What is Bismuth Bronze?
Bismuth Bronze can be made in many more ways than just adding tin to copper – in the modern age, it now designates a class of unique copper alloys. These alloys and their beneficial qualities allow them to apply to many sectors and industries, and for this reason, it can be daunting when choosing a bronze for any one job. This article will help simplify things by outlining one type of bronze, bismuth bronze (also known as lead-free bronze), and its strengths, properties, and uses. By doing so, this article will help designers interested in bismuth bronze in guiding them through their material selection, as well as providing extra information on such a useful type of bronze.
But before we continue, it may be helpful to review our article on the types of bronzes, as it shows the scope of bronze and its general characteristics.
Figure 1: Qualitative breakdown of bismuth bronze. This chart is a visual representation but note that specific alloys can contain other elements, depending upon manufacturing methods.
Bismuth bronze (also known as lead-free bronze/lead-free brass) is typically composed of 1-6% bismuth, 1% nickel, 2-4% tin and zinc, as well as various other elements in small quantities. It is primarily manufactured to address the concerns of mainstream leaded bronzes, namely that they are both difficult to recycle and harmful in certain applications. Note that lead is still present in most bismuth bronzes, but only at low quantities that pose no real threat (around 0.25%). They are comparable to leaded bronze in both machinability and workability, though bismuth can embrittle copper in large doses. Bismuth bronzes have high thermal conductivity, are corrosion resistant, safe to use for drinking water applications, and interchangeable with older alloys, meaning bismuth bronze can be used to replace non-compliant parts. It can be cast, brazed, soldered, and joined in much the same way as leaded bronzes, and works in many of the same plumbing, fastener, and bearing applications.
Resistances and weaknesses
Bismuth bronze has great machining qualities, thanks to the high lubricity of the included bismuth. They are also resistant to saltwater corrosion and pour well into molds. The material will polish better than other bronzes and is a practical alternative to lead bronzes (albeit, with some strength loss – a result of replacing lead with bismuth). They tend to be more expensive and more difficult to recycle, making them a hard-financial sell if leaded bronze can suit the application; however if not, bismuth bronze is a fantastic non-toxic alternative that is slowly phasing out traditional leaded alloys.
Mechanical Properties
Table 1 below displays some important mechanical properties for bismuth bronze; this next section will explore each of these properties and how they relate to common bismuth bronze applications.
Table 1: Summary of mechanical properties for bismuth bronze – these values are aggregates from many alloys and are for general illustration purposes.
56 W/mK
Mechanical Properties
Metric
English
Tensile yield strength
100-200 MPa
14500-29000 psi
Percent Elongation
15-30%
Thermal Conductivity
32 BTU ft/hr ft2 F
Hardness (Brinell)
50-70
Machinability
80-90%
The tensile yield strength is a common parameter that represents a material’s strength in tension. It describes the force value before which no plastic deformation occurs and is a good “first-glance” measure of a material’s strength. It should be no surprise that bismuth bronze has a low yield strength when compared to other bronzes, as it is designed to be easily formed and machined, and not selected for its strength profile. This does not mean it is weak per se, but consider a stronger bronze such as manganese bronze if high strength is of priority.
Percent elongation is a value that shows how much a material will deform before fracture, i.e. how ductile the material is. It is given in a percentage because it represents the final length compared to the original length; for example, if a specimen is 1 inch before a tensile test and 1.5 inches after, its percent elongation is ((1.5-1)/1) x 100 = 50%. A higher percent elongation allows for easier machining, forming, and overall workability. Bismuth bronze has a comparable percent elongation to traditional bronzes, meaning it can function similarly in most, if not all applications.
Thermal conductivity is a value that describes a material’s ability to conduct heat. This value has implications for joining procedures because poor conductivity introduces risks of joint failure. Luckily, bismuth bronzes function similarly to leaded bronzes and have an extensive history being soldered and brazed effectively. This means that bismuth bronze can work just as well in plumbing applications, with the added benefit of protecting from lead exposure.
Material hardness is determined using one of many available indenter machines, each with their own hardness scales. Table 1 provides hardness values for the Brinell Hardness indenter, which is used to rate various metals such as iron, steel, and bronze. For reference, the Brinell hardness of cold-worked copper is 80; bismuth bronze is, therefore, softer than copper and will more easily deform, scratch, or dent. This further proves why this material is so easy to machine, as it gives way to such stresses. It also means that this alloy is barred from being used in especially abrasive applications, as scratches will cause bismuth bronze to weaken, even despite its self-lubricating qualities.
Bismuth bronze has excellent machinability and this is one of the primary reasons to specify this copper alloy. Typically, metals are rated for machinability against a standard material, which is assigned a score of 100% (for bronze, this material is UNS C36000 free cutting brass). Anything with a machinability rating close to 100% will be similarly easy to machine, and this is abundantly clear with bismuth bronze’s score of 80-90%. It will not wear down bits fast and will behave during milling, making it well suited for high production machining operations. This mechanical property is doubly important, as designers who once specified leaded bronzes can now simply switch them out for a non-toxic substitute, without the need to buy new equipment or further train their machinists.
Applications of Bismuth Bronze
Bismuth bronze has been around for many years but is still finding more applications. Even though many of their uses are limited to tightly regulated plumbing components, bismuth bronze is being researched as an environmentally friendly substitute for traditional copper alloys. Below is a list of some common applications of bismuth bronze, but know that many more exist and will exist as time continues.
Some applications include:
Lead-free bushings and fittings
Lead-free pipes
Continuous castings
Piano strings
Mirrors
Consider bismuth bronze if machinability and sustainability are of concern for your project. Contact your supplier and let them know you are interested, and they will confirm if bismuth bronze is a good match or if another copper alloy will function better.
Summary
This article presented a brief summary of the properties, strength, and applications of bismuth bronze. For information on other products.
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