I. Introduction
Bronze and copper have long been used in manufacturing applications. Both were discovered long before aluminum or steel came onto the scene and continue to play an integral part of modern manufacturing today. Bronze is an alloy of copper which makes determining their differences difficult due to both being known as red metals; nevertheless there are subtle differences in their properties.
Engineers and product designers depend heavily on understanding the differences among metals to select the optimal material for any specific application, depending on its properties. When toughness and strength are needed, bronze’s high tensile strength makes it the optimal choice, while copper’s formability and malleability determines its applications.
This article will explore the differences between copper and bronze in terms of their chemical composition, physical and mechanical properties, formability and their applications.
II. Composition
A. Chemical makeup of bronze
Bronze is an alloy of copper, meaning it is made up of copper and tin as the main elements. The chemical formula of bronze is CuSn. Apart from these two elements, other elements can be added in different proportions to make up different types of the bronze alloy. Some of these elements include Nickel, Lead, Chromium, Aluminum, Manganese, Phosphorus, Silicon, etc. The bronze alloy assumes the name of the other element added, for example, when manganese is added, it becomes manganese bronze. All of these additional components affect the properties of the bronze alloy. Adding phosphorus enhances the shiny color of bronze while chromium increases the strength and endurance of the alloy.
B. Chemical makeup of copper
Copper, unlike bronze, is a naturally occurring non-ferrous metal. Copper appears in its pure form because it is mined straight from the earth. The chemical formula of copper is Cu. Although copper is referred to as a pure metal, when mined raw from the earth, it can contain some impurities which are removed during processing. Most people prefer using copper in manufacturing products because of its pure form. It is also used as a core alloying material for bronze and brass.
III. Physical Properties
A. Color and appearance of bronze
Bronze has mainly a reddish-brown color because of its composition of copper and tin. Bronze has other hues of brown depending on the other elements that might be added such as dark brown, gold or orange. When left in the open, bronze can oxidize changing its color to a dark golden hue. This is the easiest way to differentiate bronze from copper by just looking at both metals. Another observable feature of bronze is that it has faint rings on its surface.
B. Color and appearance of copper
Copper has a bright red color that ranges in different hues depending on its purity. Brighter red hues indicate higher quality of copper while darker hues indicate lower purity. Copper has a cubic crystalline structure that reflects orange and red light and absorbs the other colors emitting a bright red appearance. When exposed to moist air, copper oxidizes forming a green film layer called a patina that protects it from further oxidation.
IV. Mechanical Properties
A. Strength and hardness of bronze
Bronze has better strength and hardness properties than bronze. Hardness of a material is defined as the ability of the material to resist deformation when a specified force is applied to the material on a flat surface. According to the Brinell hardness scale, bronze has a hardness of 40-420. The tensile strength of bronze ranges between 350 to 635 MPa. Although bronze is harder than copper, it is also more brittle. This means that it can break when forming or machining. Bronze has also a higher yield strength compared to copper. Yield strength refers to the ability of a metal to resist permanent deformation when exposed to a specified amount of force. The yield strength of bronze is 69-800 MPa. This makes bronze suitable for applications which require high metal strength.
B. Strength and hardness of copper
Although both copper and bronze have almost similar properties, copper has a relatively lower strength than bronze because of its pure state. According to the Brinell hardness scale, copper has a hardness of 36. Copper has also a lower tensile strength and yield strength compared to bronze. Copper has a tensile strength of 210 MPa and a yield strength of 33 MPa. Because of its softness and high machinability, copper is suitable for manufacturing electrical components which require precise and accurate machining. CNC Machining is the preferred method of machining copper because of its computer aided precision. Copper has better thermal and electrical conductivity than bronze, making it suitable for manufacturing electrical wires.
V. Corrosion Resistance
A. Corrosion resistance in bronze
Bronze has excellent corrosion resistance properties because it is an alloy of copper and tin. The presence of copper enables it to oxidize forming a mottled patina. This is a protective film that forms on the surface of bronze and prevents further oxidation and corrosion. Because of its corrosion resistance properties, bronze is useful in manufacturing parts that are used in salt water environments such as marine fitting and equipment. Some of the parts that are constructed using bronze include steam pipes where there is regular heating and cooling, marine shafts, bearings, and propellers. Although bronze is corrosion resistant, this can change when it is exposed to chlorine compounds. These chlorine compounds eat the copper and bronze, and it degrades. This is called the “Bronze disease”.
B. Corrosion resistance in copper
Copper has also good corrosion resistance properties because it oxidizes to form a mottled patina. When not oxidized, copper can corrode or cause corrosion to other metal parts nearby. This is why most copper products are treated or painted to increase their resistance to corrosion. Since both copper and bronze do not have iron present, they have excellent corrosion resistance properties. Bronze has better corrosion resistance than copper because it is an alloy and has other elements such as nickel and silicon which improve its resistance to corrosion.
VI. Machinability and Workability
A. Machining and forming characteristics of bronze
Machinability refers to the score of how well a material behaves during machining, or how well a material reacts to the stress of machining. Most copper alloys have good machinability properties. Bronze specifically is a hard metal that is ductile and brittle because of tin and other additives that might be present. You should take care when machining bronze because it can crack or break during machining.
Regarding weldability and workability, bronze is a hard and study metal. It may crack when welding. To avoid this, you should use shielded metal arc welding (SMAW). Silver soldering is the suitable method to weld copper alloys such as bronze and brass.
Rapid prototyping can be used to test the machinability and formability of various alloys of bronze so as to test the one with the best properties for a particular application.
B. Machining and forming characteristics of copper
Copper has better machining and forming characteristics than bronze because of its high ductility and malleability. Copper is mainly used by product designers during mold design and mold manufacturing because of its easy forming. Copper is a relatively soft metal which means that it can take any form easily without deformation. Welding is also easy for deoxidized copper. The common methods used to weld copper are MIG and TIG. You can also use oxyacetylene and MMA to repair welds of tough copper patches.
VII. Applications
A. Common uses for bronze
As a copper alloy, bronze has many applications. Bronze is commonly used to make musical instruments because of its corrosion resistance, toughness and the golden hue finish. It is also used to make marine ships’ parts such as propellers and bearings because of its resistance to friction, wear and tear resistance and hardness. Bronze is used in automotive engineering to make automotive parts such as springs, bearings, bushings and small electric motors.
Bronze is also used to make bronze head hammers and mallets because it does not produce sparks when struck and it does not deform easily because of its high tensile strength and hardness. It is also used to make sculpture casts because of its hardness, golden finish and corrosion resistance. Bronze was commonly used by the ancient Greek for sculpting.
Bronze has different alloys that have different applications as explained below:
863 Manganese Bronze – This bronze alloy is known for its high tensile strength and is suitable for heavy duty applications that require durability and resistance to wear and tear. Some of the common applications of the 863 Manganese Bronze in the manufacturing industry include load bearings, bridge pins, screw-down nuts, stem valves, components of hydraulic cylinders and gibs cams.
907 Tin Bronze – This bronze alloy has very high corrosion resistance and is mostly used in steamy areas such as construction of steam pipes. Its corrosion resistance makes it suitable for use in parts that are exposed in seawater such as marine and ship parts. Common parts made from 907 Tin Bronze include bearings, marine fittings, bushings, piston rings and pump components.
917 Tin Bronze – This bronze alloy has a very high load carrying capacity. It performs well in both slow moving and irregular load capacity. The alloy is also corrosion resistant but requires regular lubrication to minimize wear and tear. Parts made from the 917 Tin Bronze include worm drive wheels, bridge turntables, slow and high speed bearings, gears and moving bridge components.
955 Aluminum Bronze: This is the most superior bronze alloy. It exhibits the highest properties in corrosion resistance, tensile strength, weldability, thermal conductivity, and stress resistance. The 955 Aluminum Bronze is used to make: aircraft engines, landing gear components, bushings, worms, and agitators.
B. Common uses for copper
Copper occurs in a pure state. This is why most people prefer using copper in their projects. Where strength and hardness are required, copper is converted into one of its alloys; bronze or brass. Copper was commonly used in boat hulls to control sea fish and weeds from growing on the boats. The common applications of copper in the manufacturing industry include:
Manufacturing wires and cables – Copper has very high thermal and electrical conductivity. In fact, it is used as the standard of measuring electrical conductivity in metals because it has 100% electrical conductivity. Because of this, copper is the perfect element for manufacturing electrical wires and cables. It is commonly used in telecommunication, generation of power, transmission distribution, making circuits and other electrical components which use wiring.
Electronic devices – Copper has been used in place of aluminum in circuit boards because of its better electrical conductivity. It is also used in air conditioners and heat exchangers because it has superior thermal conductivity and heat dissipation. Other applications include electromagnetics, vacuum tubes and cathode ray tubes among others.
Architecture – Since the old times, copper has been used in building construction to make doors, vaults, roofs, roof gutters, domes etc. because of its corrosion-resistance and waterproof properties. Copper is used as an added advantage to both interior and exterior design in modern day architectural design. It is used to make countertops, kitchen fittings, handrails among others.It is also used in building construction for radio frequency shielding, exterior wall cladding and expanding building joints.
Antimicrobial use – Copper is excellent in antimicrobial activities which prevent organisms such as Escherichia Coli. The copper alloys that prevent this organism through the antimicrobial activities are used in the health sector to make parts such as hospital sinks, toilets and over-bed tables among others. Countries which use this application include China, UK, Japan, Korea, Brazil etc.
VIII. Conclusion
Bronze and copper are metals of the same family with almost similar properties because bronze is a copper alloy. It can be challenging to notice the difference between these two metals although there are subtle differences. One of the main differences is in their chemical composition. Copper is a pure metal occurring in a natural state while bronze is composed of copper and tin, although other elements such as nickel, manganese and aluminum might be added to achieve certain properties. Bronze has a higher tensile strength and toughness than copper although copper has better electrical and thermal conductivity. Both metals have good corrosion resistance properties because they oxidize to form a mottled patina but copper has better machinability and formability properties than bronze, because it is relatively softer and more ductile while bronze is tougher and brittle.
Ultimately, when selecting the material for your project, you should consider the properties you need. For applications where there is a lot of load carrying and stress, strength and toughness are vital. In this case, bronze is your best option. Where you need high electrical and thermal conductivity, and machinability and forming, then copper is the best option.
