As the main consumable for metal 3D printing, metal powder has a crucial impact on the quality of printed products. 3D printing of precise and complex parts in aerospace, defense, and medical fields has high requirements on powder properties such as particle size, morphology, and purity. This paper introduces the basic requirements and main powder making processes for several commonly used high-quality nickel-based, cobalt-based alloy and titanium alloy metal powders for 3D printing in the aerospace field.

Introduction of 3D Printing Metal Powders for Aerospace

Unlike traditional metal material manufacturing technology with huge equipment, long processes, high energy consumption, pollution, and low material utilization, metal 3D printing has the following advantages: (1) high overall material utilization; (2) no need to open moulds, few manufacturing processes and short cycle times; (3) parts with complex structures can be manufactured; (4) free design according to mechanical property requirements, without considering manufacturing processes. In recent years, metal 3D printing has been developed by leaps and bounds.

Metal 3D printing is mainly used to provide rapid production of models for industrial design and the processing of complex moulds, as well as the production of small batches, complex structures, high performance, and large metal components. Metal 3D printing uses metal powder as an additive material and uses rapid forming methods such as selective laser melting (SLM), electron beam selective melting (EBSM) or laser near-net forming (LENS) to rapidly transform directly from a computerized digital model into a solid part. High-quality 3D printed metal parts for aerospace, defence, medical, automotive and electronics applications need to have high strength, dimensional accuracy, water tightness and lightweight. In addition to the printing equipment, the quality of the metal powder, including powder sphericity and cleanliness, particle size distribution, oxygen content, flowability, and bulk density, also have a significant impact on the quality control of the metal 3D printing process. This paper focuses on the requirements of metal powders for high-quality 3D printed parts for aerospace applications and the

This paper focuses on the requirements for high-quality 3D printed parts for aerospace applications and the powder preparation process.

Metal Powder Requirements for High-Quality 3D Printing

Metal powder is the most important raw material for 3D printing of metal parts, and powder properties are one of the most important factors affecting the quality of metal 3D printing products. Powder materials with a diameter of less than 1 mm are generally considered suitable for 3D printing, but high-quality 3D printed metal products have higher requirements for the shape, particle size and purity of the powder. The main types of 3D printing equipment are powder spreading, coaxial powder feeding and lateral powder feeding, depending on the powder replenishment method. The lateral powder feeding 3D printing final part

The shape and dimensional accuracy of the final part is low, the laser energy cannot be fully utilized and the power utilization rate is low. Therefore, high-quality metal 3D printing equipment mainly uses two types of powder replenishment methods, namely powder spreading or coaxial powder feeding.

Several Important Indicators for 3D Printed Metal Powders for Aerospace

Purity. Ceramic inclusions can significantly reduce the performance of the final part, and these inclusions generally have a high melting point and are difficult to sinter, so a powder-free of ceramic inclusions is required. In addition, the oxygen and nitrogen content needs to be strictly controlled. The current powder preparation technology for metal 3D printing is mainly based on atomization (including techniques such as aerosolization and rotary electrode atomization), where the powder has a large specific surface area and is easily oxidized. In aerospace and other special applications, the customer’s requirements for this indicator are more stringent, such as high-temperature alloy powder oxygen content of 0.006% to 0.018%, titanium alloy powder oxygen content of 0.007% to 0.013%, stainless steel powder oxygen content of 0.010% to 0.025% (all mass fraction). For titanium alloy powder, nitrogen, hydrogen and titanium at high temperatures will form TiN and TiH2, reducing the plasticity and toughness of titanium alloy. Therefore, the atmosphere should be strictly controlled during powder preparation.

Powder particle size distribution. Different 3D printing equipment and forming processes require different powder particle size distributions. At present, metal 3D printing commonly used powder particle size range is 15 ~ 53 μm (fine powder), 53 ~ 105 μm (coarse powder), some occasions can be relaxed to 105 ~ 150 μm (coarse powder). 3D printing with metal powder particle size selection is mainly based on the different energy sources of metal printer division, to laser as the energy source of the printer, because of its focus spot fine, easier to melt fine powder. The printer with laser as the energy source is suitable for using 15-53 μm powder as consumables, and the powder replenishment method is to lay powder layer by layer; the printer with the electron beam as the energy source is suitable for using 53-105 μm coarse powder as the main powder because the focused spot is slightly coarser and more suitable for melting coarse powder; for the coaxial powder feeding type printer can use 105-150 μm powder as consumables.

Powder morphology. Powder shape and powder preparation method is closely related, generally from the metal gas or molten liquid into powder, powder particle shape tends to be spherical; from the solid-state into powder, powder particles are mostly irregular shape; and by the aqueous solution electrolysis method of powder, preparation is mostly dendritic. Generally speaking, the higher the sphericity, the better the fluidity of the powder particles. 3D printing metal powder requires asphericity of 98% or more so that it is easier to spread and feed the powder when printing.

Powder fluidity and loose packing density. Powder fluidity directly affects the uniformity of the powder laydown and the stability of the powder feeding process during printing. The fluidity is related to the powder morphology, particle size distribution and bulk density.

The larger the powder particles, the larger the particle size distribution and the density of the powder. The larger the powder particles, the more regular the particle shape and the smaller the proportion of very fine powder in the particle size composition, the better the flowability. The density of the particles remains the same, but the relative density increases and the fluidity of the powder increases. Particle surface adsorption of water, gas, etc. will reduce the powder mobility. Loose packing density is the powder specimen naturally filled with the specified container, the mass of the powder per unit volume. In general, the coarser the powder size, the higher the bulk density, and the coarser and finer the powder, the higher the bulk density. The effect of bulk density on the density of the final metal printing product is not conclusive, but an increase in bulk density improves the flow of the powder.

3D printing technology is suitable for a variety of materials, a wide range of applications, high material formation rate, is a rapid development of an emerging material manufacturing technology in recent years. Metal 3D printing, as an important part of 3D printing, has been highly valued and studied by relevant universities, research institutes and enterprises. Metal powder is an important consumable for metal 3D printing and has been successfully applied in the aerospace field.