Source: Laser World
Introduction: Today, a large number of steel products have begun to be manufactured using additive manufacturing techniques. Due to the difference in the microstructure of the matrix material and the difference in phase composition (austenite, ferrite, martensite), the types of precipitated phases are different (intermetallic compound precipitated phase, carbide precipitated phase), resulting in steel Additive manufacturing organization and performance changes are very wide.
This is especially true when comparing additive-manufactured steel to traditionally manufactured steel. However, the time-temperature change process experienced by steel in the additive manufacturing process is very different from the traditional manufacturing process. As a result, the structure of additive manufacturing is very different compared to traditional processes. This includes non-equilibrium solidification caused by rapid solidification, which leads to the refinement of tissues, changes in morphology, and changes in crystal crystallization. This change in microstructure, in turn, requires additional or post-heat treatment and alloy composition design to adjust. In this review, we first review the different types of steel manufactured using additive manufacturing and their microstructure, mechanical properties, corrosion properties, heat treatment, and corresponding applications. Alloy types include austenitic, dual phase steel, martensite, precipitation strengthened steel, TRIP / TWIP steel, martensitic failure hardened steel, carbon bearing steel, and oxide dispersion strengthened steel (ODS). At the same time, the introduction of additive manufacturing steel not mentioned in the previous literature was also introduced. The performance and potential applications of iron-based functional materials are also introduced.
The advantages of metal additive manufacturing are compared with the traditional synthesis and forming process, because it can produce personalized products with complex shapes in a short period of time. For example, customization for personalized human implants has been applied. It can quickly manufacture key components with complex shapes and meet individual needs and can achieve weight reduction, or apply AM technology to the repair of key high value-added components of aircraft engines. In applications in these fields, alloys are generally biocompatible materials, high temperature materials, and lightweight materials such as Ti, Ni, Al, and magnesium alloys. As a result, reviews of alloys used in additive manufacturing technologies have also focused on the above materials. With the exception of two articles. There are few reviews to review the additive manufacturing of steel. These two reviews of additive manufacturing containing steel are also different from this article.
However, since the Iron Age more than 3,000 years ago, the most widely used and most successful alloy was steel. A review of alloy design, microstructure, properties, and processes to steel from the perspective of additive manufacturing has not aroused corresponding industry attention.
In this review, we are committed to involving the different types of steels that are additively manufactured today. We mainly focus on organization, performance and electrochemical performance. In particular, the focus is on the effects of processes (thermal and chemical) on the structure and performance of the additive manufacturing process. Unlike the review by Fayazfar et al., We do not describe in detail the effects of additive manufacturing process parameters. We only focus on melting additive manufacturing, especially laser additive manufacturing (LPBF, LMD) and electron beam additive manufacturing (EPBF).
Here, we do not directly introduce the base material. Especially for direct energy deposition, cladding or surface strengthening processes, a large number of secondary strengthening phases have been added and used. If these are also introduced, the paper is too long. In addition, we mainly discuss materials with uniform chemical composition in this article, which means that we are not involved in additive manufacturing of functionally graded materials or multi-materials. It should be noted that compared with the connection of dissimilar materials in welding, functionally graded materials have similar principles. Although many years of research have been done in this field during welding, there are still certain challenges in the field of additive manufacturing.
In general, as an engineering material, steel is mainly used in applications where the following needs are required:
- In harsh environments where corrosion resistance, wear resistance and hardness are required;
- Where high quality and low price are needed;
- Unmatched microstructure, which requires super hard martensite to multi-phase alloy;
- Occasions for functional requirements, such as ferromagnetic materials or Kovar alloys.
In order to meet the durability requirements of the above harsh environment, stainless steel is a good choice. Therefore, this type of alloy, especially 316L alloy, has been widely used. And it is mostly used in the condition of medium temperature corrosive environment. For service occasions that require corrosion resistance and high mechanical properties and hardness, martensite precipitation-strengthened stainless steel is more suitable, such as 17-4PH steel and 15-5PH steel. This type of alloy is mostly used in marine engineering, power and extrusion die industries.
In the tooling and die-casting industries, materials with high tensile strength, high hardness, and wear resistance are needed, and die steel that meets this requirement came into being. The biggest advantage of AM for this type of alloy application is that it can produce smooth and complex internal cooling channels close to the outer surface. For example, when it is used in the extrusion die industry, the heat can be quickly dissipated, thereby improving the production efficiency of the product and the service life of the die. Due to the high carbon content, mold steels manufactured by traditional industries are prone to cracks and are not easy to manufacture. Therefore, the additive manufacturing of mold steels urgently needs carbon-free maraging steel, especially 18Ni-300 type steel.
The appropriate price of stainless steel and the advantages of easy processing make austenitic stainless steel the preferred material in additive manufacturing stainless steel.
In the above main classification, other alloys are also involved in additive manufacturing, such as martensitic stainless steel, TRIP / TWIP steel and ODS steel. Electromagnetic alloy or Kovar alloy used for functional materials, such as Fe-Si, Fe-Ni or Fe-Co alloy.
In order to allow readers to better understand and master the knowledge in this article, we will classify the steel according to the microstructure at room temperature, starting from austenitic stainless steel to TRIP / TWIP, which is a type from austenitic stainless steel to bidirectional The transformation of steel, this steel contains both austenite and martensite. Next, the martensite and precipitation strengthened stainless steel were introduced. This is just a type of martensite that is completely similar to martensite if it is manufactured using traditional processes. Martensite and carbon bearing steel are used for aging / tempered martensite. Finally, the ODS alloy is also introduced, this particle reinforced ferritic stainless steel. This classification is related to the microstructure at room temperature, especially the microstructure obtained according to the traditional process. This way the reader will read it very clearly.
In each chapter, we introduce the microstructure of the material, the phase composition in the as-deposited and heat-treated state, and its crystal structure. The static mechanical properties are also summarized, including fatigue performance. At the appropriate time, some special other properties are also introduced, such as ferromagnetic materials and corrosion-resistant materials. At the end, the future development is also prospected.
P.Bajaj, A.Hariharan, A.Kini1P.Kürnsteiner, D.Raabe, EAJägle.Steels in additive manufacturing: A review of their microstructure and properties [J] .Materials Science and Engineering: A, Volume 772, 20 January 2020 , 138633.