Question

Manufacturing Technology

1. Find out at least 3 types method to produce for powder technology

2. Hot Pressing Forming(HPF)
(i) What is that
(ii) How its work

3. What mean by surface energy in sintering

4. What do you mean by inpregnated and infiltration

Answer 1

Solution:

Powder Metallurgy is a manufacturing process in which the shapes required are generally manufactured from metallic powders. This process include the following steps:

1. Powder production
2. Compaction
3. Sintering and
4. Secondary operations

Secondary operations are not mandatory, they depend upon the final geometric forms required. Among the 3 basic operations, preparation of powder is the toughest process. There are several methods of powder production and depending on the shape of the powder particle required the process is adopted. Some of the powder production processes are:

• Mechanical pulverization: used for brittle materials like antimony.
• Electrolytic process: Used for producing Cu, Iron and silver powders.
• Chemical reduction: Generally used for Fe, Cu, Molybdenum, Tungsten
• Atomization: Generally used for both ferrous and non-ferrous alloys such as stainless steel, superalloys.

In mechanical pulverization, brittle materials and alloys can be crushed to approximately 0.001mm. Generally, milling operation is used which involves impact, attrition, shear and compressive forces for crushing the large sized particles. Mechanism of milling involves the following stages:

1. Micro forging
2. Fracture
3. Agglomeration
4. Deagglomeration

Milling equipment includes crushers and mills such as Jaw crusher, Gyratory crusher, roll crusher, Ball mill, Vibratory ball mill, Attritor, Rod mill, Hammer mill, Planetary mill. Out of all these ball mill is most commonly used. Apart from milling, machining, shotting and graining are the other methods in mechanical pulverization.

Electrolytic Process is used for copper, iron, tungsten, tantalum, zinc, silver and tin powders. In this process electrolysis is carried out and the required powder is deposited at the required fineness. After a specific time interval, powder is removed from the electrode and collected in the vat. The main advantage is that, the powder produced is resistant to oxidation.

Atomization is a process in which the molten metal is forced through the orifice and is broken by a jet of compressed air. A very fine powder can be made by this process, but it employs specially designed nozzles and apparatus. Using inert gases reduces the risk of oxidation. Air can also be used but causes a little corrosion. some of the atomization methods are:

1. Water atomization
2. Gas atomization
3. Vacuum atomization
4. Centrifugal atomization
5. Rotating sik atomization
6. Ultra rapid solidification process

2. Hot pressing Forming

The powder produced is compacted with help of mechanical or hydraulic means. These compaction methods can be broadly classified into the following:

a)Methods with application of pressure

1. Cold die compaction

2.Isostatic pressing

i)Cold isostatic pressing

ii)Hot isostatic pressing(HIP)

3.Powder rolling

4.Powder extrusion

5.Explosive compaction

b)Methods without application of pressure.

1.Loose powder sintering in mould

2.Vibratory compaction

3.Slip casting

4.Slurry casting

5.Injection moulding

HIP is a method of compaction which involves application of pressure at elevated temperature to obtain the accurate shape of the object required. It consists of a high temperature furnace, pressure vessel, vacuum pumps, pressurizing systems, metering pumps and controls.

HIP is carried out at temperatures of about 0.5-0.7 times melting temperature of the material and the pressure employed is about 100-500MPa. The furnace is kept inside a pressure vessel and argon is supplied which acts as pressurizing gas. The pressure vessel would be insulated. Generally, ceramics, nickel based alloys, tungsten alloys are compacted.

Due to application of pressure with the gas, the powder particles are plastically deformed along the surface of contact. Due to this a metallurgical bonding takes place between the particles. Densification of the particles take place due to plastic deformation followed by creeping followed by diffusion of the particles along the area of contact.

3. Surface energy in sintering:

Sintering is a process in which the green compact of the desired component is produced under specific temperature and time. The different types of sintering are

1. Solid state sintering
2. Liquid phase sintering
3. Activated sintering
4. Reaction sintering

In solid state sintering process, the mechanism involved has following steps:

• Formation of necks at the contact points of adjacent atoms.
• Growth of necks and formation of skeleton of solid particles.
• Pores formed at the center gets isolated.

For the above mechanism to take place, the main driving force is the excess surface energy. This surface energy can be reduced by transportation of material from different areas so that the pores are minimized or removed. The surface transportation can take place by diffusion, plastic flow, viscous flow or evaporation and condensation.

Due to sintering, density, strength and ductility of the particles is increased.

4. Infiltration:

Infiltration is the process in which porosity of the sintered components are reduced by addition of filling the pores with a material of low melting point by capillary action. For example, for improving properties of iron, copper is used as infiltrate.

Impregnation:

Impregnation is the process in which the components such as bearings, bushings are impregnated with oil or grease. These components are immersed in the fluids and are internally lubricated up to 30% and when they are in use, continuous supply of lubricant in the form of oil or grease is achieved due to capillary action. Universal joint is one of the example of grease impregnated powder metallurgical part.