Many engineering applications, particularly in extreme environments, require components with properties that vary with location in the part. Functionally graded materials (FGMs), which possess gradients in properties such as hardness or density, are a potential solution to address these requirements. The laser-based additive manufacturing process of directed energy deposition (DED) can be used to fabricate metallic parts with a gradient in composition by adjusting the volume fraction of metallic powders delivered to the melt pool as a function of position. As this is a fusion process, secondary phases may develop in the gradient zone during solidification that can result in undesirable properties in the part. This work describes experimental and thermodynamic studies of a component built from 304L stainless steel © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ incrementally graded to Inconel 625. The microstructure, chemistry, phase composition, and microhardness as a function of position were characterized by microscopy, energy dispersive spectroscopy, X-ray diffraction, and microindentation. Particles of secondary phases were found in small amounts within cracks in the gradient zone. These were ascertained to consist of transition metal carbides by experimental results and thermodynamic calculations. The study provides a combined experimental and thermodynamic computational modeling approach toward the fabrication and evaluation of a functionally graded material made by DED additive manufacturing.

Interest in additive manufacturing (AM) has dramatically expanded in the last several years, owing to the paradigm shift that the process provides over conventional manufacturing. Although the vast majority of recent work in AM has focused on three-dimensional printing in polymers, AM techniques for fabricating metal alloys have been available for more than a decade. Here, laser deposition (LD) is used to fabricate multifunctional metal alloys that have a strategically graded composition to alter their mechanical and physical properties. Using the technique in combination with rotational deposition enables fabrication of compositional gradients radially from the center of a sample. A roadmap for developing gradient alloys is presented that uses multi-component phase diagrams as maps for composition selection so as to avoid unwanted phases. Practical applications for the new technology are demonstrated in low-coefficient of thermal expansion radially graded metal inserts for carbon-fiber spacecraft panels.

The current work provides an overview of the state-of-the-art in polymer and metal additive manufacturing and provides a progress report on the science and technology behind gradient metal alloys produced through laser deposition. The research discusses a road map for creating gradient metals using additive manufacturing, demonstrates basic science results obtainable through the methodology, shows examples of prototype gradient hardware, and suggests that Compositionally Graded Metals is an emerging field of metallurgy research.

Spontaneous Sn whisker growth on Cu leadframe finished with Pb-free solder is a serious reliability problem in electrical and electronic devices. Recently, Fortune magazine had an article to describe the urgency of the problem. The spontaneous growth is an irreversible process, in which there are two atomic fluxes driven by two kinds of driving force. There are a flux of Cu atoms and a flux of Sn atoms. The Cu atoms diffuse from the leadframe into the solder finish driven by chemical potential gradient to form intermetallic compound of Cu 6 Sn 5 in the grain boundaries of the solder, and the growth of the compound at room temperature generates a compressive stress in the solder. To relieve the stress, a flux of Sn atoms driven by the stress gradient diffuses away to grow a spontaneous Sn whisker which is stress-free. The typical industry solution is to insert a diffusion barrier of Ni between the Cu and solder to prevent the diffusion of Cu into the solder. It is insufficient, because we have to uncouple the irreversible processes and stop both the fluxes of Cu and Sn. A solution is presented here.