SLM lattice structures: Properties, performance, applications and challenges

Metal additive manufacturing in aerospace: A review

Propulsion system development requires new, more affordable manufacturing techniques and technologies in a constrained budget environment, while future in-space applications will require in-space manufacturing and assembly of parts and systems. Marshall is advancing cuttingedge commercial capabilities in additive and digital manufacturing and applying them to aerospace challenges. The Center is developing the standards by which new manufacturing processes and parts will be tested and qualified. Rapidly evolving digital tools, such as additive manufacturing, are the leading edge of a revolution in the design and manufacture of space systems that enables rapid prototyping and reduces production times. Marshall has unique expertise in leveraging new digital tools, 3D printing, and other advanced manufacturing technologies and applying them to propulsion systems design and other aerospace materials to meet NASA mission and industry needs. Marshall is helping establish the standards and qualifications “from art to part” for the use of these advanced techniques and the parts produced using them in aerospace or elsewhere in the U.S. industrial base.

Additive manufacturing

Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.

Steels in additive manufacturing: A review of their microstructure and properties

The application of additive manufacturing (AM) in construction has been increasingly studied in recent years. Large robotic arm- and gantry-systems have been created to print building parts using aggregate-based materials, metals, or polymers. Significant benefits of AM are the automation of the production process, a high degree of design freedom, and the resulting potential for optimization. However, the building components and 3D-printing processes need to be modeled appropriately. In this paper, the current state of AM in construction is reviewed. AM processes and systems as well as their application in research and construction projects are presented. Moreover, digital methods for planning 3D-printed building parts and AM processes are described.

Additive manufacturing in construction: A review on processes, applications, and digital planning methods

Density and mechanical properties in selective laser melting of Invar 36 and stainless steel 316L

Invar 36 has gained considerable popularity in many industries, including the aerospace industry, because of its low coefficient of thermal expansion. In this paper, a brief overview for the research needs in metal additive manufacturing is presented. A thorough study for the influence of process parameters on the quality of the parts produced is presented. This study is beneficial for the long-term growth of the additive manufacturing industry. The paper aims to select the process parameters that can be used to fabricate dense parts from Invar 36 (UNS K93600) using the selective laser melting process. In this research, a group of cubes was fabricated using different process parameters from Invar 36 powder using a selective laser melting machine. The density, microstructures, and surface features of these cubes were measured. Experimental observations were drawn from the results of the preliminary analyses. The influence of the process parameters on the density of the parts produced is discussed in this paper.

The selection of process parameters in additive manufacturing for aerospace alloys

Additive manufacturing is a layer based manufacturing process aimed at producing parts directly from a 3D model. This paper provides a review of key technologies for metal additive manufacturing. It focuses on the effect of important process parameters on the microstructure and mechanical properties of the resulting part. Several materials are considered including aerospace alloys such as titanium (TiAl6V4 “UNS R56400”), aluminum (AlSi10Mg “UNS A03600”), iron-and nickel-based alloys (stainless steel 316L “UNS S31603”, Inconel 718 “UNS N07718”, and Invar 36 FeNi36 “UNS K93600”).

A Review of Metal Additive Manufacturing Technologies

This paper presents an experimental study on the metallurgical issues associated with selective laser melting of Invar 36 and stainless steel 316 L and the resulting coefficient of thermal expansion. Invar 36 has been used in aircraft control systems, electronic devices, optical instruments, and medical instruments that are exposed to significant temperature changes. Stainless steel 316 L is commonly used for applications that require high corrosion resistance in the aerospace, medical, and nuclear industries. Both Invar 36 and stainless steel 316 L are weldable austenitic face-centered cubic crystal structures, but stainless steel 316 L may experience chromium evaporation and Invar 36 may experience weld cracking during the welding process. Various laser process parameters were tested based on a full factorial design of experiments. The microstructure, material composition, coefficient of thermal expansion, and magnetic dipole moment were measured for both materials. It was found that there exists a critical laser energy density for each material, EC, for which selective laser melting process is optimal for material properties. The critical laser energy density provides enough energy to induce stable melting, homogeneous microstructure and chemical composition, resulting in thermal expansion and magnetic properties in line with that expected for the wrought material. Below the critical energy, a lack of fusion due to insufficient melt tracks and discontinuous beads was observed. The melt track was also unstable above the critical energy due to vaporization and microsegregation of alloying elements. Both cases can generate stress risers and part flaws during manufacturing. These flaws could be avoided by finding the critical laser energy needed for each material. The critical laser energy density was determined to be 86.8 J/mm3 for Invar 36 and 104.2 J/mm3 for stainless steel 316 L.

A study of thermal expansion coefficients and microstructure during selective laser melting of Invar 36 and stainless steel 316L

Metal additive manufacturing has employed several technologies and processes to advance from rapid prototyping to rapid manufacturing. Additive manufacturing technologies compete with traditional manufacturing methods through their ability to produce complex structures and customized products. This paper aims to study the characteristics of stainless steel 316L (UNS S31603) parts produced using a selective laser melting machine. In the aerospace industry, turbine blades are typically manufactured from nickel-based alloys, titanium alloys, and stainless steels. Several geometries typical of airfoil blades were examined. The main goal is to investigate the material characteristics and surface features of the airfoil blades. The study included the geometrical errors, surface microstructures, material compositions, material phases, and residual stresses of the samples produced. The characteristics of the parts produced were investigated based on experimental observations. The paper also discusses the influence of the part dimension and orientation on the profile error, surface microstructure, and residual stress.

Mostafa Yakout

Papers

Density and mechanical properties in selective laser melting of Invar 36 and stainless steel 316L

The selection of process parameters in additive manufacturing for aerospace alloys

A Review of Metal Additive Manufacturing Technologies

A study of thermal expansion coefficients and microstructure during selective laser melting of Invar 36 and stainless steel 316L

On the characterization of stainless steel 316L parts produced by selective laser melting