Direct-chill casting

Aluminium remelt and DC casting plant

 

An in-house remelt and casting shop

In the production of extruded aluminum products, technological scrap inevitably occurs. It includes extrusion butts, cut out transverse welds, crumpled ends of profiles from a stretching machine and others. The total amount of such relatively pure aluminum scrap is usually up to 25%. Therefore, most aluminum extruders have their “In House” casting shop remelting their own aluminum scrap, and, if necessary, purchased small pig ingots, sow ingots or good quality aluminium scrap and recycling aluminium scrap. The final product of such workshops are new extrusion ingots.

Terms and definitions

According to the document [2]:

  • extrusion ingot 
    ingot, intended and suitable for extruding, typically of solid circular cross-section, sometimes with a central hollow or a flattened cross-section
  • extrusion billet
    extrusion ingot cut to length
  • extrusion log
    extrusion ingot not cut to length.

DC casting

All aluminium extrusion ingots is produced by direct-chill (DC) casting. DC casting is called because the produced extrusion ingots are directly chilled with cooling water [1]:

  • primary cooling at the rear face of the mould and
  • secondary cooling onto the ingot surface as it extends from the mould.

A typical remelt plant of aluminium extruder

A typical remelt shop shown schematicaly on Figure 1. It have the following items of equipment:

  1. Scrap charging system.
  2. Melting and holding furnace.
  3. Luaunders to feed metal to casting machine.
  4.  Metal filtration.
  5. DC casting machine.
  6. Cooling water system
  7. Crane to remove cast ingots.
  8. Log laydown area.
  9. Charger for homogenesing furnace.
  10. Homogenesing furnace.
  11. Log Cooler.
  12. Log saw (not shown).
  13. Pollution prevention and control.



Figure 1 – The typical remelt shop of an aluminium extruder [1]

 The layout shown on Figure 1 is only representative. There are many possible units of equipment and  variations of their layout. The sizing and selection of equipment depend upon the intended annual tonnage, site layout, purchasing budget and proposed working hours [1].

Casting cycle

The overall cycle time which determines the production for a casting operation is controlled be three factors:

  • Scrap charging rate
  • Furnace melt rate
  • Speed of casting

Scrap charging rate

Scrap charging is carried by out by many different methods. In small plants usually by a combination of manual and mechanical means. The example of simple mechanical charger is shown on Figure 1. .

Furnace melt rate

Furnace melt rate is dependent upon

  • burner capacity
  • the heartrh area.

The size of the hearth determines the area available for heat exchange between the combustion gasses and the furnace charge.

Speed of casting

Speed of casting varies with the diameter of the ingot. The diameter of 178 mm for 6061 alloy has an approximate casting speed of 120 mm per minute. This gives for a ingot length of 6500 mm a 55 minute casting time. A further 15 to 20 minutes is required to strip the casting pit of ingots. A further 10 to 15 minutes is required to prepare the casting machine for the next drop. Therefore, this gives a total casting cycle of no more than 1,5 hour.

Two melting/holding furnaces

The controlling factor in the production cycle time is the rate at which metal can be passed through the furnace. This time is always longer than the time for a drop.

Therefore , in most cases, at least two furnaces feed each caster as the casting pit operations can be completed in less time than it takes to clean the furnace, fill a furnace, skim, alloy, and sample the melt ready for casting. Figure 2 shows such an arrangement. In new aluminium smelters, the trend is towards three large furnaces per casting pit so that the pit is never waiting for metal.

Figure 2 – General layout of two furnaces and melt treatment units
in relation to a casting machine [3]

Sources:

  1. D.C. Casting remelt shop handbook, Ashford Engineerig Services, 1997
  2. Global Advisory Group GAG – Guidance GAG Guidance Document 001 – Terms and Definitions – 3rd Edition 2011-01
  3. Direct-Chill Casting of Light Alloys – Science and Technology /J.F. Grandfield, D.G. Eskin, I. F. Bainbridge – 2013