Rule-based DFM analysis for forging is the controlled deformation of metal into a specific shape by compressive forces. The forging process goes back to 8000 B.C. and evolved from the manual art of simple blacksmithing. Then as now, a series of compressive hammer blows performs the shaping or forging of the part. Modern forging uses machine driven impact hammers or presses that deform the work-piece by controlled pressure.
The forging process is superior to casting in that the parts formed have denser microstructures, more defined grain patterns, and less porosity, making such parts much stronger than a casting. All solid metals and alloys are forgeable, but each will have a forgeability rating from high to low or poor. The factors involved are the material's composition, crystal structure and mechanical properties all considered within a temperature range. The wider the temperature range, the higher the forgeability rating. Most forging is done on heated work-pieces. Cold forging can occur at room temperatures. The most forgeable materials are aluminum, copper, and magnesium. Lower ratings are applied to the various steels, nickel, and titanium alloys. Hot forging temperatures range from for refractory metals.
Types of forging Open die forging In open die forging a cylindrical billet is subjected to upsetting between a pair of flat dies or platens. Under frictionless homogeneous deformation, the height of the cylinder is reduced and its diameter is increased. Forging of shafts, disks, rings etc. are performed using the open die forging technique. Square cast ingots are converted into a round shape by this process. Open die forging is classified into three main types; cogging, fullering and edging.
Close die forging Also known as impression die forging, impressions are made in a pair of dies. These impressions are transferred to the work-piece during deformation. A small gap between the dies called flash gutter is provided so that the excess metal can flow into the gutter and form a flash. Flash plays an important role during the deformation of the work-piece inside the die cavity. Due to the high length to thickness ratio of the flash gutter, friction in the gap is very high. Because of this, the material in the flash gap is subjected to high pressure. There is high resistance to flow. This in turn promotes effective filling of the die cavity. In hot forging, the flash cools faster as a result of it being smaller in size. This enhances the resistance of the flash material to deformation resistance. As a result of this, the bulk of work-piece is forced to deform and fill the die cavity more effectively – even intricate parts of the die cavity is filled.
Workpiece temperature During the forging process, the workpiece is either above the recrystallization temperature or below it. The former is known as Hot forging while the latter is known as Cold forging.
Hot forging Hot forging is defined as working a metal above its recrystallization temperature. The main advantage of hot forging is that as the metal is deformed the strain-hardening effects are negated by the recrystallization process.
Advantages Decrease in yield strength, therefore it is easier to work and takes less energy (force) Increase in ductility Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneity Pores may reduce in size or close completely during deformation In steel, the weak, ductile, FCC austenite is deformed instead of the strong BCC ferrite at lower temperatures
Disadvantages Undesirable reactions between the metal and the surrounding atmosphere Less precise tolerances due to thermal contraction and warping from uneven cooling Grain structure may vary throughout the metal due to many various reasons
Cold forging Cold forging is defined as working a metal below its recrystallization temperature, usually around room temperature.
Advantages No heating required Better surface finish Superior dimensional control Better reproducibility and interchangeability Directional properties can be imparted into the metal Contamination problems are minimized
Disadvantages Higher forces are required Heavier and more powerful equipment and stronger tooling are required Metal is less ductile Metal surfaces must be clean and scale-free Intermediate anneals may be required to compensate for the loss of ductility that accompanies strain hardening The imparted directional properties may be detrimental Undesirable residual stress may be produced
Categories of tolerances Group 1 Length, width and height tolerances, mismatch tolerances, residual flash (and trimmed flat) tolerances, and pierced hole tolerances.
Group 2 Thickness tolerances and ejector mark tolerances.
Group 3 Straightness and flatness tolerances and tolerances for centre-to-centre dimensions.
Group 4 Fillet and edge radii tolerances, Burr tolerances, surface tolerances, tolerances on draft angle surfaces, eccentricity tolerances for deep holes, eccentricity tolerances for pierced holes, tolerances on concentric bosses, tolerances for unforged stock, and tolerances for deformation of sheared ends.
Deviations of forms The tolerances for lengths, widths, heights, and thicknesses cover IL only the diligences of dimensions, but also the deviations of form which are: a) Out of round, b) Deviations from cylindricity c) Deviations from parallelism, and d) Other deviations from the specified contour. The deviations arc not to exceed the limits given by the tolerances. In extreme cases they may cover the whole fields of tolerances unless other is agreed to between the supplier and purchaser. Where restrictions deviations of form have been agreed upon, this shall be noted on the drawing.
Design procedure Information required by forger In order to assist the forging supplier to utilize his experience to the best effect, both in designing the dies and tools and in establishing forging inspection procedures, rt is in the purchaser's interest to supply the following information: A finished machined drawing; Details and dimensions of machining locations (prior notice should be given of any subsequent changes in these location points) Anv other relevant information on machining operations and function of the component.
Preparation It is recommended that the drop drawing which should then be submitted forger should prepare the forging to the purchaser for approval, and, if necessary, for joint consultation.
In instances where the purchaser wishes to prepare his own fully dimensioned forging drawing, it is no less necessary that the drawing of the finished machined component and the other information referred to above should be made available to the supplier.
Indication of dimensions on drawings It is imperative to note that, with the exception concerning draft angle surfaces the tolerances indicated in this standard shall be applied only to those dimensions specifically indicated on the agreed forging drawing.
For this reason, the method of indicating dimensions on the forging drawing has a vital bearing on the dimensional control of the forging.
Tolerances for dimensions not shown on the forging drawing may not be taken from the standard but may be determined, if required, only by calculation based on the dimensions and tolerances which are already shown on the agreed forging drawing.
Indication of tolerances on drawings All forging drawings should be endorsed, ‘Tolerances conform with IS: 3469 (Part II)-1974 unless otherwise indicated
For correct endorsement of forging drawings the following form of presentation of tolerances at the foot of the drawing is recommended:
Category: 1.Lengths and overall diameters 2. Widths 3. Heights 4. Mismatch 5. Residual flash and trimmed flat 6. Thickness 7. Straightness 8. Flatness 9. Fillet and edge radii 10. Surfaces
Any tolerances which are only applicable to specific dimensions shall be indicated on the drawing against the particular dimensions concerned. Ejector mark tolerances and burr tolerances should be shown on the forging drawing against the specific locations. Any special tolerances agreed between the purchaser and the supplier shall be indicated clearly on the forging drawing and shall, wherever possible be entered against the specific dimensions concerned.
Importance of drawings The drawing of the forged part which has been accepted by the purchaser is the valid document for inspection of the forged part. This drawing is also the only valid document for tolerances on parts of the forging remaining unmachined
Processes There are many different kinds of forging processes available, however they can be grouped into three main classes: 1. Drawn out: length increases, cross-section decreases 2. Upset: Length decreases, cross-section increases 3. Squeezed in closed compression dies: produces multidirectional flow. Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.
Open-die drop-hammer forging Open-die forging is also known as smith forging. In open-die forging a hammer comes down and deforms the workpieces, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the working surfaces of the forge that contract the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore, the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape but may have a specially shaped surface for specialized operations; for instance the die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool. Open-die forging lends itself to short runs and is appropriate for art smiting and custom work. Other times open-die forging is used to rough shape ingots to prepare it for further operations. This can also orient the grains to increase strength in the required direction.
Impression-die drop-hammer forging Impression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities; this is called flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is trimmed off.
In commercial impression-die forging the workpiece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called edging, fullering, or bending impression. The following cavities are called blocking cavities in which the workpiece is working into a shape that more and more resembles the final product. These stages usually impart the workpiece will generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and rather machine the final features.
Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.