Conformados de los metales

METAL FORMING

 

 1. Introduction to metal forming

Because metals must be formed in the plastic behavior is necessary to overcome the yield strength so that the deformation is permanent. Por lo cual, el material es sometido a esfuerzos superiores a sus límites elásticos, estos límites se elevan consumiendo así la ductilidad. Therefore, the material is subjected to stresses above its elastic limits; these limits are raised consuming and ductility.

In the metal forming should take into account certain properties, such as low yield strength and high ductility. These properties are influenced by temperature: when the temperature increases, the yield stress decreases and increases ductility.

There is a large group for this manufacturing process in which the tools, usually a given conformation, exert forces on the workpiece to force them to take the form of the given geometry.

Below is the kind of distinctions to consider when studying metal forming processes:

-          Cold Working

Refers to work at room temperature or lower. This work effort occurs when applying a higher yield strength than the original metal, while producing a deformation.

The main advantages of cold work are: better accuracy, lower tolerances, better surface finishes, chances of obtaining desired directional properties in the final product and party harder.

However, cold working has some disadvantages in that it requires greater strength because the metal increases strength due to work hardening, causing the effort required to continue the deformation increases and counteracts the increase of the resistance reduction ductility and increased tensile strength limit the amount of operations that may be formed to make the parts.

-          Hot Working

Is defined as the plastic deformation of the metallic material to a temperature higher than the recrystallization. The major advantage of hot work is to obtain a nearly unlimited plastic deformation, which is also suitable for casting large parts because the metal has a low yield strength and high ductility.

Proceeds from hot work are major changes to the shape of the workpiece, lower forces and efforts required to deform the material, choice of working with metals that fracture when cold worked, usually isotropic strength properties and finally, do not occur due to hardening of parts work processes.

However, the tolerances and surface finish are usually lower compared to cold working parts are worked anisotropic behavior. Likewise, it is more difficult to register the control of dimensional accuracy due to a combination of elastic deformation and thermal shrinkage of the metal, whereby the design of the piece is necessary to consider a larger dimension to start an operation.

In practice, the hot work is performed from slightly higher temperatures 0.5Tm (half the melting temperature). The deformation process itself generates heat that increases temperatures in localized areas of the parts, which can cause undesirable merging of these regions.

2. Forming operations or preformed sheet metal

The sheet forming processes are operations performed on sheets, strips and rolls, conducted at room temperature with punches and dies systems. Some of them are: cutting, folding and stuffing.

2.1 Court Operations:

Cizallado Shearing

Sheet cutting operation which consists in reducing the sheet to a smaller size. To make the metal is subjected to two cutting edges

Die

In the die cut sheets by subjecting them to shear forces, developed between a punch and a die, shearing and differs from the latter only decreases the sheet size without giving any way.

 

The finished product of the film can be die punched or cut pieces.

The parameters taken into account in the die are form and materials of the punch and the matrix, the speed and force for punching, lubrication, material thickness and the clearance or gap between the punch and die. The determination of the light will influence the shape and quality of the cut edge. The more light there is, the cut edge will be rough and cause a larger zone of deformation in which the cure will be higher.

 

The burr height increases with increasing light... The edges of blunt tools also contribute to the formation of burrs, which decreases when the speed of the punch increases.

In some operations of punching the perforated sheet tends to accumulate between the straight portion of the matrix, packaging exerting a force that opposes the force of punching. Therefore, the cutting force should increase as more operations are performed.

2.2 Bending:

The bending deformation of metal sheet is about a certain angle. Angles can be classified as open (if you are above 90 degrees), closed (less than 90 °) or straight. During operation, the outer fibers of the material are in tension, while the interiors are in compression. The bend produces no significant changes in the thickness of the metal sheet.

There are different ways of folding, the most common are: two forms bending and folding sliding.

Doubled between forms

In this type of folding, the sheet metal is deformed between a V-shaped punch or other form and a die. It can bend with this punch from angles very obtuse to acute angles. This operation is used for operations generally low volume production.

 

Folding Sliding

In the folded slide plate to press the sheet metal as the matrix or punch while it exerts a force that bends around the edge of the die. This type of bending is limited to 90 ° angles.

When removing the bending force, the blade tries to regenerate thanks to elastic property called memory metals, restoration or recovery. This property is not only observed on plates and flat plates, but also on rods, wires and rods with any cross section.

Bending Punches

                         

The punches are distinguished by the following characteristics: bending radius and angle. The bending angle is measured between the two sides forming the edge of the core around which the sheets are folded and determines the bending radius of said chamfer edges.

Sausage:

The inlay consists of placing the metal sheet on a die and then pressing it into the cavity using a punch having the shape in which the sheet will be formed.

 

The number of drawung steps is dependent on the relationship existing between the disc and the magnitude of the dimensions of the drawn component, ease of drawing, the material and thickness of the chapa 1. That is, the more complicated forms and more depth is needed, more stages will be included in this process.

 

3. Volumetric deformation processes

The blocks forming processes causes deformations and drastic changes in material ways. The initial shapes of the pieces are in this case rectangular and cylindrical rods to deformation operations as rolling, forging, extrusion and drawing (drawing).

Deformation processes are performed in block cold working operations and heat. Cold working is done when there is a need to improve the mechanical properties or to achieve a good surface finish. Hot work is required when the volumetric strain involves large workpieces.

Laminate

The laminate is a process which reduces the thickness of a long piece through compressive forces exerted by a set of rollers that rotate pushing and pulling the workpiece between them.

 The laminate can be a result of the finished part (such as the foil used wrapping foods and cigarettes), and others, is the raw material of subsequent processes such as punching, bending and stamping.
 

Generally, the hot rolling is performed. This process begins with a continuous casting where the steel is reheated in a pit of thermal diffusion, then the steel passes through a series of rollers that grind the material (lamination process), and finally the film is stored on rolls.

 

 

Wrought

The floor was the first of the processes of direct compression type and is probably the oldest method of metal forming. Involves applying compressive stresses which exceed the yield strength of the metal.

In this forming process the material is compressed between two dice, to take the desired shape. There are three types of forging operation:

·         In the open die forging a material is compressed between two planes, allowing the material to flow without restriction is its sides.

·         Forging die in the printer, the die exerts a force on the understanding of the work piece surface, causing it to take the shape of the die. Although the restriction increases in metal, it may flow beyond this printer as causing excess material (flash).

·         In the die forging barbless completely restricts the material inside the cavity and there is no excess burrs.

Most operations are performed in hot forging, the deformation given demand in the process and the need to reduce drag and increase the ductility of the metal. However this process can be performed cold, the advantage is the greater strength of the component, resulting from strain hardening.

Extrusion
Extrusion is a process by which the compression work metal is forced to flow through a die opening to shape cross-section.

Examples of this process are hollow sections, as pipes, and a variety of shapes in cross section.

The Extrusion basically depend on the geometry and the material to be processed. There is a direct extrusion process, indirect extrusion, and in both cases the hot metal extrusion (high temperature).

In direct extrusion, is deposited in a container called a slug rough billet, which is compressed by a piston.

By being compressed, the material will be forced to flow through the other end into the shape having the given geometry.

 

Or conversely indirect extrusion die consists of a printer that is mounted directly on the piston.The pressure applied by the piston in the opposite direction to the flow of material. The billet is placed in the bottom of the given container.

In indirect extrusion, the billet does not move through the container, therefore, no friction on the walls thereof and the piston force is smaller than in the direct extrusion.