**7. Vacuum induction melting (VIM)**

The formation of bifilms in vacuum casting is practically universal, because ingots and castings poured in a vacuum furnace are nearly always top poured. In huge industrial VIM installations, the fall can be many meters, creating much damage to the metal. For instance, all the metal used by the aerospace industry for remelting for the casting of turbine blades is damaged during VIM preparation of the Ni-base alloys; the metal is top poured, falling many meters, down long vertical steel tubes; the larger the diameter of the tubes the worse the damage to the alloy by splashing and entrainment.

In probably all the leading R&D institutions in the world, metals and alloys for research are melted and poured in laboratory VIM furnaces, the top pouring, with the metal falling by a meter or more, fundamentally undermining or complicating nearly all metallurgical R&D worldwide (**Figure 6**). It has greatly contributed to the lack of understanding of more complex failure forms of metals such as fatigue, stress corrosion cracking and hydrogen embrittlement among others as a result of all researchers being unaware that their research materials were densely pre-cracked [2].

It is with great regret therefore that we have to conclude that the preparation of most metals and alloys by vacuum casting is a snare and delusion. It would be easily possible to make castings in air of far greater perfection by simply avoiding surface turbulence during the casting process. This is most effectively achieved by

**11**

**Figure 7.**

*VAR and ESR secondary remelting processes.*

*Perspective Chapter: A Personal Overview of Casting Processes*

abandoning gravity pouring and adopting counter-gravity filling of the mold. The world needs to convert its casting operations to counter-gravity casting. The suffering of the casting world from the ubiquity of casting defects will then be of interest only

The secondary remelting processes for steels and Ni alloys are designed to deliver a premium quality of metal in the form of an ingot. Their starting material is a reasonably good metal in the form of a consumable electrode which is slowly and progressively remelted by arc, plasma, electron beam, or joule heating in a liquid slag layer etc. As the tip of the electrode melts, a new ingot is then slowly built up drop by drop within its protective environment of vacuum or slag. The ingot solidifies tolerably rapidly because of the use of a water-cooled mold. At the time of writing, it requires to be noted, with regret, that none of the secondary remelting processes are totally reliable. All can have serious crack defects which can survive the subsequent forging or rolling, and the heat treatment, making these products unreliable in service. Some, as we shall see, can be seriously

VAR is probably the most widely used of all the secondary remelting processes (**Figure 7**). The marketing of VAR benefits from its name: engineers are attracted to

However, the VAR process is particularly susceptible to its slightly oxidizing vacuum conditions, growing an oxide skin on the horizontal ledges formed by the slow layer-by-layer advance of the solidifying liquid. This variety of advance occurs because of the strength of the oxide on the advancing meniscus as it rolls over the solidified or solidifying metal around the edge of the ingot. The vertical advance occurs by the horizontal flow of the liquid front, gradually spiraling upwards, advancing vertically by the 8 mm high steps corresponding to the height of the meniscus. This is the height which surface tension can support against the hydrostatic pressure due to this depth [1]. As the meniscus rolls over the oxide film on the

*DOI: http://dx.doi.org/10.5772/intechopen.93739*

for historians.

unreliable.

**8. Secondary remelting**

**9. Vacuum arc remelting (VAR)**

the concept of 'vacuum' suggesting cleanness.

abandoning gravity pouring and adopting counter-gravity filling of the mold. The world needs to convert its casting operations to counter-gravity casting. The suffering of the casting world from the ubiquity of casting defects will then be of interest only for historians.
