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 |  موضوع: مجموعة أسئلة وإجابتها عن تكنولوجيا المساحيق - Powder Metallurgy الأربعاء 01 مايو 2013, 4:38 pm | |
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مجموعة أسئلة وإجابتها عن تكنولوجيا المساحيق
Test on Powder metallurgy
ويتناول الموضوعات الأتية :
1. Describe the process of 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.
2. Describe the Sintering Process as related to Powder Metallurgy Technology.
Solid state sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat, bonding takes place between the porous aggregate particles and once cooled the powder has bonded to form a solid piece.
Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words Solid State in Solid State Sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.
One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.
3. (i) Write down one major advantage of Powder metallurgy as compared to other
Manufacturing processes.
(ii) List down FIVE disadvantages and any two limitations associated with Powder
Metallurgy processing
Advantages
The greatest advantage powder metallurgy methods have over other processes is that it does not waste material - P/M process is capable of less than 3% scrap losses. Almost all the material not used to make the product is left in its powdered form and can be recycled for use with the next product.
Limitations
There disadvantages associated with P/M processing. These include:
• high tooling costs
• expensive raw materials (powders)
• variation in material density and mechanical properties across the volume
• relatively long parts are difficult to manufacture
• Difficult storing and handling of powders (degradation with time and fire hazard with particular metallic powders).
There are some limitations on the shape of the parts. Undercuts, cross holes, and screw threads are best provided with secondary operations. In general, P/M parts have low resistance to shock loading and lower physical properties than wrought metals.
4. Describe briefly three Powder Metallurgy processes
After the metallic powders have been produced and classified, the subsequent P/M process sequence consists of three major steps: (1) blending and mixing of powders, (2) compaction, and (3) sintering, in addition to a number of optional secondary finishing and manufacturing operations used to improve either the shape and size or the material or functional properties of the P/M part.
Blending and mixing
These are two identical opera¬tions in which powders are thoroughly mixed. The only difference between blending and mixing is in the composition of the powder mixture produced:
• Blending: mixing powders of the same chemical composi¬tion but different sizes;
• Mixing: process of combin¬ing powders of different chemistries
The blending and mixing, which are done under controlled conditions (i.e. in an inert atmosphere, or in a liquid to avoid contamination and deterioration), is done to...
• Produce an uniform distribution of par ticle sizes and shapes;
• Allow for different metals to be mixed to obtain specific physical properties;
• Improve metal powder interaction and prolongs the life of dies used when metal powder is blended with lubricant.
In the process of mixing, some other ingredients are usually added to the powders:
• Lubricants: to reduce the particles-die friction;
• Binders: to achieve enough strength before sintering;
• Deflocculants: added to improve the powder’s flow characteristics during
feeding.
Compaction
The blended powders are compacted to form the shape of the desired part (Fig. 11.8). The work part after compaction is called a green compact or simply a green, the word green meaning not yet fully processed.
The density after compaction (a.k.a. green density) is much greater than the starting material density, but is not uniform in the green. The density and therefore mechanical properties vary across the green volume and depend on the compaction pressure
Sintering
Sintering is a process specifically for powdered materials, including metals, ceramics, and plastics. In this process, the compressed powder is heated to a temperature close to but not at melting, in a controlled-atmo¬sphere furnace. This is done so that particles may bond by solid state bonding, but not melt. The furnaces have three chambers:
• A burn-off chamber to volatilize the lubricants in the green compact in order to improve bond strength and prevent cracking. It slowly raise the temperature of the compound in a controlled manner.
• A high-temperature chamber for sintering. It is the site of actual solid-state diffusion and bonding between the powder particles. The time during the second stage of sintering must be sufficient to produce the desired density and final properties.
• A cooling chamber. The furnace atmosphere should be properly controlled to obtain successful sintering and optimum properties.
Sintering is an important process for the green compacts. After compaction, the density and strength of the material are low. Sintering increases both properties of the material. During the metal sintering process, a wide variety of physical, chemi¬cal, and metallurgical phenomena occur within the mass of metal powder particles. These phenomena are influenced by the sintering conditions, such as time, temperature, and atmo¬sphere, and the chemical composition of the powder mass. The primary driving force for sintering is not the fusion of material, but formation and growth of bonds between the particles.
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