Copper and Brass Forgings

• Introduction
• Advantages of Copper and Brass Forgings
• The Hot Press Forging Process
• Dies and Die Design
• Forgings and Other Products
• Tolerances (Table)
• Inquiry or Purchase Order Information
• Glossary

Introduction

Copper and brass forgings are quality parts, comparing favorably in material integrity, dimensional tolerance and surface finish with products made using other major metalworking processes. As a result of hot working, forged materials have superior density and freedom from flaws. Forming under heat and pressure in precise closed dies produces dimensional accuracy-always repeatable, part to part and lot to lot. Excellent surface finish with a clean lustrous appearance is readily available with copper and brass forgings.

This material will help design engineers and purchasing departments maximize the utility and cost advantages inherent in the specification of brass and copper forgings. It reviews competitive processes, comparing their advantages and disadvantages with forging. A a tabular guide to the tolerances commonly specified for forgings of different types is included. An extensive glossary of terms used in the industry and information required for ordering are included to improve communication between user and supplier.

Unique Combinations of Properties

Copper, brass and bronze forgings offer the designer unique combinations of properties that other metals cannot match. Alloys can be selected to utilize the following unique characteristics:

• High electrical and thermal conductivity
• Superior corrosion resistance
• High ductility
• Outstanding machinability
• Excellent joining and plating characteristics
• Superior polishing and finishing characteristics
• Non-magnetic properties
• Non-sparking characteristics
• Attractive solid colors-not just surface

Advantages of Copper and Brass Forgings

Forgings of copper and brass offer a number of outstanding advantages over parts produced by other manufacturing processes. These advantages result from the inherent properties of copper and copper alloys plus additional improvements in mechanical and physical properties imparted by the forging process.

High Strength

In making a forging, the metal is worked twice under tremendous pressures-first during rod extrusion and then during forging. The double working under pressure compacts the metal and produces a very dense and refined grain or fiber structure.

The tensile strength of the parts is thereby increased, and resistance to impact and abrasion is enhanced.

Leak Resistance

The dense non-porous forged metal permits the designer to specify thinner sections without the risk of leaks due to flaws and voids. Often the thinner forged parts result in lighter weight and lower piece cost compared to other forming processes.

Close Tolerances

A forging produced in a steel die with close tolerances offers several advantages. Overall part dimensions are held closer than in sand casting. Dimensions show minimum variation from part to part and permit automatic chucking and handling in subsequent operations. The precise designs on the die surface can produce sharp impressions on the forging surface, which is not economical with other forming processes.

Low Overall Cost

Mass production of forged parts lends itself to maximum savings. However, smaller quantities of copper alloy forgings can also prove economical when specific design problems must be solved. These problems include leak integrity, close tolerances, high strength with low weight, and non-symmetrical shape.

The Hot Press Forging Process

Rod Cutting

At the brass mill, billets cut from a cast ingot are hot extruded into rod which is further processed before delivery to the forge shop. There the rod is cut into slugs of precise length to insure filling the die cavity during forging.

Slug Heating

The slugs are then heated to forging temperature. To insure a quality part, the forger must carefully control the slug temperature, as too cold a slug will not fill the die cavity completely whereas too hot a slug will result in a porous surface.

Forging

The dies are heated to insure proper metal flow, and the heated slug is placed in the lower half of the die cavity. With a downward stroke of the forging ram, the upper die is forced against the slug to form the part. Since copper and copper alloys are readily forged, most commercial forgings are produced with a single closing of the press, no re-strike or reheating being required. This permits the use of single cavity dies rather than the more expensive progressive dies required by hard-to-forge materials, and reduces labor costs, die costs, and heating costs. Forging rates vary between 100 and 1,200 pieces per hour with the majority of forgings being forged at 200 to 600 piece rates. Where quantities are large and the design is simple, considerable savings result from the use of multiple cavity dies, with more than one part being produced at a time.

It is during this process that the dense grain structure of the hot wrought extruded brass rod is further hot worked to assure a dense uniform product with excellent physical and mechanical properties. These superior properties frequently permit the use of forged parts which are lighter than those produced by other processes.

Trimming and Clipping

To insure complete filling of the die cavity, the weight of the forging slug slightly exceeds that required by the finished forging. To allow for this excess metal, a gutter or relief area is provided in the die surface surrounding the die cavity. During forging, the excess metal is extruded into the gutter as overflow or flash which is removed later in a trimming operation. Good forging die design keeps the amount of flash to a minimum for minimum scrap. Since the flash thickness varies with the ram pressure on the die, the amount of excess metal, the slug temperature when forged, and the die temperature, the flash thickness dimension customarily has twice the tolerance allowed for other dimensions of the part. Where tighter outside dimensions are specified, a close trimming operation known as shaving can be substituted.

Dip Finishing

During the forging process surface oxides form and the parts are coated with lubricants picked up from the dies. Both these surface layers are dissolved by dipping the parts in a series of special solutions. After the forgings are clean, they are generally dipped in a passivating bath to insure a clean surface for subsequent handling.

Due to the stringent requirements of the Environmental Protection Agency, some forging suppliers have substituted a steel ball tumbling operation for dipping to clean forgings.

Packaging

The most common containers for bulk packaging are cardboard boxes. For better dent control during shipping, forgers have developed several improved packaging methods such as layering, nesting, and egg crate packaging, where each individual forging is in a separate compartment. These special packaging methods reduce shipping damage although they increase shipping costs slightly.

Dies and Die Design

In developing a forged part, the design engineer should be familiar with both the limitations and the flexibility inherent in the use of a die to hot form a part.
During each forging operation the dies are subjected to high temperature metal being forced across the die surface. The dies are therefore made of special alloy tool steels which can be hardened to withstand the temperature and pressure cycling required. The average life of a single cavity die is approximately 15,000 pieces, depending on part tolerances and design.

Die Design

It is customary to design dies so the parts produced in a new die are at the minimum end of the tolerance range. As the die wears, the part grows through the range until it exceeds the tolerance and the die must be reworked. If the metal must be forced around sharp corners or small radii during forging, die wear increases several fold. It is therefore best to design parts with generous radii and eliminate abrupt directional change if possible.

Draft

When designing a forging, a draft angle must be incorporated to permit removal from the die. Recommended draft angles are shown on the Tolerance Table on page 12. The tolerance on the draft is one-half a degree. Where designs permit no draft, the forging supplier should be consulted so that press and die combinations can be developed to produce "no draft" forgings.

Webs

Web sections are thin sections in the forging parallel to the forging plane. They are left between opposing die bosses to cushion extreme localized pressures between the two moving parts.

Webs are subsequently removed by piercing or machining.

Construction

The most common die design involves a set of die blocks with each half containing half of the die cavity. Cavities are cut into tool steel blocks which are then hardened and polished prior to use. The blocks are held in the press by holders which are unique to each forging company. During long runs, dies may require removal from the press to clean and polish the interior to maintain surface quality and dimensions.

Surface designs and lettering can be inscribed in the die surface to produce a pattern or identification on the forging. Letters may be raised or depressed, but they must always be in a plane perpendicular to the direction of die travel.

Multiple Parts

Where similar parts require only minor modification, die inserts can produce two or more individual designs with minimum tool investment.

In some designs it is possible to make two different parts by altering only one-half the die. This can be cost effective where the change involves simply a hole or boss diameter in the part.

Design Changes

Occasionally a change in configuration is required during the life of a part. Before ordering a complete new die, consideration should be given to making the change through simple die inserts or restricting changes to one-half the old die set.

If a design change is required, it is much more economical to make the forging larger rather than smaller, thus allowing the existing die to be modified. A smaller forging requires adding metal to the die which is expensive and often reduces die life.

Trimming or Clipping Dies

After the part is forged and cooled, the flash must be removed from the outer periphery prior to dip finishing. The removal of flash is done in a small press with a trimming die, which is frequently not finished until after the first samples are approved. This delay permits minor changes in the trimming die outline if required.

Where a particularly close trim is necessary, the trimming or clipping die can be designed to shave the forging, completely eliminating any flash extensions. Shaving is often a slower process and may increase piece cost, although reducing the need for subsequent finishing requirements.

Die Purchases

When a customer purchases a set of dies, the forger also makes a trimming die set which is usually included in the forging die price. It is common practice in the United States for the forger to maintain or replace both dies, when required, at no additional charge to the customer.

Parting Line

During the forging process, when the upper and lower sections of the closed die are brought together, the planes of separation are called the parting line. Care should be taken when specifying the parting line location, as it can affect the initial cost, ultimate die wear, ease of forging, grain flow, related mechanical properties, and machining requirements for the finished part. Its location must be designated on all forging drawings.

The ideal parting line is a single plane, but multi-level or irregular partings are sometimes needed. When irregular parting lines are required due to shape geometry, an inclined transition must be provided between levels. A vertical break between parting line levels cannot be trimmed.

Holes and Cavities

Cored holes or cavities can be designed into forgings from either the top, bottom, or both sides of the flash line-provided minimum web thickness requirements are observed.

Forgings and Other Products - A Comparison

Pressure-formed forgings owe their durability and overall quality to the unique pattern of grain flow that is an inherent result of the forging process. During initial rod extrusion, the metal is shaped under extreme pressure causing the grain structure to elongate and form new grains that have a fiberlike quality. The forging process re-orients these fibers in conformance to the contour of the part. This double working compacts the metal into a dense non-porous part that will withstand oil, gas or air at high pressures without leaks. It also improves mechanical properties, providing higher impact strength and fatigue resistance.

Castings

Castings provide great flexibility for the production of non-symmetrical parts.

Compared to forgings, sand casting surfaces tend to be rougher and tolerances tend to be greater, and sand inclusions and porosity are sometimes encountered. Where quantities are limited, pieces weigh over 15 pounds and casting surface roughness and tolerances are acceptable, castings would be the economical alternative.

Screw Machine Products

Screw machining produces primarily round parts with good material integrity. Tolerances of ± .003 inches and better can be easily met. Due to long setup times, this process is usually not economical for runs of less than 10,000 pieces. Also, screw machine parts are generally limited to about two pounds maximum weight. Whenever scrap accounts for more than 40 percent of the starting weight of a part, it is likely to be more economical to use a forging.

Stampings

Stamping parts from strip is generally economical with long runs of parts weighing under one quarter pound. Tolerances may typically be held to ±.002 inches with practical wall thickness to 0.125 inches. Forging should be considered when section thicknesses greater than one quarter inch are required.

Machining

Machining, which includes drilling, tapping, milling, grinding, etc., is often combined with forging to produce a final part. These secondary operations are usually provided by the forging supplier. When intricate parts are produced by assembling two or more machined parts, the assembly should be examined to determine if it could be more easily produced as a single forging.

Definition of a Forging

A forging is a metal part that has been heated and formed into a predetermined shape between a set of dies. The process involves heating a metal slug and then pressing it into shape within a die cavity. Not only is the metal shaped, but the forging process also improves the mechanical and physical properties of the part. Forging today is performed on high-speed automatic equipment.

The forging process referred to throughout this publication is performed in presses, also identified as closed-die or impression die forging. While there are other methods of producing forgings, they are unlikely to yield parts suitable for current industrial products.

Inquiry or Purchase Order Information

To assist the supplier in providing forgings, the following information should accompany inquiries and purchase orders:

1. Part name and end use
2. Current engineering drawing with notations as to finishing, lettering, heat treatment, minimum properties, and present method of manufacture
3. Sample part if available
4. Quantities, both the initial order, subsequent release schedules, and annual requirements
5. Material, by alloy number, with alternate materials permitted
6. Finish, surfaces to be machined and description of special finishes
7. Dimensional tolerances, commercial tolerances and any critical dimensions which apply
8. Specifications which apply such as ASTM, ANSI, SAE, Federal, or Military
9. Operating environment such as:
   • Pressure requirements
   • Temperature requirements
   • Corrosive atmosphere
10. Physical or mechanical requirements

Packaging instructions, where critical surfaces must be protected by spacers, layering or nesting.

Since the cost of dies and die life are very closely associated with tolerances, the purchaser or user is urged to specify the maximum drawing tolerances allowable. Where tradeoffs on tolerances may occur, the forging supplier can suggest the most economical solution.

Glossary

For an extensive list of terms on forgings, see Forgings Guide Glossary of Terms.