More and more metal additive manufacturing parts are used in aircraft. Metal additive manufacturing is a near-net-shape manufacturing process based on a three-dimensional data model of a product. A three-dimensional printer slices the three-dimensional data of a product, melts the metal powder through a high-energy heat source such as a laser beam or an electron beam, and quickly solidifies it, forming a surface with a point-to-point moving line, and finally stacking it layer by layer to form a three-dimensional product entity. As the smallest material unit in metal additive manufacturing, the relevant physical and chemical properties of metal powder seriously affect the overall performance of the final manufactured parts. However, at present, there are relatively few studies on the basic properties of powders related to additive manufacturing, and the powder properties and the performance of the final product are only qualitatively described, and there is no quantitative evaluation system. Secondly, the general phenomenon of metal additive technology in reality is that foreign countries are better than domestic development, which is also reflected in the quality of powders. At present, in the three-dimensional manufacturing process of titanium alloy and high-temperature alloy powders, the use of domestic powders often results in unstable performance. Therefore, it is particularly necessary to study the basic properties of metal powders used in additive manufacturing, which is also the key to the airworthiness certification of parts.

1. Powder property detection and analysis

The basic properties of metal powders for additive manufacturing are shown in Figure 1: appearance, composition, particle size, sphericity, fluidity, density and hollowness, etc. These factors work together to affect the overall performance of the product.

1.1 Appearance test analysis
Most metal powders for additive manufacturing have a grayish appearance and a rough and uneven particle surface, as shown in Figure 2. These characteristics are conducive to the powder’s absorption of laser or electron beam energy; however, the rough particle surface also affects the powder spreading and fluidity during the additive manufacturing process, which in turn seriously affects the density and mechanical properties of the product; most additive manufacturing powders are tens of microns or even a few microns, which makes it easy for the powder to adhere to the pellets and is not conducive to uniform spreading of the powder; the powder particles are small and are easy to absorb water vapor and get damp during packaging and transportation. Powder appearance testing mainly relies on visual inspection, light microscopy and electron microscopy, and its main purpose is to observe the overall appearance, microscopic surface and batch stability of the powder. Due to its small depth of field, optical microscopes are generally less used for the observation of the surface of macroscopically continuously distributed particles such as metal powders for additive manufacturing.

1.2 Powder composition test analysis
Table 1 is the ASTM standard for the detection of powder composition in common additive manufacturing. It mainly uses X-ray emission spectroscopy, inert gas melting and inductively coupled plasma atomic emission spectroscopy to analyze powder elements. The powder composition is currently mainly based on the national or American standards of mature parts (bars, plates, forgings and castings, etc.). However, the composition range of such documents is relatively wide, and it is difficult to use them as the final control standard for products. Secondly, in the additive manufacturing process, powder is an intermediate form of the part. The powder is rapidly melted under the action of strong energy such as laser or electron beam. Finally, it is rapidly cooled and accumulated into the final product. Although the melting and solidification time of the powder is short, the melting process will inevitably lead to a reduction in easily burned and volatile elements. Therefore, the control of such elements in the powder must be different from that of the part.

1.3 Powder particle size detection and analysis
The particle size of additive manufacturing powder is continuously distributed. Whether its particle size is reasonably distributed directly affects the powder spreading and the density of the product. According to the fuller curve formula of the particle closest packing theory of continuous particle size distribution, it is shown in formula (1) (please see the picture in the article for the formula). In the formula: P is the sieve pass; di is the diameter of each graded sieve hole or graded particle size; D is the maximum particle size of the powder; n is an index, which depends on the shape characteristics of the powder.

Only when the powder particle size distribution satisfies this formula or is close to it, can the powder be effectively densely packed when spreading, so as to ensure the density of the final product.

The main testing methods for powder particle size are sieve analysis, laser particle size analysis and image analysis. As shown in Figure 3, the main basis of the sieving method is the diameter of the cylinder with the minimum limiting diameter of the powder. According to the standard ASTM B214, the sieving method is applicable to inner spherical powders with a particle size greater than 45 μm. The particle size of the powder-feeding type powder is about 100 μm, which is generally applicable to this standard, but the particle size of the powder-spreading type powder is about 50 μm, which is not applicable to this standard.

ASTM B822 uses laser to analyze the particle size of metal powders, and its principle is shown in Figure 4. When the laser encounters powder particles during propagation, it will scatter, and its scattering angle θ is inversely proportional to the particle size of the powder. The scattered light in the same area comes from the powder of the same particle size. By analyzing the light intensity and the position of the scattered light, the parameter information of the powder particle size and the powder quantity can be obtained. Figure 5 shows the results of laser particle size analysis of a certain powder-feeding type (a) and powder-spreading type (b).

The image analysis method is to analyze two-dimensional photos of powder particles and fit the powder boundaries through analytical geometry to obtain the powder equivalent area diameter, equivalent perimeter diameter and Feret diameter (the distance between two parallel tangent lines of the powder boundaries). However, there are currently no relevant standards to regulate the operation of this analysis test and the elimination of human interference.

1.4 Sphericity detection and analysis
Sphericity is a parameter that relatively qualitatively describes the shape of powder. At present, most of the metal powders used in additive manufacturing are prepared by gas atomization technology, and the powder sphericity is relatively high. Figure 6 is an electron micrograph of a certain grade of titanium alloy powder. It can be seen that most of the powders are spherical, the powder particle size is multi-level distribution, a few powders have small particles of powder bonded to the surface, and a very small number are irregularly distributed. The closer the powder is to a sphere, the easier it is to spread, and the more beneficial it is to the performance of the product. Based on the method of analyzing the sphericity of powders based on two-dimensional powder images, the actual powder is usually an elliptical irregular powder. One way is to fit the Legendre ellipse through analytical geometry, and define the ratio of the major and minor axes of the fitted ellipse as the sphericity. The closer the ratio is to 1, the better the powder sphericity. Another method is to measure the circumference S of the outer contour of the known powder and the powder area A, and then define the sphericity α: α = S2 /4πA (see formula 2 in the figure). If α is close to 1, the powder to be tested is approximately spherical. The diagram of the graphical analysis is shown in Figure 7.

1.5 Powder density detection and analysis
Powder density is divided into loose density, tap density and true density. Loose density is the mass per unit volume of powder under free stacking. The loose density is mainly carried out according to the standards ASTM B212, B329, B417 and B703; the tap density (ASTM B527) is the mass per unit volume of powder when the volume remains unchanged under external vibration. The ratio of them to the true density of powder (ASTM B927) can qualitatively determine the density of the final product. The powder density detection and analysis standards are shown in Table 2.

1.6 Powder flowability detection and analysis
Flowability is an important indicator to measure the spreadability and conveyability of powder. Flowability can be expressed by the time it takes for a certain mass or volume of powder to pass through the instrument, or it can be reflected by the accumulation of powder. The better the powder fluidity, the more conducive it is to spreading and conveying during the powder additive manufacturing process, and the more conducive it is to obtaining additive manufacturing parts with good structure density and performance. The powder property tester is mainly determined by the cone angle formed by the powder when the powder is freely piled. Experience shows that the larger the angle, the worse the powder fluidity.
The powder fluidity can also be measured by using a Hall rheometer (ASTM B213) and a carney (ASTM B964) rheometer to measure the time it takes for 50 g of powder to flow through, or by using an Arnold meter and a Hall rheometer (ASTM B855) to measure the time it takes for 20 cm3 of powder to flow through to indicate the powder fluidity. It is particularly important to note that the powder with good fluidity uses the Hall flow meter, and the powder with poor fluidity uses the Carney flow meter (the Carney flow meter and the Hall flow meter have the same dimensions except for the larger diameter of the powder outlet, and the Carney funnel aperture ϕ 0.20in (≈5mm) is larger than the Hall funnel ϕ 0.10mm (≈2.5mm)).

1.7 Powder hollowness detection and analysis
The powder hollowness is the proportion of the unfillable space surrounded by metal to the total powder, which affects the final density of the powder. The powder hollowness can be expressed by the number ratio and volume ratio of the hollow powder. At present, metallographic and CT are mainly used for testing. CT testing is relatively simple but expensive, while metallographic testing is prone to cause powder to fall during sample preparation, affecting the final powder hollowness result.

2 Other standards related to powder
The national standard has relevant powder composition, particle size, fluidity and density testing standards, but they are generally similar to ASTM standards. In recent years, ASTM has released standards related to additive manufacturing. Among them, the powder standards need to pay attention to the control of powder raw materials, the record of powder use, and the negotiated regulations on the number and proportion of powder reuse. In principle, powders used in additive manufacturing can be recycled indefinitely, but in actual production, the number of powder uses for more precise parts is generally not more than three times, but the maximum number of powders should not exceed 10 times. Other standards related to powder are shown in Table 3.

3 Conclusion
The appearance, composition, particle size, sphericity, fluidity, density and hollowness of metal powders seriously affect the performance of additive manufacturing parts. The national and ASTM standards for composition, particle size, fluidity and density testing are relatively complete, but there is no relatively qualitative powder parameter testing standard for appearance, sphericity and hollowness. Although a number of additive manufacturing standards have been introduced, there is no relevant testing standard for the commonly used powder image analysis method. Under the existing testing methods, the various properties of the powder are parameterized, but the meaning represented by the final test results cannot be described, and the evaluation standards are not yet unified. The general basis for powder testing is to measure the quality of powder based on the existing favorable, convenient and fast testing methods with little volatility, so it is very necessary and meaningful to establish a set of testing and evaluation standards suitable for additive manufacturing powders.