Advantages and Disadvantages of Powder Metallurgy





The process of manufacturing of shaped components or semi-finished products such as bar and sheet from metal powder is called as Powder metallurgy. 
The technique of powder metallurgy combines unique technical features with cost effectiveness and generally used to produce sintered hard metals known as ‘carbides’ or ‘tungsten carbides’. 
This technique deals with the production of metal and non metal powders and manufacture of components. 

Powder metallurgy is generally used for iron based components.  
The powders used as raw material can be elemental, pre-alloyed, or partially alloyed. 
Elemental powders like iron and copper are more compressible and produce pressed compacts with good strength.  
Pre-alloyed powders are harder but less compressible therefore require higher pressing loads to produce high density compacts.
Powder metallurgy technique has many advantage as well as limitation. 

Some of the Advantages of Powder Metallurgy are as follows;

1.    Powder metallurgy produces near net shape components. The technique required few or no secondary operations. 
2.    Parts of powder metallurgy can be produce from high melting point refractory metals with less cost and difficulties.
3.    The tolerance of components produced by this technique have quite high tolerance, therefore no further machining is not required. 
4.    This technique involves high Production Rate along with low Unit Cost.
5.    It can produce complicated forms with a uniform microstructure.
6.    Powder metallurgy has full capacity for producing a variety of alloying systems and particulate composites.
7.    This technique has flexibilities for producing PM parts with specific physical and mechanical properties like hardness, strength, density and porosity.
8.    By using powder metallurgy, parts can be produced with infiltration and impregnation of other materials to obtain special characteristics which are needed for specific application.
9.    Powder metallurgy can be used to produce bi-metallic products, porous bearing and sintered carbide.
10.    Powder metallurgy makes use of 100% raw material as no material is wasted as scrap during process.

Disadvantages of Powder Metallurgy:

1.    The production of powder for metallurgy is very high.
2.    The products of metallurgy can have limited shapes and features.
3.    This technique causes potential workforce health problems from atmospheric contamination of the workplace.
4.    The tooling and equipments require for powder metallurgy are very expensive, therefore becomes main issue with low production volume.
5.     It’s difficult to produce large and complex shaped parts with powder metallurgy.
6.    The parts produce by powder metallurgy have low ductility and strength.
7.    Finally divided powder like aluminium, magnesium, titanium and zirconium are fire hazard and explosive in nature. 
8.    This technique is not useful for low melting powder such as zinc, cadmium and tin as they show thermal difficulties during sintering operations. 
 

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From Wikipedia

Powder metallurgy

Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. First, the primary material is physically powdered, divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure (via cold welding) very near the dimensions of the object ultimately to be manufactured. Pressures of 10-50 tons per square inch are commonly used. Also, to attain the same compression ratio across more complex pieces, it is often necessary to use lower punches as well as an upper punch. Finally, the end part is formed by applying pressure, high temperature, long setting times (during which self-welding occurs), or any combination thereof.

Two main techniques used to form and consolidate the powder are sintering and metal injection molding. Recent developments have made it possible to use rapid manufacturing techniques which use the metal powder for the products. Because with this technique the powder is melted and not sintered, better mechanical strength can be accomplished.

History and capabilities

The history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron (Fe) powders were fused into hard objects as early as 1200 B.C. In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.

A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings.

In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing (e.g., tool wear, complexity, or vendor options) also may be closely regulated.

Powder Metallurgy products are today used in a wide range of industries, from automotive and aerospace applications to power tools and household appliances. Each year the international PM awards highlight the developing capabilities of the technology.

Isostatic powder compacting

Isostatic powder compacting is a mass-conserving shaping process. Fine metal particles are placed into a flexible mold and then high gas or fluid pressure is applied to the mold. The resulting article is then sintered in a furnace. This increases the strength of the part by bonding the metal particles. This manufacturing process produces very little scrap metal and can be used to make many different shapes. The tolerances that this process can achieve are very precise, ranging from +/- 0.008 inches for axial dimensions and +/- 0.020 inches for radial dimensions. This is the most efficient type of powder compacting.(The following subcategories are also from this reference.) This operation is generally applicable on small production quantities, as it is more costly to run due to its slow operating speed and the need for expendable tooling.

Compacting pressures range from 15000|psi|abbr=on|lk=on to 40000|psi|abbr=on|lk=on for most metals and approximately 2000|psi|abbr=on|lk=on to 10000|psi|abbr=on|lk=on for non-metals. The density of isostatic compacted parts is 5% to 10% higher than with other powder metallurgy processes.

Equipment

There are many types of equipment used in Powder Compacting. There is the mold, which is flexible, a pressure mold that the mold is in, and the machine delivering the pressure. There are also controlling devices to control the amount of pressure and how long the pressure is held for. The machines need to apply anywhere from 15,000 psi to 40,000 psi for metals.

Geometrical Possibilities

Typical workpiece sizes range from 0.25|in|2|abbr=on to 0.75|in|2|abbr=on thick and 0.5|in|2|abbr=on to 10|in|0|abbr=on long. It is possible to compact workpieces that are between 0.0625|in|2|abbr=on and 5|in|0|abbr=on thick and 0.0625|in|2|abbr=on to 40|in|0|abbr=on long.

Tool style

Isostatic tools are available in three styles, free mold (wet-bag), coarse mold(damp-bag), and fixed mold (dry-bag). The free mold style is the traditional style of isostatic compaction and is not generally used for high production work. In free mold tooling the mold is removed and filled outside the canister. Damp bag is where the mold is located in the canister, yet filled outside. In fixed mold tooling, the mold is contained with in the canister, which facilitates automation of the process.

Hot isostatic pressing

Hot isostatic pressing (HIP) compresses and sinters the part simultaneously by applying heat ranging from 900°F (480°C) to 2250°F (1230°C). Argon gas is the most common gas used in HIP because it is an inert gas, thus prevents chemical reactions during the operation.

Cold isostatic pressing

Cold isostatic pressing (CIP) uses fluid as a means of applying pressure to the mold at room temperature. After removal the part still needs to be sintered.

Design Considerations

Advantages over standard powder compaction are the possibility of thinner walls and larger workpieces. Height to diameter ratio has no limitation. No specific limitations exist in wall thickness variations, undercuts, reliefs, threads, and cross holes. No lubricants are need for isostatic powder compaction. The minimum wall thickness is 0.05 inches and the product can have a weight between 40 and 300 pounds. There is 25 to 45% shrinkage of the powder after compacting.

Powder production techniques

Any fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered parti

Nickel-zinc battery

The nickel-zinc battery (sometimes abbreviated NiZn) is a type of rechargeable battery that may be used in cordless power tools, cordless telephone, digital cameras, battery operated lawn and garden tools, professional photography, electric bike, and light electric vehicle sectors.

Nickel-zinc battery systems have been known for over 100 years. Since 2000, development of a stabilized zinc electrode system made this technology viable and competitive with other commercially available rechargeable battery systems.

History

Thomas Edison was awarded for a rechargeable nickel-zinc battery system in 1901.

The battery was later developed by an Irish chemist, Dr. James J. Drumm (1897–1974) and installed in four 2-car Drumm Railcar sets between 1932 and 1948 for use on the Dublin-Bray line. Although successful they were then withdrawn when the batteries wore out. Early nickel-zinc batteries were plagued by limited number of discharge cycles. In the 1960s nickel-zinc batteries were investigated as an alternative to silver-zinc batteries for military applications, and in the 1970s were again of interest for electric vechicles. A company called Evercel Inc. formerly developed and patented several improvements in nickel-zinc batteries but withdrew from that area in 2004.

Applications

Nickel-zinc batteries have a charge/discharge curve similar to 1.2V NiCd or NiMH cells—but with a higher 1.6V nominal voltage

Presently this battery technology has limited consumer availability, with only AA cells offered for the digital camera market in some camera stores. Both D-cells and sub-C cells are currently used in commercial applications.

Nickel-zinc batteries perform well in high drain applications, and may have the potential to replace lead-acid batteries because of their higher-energy-to-mass ratio and higher-power-to-mass ratio (up to 75% lighter for the same power), and are cheap compared to nickel-cadmium batteries (expected to be priced somewhere in between NiCd and lead-acids). NiZn may be used as a substitute for nickel-cadmium. The European Parliament has supported bans on cadmium-based batteries; nickel-zinc offers the European power tool industry an alternative.

Electrochemistry

Charge Reaction: 2Ni(OH)2(s) + Zn(OH)2(s) ↔ 2Ni(OH)3(s) + Zn(s)

Note that the stoichiometry above is different than below, but the reactions are identical. Water is consumed and generated on the charge and discharge cycles.

Discharge Reaction: H2O + Zn + 2NiOOH ↔ ZnO +2Ni(OH)2

Electrochemical open circuit voltage potential: ~1.73V

Battery Life

The tendency of the zinc electrode to dissolve into solution and not fully migrate back to the cathode during charging has, in the past, presented challenges to the commercial viability of the NiZn battery. The zinc's reluctance to fully return to the same location of the solid electrode adversely manifests itself as shape change and dendrites or whiskers, which may reduce the cell discharging performance or eventually short out the cell, resulting in low cycle life.

Recent advancements have enabled manufacturers to prevent this problem. These advancements include improved electrode separator materials, zinc material stabilizers, and electrolyte improvements. One manufacturer (PowerGenix) claims battery cycle life comparable to NiCd batteries

Battery cycle life is most commonly specified at a discharge depth of 80 percent of rated capacity and assuming a one hour discharge current rate. If the discharge current rate is reduced or if the depth of discharge is reduced then the number of charge/discharge cyles for a battery increases.

When comparing NiZn to other battery technologies it is important to note that cycle life specifications may vary with other battery technologies depending on the discharge rate and depth of discharge that were used.

Advantages

Nickel-zinc cells have an open circuit voltage of 1.8 volts when fully charged and a nominal voltage of 1.65V. This makes NiZn an excellent replacement for electronic products that were designed to use alkaline primary cells (1.5V). NiCd and NiMH both have nominal cell voltages of 1.2v, which may cause some electronic equipment to shut off prior to a complete discharge of the battery because the minimal operating voltage is not provided.

Due to their higher voltage, fewer cells are required (compared to NiCd and NiMH)to achieve a given battery pack voltage, reducing pack weight, size and improving pack reliability. They also have low internal impedance (typ. 5 milliohm) which allows for high battery discharge rates.

NiZn batteries do not use mercury, lead or cadmium, or metal hydrides (rare earth metals) that are difficult to recycle. Both nickel and zinc are commonly occurring elements in nature. Zinc and nickel can be fully recycled.

NiZn cells use no flammable active material or organic electrolyte.

Properly designed NiZn cells can have very high power density and low temperature discharging performance.

Disadvantage

Currently, only Sub C and AA NiZn cells are available. Compared with other secondary systems, nickel-zinc cells have lower volumetric energy density.. Nickel is more costly than lead.

Charging

NiZn technology is well suited for fast recharge cycling as optimum charge rates of C or C/2 are preferred .

Known charging regimes include constant current of C or C/2 to cell voltage = 1.9V. Maximum charge time is 2½ hours. Trickle charging is not recommended as recombination is not provided for and excess hydrogen will eventually vent adversely affecting battery cycle life. Charge is reinitiated after cell voltage has fallen below 1.6V.


Organic fertilizer

Organic fertilizers are naturally-occurring fertilizers (e.g. compost, manure), or naturally-occurring mineral deposits (e.g. saltpeter).

Naturally-occurring organicfertilizers includemanure, slurry, worm castings, peat, seaweed, humic acid, and guano. Sewage sludge use in organic agricultural operations in the U.S. has been extremely limited and rare due to USDA prohibition of the practice (due to toxic metal accumulation, among other factors).

Processed organic fertilizers include compost, bloodmeal, bone meal, humic acid, amino acids, and seaweed extracts. Other examples are natural enzyme digested proteins, fish meal, and feather meal. Decomposing crop residue (green manure) from prior years is another source of fertility.

Discussion of the term 'organic'

There used to be a distinction between the term "organic" and the term "pesticide free". Organic simply dealt with the use of fertilizer types. Once the term "organic" became regulated, many other factors were added. "Pesticide-free" is not at all related to fertilization (plant nutrition), but has become a legal inclusion.

Likewise, in scientific terms, a fish emulsion can be a good organic fertilizer :), but in some jurisdictions fish emulsion must be certified "dolphin safe" to be considered "organic".

Natural sourcing

Animal-sourced Urea and Urea-Formaldehyde (from urine), are suitable for application organic agriculture, while pure synthetic forms are not deemed, however, pure (synthetically-produced) urea is not. The common thread that can be seen through these examples is that organic agriculture attempts to define itself through minimal processing (e.g. via chemical energy such as petroleum—see Haber process), as well as being naturally-occurring or via natural biological processes such as composting.

Cover crops are also grown to enrich soil as a green manure through nitrogen fixation from the atmosphere; as well as phosphorus (through nutrient mobilization) content of soils.

Powdered limestone, mined rock phosphate and Chilean saltpeter, are inorganic chemicals in the technical (organic chemistry) sense of the word, but are considered suitable for organic agriculture in limited amounts..

Advantages

Although the density of nutrients in organic material is comparatively modest, they have many advantages. The majority of nitrogen supplying organic fertilizers contain insoluble nitrogen and act as a slow-release fertilizer. By their nature, organic fertilizers increase physical and biological nutrient storage mechanisms in soils, mitigating risks of over-fertilization. Organic fertilizer nutrient content, solubility, and nutrient release rates are typically much lower than mineral (inorganic) fertilizers. A University of North Carolina study found that potential mineralizable nitrogen (PMN) in the soil was 182–285% higher in organic mulched systems, than in the synthetics control.

Organic fertilizers also re-emphasize the role of humus and other organic components of soil, which are believed to play several important roles:

  • Mobilizing existing soil nutrients, so that good growth is achieved with lower nutrient densities while wasting less
  • Releasing nutrients at a slower, more consistent rate, helping to avoid a boom-and-bust pattern
  • Helping to retain soil moisture, reducing the stress due to temporary moisture stress
  • Improving the soil structure
  • Helping to prevent topsoil erosion (responsible for desertfication and the Dust bowl

Organic fertilizers also have the advantage of avoiding certain problems associated with the regular heavy use of artificial fertilizers:

  • The necessity of reapplying artificial fertilizers regularly (and perhaps in increasing quantities) to maintain fertility
  • Extensive runoff of soluble nitrogen and phosphorus, leading to eutrophication of bodies of water (which causes fish kills)
  • Costs are lower for if fertilizer is locally available

According to the PPI institute website, it is widely thought that organic fertilizer is better than inorganic fertilizer. However, balanced responsible use of either or both can be just as good for the soil.

Disadvantages

Organic fertilizers have the following disadvantages:

  • As a dilute source of nutrients when compared to inorganic fertilizers, transporting large amount of fertilizer incurs higher costs, especially with slurry and manure.
  • The composition of organic fertilizers tends to be more complex and variable than a standardized inorganic product.
  • Improperly-processed organic fertilizers may contain pathogens from plant or animal matter that are harmful to humans or plants. However, proper composting should remove them.
  • More labor is needed to compost organic fertilizer, increasing labor costs. Some of this cost is offset by reduced cash purchase.

Conventional farming application

In non-organic farming a compromise between the use of artificial and organic fertilizers is common, often using inorganic fertilizers supplemented with the application of organics that are readily available such as the

Particle size distribution

The particle size distribution (PSD) of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size. PSD is also known as grain size distribution.

Significance

The PSD of a material can be important in understanding its physical and chemical properties. It affects the strength and load-bearing properties of rocks and soils. It affects the reactivity of solids participating in chemical reactions, and needs to be tightly controlled in many industrial products such as the manufacture of printer toner and cosmetics.

Significance in the Collection of Particulate Matter

Particle size distribution can greatly affect the efficacy of any collection device.

Settling Chambers will normally only collect very large particles, those that can be separated using sieve trays.

Centrifugal Collectors will normally collect particles down to about 20 μm. Higher efficiency models can collect particles down to 10 μm.

Fabric Filters are one of the most efficient and cost effective types of dust collectors available and can achieve a collection efficiency of more than 99% for very fine particules.

Wet Scrubbers that use liquid are commonly known as wet scrubbers. In these systems, the scrubbing liquid (usually water) comes into contact with a gas stream containing dust particles. The greater the contact of the gas and liquid streams, the higher the dust removal efficiency.

Electrostatic Precipitators use electrostatic forces to separate dust particles from exhaust gases. They can be very efficient at the collection of very fine particles.

Nomenclature

�p: Actual particle density (g/cm3)

�g: Gas or sample matrix density (g/cm3)

r2: Least-squares coefficient of determination. The closer this value is to 1.0, the better the data fit to a straight-line.

λ: Gas mean free path (cm)

D50: Mass-median-diameter (MMD). The log-normal distribution mass median diameter. The MMD is considered to be the average particle diameter by mass.

σg: Geometric standard deviation. This value is determined mathematically by the equation:

σg = D84.13 / D50 = D50 / D15.87

The value of σg determines the slope of the least-squares regression curve.

α: Relative standard deviation or degree of polydispersity. This value is also determined mathematically. For values less than 0.1, the particulate sample can be considered to be monodisperse.

α = σg / D50

Re(P) : Particle Reynolds Number. In contrast to the large numerical values noted for flow Reynolds number, particle Reynolds number for fine particles in gaseous mediums is typically less than 0.1.

Ref : Flow Reynolds number.

Kn: Particle Knudsen number.

Types

The way PSD is expressed is usually defined by the method by which it is determined. The most easily understood method of determination is sieve analysis, where powder is separated on sieves of different sizes. Thus, the PSD is defined in terms of discrete size ranges: e.g. "% of sample between 45 μm and 53 μm", when sieves of these sizes are used. The PSD is usually determined over a list of size ranges that covers nearly all the sizes present in the sample. Some methods of determination allow much narrower size ranges to be defined than can be obtained by use of sieves, and are applicable to particle sizes outside the range available in sieves. However, the idea of the notional "sieve", that "retains" particles above a certain size, and "passes" particles below that size, is universally used in presenting PSD data of all kinds.

The PSD may be expressed as a "range" analysis, in which the amount in each size range is listed in order. It may also be presented in "cumulative" form, in which the total of all sizes "retained" or "passed" by a single notional "sieve" is given for a range of sizes. Range analysis is suitable when a particular ideal mid-range particle size is being sought, while cumulative analysis is used where the amount of "under-size" or "over-size" must be controlled.

The way in which "size" is expressed is open to a wide range of interpretations. A simple treatment assumes the particles are spheres that will just pass through a square hole in a "sieve". In practice, particles are irregular - often extremely so, for example in the case of fibrous materials - and the way in which such particles are characterized during analysis is very dependent on the method of measurement used.

Sampling

Before PSD can be determined, it is vital that a precisely representative sample is obtained. The material to be analyzed must be carefully blended, and the sample withdrawn using techniques that avoid size segregation. Particular attention must be paid to avoidance of loss of fines during maniputation of the sample.

Measurement techniques

Sieve analysis

This continues to be used for many measurements because of its simplicity, cheapness, and ease of interpretation. Methods may be simple shaking of the sample in sieves until the amount retained becomes more or less constant. Alternatively, the sample may be washed through with a non-reacting liquid (usually water) or blown through with an air current.

Advantages

This technique is well-adapted for bulk materials. A large amount of materials can be readily loaded into 8|in|mm|adj=mid|-diameter sieve trays. Two common uses in the power industry are wet-sieving of milled limestone and dry-sieving of milled coal.

Disadvantages

Many PSDs are concerned with particles too small for separation by sieving to be practical. A very fine sieve, such as 37 Î¼m sieve, is exceedingly fragile, and it is very difficult to get material to pass through it. Another disadvantage is that the amount of energy used to sieve the sample is arbitrarily determined. Over-energetic sieving causes attrition of


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