According to SmarTech, the global supply chain for additive manufacturing of aluminum alloy materials appears to have "crossed the threshold" and become the next generation opportunity to support additive manufacturing technology.
But for a long time, in 3D printing aluminum alloy materials, only a few Al-Si based casting alloys have achieved crack-free processing. Forged aluminum alloys with poor weldability, due to the high thermal gradient that promotes columnar growth and thus cause hot cracks, the additive manufacturing applications of forged aluminum alloys are greatly restricted.
This restriction is being broken. The high-strength aluminum alloy 3D printing materials that have been commercialized since 2019 have opened a new door to the processing of parts that must be realized by forging. Combined with the design freedom released by 3D printing, forged aluminum alloy additive manufacturing technology It will gain huge imagination in the field of pressure vessels, hydraulic manifolds, brackets, and high-strength structural parts.
The development of new 3D printing forged aluminum alloy materials continues to develop. Hay Think will share the latest research results of French scholar Mathieu Opprecht and others in 3D printing AI6061 aluminum alloy to eliminate hot cracks.
The first step towards a fully equiaxed crystal structure
When using selective laser melting 3D printing technology for forging aluminum alloy forming, it is prone to severe cracking. In recent years, in order to obtain dense parts with high mechanical properties, many studies have focused on the optimization of process parameters, but the range of materials processed in this way is still quite limited. Among forged aluminum alloys, Al6061 and Al7075 have extremely high crack levels during selective laser melting.
The latest research from French scholar Mathieu Opprecht et al. showed that adding a certain amount of yttrium stabilized zirconia (YSZ) can induce grain refinement, change the microstructure of 3D printed 6061 aluminum alloy material, and eliminate hot cracking. Related papers are published in Acta Materialia journal.
Two methods of grain refinement
The paper shows that there are two methods for grain refinement. The first method is to control thermal stress during the printing process. The second method is to enhance the heterogeneous nucleation by changing the alloy composition or directly adding a nucleating agent to the base powder.
Whether it is a welding process or a selective laser melting process, the causes of thermal cracks are roughly the same. In both cases, the process parameters will cause thermal stress, which is the key factor causing cracks. However, it is difficult to control thermal stress through process parameter control. The paper stated that in order to significantly reduce thermal stress, it is necessary to greatly reduce the temperature gradient. In the selective laser melting process, this goal cannot be achieved through changes in process parameters or environment. In the heat treatment process, the alloying elements used to generate the strengthening phase usually increase the solidification temperature range, which is also very disadvantageous in the previous research. In addition, in the 3D printing process, a high temperature gradient usually causes a columnar structure elongated in the structural direction, which promotes the occurrence of thermal cracking.
In this study, the researchers used the second method, which is to add different amounts of yttrium stabilized zirconia (YSZ) to the Al6061 base powder. Experiments have found that the grain refinement effect depends on the amount of YSZ added. Starting from 1% (volume fraction), SEM and EBSD images show the microstructure of equiaxed columnar grains with bimodal distribution. The results show that adding 2% (volume fraction) of YSZ can completely avoid cracks on the boundary of the molten pool. Based on TEM and DRX research, the paper provides new insights into the use of additives in the 3D printing process. The article discusses the experimental results on the basis of many existing solidification models, focusing on the necessary conditions to realize the equiaxed solidification scheme.
The paper shows that follow-up work is still needed to further study the mechanical properties of printing materials, such as the influence of Al3Zr precipitation phase and grain structure. In addition, regarding this new alloy, another meaningful research perspective is post-heat treatment. Studies have shown that a large amount of Zr is enriched in solid solution. Through proper aging treatment, it is expected that nano Al3Zr phase will be precipitated, which will make the matrix hard.
According to the existing experimental results, this method can also be applied to other aluminum alloy 3D printing processes that are sensitive to thermal cracks. This work has achieved the first step towards a complete equiaxed crystal structure, which is very beneficial to the improvement of the mechanical properties of the material.
Hay Think Extended Information
For example, Martin et al. successfully 3D printed crack-free parts by adding a nucleating agent to Al7075 powder. Another typical successful case is the high-strength aluminum alloy-Scalmalloy developed by Airbus APWorks for 3D printing, which is a 5xxx aluminum alloy modified by Sc and Zr. The precipitation of the favorable core phase of Al3Zr / Al3Sc is the main reason to eliminate thermal cracking.
Scalmalloy material has been proven to be effective, but the rare earth element Sc on which the material relies is expensive. In the research work of French scholars shared by 3D Science Valley in this issue, one of the goals is to use low-cost nano-level YSZ particles as a nucleating agent to eliminate cracks in crack-sensitive aluminum alloys. The yttrium stabilized zirconia (YSZ) powder was chosen because of its large-scale production conditions, easy handling, low cost and availability of Al3Zr phase precipitation.
Al3Zr is a well-known aluminum core phase in the casting and welding process of αAlFCC. In the research results shared by Hay Think in this issue, scholars have conducted a comprehensive study on the grain refinement effect of Al6061 aluminum alloy with YSZ quantum dots during processing.
In addition, there is a high-strength 3D printed forged aluminum alloy material that also uses a zirconium-based nucleating agent to achieve grain refinement and eliminate cracks. The material is a high-strength 7A77.60L aluminum powder for 3D printing developed by HRL laboratory, which has been officially put on the market. HRL laboratory selected zirconium-based nanoparticle nucleating agents and combined them into 7075 and 6061 series aluminum alloy powders. The formed material has no cracks and is equiaxed (that is, the grains are approximately equal in length, width, and height), achieving a fine-grained microstructure and having comparable material strength with the forging material. This 3D printed aluminum alloy material The average yield strength is as high as 580 MPa, the ultimate strength is more than 600 MPa, and the average elongation is more than 8%.