Hot Extrusion

Hot Extrusion: Hot extrusion is done at fairly high temperatures, approximately 50 to 75 % of the melting point of the metal. The pressures can range from 35-700 MPa (5076 - 101,525 psi). Due to the high temperatures and pressures and its detrimental effect on the die life as well as other components, good lubrication is necessary. Oil and graphite work at lower temperatures, whereas at higher temperatures glass powder is used.
Typical parts produced by extrusions are trim parts used in automotive and construction applications, window frame members, railings, aircraft structural parts.

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Cold Extrusion

Cold Extrusion: Cold extrusion is the process done at room temperature or slightly elevated temperatures. This process can be used for most materials-subject to designing robust enough tooling that can withstand the stresses created by extrusion. Examples of the metals that can be extruded are lead, tin, aluminum alloys, copper, titanium, molybdenum, vanadium, steel. Examples of parts that are cold extruded are collapsible tubes, aluminum cans, cylinders, gear blanks. The advantages of cold extrusion are:

•	No oxidation takes place.
•	Good mechanical properties due to severe cold working as long as the temperatures created are below the re-crystallization temperature.
•	Good surface finish with the use of proper lubricants.

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Extrusion

Extrusion is the process by which long straight metal parts can be produced. The cross-sections that can be produced vary from solid round, rectangular, to L shapes, T shapes. Tubes and many other different types. Extrusion is done by squeezing metal in a closed cavity through a tool, known as a die using either a mechanical or hydraulic press.
Extrusion produces compressive and shear forces in the stock. No tensile is produced, which makes high deformation possible without tearing the metal. The cavity in which the raw material is contained is lined with a wear resistant material. This can withstand the high radial loads that are created when the material is pushed the die.
Extrusions, often minimize the need for secondary machining, but are not of the same dimensional accuracy or surface finish as machined parts. Surface finish for steel is 3 µm; (125 µ in), and Aluminum and Magnesium is 0.8 µm (30 µ in). However, this process can produce a wide variety of cross-sections that are hard to produce cost-effectively using other methods. Minimum thickness of steel is about 3 mm (0.120 in), whereas Aluminum and Magnesium is about 1mm (0.040 in). Minimum cross sections are 250 mm² (0.4 in²) for steel and less than that for Aluminum and Magnesium. Minimum corner and fillet radii are 0.4 mm (0.015 in) for Aluminum and Magnesium, and for steel, the minimum corner radius is 0.8mm(0.030 in) and 4 mm (0.120 in) fillet radius.

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Forging Quiz

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CERAMIC AND CARBIDES DIE MATERIALS

CERAMIC AND CARBIDES DIE MATERIALS

Potential use of ceramics and carbides has been found be gaining interest for use in

warm and hot forging applications. Ceramic inserts and coatings are well established in

the machining industry for reducing tool wear and enhancing the tool per formance.

Some of the ceramic materials have marked improvements over the traditional hot work

die materials (Cr-Mo-W based steels) used in hot forging

 

 

 

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DIE MATERIALS FOR FORGING OF STEEL

DIE MATERIALS FOR FORGING OF STEEL

There are various tool steels which are used in forging. Although in hot and warm

forging, mainly hot work die steels are used due to their ability to retain their hardness at

elevated temperatures with sufficient strength and toughness to withstand the stresses

that are imposed during forging. There have also been some successful applications of

other materials such as ceramics, carbides and super alloys although their application is

limited due to design and cost of manufacturing. The selection of die material grade and

subsequent treatment affects the mode of failure and rate of tool failure. 

 

 

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Forging defects

Forging defects:

Though forging process give generally prior quality product compared other manufacturing processes. There are some defects that are lightly to come a proper care is not taken in forging process design.

A brief description of such defects and their remedial method is given below.

 Unfilled Section:

In this some section of the die cavity are not completely filled by the flowing metal. The causes of this defects are improper design of the forging die or using forging techniques.

Cold Shut:

This appears as a small cracks at the corners of the forging. This is caused manely by the improper design of die. Where in the corner and the fillet radie are small as a result of which metal does not flow properly into the corner and the ends up as a cold shut.

Scale Pits:

This is seen as irregular depurations on the surface of the forging. This is primarily caused because of improper cleaning of the stock used for forging. The oxide and scale gets embedded into the finish forging surface. When the forging is cleaned by pickling, these are seen as depurations on the forging surface.

Die Shift:

This is caused by the miss alignment of the die halve, making the two halve of the forging to be improper shape. 

 Flakes:

These are basically internal ruptures caused by the improper cooling of the large forging. Rapid cooling causes the exterior to cool quickly causing internal fractures. This can be remedied by following proper cooling practices.

Improper Grain Flow:

This is caused by the improper design of the die, which makes the flow of the metal not flowing the final interred direction.

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Flashless (Enclosed Impression Die) Forging

Flashless Forging

Impression die forging is sometimes performed in totally enclosed impressions. The process is used to produce a near-net or net shape forging. The dies make no provision for flash because the process does not depend on the formation of flash to achieve complete filling. Actually, a thin fin or ring of flash may form in the clearance between the upper punch and die, but it is easily removed by blasting or tumbling operations, and does not require a trim die. The process is therefore called "flashless forging", and is sometimes called "enclosed die forging".

Enclosed dies are illustrated in Figure 5-14. In some cases the lower die may be split, allowing as-forged undercuts. Split die arrangements are illustrated in Figure 5-15.

The absence of flash is an obvious advantage for flashless forging over the conventional impression die process, but the process imposes additional requirements. For example, flashless forging is usually accomplished in one operation, and does not allow for progressive development of difficult-to-forge features through several stages of metal flow. In addition, the volume of metal in the workpiece must be controlled within very narrow limits to achieve complete filling of the cavity without developing extreme pressures. It takes some very well controlled preforming steps to accomplish this precise weight control in the final die.

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Precision Forgings vs. Conventional Forgings?

Precision Forgings differ from Conventional Forgings in many ways.  While Conventional forgings are typically machined on all surfaces, a Precision Forging is often characterized by very slight draft angles (0 to 1 degree), thin cross-sections, close tolerances, small radii, and excellent surface finishes.
While the tooling for a precision forging is typically more costly to produce and maintain, the advantages of precision forgings over conventional forgings pay multiple dividends.  The net or near-net shape of the precision forging can greatly reduce machining times and produce a part that is nearly ready to put right into service.  Additionally, the optimal grain structure of a precision forging increases fatigue life, and produces superior stress and inter-granular corrosion resistance.

Precision Forging

Precision Forging

Modern technological advances in the metal forging process and in the design of die, have allowed for the development of precision forging. Precision forging may produce some or no flash and the forged metal part will be at or near its final dimensions, requiring little or no finishing. The number of manufacturing operations is reduced as well as the material wasted. In addition, precision forging can manufacture more complex parts with thinner sections, reduced draft angles, and closer tolerances. The disadvantages of these advanced forging methods are that special machinery and die are needed, also more careful control of the manufacturing process is required. In precision forging, the amount of material in the work, as well as the flow of that material through the mold must be accurately determined. Other factors in the process such as the positioning of the work piece in the cavity must also be performed precisely.

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Impression Die Forging

Compression of workpart by dies with inverse of desired part shape
Flash is formed by metal that flows beyond die cavity into small gap between die plates
Flash must be later trimmed, but it serves an important function during compression:
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As flash forms, friction resists continued metal flow into gap,constraining metal to fill die cavity

Impression‑Die Forging Practice

•Several forming steps are often required
-With separate die cavities for each step
•Beginning steps redistribute metal for more uniform deformation and desired metallurgical structure in subsequent steps
•Final steps bring the part to final geometry

edging

Edging is also an open die forging process often used in manufacturing practice, to prepare a work for sequential metal forging processes. In edging, open die with concave surfaces plastically deform the work material. Edging acts to cause metal to flow into an area from both sides. Edging and fullering both are used to redistribute bulk quantities of the metal forging's material.









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Cogging

Successively reducing the thickness of a bar with open die forging
•Also called drawing out
•Reducing the thickness of a long section of a bar without
excessive forces or machining

a cogging operation on a rectangular bar. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil. Note the barreling of the workpiece.

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