Plastic- Extrusion of Aluminium Alloys Flashcards
What does extrusion generally involve?
A round billet is placed into a chamber and forced through a die opening by an arm. The die dictates the final shape of the product and may be simple (round) or more complex (I or T-section)
Temperature for extrusion
Normally carried out at elevated temperatures but some materials like lead can be extruded at room temperature
Temperatures for extrusion of Al and steel
Al: 350-550C
Steel: 1000-1200C
Batch or continuous process
Each billet is extruded individually so extrusion is generally a batch process. Although billets of certain metals can be welded onto the end of the previous one to make a semi-continuous process
The two main types of extrusion and their descriptions
Direct: billet slides relative to container wall.
Indirect: die moves towards the billet and so there is no relative billet/container motion.
The two have different friction conditions
Two more niche types of extrusion
Impact: similar to indirect extrusion and used for making hollow shapes.
Hydrostatic: chamber is filled with a fluid that transmits the pressure to the billet, so no friction at container walls, used for difficult to deform metals
Problem of surface cracking
Can occur if the extrusion temperature is too high or too low or if extrusion speed is too high to allow the material to deform
Problem of worn/scratched die
Leads to scratches, gouges and/or defects on the extruded material
Problem of piping and how to rectify
Occurs when impurities in the billet (could be surface oxides) move towards the centre like a funnel.
Rectifying involves changing the flow pattern or machining the billet surface prior to extrusion. Also could leave a discard i.e not extruding up to 10% of the billet
How do outer surfaces move during direct extrusion and how does this influence the extrusion process?
Initially friction causes them to remain relatively stationary whilst the central portion of the billet flows toward the die. At the end of the extrusion process, the outer layers flow toward the die. This material consists of oxides and contaminants from the billet surface. So extrusion stopped to leave discard before these oxide and contaminant layers flow into the extrudate. Want as short a discard as possible to reduce scrap.
What does case study 1 want to produce and from what?
Want a 25mm diameter rod for machining stock to be produced from a 150mm diameter 1m long billet of Al. Want a high production rate and no surface defects
Describe the arrangement for direct extrusion
Billet is in container. Dummy block between billet and ram. On opposite side of billet is the die which is held in place by a die holder. The ram forces the billet through the die and the extrudate comes out of the die. This extrudate needs supporting as it comes out.
What temperature is the container at and why?
Normally 50C less than the billet T so it acts as a heat sink for the temperature rise during extrusion
What are the extrusion variables that need considering?
Initial billet temperature
Initial billet diameter
Press capacity up to 120MN (14000tons)
Extrusion ratios typically from 10 to 100
Ram speeds up to 0.5m/s (100ft/min)
Lubrication
Describe the arrangement for indirect extrusion
Billet is in a container closed at one end. At the other end is the die. The ram presses against the outside of the die to force it into the billet and the extrudate comes out of the die in the opposite direction.
Extrusion pressure vs ram displacement graph for direct and indirect extrusion
Both quite steep linear initially up from origin. Indirect then goes very shallow negative gradient until near the end. Direct continues up further (getting less linear) then comes down steeper until nearly joins indirect line near end. At end both curve up steeply. Direct always higher pressure than indirect (apart from beginning when equal)
Explanation of why the extrusion pressure curing direct extrusion varies like it does
As extrusion proceeds the pressure decreases quite rapidly as the area over which friction is occurring is reduced.
Comparison of surface finish for direct and indirect
Direct means a better surface finish can be achieved but this comes at the price of much higher extrusion loads.
For indirect, the billet surface tends to form the extrude surface. This means that if the billet has a good surface then the extrudate will have a good surface. Therefore billet often pre-treated e.g scalping
What is true for temperature and pressure for both direct and indirect extrusion?
Temperature rises during extrusion which reduces the pressure requirements.
Pressure increases at the end of the extrusion cycle.
Why is extrusion process often terminated before all the material has been extruded?
To protect equipment from damage due to the large increase in pressure at the end
What is DMZ and where is it?
Dead metal zone.
Either side of the die opening for direct extrusion
Where are the deformation zones for direct and indirect extrusion?
Direct: isosceles triangle with a point at the die opening and the adjacent two sides enclosing the DMZs, other side is horizontal if perpendicular to die opening.
Indirect: same isosceles triangle but the other way around so horizontal side is next to die opening and there is no DMZ
Flat faced vs conical extrusion dies
Flat faced has flat faces on the side of the die that touches the billet..
Conical has sloped sides touching the billet. Lubricant can be put on the tapered surface. Only use conical dies for lubricated extrusion of hard materials as they reduce the pressure requirement for these materials
Extrusion equipment and homogenisation treatment available for case study 1
Extrusion equipment: direct or indirect 10MN press, ram speed of 3.5, 7.0, 17.5 or 35mm/s, flat faced or conical dies.
Homogenisation: 24h at 450C with fast cool, 24h at 450C with slow cool.
