Lecture- 8

Mechanical working of metals Hot working processesRollingDrawingForgingExtrudingPiercing Spinning

Mechanical working of metalsPlastic deformation or other physical change to which metal is subjected, by rolling, hammering, drawing., etc. to change its shape, properties or structure.

There are many possible process of shaping and forming metals.

The choice of method or methods to be employed is influenced by the following factors.i.Kind of metalii.The complexity of the shapeiii.The intended useiv.Relative costs of various methods.

Metals are subjected to mechanical working for two general purposes. To reduce the original blockii.To improve the mechanical properties of the metal through-Refinement of grain size, -Directional control of "flow lines" and -Break-up and distribution of unavoidable inclusions, particularly in steel.Mechanical working processes

Operations are performed on metal to produce new shape or to improve the properties of other metal. Shaping in the solid state may be divided intoi.Non-cutting or chipping, shaping, eg forging,rolling, pressing etc.ii.Cutting shape, eg - machining operations.

Non-cutting shaping are preferred to as mechanical working process.

Mechanical working of metal is simply plastic deformation performed to change dimension properties and or surface by means of mechanical pressure.

It may be either hot working or cold working.

Subjecting metals to forces, which cause into undergo plastic deformation when it is above the above the recrystallization temperature.

Hot working

Rolling ForgingDrawingExtrudingPiercingSpinningPrincipal methods of hot working

Hot rolling, also called hot working, is a metallurgical process of in which large pieces of metal such as slabs or billets are deformed between roller at high temperature to form thinner cross sections. RollingBillets- A small, usually rectangular bar of iron or steel in an intermediate stage of manufacture.

Hot rolling is a rolling operation carried out at a temperature exceeding the recrystallization temperature and permitting large amount of deformation. Hot rolling

Rolling millA machine used for rolling metal is called rolling mill.A typical rolling mill consists of a pair of rolls driven by an electric motor transmitting a torque through a gear and pair of rolls. A metal bar passing through the rotating rolls is squeezed, and it elongates while its cross section area decreases.

ForgingHammer forging (drop forging) is forming a preheated workpiece by using impact energy of the falling hammer forcing the metal to fill the space between the punch (a part attached to the hammer) and the forging die (a part attached to the anvil).Forging is a compressive metal forming process, involving shaping a metal piece by hammer.

Drawing is a metal forming process involving pulling a workpiece (cold or hot) through a die providing reduction of the cross section of the workpiece. The drawing process employs a series of dies with opening of similar shape and of size which is slightly reduced in each consequent die. DrawingAs the cross section decreases after passing each die, the length of workpiece increases, requiring corresponding increase of the speed. Drawing is used for production of wires (diameter down to 2m), rods and other products from brass and steel.

Deep drawing Deep drawing is a cold metal forming process in which a flat metal blank is forced into a die cavity by hydraulically driven punch. A cup-like cylindrical part, having the depth greater than its diameter, is formed as a result of deep drawing.

ExtrusionExtrusion is a metal forming process involving shaping a metal billet (hot or cold) by forcing it through a die with an opening. In hot extrusion process the metal billet is preheated to the required temperature: 840F - 930F (450C - 500C) for aluminum alloys and 1300F - 1470F (700C - 800C) for Copper alloys. Extrusion is used for manufacturing round and hexagonal rods, tubes, various profiles (mainly aluminum) and other shapes.

Direct and indirect extrusion In the direct extrusion the extruded metal flows in the direction of the ram motion.In the indirect extrusion the metal flows in the direction opposite to the ram motion.

Rotary piercing is a metalworking forming process for forming thick-walled seamless tubing Piercing is employed to produce seamless tubing, which is the natural form from which to make any thin walled round objects. It is also called seamless tubing. The process of making hot pierced tubing consists of passing a hot rolled billet in between two conical shaped rolls and over a mandrel, which assists in the drilling punching or pressing. It is then pushed into two piercing rolls, which impart axial as well as rolling movement to the billet and force it over the mandrel. Piercing

The alternate squeezing and bulging of the billet open up a seam in its centre. The first pass makes a rather thick-walled tube, which is again passed over a tapered plug and through grooved rolls in a two high mill where the thickness is decreased and the length is increased. If more accuracy and better finish are desired, the tube may run through sizing dies or rolls. After cooling, the tubes are usually placed in a picking bath of dilute sulphuric acid to remove the scale and oxide.

Rotary piercingThe billet is gripped by the rolls, which rotate and advance it over the piercer point, forming a hole through its length.

SpinningA cold-forming operation in which a rotating disk of sheet metal is shaped over a male form or mandrelForm attached to the Headstock spindleFinal shapeSteps in spinningOriginal blank of sheet metalFollower held in tailstock

Spinning them from sheet metal can make parts that have circular cross-sections. The principle of metal spinning. A heated circular blank of sheet metal is lightly held against a chuck by the pressure of a freely rotating pad on the lathe tailstock. This chuck may be made of plaster, wood or metal and is revolved on the spindle of a lathe.

A rounded stick or roller is pressed against the revolving piece and moved in a series of sweeps. This displaces the metal in several steps to conform to the shape of the chuck. Once the operation is started, considerable frictional heat is generated which aids in maintaining the metal at a plastic state. Spinning is a highly specialized art. To avoid excessive thinning of the metal, the pressure of the forming tool should be directed sometimes backward towards the tailstock as well as forward towards the hand stock.

Advantages of hot workingPorosity in the metal is largely eliminated. Coarse grains are refined. Slag and other impurities are squeezed into fibers and distributed throughout the massiv. Mechanical properties such as toughness, ductility, elongation, percentage, reduction of area percentage and resistance to shock and vibration are improved due to the refinement of grains.

It requires expensive tools.

It produces poor surface finish. This is due to rapid oxidation and scaling of the surface of the metal at high temperatures.

iii. Close tolerances cannot be maintained due to poor surface finish.Disadvantages of hot working