Powder metallurgy of aluminum

Powder metallurgy of metals

Powder metallurgy is a versatile production technology, which, is an:

• one side, a cheaper and more sustainable alternative to traditional metal processing methods;
• on the other hand, technology, which is capable of creating materials with chemical composition and microstructure, not achievable with the use of other technologies.

Typical Technology

Typical traditional powder metallurgy technology includes:

• powder production;
• pressing it into a compact workpiece for subsequent sintering;
• sintering of powder particles of this "compact" at high temperature.

In addition to these process steps, other powder compaction steps or some conventional metal processing techniques after a sintering operation may be included., for example, forging, rental of, extrusion [1-3].

Three reasons

Powder metallurgy in a broad sense is the skill and science of the production of metal powders and the use of these powders for the manufacture of semi-finished materials and finished products with complex shapes.. There are three reasons for the decision to use powder metallurgy as a production process:

• produce economical, precise and complex details;
• produce materials with unique microstructure and properties and
• produce materials, which are difficult or impossible to produce in other ways.

For aluminum alloys, powder metallurgy can be applied for one or more of these three reasons., especially on the second and third. Traditional aluminum powder alloys are processed using the "press-sintering" technology. They are mainly used to improve the economic performance of the production of complex parts.. Using this technology, most of the products of powder metallurgy are produced (Figure 1).

The figure 2 presented main technological chains of industrial production of parts and products from powder metal materials.

Figure 1 - Aluminum products,
made by powder metallurgy methods [1]

Figure 2 - Traditional and additive technologies of powder metallurgy [2]

Additive technologies

New additive technologies, also called "3-D printing", also designed to reduce production costs in complex parts. These technologies are playing an ever-increasing industrial role. As it shown on the picture 1, these alloys, skipping the stage of pressing and sintering the powder, immediately form the finished part. Powder metallurgy also employs "fast solidification" technology and composite aluminum alloys, many of which require additional technological steps to obtain finished parts and products from them.

Metallurgy of powdered aluminum

Production of products from aluminum powders

The production of powder aluminum alloys is carried out mainly by two methods.. For chemical compositions with alloying elements, which have high solubility in the liquid state, fast solidification technology is applied. For alloying elements with low liquid solubility, solid-state mixing is usually used..

Main stages of production

Products from aluminum powder materials can be manufactured using various technologies, but the general stages of their production are as follows:

• powder production
• mixing powders with added lubricants and fillers
• cold or hot seal, usually followed by degassing
• sintering or hot pressing
• various additional treatments

An example of one of the production processes of aluminum powder metallurgy from the manufacture of powder to the finished product is shown in the figure 3.

Figure 3 - An example of a typical production technology
aluminum powder materials and products from them [3]

Applications for powder aluminum products

Most powder aluminum alloys can be classified into three main areas of application for products and parts:

• reduced density;
• high strength;
• high operating temperature.

Low density

Density reduction is achieved by adding lithium. It is known, that some industrial aluminum-lithium alloys (for example, 2090 и 8090) is produced by the classical method with smelting ingots. However, using powder metallurgy methods they achieve:

• supercooling beyond the equilibrium solubility limit of lithium in aluminum
• increasing strength by dispersed hardening with very small particles of the secondary phase.

These powdered aluminum-lithium alloys are largely capable of replacing classical alloys. 2024 и 7075.

High strength

High-strength alloys produce:

• by the method of ultrafast solidification of alloys of the 7xxx series with additional alloying with transition elements, such as cobalt, zirconium or nickel

• mechanical alloying with aluminum carbides or aluminum oxides of pure aluminum or aluminum-magnesium alloys.

These alloys are intended to replace alloy 7075-T6.

High temperature strength

Alloys for operation at elevated temperatures are obtained by ultra-fast solidification of aluminum alloys with additions of rare earth or transition metals by mechanical alloying.

Economics of Powdered Aluminum

Despite interesting technical capabilities, actual economic interest in powdered aluminum in the aluminum industry is very insignificant. The main barrier for the widespread use of aluminum powder products is their high cost - roughly 1,5-2 times more, than products from classic aluminum ingots. This is due to the need to use special equipment and safety measures when handling metal powders.

The small size of the workpieces is another obstacle., which can be processed into products by metal forming methods, and limited availability. A certain obstacle to the use of aluminum powder components in critical load-bearing structures due to the lack of reliable non-destructive testing methods for detecting small pores or other defects. The industrial application of products made of powdered aluminum is currently mainly limited [3]:

• high-strength alloys for military aircraft parts
• wear-resistant alloys for automotive engines
• alloys with a high modulus of elasticity to increase the rigidity of parts and products
• high temperature engine alloys, as well as aircraft fuselages.

Modern technologies for the production of aluminum powders

Powdered aluminum alloys

Most industrial powder aluminum alloys are based on wrought aluminum alloys of the 2xxx series, 4xxx, 6xxx or 7xxx, but always contain magnesium alloying additives, what is necessary to reduce the formation of aluminum oxide on powder particles during sintering [2].

Basic methods

Powdered aluminum materials can be produced using several manufacturing methods.. The main such industrial methods, that is, such, which produce powders, which are available on the market, are:

• atomization;
• "Spinning" the melt into ribbons followed by spraying the ribbons into flakes;
• mechanical alloying.

Atomization

Atomization involves the formation of powder from a stream of molten metal, which breaks into drops. Can be formed as pure aluminum powders, and from pre-alloyed powders. In fact, this method produces most of all powders.. An important aspect of this method is the rapid solidification of the powder from the melt.. To implement the method, the following methods are used:

• Water atomization
• Gas atomization
• Vacuum atomization
• Air atomization
• Centrifugal atomization

Water and gas atomization methods are the main.

Fast curing to tape and spraying

Another method of producing aluminum powder is to obtain a rapidly solidified tape and then grind this tape into flakes., which can be pressed and extruded.

Mechanical alloying

An interesting method for the production of oxide dispersion-strengthened material for use in parts at high temperatures is mechanical alloying. In this method, the powder is welded repeatedly, breaks down and weld again.

Gas atomization

The figure 4 shows a schematic diagram of a vertical atomizer using an inert gas. Superheated melt is prepared in a vacuum induction furnace and poured into one or more nozzles. Rapidly expanding gas breaks up the liquid jet, which first turns into a thin sheet and then forms bundles, ellipsoids and spheres. This powder is collected, and the air passes through the cyclone, where it is cleaned of fine powder, and goes to reuse. This gas atomization process has a large number of variables., such as:

• alloy composition
• metal feed rate
• melt temperature
• melt viscosity
• gas pressure and temperature
• type of gas
• nozzle geometry.

Figure 4 - Atomization of powder materials with inert gas [3]

The main advantages of gas atomization are:

• homogeneity of powder material,
• no pollution due to inert gas atmosphere and
• spherical shape of powder particles.

Water atomization

Water atomization is similar to gas atomization, but a stream of molten metal is broken by jets of water (figure 5). Cooling rate is higher, than with gas atomization and, Consequently, chemical segregation in powder is less. The shape of the powder particles is more irregular, the surface of the particles is coarser and more oxidized. Increased water velocity leads to a decrease in the average particle size.

Figure 5 - Water atomization of powder materials [3]

Mechanical alloying

In mechanical alloying, a mixture of balls and pure aluminum powder is used to create a microalloyed composite powder on an attritor or in another high-energy ball mill (Figure 6).

Figure 6 - Mechanical alloying of aluminum powders [3]

Multiple grinding, cold welding, breaking and re-welding the metal powder results in a homogeneous powder material with a uniform distribution of inclusions. A good balance between grind and weld is achieved by the correct selection of organic fluids, which significantly affect technology. Mechanically alloyed powders are very difficult to compress, since their cold plastic deformation requires great efforts. This method is not particularly productive., but remains attractive, this is how unique alloys can be produced with it, which are mainly used for products and parts, working at high temperatures.

Advantages of Fast Curing

An important characteristic of atomization is the rapid solidification of the powder from the melt (10² – 10⁴ K / s for gas atomization, 10⁴ – 10⁶ K / s for water atomization). One of the clear advantages of an increased solidification rate is the refinement of microstructural components.. The relationship between the cooling rate and the distance between dendrid branches is approximately linear (Fig. 7). This structural comminution has an important effect on the properties of powder materials such as, increased strength, higher fatigue strength and increased corrosion resistance. Another effect of an increased cooling rate is the expansion of the limits of dissolution of alloying elements in a solid solution above equilibrium values ​​(figure 8). This makes it possible to obtain metastable alloys with a chemical composition, which cannot be achieved with the classical metallurgical ingot casting method.

Figure 7 - Reducing the distance between the branches of dendrites
with fast hardening [3]

Figure 8 - Increasing the solubility of alloying elements
with fast hardening [3]

Porosity problem

The structure and porosity of powder aluminum alloys depends on the chemical composition of the alloy and the characteristics of the powder., as well as technological conditions during pressing, sintering, additive processes and secondary pressure forming operations.

Almost all properties of powder aluminum alloys depend on their residual porosity. Residual porosity can reduce quality values ​​to levels below, than traditional wrought or cast alloys. It is not only the reduction or elimination of residual porosity that is important, but also full understanding and control of its presence in parts and products.

Secondary technological operations, such as forging, rolling and extrusion, able to increase density and reduce residual surface porosity. As a result, it is possible to improve many properties of powder materials.. With careful control of the technological parameters of powder metallurgy, the properties of aluminum powder materials can exceed the properties of traditional wrought aluminum alloys [2].

Sources:

1. Powder Metallurgy / Joseph W. Newkirk // Handbook of Aluminum Volume – Physical Metallurgy and Processes, ed. G. E. Totten and D. S. MacKenzie – 2003
2. Powder Metallurgy Aluminum Alloys: Structure and Porosity / W. Judge and G. Kipouros // Encyclopedia of Aluminum and Its Alloys, ed. G.E. Totten, M. Tiryakioğlu, O. Kessler – 2019
3. Aluminium Powder Metallurgy / TALAT Lecture 1401, 1994