Page 17 - MetalForming March 2019

The width of the strip used to make the component part also can affect formability, increasingly more impor- tant with the trend toward smaller elec- tronic parts. Narrower strip can make sharper bends, but the effect is signif- icant only when the strip becomes quite narrow. For example, when the width- to-thickness ratio drops below 8:1, more localized necking occurs and thus sharper bends become possible, according to the Copper Development Association Design Guide. Clean cut edges become important at this ratio with fractures likely to initiate at the edge.
The formability of copper and cop- per alloys (including bronzes and brass) depends on alloy and temper. Some alloys, such as C51000 (phosphor bronze 5 percent), can display very sig- nificant directionality after rolling.
For deep drawing applications, the deepest draws occur with alloys C52100 (phosphor-bronze 8 percent), followed by the brasses in order of decreasing zinc level. The LDR of cartridge brass (C26000) increases with increasing grain size.
Beryllium copper (BeCu) in the cold worked condition exhibits sharper cuts and less burrs than in the aging tem- pers. NGK Berlyco recommends a punch-and-die clearance of 5 to 10 percent of material thickness per side. When using dead soft (annealed) mate- rial, clearance should be reduced to 5 to 6 percent per side. Carbide tooling can be used where delicate parts with superior finish, or a high degree of dimensional precision, is required. In all cases the tooling must be kept sharp.
Depending on the depth of the draw, dead-soft to half-hard (1/2H) material can be used. When required, excessive draw depths may mandate the inclu- sion of an intermediate anneal between successive drawing operations because the copper beryllium has a tendency to work harden more rapidly than other copper-base alloys. For more process-
ing information, refer to NGK Berlyco’s Design Guide. (www.ngk-alloys.com/ NGK_Berylco_Design_Guide_En.pdf ).
Nickel Superalloys: Stronger and Complex
Three types of nickel alloys com- monly stamped include: Hastelloy (nickel-molybdenum alloy), Inconel (nickel-iron-chromium alloy) and Monel (copper-nickel alloy). All are superalloys, also known as high-per- formance alloys. Generally stronger, they work harden more rapidly com- pared to the austenitic stainless steels and, therefore, require greater force to achieve the same amount of deforma- tion. The higher yield strengths of these alloys may also result in greater spring- back.
Another difference: nickel-alloy radii, fillets and clearances are usually larger than those for brass and steel. Also, superior surface finishes often call for such soft die materials as alu- minum-bronze, nickel-aluminum- bronze and zinc alloys.
Rapid work hardening also neces- sitates more frequent annealing treat- ments between forming steps to reach the final product shape. Not recom- mended: lubricants that contain white lead, zinc compounds or molybdenum disulfide when annealing is required because they are difficult to remove and can cause lead, zinc or sulfur to diffuse into the alloy during the anneal- ing process, resulting in severe embrit- tlement.
All die materials, lubricants or for- eign matter should be carefully removed from the workpiece before any intermediate or final annealing treatments.
While there’s plenty to be aware of when stamping nonferrous alloys, understanding how the materials and their properties affect the metal stamp- ing process equips designers and tool- makers to deliver reliable stamping processes. MF
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Tooling by Design