Part II: An alloy well-suited for today's trend toward
By L. Michael Halleran
|In the spring 1996 issues of SPRINGS, beryllium copper was presented as an alloy in great demand for demanding applications. This article provides test data on the material's mechanical properties and recommends pretempered beryllium copper wore for overall performance advantages.|
|Since the 1960s, many great
strides have been made in the production and manufacturing of high quality metal formed
parts. The progress of past decades has come to the point where speeds have increased a
hundred-fold to thousands of strokes per minute, wires are pointed and formed on bandoleer
strips, and computer-programmed coilers and multiple slides produce countless varieties of
wire forms without secondary operations.
Implicit in the versatility and increased speeds of metalforming technology has been the ability of finer, thinner, narrower, smoother, straighter metals to be bent, cut, punched and machined faster, and for longer periods of time, without costly interruption or down time.
Indeed, higher production rates of manufacturing demand much more consistent materials, improved mechanical properties, stringent tolerance envelopes and near-perfect surfaces.
Beryllium copper, a high strength copper-base alloy system (0.5-2.0% Be, balance Cu), has long been a technology front runner for formability, conductivity and strength.
As a select engineering raw material in years past, the high strength levels from beryllium copper alloys have been reached through heat treating (precipitation age-hardening) cold worked and formed parts to peak mechanical properties. Today, it is still widely recognized that maximum strength properties can only be developed this way. As such, the beryllium, which is placed into solution in prior annealing, precipitates gamma phase Cu-Be composition into the grain boundaries. It is the strength in the grain boundaries that gives beryllium copper its strength. In addition, the lack of soluble beryllium copper in the copper matrix explains the slight improvement in conductivity.
During the precipitation hardening process, the result of movement of beryllium out of solution allows the density of the alloy to actually increase from .297 lb./cu. in. to .302 Ib. cu. in. (8.26 gm./cu. cm. to 8.37 gm./cu. cm.). Therefore, parts made from age-hardenable beryllium copper will shrink, and maybe distort in length or volume as a result of age-hardening.
The shrink distortion from heat treatment occurs when varying degrees of worked and non-worked areas, or non uniform worked areas are present. The distortion occurs when areas with greater cold work age-harden and shrink faster than areas with less work or no cold work. Although it can occur to a much lesser extent in hard tempers with equivalent cold work, the effect is more dramatic in parts having soft tempers with concentrated cold work in bend sections. Distortion in soft tempers may be influenced by the longer time in the stress relief phase due to the longer time required for peak-aging completion.
Raising the temperature to 600-800 F and aging for a shorter than usual time may reduce distortion in either case. Trial lots in 25 increments is suggested. No shrinkage occurs when stress relieving previously age-hardenedmaterial. (However, distortion may occur in the stress relieving process because high-stressed material is relieved more quickly than that which is low-stressed.)
Most experts will agree that cold-worked and age hardened beryllium copper does produce maximum mechanical properties. The table on page 76 confirms that C172 in extra hard and aged temper TH06 has a tensile strength ranging from 205-235 ksi. (1415- 1620MPa). Unfortunately, total cost and quality complications often do not allow for such straightforward thinking and processing. In fact, as wires are used in finer applications and in smaller diameter sizes than ever before, the advantages of "formed and heat treat" are increasingly fewer.
Therefore, springmakers still get the most benefits from using supplier-produced pretempered (ASTM designation TL08) wire. It enables metal parts to be finished directly off presses, slides and coilers, and sent straight to shipping or parts assembly. Pretempered beryllium copper is precipitation age-hardened, then drawn or rolled to final properties.
Pretempered beryllium copper is produced to distinct combinations offomiability and strength. All wire must be capable of fabrication to a I? bend without severe "orange-peeling". A 11 bend is equivalent to bending the wire around its own diameter in a tight-wrapping fashion.
The pretempered section of the table is the product of years of process-data collection. The table designates six combined size and property ranges produced for the spring industry. If needed, process modifications can enhance one mechanical property slightly over another. The pretempered wire size ranges have been designed to accommodate the different stresses associated with bending different diameter wires around their own diameter. These stresses are directly proportional to the thickness and yield strength of the wire itself. As miniaturization and micro-miniaturization continue at a hare's pace, sublime issues will continue to push the advantage onto pretempered wire's side.
A growing issue is wire surface and especially smoothness. "Form and heat-treated" and even "pretempered, formed and stress relieved" springs, when exposed to elevated temperatures, lead to the
necessity of solving more complicated issues, such as removing heat treatment oxides without over-etching the surface. Over-etching beryllium copper should be avoided at all costs, due to the already significant notch-sensitive nature of it and other copper-based alloys. Although peak-aged beryllium copper has its maximum strength, it is most resistant to further bending.
Pretempered beryllium copper is inherently tougher and more resilient than peak-aged. Sometimes, using silver or other preplated wire may reduce the coiling stresses below the level needed to stress relieve. Premium surfaces can be realized if bypassing stress relief is possible.
Miniaturization is here to stay. The vision for the future includes not only a continuation of this trend, but also an increase in the specialty features of commonly processed materials, many of which can be achieved with beryllium copper. These include: fine special shapes, narrow widths, multiple gages, rolled-in features, textures and patterns, in concert with precious and semi-precious metal alloy platings, some on specialty cladded base metal combinations.
This article is reprinted from Springs, the magazine of spring technology, official publication of the Spring Manufacturers Institute, Summer, 1998, Vol. 37 No. 3, pages 75-79. All rights reserved.
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