Additive alloy melting: Speeding up future alloy development
Miguel Cristóbal (IMDEA Materiales & CENIM, CSIC)
Members of IMDEA Materials Institute and of the National Center for Metallurgical Research (CENIM-CSIC) are working together on the development of a novel method to accelerate the design of metallic alloys named Additive Alloy Melting, or ADAM. This method allows to limit significantly the time required to fabricate new alloys that meet the increasing demands of the energy, transport or biomedical sectors, which is currently carried out by costly and time consuming trial and error methods. The mentioned time reduction is achieved by casting several alloys with different compositions in one single step. ADAM is being used in the development of new heat resistant steels based on the AISI304 alloy, the most common stainless steel.
ADAM is carried out in an arc furnace, where the energy of a powerful electric arc is used to melt the components of the alloy. This furnace is capable of melting all kinds of metals, from aluminum to tungsten, the metal with the highest melting point. Melting is performed under an argon atmosphere in order to protect the metal from oxidation. A special copper crucible like the one represented in the image below is employed. This image represents the process by which the alloys are produced. The crucible (orange) is closed by a moving piston (dark grey) that can be shifted up and down.
ADAM processing consists of the following steps:
- A) First of all the raw materials of the different alloys that are melted are weighed. They could be pure elements and/or alloys. Then the chamber is evacuated and argon is introduced. This evacuation-purging cycle is repeated several times.
- B) Subsequently, the first alloy (1) is melted and poured into the hole. The arc is kept for a few minutes to facilitate alloy homogenization. Then the resulting alloy is allowed to cool and solidify.
- C) The piston is lowered and the process is repeated with a second alloy (2).
- D) Thus, an ingot formed by different layers of different alloys is obtained. The process is then repeated up to 5 times. An example of the stacked alloys thus obtained is shown in the right part of previous image
A distinct feature of this method is that each alloy is produced independently from the rest. Thus, in a single operation, completely different alloys can be cast. Several studies have shown that high precision can be achieved with this method in terms of composition.
Alloying elements in cast materials are generally not distributed homogeneously at a microscopic level. This phenomenon is known as segregation. The following image, obtained by Electron Probe Micro Analysis, shows the distribution of different alloying elements in an area of 250×250 microns of the AISI304 steel produced by ADAM. It can be appreciated that they are distributed heterogeneously, from areas with higher content (yellow) to areas with lower content (blue).
ADAM is currently being used to develop new heat resistant steels based on AISI304 modified with copper and niobium with the aim of increasing their high temperature performance. The development of new heat resistant steels is crucial for the energy sector. Future challenges include increasing the amount of material produced, and also devising a way to heat treat the produced material in one step, thus further accelerating the different steps of the alloy design process.
This work has already been presented at the Advanced Materials and Processing Technologies (AMPT) conference in 2015 and at the National Materials Conference in Gijón in 2016. It has also been part of the Master thesis of one member of the DIMMAT project.