the desired pseudoplastic...

Milling Environment

Surface-active agents and lubricants generally are used to nullify the forces of welding (or autohesion) and thereby inhibit agglomeration. Surface-active agents adsorbed on a particle surface interfere with welding and lower the surface tension of solid material. Because the energy required for milling is equal to the product of new surface area generated times the surface tension, a reduction in surface tension results in the use of shorter milling times and or finer powders. Corrosion...

Types of Atmospheres

Conventional atmospheres used in sintering are endothermic gas, exothermic gas, dissociated ammonia, hydrogen, and vacuum. According to the Metal Powder Industries Federation, the most common atmosphere, endothermic gas, was used by - -83 of the industry in 1976 for sintering carbon steels (Ref 4). Endothermic gas is generated directly from partial combustion of natural gas. Exothermic gas, also a product of incomplete combustion of natural gas, was used by 5 of the industry for sintering...

Atomic Absorption Spectrometry

Atomic absorption spectrometry (AAS) is generally used for measuring relatively low concentrations of approximately 70 metallic or semimetallic elements in solution samples. The basic experimental equipment used is essentially the same as that of 30 years ago--enhanced by modern electronics, background-correction schemes, and alternate types of atomizers. The predominance of AAS in general-purpose trace-metal analysis has recently been somewhat eclipsed by modern atomic emission spectrochemical...

Mechanism of Milling Phenomenology

One type of milling mechanism consists of an initial microforging stage in which particles are deformed in the absence of agglomeration by welding and fracture. Eventually, particles become so severely deformed and embrittled by cold work that they enter a secondary stage, during which the particle fracture by a fatigue failure mechanism and or by the fragmentation of fragile flakes. Fragments generated by this mechanism may continue to reduce in size in the absence of strong agglomerating...

Container Fabrication

Tooling and Container Component Fabrication. Once the design has been established, the metal container is fabricated. This is not a trivial step because the container must be producible in an economical fashion or the finished part cannot be manufactured. The most economical and easily formed container material is low-carbon steel however, other materials (e.g., stainless steel, nickel alloys, titanium, etc.) can also be used. The process is constrained by existing metalforming techniques...

Fig 3 Scanning electron micrograph of the filamentarytype nickel powder produced by carbonyl decomposition 1000x

Extra fine nickel powders (e.g., Inco type 210) also have filamental morphology, but the filaments are finer and specific areas range from 1.5 to 6 m2 g depending on the grade. Semi smooth high-density nickel powders are available in fine and coarse sizes. The fine is 10 to 20 'm in diameter (Fig. 4) the coarse is -16+40 mesh. Powder apparent density ranges from 3.5 to 4.2 g cm3. Fig. 4 Scanning electron micrograph of high-density (fine) nickel powder produced by carbonyl decomposition. 1000x...

Palladium Powders

Palladium is used in the electrical and electronics industries in electrical contacts, multilayer ceramic capacitors, thick-film paste conductors, resistors, thermocouples, and heating pads. The corrosion resistance, high melting point, contact resistance, and reasonable electrical conductivity of palladium account for its use as an electrical contact material where long life and reliability are essential. Palladium can be preferred for low-current, long-life relays operating at low contact...

Sliding at Impact Points

Two key factors in chute design are the chute angle and the smoothness of the chute surface at the point of impact. Too shallow or too rough a surface in a chute impedes flow. The required minimum chute slope can be determined by placing a ring-type device containing a sample of the powder on a representative sample of the chute wall surface and applying a predetermined vertical load to simulate impact. Once the weight has been removed, the plate is raised to determine the angle at which the...

Process Control

A key aspect of process control philosophy is that shop floor operators be given as much ownership of their process as possible. Ideally, the operator gages the process, charts it, takes corrective action when it goes out of control, and makes the appropriate adjustments to the process when necessary. Certain process control procedures, however, cannot be practically conducted by the production operators. Some measurements cannot be accurately determined on the production floor. Examples...

Iron Copper and Iron Copper Graphite

The sintering of iron-copper mixtures involves (a) solid bonding of iron to iron, (b) solid bonding of copper to iron, (c) melting of copper, (d) solution and diffusion of copper in solid iron, and (e) solution and precipitation of iron in liquid copper. At typical sintering temperatures of 1095 to 1120 C (2000 to 2050 F), 7.5 to 9.0 Cu is soluble in iron. However, with this copper content and typical sintering conditions, some of the molten copper remains undissolved, or free, and can dissolve...

The Hoganas Process

The Hoganas process uses pure magnetite (Fe3O4) ore found in northern Sweden, which has an iron content of approximately 71.5 after beneficiation by grinding and magnetic separation. The impurities present in small amounts are not in solid solution in the oxide, but exist as discrete phases. The ore is of consistent quality and is available in sufficient quantities for continued usage. Coke breeze or another carbon source that provides the reducing agent is required to produce sponge iron...

Apparatus

Lea and Nurse (Ref 21) developed the apparatus shown in Fig. 4 to provide permeability measurements. The powder was compacted in the sample cell to a predetermined porosity. Air was permitted to flow through the bed, and the pressure drop (h1) was measured on the first manometer the air then passed through a capillary flowmeter, across which another pressure drop was measured as h2 on a second manometer. Fig. 4 Lea and Nurse permeability apparatus with manometer and flowmeter The capillary...

Cold Sintering of Metal Matrix Composites with Partial Surface Reduction of Ceramic Particles

Bonding integrity between a metal matrix and ceramic particles can be improved by partial (surface) reduction or metallization of the ceramic particles prior to consolidation of the composite to full density. Such an approach has been applied to the processing of iron-base matrix composites (cermets) reinforced with Cr2O3 and VC particles (Ref 31, 46). A schematic of the process is shown in Fig. 16. Metallization of ceramic particle surfaces result in an increased composite strength. The...

Properties and Applications

One of the most important properties of pure tungsten carbide is its high hardness (for properties, see Table 2), which is the basis for the application in hardmetals. Linear coefficient of thermal expansion (20-400 C), 10-6 K Hardmetals, also called cemented carbides, achieve an optimized balance between the high hardness of the tungsten carbide and the high toughness of a ductile binder metal, such as cobalt, nickel, or iron, whereby optimal interfacial bonding is achieved (Ref 56, 57). To...

Commercial Processes

Currently, Inco Ltd. produces high-purity nickel powders by the thermal decomposition of nickel tetracarbonyl. Gaseous nickel tetracarbonyl is formed by reacting carbon monoxide with nickel concentrates under controlled conditions subsequent thermal decomposition of the gas permits recovery of the nickel as a fine metallic powder and nickel pellets. The process, which affords a high degree of purity with respect to metallic elements other than nickel, produces nickel powders with extremely...