Melting and Pouring of the Reactive Metals

Abstract:
A very reactive metal in the molten state, titanium must be melted and poured in a vacuum to yield a quality casting. Most titanium investment cast parts are made with the vacuum arc remelting (VAR) technique, which uses a vacuum arc furnace to melt a portion of a titanium electrode into a water- cooled copper crucible.

Traditional Titanium (Reactive metals) Casting

A very reactive metal in the molten state, titanium must be melted and poured in a vacuum to yield a quality casting. Most titanium investment cast parts are made with the vacuum arc remelting (VAR) technique, which uses a vacuum arc furnace to melt a portion of a titanium electrode into a water- cooled copper crucible. When the desired amount of metal is melted, the remaining electrode is quickly retracted and the crucible tilted to pour the metal into the mold.

The consumable electrodes are generally forged billets, wrought revert material, or selected foundry returns, the extra processing of which tends to increase the cost of the raw material. Because the VAR method tends to yield a melt of inhomogeneous temperature, it often must be poured into centrifugal or preheated static molds.

Titanium Casting

During the last several decades titanium and its alloys have established footholds in the aerospace, energy, and chemical industries. Typical applications include aircraft structural and engine components, steam turbine blades, offshore drilling components, marine components, and pumps and valves for the chemical process industry.

Titanium’s uses have been somewhat limited because of the relatively high cost associated with the metal, but continually improving manufacturing techniques resulting in near net shapes and greater precision have helped improve titanium’s affordability, thereby broadening its markets.

Important to know:

  1. Titanium is the 4th most abundant metallic element in the earth’s crust.
  2. Titanium’s melting point is 1667°C.
  3. Titanium’s density is about one half that of steel, but that its mechanical properties are similar.
  4. There are no true titanium casting alloys; titanium castings are produced from wrought alloy formulations.
  5. The mechanical properties of titanium castings are generally comparable, and sometimes better than, titanium forgings.

USBM - Titanium Induction Melting Method

The crucible consists of four circular quadrants brazed to a copper plate to form a crucible. The quadrants and base plate have integral cooling passages through which water is circulated, and refractory cement is packed where the quadrants meet. Induction coils surround the copper crucible, and the whole assembly is installed in a vacuum chamber.

A titanium charge is placed in the crucible with granular calcium fluoride added to act as an electrical insulator between the titanium and the crucible to prevent arcing and crucible damage. The chamber is then evacuated and power is supplied to the induction coil. Here is where the segmented crucible becomes important, because if it weren’t nearly all the field generated by the induction coil would dissipate before it could melt the charge.

When power is applied, the titanium charge heats up, but the calcium fluoride melts first, effectively coating the crucible. As the titanium melts, the water cooled crucible freezes the outer shell of the melt, forming a titanium "skull" similar to that created in arc melting.

Named for the "skull" of material that lines the crucible after each pour, ISM is purported to offer several advantages over other reactive metal melting and casting techniques. The charge material, for example, does not have to be an expensive electrode. Rather, the charge stock can be in any form as long as it fits into the crucible. The charge is typically composed of revert and titanium scarp.

Also, titanium dioxide can easily be added to the melt to obtain desired oxygen content in the castings, and alloying in general is more easily accomplished. For example, high vapor pressure alloying elements such as manganese can be added late in the melt.

Additionally, the molten pool can be held for a long time to allow additions with high melting points, such as tungsten, to fully dissolve. These advantages has been used to produce many different reactive alloys. Many alloys have been based on Ti-Al-type formulations, but Zr, Nb, Cr, V, Ni and Al alloys has also been produced, to name a few.

Refractories

Manganese Bronzes (High Tensile Brasses)

Refractories are: nonmetalic mineral materials (usually metal-oxides, carbonates or silicates) + mineral bonding materials + additives.

The well-known refractories used in metallurgy (presented with the leading chemical component):

  • Graphite -C
  • Silica -SiO2
  • Alumina -Al2O3
  • Chrome - Cr2O3
  • Dolomite - (CaMg)2 CO3
  • Magnesite -MgCO3
  • Carborundum - SiC
  • Periclas -MgO (sintered-calcined Magnesite)
  • Zirkonia -ZrO2
  • Zirkon -ZrSiO4
  • Olivine -(MgFe)2 SiO4 (mixture of 2MgOSiO2 -Forsterite and 2FeOSiO2 -Fajalite)
  • Talc -Mg6 (OH)4 (Si8 O20)
  • Fire-clay -Al2O3 x SiO2 x nH20 (mix. of alumo-silicates)
  • Chamotte -Calcined fire-clay (main component is Mulite- 3Al2O3 x 2SiO2)

Some well-known bonding materials and additives:

  • organic resins -natural or synthetics
  • tar
  • sodium silicate
  • boric acid
  • phosphoric acid
  • cement, water, etc.

Necessary thermo physics and chemical properties of refractories:

  • density
  • strength
  • thermal-shock resistance
  • resistance to erosion
  • refractoriness
  • thermal conductivity.

Physical form of refractories:

  • plastic (mortar) - castable refractory
  • nonplastic mixes - powder refractory linings
  • monolithic linings - cast of a single piece for all or a big part of furnace; it is made of ramming or ganning mixes.

Applying methods:

  • manual tamping
  • ramming (mechanical applying)
  • gunning (mech. applying)
  • casting
  • masonry techniques.

Chemical nature:

  • acid - silica, fire-clay
  • basic - chrome-magnesite, magnesite, magnesite-chrome
  • neutral - alumina, fused alumina, chrome
Classification by:
  1. heat resistance-magnesite, alumina, silica
  2. thermal-shock and slag resistance- alumina, chrome, magnesite, olivine, silica

Common damages:

  • erosion
  • building - build up of a slag on the walls of furnace or anywhere else in the vessels
  • cracking - loss of a part of the lining

Some terms:

  • high thermal conductivity -possess materials through which heat passes easily
  • insulating refractories - poor heat conducted materials
  • sealer washes - fine grained refractories for filling cracks
  • patching and maintenance materials - for eroded areas
  • glazing - ceramical change during lining fritting and sintering
  • organic binders - clay, molasses etc. remain sticky at lower temperatures
  • chem. bonding agents - phosphoric acid-set up hard as they dry
  • sinteric acid-boric acid (1-3%) - hastens glazing

Developing tendencies:

  • Fall - Magnezite, Silica, Fire-Clay, Dolomite
  • Rise - Carborundrun, Almnina, Zirkonia, Graphite.

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