Metal forming

Metal forming 

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 – one that performed at relatively low temperatures, cold working; ad the other performed at high temperatures, hot working. Hot working is responsible mainly for substantial change in cross section without material getting strengthened, while during cold working, fine details are achieved along with material getting strengthened. 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. In open die forging, two dies having same shape is employed, usually, over large work-pieces; while in closed die forging, there may be more than two pieces of die put together having finished shape. Forged products have outstanding grain structures and very good mechanical properties. Typical products include: crane hooks, wrenches, crank shafts, connecting rods.

Rolling: 

Most widely used forming technique because of high production rate and close dimensional control of final product. It involves passing a piece of metal between two rotating rolls. Deformation is terms of reduction in thickness resulting from applied compressive forces. This technique is typically employed to produce sheets, strips, foil, Ibeams, rails, etc.

Extrusion: 

      In this technique a piece of material is forced though 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. Typical extrusion products are: rods, (seamless) tubes, complicated shapes for domestic purpose.

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. Typical drawing strand includes number of dies in a series sequence. Rods, wire, and tubes are commonly produced using drawing technique.

Machining 

   This technique employs removable of metal from selected areas of the workpiece to give final shape to the product. This is in direct contrast with metal forming where metal is moved and volume is conserved. 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.

Joining 

       There been many joining techniques, especially for metallic materials. These include: welding, brazing, soldering, and riveting. 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. These techniques are employed to join two pieces of same metal with complicated shapes, or of different metals because of difficulty in fabricating them using one of the previous methods. This may be employed when on-part fabrication is expensive or inconvenient.

Powder metallurgy 

     In this technique, metal powders or mixture of metal powers at desired relative amounts are compacted into the desired shape, followed by sintering in controlled atmosphere to produce a denser product. It is makes it possible to produce a virtually non-porous product where diffusional processes controls the efficiency of the process. It is suitable, especially, for metals with low ductility/high melting points. Other advantage include: close dimensional tolerance of complicated shapes. Usually products are less dense than wrought products because of porosity. However, it is advantageous as pores can retain oil for self-lubrication of bushes, and high damping capacity.