Brass stampings: machining, plating and spring properties

What brass brings to a stamped part

Brass is a copper zinc alloy. It is cheaper than pure copper, stronger than pure copper, and easier to cut and form than most bronzes. It plates well, solders well and takes a polish. Those properties made it the traditional metal for socket contacts, switch parts, terminals, shields and decorative hardware.

The trade-off is conductivity. Zinc lowers conductivity compared with pure copper, so brass carries less current for the same section. It also has modest spring properties and loses force faster than phosphor bronze when it gets warm. A brass terminal that feels right on the bench can relax in a warm enclosure and lose contact pressure over a few thousand hours.

The grades that matter in a press shop

Four families cover most stamped brass parts. Cartridge brass is the general forming grade. Yellow brass is a close relative with slightly more zinc. Phosphor bronze is the spring grade. Nickel silver is the corrosion and appearance grade. The table sets out the split.

Temper, spring and stress relaxation

Brass strip is supplied in tempers from annealed to hard. The harder the temper, the higher the yield strength and the more spring it holds, but the tighter the bend limit and the greater the springback. A hard temper brass contact can be formed to a small radius and still hold a useful force, provided the bend line is across the rolling direction and the edge is sound.

Spring properties are not only about the initial force. Stress relaxation is the loss of force under a sustained load at temperature. Brass is more sensitive to this than phosphor bronze or beryllium copper, which is why brass is a poor choice for a contact that sits compressed in a warm housing for years. If the design needs a guaranteed force after ageing, test the actual grade at the service temperature.

For parts that are formed and then machined, the temper also affects the secondary operation. A hard temper part can chip or burr when tapped or drilled, and the work hardened skin can deflect a small drill. A stress relief anneal before machining may be cheaper than scrapping tools and parts.

Dezincification and stress corrosion

Brass in contact with water can lose zinc from the surface, leaving a porous copper rich layer. It is a concern in plumbing and in parts that see standing water, particularly warm water with chloride. Standard cartridge brass is not the right choice for that duty; dezincification resistant grades exist for it.

The second risk is stress corrosion cracking. A brass part that has been cold formed carries residual tensile stress at the bend. In the presence of ammonia, amines or certain cleaning chemicals, a crack can start and run through the section, often while the part is in storage.

Stress relief is the control. A low temperature anneal after forming removes most of the residual stress without softening the part enough to lose its spring. The temperature and time depend on the grade and the amount of cold work, and the supplier should confirm them with a hardness check rather than a guess.

• Avoid brass for parts that sit in warm, chlorinated water unless the grade is dezincification resistant
• Specify stress relief after severe forming for parts that see ammonia or amine exposure
• Keep cleaning and passivation chemicals away from formed brass unless they are proven compatible
• Do not store formed brass in closed containers with wet packaging

Plating and the finish that stays

Brass plates readily, which is both an advantage and a trap. A thin tin or silver finish over brass can look perfect and still fail a porosity test, because the brass surface underneath was not clean or the deposit was too thin at an edge. Zinc in the substrate can also diffuse into a finish at elevated temperature and change its colour and solderability.

Nickel is a common barrier layer under tin, silver or gold. It slows diffusion and gives a harder surface for a wiping contact. For a low resistance contact, silver over nickel over brass is a practical stack. For a solderable terminal, tin over nickel over brass is common.

The stamping process has to protect the finish. Sharp burrs, torn edges and burnished surfaces are hard to plate evenly, and a burr can act as a high current density point that builds a thick, brittle deposit. If the part is plated after stamping, agree the edge quality and the racking method. If it is pre-plated, protect the coating at every bend.

Stamping and secondary machining notes

Brass blanks cleanly with a modest clearance and leaves a good edge. It work hardens less than stainless and does not gall as readily as pure copper, so die maintenance is relatively light. The common defects are slivers, burrs on a thin web and cracking at a tight bend in a hard temper.

Secondary machining is where brass earns its reputation. It turns, drills and taps with short chips and a good surface finish, which is why so many brass parts are stamped and then machined. The catch is that free machining grades rely on lead, and leaded brass is brittle in forming. Do not specify a free machining grade because the part has a tapped hole. Use a formable grade and accept a shorter tool life, or move the part to a turned process.

Lead content is also a compliance question. Some brass grades carry lead for machinability, and the applicable restrictions depend on the market and the application. Ask for the material declaration and the lead content before the drawing is released.

What to put on a brass stamping drawing

Name the grade and the temper, not just brass. Add the strip thickness and tolerance, the finish and its thickness, the bend radii and the grain direction where it matters. If the part is a contact, add the force at the working position and the maximum operating temperature.

For a part that will be machined after stamping, say which features are machined and what the datum is. A stamped part with a machined hole needs a clear relationship between the two, or the hole will drift with the stamping tolerance. For a part that will be plated, say whether the edge is visible and what coverage is required.

Finally, decide whether the part is really a brass part. If it needs high conductivity, use copper. If it needs spring at temperature, use phosphor bronze or beryllium copper. If it needs corrosion resistance in water, use a dezincification resistant grade or a different alloy family. Brass is a good default, not a universal one.