Advanced aluminum-ferroalloy for 3D printing

3D printing objects and machine parts is a high-tech and advanced cutting-edge technology, which can print almost everything, from buildings, bridges and cars to teeth, etc., in addition to plastic, garbage and ceramic particles, metal powder / wire materials such as aluminum, titanium, stainless steel and superalloy powder are also commonly used. Traditional aluminum alloy powders can be used to print objects and structures, and some aluminum alloys that can not be used in conventional 3D printing have been developed. For example, Fe is a harmful impurity for more than 98% of deformed aluminum alloys. However, aluminum alloy for 3D printing is a useful alloying element, because it can significantly refine the microstructure, hinder grain growth, and greatly improve the mechanical properties of the printed parts.

3D printing, also known as Adding Manufacture or Additive Manufacturing,AM, is a new rapid prototyping manufacturing technology, which has attracted wide attention from all walks of life and media at home and abroad. it is a technology based on digital model text, using adhesive materials such as metal powder or plastic, and building all kinds of objects by layer-by-layer printing. 3D printer was introduced in the mid-1990s. It is a rapid prototyping device that uses light curing and paper lamination technology. Its working principle is roughly the same as that of conventional printers. The machine is equipped with "printing materials" such as liquid or powder. After being connected to the computer, the "printing materials" are superimposed layer after layer through its control, turning the blueprints on the computer into real objects and machines.

The 3D printing process can be divided into two steps. First, a special "glue" is sprayed in the area that needs to be formed, and then the powder is evenly sprayed. The powder will quickly solidify when it meets the glue, and the area without glue will remain loose. Repeat this process and the real thing will be printed out according to the blueprint. Because its layered processing process is very similar to inkjet printing, it is called 3D printing, and the corresponding machine is called 3D printer.

Preparation of aluminum powder

In addition to plastic and ceramic particles, metal powders such as aluminum, titanium, stainless steel and superalloy powder are also commonly used in 3D printing materials. In the 3D printing process, the 3D printed powder with complex geometry and structure is prepared by conventional gas atomization, and the small droplets are quenched in the atomization process to form fine grains, and the grain size of the powder is generally 20 μ m ~ 30 μ m. At present, there are not many aluminum alloys suitable for laser melting printing (SLM,selective laser melting). Only a few alloy atomized powders are not suitable for this kind of printing, the products without small holes and hot cracks can be printed by SLM method. AISi10Mg alloy is most used, but it is not an aluminum alloy developed for 3D printing, it is a near-eutectic conventional alloy, because it contains a lot of eutectic. Therefore, there are no small holes and hot cracks in the printed products, so it is urgent to develop aluminum alloy suitable for 3D printing of SLM process.

Advanced aluminum-ferroalloy for 3D printing

In recent years, a batch of new aluminum alloys suitable for 3D printing have been successfully developed, which will not form porosity or hot cracks in the products. There are two strategic measures to develop 3D printing aluminum alloy: one is to ensure that enough melt can flow into the gap between grains during high-speed solidification of droplets, and the other is that the grains formed should be fine. in order to achieve the first goal, the alloy should have enough eutectic. Fine-grained products not only have high strength and good plasticity, but also improve the fluidity of the melt without forming loose and hot cracks. In W. The 3D printed grain diagram of the alloy containing Fe prepared by Fragner et al shows that in the microstructure of AlSi9Cu3MgFeZn alloy (A380, A226) containing 1.2%Fe, the size of the Fe-rich intermediate metal phase β-AlFeSi is several hundred μ m, and the size of β-AIFeSi phase is less than 1 μ m.

The most important thing in designing alloys and production processes is to ensure that the melt has a high cooling rate, preferably 104K/s~106K/s, that is, about 1000 times the cooling rate of conventional die casting. At this cooling rate, the intermediate metal phase (IMPs,intermetalic phases) will become heterogeneous nuclei and hinder grain growth. The elements that can form this intermediate phase are Zr, Sc, Ti, SiC, and the intermediate metal phases formed by them are:

Al+Zr "Al3Zr", "Al+Sc" Al3Sc, "Al+Sc+" Al3 (Sc,Zr), "Si" Mg2Si in 7075 alloy, "Si" Mg2Si in 5XXX alloy.

Iron-rich phases are harmful because they reduce the plasticity of the alloy, and its quantity and size should be strictly controlled. When the AISi9Cu3MgFeZn (A380, A226) alloy containing 1.2%Fe is cooled slowly (1K/s~10K/s), it is equivalent to the cooling rate of die casting, and the β-AIFeSi phase in the structure is flaky or needle-like, its tensile strength 160MPa, elongation is very low, only 0.7%, because its rich Fe intermediate phase is as large as hundreds of μ m, resulting in stress concentration. The microstructure of selective laser melting (SLM) alloy is very fine, the size of Fe-rich mesophase (IMP) is less than 1 μ m, the mechanical properties are greatly improved, the yield strength is 220 MPa, the elongation is 1.9%, and even 5.3% (2).

Thus it can be seen that Fe is a useful alloying element for selecting aluminum alloy powder for laser 3D printing. Because of the fast cooling rate of this process, fine microstructure can be formed. Austrian Nonferrous Metals Company (Amag) and its partner company have designed and prepared cast aluminum alloy and deformed aluminum alloy with high content of Fe for 3D printing. The content of Fe and Mn in the alloy is so large that the solidus temperature TsTMP of the alloy is higher than or close to the liquidus TL temperature of the alloy when choosing laser 3D printing. This ensures that IMPS 3D printed parts have fine microstructure and better mechanical properties.

3D printing aluminum alloy with high Fe+Mn content

According to the role of Fe and Mn elements in aluminum alloy, Austrian Nonferrous Metals Company should not only accurately control the content of Fe and Mn, but also modulate the content of Si, Cu, Mg, Mn and Zn when preparing some aluminum alloys with poor performance. The new advanced high-speed rail 3D printing aluminum alloy developed by Amag Company is based on AA5083 (AIMg4.5Mn0.55) alloy, and refers to the composition of AM alloy Scalmalloy, which is a kind of AA5083 alloy containing Sc+Zr, although it has good comprehensive properties, but the price is high. Alloy 5083 is a kind of alloy formed in the 1930s, which has been widely used and is still a commonly used alloy today. Its composition (mass%) is Si0.40,Fe0.40,Cu0.10,Mn0.40~1.0,Mg4.0~4.9,Cr0.05~0.25,Zn0.25,Ti0.15, other impurities are 0.05,0.15 in total, and the rest are Al. Now it has formed a large family with eight members: 5083, 5183, 5183A, 5283, 5283A, 5283B, 5383, 5483.

The chemical composition of the new alloy and the comparative benchmarking alloy-Scalmalloy, alloys with low Fe (0.1%), and alloys containing 0.45%Zr are shown in the table below. The composition of alloy A3 is similar to that of Al alloy, but the content of Fe is very high, and the content of Mn is adjusted. The composition of A2 alloy is similar to that of Scalmalloy alloy, but the Fe content is as high as 0.7%.

In the optical and electron microscope microstructure images of A1 and A3 alloys, it can also be clearly shown that the grains of alloys containing 1.3%Fe are fine, and A3 alloys have no visible hot cracks, indicating that Fe has a good effect. The Fe content of A1 alloy is very low, only 0.1%, coarse grains and hot cracks.

The optical and electron microscopic microstructure images of A3 alloy (1.3%Fe) show that the grain is fine, the Fe (+ Mn) phase is submicrostructure (submicroscoptically fine), which is very fine, and it is mainly located at the grain boundary, which hinders the grain growth, so the grain size of the alloy is fine, there are no cracks, the mechanical properties are good, and all indexes are in a high position. A3 alloy has fine grains and high tensile strength. A1 alloy has low plasticity and low elongation because of hot cracks.

The formation of fine grains is due to the work of Sc and Sc+Fe. The mechanical properties of A2 alloy are higher than those of Scalmalloy alloy because it contains 0.4% Fe. However, the Fe content of Scalmalloy alloy is low, and A3 alloy and Scamalloy alloy have roughly the same mechanical properties, but the latter alloy has expensive Sc and should be used less. Fe in A3 alloy is a commonly used alloying element and can be brought from the scrap used. Scalmalloy has high strength properties, yield strength 475MPa and tensile strength 520 MPa after artificial aging at 325 ℃ / 4h, but A3 alloy has no aging reaction.

Postscript

3D printing technology is an omnipotent high-precision and new rapid prototyping technology, which has attracted close attention from all walks of life in various countries and regions. China has done a lot of work in the application, research, development and promotion of 3D printing technology. it is the first to print the connecting ring of 10m class 2219 aluminum alloy rocket launch fuel tank in the world. As early as August 2015, the Premier of the State Council hosted a special lecture of the State Council to discuss issues such as accelerating the development of advanced manufacturing and 3D printing. In recent years, great achievements have been made, but there are still some gaps compared with developed countries. The 5XXX series aluminum alloy for 3D printing with high iron content introduced in this paper may be helpful to interested readers, but the research results in this field have not been reported in our country.