a) Centre-burst:
- Internal crack caused by excessive tensile stress at the centre, which is caused by a high die angle, low extrusion ratio, and imperfections in the work metal, which serve as crack defects beginning locations.
- Arrowhead fracture, centre cracking, and chevron cracking are some other names for this condition.
b) Piping:
- Under direct extrusion, the development of a Sink hole at the end of the billet.
- Direct extrusion is connected with piping, which is a fault.
- To eliminate piping, a dummy block with a little smaller diameter than the billet is used. Tailpipe and fishtailing are two more names for this problem.
c) Cracking on the surface:
- This flaw is caused by high part temperatures, which cause surface fractures to form.
- They frequently arise when the extrusion speed is too fast, resulting in significant strain rates and heat buildup.
- High friction and surface chilling of high-temperature billets in hot extrusion are two more factors that contribute to surface cracking.
Process Variables in Direct Extrusion
- The die angle
- Reduction in cross-section
- Extrusion speed
- Extrusion pressure
- Billet temperature
- Lubrication
Pros
- Extruded products come in a wide variety of shapes and sizes. Extrusion can be used to make cross-sectional shapes that are not practical to roll, such as those having re-entrant sections.
- Because the dies can be easily removed and replaced, no time is lost when changing shapes.
- Extruded parts have a higher level of dimensional accuracy than rolled parts.
- The ductility of metals is higher in extrusion because the metal in the container is in composite compression, which is especially useful when working with poorly plastic metals and alloys.
- In comparison to rolling, where the reduction each pass is typically 2., very significant reductions are conceivable.
- It is possible to manufacture small pieces in big quantities. A long gear, for example, is extruded and then sliced into a number of individual gears to make a simple pump gear.
- Because it is more exact than forging, it does not require draught or flash for trimming, nor does it necessitate unnecessary machining.
Cons
- Extrusion waste is higher than rolling waste, which is roughly 1 to 3 percent.
- Because the axial and outer layers of blanks flow differently, inhomogeneity non structure and attributes of an extruded product is increased.
- Extrusion tooling has a lower service life because to high contact stresses and slide rates.
- Tooling expenses are rather high due to the use of expensive alloy steel.
- Extrusion is generally less productive than rolling, especially in continuous variants.
- When compared to other processes, extrusion has a higher cost.
Limitations
- Extrusion is more extensively utilised in the production of solid and hollow sections from non-ferrous metals and alloys that are poorly plastic (aluminum, copper, brass and bronze etc.)
- Aluminum, magnesium, zinc, and copper alloys are examples of ductile non-ferrous metals that can be extruded. Other metals can be extruded as well, although the force required is substantial, even when hot extrusion is used.
- With the development of molten-glass lubricants, steel and other ferrous alloys can also be successfully treated.
- Fabrication of complex-configuration sections and pipes.
- Production in medium and small batches
- Parts with excellent dimensional accuracy are manufactured.
- Extruded items include rods with diameters ranging from 3 to 250 mm, pipes with diameters ranging from 20 to 400 mm and wall thicknesses of 1 mm and above, and more complicated shapes that cannot be achieved using other mechanical methods.
Products made using extrusions
Many commercial aluminium items (structural shapes, door and window frames, etc.) are created using this method. Aluminum is arguably the most suited metal for extrusion (hot and cold).
Railings for sliding doors, tubes with diverse cross sections, structural and architectural designs, and door and window frames are examples of extrusion items.
Extruded items can be trimmed to any length, resulting in separate parts like brackets, gears, and coat hangers.
