Intricate parts from the inside out

Sept. 4, 2003
Additive metal-deposition process lets designers use novel shapes, features, and material combinations to build parts that would be difficult or impossible to machine.

Richard Grylls
LENS Team Leader
Optomec Inc.
Albuquerque, N.M.

Designers are familiar with how a concept becomes a real product: Part of the process involves throwing it 'over the wall' so manufacturing can determine how, or if, the part can be built. A new design often bounces back and forth several times between designers and production engineers until it's transformed into something 'manufacturable.'

The problem is, key features that reduce part weight or add strength in critical areas may be lost in the shuffle. In the worst case, a good design may be discarded because manufacturing limitations make it impractical to produce.

A new low-volume production technology reduces the manufacturing-related limitations on designs. Laser Engineered Net Shaping, or LENS, makes near-net-shape metal parts directly from CAD data. Depositing metals in an additive process, it produces parts with material properties equal to or better than those of conventional wrought materials.

Unlike machining, which makes parts from the outside in, LENS parts are built from the inside out. This lets designers prototype and manufacture metal parts with shapes and features that would be difficult or impossible to machine. The technique, for example, can build ultrathin parts more than an inch tall with depth-to-diameter aspect ratios up to 70:1. This contrasts to machining aspect ratios that are limited to about 10:1.

In addition, it easily manufactures products from hard-to-machine materials such as titanium. And it deposits material combinations that were once impractical and/or prohibitively expensive. In a single step, the process produces multimaterial structures with gradual material transitions and negligible internal stress.

Process basics

Originally developed at Sandia National Laboratories, Albuquerque, LENS machines dispense metal powders in patterns dictated by 3D CAD models. Guided by these computerized blueprints, the systems create metal structures by depositing them a layer at a time.

To start the process, a high-powered Nd:YAG laser beam strikes a tiny spot on a metal substrate, producing a molten pool. A nozzle blows a precise amount of metal powder into the pool to increase the material volume. A layer is built to the CAD geometry as the positioning system moves the substrate under the beam in the XY plane. The lasing and powder-deposition process repeats until the layer is complete. The system then refocuses the laser in the Z direction until the unit builds layer upon layer a metal version of the CAD model.

The deposition process takes place inside a sealed chamber, where environmental variables are tightly controlled. For example, the chamber can maintain an argon atmosphere with oxygen levels of less than 10 ppm. This is essential when manufacturing parts made from aluminum, which are plagued by an oxide that prevents the material from properly wetting the deposited layer. Thanks to the almost oxygen-free atmosphere inside the chamber, the system produces aluminum parts that can't be made by conventional manufacturing processes.

Material advantages

A variety of materials, including titanium, stainless steel, tool steel, cobalt, and Inconel are candidates for the process. These metals cool quickly and solidify with fine-grained microstructures. They have greater strength than wrought materials, with no loss of ductility. Metallographic analysis shows the average grain size of LENS-deposited
H-13 tool steel is just 3µm - an order of magnitude smaller than those typically found in H-13 tool steels. Compared to material property data found in reference sources such as the ASTM Handbook on Powder Metals overall material properties of LENS parts equal or exceed those of wrought materials letting designers build smaller, thinner, lighter, and less-expensive parts.

The small grain size and homogeneity of LENS materials may be key to their superior corrosion resistance. This is especially important to designers molding polyvinyl-chloride (PVC) parts. PVC emits chlorine, which combines with moisture in the air to form hydrochloric acid which attacks and ruins injection-molding tools.

To make PVC injection molds last longer, manufacturers use corrosion-resistant materials such as 420 stainless steel. In a recent test conducted by Anderson Laboratories, an independent testing laboratory, LENS-produced 420 stainless steels corroded 30% more slowly than their wrought counterparts.

Process pluses

Besides good material properties, the process offers other advantages over conventional manufacturing techniques. In contrast to machining that removes material from a block of metal, LENS deposits materials only where needed thus eliminating material waste. In addition, it makes parts directly from CAD files, without intermediate steps. This reduces manufacturing costs, product development time, and time to market.

The process also dispenses different metals in combination to create mixed-material parts. This lets designers specify different materials for different areas of a part, depending on requirements. Designers needn't build an entire part from an expensive, wear-resistant material. Instead, they can deposit wear-resistant materials on part surfaces, where needed, using a less-expensive material for the part interior.

Or consider a rotating part that must be strong at the attachment point in the center and lightweight at the outer edges. In some cases, a designer could alter the part geometry to produce the required qualities. But manufacturability considerations may limit design freedom. Part geometry needs no alteration with LENS - the process can build a two-material part with strong centers and lightweight outer edges.

LENS can produce both abrupt and gradual transitions from one material to another. The latter is advisable when the two materials have large disparity in their coefficients of thermal expansion. For example, consider a mold of copper and steel. Copper expands twice as much as tool steel when heated. If the mold has an abrupt transition between the two materials, their interface will see a great deal of stress as the mold thermally cycles during the molding process.

Programmed for gradient transition, however, a LENS machine gradually changes the expansion coefficient of the deposited layers. This reduces the possibility that the part will fail at the material interface.

LENS as a design tool

When developing most metal components, designers often make prototypes to evaluate product form, fit, and function. But most conventional rapid-prototyping systems make sample parts out of paper, polymers, ceramics, and porous metals. These prototypes serve well as a study for product form and fit. But in many cases they fall short during functional tests because their materials aren't the same as those in the actual products.

In contrast to other prototyping processes that produce parts that ultimately must be made some other way, LENS can also handle low-volume production runs. Fully functional metal prototypes generated from CAD solid models let designers develop more innovative product designs as well as modify or rework existing prototypes to test design changes - designers needn't fabricate an entirely new part with each minor design iteration.

Many low-volume end products are made of specialty materials that are difficult to fabricate. High-volume tooling is often prohibitively expensive to justify in these cases. Parts, therefore, are often the product of costly, labor-intensive manual processes. In a variety of industries, however, automated LENS machines cost-effectively manufacture small numbers of metal parts quickly.

Unlike conventional milling and machining operations, LENS machines can make parts with a wide range of complex internal geometries including hollow or honeycomb interiors. These parts are lighter than their solid counterparts, but still strong enough for application needs.

In addition, the process turns titanium powders into a wide variety of end products. Titanium's high strength-to-weight ratio makes it suitable for many low-volume products in the aerospace, defense, medical, and automotive industries. But titanium and its alloys are hard, tough materials that turn machining into a slow and costly process.

Machining titanium also generates a great deal of material waste. In the aerospace industry, for example, the typical 'buy-to-fly' ratio for machined titanium (the amount of metal that must be purchased versus the amount of metal actually needed to make the aircraft) is about 15:1. This translates to about 90% of this costly material being scrapped.

By adding precise amounts of material rather than milling it away LENS slashes production times and dramatically reduces material costs. It also produces titanium components having longer service life and better material properties than forged alternatives.

The process will soon benefit from ongoing developments to increase the economic feasibility of titanium manufacturing by boosting deposition rates. Faster deposition should make LENS considerably less expensive than other methods of making titanium parts.

Building implants with longevity
In the medical industry, titanium is used to make surgical implants. Today, these implants are normally machined or forged. But LENS can economically make implants with novel geometries and material compositions that lengthen product life, thereby boosting the quality of life for patients.

Implant designs can take advantage of improved material properties provided by the process. For example, high-strength LENS materials let designers use thinner implant stem thickness so more bone material can remain. There's a better integration between the implant and native tissue.

In addition, LENS materials can reduce implant wear at key points. In one case, LENS-made titanium implants employed a composite outer surface in areas subject to excessive wear. Conventional plating techniques deposit only thin coats of titanium carbide, necessitating a multistep plating process to deposit a thick coat. But in a single step, a LENS nozzle can deposit a thick protective coat simply by making several passes over the part. What's more, the process can make an engineered transition between two materials, creating a better bond between the plating material and the underlying surface.

The process can also be a boon for mold designers. The additive manufacturing technique makes it easy to add mold features that are beyond the capabilities of other production methods. So designers can add complex but crucial features such as curving hollow passages. These passages help reduce cooling times before parts eject from a mold. Production comes to a halt during these cooling periods, which can account for up to 70% of the total cycle time.

Normally, cooling channels are drilled into a tool after fabrication. But drilling imposes straight-line limitations on channel paths, so they can't follow the contours of the mold surface.

Thanks to LENS, however, mold designers can add three-dimensional channels that conform to the surface of a tool. Conformal channels cut cooling times to a fraction of that required by molds with straight-line cooling passages, slashing cycle times and boosting mold output.

To accelerate cooling and cut cycle times even further, the process lets designers add a thermally conductive material to a mold. In one case, the technology built a copper core into a mold with a 420 stainless-steel surface. During molding operations, the thermally conductive core rapidly conducts heat from the steel surface and transfers it to conformal cooling channels. This arrangement reduces cycle times by 15%.

LENS additive manufacturing systems dispense metal powders in patterns dictated by three-dimensional CAD models.

A high-powered Nd: YAG laser beam strikes a tiny spot on a metal substrate, producing a molten pool. A nearby nozzle blows a precise amount of metal powder into the pool to increase the material volume.

The LENS process easily produces parts with unusual or complex internal geometries, such as the honeycomb interior of the part pictured here.

Titanium hip replacement created by LENS additive manufacturing process.

The promise of mesoscale deposition

Two quite different technologies dominate microelectronic fabrication: thick film and thin film. In thick-film processes, stencil printing applies paste or ink patterns to a substrate. Capacitors, inductors, resistors, and conductive lines can be created by successive applications, followed by firing at up to 1,000°C.

Thick-film technology is simple and relatively inexpensive but has limitations. For example, the high temperatures it requires would damage flexible polymer substrates. Also, stencil printing typically can't produce lines and spaces smaller than 100 µm.

To produce tiny features on densely packed devices, manufacturers normally use thin-film processes such as sputtering or chemical-vapor deposition. In thin-film processes, materials are patterned via masks and photoresists. Although complicated and costly, thin-film processes can produce complex devices with submicron features.

But features between 1 and 100 µm wide as are common in microelectronics may be difficult to produce consistently using conventional thick and thin-film processes. IC packaging, MEMS, embedded passives, RF filter patterns, and other microelectronics can now be built using a LENS technology that was developed through the Defense Advanced Research Projects Agency (DARPA). The process is based on Maskless Mesoscale Materials Deposition (M3D) technology and can produce mesoscale electronic features.

With M3D, electronic materials deposit onto low-temperature, nonplanar substrates without masks or other thin-film equipment. Simpler and less expensive than thin-film techniques, M3D creates a wide variety of electronic components, devices, and interconnects with features as small as 10 µm. Potential applications include microsensors, microantennae, fuel cells, flat-panel displays, flip-chip packaging, and high-density interconnects.

LENS can be used to create tools, or repair damaged tools that would normally be discarded by adding replacement material to the worn or fractured surface.

A design model of a mold created with a LENS system includes conformal cooling passages. These passages are not feasible with conventional drilling processes.

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