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Additive Manufacturing
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Additive Manufacturing

Additive manufacturing, also known as direct digital manufacturing, is the focus of many of the efforts to reduce costs in the production of titanium parts. Additive manufacturing is a family of processes in which parts are first modeled in a CAD program that “slices” them into thin layers. Parts are then built up layer by layer in specialized machines according to the pattern in the program. The machines build up the layers by laser-sintering powder; or by injecting powder at high speed onto a substrate; or by injecting powder into a high temperature plasma, laser beam, or electron beam, where it is melted before striking the substrate.

The major benefits of additive manufacturing are reduced waste, speed of production, elimination of the need for tooling, and the production of near-net-shape parts. Because parts are built layer by layer, it is possible to design internal features and passages that could not otherwise be produced. Complex geometries and assemblies with multiple components can be fabricated as a single part, improving reliability and reducing labor cost.

Therefore, titanium manufacturers and fabricators are developing these technologies to reduce cost and material waste while still providing the unique benefits of titanium. Technologies such as electron beam-direct digital manufacturing, direct metal laser sintering, laser-engineered net shapes, and ion fusion formation were discussed in several presentations.

Northrop Grumman, Honeywell Aerospace, Oak Ridge National Laboratory, U.S. Army ARDEC, and W.A. Gooch Consulting reported on several programs at the conference. The Northrop Grumman presentation reported that advances in electron beam direct digital manufacturing (EB-DDM) have the potential for improved properties and reduced costs for aerospace components. During this process, the electron beam melts metal powder in a layer-by-layer process to build the part. A major benefit is that near-net-shape designs are no longer limited by conventional manufacturing capabilities, and the technology enables fabricating geometries that are challenging or too expensive to manufacture by conventional means. Furthermore, because of the material efficiency of the process, the buy-to-fly ratio is significantly reduced.

Faster deposition rates and dimensional control are key to cost savings, and the ability to reduce cost is very dependent on geometry. However, certain challenges remain to be addressed. For example, the current surface finish of electron-beam DDM metal components results in an excessively rough surface for fatigue-rated components. The uneven finish prohibits use of baseline nondestructive inspection techniques such as dye penetrant, eddy current, and ultrasonics.

Another issue is the high cost of powder raw material stock, which raises costs. Finally, the application of the technology is limited because of the lack of specific design capabilities and tools within the majority of the engineering community. At Oak Ridge National Laboratory, researchers are working with additive manufacturing providers to develop high-performance materials, low-cost feedstocks, efficient processing techniques, and in-situ characterization and controls. For example, Oak Ridge is collaborating with Arcam on electron beam melting (EBM) technology, to increase the material deposition rate and the build volume. Lockheed Martin is collaborating with Oak Ridge to build titanium brackets for an aerospace application via the EBM process.

The technology has the potential to reduce cost of the part by over 50%, and to reduce the buy-to-fly ratio from the current 33:1 down to 1.5:1. Honeywell Aerospace reported on additive manufacturing of titanium alloys by direct metal laser sintering, electron beam melting, and ion fusion formation. During direct metal laser sintering (DMLS), the powder is fused into a solid part by melting it in specific patterns with a focused laser beam having power of 200 to 400 watts. Parts are built up in layers only about 20 microns thick.

Highly complex geometries can be built directly from CAD data, without the need for tooling. Parts are net-shape, with high accuracy, good surface quality, and excellent mechanical properties. Build speed is slower than electron beam, but the surface finish is better. Typical dimensional accuracy for Ti-6Al-4V components is +/- 0.005 inch. Ion fusion formation (IFF) is based on a plasma welding torch that melts wire or powder feedstock. The hot plasma (usually consisting of argon ions) is directed to a predetermined spot on the workpiece, and the feedstock is introduced into the plasma stream to produce a pool of molten material at that location. Parts can be built in almost any three-dimensional shape, with high precision and predictable properties.

IFF could be a game-changing technology, potentially suitable for difficult- to-manufacture components, and offers the possibility of producing functionally graded materials. Products can be used as-deposited, or post-deposition processed, and/or machined. IFF equipment is not complicated to operate, has low initial capital costs, requires little maintenance, and has low operating costs. It is less expensive than electron beam and laser melting technologies, because the feedstock is melted by electrical energy rather than expensive lasers or electron beams.

Challenge areas are that wire feedstock products require post machining, and low-power control/fine feature equipment is currently not installed. Multi-axis build and thermal management capability complicate programming, but also provide more flexibility. The largest barrier to entry into the aerospace marketplace is machine size, because the build envelope of 4 x 4 x 6 feet is too small for many aerospace structures.

Although aerospace is the largest market for titanium, the armed services recognize that it is a key material to meet military needs for higher strength, lower weight vehicles and components, better ballistic performance, and improved corrosion resistance. Because of this, the U.S. Army Armament Research and Development Center (ARDEC) works to improve processes such as near net shape technologies, forging, casting, low-cost powders, and advanced machining. ARDEC’s presentation covered the ManTech titanium programs that focus on reducing costs, improving manufacturability, developing new processes, and testing new titanium alloys.

Specifically, additive manufacturing processes at ARDEC include electron beam melting, laser cladding, and laser-engineered net shaping (LENS). LENS is a process in which metal powder is injected into the focused beam of a high-power laser under tightly controlled atmospheric conditions. The focused laser beam melts the surface of the base material in a small area, and the powder is absorbed into the molten pool.

The resulting deposits range from 0.005 to 0.040 inches thick, and may then be used to build, repair, or clad metal parts for a variety of different applications. The key benefits of this process over traditional techniques are a metallurgical bond; relatively low heat input; minimal stress and distortion created by deposits; and rapid cooling rates.

The process is suitable for repairing shafts, cladding aerospace components, and manufacturing free-form parts. In addition to titanium, suitable alloys include nickel, cobalt, Inconel, stainless steel, and hardfacing alloys. Another major military application for titanium is armor. The W.A. Gooch Consulting presentation first described several titanium armor alloys, listing their chemical compositions and mechanical properties, then discussed several advanced processing methods.

For example, BAE Advanced Materials (now CoorsTek Vista), has developed a process to hot press large near-net-shape functionally gradient titanium-base tiles in a single stage. On a titanium metal substrate, titanium and titanium/titanium diboride (TiB2) powder mixtures form a titanium monoboride (TiB) hard face that grades through intermediate layers. The TiB ceramic is formed through reaction sintering between the TiB2 and titanium powders during the hot-press phase. TiB is densified as a cermet (ceramic in a metal matrix) to aid in fabrication.

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