Powder Metallurgy

Metal powder technology is one of the most established production methods nowadays in all kinds of industries.

The alloy types which can be manufactured by powder metallurgy cover a broad spectrum, ranging from soldering and brazing alloys for the electronics industry, nickel, cobalt and iron-base superalloys for the aircraft industry, hydrogen storage and magnetic alloys, up to reactive alloys such as titanium for the sputter target production.

The process steps involved in the production of metal powders are melting, atomizing and solidifying of the respective metals and alloys. Metal powder production methods such as oxide reduction and water atomization, are limited with respect to special powder quality criteria, such as particle geometry, particle morphology and chemical purity.

Inert gas atomization, combined with melting under vacuum, therefore is the leading powder-making process for the production of high-grade metal powders which have to meet specific quality criteria such as:
• Spherical shape;
• High cleanliness;
• Rapid solidification;
• Homogeneous microstructure.

ALD has the capability to combine various melting technologies with inert gas atomization which enables the production of superalloys, superclean materials and additionally reactive metals.

Metal Powder Applications:

• Ni-base superalloys for the aviation industry and power engineering;
• Solders and brazing metals;
• Wear-protection coatings;
• MIM powders for components;
• Sputter target production for electronics.
• MCRALY oxidation protection coatings.

2. Production-scale Inert Gas Atomization System. The system has a vacuum induction of 35 l effective volume.
It is equipped with a high-vacuum pumpset and a cyclone particle separator

Vacuum Induction Melting and Inert Gas Atomization

The standard design of a vacuum inert gas atomization (VIGA) system comprises a Vacuum Induction Melting (VIM) furnace where the alloys are melted, refined and degassed. The refined melt is poured through a preheated tundish system into a gas nozzle where the melt stream is disintegrated by the kinetic energy of a high pressure inert gas stream. The metal powder produced solidifies in flight in the atomization tower located directly underneath the atomization nozzle. The powder gas mixture is transported via a conveying tube to the cyclone where the coarse and the fine powder fractions are separated from the atomization gas. The metal powder is collected in sealed containers which are located directly below the cyclones.

ALD has developed atomization systems where a variety of melting processes can be combined with inert gas atomization. The atomization systems built by ALD have a modular design and are applicable from laboratory scale (1– 8 l crucible volume), through pilot production (10 – 50 l crucible volume) up to large-scale atomization systems (with 300 l crucible volume).

Basic layout of different melting alternatives in metal powder production

Large Scale VIGA Atomization Unit

The photo on this page shows a large scale inert gas atomization system. The melting crucible of this production atomization system has a maximum capacity of 2 tons. The atomization tower is connected to a melt chamber with a double-crucible door arrangement. Each furnace door is equipped with a vacuum induction melting furnace. This design allows very fast crucible changing. While one crucible is in production the second crucible can be cleaned or relined in stand-by position. This minimizes the down time for furnace change operations. Additionally, the double-door design enhances the production flexibility, because different furnace sizes can be used in the same equipment. The melting chamber is equipped with a bulk charger, two temperature measuring devices and a redundant tundish system.

Each pouring tundish, including the gas nozzle arrangement, is mounted on a tundish cart. The tundish cart can be moved sideways to a location for loading and unloading without venting the system and without breaking the ambient atmosphere. The redundant tundish configuration allows a high flexibility in case clogging of the outlet nozzle occurs. In that situation, the second preheated tundish nozzle system which is in stand-by position can be moved into the atomization position to continue the process.

1. Schematic design of a large scale atomization unit with a double-door melting furnace chamber,
2. Double-door crucible VIGA atomization unit. Each vacuum induction furnace has a rated batch capacity of 2,000 kg. A gas recycling system recovers the inert gas for reuse.

Ceramic-Free Metal Powder Production

The “standard” design of a vacuum induction melting inert gas atomization system is equipped with a ceramic melting crucible and also ceramic material for the tundish and the melt outlet nozzle arrangement. Due to the contact between the melt and the ceramic lining and nozzle material, ceramic inclusions in the melt can occur, which influence the material properties of high-strength PM-components in a negative manner. Reactive metal powders, such as titanium based alloys, cannot be produced with this method at all, due to the reactions between the reactive melt and the ceramic lining. In order to overcome the “ceramic problem” it is necessary to use melting techniques where the melt is not in contact with ceramic lining material. Additionally, a refining of the melt during the melting process would be desirable. Typical materials that need ceramic-free production processes are refractory and reactive materials, such as Ti, TiAl, FeGd, FeTb, Zr and Cr.

EIGA

In the EIGA (electrode induction melting gas atomization) process, prealloyed rods in form of an electrode are inductively melted and atomized without any melting crucible at all. The melting of the electrode is accomplished by lowering the slowly rotating metal electrode into an annular induction coil. The melt stream from the electrode falls into the gas atomization nozzle system and is atomized with inert gas. The EIGA process was originally developed for reactive alloys such as titanium or high-melting alloys. It can also be applied to many other materials.

1. EIGA Furnace, 2. Schematic view of the EIGA system, Schematic view of the PIGA system

PIGA

For the production of ceramic-free powders and for the atomization of reactive, and/or high-melting alloys, melting can also be accomplished by means of a plasma jet in a water-cooled copper crucible. PIGA stands for plasma-melting induction-guiding gas atomization. The bottom of the PIGA crucible shown above is connected with an inductively heated discharge nozzle, also made of a copper base material. This ceramic-free discharge nozzle system guides the liquid metal stream into the gas atomization nozzle, where it is disintegrated by the inert gas.

VIGA-CC

Reactive alloys like titanium or intermetallic TiAl alloys can also be melted in a copperbased cold wall induction crucible which is equipped with a bottom pouring system. The bottom pouring opening of the cold crucible is attached to a CIG system. CIG stands for cold-wall induction guiding system and is exclusively patented by ALD. VIGA-CC stands for vacuum induction melting gas atomization based on coldwall crucible melting technology.

ESR-CIG

High performance superalloys for the aircraft industry are typically produced via the so-called “triple melt process”. In the triple melt process the refining of the material is carried out by the reactive slag in the ESR melting step. The combination of the ESR remelting technique with a ceramic-free melt guiding system (CIG) represents a process technology to produce powder material with a high level of cleanliness and chemical homogeneity. In the ESR-CIG (electroslag remelting cold-wall induction guiding) process, the material to be atomized is fed in form of an electrode. The electrode is lowered into the metallurgical refining slag. As the electrode tip is melted at its point of contact with the slag, droplets of the refined metal are formed and these droplets pass down through the reactive slag layer.

The refined metal droplets which pass through the reactive slag form a liquid melt pool underneath the slag layer. The melt pool is enclosed by a water-cooled crucible made of copper. The refined liquid metal is guided through the coldwall induction guiding system and is disintegrated by a high kinetic inert gas stream in a free-fall-type gas nozzle.

1. Schematic view of the VIGA-CC system, Schematic view of the ESR-CIG system

Sprayforming Technology

Beside the conventional powder-processing route, sprayforming became more and more important during the last decade. This unique process enables the direct fabrication of semi-finished products. A number of process steps related to compaction can be eliminated, the pick up of oxygen is minimized and the risk of contaminatin is dramatically reduced compared to the powder-HIP (Hot Isothermal Pressing) route.

The principles of sprayforming technology are to atomize the molten metal into droplets and to solidify them rapidly onto a collector. By moving this collector the build up of semi-finishes product is established. Due to the high cooling rates, which occur during atomization, a fine micro-structure with no macro-segregation is achieved. Depending on the design of the atomizer, the movement of the spray nozzle(s) and the collector design various shapes, such
as billets, rings, tubes and bars can be produced.

The produced semi-finished products are subjected to secondary processing steps, such as heat treatment, rolling, forging, extrusion or HIP. The process is used extensively to manufacture billets for a wide range of appications in aluminium alloys, copper alloys, special steels and superalloys.

Inert Gas Recycling

At a certain batch size of the atomization system, recycling of the inert gas is recommended, to reduce the total inert gas consumption and thus achieve a more economical production process. ALD offers two different process technologies to recycle the inert gas.

Spryforming process using an “OSPREY” twin-atomizer

Inert Gas Recycling Based on Compressor Technology

One method of reusing the inert gas is to “drive” the gas in a closed gas circulation loop, using a suitable compressor system. Behind the cyclone and the filter system, the “dust-free” gas is repressurized using a 2-stage compressor unit. The compressors have to be gastight to prevent contamination of the recirculated inert gas. After each compressor, a gas buffer tank is used to minimize pressure fluctuations during the atomization process. This results in stable atomizing process conditions with respect to atomization pressure and gas-flow rate. In case the permissible impurity levels in the atomization gas are set very low, the oxygen, hydrogen and nitrogen contents can be monitored at several locations in the gas circulation loop.

For large-scale atomizing systems this type of gas recycling is economically operated in a pressure range up to 50 bar.

Powder cooling loop in the gas recycling system of a large scale atomization system including powder transportation and separation.

Argon Recycling Based on Liquefaction

If a higher gas supply pressure is required, the recycling concept described above has to be changed to the principle of reliquefying the argon by using evaporating liquid nitrogen as the refrigerant. In this situation, the 2-stage compressors with the pulsation buffer are replaced by a concurrent flow argon liquifier and a set of high-pressure liquid argon pumps.

The high-pressure liquid argon pumps feed the liquid argon through an evaporator into high-pressure gas receivers. Based on this technology a gas supply pressure of approx. 100–200 bar can be achieved.

Operational experience with large scale atomization systems equipped with the recycling systems described above, shows that the yield of the recycled gas for both recycling systems is in the range of 90–95 %.

1. Argon recycling system based on liquefaction, 2. Inert gas recycling system with a 2-stage compressor system, Schematic layout of the argon liquefying system with high-pressure liquid argon pumps and high pressure gas receivers

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