2. Copper Alloys

• 2.1. Introduction
• 2.2. Forming
• 2.3. Joining

2.1. Introduction

A wide variety of copper alloys are available for use in construction. The variations in color stem primarily from differences in chemical composition. Production and forming methods may affect alloy selection. Additional information is available upon request. The CDA publication Copper Brass Bronze - Architectural Applications (PDF 4MB) covers the selection process in greater detail.

Technically, alloys primarily of copper and tin are considered bronzes, while those chiefly of copper and zinc are brasses. In practice, however, the term bronze is commonly used for a variety of copper alloys, including those with little or no tin. This is because they resemble true bronzes in both natural and weathered colors. Table 2.1A lists the characteristics of some of the more popular copper alloys and their common names.

A Unified Numbering System has been developed for metals and alloys by ASTM and SAE. CDA administers the section on copper and its alloys. This system is based on wrought alloy numbers ranging from C10000 through C79999. Cast alloy numbers range between C80000 and C99999.

Nickel-silver alloys C74500 and C79600 are usually called "white bronze"; all others are considered "yellow bronze". "Statuary bronze" and "green bronze" do not refer to specific alloys, but to their naturally weathered or chemically induced colors. The former is used to describe brown to black surfaces; the latter is used for patinas.

In general, most copper alloys eventually weather to the gray-green patina. There are, however, significant variations in their natural colors and in the rate at which they form a patina. The last two columns in Table 2.1A contain information about the natural and weathered colors of the alloys. Table 2.1B is a color matching table. It depicts which alloys, in various forms, are reasonably well matched in color with the sheet, strip, and plate copper alloys.

Copper Alloy Table

Color Matching Chart

2.2. Forming

Numerous methods can be used to form copper alloys into sheet, plate, rod, wire, and irregular shapes. Table 2.2A indicates forming methods appropriate for use with common alloys. The following is a brief description of each method:

Bending: A mechanical forming process performed at room or at elevated temperatures. Bending is accomplished with the aid of rollers, bending shoes and mandrels. Its primary purpose is to produce curved sections from straight lengths of tube, rod, or extruded shapes.

Brake Forming: A mechanical bending operation usually performed on metal sheet, strip, or plate.

Castings: These are produced by pouring molten metal into a mold and allowing it to cool and solidify. This method is used to form irregular shapes. Only specially formulated alloys C80000 through C99999 can be cast.

Explosive Forming: A high energy rate forming method by which shapes are produced using only a single die. The energy is supplied by chemical explosives. Large shapes can be formed without the need for heavy equipment.

Extrusion: The process of producing a metal shape of constant cross-section by forcing the heated metal through an appropriately shaped die. In general, cross-section diagonals should not exceed six inches. The average thickness of copper alloy extrusions should be about 1/8 inch. The resulting shape can be almost any length, limited mostly by the structural requirements of the final object.

Cold Forging: A forming process in which a metal object at room temperature is shaped by repeated hammering.

Hot Forging: A method of forming metal objects in which a heated slug or blank cut from wrought material is pressed into a closed cell impression die.

Hydroforming: A forming process in which a sheet alloy is pressed between a male die and a rubber piece subjected to hydraulic pressure.

Laminating: Bonding of sheet or strip alloys to various substrates such as steel, plywood, aluminum, or rigid insulating material. The bond is typically achieved with the use of adhesives. The resulting panel can be quite strong, even with thin copper alloy material.

Roll Forming: Shapes made from sheet or strip material by passing it between multiple stands of contoured rolls. Generally, the corners are not as sharp as those achieved by extrusion.

Spinning: A mechanical forming process in which sheet or strip alloy is shaped under pressure applied by a smooth hand tool or roller while the material is revolved rapidly.

Stamping: Shaping sheet or strip alloy by means of a die in a press or power hammer.

Forming Chart

2.3. Joining

Mechanical fasteners, such as screws, bolts, and rivets provide the simplest and most common joining technique. They typically do not require specialized tools for installation, and many can be removed for disassembly. Table 2.1B lists the companion fasteners for each sheet or plate alloy by simplifying color matching and reducing the risk of material incompatibility.

Adhesives can also be used in certain applications. The lamination process of a sheet alloy onto a substrate is dependent on adhesive bonding. Relatively thin sheet alloys can be bonded to steel, plywood, aluminum, or certain types of foam, which act as rigid insulation. The strength and rigidity of the resulting composite panel is often achieved by the combined section acting as one unit.

The integrity of the bond is dependent on surface preparation, adhesive selection, bonding procedure, and joint design. Laminated panels for exterior applications should use a thermosetting or high quality thermoplastic adhesive. Edges and joints are the most vulnerable areas on a panel, as they are the most likely entry points for moisture.

There are three commonly used metallurgical methods for joining alloys: soldering, brazing, and welding. Table 2.3A summarizes the joining characteristics of each alloy for these methods.

Where the joining material is required mostly for watertightness, soldering may be used. Lead or tin-based filler metals with melting temperatures below 500 degrees Fahrenheit are typically used. Soldered joints typically depend on mechanical fasteners for strength. This method is commonly used for sealing joints in gutter, roofing, and flashing applications. Since the filler material does not match copper alloys in color, soldering should only be used in concealed joints when appearance is critical.

Brazing is a preferred metallurgical method for joining pipe and tube copper alloys. Two metal sections are joined with a non-ferrous filler material with a melting point above 800 degrees Fahrenheit, but below the melting point of the base metals. Blind or concealed joints are recommended, since the color match of filler material varies. Where this is not possible, mechanical removal of excess material may be necessary.

The final metallurgical joining method, welding, is seldom used with copper alloys because of problems with joint distortion and color matching. Welding uses high temperature or pressure to fuse the base metals together, often with an additional filler metal. Silicon bronzes are the only copper alloys which can be readily welded.

With modern equipment and processes, gas shielded arc welding is gaining acceptance for many copper alloys and use.

Joining Chart