Krista N. Amato

Metal Fabrication by Additive Manufacturing Using Laser and Electron Beam Melting Technologies

Selective laser melting (SLM) and electron beam melting (EBM) are relatively new rapid, additive manufacturing technologies which can allow for the fabrication of complex, multi-functional metal or alloy monoliths by CAD-directed, selective melting of precursor powder beds. By altering the beam parameters and scan strategies, new and unusual, even non-equilibrium microstructures can be produced; including controlled microstructural architectures which ideally extend the contemporary materials science and engineering paradigm relating structure-properties-processing-performance. In this study, comparative examples for SLM and EBM fabricated components from pre-alloyed, atomized precursor powders are presented. These include Cu, Ti-6Al-4V, alloy 625 (a Ni-base superalloy), a Co-base superalloy, and 17-4 PH stainless steel. These systems are characterized by optical metallography, scanning and transmission electron microscopy, and X-ray diffraction.

Fabrication of Metal and Alloy Components by Additive Manufacturing: Examples of 3D Materials Science

Objective This paper provides a brief review of relatively new additive manufacturing technologies for the fabrication of unusual and complex metal and alloy products by laser and electron beam melting. A number of process features and product microstructures are illustrated utilizing 3D optical and transmission electron microscope image compositions representing examples of 3D materials science. Methods Processing methods involving electron beam melting (EBM) and a process referred to as direct metal laser sintering (DMLS), often called selective laser melting (SLM) are described along with the use of light (optical) microscopy (OM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to elucidate microstructural phenomena. Results Examples of EBM and SLM studies are presented in 3D image compositions. These include EBM of Ti-6Al-4V, Cu, Co-base superalloy and Inconel 625; and SLM of 17-4 PH stainless steel, Inconel 718 and Inconel 625. Conclusions 3D image compositions constituting 3D materials science provide effective visualization for directional solidification-related phenomena associated with the EBM and SLM fabrication of a range of metals and alloys, especially microstructures and microstructural architectures.

Microstructures and Properties of 17-4 PH Stainless Steel Fabricated by Selective Laser Melting

Objective This research examines 17-4 PH stainless steel powders produced by atomization in either argon or nitrogen atmospheres (producing martensitic (α-Fe) or mostly austenitic (γ-Fe) phase powders, respectively) and correspondingly fabricated by selective laser melting (SLM) in either a nitrogen or argon atmosphere. Methods Pre-alloyed 17-4 stainless steel powders prepared by atomization in either argon or nitrogen atmospheres were fabricated by SLM. The initial powder microstructures and phase structures were examined by light (optical) microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Prototypes fabricated by SLM were similarly characterized, and in addition transmission electron microscopy (TEM) characterization was also performed. Results Martensitic powder fabricated by SLM in nitrogen gas produced a martensitic product while pre-alloyed austenitic powder produced a primarily austenitic product. In contrast, both powders produced martensitic products when fabricated by SLM in argon gas. This unusual behavior occurred because of the rapid cooling affected by nitrogen versus argon cover gas as a consequence of a 40% greater thermal conductivity of nitrogen gas versus argon gas. SLM fabricated martensitic products exhibited HRC 30 in contrast to HRC 43 when aged at 482°C for 1 hour. Austenitic products did not exhibit age-hardening. Conclusions Using an argon cover gas, SLM-fabricated products are martensitic (and magnetic) with either an austenitic or martensitic pre-alloyed 17-4 PH stainless steel powder. Using a nitrogen cover gas, the product phase is the same as the precursor powder phase (austenitic or martensitic).

Open-Cellular Co-Base and Ni-Base Superalloys Fabricated by Electron Beam Melting

Reticulated mesh samples of Co-29Cr-6Mo alloy and Ni-21Cr-9Mo-4Nb alloy (625) and stochastic foam samples of Co-29Cr-6Mo alloy fabricated by electron beam melting were characterized by optical metallography, and the dynamic stiffness (Young’s modulus) was measured by resonant frequency analysis. The relative stiffness (E/Es) versus relative density (ρ/ρs) plotted on a log-log basis resulted in a fitted straight line with a slope n ≅ 2, consistent with that for ideal open cellular materials.

Heat treatment and oxidation characteristics of Nb-20Mo-15Si-5B-20(Cr,Ti) alloys from 700 to 1400°C

Nb-20Mo-15Si-5B-20(Cr,Ti) alloys have been subjected to annealing treatment for 2 hours in a range of temperatures from 700 to 1400oC and quenched in water to perform the microstructural characterization for these two new alloys. As cast structure consists of a solid solution phase, , and silicides (Nb5Si3) in both alloys but Cr alloy also contains NbCr2 and Nb3Si phases. Heating to higher temperature introduces Ti silicides in the microstructure for Ti alloy. The oxidation in air has been conducted on these alloys in the same temperature range for 24 hours. Weight gain per unit area as a function of temperature provides the oxidation curves while characterization techniques using SEM, EDS on SEM, x-ray mapping, and XRD has yielded the analyses of the oxide scale. The scale consists of various oxides of Nb, Mo, Cr, Si, and Ti. Cr alloy appears to offer higher oxidation resistance in the selected range of oxidation temperatures.

3D Microstructural Architectures for Metal and Alloy Components Fabricated by 3D Printing/Additive Manufacturing Technologies

The layer-by-layer building of monolithic, 3D metal components from selectively melted powder layers using laser or electron beams is a novel form of 3D printing or additive manufacturing. Microstructures created in these 3D products can involve novel, directional solidification structures which can include crystallographically oriented grains containing columnar arrays of precipitates characteristic of a microstructural architecture. These microstructural architectures are advantageously rendered in 3D image constructions involving light optical microscopy and scanning and transmission electron microscopy observations. Microstructural evolution can also be effectively examined through 3D image sequences which, along with x-ray diffraction (XRD) analysis in the x-y and x-z planes, can effectively characterize related crystallographic/texture variances. This paper compares 3D microstructural architectures in Co-base and Ni-base superalloys, columnar martensitic grain structures in 17–4 PH alloy, and columnar copper oxides and dislocation arrays in copper.