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The Effective & Reliable Nature of Trimetals

April 27, 2015

THE EFFECTIVE & RELIABLE NATURE OF TRIMETALS

Combining the tensile properties of copper with the versatility of silver alloys, trimetal alloys are an effective and reliable choice for carbide-to-steel brazing, such as in toolmaking.  This article explains the value of trimetals as a viable brazing option. 

Three-Layer Composition

A trimetal shim is essentially a three-layer design, similar to a sandwich cookie, with two outer strips of brazing filler metal clad onto a copper middle in a 1-2-1 ratio.

The copper core makes the trimetal unique among alloys. Copper (Cu) withstands high temperatures, resists corrosion, and offers high electrical and thermal conductivity. The outer “jacket” consists of a combination of common filler metals such as:

  • Silver (Ag), the standard for nonferrous and ferrous applications, provides versatility, high strength and low melting points,
  • Nickel (Ni) and Manganese (Mn) aid carbide wetting and strengthen the bond,
  • Cadmium* (Cd) affords higher mechanical strength and a harder, more wear-resistant joint; can also be used at lower working temperatures,
  • Zinc is used in galvanizing and corrosion protection.

Taking the Heat

Trimetals solve one of the most challenging problems in brazing: Overcoming the differences in CTEs (Coefficients of Thermal Expansion) between carbide and steel.

At brazing temperatures, steel expands up to 3X faster than carbide and cools about 2.5X as fast. This differential places tremendous stresses on the carbide component that, if not offset, can lead to warping, separation and cracking. A weaker joint may cause part fatigue, premature wear, or breakage. Due to their unique ductile properties, trimetals compensate for these differences.

As the brazed joint cools, the plastic deformation of the softer copper core reduces the stresses on the brazing filler and the components being joined. The result is a stress-free hardened bond.

For end products that undergo high-impact wear conditions, the ductility of a trimetal bond adds durability. For example, in cutting blades the copper layer’s tensile strength adds a cushion effect that can extend the life of the tool.

Because of their resistance to corrosion, trimetals are also cost-effective alternatives to conventional pretinning. And as with any brazing application, the physical integrity of the joined pieces remains intact because the pieces are not melted in the brazing process.

Applications

Trimetal shims are ideal for joining large pieces with surface areas greater than 0.5 in.2. Common applications include carbide and diamond cutting blades, and heavy-duty blades found on bulldozers, earth graders and snow plows.

Points to Remember

When assessing a trimetal application, consider these factors:

  • Materials to be bonded
  • Type of brazing alloy
  • Usage conditions
  • Size of assembly
  • Heating method 

Thoughtful planning will ensure the success of your project.

 

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