Parts manufactured by laser additive have undergone a lot of cyclic reheating. Researchers can use rapid quenching, sequential in-situ heating and local phase transformation to create layered microstructures to achieve precise and localized control of martensite formation and precipitation. Thereby controlling the mechanical behavior. The material studied has a tensile strength of 1300 MPa and an elongation of 10%. The in-situ precipitation strengthening and local tissue control principles adopted can be widely used in the manufacturing process of precipitation hardening alloys and different additives, avoiding time-consuming and expensive post-treatment aging heat treatment, and also providing the possibility of locally adjusting the microstructure, which is Traditional heat treatment is impossible.
Laser additive manufacturing is very attractive for producing complex three-dimensional parts from metal powders using computer-aided design models. For example, by using a high cooling rate and cyclic reheating, this method can achieve digital control of processing parameters, resulting in an alloy structure.
Recently, researchers such as Philipp Kürnsteiner and Dierk Raabe from the Max Planck Institute in Germany have recently shown that this cyclic reheating, so-called intrinsic heat treatment, can trigger nickel aluminum in iron-nickel aluminum alloys during laser additive manufacturing. precipitation. Here, the researchers report Fe19Ni5Ti (weight percent) steel tailored for laser additive manufacturing. Related papers were published on Nature on June 25th, Beijing time under the title "High-strength Damascus steel by additive manufacturing".
Paper link:
https://www.nature.com/articles/s41586-020-2409-3
Parts manufactured by laser additive manufacturing (LAM) have undergone specific thermal processes. The first is rapid quenching from the liquid state, and then the internal heat treatment (IHT), that is, cyclic reheating consisting of a large number of short temperature peaks. In directional energy deposition (DED), parts are melted by a laser and powder is transported through a nozzle by a carrier gas. In DED, it provides an opportunity to adjust the microstructure locally. However, new materials must be tailored to make the best use of these special conditions, because traditional alloy compositions cannot be expected to function effectively because they have been optimized for other processing routes, such as casting or forging.
Studies have shown that IHT can trigger nickel-aluminum (NiAl) precipitation in iron-nickel-aluminum (Fe-Ni-Al) alloys. The performance of this so-called maraging steel comes from two important phase transformations. First, a soft nickel-rich martensite structure is formed during the quenching process after austenite-martensite transformation. Subsequently, the martensite undergoes a second phase hardening to form intermetallic nanoprecipitates. Therefore, commercial maraging steels (for example 18Ni-300) produced in a conventional manner and produced with LAM- require expensive aging treatments to form enhanced performance intermetallic precipitates. The iron-nickel-titanium (Fe-Ni-Ti) alloy system has extremely fast Ni3Ti precipitation kinetics, making it very suitable for in-situ precipitation hardening using short temperature peaks during IHT.
Here, the digital control of DED process parameters allows researchers to locally utilize these two phase changes and adjust the microstructure to create a new material inspired by Damascus steel. The layered structure of Damascus steel was originally produced by repeatedly folding and forging a large composite material composed of hard and soft steel, which gave the composite material excellent strength and ductility. The researchers used this concept to produce a layered microstructure by using rapid quenching, sequential in-situ heating and local phase transformation, rather than folding and forging to produce martensite-like maraging steel.
The researchers specifically designed a Fe19Ni5Ti (wt%) alloy to take advantage of rapid quenching and advanced high temperatures. By adjusting the DED process parameters to adjust the time and temperature distribution during the manufacturing process, accurate and localized control of martensite formation and precipitation is achieved, thereby controlling mechanical behavior. The studied material has a tensile strength of 1300 MPa and an elongation of 10%, and shows more excellent mechanical properties than ancient Damascus steel. The principles of in-situ precipitation strengthening and local tissue control adopted here can be widely applied to the manufacturing process of precipitation hardening alloys and different additives. This method avoids time-consuming and expensive post-treatment aging heat treatment, and also provides the possibility of locally adjusting the microstructure, which is impossible with traditional heat treatment.
In summary, the method proposed in this paper is applicable to a wide range of additive manufacturing processes. In addition, in-situ hardening using intrinsic heat treatment (IHT) can be extended to other precipitation hardening alloys. The opportunity to customize the microstructure and mechanical properties locally provides new possibilities for the manufacturing industry.