A continuing effort to devise relatively inexpensive means of manufacturing titanium-alloy parts has been focused on a free-form fabrication approach. As used here, “free-form fabrication” refers generally to any or all of a number of methods and processes denoted, variously, as rapid prototyping or three-dimensional (3D) printing.

Addition of Ductwork and a Fan to a commercially available 3D-printing machine is among the modifications made to reduce the risk of explosion from airborne titanium-alloy dust that could be generated during operation.

Freeform fabrication is a form of computer-aided manufacturing that involves the use of data on the 3D shape and size (hereafter called “3D data”) of a part that one seeks to fabricate. The 3D data are obtained from a computer-aided design (CAD) file or from a 3D digitizing system. Specialized software converts the 3D data into layered 2D data. These layered data are used by computer-controlled machinery in a process in which the part is formed by laying down one or more liquid, powder, and/or sheet material(s) in a succession of layers, point by point within each layer.

For the development of the present approach to free-form fabrication of titanium-alloy parts, a commercially available 3D-printing machine has been modified to add safety features to eliminate an explosion hazard that would be posed by airborne titanium-alloy dust. These measures consist mainly of enclosure of the volume in which the part is formed and addition of a forcedair system to purge the volume during operation (see figure).

In preparation for fabrication of a part, a titanium-alloy powder is mixed with a small amount (typically a few weight percent) of a binder material (which typically contains ammonium molybdate) in powder form and, optionally, with a carbonyl iron powder. The 3D-printing machine is used to lay down the powder mixture to form a part to nearly net size and shape.

The part as thus formed is heated to increasing temperature in a flowing argon/hydrogen atmosphere mixture with holds at various temperatures (typically from 230 to 850 °C) to remove the volatile chemical elements of the binder and carbonyl and to effect pre-sintering. Finally, the part is sintered by heating to a higher temperature (typically, 1,440 °C) in a vacuum to form a unitary (albeit somewhat porous) piece of molybdenum alloy. The iron powder contributes to reduction of porosity in that during sintering, it results in the formation of a transient iron-rich liquid phase that enhances densification.

It is planned, for future development efforts, to investigate the use of hot isostatic pressing as a means of bringing as-sintered titanium-alloy parts up to full density. It is also planned to investigate use of finiteelement modeling to simulate absorption of heat in, and shrinkage of, parts in order to devise means of controlling distortions and thereby bringing the parts closer to desired final sizes and shapes.

This work was done by Corby G. Anderson and John J. Krstulich of the University of Montana, and Stephen (Drew) Wilkerson of the Army Research Laboratory.


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Inexpensive Free-Form Fabrication of Titanium-Alloy Parts

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This article first appeared in the December, 2007 issue of Defense Tech Briefs Magazine.

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