Operaciones de conformado de metales. Metal Shaping.

Introduction


Practically all metals, which are not used in cast form are reduced to some standard shapes for subsequent processing.

Manufacturing companies producting metals supply metals in form of ingots which are obtained by casting liquid metal  into a square cross section.
-Slab (500-1800 mm wide and 50-300 mm thick)
-Billets (40 to 150 sq mm)
-Blooms (150 to 400 sq mm)

Sometimes continuous casting methods are also used to cast the liquid metal into slabs, billets or blooms.
These shapes are further processed through hot rolling, forging or extrusion, to produce materials in standart form such as plates, sheets, rods, tubes and structural sections.

 Sequence of operations for obtaining different shapes

Primary metal forming processes

-Rolling.

-Forging.

-Extrusion.

-Tube and wire drawing.

-Deep drawing

-Although Punching and Blacking opetations are not metal forming processes however these will be covered due to similarity with deep drawing process.

Rolling

Change in grains structure in rolling

Salient points about rolling
-Rolling is the most extensively used metal forming process and it shareis roughly 90%.
-The material to be rolled is drawn by means of friction into the two revolving roll gap.
-The compresive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area.
-The geometry of the product depend on the contour of the roll gap.
-Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish.
-The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough.

Extrusion

It is a relatively new process and its commercial exploitation started early in the nineteenth century with the extrusion of lead pipes. Extrusion of steels became possible only after 1930 when extrusion chambers could be designed to withstand high temperature and pressure.

In extrusion, the material is compressed in a chamber and the deformed material is forced to flow through the die. The die opening corresponds to the cross section of the required product.

It is basically a hot working process, however, for softer materials cold extrusion is also performed.

Direct and Indirect Extrusion

In direct extrusion metal flows in the same direction as that of the ram. Because of the relative motion between the heated billet and the chamber walls, friction is severe and is reduced by using molten glass as a lubricant in case of steels at higher temperatures. At lower temperatures, oils with graphite powder is used for lubrication.

In indirect extrusion process metal flows in the opposite direction of the ram. It is more efficient since it reduces friction losses considerably. The process, however, is not used extensively because it restricts the length of the extruded component.

Impact Extrusion


It is similar to indirect extrusion. Here the punch descends rapidly on to the blank which gets indirectly extruded on to the punch and to give a tubular section. The length of the tube formed is controlled by the amount of metal in the slug or by the blank thickness. Collapsible tubes for pastes are extruded by this method.


Hydrostatic Extrusion

In this process the friction between container wall and billet is eliminated, however, this process has got limited applications in industry due to specialized equipment & tooling and low production rate due to high set up time.



Drawing


Large quantities of wires, rods, tubes and other sections are produced by drawing process which is basically a cold working process. In this process the material is pulled through a die in order to reduce it to the desired shape and size.

In a typical wire drawing operation, once end of the wire is reduced and passed through the opening of the die, gripped and pulled to reduce its diameter.

- By successive drawing operation through dies of reducing diameter the wire can be reduced to a very small   diameter.
- Annealing before each drawing operation permits large area reduction.
- Tungsten Carbide dies are used to for drawing hard wires, and diamond dies is the choice for fine wires.

Tube drawing


•Tube drawing is also similar to wire drawing, except that a mandrel of appropriate diameter is required to form the internal hole.
•Here two arrangements are shown in figure (a) with a floating plug and (b) with a moving mandrel
•The process reduces the diameter and thickness of the tube.

Punching and Blanking


􀂃Punching and blanking operations are not metal forming operations but are discussed together with metal
forming because of their similarity with deep drawing operation.
􀂃Objective of punching and blanking is to remove material from the sheet metal by causing rupture, the punch and die corners are not provided with the any radius.
􀂃Tool steel is the most common material for tool and die. Carbides are also used when high production is
needed.

Comparison of metal forming processes

This is an interestic video about de metal shaping using an english weel by Lazze.