Nickel-Based High-Temperature Alloy Powder Preparation Technology

High-temperature alloy, also known as heat-resistant alloy or superalloy, refers to an alloy based on iron, nickel and cobalt, which can resist oxidation, corrosion and creep at high temperatures above 600°C, and can work for a long time under high mechanical stress. In particular, nickel-based high-temperature alloys can have better strength, oxidation resistance and corrosion resistance at high temperatures of 650~1000°C. Therefore, high-temperature alloys are currently the cutting-edge industrial materials standing at the top of the pyramid. They are not only the key materials for the hot end components of aviation engines and various high-temperature components of aerospace rocket engines, but also widely used in industrial gas turbines, energy, chemical industry and other industrial sectors.

1. Powder high-temperature alloys are the preferred materials for key hot end components of advanced aviation engines. They are mainly used to manufacture high-temperature load-bearing components of the hot end of engines such as turbine disks, compressor disks, drum shafts and turbine disk high-pressure baffles. Powder high-temperature alloys use metal powder as raw materials, and after subsequent heat processing, they are obtained. The alloy has high tensile strength and good fatigue resistance. Powdered superalloys have gone through three generations of development and have been widely used in a variety of key parts such as turbine disks of advanced military and civil aviation engines. The current international research and development of powdered superalloys has entered the fourth generation. Powdered superalloy ingots have no macroscopic segregation, uniform microstructure, fine grains, excellent mechanical properties and hot processing properties, which can effectively ensure the reliability and durability of the engine, and can be nearly net-shaped, with a short manufacturing cycle and low production cost.

1.1 Powdered superalloy production process The overall idea of ​​the powdered superalloy production process is to prepare and process the powder under the protection of an inert atmosphere, and then use a hot forming process to consolidate and densify the powder. After years of development, two process routes have basically been formed, namely, the "argon atomization method (AA) powder making + hot extrusion (HEX) + isothermal forging (ITF)" process represented by Western countries such as Europe and the United States, and the "rotating electrode method (PREP) powder making + hot isostatic pressing (HIP) direct forming" process represented by Russia.

The process route developed by my country is "plasma rotating electrode method (PREP) powder making + hot isostatic pressing (IP) forming + jacket forging/heat treatment". The process has been successfully used to produce large-sized FGH4095 powder turbine discs. The powder high-temperature alloy turbine baffles and small-sized turbine discs prepared by direct HIP forming process have passed the test run and have formed mass production capabilities. The research work on the second-generation powder high-temperature alloy FGH4096 dual-performance turbine disc has also made breakthrough progress. The "PREP+direct HIP+isothermal forging" process has broken through key technologies such as powder purity and developed a FGH4096 alloy turbine disc for a high thrust-to-weight ratio engine. Through the use of fine-grain forging and gradient heat treatment processes, a dual-microstructure disc blank with a rim grain size of 5 to 6 and a hub grain size of 10 to 11 was obtained. In order to solve the problems of excessive inclusion size and unqualified ultrasonic testing in powder high-temperature alloy turbine disks, my country has carried out research on the "extrusion + isothermal forging" process, and has made important progress. Recently, my country successfully extruded powder high-temperature alloy billets for aircraft engine turbine disks on the 36,000-ton ferrous metal extruder of the Northern Heavy Industry Group, marking a major breakthrough in my country's powder high-temperature alloy technology. Studies have shown that the original particle boundary (PPB) is eliminated during the extrusion process, and the inclusions are effectively crushed along the extrusion direction. In the subsequent forging process, the inclusions are further crushed and dispersed in the plane perpendicular to the forging direction. my country has also begun exploratory research on the extrusion deformation process of powder high-temperature alloys, especially in the finite element simulation technology. By simulating the cladding extrusion process, the influence of factors such as die structure parameters on the extrusion process can be systematically analyzed, thereby determining the optimal combination of die structure [9]. The "extrusion billet + isothermal forging" process has become one of the important development directions of my country's powder high-temperature alloy turbine disks.

1.2 Defects in Powder Superalloys and Their Effects
Inclusions, PPB and thermally induced holes (TIP) are the three major defects of powder superalloys, which seriously affect the performance of powder superalloy parts. Domestic and foreign researchers have conducted extensive research and analysis on the sources of defects and their effects on alloy properties, and have proposed corresponding measures and methods to reduce and eliminate defects.

Inclusions have an important influence on various mechanical properties of powder superalloys, especially low-cycle fatigue properties. Inclusions in powder superalloys include oxides, organic matter, foreign metals and other particles. Among them, ceramics, slag and other oxide inclusions mainly come from the refractory materials of the crucible, ladle and nozzle of the master alloy smelting and powder making device, as well as the deoxidation products in the smelting process and solid impurities in argon; rubber, fiber and other organic matter come from the pollution of the vacuum system of the powder storage tank, valve, powder preparation and processing equipment; foreign metals come from the previous batch of atomized alloys or sheath materials.

PPB is a network of carbon oxides precipitated at the particle boundaries due to the chemical reaction between O and C elements adsorbed on the surface of powder particles and the powder constituent elements during the hot isostatic pressing process [10]. This weak interface hinders the diffusion and connection between metal particles, becomes the fracture source and crack propagation channel of the alloy, and reduces the plasticity and fatigue life of the alloy. The root cause of PPB is the O and C content on the powder surface. Reducing the C content, adding strong carbide-forming elements such as Nb and Hf, and vacuum degassing of powder are the main methods to eliminate PPB. It can also be eliminated through heat treatment, optimization of hot isostatic pressing process, etc.

TIP is a discontinuous hole formed by the expansion of argon gas that is insoluble in the alloy during hot isostatic pressing. It will cause the workpiece to warp and significantly reduce the tensile, durability, creep resistance and other properties of the alloy. The residual argon gas in the powder high-temperature alloy mainly comes from the hollow powder formed by the argon gas encapsulated by the droplets during the atomization powder making process. In addition, the argon gas adsorbed on the powder surface is not completely removed or the sealing of the package is not tight, which can also cause TIP.

In summary, the three major defects of powder high-temperature alloys are directly related to the quality of alloy powders. The preparation technology of high-quality high-temperature alloy powders with no inclusions, no hollow powder and low gas content is the key to the development and application of powder high-temperature alloys.

1. Common production processes of high-temperature metal powders

There are many methods for producing metal powders, including solid crushing, ball milling, atomization, electrolysis and chemical methods. For additive manufacturing, spherical powders are needed, and atomization is considered to be the most ideal method for producing metal powders for additive manufacturing. In addition, the rotating electrode method is gradually used in the preparation of additive manufacturing powder materials.

The basic process flow of nickel-based high-temperature alloy spherical powder preparation is as follows: master alloy smelting and processing → powder making → powder screening → (electrostatic separation and impurity removal) → powder performance inspection, among which master alloy smelting, atomization powder making and electrostatic separation and impurity removal are the key links to obtain high-quality powders.
2.1 Master alloy melting technology
The high-temperature alloy master alloy melting technology plays a decisive role in the preparation of high-quality spherical powder. Impurities, oxygen, nitrogen and hydrogen content in the raw materials directly affect the quality of the powder. The melting of high-temperature alloy master alloys usually adopts vacuum induction melting technology, which has significant advantages in accurately controlling the alloy composition and removing gas impurities and harmful elements in the alloy. However, due to the use of ceramic crucibles, ceramic and slag inclusion defects will inevitably be introduced into the master alloy. In addition, the master alloy will also produce defects such as shrinkage, looseness, and segregation during the solidification process.

The above defects can be eliminated in the subsequent induction melting and pouring process of casting high-temperature alloys into part blanks, but they will have a greater impact on the preparation of high-temperature alloy powders. The inclusions in the master alloy cannot be removed during the powder making process, and defects such as shrinkage and looseness will also cause oxidation of hollow powder and powder surface. Therefore, it is necessary to take corresponding measures to eliminate the inclusions and defects in the vacuum induction melting master alloy. In the high-purity casting and forging high-temperature alloy production industry, the two-way or three-way process is generally used, namely vacuum induction melting + electroslag remelting/vacuum arc remelting. Vacuum induction melting prepares consumable electrodes with suitable chemical composition. The electroslag remelting process removes brittle oxide inclusions to obtain higher purity. Finally, vacuum arc remelting is used to obtain blanks with no macrosegregation and uniform structure. The electroslag remelting process is one of the most effective refining processes for removing non-metallic inclusions in alloys. It can effectively remove large-sized foreign inclusions and disperse and refine the endogenous inclusions. It is the preferred process for secondary refining of powder high-temperature alloy master alloys.

Russia began to widely use vacuum induction melting + electroslag remelting process to produce powder high-temperature alloy master alloy ingots in the 1990s. In 2002, the Central Iron and Steel Research Institute designed and built the world's first ultra-small section (50 mm) vacuum/inert gas protection fast electroslag furnace, and successfully prepared high-temperature alloy electroslag ingots in 2004. Studies have shown that the FGH95 master alloy prepared by the inert gas protection electroslag remelting process has a lower O content, a lower Al and Ti burnout, and a significantly reduced size and number of non-metallic inclusions.

Foam ceramic filtration technology is also an effective method for removing non-metallic inclusions in high-temperature alloys. This technology mainly uses a three-dimensional continuous mesh foam ceramic plate filter composed of a dense ceramic branch skeleton to filter the metal melt, which can filter out inclusion particles, liquid flux inclusions and some harmful metal elements in the alloy melt. Beijing University of Science and Technology, Yingkou Magnesium Materials Research Institute, Northeastern University, Institute of Metals of the Chinese Academy of Sciences and other scientific research units have used alumina-based ceramic filters to filter and purify high-temperature alloys, and can effectively remove inclusions in high-temperature alloys.

In addition, the rotary ingot casting process developed in Japan, the high-quality high-temperature alloy ingot production and manufacturing technology (BIAM high-quality process) developed by Beijing Institute of Aeronautical Materials, the electromagnetic soft contact forming purification technology and composite molten salt purification technology developed by Northwestern Polytechnical University all have good effects on removing inclusions and purifying alloys. In recent years, new purification technologies that have developed rapidly, such as electromagnetic purification technology and ultrasonic processing technology, are becoming new research hotspots in the field of alloy purification.

Some of the above alloy purification technologies have been widely used in high-temperature alloy purification, while others need to be further improved and developed. The development of aerospace engines requires powder high-temperature alloys to have higher mechanical properties and reliability, and also puts forward higher requirements on the purity of master alloys. For the production of powder high-temperature alloy master alloys, research and development of high-efficiency, low-cost, high-purity, and inclusion-free master alloy ingot smelting and purification technology is an important direction for future development.

2.2 Powder making

Powder preparation is the first process in the production of powder high-temperature alloys, and it is also one of the most critical processes. The preparation of high-temperature alloy powders with small and uniform size, good sphericity, and low gas and inclusion content can reduce or even eliminate defects such as PPB and inclusions in the alloy, and significantly improve the organization and performance of alloy discs. Therefore, advanced powder preparation technology is a key link to obtain high-quality high-temperature alloy powders, thereby eliminating metallurgical defects inside the alloy and ensuring the quality of high-temperature alloy discs.
Researchers in the field of powder high-temperature alloys have conducted extensive and in-depth research on a variety of powder preparation processes. At present, the main methods for preparing spherical metal powders are vacuum induction melting gas atomization (VIGA method), plasma rotating electrode powder making technology (PREP method), electrode induction gas atomization (EIGA method) and plasma atomization (PA method).

2.2.1 Vacuum induction melting gas atomization (VIGA method)
Vacuum induction melting gas atomization refers to the use of coil induction heating principle to melt the charge in a vacuum environment. After reaching a certain temperature, the melting chamber and the atomization chamber are filled with atomizing gas, and then the molten steel is poured into the tundish. The metal liquid flows into the atomization chamber through the nozzle, and then the high-pressure inert gas is used as the atomization medium to break up the continuous metal liquid flow, so that it quickly solidifies into fine particles, that is, metal powder.

In addition to nickel-based high-temperature alloy powders, other metal powder materials include: stainless steel 316L, 174PH; cobalt-based alloys CoCrMo, CoCrW, CoCrMoW; titanium and titanium alloys TC4, TC11TA15, TiAl4822, Ti2AlNb; nickel-based alloys FGH95, FGH96, FGH97, GH4169, rare metals, etc. 2.2.2 Plasma Rotating Electrode Powdering Technology (PREP Method) Plasma Rotating Electrode Powdering Technology is used to prepare metal powders such as highly reactive metals, heat-resistant nickel and titanium alloys. This technology adopts the principle of centrifugal atomization. In an inert gas environment, the plasma generator and the electrode generate an arc. The temperature of the arc can quickly melt the rapidly rotating rod. The molten metal is centrifugally atomized under the action of surface tension. The small droplets of centrifugally atomized liquid quickly solidify into particles in the sputtering chamber, and finally the particles are deposited and fall into the powder collecting tank.

The powder produced by plasma rotating electrode powder making technology has the advantages of good sphericity, concentrated particle size, high surface finish, low gas content and high purity.

At present, Russia's plasma rotating electrode powder making equipment is the world leader. Figure 5 is the Russian Granule 2000 plasma rotating electrode powder making equipment. Its static vacuum system, dynamic vacuum system, plasma generator, rod rotation system, cabin design, feeding design, gas system, etc. are superior to the domestic PREP equipment design. The speed of Granule 2000 plasma rotating electrode powder making equipment can reach 20000~25000 r/min.

2.2.3 Plasma atomization method (PA method)
Plasma atomization method (PA method) is a method for preparing spherical powder by atomizing metal droplets with a plasma gun. This method was first proposed by M.EntezaRian and others and applied for a patent in 1998. Now Canada's AP&C company is the global leader in plasma atomization technology. The company owns the complete set of technical patents for this equipment. Plasma atomization is essentially a gas atomization powder making technology. Its principle is: under the protection of inert gas, a plasma gun is used to heat the alloy wire, melt and evaporate it into metal vapor, and then the saturated metal vapor is quickly agglomerated, nucleated and grown through gas quenching cooling technology to obtain ultrafine alloy powder[26]. The particle size distribution of alloy powder prepared by plasma atomization is relatively narrow, ranging from 10 to 150 μm, and powder below 50 μm accounts for about 40%, and the fine powder yield is extremely high; in addition, the powder prepared by the PA method also has a high sphericity and low impurity content. The main disadvantage of the PA method is that the raw material is a finer wire, and the manufacturing cost of the wire raw material is higher than that of the master alloy rod, and the powder making efficiency is low.

Currently, the plasma atomization equipment of AP&C Company in Canada is equipped with a fully automatic monitoring system and a gas recovery device to ensure the stability of powder quality and reduce production costs through gas recovery. This technology has been applied to the large-scale production of high-quality spherical metal powders, and the types of powders include pure titanium and titanium alloys, nickel-based alloys, etc.

2.2.4 Electrode Induction Gas Atomization (EIGA) Electrode induction melting gas atomization (EIGA) is an ultra-clean gas atomization powder making technology that does not use ceramic crucibles. It has the characteristics of high gas atomization production efficiency, large output, and fine powder particle size. The principle of EIGA atomization powder making is as follows: under the protection of inert gas, the master alloy rod is installed on the feeding device and enters the conical coil below at a certain rotation speed and descending speed [12]. The tip of the rod is gradually melted by the conical ultra-high frequency induction coil to form an alloy liquid flow with a continuously controllable diameter. Under the action of gravity, the molten liquid flow directly flows into or drips into the atomization chamber below. Under the action of high-pressure inert gas, the alloy liquid flow is broken into small droplets. Under the action of its own surface tension in the atomization chamber, the small droplets are rapidly spheroidized and solidified to form metal powder. In the EIGA powder making technology, the entire melting process of the master alloy does not use refractory materials such as crucibles and guide nozzles, which reduces the introduction of non-metallic impurities; compared with the powder produced by the VIGA method, the EIGA powder has a smaller particle size, does not contain a large number of flakes, and the powder particle size Dv(50) can be controlled at about 50~100 μm, with high production efficiency.

By optimizing the electrode induction gas atomization technology process, the German ALD company has designed and developed a variety of electrode induction gas atomization powder making furnace equipment for the research and production of titanium and titanium alloy powder materials. This type of equipment has been promoted worldwide. The median particle size Dv(50) of Ti6Al4V powder prepared by Japan's OSAKA Titanium Company using the electrode induction gas atomization method is about 40 μm, the powder has high sphericity, few satellite particles, and few non-metallic inclusions, and has been applied in the field of additive manufacturing.

2.3 Electrostatic separation technology Electrostatic separation technology is a key technology for removing non-metallic inclusions from powders. Its principle is to charge powder particles through corona discharge, and then separate metal powder and non-metallic inclusions by using the difference in electrical properties between them.

The electrostatic separation device consists of two electrodes, a thin metal wire as the negative electrode, and a large-diameter metal reel that is grounded and has a certain rotation speed as the positive electrode. When the potential difference between the two electrodes reaches a certain value, corona discharge occurs between the two electrodes. After the metal powder containing non-metallic inclusions falls into the corona electric field formed on the surface of the roller through the feeder, it meets the electrons and negative ions flying to the positive electrode, causing it to carry the negative charge of the roller. Due to the high conductivity of metal powder, the negative charge obtained is quickly released after contacting the roller, and falls from the front of the roller into the finished powder collection area under the action of gravity and centrifugal force; and non-metallic inclusions are not easy to lose charge due to their low conductivity. They are adsorbed on the roller under the action of Coulomb force and electrostatic adsorption, and are brushed off by the roller brush when the roller rotates to the rear.

2.4 Powder Sieving In addition to the atomization process, another key process to determine the powder characteristics is the post-processing of the metal powder raw materials. This includes oxidative passivation of the powder particles, classification (i.e., sieving or air classification) and mixing to obtain a final uniform powder batch. Powder sieving is to obtain powder of the required particle size and remove some impurities according to the difference between the size of the required powder and the size of the impurities. The sieving machine works in a vacuum state or under the protection of inert gas to prevent the powder from being oxidized during the sieving process. The main parameters of the sieving process are the vibration amplitude and vibration frequency of the vibrating screen. In general, the raw powder of the VIGA method should be pre-screened with 100 mesh to prevent large irregular objects generated during the atomization process from damaging and clogging the screen.

In addition, there are various technologies to improve the flowability of the powder after passivation. There are currently multiple methods to quantify the rheological properties (flowability and spreadability) of powders, but the correlation between them has not been fully determined. No other methods are widely used except the simplest funnel flow method (ASTM B213/ISO 4490/ASTM B964). There is a lack of understanding in the AM community about what is an acceptable flow value for a particular metal alloy, so more quantitative research on powder flow for additive manufacturing is necessary.

3 Powder Reusability


Industries, led by aerospace and orthopedics, are rapidly adopting AM metal powders. Since AM metal powders are one of the main drivers of cost, especially in high-performance applications, there is a strong interest in powder reuse. Since only a small portion of the powder will melt and fuse into the part, the remaining powder can be used multiple times until the powder is no longer usable in a specific application.



Nickel alloy 718, which has excellent mechanical properties and oxidation/corrosion resistance, was first introduced in the 1960s to overcome the disadvantages of poor weldability of superalloys. Deng et al. reused 100 kg of nickel alloy 718 (PSD 45-106 μm) six times to manufacture parts on a powder bed fusion (E-PBF) machine. The powder was produced by electrode induction melting inert gas atomization process (EIGA), and the powder was not regenerated with the original powder during the reuse process. Compared with the original powder, the elemental composition of the recycled powder did not change significantly, only the oxygen element increased from 0.014 wt% to 0.022 wt%. After the sixth build, the chemical composition of the powder particles collected from different locations did not change, and the elemental composition still met the specific requirements of the alloy. The flowability and PSD data did not change significantly after the reuse cycle. However, the PSD data after the sixth build cycle showed a slight increase in the average diameter of the particles. It shows that the nickel alloy 718 powder is stable under electron beam and can be reused many times without significant changes in powder size and chemical composition.

According to the principle of thermodynamics, the formation of Al2O3 in nickel 718 alloy is more favorable than the formation of other oxides. During the E-PBF process, oxidation of Al, Ti, Fe or Cr seems inevitable, but very low oxygen concentrations can inhibit the formation of such oxides on the powder surface. This is because the equilibrium partial pressure of oxygen at the melting point of these elements must be below 10-7~10-9 mbar to avoid these oxidations. On the other hand, the high production temperatures in the E-PBE equipment lead to a high diffusion rate of oxygen. Therefore, there is a driving force for the reaction to occur. After L-PBF and E-PBF processes, Cr2O3 and Al2O3 were observed to form on the powder surface. Although Cr2O3 has a lower affinity for oxygen than Al2O3, its formation is kinetically favorable, especially considering its high content in nickel 718 alloy. In repeated use cycles, it can be expected that the existing oxides will gradually decompose into the most stable oxide (i.e. Al2O3) when exposed to high temperatures in the AM process.

4 Conclusion
my country has carried out a lot of scientific research and technological development work in the research and production of powder high-temperature alloys and various key technologies, especially in the key technologies of high-temperature alloy powder preparation, which has achieved breakthroughs, ensuring the organization and performance of key parts of my country's powder high-temperature alloys, and has made outstanding contributions to the localization of aircraft engines. With the goal of eliminating the three major defects in powder high-temperature alloys and improving the overall performance of powder molded parts, future high-temperature alloy powders must develop in the direction of high-purity fine powder.