Understanding Beryllium

BY JIM BENES
ASSOCIATE EDITOR

Stiffness and light weight of AlBeMet aluminum-beryllium alloy (top) from Brush Wellman makes it ideal for rotating optics mounts on Apache helicopters (left).

BERYLLIUM IS A MATERIAL LIKE NO OTHER. From a design standpoint, no other material has its combination of physical and mechanical characteristics that are so well suited to specific demanding applications. However, these same characteristics can make machining difficult. Ninety-eight-percent pure beryllium is expensive, selling from $600 to $800 per pound prior to machining, so shops must understand its machining idiosyncracies to avoid scrapping progressively expensive parts. Also, airborne beryllium particulates are toxic requiring shops to follow stringent safety precautions for safe machining.

Beryllium metal (>98 percent Be) is extremely stiff and lightweight and has a modulus of elasticity almost 50 percent greater than that of steel with only about one-fourth the weight. "Beryllium, with a tensile strength about half that of alloy steels, will not take as much load, but with a higher modulus of elasticity, it will not deflect as much under a given load," says Ron May, a former process and applications engineer at Brush Wellman Inc. (www. brushwellman.com) and now a consultant. "High stiffness and light weight translate to accurate positioning of high-end instruments and optical equipment and the ability to withstand high stresses, such as those encountered during spacecraft liftoff and in military applications." Beryllium has good thermal characteristics and nonmagnetic. Because it is transparent to X-rays, ultra-thin beryllium foil is used in X-ray lithography for reproduction of microminiature integrated circuits, and the material is used in virtually all X-ray generators. Beryllium is used in nuclear reactors as a reflector or moderator because it has a low thermal neutron absorption cross section.

Machine with care
Beryllium parts are tightly toleranced and heavily machined. Typically, 90 percent of beryllium stock is machined away during production. Though extreme, for example, a 44-pound part might result from machining a 600-pound beryllium billet. Because of the high cost of beryllium, producers such as Brush Wellman are working on programs to produce near-net-shape stock to reduce shipping costs and machining requirements.

Beryllium is not ductile. This contributes to its stiffness and stability and means it can be machined accurately. However, it also has characteristics that make it similar to powdered metal or sintered parts, and it fractures easily if not machined properly. Although it is relatively soft (Rc 11), beryllium is abrasive and hard on tools. High-quality carbide tools are required to machine this material. Cutter life is about 10 percent that when cutting 6061 aluminum. Generally, a Grade 2 generalpurpose carbide for cast iron and nonferrous materials is best. Tools must be kept sharp because dull tools create surface stresses and cracking that makes it difficult to maintain tolerance. Tool coatings are usually of little benefit because they rub off due to the abrasiveness of beryllium. Also, some shops report that the carbide substrate of some coated tools may not be of top quality, so once the coating is gone the tools quickly fail.

Stringent process controls are required with beryllium. "A general rule-of-thumb is that acceptable tolerances on all aspects of processing pure beryllium are about half those required with other materials," notes Greg Westbrook, engineering manager at LA Gauge Co. (www.lagauge.com), Sun Valley, Calif., a fabricator of beryllium components.

Beryllium is machined by all common metalworking processes; however, tapping can be difficult because of the high surface stresses involved and the lack of ductility of the material. If the machinablility factor of free-machining steel is rated at 100 percent, the machinability factor or beryllium is 55 percent. Using the same comparison, titanium and its alloys range from 18 to 38 percent and aluminum and brass are 120 to 200 percent. Grinding is not generally used to remove large amounts of beryllium, and usually is reserved for close-tolerance work and to produce fine surface finishes.

Cutting beryllium produces surface microcracks, called twinning, which extend to about 10 percent of the depth of cut. Surface damage can be prevented by proper machining procedures followed by stress relief heat treatment or chemical etching to remove damaged surfaces.

To minimize stresses, depth of cuts should progressively decrease by about 50 percent from the previous cut. For example, a typical machining sequence might include an initial rough cut to remove about 0.050 in. of material, followed by a 1,425° F thermal stress relief. A semifinishing cut would then remove about 0.030 in. of stock followedby another that removes about 0.010 to 0.015 in of material.-A finishing cut removes the final 0.005 in. of material. Because beryllium is brittle, extremely notch sensitive and can be damaged easily by improper handling, tooling fixtures and accessories must minimize bending forces and resultant deflections or vibrations from tool pressures. Soft-jawed chucks with large contact areas are best.