Sheet-Metal Characteristics:
Sheet metals are generally characterized by a high ratio of surface area to thickness. Forming of sheet metals is usually carried out by tensile forces in the plane of the sheet; otherwise the application of compressive forces could lead to buckling, folding, and wrinkling of the sheet. Unlike bulk deformation processes, in most sheet-forming processes any thickness change is caused by stretching of the sheet under tensile stresses (Poisson's ratio). This decrease in thickness should be avoided in most cases since it can lead to necking and failure. The major factors that significantly influence the overall sheet-forming operation are,
Elongation:
A specimen subjected to tension first undergoes uniform elongation that corresponds to the ultimate tensile strength. Then necking begins and non-uniform elongation occurs until fracture takes place. The material is being stretched in sheet forming, so high uniform elongation is desirable for good formability.
Yield-point elongation:
Low-carbon steels exhibit this behavior, which indicates that after the material yields, it stretches farther in certain regions in the specimen with no increase in the lower yield point, while other regions have not yet yielded. This behavior produces Lueders bands (or stretcher strain marks or worms) on the sheet, making elongated depressions on the surface of the sheet. To avoid this problem, the thickness of the sheet is reduced by 0.5 to 1.5% by cold rolling.
Anisotropy:
Anisotropy is the directionality of the sheet metal. It is acquired during the thermo-mechanical processing history of the sheet. There are two types of anisotropy: crystallographic anisotropy (from preferred grain orientation) and mechanical fibering (from alignment of impurities, inclusions, voids, and the like, throughout the thickness of the sheet during processing). Anisotropy may be present in both the plane of the sheet and its thickness direction.
Grain size:
The grain size is important because it affects the mechanical properties of the material and the surface appearance of the formed part. The coarser the grain, the rougher the surface.
Residual stresses:
Residual stresses can be present in sheet metal parts because of the non-uniform deformation of the sheet during forming. When disturbed, such as by removing a portion of it, the part may distort. Tensile residual stresses on surfaces can also lead to stress-corrosion cracking of sheet-metal parts unless they are properly stress relieved.
Spring back:
Sheet-metal parts are generally thin and are subjected to relatively small strains. Thus they are likely to experience considerable spring back, particularly in bending and other sheet-forming operations where the bend radius-to-thickness ratio is high.
Wrinkling:
Although the sheet metal is generally subjected to tensile stresses, the method of forming may cause compressive stresses to develop in the plane of the sheet, which cause wrinkling, buckling, folding, or collapsing of the sheet. The tendency for wrinkling increases with the unsupported or unconstrained length or surface area of the sheet, decreasing thickness, and non uniformity of the thickness.
Coated sheet metal:
Sheet metals, especially steel, precoated with a variety of organic coatings, films, and laminates are available and used primarily for appearance and corrosion resistance. Coating are applied to the coil stock on continuous lines, with thickness ranging from 0.0025 to 0.2 mm on flat surfaces.
Sheet metal operations:
Shearing: The operation is carried out beyond the ultimate strength. E.g. Blanking, piercing and perforating.
Bending: In this one half of the material in neutral axis is subjected to tension and other half is subjected to compression. This operation is carried beyond the elastic limits and below the ultimate strength.
Drawing: One shape is converted to another, by applying tensile load.
Stretching: The materials is prestretched to its yield point and formed around a tool of definite shape by stretching beyond the elastic limit and giving it a permanent set.
Squeezing: Metals are worked in compressive load. E.g. Drop stamping and coining.
Details of individual operations are given below.
Blanking:
The operation of making different shapes out of sheet metal is called as blanking. The raw material is called as blank. The tool contains a male member called as punch and a female member called as die.
Piercing:
The operation of punching holes on sheet metal. The holes may be circular or any other shape. It is also consists of a punch and die.
Blanking and piercing:
In this blanks and holes with smoother edges and closer tolerance are made. High cost savings are obtained. tolerances to the extend of 0.025 mm is obtained in thin metals and 0.08 in thick metals. In conventional blanking and piercing to attain the desired tolerance the parts have to be reworked. The process is slower.
Perforation:
Piercing a number of identical holes in a single or continuous rows on a sheet.
Notching:
It is a unbalanced or incomplete blanking operation, where only a portion of the metal is sheared out.
Power Brake:
It is primarily used for straight bending of sheet to form flanges. The forming pressure requires depending on material thickness, sharpness of bend, type of die used etc.
Routing Machines:
Routers are used to create blanks with contours. There are two types of routing machines. They are
In radial arm router, the work piece is held stationery and cutter moves around the work piece. But in case of fixed head router, the router had is stationery and the work is moved around cutter. this is used for smaller components. Routing time is usually few sections. In CNC routers, all operations except loading and unloading of sheet metals can be mechanized.
Stretch Wrap forming:
In this a sheet metal is stretched within elastic limits, the amount of stretch is proportional to pull. On release, the metal regains its original shape. However if the metal is stretched beyond the elastic limits it does not regain its original shape, also the amount of stretch is not proportional to pull. In this machine, the operation take place in the following stages.
Stage 1 : The material is held on the machine and stretched equally on both sides upto the elastic limit of the material.
Stage 2 : Maintaining this same pressure the materials is wrapped around a form block.
Stage 3 : If the material is unloaded, it has a tendency to spring back t original shape. Hence it is stretched beyond the elastic limits to get a permanent set.
Peen forming:
It is a dieless forming process, performed at room temperature. In these machines, the surface of the work piece is impacted by small steel shots. Every piece of shot acts like a tiny hammer. Thus stretching the upper surface. This impact pressure causes local plastic deformation and produces a residual compressive stress. This residual compressive stress along with stretching causes a concave surface on the peened side.
The size, velocity and angle of impingement of shots as well as distance between the nozzle and work piece, all these factors are controlled by specially designed machines.
Stress peen forming:
In this, the part to be machines is pre-stressed to an arc with 90 % of elastic limit and peen formed as mentioned earlier. Pre-stressing increase the effect of peen forming in one direction and decrease the effect in opposite direction.
Nozzle type machines are used for peen forming. Compressed air is used to properly steels hot. There are around 20 nozzles and each nozzle is capable of delivering around 23 Kgs / Min. The nozzle direction is adjustable. Centrifugal wheel machines are also used.
Spinning:
Metal with 3 dimensions are formed. The work piece is pressed by a blunt tool against a rotating die, whose outer contour matches with the inner contour of the finished part. Parts with hemispherical, conical, cylindrical shapes are produced by this method.
Super plastic formation:
Materials that have unusually large strains ( > 500 % ) without localized necking is called as super plastic. These materials can be formed into more complex shapes with much lower loads.
Defects that can occur with Sheet Products:
Formability of Sheet Metals:
Sheet-metal formability is defined as the ability of the metal to undergo the desired shape changes without failures such as necking and tearing. Three factors have a major influence on formability
Several techniques have been developed to test the formability of sheet metals, and the forming-limit diagram (FLD) is one of them. The region above the curves is the failure zone; R is the normal anisotropy. Sheet metal thickness affects the FLD also. The thicker the sheet, the higher its formability curve. However, in actual operation, thick blank may not bend as easily around small radii. The rate of deformation on FLD should be assessed for each material as well. The FLD is used to compare different metals. Strips of metal of different widths, covered with a grid of small circles, typically 2.5 to 6 mm diameter, are tested with a very good lubricant over a spherical punch. Sheet wide enough to be clamped on all edges undergoes balanced biaxial tensile strain over the center of the punch. As the width of the strip is decreased, the minor strain decreases. The minor strain may be +ve or -ve. The major and minor strains from the circle nearest to the tear can be considered to be a point on the boundary between safe and unsafe zones of the FLD. A typical FLD is shown below.
Dent resistance of sheet-metal parts:
A dent is a small but permanent biaxial deformation. In certain applications involving sheet-metal parts, such as automotive body panels, appliances, and furniture, dent resistance is an important issue. Dent resistance is determined by a combination of material and geometrical parameters, and is defined as:
Dent resistance = ( alpha ) Y2t4/S,
Where:
Y = yield stress
t = thickness
S = panel stiffness = ( E )(t a)( shape)
The value of a ranges from 1 to 2 for most panels. As for the shape, the smaller the curvature, the greater the dent resistance because of its flexibility. Therefore dent resistance increases with increasing strength and thickness and decreases with increasing elastic modulus, E, and stiffness and decreasing curvature.
Last updated on Tuesday, December 23, 2003 , 06:25 PM