Metals are processed by different means to achieve metals and alloys of desired shapes and characteristics. These processing techniques are further divided based on the different procedure employed.
Metal fabrication techniques are mainly four kinds:
|Casting||To give a shape by pouring in liquid metal into a mold that holds the required shape, and letting harden the metal without external pressure|
|Forming||To give shape in solid state by applying pressure;|
|Machining||In which material is removed in order to give it the required shape|
|Joining||Where different parts are joined by various means.
One of the most important miscellaneous techniques is powder metallurgy.
This technique is employed when
This is also employed as it is usually economical compared with other techniques.
Different casting techniques include: sand, die, investment, continuous casting
Sand casting: The common casting method where sand is used as casting material. A two piece mold (cope and drag) is formed by compact packing of sand around a pattern of required shape. An addition gating is provided for proper distribution of liquid metal.
Die casting: Here metal is forced into mold by external pressure at high velocities. Usually a permanent two-piece mold made of steel is used. In this technique rapid cooling rates are achieved, thus inexpensive.
Investment casting: In this pattern is made of wax. Then fluid slurry of casting material is poured over which eventually hardens and holds the required shape. Subsequently, pattern material is heated to leave behind the cavity.
This technique is employed when high dimensional accuracy, reproduction of fine details, and an excellent finish are required.
Examples: Jewelry, dental crowns, and gas turbine blades jet engine impellers.
Continuous casting: After refining metals are usually in molten state, which are later solidified into ingots for further processing like forming. In continuous casting, solidification and primary forming process are combined, where refined metal is cast directly into a continuous strand which is cooled by water jets.
This technique is highly automated and more efficient. Uniform composition throughout the casting is achievable when compared with ingot-cast products.
In these techniques, a metallic piece is subjected to external pressures (in excess of yield strength of the material) to induce deformation, thus material acquires a desired shape.
These are basically two types
Most common forming techniques are: forging, rolling, extrusion, and drawing.
Forging: This involves deforming a single piece of metal, usually, by successive blows or continuous squeezing.
Rolling: Most widely used forming technique. It involves passing a piece of metal between two rotating rolls. Deformation is terms of reduction in thickness resulting from applied compressive forces.
Extrusion: In this technique a piece of material is forced through a die orifice by a compressive force. Final product emerging from die will have the desired shape and reduced cross sectional area, and will constant cross-section over very long lengths.
Two varieties of extrusion are direct extrusion and indirect extrusion, where distinction limits to movement of tool and final product and consequent changes in required force.
Drawing: It is pulling of material though die orifice using tensile forces. Again a reduction in cross-section results with corresponding change in length.
Drawing die entrance is at angle against to extrusion die which is usually rectangular.
This technique employs removable of metal from selected areas of the workpiece to give final shape to the product. Machining usually is employed to produce shapes with high dimensional tolerance, good surface finish, and often with complex geometry.
And another important note is that when number of product pieces required is small, machining is preferred over forming as special tool cost will be less
There been many joining techniques, especially for metallic materials. These include:
In these techniques, two pieces are joined together either by adhesive/cohesive bonding and/or mechanical locking. Welding, brazing, and soldering involve melting of either parent metal or external metallic liquid (filler material) which upon cooling provides cohesive bonds. In riveting, pieces are put together by mechanical locking.
Where to use powder metallurgy?
Steps involved in this process are
Metals are very often subjected to thermal processing apart from mechanical processing.
Reasons for thermal processing
Thermal processing is also known as heat treatment.
Factors affecting heat treatment:
All these factors depend on material, pre-processing of the material’s chemical composition, size and shape of the object, final properties desired, material’s melting point/liquidus, etc.
Thermal processes are classified based on cooling rates from elevated temperatures
|Annealing||cooling the material from elevated temperatures slowly|
|Quenching & tempering||very fast cooling of the material using cooling medium like water/oil bath to retain the phase change|
Benefits of annealing:
Depending on the specific purpose, annealing is classified into various types:
Typically, the product receives additional heat treatments after machining to restore hardness and strength.
Quenching is heat treatment process where material is cooled at a rapid rate from elevated temperature to produce Martensite phase. This process is also known as hardening.
How to achieve rapid cooling?
Quenching process is almost always followed by tempering heat treatment.
Below are modified quenching processes:
In case hardening, the surface of the steel is made hard and wear resistant, but the core remains soft and tough. Such a combination of properties is desired in applications such as gears.
For large work pieces and complicated cross-sections induction heating is not easy to apply. In such cases, flame hardening is done by means of an oxyacetylene torch. Heating should be done rapidly by the torch and the surface quenched, before appreciable heat transfer to the core occurs
Advantages of laser hardening:
Carburizing is the most widely used method of surface hardening.
Why carburizing always done in austenitic state?
Uses of Nitriding: