An alloy, called FSLA (free-sintering low-alloy), was designed and implemented for use with binder jet printing. This work focuses on various heat treatments that can be utilized with the alloy to produce a range of properties for various applications. The microstructure of the alloy can be varied post-sintering, by heat treatment, to give a wide range of mechanical properties that are suitable for automotive components. Balancing the carbides in the structure along with the amounts of ferrite and martensite|bainite give a range of ultimate tensile strengths from approximatelt 490 to 1000 MPa. Unlike other low alloy steels, the material has been designed to have a high degree of sinterability which allows densities of the order of 98% to be reached while sintering at temperatures typically used in metal injection molding (1380 degrees Celcius).

The need to use low-alloy steel powders for laser-beam powder-bed fusion (PBF-LB) processing has increased as the importance of additive manufacturing (AM) has grown for prototyping and manufacturing of structural and automotive parts. Selecting an alloy that can be processed to cover a wide range of properties is important as this limits the development of build parameters and material changes in the PBF-LB equipment. A 4600 low-alloy steel, which is used in multiple powder and wrought metallurgy processes, was chosen because of the properties achievable through both carbon variation and heat treatment. The mechanical properties of the PBF-LB specimens were evaluated using variations in carbon content in both water- and gas-atomized powders. Various heat treatments showed the range of mechanical properties that can be achieved with this alloy. Microstructure was evaluated for samples built with both powder types and is discussed in relation to build parameters and the mechanical properties.

Iron-copper and iron-copper-carbon have been significant alloy compositions in the ferrous powder metallurgy industry for decades. In most applications in which these alloys are used, the copper is mixed into an iron or low-alloy steel base powder as an elemental particulate additive, with alloying accomplished by diffusion during sintering. In this manuscript, the effect of sintering on the development of the microstructure is investigated, where the copper particles melt and flow into the interconnected pore structure, relocate within the iron-base matrix, and create more interfacial area for diffusion. With this combination of enlarged copper-iron contact area and the presence of the liquid phase, diffusion is improved and enhancements in both physical and mechanical properties are realized. The mechanisms for both alloying and the evolution of the microstructure are described through changes in both compact temperature and the time at sintering temperature. It is demonstrated that alloying strengthens not only the surfaces of the particles, but also many of the most vulnerable sites in the compact, i.e., the locations where neighboring particles are forced into contact during compaction. These sintered particle-to-particle contacts are frequently small in cross-sectional area and often the weakest sites in the final microstructure. Fractography is used to show the change in fracture behavior as copper diffusion is affected by sintering times and temperatures.

Characterization of metal powders for the aerospace industry (titanium, superalloys, stainless steel, and high-strength low-alloy steels) involves documenting the average particle size, particle size distribution, surface area, flowability, apparent density, tap density, moisture content, and porosity in a powder. These attributes are in many cases, considered minimum required information by the end user. Frequently, the manufacturer of these powders provides a certificate of analysis, which contains this information. Additional characterization utilizing metallographic techniques is often provided to optimize the powder for each application. This chapter will review basic powder testing used as quality control tools for powders as well as the advanced metallographic techniques used to optimize the powder for a specific process.

A new gas atomizer using induction skull melting has been utilized to produce a series of standard ASTM titanium alloys and new research alloys. The utilization of titanium scrap to produce a more cost-effective end product is explored. The effect that various types of scrap have on the resulting oxygen, nitrogen, carbon, and hydrogen levels is studied. In addition, the segregation and oxidation of volatile elements such as, aluminum, vanadium, and tin are studied using chemical analysis and scanning electron microscopy (SEM) with energy dispersive techniques. The microstructure and nonmetallic inclusion level are characterized in the as-atomized powder. Shape, porosity, and inclusion analysis of the various grades of powder is performed via standard metaliographic and image analysis techniques.

Customized test techniques using combinations of light and electron microscopy, computed tomography, and light scattering have been developed to evaluate the characteristics of both the powder feedstocks used in additive manufacturing and the finished parts. By combining these techniques, a more accurate assessment of additive manufacturing products is possible because each provides specialized benefits to the evaluation. Some are limited to an individual type of data, while others are more versatile and provide information in several areas of interest. The high-resolution capabilities of the microscopy techniques increase the accuracy of linear and areal measurements, and the scanning electron microscopy enhancement of performing chemical analysis provides additional unique benefits. Computed tomography has the ability to view particles and parts as three-dimensional features, finding internal defects, porosity, etc. The light-scattering methods are valuable in being able to estimate particle size into the range from nanometer to millimeter. This technical paper will present both benefits and drawbacks of these techniques in hopes of finding the best combination of tests to characterize the additive manufacturing materials and products efficiently and accurately.

The choice of alloying method for ferrous powder metallurgy alloys is often dictated by the oxidation potential of the alloying elements used in powder and compact manufacture. Silicon is effective in improving properties of ferrous PM steels; however, if added to the melt prior to atomization, the likelihood of oxidation is high. Alternatively, Si-rich and more complex particulates can be combined with the base powder, i.e., iron or low-alloy steel, with alloying occurring by solid-state diffusion during sintering. The effectiveness of these additives to improve material properties is dependent on their distribution throughout the material volume, as determined by diffusion of each added element. In this study, the distribution of silicon was quantified using energy-dispersive spectroscopy on an iron-based alloy composition. These data were compared with the volume fraction of the various transformation products in cross sections of hardenability samples using both light and electron microscopy. Predictions on the effects of local chemical composition, cooling rate, and microstructure are made.

The metallic powders used as feedstock for the additive manufacturing (AM) process require evaluation of several morphological and chemical composition characteristics to ensure predictable material behavior during the manufacturing process and performance in use. The size, shape, and internal porosity of the particles are several traits requiring evaluation. Often, numerical estimates representing these characteristics are made using the digital images generated with X-ray computed tomography (CT) scans or automated image analysis (AIA). Image analysis offers several advantages over CT scans. These include lower equipment and individual test costs, higher image resolution due to higher system magnifications, and faster analysis time. In addition to the morphological analyses, particle-to-particle chemical composition uniformity is essential to maintaining the proper microstructure of the part. A procedure using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) is described to determine the presence of particulate cross-product contamination and any foreign materials.

Although the binder-jetting additive manufacturing (AM) process differs from the traditional powder metallurgy (PM) process of pressing and sintering, the technological intersection of the two processes is a need to sinter the "green" parts. In this study, comparisons are made between the two production methods with respect to overall density, mechanical properties, microstructure, and pore structure as a result of sintering variations. Vacuum sintered, binder jetted SS-316L tensile test samples exhibited robust mechanical properties in relation to their sintered density (similar to 7.4 g/cm(3)) due to the unique combination of fine powder size (d(50) similar to 9 mu m), production method, and sintering method. This is analyzed and explained through porosity and microstructural analyses in comparison with typical press-and-sinter results. The tensile test bars were sintered in a low partial pressure of hydrogen at temperatures from 1,260 degrees C to 1,380 degrees C (2,300 degrees F to 2,516 degrees F).

An integral part of producing titanium parts by additive manufacturing (AM) is providing test data that demonstrate the quality, uniformity, and consistency of the products. Typically, chemical, mechanical, physical, and metallographic tests are performed to ensure parts meet or exceed the expectations and requirements of each application. Each test discipline provides unique information on the properties and expected behavior of the AM produced parts. The contributions of metallographic testing on quantifying some of the attributes of the parts are discussed in this paper. The microstructural constituents of internal porosity, nonmetallic inclusion content, and surface texture are evaluated and discussed. Light-optical microscopy using automated image analysis (IA) and stereo microscopy techniques are employed in this analysis.

Metal powders used in additive manufacturing are required to have a different set of physical particle characteristics corn pared with the more traditional powder metallurgy (PM) grades. The desired particle-size distribution is finer and the shape nearly spherical, in contrast to the PM-grade particles that are larger in size and more irregular in shape. Fine, nearly spherical particles are desired for additive manufacturing since they flow faster, with more uniformity, and fill the additive manufacturing powder beds more efficiently than the regular powder metallurgy grades. As a consequence, metallographic testing designed to evaluate the shape and surface texture of the particles requires preparation techniques that accommodate both the fine particle size and the minimized surface area. From mounting the loose powders, through grinding and polishing, a procedure is offered in this report to account for these particle characteristics. Once the samples are prepared, automated image analysis techniques are presented to evaluate the shapes and surface textures of the particles. These explore the use of measurement ratios, comparison with a known geometric shape, and a technique measuring a systematic arrangement of radii drawn from the metallographically prepared, two-dimensional particle centroid. Graphic and statistical treatments of the data are offered.

The microstructures of powder metallurgy steels are typically pearlitic in the as-sintered condition or martensitic after-heat treatment (quench-hardening or sinter hardening). By design of the alloy system, a bainitic microstructure can be produced in a powder metallurgy steel after sintering. In the wrought ferrous industry, bainitic steels are known to exhibit high strength and hardness and a high level of toughness. The aim of this research was to develop a bainitic microstructure in a powder metallurgy steel with a high level of strength and toughness and acceptable ductility. The study focused on alloy composition and attendant mechanical properties and microstructure. In addition, processing routes were examined to achieve bainitic and mixed (pearlitic/bainitic/martensitic) microstructures with the potential to enhance mechanical properties.

AbstractMetallographic analysis is primarily a collection of visual and imaging techniques that provide an insight into the background of a material or part and its behavior. Metallic specimens, both porous and pore-free, are opaque, and as a result, an optical examination must be performed on carefully prepared planar (two-dimensional) surfaces. This article discusses the preparation sequence of ferrous powders, which is normally separated into several well-defined steps: sample selection, sectioning, mounting, grinding, polishing, drying, and chemical etching and/or coating. It provides several suggestions to promote and encourage the safety of those performing metallographic preparation and analysis.